Posts by Rune

Rune · Interplanetary transportation

Thanks for the replies, guys! Good to know all that writing got read.

Glandu wrote:

waow. That's the kind of things I'm not equipped to understand. I just want to know if I read it right : 40 million metric tons of atomic bombs?

Yup, you read right. The ship has a mass ratio of 5, and the propellant is comprised of the pulse units themselves, so 4 tons of bombs for every ton of ship. And by the efficiency figures I quoted, most of that is Deuterium turning into helium before impacting the plate. I know, it sounds insane, but look at that jet power! This is what interstellar propulsion looks like in the real world.

Mark Friedenbach wrote:

Is there any way you can reasonably work the numbers to get it down to a century or less (0.2c)? Any longer than that and it is possible that regular technological progress would allow a later generation to leave after you and get there first (using, say antimatter propulsion).

I think I already went as far as you could go maximizing parameters. Mass ratio looks already absurd enough when you consider the fuel... you know how the rocket equation works, I'm sure, so you can see how you might need a mass ratio in the millions if you can't improve on the isp. And remember that the maximum theoretical debris velocity in a pure D-D reaction is only twice the one needed to get my effective isp... if that is the only efficiency loss. I'm sure you know that translates to an exhaust speed of some 7,500km/s, and I see it as very difficult to increase that number by a factor of ten.

Theoretically, yes, you could go even bigger and faster than that with antimatter, after all I am only converting a tiny percentage of the mass into energy. But not without first finding out a way of producing and storing that much antimatter. And I'm talking millions of tonnes of antimatter here to manufacture these "advanced pulse units". And you also run into the problem of a significant fraction of the energy being released as weird particles like muons and pions that don't care much that the pusher plate is on their way.

Any other drive runs into the problem of the jet power: efficient as you can be, I am talking about a hundred Petawatts (with a "P", yes). No matter how little leaks from the drive system, it is more than enough to melt any ship. In the case of pulse propulsion, however, the magic of open systems and nanosecond detonations solves both: whatever little fraction gets deposited on the ship as heat goes into ablating a few microns of plate, nothing else. Consider this: going by jet power, the bomb exploding every three seconds behind the ship is the equivalent of some 72 megatons of TNT, which would make it bigger than the Tsar bomba, the most powerful nuclear test ever. Any other drive system has to match that energy at least, and improve on energy density if it wants to increase the specific impulse. BTW, if you can't make bombs big enough for the thrust required, you could increase the frequency. As long as you get the equivalent of a 24 megaton bomb going off every second. And also, you'll have to forgive me, but I caught a typo, the effective jet power should be ~90.2x10^15 watts, so quite different than I last wrote (If I haven't screwed with the decimal points again).

GW Johnson wrote:

You're exactly right about the nuclear "shaped charges", that's exactly how to get a spindle shape of EM radiation pulse from the explosion. In space there is no blast wave at all, just that pulse of intense EM radiation. If it's spindle-shaped, your pusher plate can be made to intercept one end (half) of it, at the design detonation distance. It's a diameter-at-range thing (constant angle).

I know, it's some kind of lensing effect produced by the plasma distribution at the beginning of the blast, originally thought up for the Casaba Howitzer, right? Or something similar, my particle physics are not what I would like them to be. It only depends on the bomb composition and mass distribution, so I figure you can get the same effect on H bombs. Lucky, too, otherwise efficiency drops by a huge factor.

GW Johnson wrote:

The hardest thing about your starship isn't the ship or its propulsion, it's the closed ecology life-support, and the organized "miniature industrial society" setup to maintain all the technologies and hardware and pulse "fuel" for centuries. We get those items down right, and multi-generational interstellar flight becomes a realistic thing to consider doing. We may even have some reliably-identified more-or-less-Earthlike target exoplanets to visit, by the time we get that support stuff "right", if we started right now and worked on it (but, we are not doing those things).

Yeah, we completely agree on that point, this is the critical point in the concept. But the way I see it, most of those problems also need to be solved for independent orbital settlements anyway. So perhaps building the passenger section and testing it as the first orbital city is a way to build confidence that keeping a few thousands humans living for hundreds of years in a completely closed artificial environment is something feasible. You know, just like Earth but artificial and as tiny as we can make it. Once you know how to do that, then it's a matter of starting to assemble bombs and building pusher plates, and we are set to start seeding the galaxy with humans.

GW Johnson wrote:

You did a really good job thinking about a lot of really tough issues on that one.

Thanks! You'll get me to blush. I am a big fan of orbital settlements and pulse propulsion, so I have been thinking about this kind of things for a long time. I think that shows, I only referenced a tiny fraction of the sources I used because, honestly, I don't remember where I saw most of them, they have become general knowledge stuck in my brain.

GW Johnson wrote:

I think you're much younger than I am, being bothered by "exams". That's over 4 decades ago for me. You might actually live to see one of these pulse ships built, if the politics changes to support it. I probably won't. I'll be lucky to live to see a manned Mars mission of any sort. Maybe not, the way things are going right now.

Yeah, I'm in my twenties trying to finish my engineering degree in aeronautics one of this days. Hopefully it'll be next year? One can try. But I don't think any of this is happening in the 21st century, it needs a huge solar system economy to be feasible, and the solar system will keep us busy for a long time yet. Enough to develop the habitat technology, by the way, and test it thoroughly. But, you know, perhaps significant life extension IS achieved during my lifetime (or yours), so why not?

Rune. It was fun to think it up, at the very least.

Rune · Interplanetary transportation

AKA Orion interstellar craft. Good stuff. It is, to my knowledge, the only proposed propulsion system ever capable of launching an interstellar mission without some technical miracle involved somewhere. It also comes from the fifties, and the bomb handling mechanisms were supposed to be copied from those in a coca-cola factory (the pulse units would look like metal cans, only maybe bigger). Big pistons, a control room more reminiscent of a battleship... the cool factor in all of this is completely off-the-scale, at least in my opinion, so I just HAD to take a stab at it, even if the information about it is... shall we say scarce. It is unsurprising how difficult it is to get to real numbers about anything related to nuclear bombs. So here it goes, if only to fuel some decently-researched sci-fi story set in a generation ship I probably will never write. WARNING: this is a very big post, written over a few days: I hope you enjoy it, but I don't expect everybody to endure it, even though it would be nice if the people that take the time to read leave some comment, if only to say how nuts I am. Enjoy! ^_^

The best and pretty much only solid source I could draw on, was the paper by Dyson on interstellar propulsion (Here it is, BTW). I figured if I went to the father of the concept, I couldn't really get the basic assumptions that wrong. Of course, the paper is not that technical, and in fact it is intentionally misleading (again, weapons of mass destruction involved). But apart from that, there is surprisingly little to draw on. I could get the isp from atomic rockets' , but to be frank not only are there many listed, I have no idea where he got them from. So I start my ass-pulling early by saying I ended up feeling comfortable by saying sufficiently big (>10mT) almost-pure hydrogen bombs (or as close as we can get to them) in a big-ass plate (greater than 100m diameter, and probably closer to a km) would give an effective isp of about 750.000s. For a frame of reference, that is about 50% of what a perfect deuterium explosion would get if all the debris went either against the plate perpendicularly, or away from it (it would be half that if the bang went in a perfect sphere). With shaped charges and reasonably big and efficient bombs, I figured you could get close to that, but if not, then you have to adjust travel speed and time accordingly, either up or down (I'm not going to be ONLY pessimistic).

Then, set a mass ratio, apply Tsiolkovsky, and you get a travel speed. I went with a mass ratio of 5. Why? Well, mostly because you have to pick one, but also because that works out to about 4% of c as total delta-v, and cruising at ~2% of c you could get 20ly away from Sol in 1000 years. There are a lot of stars inside a 20ly sphere centered on sol (135 stellar objects, in fact), and even if you don't find earth-like planets in those, you will see this ship would need no clone of Earth to colonize a target system (as in establish a self-sustaining human civilization), as long as there are accessible resources. I will bet whatever you want any given star system has sufficient resources to do that in the form of asteroids and comets. And a millennium seems like a good round number to plan for in multi-generational ships, too. I mean, once you can't make it there in a single generation, using tens of them is not that much of a stretch, many countries and organizations have existed for that long in a stable form. Though maybe isolated island communities is a better analogy. Whatever, I figured it could be done. By all means, if you have a more solid argument for other speeds and travel times, or how long could such a mission take, please feel free to share. Or if you just want to point out why I'm wrong, that's cool too

I didn't go much farther than 1000 years in my thinking because I thought that was going to be a big enough engineering problem: build a machine to be used for a whole millennium! Now that's a tall order. That kind of drove the next point in the concept: size. To put it into one sentence, it's got to be big enough to pretty much rebuild itself in flight. I mean, there is no sane person that would expect any machine to work for centuries, but that doesn't mean that new machines can't be built when (and from) the old ones fail. For one, I am not sure about storing nukes for centuries and then expect them to be viable and go off when you want them to. Too much time for unstable isotopes to go away, even if you stored them in perfect isolation from radiation sources and eroding atmospheres. So I guess most of the bombs for deceleration, or at lest their fissile detonators, will be built (or significantly re-built) as they are needed during the final years of the journey. Which in turn means that the ship has to be capable of supporting a nuclear industry on it's own from the supplies it carries. It would have to anyway, since I am guessing the power for the long interstellar voyage would be provided by nuclear reactors, and they can be very long lived, but even so they would need to be refueled , maintained, and such. Same goes for ECLSS (though I expect most of the work there will be done by the agriculture/waste management systems) and every other thing on the ship: you can't expect it to work forever, so you have to be able to build the replacement, preferably from the materials of the original.

So to set up a truly independent colony you have to send something very close to a truly independent colony. At least big enough so you can maintain a sufficient pool of skills to run everything, maintain anything, and build the replacement parts you will need in the future. How much people is that? Honestly, no idea. I mean, the amount of different skills that go to make a sophisticated society... They will have access to virtually unlimited amounts of data in the ship's library, but access to an engineer course or manual does not make you an engineer able to service a nuclear reactor or build pieces for one... In the same vein, the ship will be built so all pieces are fairly low-tech and easily replicable, but there is only so much you can simplify the design of a guidance computer, for example, or the firing mechanism of a nuke, or an antibiotic, or a thousand other things. To keep on the theme of a nice, round number, I went with an average crew of one thousand. A few hundred people are a viable genetic sample, especially if augmented with frozen embryos from earth, and it seems like a reasonable minimum number to have the necessary skill set. It seems a bit on the low side of things, considering the construction job ahead of them during the trip (at the very least, thousands of H bombs, or at least their fission fuses), but I used such big margins everywhere else, that I imagine you could probably fit several times more people in the same mass budget. But hey, the reasonable thing would be to send these kind of ships in groups anyway (say, three ships providing support and backup to one another), so you could just build a fleet of them in the worst case, and specialize each one on one aspect of production. See where I was going with "these ships don't need planets to colonize"? They are colonies themselves, so to settle a target system they just have to use the capabilities they already have to build copies of themselves without the propulsion systems. Landing and/or terraforming can come later if planetary chauvinism is still a big thing for them (which I suppose it will).

A hab for one thousand people for what essentially amounts to forever... small thing to design, right? First, let's start with the artificial gravity requirement: 1G provided by rotation sets the usual 100m minimum radius for 3rpm's. Having rotational stability means we can not make it much longer than that if we want it to rotate stably on it's own without adding another cylinder (a fact often overlooked when designing these things), so let's play a bit with areas next. This paper (a fascinating read on space habitats on its own right) states the maximum ratio between cylinder length and radius is 1.3, so let's start small: a 100m radius, 130m long cylinder has an exterior surface of a shade over 80,000 square meters. Each of the two endcaps have about 31,500 more. And here is where the second requirement, radiation protection, quicks in to simplify the problem considerably. Let me explain further: in order to protect humans from radiation long-term (as in, generations), you need, basically, a lot of material between you and the radiation. Sources claim anywhere between 5mT per square meter behind the Van Allen belts, to 40mt wherever you put it, to more or anything in between. That is HUGE!! It means, pretty much, that you can forget about structural weight: whatever mass you need of structure is both going to count towards your radiation protection, and be completely insignificant compared to the rest of it. After much thought on this, I picked a more-or-less average number of 10mT/m2, and afterwards decided to double it, just to be extra safe. I figure if you have 10 meters of water and another 10mT of structure, machinery, supplies, soil, and everything else covering every square meter of the exterior of the habitat section, the passengers are going to be safe from just about everything imaginable. Plus, the interior atmosphere, even in this tiny design, does its small part as additional protection peaking on the cylinder inner surface (it may seem weird, but it's just like on earth, where the ground shields you very well from half of the sky). When you go to the big cylinders like O'Neill colonies the atmosphere starts getting so massive that you can actually use almost no shielding apart from structure, but I figured in this case I would ignore that effect since it would be both much less and I wanted to leave as much margin as I could.

And how many square meters per person anyway? Well, again you have a source for every taste, anything from to 100 to 200 square meters or more, varying a lot depending on whether or not you count arable land towards that or not. To make an efficient use of all that radiation protection, I am going to put the interior to good use adding reduced gravity inner cylinders for agricultural production and industry and such (including a 0-g recreational/industrial area right in the middle). So I figure I could get away with a low number, but use a reasonably high one anyway. Let's say more than 150 square meters for every person (low-density city-like), 160 so it fits nicely with the hull area into a round number (remember? 80,000m2). That would work out to about 500 people in the smallest cylinder I could think of, with generous margin everywhere. And I mean generous, 3 inner cylinders starting a healthy 50m up and spaced 10m, give you another 96,000 square meters to grow stuff in. That is several times what is required in terms of food production, and I imagine it can work as oxygen supply. I figure you will still need air filtrations systems, but perhaps air circulation and humidity control can be ingeniously designed to be passive and mechanics-free.

So for our crew of one thousand people, I am going to add a final barrier of redundancy and safety by picking two separate, independent cylinders of just those dimensions: 130m long, 100m radius. By counter-rotating them you give the ship both no net angular momentum and at the same time stability, and they can also be spun up and kept there by electrical means having one push on the other. Technically they don't even have to be physically joined between them and with the propulsion section, they could detach once on cruise, but I imagine some kind of rigid axis would exist, if only for transportation purposes. That would turn out to about 160,000 meters of usable, 1g shielded surface, plus 2 shielded endcaps (you can save two whole endcaps if you keep the two cylinders in line), plus more than 25,000m2 in margin, 'cause I like both margin and round numbers, which comes up to some 250,000 square meters in total. Since area dictates total mass, we are taking about 5 million metric tons of habitat in total, the vast majority of it in plain radiation shielding with perhaps a double use like being a lake or raw materials for future production of stuff. Said shielding could rotate with the habitat sections or stay put, saving some structural weight, but considering the mass budget, I think just spend a bit more in structure that you can certainly afford, and save on complexity and failure modes. Some external ballast running on cables on the exterior could compensate for the irregular and changing mass distribution on the inside (otherwise, prepare for the equivalent of quakes as the ship wobbles).

Thermal control for such a closed system is going to be about getting the power not used by the plants out, mostly, but by happy coincidence the pusher plate is a stupendous and gargantuan conductive radiator in case the huge external surface is not enough. I mean, lighting up the place at a healthy 72W/m2 in LED's needs a bit over 25MW, and providing and additional, I don't know, 50kw/person (a huge overkill by at least a factor of 5 based on current US consumption), another 25MW. Pitifully low numbers when you stop to think about the rest of the project. Rejecting that much power with ISS-like radiators would need about 250,000m2 of them. By coincidence, wink, wink, the same as our shielding area. But I still think making the pusher plate do double duty is a neat solution. Besides, generating that energy has probably generated at least as much in waste heat from the powerplants in the first place, so the total area may need to be up to three times bigger (that's 33% efficient thermal-to-electric conversion, which seems reasonably good and feasible with simple high temperature turbine systems). But who cares, a big, efficient pusher plate (1km diameter? why not, the bigger the better) would have much more surface than that. The fact that we are talking about cold interstellar space versus "hot" inner system operations also eases things here, since any radiator should work much better than at Earth's orbit.

So things start to take shape. The "payload" is about 5 million metric tons of radiation shielding with a bit of structure, piping, and machinery keeping it together, and a touch of human and plant flavor inside, in the form of two counter-rotating habitat cylinders set up in line. All this in front of a non-rotating magazine section containing the "fuel", with the shock absorbers coming afterwards (good place to put the power reactors, too, if they don't need gravity to work) and the 1km diameter pusher plate completing the picture at the end where the fun stuff happens. Total length, anything from ~300m (you keep the bombs around the habitat section) to perhaps 500-600 depending on magazine size. Width, well, you have the 1km wide plate, but the rest should fit within 200m, which is the diameter of the habitat section.

A few more notes on the propulsion system to complete the picture. I have no idea how the diameter of the pusher plate affects the overall propulsion efficiency, so I had to pick a radius out of thin air, mostly. Basically, because it depends on the bomb collimation factor and its power, and to know those I would have to start by designing the pulse units, something which I am very poorly equipped to do. By applying the weight fractions Dyson also pulls out of thin air for pusher plate mass and shock absorbers, I get 3.33 million mT for the plate and 0.2 million for the shock absorber system. By picking plain steel for the plate (7.85g/cm3), and assuming it is a perfect cylinder 0.5m tall, I get a radius for the plate of about 500m. The plate is not going to look anything like that, of course, but the exact geometry is not going to be spherical either with well-collimated shaped bombs. The most I assume about the bombs is that they are more than 1MT per metric ton, BTW, and powerful enough to provide the quoted isp. Which basically means that they are a lot of deuterium, and one bomb weighting 20mT explodes every 3s to propel the ship at a minimum of 0.05G at full weight and 0.2G with the last bomb. That translates to an effective jet power of... wait for it... 96x10^6 freaking terawatts, which should provide an acceleration time of 230 days to ~2% of c. Must make a bright star at departure.

Oh, and BTW, I finish up with a total ship mass of 50 million tons. 40 in pulse units, 5 for payload, 5 for the propulsion system. I know, 3.33+0.2 doesn't come up to 5, but you will need some sort of magazine, and a lot of auxiliary equipment like nuclear reactors and radiators. Call that additional margin, structure, whatever you want.

As a final addendum, you should note that a bit more than a fifth of the total ship's weight, about 12million metric tons of bombs, which is mostly deuterium, stays with the ship until the deceleration "burn". That is the part of the bombs that might have to be manufactured during flight in the probable case that you can't guarantee nukes working hundreds of years after production. You may also note that that would be a much more than decent radiation shield, and that using it as such and dispensing with most of that would instantly multiply several times the payload capacity in terms of people. But, you know, I like to provide a robust starting point on what is feasible. If it turns out you can put ten times as many people in a bigger, lighter hab section, then great: just increase radius and length, and see where structural fractions to handle the loads get you. But this is definitely doable, and retains radiation shielding the whole time, even after arrival at the target system, which seems to be a safe bet.

Phew, that was a really, really long post. Must be the exams inspiring. What do I get out of this, as a final conclusion? First, that the ship is so dirt-simple in technological terms, that this is actually doable, just a matter of price. But more importantly, that someday, when a project of this size is within our possibilities economically speaking, is is perfectly viable to settle other stars if we put our mind to it. It doesn't even take that much. And simple economics and compound interest says that point in time is not really that far away... Hope you enjoyed!!

Rune. Ad astra... right?

Rune · Interplanetary transportation

Rxke wrote:

'we got a dragon by the tail!'

I was just about to post that.

Rune. NOW I can finally have lunch. (16.00 here)

Rune · Interplanetary transportation

Oh, no, GW, turns out ion tugs are on the rise, and have been for some time. Just a few months ago Boeing announced it was rolling out the new 702 GEO comm birds as all-electric, to launch in pairs in Falcon 9's. Xenon ion thrusters with 3,000-5,000s isp are quite commonplace now for stationkeeping, especially on the russian side of things because they developed them first way back when. I think the failure in the last USAF's AEHF satellite, where they lost the apogee engine and had to salvage the mission by using the station-keeping ion thrusters to get to orbit, gave electric propulsion a big boost confidence-wise... and the sat a radiation dose it was definitely not designed for, we'll see how long it lasts. For unmanned satellites everything, of course, and mainly because of the station-keeping advantages (though they do manage to fit two GEO commsats on a single Falcon, using this and other weight savings). Solar panels are also getting better all the time, giving nuclear power-to-weight ratios a run for their money, though I am most definitely not up to date on that.

Rune. As I said, they have their uses.

Rune · Human missions

GW Johnson wrote:

PS - don't forget about nuclear explosion propulsion. Works better the larger the ship. 10^4 -- 10^5 tons, that's the proper class for pulsed propulsion.

Never do. I have the numbers run (very approximately, of course), for a beautiful, 50 million mT starship that can get >1000 people inside a rotating O'Neill cylinder-style colony, to a whooping 2% of c and back in about 230 days each (before you ask, LOTS of hydrogen nukes). And with style, I budgeted 80,000 square meters of 1G surface living area, and more than that as reduced G agricultural areas. Trip times would be a few generations to any interesting star, but boy, that is a true starship.

Rune. Off-topic as hell, but you'll forgive me.

Rune · Interplanetary transportation

I'm watching Dragon fly, silhouetted against the ocean, from the comfort of my room. Nice, this internet thing.

Rune. Good to see you up there!

Rune · Interplanetary transportation

Terraformer wrote:

What about Nuclear Electric Propulsion? If you're going to have NTR's, you'll need the lightweight reactor anyway, and surely NEP would allow you to convert the nuclear energy into kinetic energy much more efficiently? What sort of power-mass ratio can we get from a nuclear reactor that we could build today?
Do remember that only a small minority complain about placing nuclear reactors on board ships.

Well, NEP's have the worst problems of both worlds. First, the reactor is quite different, it is not cooled by the propellant and therefore requires huge radiators, so no weight advantage vs a solar system, at least in the inner planets. Same T/W, too, so huge flight times also. It is inherently more radiation-robust, of course, but that is a small consolation for me. And don't get me started on efficiency... electric drives are about 60% efficient, and nukes never get above 50%. So about 70% of your energy gets wasted as heat. Tough that is better than solar panels, of course. A thermal rocket is in the very high nineties, in practical terms 100%, courtesy of the thermodynamic trick of open cycles and regenerative cooling.

And about complains about nukes... well, honest-to-god nuclear reactors have been flown and nobody on the general public seems to have noticed in the first place. My point of view? They will buy whatever you sell them. Hate it, too, but that is always a fraction of any response.

Rune. I think the people that ought to know better are the most scared of trying to use it.

Rune · Interplanetary transportation

Impaler wrote:

When I said the performance of the NTR was reduced I was referring to the Payload Mass, because percent (6% in the paper I site) of a NTR vehicle will be the Engine/Core that's mass which is unavailable for Propellent or payload. I was not implying anything about a low or lowered Thrust/Mass ratio.

I get your point, but you will admit, for a reusable vehicle that refuels through depots (whose propellant is supposed to be cheap), empty weight is important. I mean, it is what you have to launch from earth. Also, in general, the price of aerospace-grade equipment is a function of weight. 50mT of high-power electrical equipment is going to cost at the very least as much as a single 500kg engine and a big empty ~10mT fuel tank.

Impaler wrote:

The Radiation belts of Earth are clearly not a concern for electronics of spacecraft that transit the belt slow or fast as many SEP craft have already done this as has every chemical rocket too.

Yup, they just size the solar panel 40% bigger and rad-protect all the electronics to account for the radiation damage. Good luck reusing a solar tug after crossing the Van Allen belts a few times.

Impaler wrote:

The power needs for a cargo hauling SEP system are only 1 MW and that's for a 100mt vessel with a payload fraction above 50% doing a slow multi-year trips to Mars, that's an amount of power only about 3 times what the ISS generates so Nuclear is NOT required for slow cargo, I said that SEP is not appropriate for fast crew transport. I also didn't say NASA was pro-NTR, I said the paper was pro-NTR in that it advocates for the use and development of NTR/NERVA and it rates the Tech at TRL 6 which means demonstrated but not yet at a flight ready versions.

That is what bugs me. You yourself say SEP is probably never going to get a man on the way to mars. Yet you want to develop it as a way to, ultimately, get people to mars. I say if you can't develop the rational propulsion choice to get people there cheaply, then at least go with the one (developed already) that can, plain chemical expendable rockets. Don't overcomplicate things and get lost in the fancy engineering like NASA has been doing for the last 40 years. Just pick something that can get the job done and actually use it to get the job done for a change.

Tough I do admit, let me be clear on that, that if you restrict SEP to unmanned payloads and high orbits/interplanetary flights in the inner solar system, it not such a bad propulsion system. My point of contention is that nukes are a better, more universal one, that can handle manned payloads.

Rune. So don't forget them!

Rune · Interplanetary transportation

Impaler wrote:

Third the performance of ISP in the ~900 range on paper only double that of chemical is pulled down by the large non-propellent engine mass. Thus the whole systems performance in payload delivery is not sufficiently large compared to Chemical propulsion (~50% more payload) to be sufficiently advantageous to justify the development of the technology even if the first two barriers did not exist. More chemical rockets would probably be cheaper then development costs of such a technology.

Wait, what? I stopped reading here. First, T/W in a NTR goes from ~10 if you use a straight NERVA design to over ~30 if you have fancier cores and turbopumps. So much, much, MUCH lighter engine group mass and unfueled "tug" mass (I prefer propulsion stage or module, 'cause that's what they are). Also, Oberth savings (missions to in cislunar space can cost x4 in delta-v if you don't use it) and you don't fry your electronics, solar panels, and battery system by staying for a year in the Van Allen belts, slowly spiraling your way out of LEO.

Second the "small payload advantage" can make a MTV single stage, and hence fully and rapidly reusable, using a 875s core like NERVA's and a storable soft cryogenic propellant like methane. Is that really such a "small advantage"? Because for me that opens Mars to mass deliveries of cheap payloads with a modest fleet of reusable propulsion stages. Doing that with chemical propulsion looks very, very on the verge of what is possible at difficult to believe mass ratios and using every delta-v saving trick in the book (aerocapture into a very high elliptic orbit at Mars, departure from lagrange points, you name it).

You actually point out yourself the need for nuclear reactors when considering either manned or big cargo electric ships, but you don't state a very important point: a >>1000s isp propulsion system is really good to move to the belt and beyond. Which is where the sun intensity drops like a rock. Good luck not using nuclear reactors there, and that is why I think that SEP is a dead end for anything other that unmanned small ships. Also, the "frying itself on the radiation belts" thing because it takes it years to leave a planet doesn't help either.

And about a NTR's readiness level... do you know what a solid core rocket is, basically? Pick a chemical rocket, switch the combustion chamber for a nuclear core with a cooling loop instead of a preburner to run the turbopump, and you are done. Oh, yeah, and eliminate the oxidizer loop. That simple. The only item that has been upgraded in the following designs after NERVA has been the core. The rest is the same LH2 turbomachinery taken from the J-2, and slightly upgraded as the J-2 itself got upgraded. And if you don't want to certify a new core to use the fancier materials available nowadays, as I said, a NERVA or DUMBO core can do very nicely for anything this side of the belt. The only expensive thing in all of this is the facility where you test-fire the thing, and the industry capable of manufacturing the core fuel elements (that is, the nuclear one, though US' is a bit on the down slope nowadays, maybe you'd better talk to the germans, they are experts in fuel). I would also bet that with today's material science and a modest deign goal, the exhaust ends up looking so harmless that open tests in some nuclear site are perfectly acceptable form a rational environmental point of view (never mind you can convince some hardcore greenie of that) and an earth launch doesn't look that dirty. Though, to be clear, I wouldn't never ever launch an activated reactor, that is just asking for trouble. So NASA can say it's at TRL 6 all they want. I say someone with "cojones", and access to a decent nuclear fuel industry, could have one flying in a very short time. Most of the technical stuff, other than fuel element production, is either open source by now, or depends on your particular design and you have to engineer yourself. Of course, NTR core grade nuclear material is also weapons-grade nuclear material. Hence the bad language in the previous phrase and the need of a government, which always complicates this things.

And as a last note, NASA has never been pro-NTR. They have always seen it as a crude thing that will get killed by politics, IMO, and are only interested in the super-complicated state-of-the-art things like NEP for the outer planets. They were the ones that killed the most promising solid core concepts like DUMBO (which, with much simpler turbomachinery and lower design goal, looked like it had better isp than NERVA at better T/W).

Rune. Ok, I'll admit. I went back and re-read very politely most of your post to reply adequately.

Rune · Interplanetary transportation

And it's off! Second time is the charm. Now to wait for the HD video to see those Merlins lighting up the night properly.

Rune. Good luck up there, Dragon, you know it is your year.

Rune · Human missions

Quite true, Impaler. While having a ~50mT launcher is nice to loft the big things in one piece (let's say Hab, propulsion module, and lander(s)), it makes a lot of sense to use the existing 10-20mT launchers that most nations do already have to launch the heavy dividable stuff like fuel. Just one... call it small discrepancy. I wouldn't launch propulsion stages, I would launch tankers with a standard docking interface. First, most of the countries that could pitch in already have ships that could be upgraded into these things (part of the technological/industrial pie to split). Second, a big tank can launch empty attached to a small heavy engine with no problems, especially if the tank is made to hold dense, storable propellants like methane. It also makes sense to launch just one tank, to minimize plumbing and interfaces in the final thing and make it mass efficient (I think ISS taught us that with its $100B price tag). Third, every country could design it's own, or use several sizes, and use them for other things, and that makes the industry that much bigger and sustainable. A nice bonus.

Rune. Good candidate for ISS-style barter agreements, too: You build the lander and provide X mT of fuel, you the Hab and Y mT, you launch the big stuff, you the crew... you can build on everyone's specialties.

Rune · Human missions

Sure, put the Hab on wheels. It is a sound idea. That way it can go after the lander, and the ascent vehicle if they are not the same (in most ISRU plans they are not). It is conceivable that they end up a few kms away from each other, and that way you can relax the landing precision requirements in the first place. But I would put it on an earlier cargo flight, and land it unmanned. Why? Well, first you wouldn't need to make it safe enough for people. Just safe enough that you think it is likely to be pulled off at the first try. If you fail, you send another and delay the whole thing two years, big-ish deal and certainly big money, but no deaths.

Second, you can actually test that all the hab systems work before you commit yourself to a surface stay remotely. It is also good to test that your ascent vehicle looks good before you land, that is why I like the rotation of landers in Mars Direct. I wouldn't make them the return vehicles all the way to earth (makes them big, and non-reusable or single stage), but otherwise I like the concept of having one ready and bringing another in case that fails.

You also do need a manned ship to take off, period, and it has to land at some point, so it makes sense to put the astronauts in that one, and since it has to take off again it makes sense to make it light. A lander crew compartment in one of my scenarios would look more like a soyuz's than anything else.

So, Hab on wheels, that I agree. But I wouldn't call that a rover... in fact if the mobility system was detachable (I'm thinking something like ATHLETE, which is already built and everything), it could transport the astronauts in a light inflatable tent on short trips around the area, while the hab stays put, with it's heavy equipment like labs, supplies, long-term-redundant-as-hell ECLSS and power systems. Probably turning the atmosphere into fuel for the mobility system (I'm thinking methane/LOX fuel cells make sense here, you get the best of both worlds, electrical power and chemical energy density) and the lander that just came down if you have a working ISRU system, too. That would be cool to develop, and you can prove it works without endangering anyone if you have an unmanned flight two years before carrying the hab and the return lander ahead of the crew. The fact that the landers would transport the hydrogen for the sabatier reactors on theirs way down in their empty fuel tanks, eliminating the need to dig anywhere for water and lots of equipment with it, is quite important and drives the architecture a bit, too.

Rune. Remember, doing it in a single flight doubles the size of the propulsion system to push it to mars. Why not use both "MOR" AND "MSR"? (I think those would be the acronyms for Mars Orbit Rendezvous and Mars Surface Rendezvous)

Rune · Human missions

I knew your ship was rigid, GW, but you also tend to over-kill the requirements a bit elsewhere, in my humble opinion. I mean, when I see a 6000s gas core talked about, it is to go to the outer planets or something! You could go LEO-LMO-LEO propulsively with a solid core in a single stage (and no heatshield, just maybe an aerobraking period at the end of each leg to lower and circularize the parking orbits). I know 'cause I run the numbers elsewhere, and using methane as fuel. If you tweaked the architecture and left from L1/2, and waited on Mars in a very high orbit (like near Deimos), even better if it is highly elliptical, so your lander doesn't need lots of delta-v to land, only to take off, then a couple of chemical stages or maybe even one and a bit of aerobraking will do. Higher fuel mass, of course, and I haven't worked out proper numbers, but we have some first stages with mass ratios of about 20. When I try to work out masses and volumes, and since I assume a reusable nuclear stage would actually be, you know, reused (and probably tested while you are at it) to position cargo and Hab elements on the ground, maybe even backup landers, in previous unmanned missions, my MTV turns out "a bit" smaller, lighter, and much more compact. I mean, a ~50mT Hab (a transhab is 3 stories high) with a ~25mT mostly unfueled lander pushed by either a nuclear methane stage or a storable chemical return stage does not look like it would be +100m long, even at low diameters. That stack would need something else to spin to meaningful G, and would be, overall, less than 1000mT total weight depending on propulsion (say two MR3 chemical stages at worst?), probably much less if that propulsion is nuclear, getting IMLEO into the low hundreds of tons. And by sending previous unmanned cargo missions you can get as much surface equipment, and as many "landing sites" as you need.

Tough if we go with Impaler's 0.25G as an acceptable thing (I wouldn't), even that small length would be enough. And easier to boost to mars and refuel when it gets back that something that multiplies several times your IMLEO just in unnecessary shielding. You do know a few cms of water are good against solar flares only, right Impaler? Cosmic rays go through anything not measured in tens of tons per square meter. That is why everybody talks about smaller radiation shelters.

So really, you do not need lots of complex hardware and billions of dollars to go to Mars, like the thread is all about. BTW, RobS, I agree with most of what you said at the beginning of it, even if I have some different ideas about how to go about it . If you are smart, and don't get seduced by "what would be very cool to have", then you actually need to develop only a few relatively simple items (say ~4: manned lander, cargo delivery lander, hab and in-space stage), most of them very technologically mature already. I am starting to believe you don't even need a costly nuclear rocket development program... and it is me saying that, you know, the nukie.

Rune. Though it would be the first "cool thing to have" I would add, since it totally makes it doable with single stage reusable vehicles.

Note: Throughout the post, I am referring to the literal definition of aerobraking, as opposed to aerocapture or aerodynamic reentry deceleration. That is mostly free in mass since it requires no heatshield.

Rune · Human missions

Ah, ok, you are advocating something with a catastrophic failure built-in. No worries . What is the relative speed of the rotating hard surface respective to the inflatable shell? And here I am thinking of a kevlar line with cables embedded on it (or, you know, go wild and use wi-fi) released in case of emergency by explosive bolts. 'Cause rocket stages can explode, for example, and you asked for it. With independent control authority in the spent departure stage, the unspent return stage, and the Hab, all of them able to talk to each other. If it is a single stage it is partially full, BTW, and with all the redundancy expected of such hardware. I must be crazy, right?

Have you looked at the .pdf on the atomic rocket site I linked to? It's a fancier alternative with an electric behemoth of a ship that doesn't stop spinning unless it's either at earth or mars orbit and some other vehicle has to dock. The total propellant required to spin or despin is a whooping 200-400kg depending on the attitude control system used. That is, 1-2mT for a huge electric ship with a multi-megawatt nuclear reactor in a full mars mission. Everything designed for 1G except the thrusters, which are kind of in the middle, and it turns out that simplifies building things like radiators or booms. Not least because they work at Earth surface, too. You might like it, seriously.

Rune. If you don't like to work out the dynamics of a tether system 'cause, I dunno, you are feeling lazy, you could always use a rigid extensible boom. As GW said, it's not that long, not that much mass.

Rune · Human missions

Oh, and in case it isn't clear (I don't know why it would be since I barely mention it at all!). I also think the lander is the pacing item for any Mars mission, for sure. Radiation we know what to do, propulsion we know what to do, artificial gravity we know what to do (it's untested, but as long as Newton works, I will trust a spinning ship to do the job). It's the ship capable of landing on Mars and taking off again that we have never built.

Can it be done? I think so, even with chemical engines as I said before. I mean, at most (during launch from Mars) we are asking about 6km/s out of it. That is about Deimos orbit from the planet surface, a sufficiently high orbit for pretty much any MTV to park in. Using your "standard" Methane/LOX engine that every mars mission assumes, even when there is none built at this point (though apparently LH2/LOX turbo-machinery works with little modifications, and there are plenty of tests), at say 350s isp (easy for methane with just a big nozzle) that is a mass ratio of 5.75. A tad more than 17% of launch weight for payload, structure, engines and everything else. When you compare it to the reusable SSTO's we are contemplating on building here, and take into account LCH4/LOX is a fairly dense, only mildly cryogenic combination, it doesn't sound that ridiculous. Yeah, it would be nice if the thing aerobraked on it's own before a short powered landing to reuse it for more than one mission and transport it to mars (mostly) empty, but it is not really necessary if you can't crack the heatshield mass issue, you could ditch it before landing and use the landers only for a single ascent-descent mission. Only the working ISRU unit, and as I said before, you can validate that with a sample return mission.

As for the architecture to use it, something like DRM (mars semi-direct, by it's common name). Redundancy by having a prepositioned fueled lander waiting the one bringing the astronauts in, which stays to be used on mission two. Simple and robust, and if you can reuse them you can get a lot of missions out of every couple of landers, since chemical engines are easier to design for a lot of restarts. We NTR fans should remember a NERVA, for example, was designed for mere 10 restarts after which it would have burned out enough of its core to be disposed of.

And Terraformer, you beat me to it by seconds.

Rune. There, I finally talked about Mars landers. About time.

Rune · Human missions

That is simple, Impaler? Wow. Different definitions of simple. I mean, I would say taking a mars/moon hab, a couple tethers, and using the propulsion stage as counterweight is just a bit simpler. As in, the only new design is the tether system. Hell, you can even land the hab later, and that way you don't have to take out of the design the landing systems. And you need to design a hab if you are going to Mars, right? Always good to have one radiation shelter with you, even when you are on the surface. I mention tether because some versions of the ship may turn out quite compact, and there is a limit to the rpm a human can take and be comfortable. That is, not nauseated all the time. Conservatively, I think, that is around 3rpm.

Oh, and you don't need to identical counter-rotating sections. Have you heard of IMU's (inertial maneuvering units)? Big-ass gyroscopes for attitude control. Really any kind of flywheel would do, and the smaller the lower the friction losses. But having angular momentum stored in your ship itself by spinning it whole seems even easier, since it adds only the fuel required for a few spin-despin maneuvers. And have you considered what a spinning atmospheric seal rated for vaccum and 2+years of operation under stress with minimal losses would look like? I wouldn't like to have to design that.

And before someone points out a spinning ship is difficult to control, you can provide course correction on such a tethered ship... with ion thrusters, even. The reference I take for that is on Atomic Rockets' section in TransHab, where he links to a very enlightening study about a Transhab redesigned for 1G at the end of an extensible, flexible boom. The whole thing is like less than 40mT, supplies for a reference Mars mission using ion propulsion included. Go look it up, hurry! (For the lazy ones that don't care to use google.)

GW, to reuse a significant fraction of the ship, and not go throwing advanced in-space engines and tanking around, you really need to have a nuclear in-space stage for a Mars-and-back ship, unless you have refueling stations there, and that supposes either a lot of expendable missions to fill the station, or working ISRU by the time you send the first mission (which I always thought was stupid). So not only the lander. In fact I see as easier to make ISRU work for a lander and get it in a single stage using methane/LOX rockets. With proper redundancy (a fueled lander waiting by the time the astronauts land in the now empty one), and validating the method with a robotic sample return mission (perhaps in the orbital test flight of the reusable mars transfer nuclear stage?), I think it could be good enough. Lowers the required T/W on the NTR so you can build it shielded enough to convince a greenie it is safe to launch. Also, smaller, so less nuclear fuel and megawatts in there.

That being said, with enough Falcon Heavies you could fill a Mars depot from earth with ~10mT loads at a reasonable price (14mT on mars transfer, some expendable tanker using soyuz tech... you get the idea). Pick storable fuels, maybe augment them with supplemental ion/arcjet systems for the coast period (again, you can use a continuous thrust system on a spinning ship), and you can actually do an all-chemical mission with currently validated technology. Look at that! You also get very busy launch windows, I might add, which is a kind of complexity in its own way. To return a ~50mT Hab from High mars orbit would take... say between 5 and 10 fueling flights? Just order-of-magnitude approach to pick architecture, I don't want to get into details in case I lose sight of the whole picture. I throw in arcjets because I've been looking at them recently and it seems all very nice: all electric, using any gas, and NTR-like isp (so NTR-like thrust for the same power). For course corrections and a small isp boost it looks nice, but it is just a small personal twist, not the central point I want to make.

Rune. Which is, I think, KISS. So maybe no arcjets, and all chemical. Wouldn't that be old school?

Rune · Interplanetary transportation

GW Johnson wrote:

Rune:
I've seen a lot of re-entering satellites, and I watched Columbia re-enter in pieces, although I didn't know it was Columbia until about 10 minutes later. Thinking back on what I saw, and watching the video footage others took over-and-over, I pretty well figured out what I saw.
The ship lost its wing to the foam impact damage at about M12 over the Texas-New Mexico border. (It was photographed over New Mexico at M15 intact but streaming debris from an obviously-failing wing.) It tumbled and immediately lost its other wing, vertical fin, and bay doors to the hypersonic wind blast.
2-3 seconds later the windshield caved in, ripping the top off the flight deck, and the 4 astronauts there were ripped out from under their seat belts in pieces. 3000-5000 psf q does that. No time to burn, just blunt wind blast pressure forces. Those 4 torn-apart astronauts were the body parts that rained down just east of Dallas, a little cooked, but not burnt.
We knew about the vulnerability of the windscreen to direct hypersonic stream impingement when I was a grad student in 1973. Found it in wind tunnel tests, and found the narrow range of AOA where the stream safely jumps over the cockpit, as a separated flow. Lose attitude control, you're dead.
When it was over Dallas at about M6 or 7, that's when I saw it from outside Waco, Texas, to my north about 100 miles slant range. The heavy engine thrust structure had already separated. It and the fuselage (cabin still attached) led the debris stream. The wings were fluttering along behind, along with the chunks of bay doors and the fin, and a whole cloud of smaller pieces, maybe 2 dozen or so. The fuselage and thrust structure were tumbling. I could not see them tumble, but they were leaving characteristically-braided contrails.
I watched it go eastward until the contrails dimmed as the hypersonics faded into "mere" supersonics. Between Dallas and Tyler, I saw the fuselage break up as a fan of pieces, leaving the cabin tumbling alone and still mostly intact, except for the lost flight deck roof. A contact at NASA confirmed to me that the three on the mid deck were still alive at that point, although I hope the gee-force pounding had beaten them unconscious. That's the last I saw of it, visually.
Seconds later the cabin decelerated to about M1/20kft, and was crushed by the rapidly-rising wind pressures again. My contact at NASA said that's when the 3 mid deck astronauts died by blunt force trauma, not upon impact with the ground seconds later. If stabilized so as not to tumble, it would not have crushed like that.
Clearly, lots of the structures survived the re-entry in recognizable condition. This includes uniform patches and plastic parts from the interior. No, this stuff doesn't burn up on re-entry the way all the "experts" always said it did all these years. There isn't time to burn, it decelerates quite rapidly. The pieces literally heat-sink their way through reentry on a transient.
As for crew survival, you separate the cabin from the cargo bay with a shaped charge, and stream an inflated drogue from the nose to take the spin off the cabin. If there’s enough warning time, the flight deck crew can evacuate down to the middeck. Otherwise, windshield failure and flight deck roof loss is very likely before the drogue can stabilize it. As it slows to “mere” supersonic speed, you blow the hatch. All survivors on the mid deck have but seconds to jump before impact, but that’s better than no chance at all.
It was the same with Skylab in 1979. Fragile thin-shell aluminum remained intact as one single radar return down to 40 nautical mile altitude, about halfway through reentry (around M12, just like Columbia). Minutes later, although the solar wings and telescope mount were gone, the main body was still in one piece when it completed reentry just off the western Australian coast.
It finally broke up over land at about M1/20kft, while ballistically falling into rapidly rising q at low altitudes as the path angle quickly steepened downward. Of 85-90 tons at reentry, they picked up 75 tons of debris in Australia. Nope, these things most definitely do not "burn up".
GW

Ingenious though the idea may be, what I get out of it is mainly that you can't make a shuttle-like vehicle safe. Not as safe as a capsule. That "Lose attitude control, you're dead", which would also be true for skylon I'm sure, will give old-fashioned capsules an edge forever, and the added point of a rocket escape system finishes the discussion, IMO. The capsule is just more fail-safe from the beggining, about the only system you really need working to survive a crash is an emergency parachute, or a rocket landing system. You can put both on, and add redundancy. Proof? Recently the russians had two soyuzes lose all control during reentry and executed ballistic flights (that is, they fell like rocks). They didn't even ground the craft while they worked the problem, and it didn't make the news, much less kill anyone.

And if someone tells me a skylon can be flown several times and reused, and therefore is more reliable, the same can be said of a proper reusable capsule. So if skylon is ever to be made into a manned transport, the payload I suggest is a independent capsule with an escape system. But, you know, skylon's proper use is to put bulk stuff like propellant cheaply on orbit in huge quantities and regularly, not to carry people in style, and that hopefully it can do.

And Impaler, 12 mT is nothing to be sneered at. You can't launch a lunar mission on that, sure, but you can service 99% of the satellite market (with the possible exception of the huge DOD GEO comm spy birds). Hell, most LEO satellites are just a couple of tons. Buy, I don't know, Fregat stages for the final insertion, and you can service anywhere up to GEO anything that is currently commercially built. Arianne V's 20mT to LEO payload mainly means they need to dual-launch big GEO birds and have room to spare for several secondary payloads to not waste room.

Rune. Oh, and thanks for the detailed description, GW. I actually listened to a classmate analyze the accident in class last year, but from the "why the leading edge failed" POV, nothing on what had happened afterwards.

Rune · Human missions

GW Johnson wrote:

Radiation shielding: doesn't take very much dirt to shield solar particles. I was thinking equivalent to 20 cm thickness of water. At an effective sp.gr about 2 for "dirt", that's only about 10 cm of said "dirt". Cosmic rays, that's also about as good as it gets, unless you go with meters and meters of thickness, due to the secondary particle shower. There's going to be some sort of waste crap that can serve as "dirt" almost anywhere you go.
It's the career limit on cosmic rays you worry about for long-term exposure, since the thin shield only cuts it down crudely by a factor of 2. Otherwise, you have to have a really thick shield. Solar minimum GCR is 60 REM/year. Cut that in half to 30 with a thin shield. If the astronaut is already old enough to take the high career limit of 400 REM, it'll take over 10 years to accumulate, as long as he doesn't go outside. Younger people are allowed less, and cannot stay as long.

Hey, just to be clear, I'm with you there. Ships don't need that much, you can probably make do with a water-lined storm shelter. But in the long run, you want to raise kids there (wherever there is), or we are forever confined to earth, so yeah, eventually slag used for rad shielding is going to be both valuable, and necessary. Perhaps even scarce compared to, say, metal alloys, if we go by the percentage of mass of virtually every proposed orbital habitat.

GW Johnson wrote:

Refueling is "automated" with Progress, I know, but it's sort of a restricted case, and not everybody is doing it yet. Besides, overseeing the tasks, and making sure everything happens correctly with refueling, is a good excuse for astronauts to fly. So, why not?

I one word? Cost. Adding a manned capsule is adding about 7-10mT to the launch mass. Considering the size of current launchers, you would be throwing away between 50 and 15% of your payload each flight just for starters (and 15% is a slimmed Dragon in a Falcon Heavy, and none of those is flying), when a couple tons of service module can do the job. And we can make better refueling systems than those of Progress, or we haven't advanced anything since the eighties, and I don't accept that. And just to drive the point home: if it's an unmanned probe no one cares about, mission control can be 5 guys in a trailer. If it's manned... well, see Houston. That's the kind of thinking that got us the shuttle, the 125mT Heavy lift launch system that could deliver 25mT of useful cargo and risked people to do it.

If "you" (I love using this hypothetical "you", have you noticed?) want to take advantage of low launch costs, then please do: Mass-produce a fleet of 5-10mT unmanned probes, at the very least several tens of them, enough so each costs maybe 50M (and that would be at least an order of magnitude reduction in costs for each, so maybe hundreds to make that happen), and launch them in packs of three in Falcon Heavies in direct trajectories. There, triple-redundant asteroid prospecting at 300M a pop. Nasa might even buy some as low-cost missions. And in 5mT you pack as much mass as a geosync comm bird, so don't tell me you can't fit enough stuff to do a decent inspection, maybe even return samples, and certainly serve as communication relays for other probes if they are situated right. Hell, the probe that returns the 7m 500mT rock in the famous paper is about 17mT if memory serves right, and 13 of those are Xenon. So maybe some probes can have bigger tanks and electric drives and return sizable rocks to cislunar space directly (that would be a whole FH launch, but what the hell, 500mT of resources at perhaps 300M if you do enough of them).

All of that only works if you get enough volume to dilute the costs of extended ground support, however minimal it is. So the big problem is, as always, will there be enough customers?

Rune. It's funny how I switch from speaking about the far future and the present day in a couple of lines.

Rune · Human missions

GW Johnson wrote:

What about the stony minerals? Any use for them that anyone can see?

To make the radiation problem go away for any long-term habitat outside the Van Allen belts takes about 10-20mT/m^2 of any relatively light element... honestly, I can't think of a better use. And I mean go away as in it's as safe to live there as on the surface of Earth. Well, maybe you could also use some for glass for the windows in the greenhouses. Or dirt for the plants to grow in. I see it at least as crucial a resource as water, really.

So, you know, not to make or refuel ships. To make homes. The gravity wells are well stocked with radiation shielding, but any really good orbital location has this single defect of lacking... well, stuff. Fix that, and planetary chauvinism has a much harder case in colonization discussions.

GW Johnson wrote:

How to reach an NEO with what we have this year or next, to support prospecting for mining? Hmmmmm.
How about launching two Falcon-Heavies? One has the unmanned spacecraft (whatever it is) that will go to the NEO. The other payload is nothing but a giant propellant tank made out of a Falcon second stage, but without engines. The vehicle with the NEO spacecraft keeps its second stage, using it to circularize in LEO at near-depletion of propellant. The second vehicle is just a refuelling tanker.
Once refueled in LEO, the spacecraft uses the refueled Falcon second stage as its propulsion for departure and rendevous, much like we used the Saturn S-IV-B stage decades ago. Except, I'd plan to use it for the rendezvous with the NEO, too, that's typically a significant delta-vee. Then the spacecraft itself can prospect the NEO, collect samples, and perhaps return.
I'm thinking we'll need men to oversee the refuelling transfer in LEO. Certainly to safely dock the two big "spacecraft" and make the fluid hookups. So, the tanker probably has a manned Dragon on its nose. Crew of 2?
This is not an idea for sending men to an NEO, just a prospecting robot. Sending men requires a different kind of vehicle, one resembling a manned Mars transfer vehicle. That's a different problem, but the same basic transfer vehicle assembled in LEO could serve. The difference is that you don't need a lander to visit NEO's.

Doubtful. Progress seems to have no problem performing refueling operations on it's own, and I'm sure we can even improve on that. Other than that, a Falcon Heavy could launch a pretty hefty payload to Earth escape directly, and it's almost nothing from there to a particular asteroid. Whomever can't pack a decent robotic prospector in >10mT (or several if they haven't got to get samples back, or an orbiter/lander combo) deserves their aerospace engineering grade rescinded. I mean, an Atlas V with half the lift launched Curiosity, it's cruise stage, and the descent stage to Mars. Asteroids are closer, and Falcon H is way bigger. Keep it simple.

And Clark... well, I agree, nothing more really.

Rune. Must be one of these choir guys. ^^

Rune · Interplanetary transportation

GW Johnson wrote:

Louis:
Both shuttle accidents showed pressure cabin separation from the rest of the debris (I witnessed this with my own eyes during Columbia's destruction, right from my front yard). Structurally, the weak point was the cabin to cargo bay joint, where the structure went from a closed tube to an open tube (no strength in the bay doors).
If you take the spin off the pressure cabin (pressurized or not, the crew should be in suits, who cares if it is punctured in some way), you can use the compartment between cockpit (two levels) and cargo bay as a sacrificial "heat shield", with nothing more than a stabilizing drogue from the nose.
Once the noisy hypersonics quiet down, you are are low-supersonic decelerating toward M1/20kft max q, and only dozens of seconds from impact. You quickly blow the hatch, and jump out on personnel chutes with an oxygen bottle. No wings, so we don't need the silly pole and tractor rocket motors.
We have known since WW2 that crews were unable to bail out from spinning airplanes due to centrifugal forces. In fact, that problem was the original rationale behind ejection seats.
But for a shuttle pressure cabin, a de-spin drogue is simply more practical.
We have also known since WW2 that bailout from a non-spinning airplane is easy. Just don't do it above about M1 or thereabouts, because of the nonsurvivable wind blast (known since the early 50's). Which transonic point is some 20kft on the typical ballistic re-entry trajectory, even for debris.
GW

Question: wouldn't the hypersonic airflow shake the very non-aerodynamic broken cabin into complete destruction? Never mind the superheated air finding the thermally soft spots, wherever they are, and burning through them like a blowtorch, getting the heat into the airframe, the astronauts would be long dead plastered on the wall by then in this scenario. Unless you redesign the whole primary structure with designed failure points that result in a self-stabilizing detached cabin, or something. Also, how do you test that this works, never mind modeling it for design? You sacrifice a couple orbiters?

Rune. Just nit-picking, but I imagine these are the kind of things that went through their minds.

Rune · Interplanetary transportation

Well, I'd rather see both companies compete to bring the cost down and not get stuck with a single provider of reusable launch services. You know, bring the cost down as much as it is possible. Plus, different business models. Reaction engines wants to sell the vehicles, SpaceX wants to sell launch services. Other than that, both should be similarly comfortable for a passenger. Oh, and skylon is almost every bit a bomb as a rocket is, only it has less oxidizer at the point where the atmosphere can burn the H2 with no problems, and no escape system (it is a rocket most of the way to orbit, after all).

Rune. Let the best rocket win.

Rune · Human missions

GW Johnson wrote:

Hi Rune!! How are you?

Glad to be back Other than that, quite busy with exams

Mark Friedenbach wrote:

Take a closer look at the names involved. It's a roll-call of PRI's known employee roster.

I knew there was something fishy... Well, PR is PR. Might as well get used to it.

Mark Friedenbach wrote:

Also, the $2.4bn costing of the retrieval mission was by NASA costing standards. PRI has stated that they think that their unique operational structure and other cost savings could reduce that price tag by 1 or 2 orders of magnitude, putting it presumably between $24MM and $240MM, which seems more sustainable. No one outside of PRI knows if they can actually achieve that, but it's not impossible and it is what they claim.

Well, at $24M a mission, it would be pretty much free fuel, and the best business in the history of ever... However, I doubt that. For one thing, it's a lengthy mission, meaning lengthy payrolls. Also, lots of ground support/telescope time involved to blow the budget way over the hundreds of millions. Though I have to admit they have gone the smart route and gone about the business of decreasing the price of telescope time first, otherwise their business would have been a no-go from the start. Kudos for that. Have I mentioned a single mission uses up 12 years worth of current worldwide xenon production? That has to be expensive. Yeah, it can be done more cheaply, that's for sure. Nevertheless, I'll buy a two-order magnitude reduction from forecasts for a first mission when I see it. Can it be profitable? I hope so, but I have no idea, which probably means it's close call in search of the right approach to pull it off.

Mark Friedenbach wrote:

It wasn't me that said it, but I'll concur that mass drivers are a severe navigational problem--the ejected masses would be large enough and moving fast enough to destroy any future space craft that crosses paths. It'd be a simple solution today, but certainly come back to bite us in the future.

Hum. Think about the problem for a second. When you are using a mass driver, you are throwing away rocks with masses on the order of kilograms, of which the solar system is literally full of, at several km/s away of your present orbit. Considering where you would use it, the "propellant" either burns in the atmosphere of some inner planet or attains independent solar orbit. I mean, we are talking about going to hunt asteroids at, at most, hundreds of m/s away of earth orbit. You are throwing the rocks kms/s away... No freaking way you are increasing the meteoroid threat in interplanetary travel. Which, considering today's sensor systems, is quite insignificant IMO, but ever present. You could say there is more rubble out there, and more dangerous, than we could ever throw. Fortunately, space is BIG.

Mark Friedenbach wrote:

Otherwise, great analysis. Have you submitted your resume to them yet?

Thanks! And I have half a mind to do so, once it's a bit more finished. Not like I am going to get a really interesting aerospace job here in Spain any time soon...

Mark Friedenbach wrote:

I have said for a long time that the only thing holding back a true opening of the space frontier is 1) cheap access to space, and 2) in-situ resources once you get there. SpaceX & competitors are definitively solving #1, and if nothing else PRI is removing the giggle factor from asteroid mining, which solves #2. We are finally on the verge of a real entrepreneurial space frontier, and the accompanying gold rush.

Hear, hear!

Rune. I hope to have bragging rights for hearing here about your space startup some day!

Rune · Interplanetary transportation

I've also been following Skylon for a number of years. They are certainly shooting high in their ambitions! Full-fledged SSTO, HTOL, no less. The whole shebang. It is a harsh road they are traveling, especially funding-wise, but it could very well end up working and getting european space efforts into the history books. The first real SSTO would indeed be a very big deal, and the precooler seems pretty much done (a full-sized prototype is undergoing testing right now, as is shown on the video). It's a definite maybe, and I hope they succeed, get their funding, start selling spaceplanes, and enter my personal hall of fame.

Rune. As a side project, they are also developing the world's first hypersonic antipodal airplane, the LAPCAT A2. O_O

Rune · Human missions

Hey guys! I have been away for while, but know I am always lurking somewhere not far away, and this Planetary Resources thing has inspired me, so here I am

One quick note before starting: the price for water you have established, based on Falcon Heavy launching it, is the price for anything, actually. Styrofoam for packaging food, or dirt, or toilet paper, or radiation shielding material (which asteroid rubble is very good for), would cost the same if they have to be lifted from earth. All you have to do is find a customer in orbit, and deliver it for less money.

The actual plan they are talking about implementing (basically just prospecting at this point, and quite remote and low cost at that) is not much to write about (though flooding LEO with cheap telescopes is a good, and necessary thing), but the intention behind it to seriously look at industrializing space is good to see any day. And there is some amount of real money behind it! Ok, not that much, but some.

Now onto how I would do it, which is always the fun part. Have you all read the paper Mark linked to? It's not actually done by Planetary Resources, but by a bunch of non-profit space advocacy groups, but it has surfaced with such good timing that you have to conclude there is some correlation. If you want to skip the read (an interesting one), I can just jump to the conclusions: you can launch in a Atlas V a probe, and ~5-10 years later said probe will deposit a ~500mT, ~7m diameter carbonaceous condrite rock into high lunar orbit, neatly packaged in a bag, de-spun, and with station-keeping capabilities courtesy of the probe. Bagged and tagged, right? Said rock is, more or less, about 40% volatiles (lets say half of that is water, the rest carbon-bearing volatiles and ammonia), 40% silicates and other "useless" rabble, and 20% metals, mostly nickel-iron. Wow, lots of stuff for a small rock, right? I mean, the rock wold fit in some living rooms I've seen.

Problem is, that is nowhere near enough to pay for the mission, even if the processing and delivery to customers was free. The paper assumes a cost of about 2.6 billion for the entire mission (just an atlas launch, but a lot of ground control and observation time with costly telescopes). Oh, and the tug's 13mT of Xenon propellant represents about 12 years of present-day production, I found out. Clearly unsustainable.

However, it would be a great practice run, and would yield tons of data about what is out there and how to process it. A good first step, and it actually yields usable material in a very accessible and safe place. Stuff lost/forgotten/discarded into high lunar orbit is eventually perturbed into the surface of the moon, not earth's. Even if there is no station at L1/2 to serve as "base-camp", any GEO launchers could reach it with meaningful payload, and a Falcon Heavy could put a Dragon there. Returning from there to LEO/earth is likewise easy-ish. And what have you left there of value? Well, 100mT of water for starters. And about 200mT of quite decent radiation shielding and soil matrix. It would actually make a lot of sense to do something like this just to build a decent, long-term manned station outside the Van Allen belts.

Seems to me those resources would enable subsequent asteroids to be captured and processed much more cheaply. First, the second generation of asteroid tugs, the guys that pick the rocks and bring them back, would likely be reused several times and fueled with "cost-free" water extracted from previous asteroids. And they would likely use volatiles produced in-situ as the reaction mass to tow the asteroids back in much more reasonable, but still multi-year, timelines, all robotically like the first one. Serviced at the lunar orbit end of their missions, the tugs cold last for a really long time. I mean, comsats already last routinely about 15 years without any servicing. Engines I don't know, from electrical solar powered arcjets working with hydrogen/water/ammonia to nuclear engines with the same propellants, to mass drivers using buckets and rubble and of course the usual crude chemical engines (which would consume a lot of the water to bring them back). Even ion engines, if you up the power levels significantly and solve the fuel problem. Hum. That gets me thinking... have I found a use for VASIMIR that actually makes sense? Can't be. In any case, whatever works best and you can use, both from an engineering, political, and economical standpoint. I would say arcjets can do quite decent in all those categories, but real analysis will figure that out.

More notes, as I come up with them: I think someone pointed out that using mass drivers could be a navigational hazard: it's more of a navigational hazard just being out of earth's atmosphere, and the rocks you are throwing out are going to be small and several km/s away from your orbit by definition, so no. Also, Void (it was you, right?), I think you have just re-invented magnetoplasmadynamic engines, or in general any other form of plasma engine, all of which would be more efficient that what you suggest, and could directly make the plasma from water without involving electrolysis or chemical engines. Low-thrust, though, and power-hungry. Think of them as multi-propellant ion engines, like solid NTR's are multi-propellant chemical engines: same order of magnitude but better isp, a bit lower thrust/weight, a lot more power involved.

Well, that's for fuel and logistics. The rest of the asteroid would be processed very close to home with reusable machinery in a, I guess, optionally manned station in L1/2 protected with free radiation shielding, kept there with free station keeping fuel, and supplied with free water and other volatiles (ammonia for the cooling systems, O2 for the crews, CO and other stuff for a variety of industrial processes). Imported solar power, machinery and everything else, of course. I'm doubtful on the usefulness of in-situ food production, it's such a small mass item compared with other stuff. All of that would cut the mass lifted from LEO to only space-rate hardware and crews, delivered to L1, and cut all subsequent mission costs for... well, everything, to a fraction of what they would otherwise be. Who knows, maybe the business case could even close right here if you find someone wanting to do something else in space (my first most radical and pie-in-the-sky choice, a space settlement UN agency ^_^) and he pays you for everything you are already producing to bring the asteroids home. It's certainly good to have a financially-solvent mid step, and preferably several along the way. Selling asteroid information and observational capabilities first, water and rocket fuel afterwards, satellite recovery and servicing, radiation protection...

And I haven't even started with metal refining! Probably that would take a much longer time to be established, surely that would need much more significant machinery to be placed and maintained at L1, and basically I leave it for the future to solve. As some vague directions, I guess fuel tanks, habitats, and solar power units would be the first things to be produced in space, in that order for reasons of complexity. Not much sense to ship back to earth anything other than science-relevant samples and maybe, in the very long run, the by-product Platinum-group metals and finished products. Got you all there, right? Platinum is the least important thing, IMO. In fact, by the time someone is doing all this, they are probably going for the big ones (>50m diameter) where it makes more sense to move your whole refinery there and ship stuff back. Presumably said refineries are mass-produced from standard templates at L1/2/4/5 from what has been learned in the previous decades. And we are definitely well into the later half of the century, too, and space is already a significant fraction of humanity's GDP and being "settled" by any measure you care to use. Here's hoping all that happens, and the faster the better.

So, to sum it all up, why the hell would you mine asteroids? In a phrase, to settle space for its own sake. Or put into other words, to build self-sustaining space colonies. I think that is worth it. And I think there must be some way you get someone (governments, corporations, the public, whomever) to pay for it. Who knows, in the long run they may even get a return on their investment, humanity for sure will.

Rune. I want to see a O'Neill cylinder being built!

Rune · Life support systems

JoshNH4H wrote:

I believe Rune is in favor of imported Uranium

Well, I am in favor of refueling the imported reactors for as long as they work. That is, I believe if you are going to import any powerplant (and to establish a first settlement you will have to), nuclear power is the most efficient and robust way to go. And I mean big, badass, tens of MW's and bigger powerplants to have a robust energy base to fiddle with ISRU in a big scale. Plus, keeping a reactor going is ludicrously cheap in terms of fuel mass, and said mass is only needed every couple of years (happy coincidence, the time between launch windows). Now, by the time the first imported reactors are ready to be decommissioned, maybe 50 years down the line, and replaced as the main powersource, the whole argument about whether building new reactors or solar thermal (or wind, or PV, or whatever) powerplants is not that easy of a decision. That's probably when martians start building proper martian cities and all of that, so lots of new decisions to be made based on different premises. In fact I'm not sure it would not go the way I argue against, when the final pro/cons analysis is done, in most cases. But it's a timeline thing, every thing at its proper time, and for the proper applications. Hope that clarifies my position.

JoshNH4H wrote:

Is it bad that my arguments for why CSP might be better than nuclear are starting to convince me that it might be worth importing fissile material?

Dude, you are not the only one with a similar problem. My take is it's healthy to look at things from the other side of the fence every now and then.

Rune. I hadn't thought about Asimov's belt-worn reactors in years. Thanks for the reminder! (They must have been inspired by RTG's, is my take on them)

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#51 Re: Interplanetary transportation » Nuclear Pulse Propulsion starship. » 2012-05-30 07:36:38

#52 Interplanetary transportation » Nuclear Pulse Propulsion starship. » 2012-05-29 09:49:23

#53 Re: Interplanetary transportation » Falcon 1 & Falcon 9 » 2012-05-25 08:00:40

#54 Re: Interplanetary transportation » Forget NTR, SEP is the Future » 2012-05-24 18:18:59

#55 Re: Human missions » Space X to Lead Mars Consortium? » 2012-05-24 05:33:20

#56 Re: Interplanetary transportation » Falcon 1 & Falcon 9 » 2012-05-24 05:19:28

#57 Re: Interplanetary transportation » Forget NTR, SEP is the Future » 2012-05-24 05:16:25

#58 Re: Interplanetary transportation » Forget NTR, SEP is the Future » 2012-05-24 05:04:25

#59 Re: Interplanetary transportation » Forget NTR, SEP is the Future » 2012-05-22 20:23:20

#60 Re: Interplanetary transportation » Falcon 1 & Falcon 9 » 2012-05-22 06:40:41

#61 Re: Human missions » Control cost or go home » 2012-05-21 10:01:07

#62 Re: Human missions » Land Crew in large pressurized Rover » 2012-05-21 09:39:12

#63 Re: Human missions » Control cost or go home » 2012-05-18 05:52:48

#64 Re: Human missions » Control cost or go home » 2012-05-17 14:05:18

#65 Re: Human missions » Control cost or go home » 2012-05-17 12:34:15

#66 Re: Human missions » Control cost or go home » 2012-05-17 12:01:45

#67 Re: Interplanetary transportation » Reaction Engines » 2012-05-14 10:44:11

#68 Re: Human missions » Planetary Resources Inc. » 2012-05-05 09:26:28

#69 Re: Human missions » Planetary Resources Inc. » 2012-05-04 10:18:53

#70 Re: Interplanetary transportation » Reaction Engines » 2012-05-02 07:51:38

#71 Re: Interplanetary transportation » Reaction Engines » 2012-05-01 06:21:19

#72 Re: Human missions » Planetary Resources Inc. » 2012-04-29 19:24:20

#73 Re: Interplanetary transportation » Reaction Engines » 2012-04-29 18:40:33

#74 Re: Human missions » Planetary Resources Inc. » 2012-04-29 10:28:29

#75 Re: Life support systems » Wind power : possible ? » 2012-03-13 19:23:12

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