New Mars Forums

I was bothered enough by all this to dedicate a 5-min google search to it. This is the best that turned out (Guided Entry Performance of Low Ballistic Coefficient Vehicles at Mars). For the nice simple chart, go to page 2. Problem is, the worst beta they work with is about 150kg/m2. That would be MSL-like, or about 11.8mT if you used a 10m diameter heatshield, which is quite ridiculously low for the roomiest rocket I can think of (SLS). The good part, I found out how THAT works: 540m/s at 20km of altitude, and they pop chutes. Maximum load, 10G. Since I just calculated the drop from 1000m/s, applying the 10G limit I find out the lowest you can turn on the engines is 10km, which is almost twice as fast at half the altitude. Looks like you can make do with a worse beta than 150kg/m2. How much worse, twice that? Three times? I wish I knew.

Rune. Somebody else has sources? 'Cause I can't believe nobody has worked this out already.

That is all fine and good, and in fact I knew it or most of it, but when you have the fuel elements in direct contact with the working fluid, that in my book is an open system. Excuse me for putting words in your mouth, but that is my read of your posts ^^'. And yes, NERVA fuel claddings were awesome. Some even more awesome can be built today. None of them will be perfect, and the core elements will have a measurable rate of degradation and corrosion, especially if they work on atmospheric air instead of chemically pure hydrogen. And that is why engineers looking into the problem of flying bombers with nuclear engines bothered with heat exchangers and secondary loops, so they could work with closed systems and contain 100% of the reaction products. Because 99.999% fuel containment is not enough, when you are going to have tens of thousands of birds flying around cities. A few rocket launches, maybe you can get away with it, but not civil aviation.

As an incidental note, I must say at this point that Reaction Engines' 400MW precooler (something like 500kgs in weight, they one they developed for Skylon, you must have heard of it) would work wonders on the system weight of the old nuclear bombers idea with a primary coolant loop running conventional airbreathing engines. Who knows, that technology might even make the whole thing light enough to fly, though I doubt that could ever be economically competitive with, say, standard jets that let their nuclear fuel factories on land to run on chemical batteries (AKA fuel). But perhaps if you wanted super-endurance...

As to irradiation, have a look at the shit suspended in your backyard's air. I myself live in Madrid, Spain, so I can assure you I breath a very unhealthy dose of heavy suspended particles every day. Quite a lot of other nasty stuff, too. Of course light elements are not dangerous after irradiated. In an ideal world, yes, radiation cannot make radioactive air. In this one, nuclear planes will never fly because of it.

And last but not least to the proliferation issue... let me clarify: no further NUCLEAR refinement or enrichment. Chemical treatment alone will separate the uranium from the cladding and the ceramic matrix very quickly, and almost anyone could do it, it's the easy part in fuel processing. The hard thing is to get enough percentage of the desired isotopes among the uranium you refine (or plutonium), which is what the compact engines need. And what Iran is getting scolded for trying, too.

Rune. And I'm as pro-nuke as you can find them, and seriously, I don't see it.

Next week when Senzhou lands the same way Soyuz does (i.e, heatshield, chutes, AND terminal solid rockets) you might get a surprise: it's almost the standard option outside the west... there is way more operational data for that than for shuttle-style gliding landings!

Curiosity aerobrakes with it's heatshield while maintaining attitude control with RCS thrust. Then it deploys supersonic drogues, then supersonic chutes, then it uses a terminal rocket system for final approach. What exactly did you say they haven't used there? Because my point of contention with MSL's EDL is the use of chutes, I would have substituted them for more rocket fuel to prove manned systems, and not built the most complex landing system imaginable.

Rune. Again, why chutes for big payloads.

Solid rockets may be reused, but I think we could take a page from the SRB's and stop deluding ourselves that that is going to save any money.

Other than that, the gun is neat. Makes me think of a launch loop in small scale. Maybe by building things like these we will build confidence for the time when the magic material appears and we can build the elevator and be done with atmospheric rocketry.

Rune. "Reusing" a solid rocket engine in the real world means rebuilding it.

Are you kidding? All your answers are a google away, if you know the keyword... in this case, it's "BBCode" (Bulletin Board Code). Check the link, and you will find out posting anything is a cheat sheet away. Though I haven't figured out tables myself, that's mostly 'cause I'm lazy.

I know I have seen a NASA paper with reentry charts of speed vs. altitude and aerodynamic heating or something like that, but I have no idea when or where, so I have been after some decent source on that for a long time. I am also coming to believe that turning on a rocket engine in the middle of hypersonic flight is not that big of a deal (my reasoning: Dragon and Soyuz control attitude with a RCS system all the way through reentry, it may not point directly downwards, but those are rockets turned on and off while hypersonic). So why not just turn the engines on when you hit your natural terminal velocity at reasonable altitude, and forget chutes altogether. The mass ratio won't look that bad for reasonable terminal velocities, like 1,000-2,000 m/s, and acceleration profiles can't be that bad (one of the reasons I would like to see at which altitude MSL hits terminal velocity and opens chutes). T/W requirements can't be that much more than in a terminal landing system, so the only added weight is fuel in a bigger tank: less systems, more reliability, I would like to work out weights.

So I really, really would like to see some numbers run on it, which is why I'll check it out when you post it.

Rune. On an unrelated point, I wouldn't know what to do with a slide rule.

The smaller you go, the better your ballistic coefficient you get, so lower terminal velocity. Make it feather-like, and you need no parachute. A few grams of carbon fiber with a few tens of centimeters of diameter would land pretty softly on their own. So no lower boundary, a penny-sized piece of carbon-carbon would land much easier than MSL.

What could you do with that, now that's another question (my answer is very little, since a comm system capable of reaching orbit is too heavy to fit, never mind sensors). But a paper plane can theoretically survive earth reentry if the paper can survive the thermosphere.

Rune. They tried that from ISS, actually, though I heard no more about it.

The last Skylon published revision, the C2, is specifically sized (15mT to LEO) so you can fit in the payload bay a standard ~6mT GEO commsat and a SUS (Skylon Upper Stage), a reusable H2/LOX stage that performs a rendezvous with Skylon after it has inserted the satellite into GTO. Running the numbers (up to two launches a week per airframe, 200 flights per airframe) you come to the conclusion that a single Skylon could service the whole international launch market as it exists today, if it performs as expected. But they are hoping for "a little" more competition. Worst case scenario, with 30 vehicles sharing 30% of the present-day market that doesn't grow a single bit, they figure they could only lower price to about 40% of the one today (total cost including range and hidden subsides). But don't trust me, go check the stuff on their web, I see you haven't in quite a while.

The latest revision they are actually working on is rumored to be about 20mT to LEO (or so I've gathered). Which is about what the shuttle was promised to be way back when it was first conceptualized, only in a HTOL single stage. But will any big engine company AND a big aircraft company buy an idea "not invented here"? That is the question now. REL are only planning to become a minor supplier to Skylon production, providing just the concept and the precoolers to the project.

Rune. Only a madman would develop a commercial system for LEO only. The big commercial bucks are to GTO, as of today.

Don't sweat it, you are being more polite than I am, and we are all here for fun! Discussions are better when both parties can present pointy arguments without any getting personally offended...

...and it seems it is is me who needed the scolding. I thought it made for a bad nuke! Doing a proper job and looking up the bare minimum in looking up things I retract what I said: Thorium reactors would make for great fission detonator production. A second stage of deuterium/lithium (for tritium production during the blast), which is stable so we can store it indefinitely in principle, and that looks like a good pulse unit production scheme, or as good as I can think up with my civilian knowledge of nuclear weapons.

Yeah, if we go by the recent advances you quote and we extrapolate a more-or-less linear increase (always a tricky thing to do, nature usually doesn't work that way), then interplanetary ships fueled before departure start to look very feasible. Not that you need the isp on interplanetary travel, but you could use tiny sub-kiloton clean fusion nukes while lifting off a planetary surface, for example. Don't bother with Helium 3, we don't need that to blow up hydrogen bombs.

Always remember that the containment system will take an appreciable percentage of the total ship mass, of course, though how much it is very difficult to say at this point. And it will likely need some power of it's own, which presents the usual problems (if you start needing a fixed percentage of your jet power as electrical energy to sustain your propulsion scheme, however good your efficiency at producing it is, when you scale things up to interstellar the waste heat problem becomes astronomical). That is what I don't buy about inertial confinement schemes: a very important part of your jet power is going back and forth though a whole electrical system on each pulse, so efficiencies being what they usually are, I don't see how you can work that out to interstellar travel without radiators the size of moons, never mind antimatter storage or production.

So, you know, it may be that in the end the "mercury orbit power station" for antimatter production gets built to provide fuel for the whole solar system and the interstellar arks at departure. As a sci-fi fan, I can see the appeal, believe me. But if I had to go by economics... I have to say I never much liked antimatter. One of those start trek thingies I never bought as happening in the real world. The way I see it, someone will figure out the materials problem for a space elevator sooner, and then we can move the messy "nuclear rocket of mass destruction" stuff out of the gravity well and shut every greenie up without invoking stuff like practical fusion or practical antimatter economy. Just unobtanium cables

As to proliferation concerns, there is pretty much only one thing that I can say with certainty: we are going to have to solve those, or stay confined to the inner system. The energies involved in serious space travel, and Jon's Law ("Any interesting space drive is a weapon of mass destruction") conspire to make any race stupid enough to blow up itself very capable to do so as soon as they start fiddling with merely interplanetary speeds. I mean, try dropping rocks at 100km/s, and you don't even need the nuke on the pointy end. So an ion propulsion system can be just as dangerous as the meanest ICBM. We better get a handle on how to solve our differences of opinion without blowing stuff up.

Rune. It's weird how Orion is technologically simpler the more you ask of it. Bigger is better!

Mainly, physics. The mass ratio of a rocket designed for Hohmann transits is completely incapable of achieving the much higher delta-v's of anytime travel. Sure, you can stage things and any velocity is achievable in theory. But there are limits to what is reasonable, or economical. A reusable chemical stage would launch once every two years, and would need refueling at both ends of the trip, period. No two ways around that, the window may be a bit wider or a bit narrower, but it will be a window, and it will happen every two years or so. If you want magic, invoke fusion like everybody else does.

A plane carries hundreds of people. So do trains, or ocean liners, or the old wooden sailing ships. So hardly a superliner, just a decent-sized transport for a mature transportation system. I assume things like that (and bigger) will be flying by this time. Probably built in orbit in the first place, from materials not brought from Earth. But I would bet the oldest of them is no more than a couple of years into service. We are in the biplane era at most in manned spaceflight, if that much. Maybe in the glider era that preceded it.

Contrary to what you think, Mars is going to be a VERY high consumption society, if you measure that consumption by the mass of the materials needing to be manufactured. Why? Well, because, just to name a few things, here in Earth you don't have to manufacture air, soil, or light. I don't care how much frugal the martians are on their needs, here on earth the square meters you need to live in came for free, the soil in which the food you eat is grown came for free, and the air you fill your lungs with is free. Or more correctly, a huge biosphere paid for it for you. In mars, every ton of air and soil has to be manufactured (of course with materials form Mars! no other way), every square meter of livable land has to be walled and protected, and most likely lit with additional lights. That is a huge added cost, and the machinery needed to produce all of that, and the new generations of machinery needed afterwards, is going to be huge. The size of the smallest possible >90% independent colony is likely to be much bigger also that you suppose (or I, for that matter).

And both need a full biosphere.

Rune. To make a pie, you must first create the universe, you know. 

You all know we are talking fifty years after the first landing here, right? I mean, 25 launch windows. I see a lot of wild optimism here, thankfully tempered by some. No freaking way a self sufficient colony numbering in the thousands is up and running by then, IMO. Try a hundred years, and maybe we are talking. Maybe, just maybe, the exponential population growth is starting about this time. I mean, maybe there are permanent residents by then, a few at most. But most of the money going to Mars in those days is going to be from the government, and for scientific/prestige reasons, I expect.

Eventually, yes, all populations have a geometric growth if the resources are available to do so. Plain mathematics. But establishing a seed for that is going to be much more difficult than some optimists realize, like Glandu, Impaler and others are saying. Maybe we'll get around to trying that 50 years after the first landing. I can guarantee you that is not a given, but if we are being optimistic I'll grant that. But even if the best of cases, by this time frame we are beginning to build in Mars the truly large-scale civilization blocks that will enable that growth, not in the middle of the process. Factories (manufactured with as much martian mass as you can, but mostly imported in terms of cost and man-hours to build them) capable of sustained growth and meaningful production will be coming online by then, and the reusable system that gets people there and back will be entering its mature stage of operations and expanding. The construction methods for building on Mars will start to look something close to standard. It would be a miracle if the legal mess of colonizing another planetary body is solved by then, but that kind of thing always trails after reality anyhow.

The most I would say about this time frame, is that by now the picture should be much more clear as to how the colonization effort will proceed. Most of the important questions about that will be able to be answered by this time, and the next step will be decided then and only then. I would very much like to be there to see that!!

Rune. Imitation is the best form of flattery.

And splashdown! Congrats to the SpaceX team on a very successful mission. Year of the Dragon, right?

Rune. Oh, and 3 for 3 for the Falcon. That is even more significant.

Hehe... I can see how you might be, a little. Yes, you are exactly right: one pulse every three seconds, and each pulse is a 72 megaton thermonuclear explosion. But this is a big ship, you must remember. The shock absorbers even out the thrust over the three seconds between each pulse, and the resultant acceleration turns out to be a gentle 0,05G (more or less) when the ship is fully loaded, 0,2 with empty magazines. Surprising, right? Think that no matter how energetic the reaction, individual bombs mass 20mT, so they cannot really impart that much momentum to a million-ton spaceship.

Dyson actually limits momentum exchange for practical reasons like material strenght, and also 1G maximum thrust, to about 30m/s for each pulse. This would actually be significantly less.

Rune. Hope that helps!!