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Josh:
Yep, anywhere to anywhere else inside Texas is many hours to drive, even with interstate highways. Used to take 3-4 days before interstates. Big place. Flying is definitely the right choice.
To answer your question, my paper at the 2011 convention was about a similar orbit-based mission using nuclear transit propulsion and nuclear landers. The transit ships were about 1/2-or-less the mass of the chemical transit ships (not a surprise), and a smaller lander carried a much larger payload. Mission objectives were broadly similar: the nuke mission made up to 16 landings, which means a visit to Phobos was easy. That one didn't establish a base, although those components easily could have been used to do that task.
GW
I saw in the discussions some fears that the reactor might fail, I guess based on incidents like Three Mile Island, Chernobyl, and now Fukushima. The US Navy nuke program has a sterling track record, so I thought I’d offer some of the experiences and data.
As for surface transport, you have electric drive, and you have mechanical drive. We could easily do electric drive, using the marine and RR technologies and a suitable battery or battery-substitute. Actually, chemical storage and a fuel cell is the best.
For mechanical drive, that’s just an engine and a transmission. The heat of combustion of LCH4 with LOX is known, but I don’t have the number available. You’ll need a diluent gas to cut chamber temperatures a bit. The best combustion engine to couple to a variable-speed requirement is piston, not turbine. Operating on a smooth paved highway at steady cruise, you could expect around 15-to-20% energy conversion efficiency. Maybe 25% peak, but I’ll believe it when I see it. Start/stop rough-ground, you might get around 5-10% energy conversion efficiency, and that’s only if you don’t stop and idle a lot.
With LOX-LH2 you could do exactly the same piston mechanical drive, but we already know that trade works out far better as fuel cell electric drive. Don’t forget the diluent gas: we can handle 2000 C pulsed flames, but not 3000 C.
GW
The fleet of ships idea dating from the 1950’s was actually Ernest Stuhlinger’s idea, not von Braun’s. (They knew each other.) The ships were to be ion thruster-powered. There were 6 ships, each with a crew of 20, and each carrying one one-shot/throwaway 2-stage chemical lander, for a total of 6 landings. The landers were based on von Braun designs using hydrazine and RFNA. The old Disney “Tomorrowland” program “Mars and Beyond” showcases that Mars mission design concept very well for the public, actually. Not a bad design for what they thought they knew in 1955.
That kind of fleet and 100-something crew would have required thousands of launches to send tens of thousands of tons to orbit, for assembly by hundreds, if not thousands, of other people. Considerably more infrastructure than the fleet’s mass would have to exist in orbit to carry out such a task. It’s just my opinion, of course, but that’s well into the unaffordable “battlestar galactica” realm.
I went “slender baton” with simple module-docking, and 4 ships (3 unmanned) in my fleet, because my crew was only 6. Total fleet mass as assembled was under 4000 tons, and that’s an upper-bound, Cadillac-return mission (6 landings plus a Phobos visit, plus a base at the best site, while there in the one trip). All that you can get while assuming that your ISRU/ISPP does not work at all. If it does work, you get to fly suborbitally to even more landing sites, and your base is quite the attractive place for anybody making a second trip.
These are all things that mission planners really need to worry about. And crunch numbers about. You can do it with something a bit better than a bounding analysis, but less than a full-blown design process / design analysis.
GW
Sending robot ISRU experiments is a right thing to do, but not the only thing to do. Has anybody associated with that project said how they’re going to get real ground truth concerning fresh water supplies? Colony death is certain and sooner without it.
GW
Things to worry about:
Any base you build will require a source of fresh potable water for drinking, and for watering crops in any sort of greenhouse. Salt water won’t do, because desalination is so energy-intensive, and therefore heavyweight in required equipment. Simple fact-of-life, sorry. They cannot survive, much less thrive, without fresh water.
Being below a 6 mbar datum determined by the triple point does not make exposed 0 C water or ice stable. What makes the condensed phases stable at 0 C is water vapor pressure at 6 mb. You’ll have to cover the exposed water or ice in such a way as to let the vapor displace 6 mbar worth of atmosphere. This will very likely impact how you go about prospecting for ice resources, and in how you store them and ship them after you find them.
A lot of the soils seem to be very salty in one way or another. Any water dispersed in that soil will be salt-contaminated. Not only do you have to dig up and process an enormous soil volume to get a much smaller water volume, you will have to desalinate that water before you can let people or crops drink it. It sure does make sense to prospect for buried freshwater glaciers before committing to building a base. That’s ground truth supporting an informed decision.
I have never seen a robot drill rig. I have seen a lot of rigs that need to pull the drill string quite frequently to replace worn out drill bits. Less frequently, the drill string breaks. Can you imagine a robot that can extract the broken-off drill pipe string from way down in that well? Sorry, I can’t. Men will have to do the deep drilling, and it will take a lot longer than you expect. Ask the oil and gas guys if that’s not the gospel truth.
It looks like Josh’s map says an orbit inclined 40 degrees or so would take you to every possible place where there might reasonably be buried freshwater glaciers, excepting the poles themselves. There are several such sites, none at high altitudes, but several not so near the equator. Every one of them requires real ground truth to support an informed decision about whether or not to attempt a base there. Would it not make a lot of sense to land a crew at each site and drill for that ground truth, before actually landing everybody plus base-building supplies at the best site? That leads immediately to orbit-basing until you select that best site. And that’s why I think making only one landing immediately upon arrival (from orbit or direct) seems like a silly, wasteful idea.
It is quite likely there will be one (and only one) government-funded mission to Mars. That trip has to find the place to plant a base that can thrive, or there’s no point for any of the commercial/private groups to go. There’s lots of indirect evidence of water all over Mars, but it’s not certainty. Certainty requires ground truth. Does it not make sense to plan on getting that ground truth and finding the base site “for sure” in that first (and only) government trip? How do you get that out of a NASA flag-and-footprints stunt, or out of a direct landing at only one site? I don’t see much sanity in those ideas. It’s just my opinion only, but there it is.
GW
Aug 7 to 10 is on my calendar, thanks. A warning to all: Houston is very humid and very hot in August. Come prepared to sweat. A lot.
Josh: do you think a version of my Mars mission 2013 posting might make a good paper or two?
GW
Us older types have a hard time with the ever-shifting social media stuff. Shoot, I have a hard enough time using a laptop and keeping up with email. Y'all should be quite proud of me: I have a website and a blog site, too. (Can't figure out how to update the website, though.) My wife uses Facebook, but I have not got a clue how to do that. No smart phone, no Twitter, none of that stuff. I'm not yet dead, and don't plan on dying for several years yet. And I'm more typical of my age bracket than most younger folks want to admit. So, there's several of us old codgers still trying to contribute. I'll be on the forums, I think. Most of us older types will, I'd bet.
GW
A nuclear power plant on the order of what fits inside a submarine has proven to be a very reliable system. Some are steam turbine drive, others have generated electricity for electric drive. There have been no reactor failures since SSN-571 Nautilus ca. 1954. Only SSN-575 Seawolf had a sodium-cooled system, and it got replaced with pressurized water after a year or two. The only engineering problems have been leaks in the steam loops that drive the turbine. Even the turbines are now very, very reliable, as long as scheduled maintenance and repair are done at intervals of decades. I'd say a pressurized-water nuclear power plant generating electricity could run very well without much trouble for decades. They already do, if done to USN, not commercial, standards.
We developed low rpm electric motors decades ago, for use in submarines without reduction gears, starting in Tench-class fleet subs in 1944, and by refit in the older Balao's and Gato's. These not only proved to be quieter, they were also more reliable. They're still in the modern nuke boats that have electric drive. You're talking thousands of horsepower (or KW) at about a hundred rpm. This sort of thing could turn wheels about as easily as propellers. But you pay for it, they're big, and heavy. Doesn't matter, having such a technology available to you on Mars would be well worth it.
Scaled down a bit, this same electric drive system with low-rpm motors has been in diesel-electric locomotives since about 1933. So you don't have to transport marine weights and volumes, the railroad stuff is a lot smaller. In fact, the same technology is small enough to fit inside road cars and trucks, and has been since about 1973. So, if you want an electric-drive truck to run of graded dirt roads on Mars, yes we can build such things. As has been true since the beginning, it's all about the battery and how you keep it charged.
To run a truck on tires on a gravel road is a lot "draggier" proposition than we have on the paved highways here. You have lower gee, for a lower normal force, yes, but the effective friction coefficients are going to be around 3 to 10 times higher than we see for tires on pavement. So, even at reduced speeds and reduced gee, you will still need around 10-ish KW to move a big rig (or mobile habitat) on Mars. It's even worse if the road is not graded (going overland without roads). That's an awfully big solar panel on a smallish vehicle; plays havoc with design layouts.
Putting the nuke on the vehicle has two serious problems: radiation and size. Nukes never scaled down. About the smallest size into which a naval system fits is a 9 or 10 meter diameter submarine compartment about 2-3 times as long. That's why nuclear trains, and nuclear road vehicles, have never been built. Land with a nuke plant to start your first base. You can add stationary solar to the other sites you think advantageous later. It'll take a mix of both types to get started.
You might use chemical energy storage for the electric-drive vehicles, of which the most efficient is the fuel cell. The truly practical ones use hydrogen and oxygen (my, isn't it convenient there's buried ice all over Mars?). The others that use hydrocarbons are very complicated, and have never been put into service, for a variety of very good reasons. But hydrogen-oxygen has been in service, for a long time now. And I think that points the way you should go.
Put a solar-powered electrolysis plant on top of every buried glacier, and add both compressed-gas and liquified-gas infrastructure at these sites. Use them as filling stations for your electric-drive surface vehicles that use the (easier-to-deal-with) compressed gases. Transport with those vehicles the liquified gases to those sites where your LOX-LH2 flight vehicles operate. Pave the roads (a whole 'nother issue) as fast as practical, to reduce surface transport energy costs.
Hey, that's part of how a base might evolve into a permanent settlement.
GW
Midoshi:
How's MAVEN doing lately?
GW
A wheel makes sense if all the modules in it are human habitation spaces and the supporting supplies. The central core needs to be the propellants and propulsion. The problem with chemical or even solid-core nuclear propulsion is that the mass and volume of propellants is order-of-magnitude 10^2 (or more) larger than everything associated with the crew. That plays havoc with vehicle layout, and in the past has led to "battlestar galactica" designs from NASA and others that would be completely unaffordable at this time in history. Gigantic truss structures are nothing but inert weight that drives you to gigantic amounts of propellant required, very, very quickly.
In the simple "slim" baton that spins end-over-end, you have multiple decks in each human-habitation module, all at one end. Let them use ladders to get from deck to deck, to get really good and aerobic exercise "all the time". Put the gym and congregating areas in the farthest deck at one full gee, and the working areas just above at near one full gee (yep, there is a strong radial gradient in small sizes, say 100 to 200 meters long). Put the sleeping quarters highest up at the least gee, since the bed rest studies seem to show there is little gee benefit while you are prone asleep. Put the stored supplies above that, at the least gee, usually around 0.5 gee in the designs I have looked at.
Those same considerations would apply to cable-connected spinning designs. I just prefer the semi-rigid baton for enhanced safety and convenience. The downside of the baton is that it requires you to go to a modular approach, so that you can stage-off empties, and then redock/reconfigure your baton. I am afraid of the transients in cable-connected designs: spin-up and de-spin in such structures requires multiple super-well coordinated thrusters operating simultaneously. An accident with a stuck thruster like what happened on Armstrong's Gemini flight would cause loss of vehicle and crew. Plus, what if a meteor cuts the cable while you are spinning? How do you achieve multiple-cable redundancy for things like that? I honestly don't know. But I do know we can dock lots of modules together, and there is a very promising layered foam-and-foil meteor armor that is super lightweight.
I usually allow something on the order of 60 to 100 cubic meters of pressurized volume per person. Some of that is in common areas for congregating or working together, some of that is for private personal spaces in which to be alone. I don't know a figure for that last split, but the overall number is what drives rough-out design analyses, anyway. I do know the longer the confinement, the more space is required for mental health. Planning for a crew to ride all the way to Mars in things as crowded as a space capsule is as insane as the crew would be on arrival. And don't forget, there is confinement while at Mars for about a year , because you cannot run around unprotected outside (so where do they live while there and how big is it?), and there is that long voyage home.
There are a whole slew of things to worry about, planning a voyage like that, and most of those issues have nothing to do with the rocket equation and which Isp you have available, except in how big and heavy your dead-head payloads are.
GW
RobertDyck said: Skin-tight spandex suit with leather boots? Black leather with spike heels? Oops! This is for Mars.
Well, imagine how fashionable it will be running around in your spandex underwear when you are inside! While a funny thought, it does encourage population increase! There's some motivation there to get on with developing the techniques and infrastructure to thrive, not just survive.
GW
I think they're raising money any way they can. I don't think they yet know how they're going to pull this off. I question the wisdom of a one-way colony-planting trip before we have attempted any experiments at trying to live off the land there. That makes it more-or-less a suicide mission, unless somebody gets there first with a base or automated equipment to try out all the technologies that might (I repeat "might") enable humans to survive there. Note also that I said "survive", not "thrive". There is a huge difference. A colony must be able to thrive, sooner or later, or it cannot be a success.
GW
Hi Quaoar:
Nope, my design is an old-fashioned "do-it-the-hard-way" design. I used propulsive burns for capture at Mars, precisely because we have not done aerocapture with men anywhere but Earth (coming back from the moon). Our atmosphere is very predictable at entry altitudes (140 km on down to around 30 km). Mars's atmosphere varies by factors of 2 or more at entry altitudes there (135 km on down to 10 or 20 km). That's too variable to trust very much with men. Even probe designs have had difficulties with this variability.
My rough-out design is posted in an article over at http://exrocketman.blogspot.com as the article titled "Mars Mission Study 2013", dated 12-13-2013, if you want to go look at more details. I have updated that article with second thoughts and extra data, and will continue to do so. As posted, it represents an upper bound on the costs for a mission that accomplishes a huge return no matter what happens, with all-chemical propulsion and all reusable-or-salvageable hardware.
When I use the term "rough-out" design, I'm talking about something better than a bounding analysis, but less than a full design analysis. I used the simple rocket equation, with empirical "jigger" factors for drag and gravity losses where applicable (the same sort of stuff von Braun was doing in the 1940's and 1950's to show what is possible before embarking upon real designs, and we all know how that turned out during the 40's, 50's, and 60's).
I used a very old entry approximate analysis originally developed for warheads. I used some very approximate terminal descent kinematics from constant-acceleration physics, modified by intuitive safety factors that hopefully are very conservative.
I did work out the appropriate volumes and shapes to go with my weight statements, just good enough to get realistic diameters and lengths for the vehicles. My volume allowance is near 60-100 cubic meters per person, in both the transit habitat and the lander, but I didn't work out the details of how much is for congregating together vs being alone. I doubled and tripled the typical allowances for for food air and water for the crew, because I think frozen food and water-based cooking are going to be required (the artificial gravity enables this, too).
GW
Hi Josh:
To answer your question about cutting transit time: I dunno. I haven't run any numbers, and I know little about the electric propulsion devices. I suspect that the key driver for tradeoffs will be solar panel weight. Do you have any figures for weight per unit power? For thrust per unit power? What about for the gravity-losses of using non-impulsive micro-thrusts?
But, the idea is an intriguing one.
GW
The volume, mass, and/or number of propellant tank modules in any design far exceeds the volume, mass, and/or number of all other types of things, in any conceivable Mars ship, chemical or nuclear. The only thing needing artificial gee is the crew hab, which is 1, 2, maybe 3 of these small components. That being the case, I saw no reason to build a spinning wheel for a habitat. That way lay "battlestar galactica" stuff.
What I came up with was something about the same volume allowance as the old Skylab for a crew hab, with a crew of 6. That's twice the crowding the old Skylab crews of 3 saw, but still spacious compared to Mir and ISS. I did a combined parallel-series stackup of propellant modules to achieve a length between about 100 and 200 meters, with pusher engines at the tail end. It spins like a baton end over end to put 1 full gee at the farthest crew deck, less at higher decks, but still over half a gee. The whole ship was in the 300-500 ton class, but assembled of modules small enough to launch with Atlas-5, Delta-4, and a few Falcon-Heavies.
No need for a hugely-expensive SLS. It's tonnage to launch multiplied by unit launch cost (per unit mass of payload), which is only low if the launcher flies fully loaded. You launch all this tonnage at the anticipated unit costs for SLS, and the economics become politically unsustainable, no matter what size objects you launch. $1000-2500/pound is something we can tolerate. 10,000/lb is not. Period. I see no point to an SLS-class launcher until commercial space sees a need for it, and does it at well under $1000/lb. And someday, they will. But not in the next several years.
The landers went separately, each pushing the landing propellant supplies as dead-head cargo. These went unmanned and unspun. Still modular, using exactly the same common propellant tank modules as the manned ship. I did not rely on in-situ propellant, and I did not jettison so much as a single tank into deep space. I recovered every single item in my design for reuse or salvage, either at Mars or in Earth orbit. No free return, either. So my mission design represents a generous upper bound on what has to be launched and assembled. The three unmanned ships, pushed by the landers themselves, were in the 1000 ton class, each. That's in the neighborhood of 4000 tons that has to be launched and assembled. A nuke would be around half of that, this was a LOX-LH2 chemical rough-out.
This verges upon "battlestar galactica" problems, yes, but it also accomplishes much more than most mission plans I have seen. I budgeted the propellants from Earth to spend the first 5 or 6 months at Mars making 6 separate landings plus a visit to Phobos from low Mars orbit. The best site explored (presumably with lots of massive buried ice to mine), would be selected for a base. The remainder of the year at Mars would be spent there at the best site, building that base, all 6 on the surface with all 3 landers. If the ISPP actually works at the base site, it produces the extra propellants to support suborbital lander flights to explore additional sites. Icing on the cake, it would be. But we get a huge return, even if it doesn't work. Including a functioning base waiting for folks to return on the next trip.
As I said, an upper bound. You get exactly what you pay for. But you must also remember that nothing is as expensive as a dead crew. So, don't be too cheap. Cheap kills. We've already seen it.
GW
The CO2 would have to be stored as a compressed liquid, at several hundred psi (several 10's of atm). That's a very heavy tank structure. Water can be stored at a hair above 0 C at a hair above 6 mbar pressure. That's a very light structure, whose strength will depend more of liquid depth under acceleration than anything to do with storage pressure requirements. Plus higher molecular weight lowers Isp in a NERVA-like engine. Water is 18, CO2 is 44.
Those things being true, I think I would look for a place on Mars to land where there is massive buried ice, preferable not salt-contaminated. Just mine or drill for water (which requires steam extraction up the well head pipe), I'm not sure which extraction method is easier. About all you need do is filter out the dirt and keep it from freezing. Not every place on Mars will have buried glaciers, but any base is going to need lots of fresh water. So why not just land on one, and put the base there?
That does mean we need some ground truth to find such buried glaciers "for sure" before we start landing components for a base, now, doesn't it?
I'm not sure there is a way to get effective shielding in a nuclear lander, because it needs to be short and squat for landing stability on rough ground. If it were a long slender shape, the propellant and tankage structure actually maker a pretty good shield. Trouble is, things like that tip over way too easily upon landing.
If you really want a nuclear lander, you have to limit your inherently-unavoidable radiation dose by limiting the time you are aboard. That means you have to pitch your surface habitat remote from your ship. There's just no way around that dilemma with solid core nuke rockets. Might be fixed with open-cycle gas core, but we don't have those things yet. But it does buy a very large payload fraction for a ferry design.
GW
Acetylene is safest-stored dissolved in a suitable liquid. Most acetylene bottles used in welding have the gas dissolved in acetone, under roughly 10-20 atm pressure at Earthly temperatures. If you don't do something like this, you are almost guaranteed a tank explosion. Acetylene can be quite tricky to handle, more so than the article indicates. Rocket designers are not used to handling this stuff, it's the Earthly mechanical engineers dealing in compressed gases generally, who have the relevant experience. And none of them are used to creating high strength/weight structures.
GW
If there were such a thing as a lightweight electric power supply in the megawatt range (and there is not), then electric propulsion (and there are many types) could be used for faster transit to Mars. I don't know if 39 days would be feasible, but even 90 days would be good. This problem needs solution, but we cannot wait on its eventual development. You add that later when you have it in hand.
At the few-kilowatt level, we already have fairly lightweight electric power, and we'll need it anyway for any manned mission to Mars. (Shuttle's panels were 25 KW, I believe.) Why not combine electric and conventional propulsion in the same vehicle? Do the departure and arrival burns at Hohmann-transfer levels with conventional rockets, but also add continuous acceleration and deceleration during the long transit with electric propulsion. That would cut down total transit time.
GW
The three things I always see proposed for artificial spin gravity are (1) gigantic battlestar galacticas that spin about longitudinal axes, (2) cable-connected lightweights, and (3) truss-connected battlestar galacticas.
Battlestar galacticas are not practical at all at this time in history: too expensive, in all senses of the word. That eliminates ideas (1) and (3).
Idea (2) is almost practical, except for instabilities and difficulties spinning up and spinning down, the transients. Otherwise, it looks pretty good steady-state. Proposing to do this with two relatively tiny capsules ignores both living-space-to-stay-sane and radiation shielding for a long voyage (6 months or more 1-way to Mars). You can better do this with a proper habitat module at one end of the cable, and something else heavy at the other end (maybe the propellant and engines you have to have, anyway). God help you if the cable breaks (meteroid impact!!!), because no one else can.
Because of the need to do course corrections, there will be more than one very difficult-to-execute spin-up and spin-down operation. More than one of each maneuver vastly increases the probability of an accident, because these probabilities multiply, they do not add. That is why rigid structures are very much preferred for spin gravity designs. Much lower probability of a transient spin-up or -down instability accident.
There is a 4th idea that can address all of this successfully, which can easily be implemented with multiple docked propellant tanks, far more easily than with bigger stages. Using a combination of serial and parallel docking configurations, you can always create a long baton, fatter or skinnier as the number of tanks changes, with the hab at one end and the engines at the other. This structure is at least semi-rigid, quite unlike cable-connected assemblies. The spin-up/spin-down risks are therefore way-to-hell-and-gone lower than any imaginable cable-connected system. Doing course corrections is thus way less risky with a more rigid spinning structure, and the baton shape is well known to be quite stable while spinning (see the twirlers at any football game Friday nights all over the US).
The trouble with idea (4) (spinning baton) is that it is inherently not amenable to the overly-minimalist configuration proposals. I don't see that as a serious objection, because those overly-minimalist proposals ignore the living space and radiation protection issues. Either is likely to kill crews, more especially the solar flare radiation. But insanity also kills, as we have seen for a long time now with mass shootings by people known to have mental problems. Ask anyone who has ever served time in solitary confinement about its effects on sanity. He will confirm what I say about long-term confinement in quarters too close.
Overly-minimalist proposals being infeasible on the confinement and radiation-protection issues, I don't see any realistic objection to using the modular ship idea to provide a spinning baton shape for artificial gravity. Its only problem is that we are forced to design for 1 full gee at the farthest hab deck, because we do not have one single shred of direct evidence that partial gee will be sufficient. 1 full gee at 4 rpm requires 56 m radius, scale your ideas from there. We already know how to build things that docked-module way: ISS is also docked modules, and the Saturn-1 first stage had parallel-connected tanks, and we do strap-on boosters.
Any spacious hab design will have the room to add water and wastewater tankage in such a way so as to provide a 20 cm water shield around at least part of the hab as a radiation shelter. Could be externally or internally mounted. Lots of design freedom there.
I would suggest using Bigelow-syle inflatables to be docked together to create a really big hab module. I would also suggest that the packaged core equipment in each module be deployed in such a way as to remain within the core space of the fully-erected inflatable structure. That way, access to the pressure shell is unimpeded, for rapid repair of meteroid punctures. You cannot afford the time to move stuff out of the way: it'll depressurize before you can patch it if you clutter the pressure walls. That same free-access consideration applies to rigid-shell hab modules, too. Basic safety "at work" here.
I would also suggest that the vehicle control station be the radiation shelter. That way critical maneuvers may be performed no matter the solar weather.
None of this would yet apply to a surface hab on Mars. But depending upon what we finally learn about "how much gee is enough", we might have to add some sort of spinning frisbee shape as part of permanent settlements on Mars and the moon. No one knows yet. We can solve that problem if and when it crops up.
Just ideas and suggestions from an old, and very widely-experienced, aerospace engineer.
GW
What not having any experience between 0 gee and 1 gee really means, is that for any mission too long to endure 0 gee safely, then your artificial spin gravity design must be 1 full gee, no fractional values. Period. Not until we have the experience to know whether 0.38 gee or 0.16 gee (or whatever) is enough or not.
Spin gravity designs must get this 1 full gee at a tolerable spin rate. That's a fuzzy limit, but untrained civilians seem to be able to tolerate 3, perhaps 4, rpm quite handily for extended periods. If you design for 1 gee at 4 rpm, you need a 56 m radius from the center of gravity to your 1 gee deck location. We know enough from the bed rest studies to understand that the sleeping quarters can be fractional gee, it's your day shift work station that needs to be the 1 full gee.
The time you can endure 0 gee is also a fuzzy limit, very fuzzy indeed. It depends upon how much health damage you can tolerate. There is bone density loss, heart and circulatory system damage, and very good reasons to suspect both immune system and vision damage. No telling what else. For a 3-4 gee ride home from LEO, 6 months to about a year, perhaps a little more, seem to be tolerable.
For the 9-15 gee free return from Mars, the astronauts would have to be in a whole lot better physical condition than is typical from 6 months on ISS. Yet, we are talking about a nominal 2.5 year mission to Mars. That's 6 to 8.5 months one-way. I think everybody knows that doing this in free fall is a bad idea, probably fatal, but there are still a lot of folks who just won't face that fact yet. Including some at NASA, disappointingly enough.
This issue points out very clearly what a stupid decision it was not to include the medical centrifuge module in the ISS. Actually, a spinning space station design would have been even better. In many ways, it would appear that NASA and the others had, and still do not have, any intention of flying a crew of people to Mars. If they did, we would know by now (by direct experiment) whether 0.2 or 0.5 or whatever fractional gee was enough.
That and all the excuses over radiation exposures are what tell me the government agencies do not want to send men to Mars. That is why the Dennis Tito 500 day flyby mission may well take place: to shame the government agencies into actually doing something. I think the Tito mission may well be a suicide mission (because of microgravity diseases and radiation), but that is what it may take to spark some serious efforts at NASA and the rest for a Mars landing. Sad, but (unfortunately) true, I believe.
GW
If you do establish some sort of base on that first mission, wouldn't you establish some sort of greenhouse? Bury him in there, without a coffin. Let his body feed the plants.
GW
Hi Louis:
First time I'd seen it, thanks. Any manned settlement Mars One establishes is going to need a large supply of water (locally as ice). They need to develop some ground truth about buried glacial deposits of ice at all the sites they are considering. I've never seen a lander or an orbiter equipped to do that. Saw nothing in the article about that, either.
Water content in the Martian soil suffers from two really serious problems: (1) 1-2% ice-in-soil is an awfully diffuse resource to recover, and (2) an awful lot of that soil-bound ice seems to be far too salty for human or agricultural use without some sort of chemical cleanup.
In contrast, a buried glacier would very likely be real freshwater. Especially if it once was pack ice from a vanished ocean.
To find out what's really down there, and how much there really is, takes a drill rig capable of drilling as much as a kilometer down. Based on the probe designs I've seen, we'll not get data like that until men go, and even then only if their rover has the drill rig on it. Needs a digging blade, too. Basically, we need a backhoe/front-end loader, but with a drill rig on it.
I surely would hate to see people settled onto a site that turns out to be a dry hole. Because then we'd have to watch them all die without realistic hope of rescue. Commercial or governmental, doesn't matter; there is nothing as expensive as a dead crew. That's been the history of it, going back to the first manned flights in the 1960's.
GW
RobertDyck:
Turbine as-we-know-it is restricted to about 2000-2200 F gas temperature at the turbine inlet, limited by weak material properties at high temperatures, while under enormous stresses. There is no way around that dilemma. Reducing temperature kills you in terms of getting any usable energy out of the machine, it just barely works as it is. And that is true of turbo-shaft machines, no different than turbojet, really. Only the compressors differ: turbo-shaft are often centrifugal flow.
Most of the turbojet and turboprop aircraft engines, which normally burn kerosene, are certificated to burn aviation gasoline as an emergency fuel for a few hours (usually no more than 100 hours). But only if you do a hot section overhaul afterwards! It seems the minute lead content in avgas (about 2 cc tetra-ethyl lead per US gallon) leads to particulate lead and lead oxide in the combusted stream. This eats the coating off the turbine blades, leading to rapid erosion and blading failure. The older grade 100-130 avgas had 4 cc TEL/gallon, and was infamous for rapidly fouling spark plugs with lead deposits. Same was true of the old leaded automotive gasolines, too. I lived through all of that.
Got a chemical formula for a typical silane? I might be able to work out what the reaction looks like in air and in CO2. Not a chemical engineer, but I did acquire a lot of practical chemistry working solid rockets and ramjets. Especially the ramjets.
GW
By the time NERVA finished, they had a plume very clean of radioactivity, and a nice Isp, just a crummy T/W. Projections for the next design, based on what they did achieve, were Isp 900-1000 s at T/W near 5.
Timberwind was a particle-bed variant of the fuel loading design. It never got tested as an engine, so there are no performance figures for it, only design analysis estimates. It appeared to have about the same Isp potential as NERVA (in the vicinity of 1000 s), but at a much better T/W, because the reactor and chamber design was so much lighter. Whether that would have actually translated into a lighter engine ready-to-fly is what the testing that was never done would have answered.
There was another fuel-incorporation design called Dumbo, that used some sort of metallic ribbons instead of ceramic rods. There was also some sort of change in the nozzle design for Dumbo, relative to NERVA. I've heard tell that a better nozzle could have been incorporated into NERVA as well, but I honestly don't know the truth or falsehood of that. Like Timberwind, Dumbo was never tested, so all we have are design projections, not trustable performance. It, Timberwind, and Nerva all had about the same Isp. Both Dumbo and Timberwind were supposed to be lighter than NERVA for better T/W.
As for plume radioactivity, again only NERVA ever got tested. Initially, it was really bad due to core cracking and erosion. But, they fixed that by the time the program was stopped. Stopped just short of first flight, I might add. Whether the other two designs would have had clean plumes is something that only testing would verify, and that was never done.
All three had the same abort/crash problems, especially after first use. You are faced with handling a very radioactive core in an emergency situation, all the way from the pad to orbital speeds. That, rather than plume radioactivity, is the real problem with using nuclear rockets for Earth surface launch. It's not that it cannot be handled, because it was, with the demonstrated 500-mph crash integrity of the reactor vessel flown on the NB-36 airplane. But that's very heavy, leading to engine T/W under 1 as a nuclear rocket.
The abort/crash problem is "solved" by the open-cycle gas core concept, but not by the closed-cycle "nuclear light bulb" concept. However, even with open-cycle gas core, you trade crash/abort safety for a radioactive plume. Plus, gas core never got to a testable device, only separate academic lab demonstrations of a couple of the tinkertoys (a gas phase core was controlled, and a plasma device demonstrated sufficient relative confinement of heavy species to assure 100% burnup).
It's not like you will contaminate the entire planet if a nuke rocket has a problem. But the irrational fear of anything not "guaranteed safe" (even though there is no such thing) is a huge obstacle that has prevented the application of this technology. And the idiotic mistakes of the nuke power plant people (all over the globe) have done nothing to help assuage these fears.
GW
I think you will find there is no turbine design that can handle solids in the stream, either silica particles or massive amounts of soot. And teflon is long gone at useful turbine inlet temperatures. Not even a radial flow design would withstand that kind of beating.
A piston engine might work for a while, but the solids will cause leaky, then burned, valves. It'll handle soot much better than a turbine (diesels do it all the time), but the silica will slowly destroy it.
Some sort of external combustion design might work, if a separator can remove the solids from the stream before it gets to the moving machinery, whatever that is. If the product stream is more solids than gases, then it won't be a very useful engine. Expansion only works with gaseous phases.
I know nothing about any of the silanes. When burning with CO2, how much of the combusted stream is gas?
GW
GW