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You know, if this fuel-peroxide diesel thing were to work, then that makes possible locomotives on Mars not too dissimilar to what we have here. Not the same, but the same basic idea: IC/electric series hybrid.
That makes railroading possible big-time on Mars, once a set of bases (nascent colony) get planted. And that makes possible bringing together resources from different regions to one place to do manufacturing. It's a real bootstrap/chicken-and-egg process, but we've already done it here.
That's a major piece of technology that needs to be nailed down. It's not a prerequisite for a first mission, but it sure would be an enabler for a real settlement.
GW
There was something about nuclear jet propulsion, NERVA, and Timberwind, earlier in this thread. NERVA was an enriched-uranium solid-core nuclear thermal rocket that received a lot of ground test and development work in preparation for a flight application that got cancelled right before it flew. The gross characteristics of NERVA as tested were Isp 750-900 sec, engine thrust/weight 3.6, and a longest-demonstrated single "burn" of 47 minutes (yep, that's minutes!). NERVA and its predecessors like Phoebus and Kiwi, were part of AEC/NASA "Project Rover" for nuclear rocket propulsion.
Timberwind was a fluidized-bed improved design concept that never got much ground testing. There were others as well, such as an alternate-geometry solid-core design called Dumbo, which I think actually had more potential than Timberwind. The consensus potential of solid-core nuclear thermal rocketry based on 1970's experience was Isp 1000 sec, at engine thrust/weight 4, and several hours of restarts and accumulated burns, based mainly on Kiwi/Phoebus/NERVA.
The entire program was prematurely cancelled in 1973 before it could fly. The intended initial application was a replacement third stage on the Saturn-5 for the S-IVB third stage. It would have at least doubled the payload capacity of the Saturn-5 to the moon.
There were two gas core designs (generically "Lightbulb" and "Open-Cycle"), and several fluidized-bed (like Timberwind) or liquid-core ideas. None got tested beyond the level of academic university bench tests. All were part of "Project Rover", but none ever made it to any sort of real engine tests. Of these, the "Open-Cycle" idea was probably the closest to reality, being limited more by its waste heat radiator design than by anything associated with gas phase nuclear reaction controllability or by its degree of gaseous (actually plasma) uranium containment.
The B-36 "nuclear airplane" thing was most definitely not a nuclear jet engine. There was one NB-36 built and flown that had a working nuclear reactor aboard. The containment vessel for it had been crash-tested on the rocket sled at Holloman AFB, NM, to 500 mph impacts into solid stone without leaks. But it was no engine, just an ordinary reactor for a power plant, inside an extraordinarily-tough containment. Just the reactor, no steam loop, no generator turbine. They did generate thermal power in flight, but no electricity. I do not remember the government project name under which this was done, but it was 1950's vintage.
The nuclear jet engine was done under "Project Pluto", which I believe was AEC/USAF, nothing to do with NACA/NASA. It was 1950's-1960's vintage stuff. There were a lot of concepts investigated, but the only one actually tested was the nuclear ramjet. There used to be a facility for it at Jackass Flats, NV, on the government nuclear test site, alongside that for "Rover" and NERVA. A little of that stuff is still there, most has been disposed of.
The government supplied the reactor core, and LTV Aerospace was the airframe prime, for a nuclear ramjet cruise missile application. It was to cruise "forever" at low altitude and Mach 3, and divert to Russia and explode if war ever broke out. There was a separate megaton-range warhead for that. The engine was tested in direct-connect ramjet test mode at Jackass Flats in full scale and at full power, but the vehicle never flew, fortunately. It was superseded by rocket ICBM's in 1962. The nuclear ramjet spewed enough radiation, and more importantly its trailing shock waves and noise were so lethally loud, it would have killed more people on the ground as it cruised, than its warhead could ever have killed by exploding.
The only other nuclear propulsion effort was USAF "Project Orion" from about 1959 to 1965, when all military space was cancelled and diverted to NASA, excepting MOL, which wasn't cancelled until 1969. This was the nuclear explosion drive, based on in-company research by General Atomics in San Diego, from about 1950-ish up to 1959. It wasn't supposed to be anything but paper studies, but General Atomics couldn't resist, and flight-tested explosion propulsion with ordinary high explosives in a 1-meter long test article. It worked great, and just as predicted.
Nuclear explosion propulsion as it was understood ca. 1960 requires a shaped-charge fission device (that we could actually build) that is actually rather unsuitable as a blast weapon. It is very peculiar: working more efficiently in very large vehicles (10,000+ tons) and not very efficiently at all in small vehicles (few hundreds to 1000 tons). In 1959 the baseline paper design vehicle for "Orion" was 185 feet in diameter and 280 feet long, weighing 10,000 tons at surface launch. It spun for artificial gravity, and was designed for several dozen crew for a 2 year round trip to Saturn and back, single stage, stopping off at the moon and Mars along the way.
I wish we'd built that one. We still could. "Orion" was viewed by NASA as a competitor to "Rover", so they "cancelled" it by lack of funding in 1965.
GW
I rather think Zubrin is wrong claiming atmospheric CO2 can be used on Mars as the dilution gas for fuel-oxygen combustion inside engines. The compression required is simply technologically infeasible on Mars. Thermodynamics requires engine cycle pressures similar to those here on Earth for decent output. Here on Earth, the source to be compressed is at 1 atm, on Mars it is at about 0.006 to 0.007 atm. We do not have compressors capable of compression ratios that high, in a physical form that could be part of a practical engine.
That's why I suggested the diesel/hydrogen peroxide scheme with the steam dilution gas "built-in" to the already-pressurized propellants. There's a reason they tried this (and not hydrazine decomposition) as submarine propulsion from late in WW2 until atomic power came along in the mid and late 1950's. It actually worked at practical levels of power, and with practical amounts of liquid storage. Peroxide storage stability is the only real hitch.
I don't know the details, but the steam torpedo power plant is a cousin of this system, and versions of it have been around since WW1. It's suitable for a one-way weapon, but not really suitable for the sub itself, or else they would have done it already.
My intuition suggests that liquid methane could be burned with hydrogen peroxide the same as diesel or kerosene can. Not sure whether compression or spark ignition might be better suited with methane (high vs low octane number) in piston engines (turbine doesn't care). But I'd think you could build a piston or turbine engine for shaft power around this combination that would work even in vacuum.
GW
Once you have the frozen CO2 from your freezer, just be sure there's very little free volume in your canister. The sublimating dry ice will pressurize the canister quite nicely, to perhaps multiple atm.
GW
RobS:
This is an excerpt from my last post from the "airbreathing engines" thread under this same topic:
"The closest thing I know to a practical piston engine system independent of atmospheric oxygen was attempted for submarines decades ago. Diesel fuel plus hydrogen peroxide actually did work from storable liquids while submerged. It's somewhat similar to the propulsion of the torpedoes, but more suited to a reusable vehicle. There were great difficulties with it, and it was superseded by atomic power in submarines, then forgotten.
Given some sort of fuel that could replace the diesel, and some way to make hydrogen peroxide on Mars, that technology might lead to practical internal combustion or gas turbine power plants on Mars. The storable liquids are pressurized at around 1 atm, like here. Hydrogen peroxide decomposes to oxygen and steam. Steam is the diluent gas that reduces stream temperatures to something you can confine with cooled steel.
It can be done, but needs some development before we take it to Mars and count on it at the risk of lives."
Given a tad of development to make fuel-peroxide really work well, the same piston-engine "Gator" could be run with stored liquids pre-pressurized to around 1 atm in their storage tanks. This shouldn't take more than a couple of years to check out and debug pretty thoroughly, if done by a Spacex-type of outfit. (A ULA-type would milk this for lots of gov't $ for about a decade.)
Rubber tires in the cold are a problem with brittle failure. But it can be done. Already was done for the Antarctic bases in the late 1950's.
GW
My engineering intuition suggests the instability of the rocket plume fired into an oncoming supersonic/hypersonic stream derives from it's being fired straight ahead into the stream: the shock layer doesn't know which way to jump, so it buzzes around.
What if you have multiple thrusters and cant them all, at least a little. Now the shock layers know exactly which way to bend things. My intuition suggests this will be fluid dynamically stable. It would make rocket braking feasible during re-entry.
Guess what! The super Draco's on the Spacex Dragon are canted about 45 degrees. It's supposed to be rated for trips to Mars.
Others seem to have the same intuition regarding rocket braking in re-entry. I bet Spacex has already tested this, too.
GW
RobertDyck:
I quite agree with you about NASA. I am pleased with what happened in Canada with the RadarSat. Sounds like Canada's space agency has not stultified with "bureaucratic disease" the way the US agency did.
Part of our problem here in the US is that we allowed Congress to make pork-barrel political footballs out of every little project NASA does, which means NASA does nothing Congress cannot play for politics. That is precisely why American astronauts have not left Earth orbit for 4 decades now.
I hope Canada can avoid that mistake - it is fatal.
As for gas turbine or any other combustion engines on Mars, if they suck the local "air", they have to compress it to useful pressures inside the engines. Those are cycle pressures measured in tens of atm. Here, the source is 1 atm, for a compression ratio of tens. On Mars, the source is 0.0068 atm (at 7 mbar), for a required compression ratio in the thousands. The most extreme gas turbine ratios are around 30-something with known technology.
The most extreme recip compressors are in submarines, which for the high pressure air banks are around 480 atm and around 480 compression ratio. Those are not exactly the transportable-weight devices you could make a part of an engine, either.
You could in a vehicle carry both a fuel and liquid oxygen, and operate a turbine or a piston engine with it. The problem is the around-6000 F (3300 C) stoichiometric flame temperatures with just about any fuel and straight oxygen, again at tens of atm cycle pressure. Except for rockets, you'll need huge volumes of colder diluent gas to bring the stream temperatures down to something the hardware can withstand. Gas turbines do this with excess air, here on Earth. Due to the difficulties of compressing on Mars, that's not really an option.
The closest thing I know to a practical piston engine system independent of atmospheric oxygen was attempted for submarines decades ago. Diesel fuel plus hydrogen peroxide actually did work from storable liquids while submerged. It's somewhat similar to the propulsion of the torpedoes, but more suited to a reusable vehicle. There were great difficulties with it, and it was superseded by atomic power in submarines, then forgotten.
Given some sort of fuel that could replace the diesel, and some way to make hydrogen peroxide on Mars, that technology might lead to practical internal combustion or gas turbine power plants on Mars. The storable liquids are pressurized at around 1 atm, like here. Hydrogen peroxide decomposes to oxygen and steam. Steam is the diluent gas that reduces stream temperatures to something you can confine with cooled steel.
It can be done, but needs some development before we take it to Mars and count on it at the risk of lives.
GW
Turbojet? That gets us right back to the question of how to build such a thing. How to build a gas turbine that thinks CO2 is an oxidizer, and can generate a useful cycle pressure level in a 6-7 mbar atmosphere.
Like I said, that will be a really tough thing to do.
I really do have to wonder whether rocket-powered vehicles augmented a tad by aerodynamic lift while cruising supersonically wouldn't be just about as practical. But I sure would like to see some sort of "air"-breathing engine developed. We'll need a lot better compressors than we know how to build right now, of that I'm sure.
GW
The high-altitude NASA craft was very subsonic. Not compatible with supersonic flight with ramjets, so the propulsion would have to be something very subsonic, like propellers. There might be a way to do that with a combination of solar PV and chemical or battery storage of electricity. I have no idea what takeoff might look like in "air" that thin. Probably JATO-assist would be required.
GW
If for your transfer vehicles you are considering propellant modules in the 30 ton class assembled by simple docking in LEO, I think that is the right approach. Unmanned supplies and equipment can go "slowboat" this way on min energy trajectories for capture into Mars orbit, and the basic transfer vehicle could be reused, but only if you do not jettison the empty modules.
Myself, I would use the NERVA design that almost flew for this, not Timberwind, which never got that far through test. Later, as better nuke engines become available, you could replace the NERVA's. But why throw away hardware if you don't have too?
That sort of transfer begs the question of propellant supply at both ends, so that the vehicle may return for reuse. For nuke rockets, hydrogen is usually considered, which is fairly easily made from water using solar PV, if the rate of production required is not too high. Both Mars and Earth have lots of water, as ice on Mars, both liquid and ice here.
Why not shoot ice payloads almost naked into LEO and LMO with light gas gun technology? That technology is powerful enough now to work on Mars, and almost ready to do that job with small payloads to LEO. So the payloads are small? So what? Use solar thermal to melt, and solar PV to electrolyze, in both LEO and LMO. As long as the individual transfer flights are many months apart, you don't care that the batch process is small scale and slow. Your propellant reserves build up over that time interval. It can be automated. Product is both hydrogen and oxygen, items very valuable.
Single stage one-way nuke transfer by min energy trajectories, with fully reusable vehicles, means you build them and launch the fueled modules from Earth only once. After that, resupply is very cheap, once the light gas guns and solar propellant satellites are in place. This is not "battlestar galactica" stuff, either. The biggest one-way payload to ship to establish this capability is the light gas gun equipment to be landed piecemeal on Mars.
Now, this does also beg the question of equipment transfer from LMO to the surface at Mars. If piecemeal components can fit inside one-way Dragons rigged as landers, that's the start. But, if you have NERVA engine technology resurrected, that also supports one-stage fully reusable lander technology, using nothing more sophisticated than direct rocket-braking descent. These things could carry a big load fueled for a two-way trip from LMO. Check it out yourself at Isp near 1000 sec, engine T/W around 4, vehicle structural fraction 20% to be tough as an old boot, and 70% propellant fraction to be fueled for up to 30 degrees out of plane, full rocket delta-vee two-ways. I got a 10% payload fraction.
If you make hydrogen both in LMO and on the surface, you could carry far heavier payloads fueled only one-way like that.
The more desirable follow-on to NERVA would be one of the gas core concepts, but that's future stuff. We could do the NERVA thing right now, but we need to pick the brains of those who did it for their engineering art. There are very few of those guys left. That was 4 decades ago they did that.
If it turns out there's significant ice inside Phobos, so much the better. No light gas on Mars is necessary.
It takes a while to bootstrap into a setup like that, but if you plan for it from the outset, the right choices can be made along the path. There's a real sustainable transport system to Mars. Simple, direct, and launch things only once (keep on using hardware once launched).
GW
Bamboo is actually a decent structural material. Good strength/weight, and rather easy to work. Joining seems best done with lashings, actually.
You will have to have a place with air and water to grow it in. There's osmotic pressures of transpiration to consider, among many things.
I doubt we'll bio-engineer anything that could live outside until Mars is terraformed. The water vapor pressure problem is insoluble at 7 mbar dry CO2.
GW
Sure. They're a good idea too.
GW
I have lots of experience designing, repairing, and operating piston and turbine engines on a lot of weird fuels. But, I cannot imagine using a slag-forming reaction (from burning tri-silane with atmospheric CO2) inside anything with moving parts. I just do not see how it could survive the grit and other solids for more than a few seconds. Blades, valves, pistons, makes no difference.
On the other hand, a device without moving parts could survive such abuse, so long as one you periodically (or continuously) get rid of the slag buildup inside. I also have decades of experience with ramjet engines, so that does come to mind. Not the ones with the baffle or colander flameholders, the simple dump combustors, which had wider flameout limits here on Earth, anyway.
I'm not sure how any "air"-breathing engine could ever produce useful power relative to its weight on Mars, because the atmosphere is so thin. Nor am I sure that airplanes would ever be practical there, for the same reason. But, under the assumption that low engine cycle pressures and high wing loads can feasibly be addressed, one might consider a ramjet airplane with a rocket booster for planetary transport on Mars.
Use the tri-silane or some other fuel with some oxygen in a separate rocket motor to takeoff and accelerate to supersonic-type ramjet speed, generally Mach 2-ish here on Earth, probably similar there. Then switch to tri-silane (or magnesium powder) burning with Martian atmospherioc CO2 in the ramjet (and shut the rocket down). You cut all power to decelerate and glide to landing, with some reserve rocket propellants to support go-around or divert.
I picked the supersonic-inlet ramjet variant because the performance potential is much higher (higher cycle pressures in the engine) than the subsonic variety. This plus other extraordinary design measures would be needed to counter the disadvantage of the extremely thin "air" there. This is still subsonic combustion, though, not "scramjet", which simply is not yet ready for prime time. Flight speeds will be in the Mach 2 to Mach 4 range quite easily.
Air transport on Mars? Maybe. That "air" is awfully thin. Engine thrust and power, and wing lift, will be more-or-less proportional to atmospheric density (as a ratio to here: pressure ratio .006 times molecular weight ratio 1.57 divided by temperature ratio something like 1.2, for .0079). The weights to be moved are proportional to gravity's pull (as a ratio to here: 0.38 gee). Power to weight, and wing loading, will be more or less in density/gravity ratio to here: 0.02.
It'll be 50 times harder than here to get decent power out of an airbreathing engine, and 50 times harder than here to balance structural weights against wing lift required to fly. Very tough to engineer.
GW
Standard military weapons-release speed is 485 knots indicated. At sea level, that's about 0.85 Mach. Definitely subsonic at all low altitudes of any interest. Condensation clouds can exist in the intake of a jet parked static on a carrier deck.
I tried ballutes as a stabilizing drogue back in the mid 80's. Ribbon chutes worked better, and were good to 2.5 Mach, if you bagged it properly for control of opening shock. Different application, same decelerators.
These were ram-air inflated devices, though. In space, you are really talking about low to medium-pressure forced inflation of a shaped balloon. Not quite the same thing. It was the ram air inflation I found unreliable, unlike a ribbon chute.
GW
As regards artificial gravity, I think you'll find we actually do have some idea of what spin rates are tolerable. Aircraft and early astronautic work defined it. That boundary is a bit fuzzy, but 4 rpm seems to be a good rule of thumb for a top rotation rate.
What we don't know is how much gee is enough. That research has never been done, and I for one do not trust people's lives and health to surrogate studies like enforced bed rest. It's just not the same.
Our only direct experiences are at 1 gee and zero gee, with the result that in zero gee bad things start happening to the body that cannot really be reversed, after somewhere in the vicinity of a year. Again, a fuzzy boundary.
Most missions to Mars involve a one-way transit time of 6-9 months, plus some extended stay on the surface. Total mission times are around 2 years. Mars has 0.38 gee, but we have no data whether that's "enough gee", precisely because those studies were never done in a spinning lab in LEO.
So, it looks to me like it's one full gee at no more than 4 rpm. Period.
That's a 56 meter radius. If you build the vehicle long and narrow and put the habitat on one end, you can just spin it end-over-end for gee. De-spin for maneuvers. Easy. No battlestar-galactica, no extended space truss structures, no complicated cable-connected anything.
Think around a dozen-and-a-half to two-dozen docked 30-50 ton modules, mostly propellant tanks, each one around 5 m diameter and 15-20 meter long, or something in that vicinity. 1 full gee at way under 4 rpm. I'd put a Skylab-like module up there for 6 folks, at one end, and a NERVA-type nuclear rocket on the other. (We know how to build those engines, by the way. They all but flew 4 decades ago.)
The "Skylab" could be 3 or 4 inflatables docked together. Just provide lots of open space inside, in which to live sanely for several months at a time.
Don't count on ISRU return propellant on the first trip, it's an untried technology as yet. Losing a crew is simply more "expensive" than carrying the return propellant with you. It is crucial to design-in a "way out" for the crew at every single phase. These people must return alive if there is ever to be another trip.
You can send the landers and their propellants and surface supplies separately, and then rendezvous in LMO with it when you get there. It's the surest way, one we already know works. "KISS" is how to ensure it all works.
So, we're looking at assembly of a small "fleet" in LEO. We know how to do that, we built the ISS that way.
Once you're in the shuttle payload range of 25+ tons, it no longer really matters very much what the module size is. What matters is cost per kg delivered to LEO. With Falcon-Heavy up to 53 tons priced at about $2000/kg, what do we need with a big, expensive government rocket development anymore?
Just thinking out loud, and trying to raise some very pertinent questions.....
GW
I was not so very much a fan of X-33's 8% structural inert fractions. Turns out I was right: those propellant tanks fractured under their own weight, when stood on end.
I am very much a fan of very low ballistic coefficients/wing loadings for re-entry. I like the idea of an extension of the venerable old Piper Cub as a re-entry vehicle. Absorbing BTU's is easy. High skin temperatures is not.
Sort of poetic justice, in a way. I rather like stick-and-rudder flying. I learned to fly in a Piper J-5, if anybody knows what one of those is.
GW
There will always be a need for paper. I don't care how "electronic" we get, you will always have to have a permanent storage medium for critical data.
And, nothing will ever replace toilet paper.
If bamboo is good for those uses, then so be it.
GW
Actually, it could be done, with a better buoyancy-defining density ratio there than here. At equal pressures and temperatures otherwise, gas density ratio boils down to molecular weight ratio. That's 2 (for hydrogen) vs 28.97 (for air here) vs 44 (for CO2 there). Mars density ratios are about 50% better.
From a practical standpoint, I would worry about the envelope's survivability: plastics have a very short life on exposure to UV, which is very harsh on Mars. You won't be able to build this dirigible (or balloon, or blimp) out of aluminum. Too heavy. Just like here.
Even here, hydrogen explosions are less of a risk than most folks suspect. The Hindenburg would have burned and crashed, even if it had been filled with helium. The forensic data (confirmed by old insurance records) suggest that the real culprit was static-spark ignition of the doped fabric skin. That dope was nitrate-base (a monopropellant explosive/pyrotechnic), and was pigmented with a mixture of aluminum powder and iron oxide (which is today known as thermite!!!!). The hydrogen just added a little to the yield of the fire. But the flame propagation speed along the Hindenburg's skin was around 10 times what fully-premixed hydrogen-air is capable of. And it wasn't even pre-mixed.
The ONLY real problem with a smallish hydrogen-air flame here on Earth is that you can't see it in full daylight. But, on Mars, you won't even have one at all in a CO2 atmosphere.
From a design standpoint, the real design problem with a Martian dirigible/blimp/balloon is that the material-of-construction weight only scales down by 0.38 (for the strength of gravity), while actual gas densities scale down by 7mbar out of 1013, roughly. A dirigible/blimp/balloon on Mars will have to be very much larger in proportion to its useful payload there, than here.
But, yes, it could be done. And safely.
GW
quote:
"Musk seems to have several things going for him: (1) a team of dedicated young engineers who want to succeed; (2) the luxury of looking at the big picture and figuring out the cheapest, easiest way to do it, without lobbyists asking to fund their special big rocket or Congressmen insisting on expenditures in their district. Musk also seems to have good instincts about what near-term technological innovations to pursue and which need to wait."
There you went and put your finger squarely on what is wrong with NASA, and what will eventually afflict ESA, JAXA, and all the rest: a government program in which tactical details (not just overall strategy) are political footballs. This went wrong for NASA in the 1970's, and they have accomplished no significant large exploration objectives since. The ones they have accomplished have all been robotic, and all have been the subject of drastic political fighting in Congress.
Let that be a lesson to all my non-US correspondent friends.
If anyone can break this deadlock, it will be Musk. But I have my doubts he can single-handedly pull off a manned Mars mission that really leads anywhere. That's because he only has the resources to fly there maybe once or twice. And, (here's the real problem) the "exploration" isn't yet done. There is a capstone exploration mission with men that you simply cannot do with minimalist mission designs. You do not need "Battlestar Galactica", but you cannot do it with one Dragon full of 1-6 men, either.
The key is limiting your government legislatures to ONLY strategic objectives. Forbid them EXPLICITLY from dictating tactical details. It is likely way too late for the US & NASA, but ESA and JAXA might still stand some chance if this is done.
This is important because most corporations DO NOT function like Musk's Spacex: they will not invest in anything unless there is demonstrable short-term profit. Mars will not show profit for a long time yet. (Sorry, but it it just won't. To expect otherwise is simply not realistic.) Neither did the Roanoke colony. Even Jamestown was a financial loss for a long time.
You have to focus on what government can do best and on what corporations can do do best. There's less overlap than most folks believe. The "smarts" lie within the corporations, not the government labs. But corporations only do speculative things if a government will pay them to do it. Chicken-and-egg......
Once you get by that conceptual hurdle, you can design practical missions to the moon, Mars, NEO's, even to the stars.
But NOT until you take that hurdle into account. Sorry.
That's about 500 years of our history talking, not me.
GW
Yep, it matters.
Days or weeks or months of micro-acceleration are incompatible with the delta-vees one computes under the impulsive (pretty near zero burn time) assumption.
GW
"Lower heating if decelerating at higher up" is really lower peak skin temperatures at lower ballistic coefficient. The total integrated BTU's (KW-hr) absorbed over the trajectory is actually higher in that case, but that's an easier problem to solve than high skin temperatures. Control of survivable gees is the real key to how this trajectory is selected.
For a winged vehicle here on Earth, a "ballistic coefficient" is better expressed as "wing loading", which is vehicle weight divided by wing planform area. To obtain the benefit of earlier deceleration higher up, this needs to look more like a small light aircraft (10-20 lb/sq.ft) than the shuttle or a jet fighter (100-200 lb/sq.ft). Peak skin temperatures are under 2000 F, which ceramics, or very, very, very heavily-cooled Inconel-X, can survive. Better odds on the ceramics.
For a space capsule here on Earth, it is vehicle weight divided by heat shield broadside area. This has typically been about 4 times the shuttle/jet fighter range (400-800 lb/sq.ft). These vehicles punch very deep into the air before they slow significantly. Peak skin temperatures (stagnation point, leading edges) is around 3500-4000 F. That's why Shuttle had ablative carbon-carbon leading edge and nose cap pieces, and why Mercury, Gemini, Apollo, and Dragon had/have ablative phenolic composite heat shields.
PICA-X on Dragon (and the other new capsules) is actually nothing but a modernized version of the 1960-ish vintage silica-phenolic, which is really nothing more than what the old capsules used in the 1960's and 1970's.
Think about it: if you can actually achieve such a low wing loading/ballistic coefficient (around 10-20 lb/sq.ft), you could survive re-entry here on Earth (from orbit) with a steel truss "airframe" and a ceramic fire-curtain cloth skin, as long as there was some sort of insulating standoff between the frame and the skin, and some sort of ventilation inside to absorb the BTU's. In other words, a ceramic fabric-skinned variant of a Piper Cub might actually be a feasible re-entry vehicle from LEO.
Anything that might work here would work on Mars. The heating there is less demanding. I see some experiments that need to be done here!!!!
GW
The original topic was wind power on Mars. The maximum achievable energy capture of a windmill I once read to be around 59% of what kinetic energy rate (kinetic power) there was in that portion of the wind stream tube intercepted by the mill. I don't remember how that limit was derived, but no real windmills actually recover that much. Sort of like a Carnot engine in thermodynamics, you can't really build anything quite that good.
Now, the KE rate (kinetic power) of a windstream is essentially 0.5 mass flow rate * velocity squared. Mass flow rate is density*velocity. So the kinetic power available is 0.5*density*velocity cubed. You will recover about half or so of 59% of that stream power in a good mill; say about 25%.
Density on Mars is quite low relative to density on Earth, and wind speeds are comparable. So the velocity-cubed factors are similar there and here. Temperatures are colder on Mars, which act to increase density by inverse proportionality, while pressures are lower, which act to decrease density in direct proportion. Say 233K there versus 288K here, and 7 mbar there compared to 1013 mbar here.
The density ratio there to here is then (7*288)/(1013*233) = 0.0085 or thereabouts. Wind power potential there is a bit less than 1% of wind power potential here, at similar speeds. Twice the speed there vs here would give you power potentials 8 times higher (about 6.8% of here).
Myself, I'd be looking harder at solar PV, solar thermal, and nuclear, than wind. The "air" is just too thin there.
GW
VASIMR is a variation on the ion drive. All the electric propulsion schemes, including VASIMR, will have vehicle accelerations measured in milli-gees or less. Some much less. The burn is definitely not a short impulsive burn, by any standards.
If your burn is impulsive, meaning delivered over an extremely short time, not a large percentage, relative to travel time, then you have no gravity losses relative to what we normally compute for the rockets and orbits we are familiar with.
Minute acceleration levels make you burn through a large percentage of your travel, incurring gravity losses during the entire burn. Your effective specific impulse is nowhere near as high as you would compute from a bench or static test.
These electric things do help quite a bit on very long trajectories, but the shorter the trip, the "crappier" they look relative to plain old high-thrust, very-impulsive rockets. Mars is not far away enough for it to look good. Itokawa was.
GW
My engineering intuition suggests that trailed drag devices won't be very effective, because of "shadowing" in the wake of the main vehicle. Even if the tow cable were miles long, this effect would still obtain, we've seen it in the persistent ionized trails the space shuttle left behind during its re-entries.
To make a towed drag device effective, it would have to be wider than than the wake left behind by the vehicle, which is somewhat wider than the vehicle itself. That means you are looking at some sort of conical ring-shaped ballute inflatable. Anything in the middle shadowed by the wake is ineffective as a drag device. But it would be inherently dynamically stable to use such a towed drag device.
I haven't go a clue how to build such a device, but it would need every bit the same heat protection as the vehicle itself, and so would its tow cable, being immersed in incandescent plasma. That's a very harsh environment for all known materials. Interesting idea, though. Certainly deserving of tests.
It would be rather smart to include test articles on ferry flights to ISS, and release them for test coming home.
GW
Quote:
"Once we have a basic energy, industrial and agricultural infrastructure in place, follow up missions would be relatively cheap."
You just made my pessimistic point.
None of the mission plans I have ever heard of since 1956 was planned to establish anything like useful infrastructure of any kind. Since the 60's, it's been flag-and-footprints with 1-12 men, and usually only one vehicle making a single landing.
One or two limited-objective missions is all that the government(s, all of them) will ever do. Until there is at least some infrastructure on Mars , business will not go. Chicken-and-egg.
Except maybe Elon Musk. He bucks the trend, and resembles the more adventurous companies of about 5 centuries ago in what he wants to try.
Hope springs eternal.
GW