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Solar-electric might well work pretty good in the inner solar system, maybe even to Mars or the Main Belt. But there is a very good reason most of the probes to the outer planets have been nuclear-powered. The lower the sunlight intensity, the bigger and heavier your solar hardware has to be. It's either that, or accept lower power and lower thrust (which is already pretty low for the weight to begin with).
As for Zubrin, he's pretty smart, and he can be very vocal. But, like all of us, he can sometimes come up very short on being right. I know he has no patience for technology development. Sometimes that's the right thing to promote, and sometimes it's wrong. When you are putting together an expedition, it only makes sense to use proven technology, otherwise it's just a gravy train, and you never really go. That's where Zubrin is coming from. I've been there, too.
But, 10+ years down the road, you'd really like to have some better technologies available to choose from. That's mostly where I come from. And I don't mind resurrecting an old one for a new use (not many come from there!). The trade-off comes from expedition schedule: just when do you really expect to go? Does that give you time to add some new proven technologies to the mix of proposals, when it actually comes time to choose? And, just when do you have to choose? All expeditions are different in that respect.
The pitfall is not "having some technology programs", it is "incorporating new technology development into your baseline expedition program". Those need to be separate functions until such time as the technology program(s) actually bear fruit. Ideally, the technology development effort is a whole separate plethora of parallel and serial projects, funded continuously over time, with the stated purpose of providing "new stuff later" when it is needed.
Funding these vs specific expeditions is not necessary a zero-sum game, either. But usually, we can only afford one or two expeditions at a time (those usually have huge price tags). Technology efforts are usually very modestly-priced in comparison to expeditions, and are also much more amenable to trading off funding vs schedule time.
GW
Hi Russel:
Have you seen the deceleration dynamics profiles I posted over at "exrocketman"? For shallow entry trajectories, velocity vs slant range (or altitude, take your pick) is an S-shaped curve transitioning from high to relatively low. Acceleration (decel, actually) is a pulse (or narrow bell curve) situated in between two zones of relatively flat and low deceleration. Initially, the V-squared effect is high but density is incredibly low, while finally V-squared is relatively low while density is significant. In between, you get high forces acting to decelerate the vehicle. With low ballistic coefficient, this pulse occurs earlier in the trajectory, at high coefficient, later. The same thing would apply to lift forces.
The convective heating rates follow the same pulsed behavior, and at pretty much the same place in the trajectory as the deceleration pulse. Earlier is a bit lower heating peak, later is a bit higher. Peak heating rates go with peak skin temperatures. But, the heating integral over time is pretty much the same no matter where in the trajectory the pulse takes place. Down to around 100 kg/sq.m on Mars from LMO, I don't see heat rates that might excuse me from needing some sort of significant and heavy heat shield. But if I got down closer to 1 or just a few kg/sq.m, I might see the opportunity to use the lighter options. That corresponds to the wing loading of the typical Piper Cub or other light aircraft, around 6 to 15-or-so pounds per sq. ft. No one has ever designed a reentry vehicle like that.
Steepening the trajectory moves you very quickly toward hitting the surface before aero drag can decelerate you to local Mach 3 (the semi-arbitrary "end-of-hypersonics" point). From 200 km on Mars, I ran one scenario at an intermediate ballistic coefficient, and at 6-something degrees depression instead of 1.6 degrees. I had surface impact between Mach 4 and 5. Same thing happens with warheads here: impact is at two-digit Mach at typical suborbital ICBM conditions.
I haven't run the vertical drop scenario with my reentry model. It starts from near-0 velocity, not 3469 m/s on Mars. But I don't think you see much drag in that thin air until too late to do much good. And remember, we have no chute designs that work much above Mach 2.5.
Your model of killing only part of the orbital velocity would fall somewhere between the two extremes I looked at. I don't know how things vary between those extremes, but I doubt it's directly proportional to velocity. KE, perhaps. Dunno.
Heat shields can be either ablatives or refractories, and can be augmented by sweat cooling or simple heat-sinking. There's a lot of options there, especially on Mars, where entry is less demanding because the KE's at interface are so much lower. Any technique we can make work here, can be made to work easier there. At very low wing loading (ballistic coefficient), it might be possible to do refractory with ceramic fabric instead of low-density tiles. But, skin temperatures must be under 2300 F with alumino-silicate fiber to avoid embrittlement and irreversible volume change from temperature-induced solid phase change. That's a big "if".
GW
I saw the same outcomes reported in a variety of news stories, too. One or two of them also mentioned that the probability of impact in 2068 increased (although it is still low).
The flip side is that the further into the future the prediction is made, the less reliable it is, no matter how precise the current observations are. This is because of a plethora of small but significant effects perturbing the orbits, things not well understood at all. Only one of these is the Yarkovsky Effect.
My best guess is that we will have warning times at most of a few years, at worst a few days, with Earth-crossing asteroids. No one is yet worrying about the highly-elliptical or even hyperbolic comets. Warning times for those will be at most days, at worst an hour or so.
We're going to need far more powerful propulsion than anything contemplated now, in order to reach these things in time to do any significant deflection. That's just going to be a fact of life, though few yet acknowledge this. The new SLS/Orion capsule is nowhere near what we're going to need. Neither would any of the electric schemes, or even NERVA or its variants. The only thing I know of with the power to do even a part of this is the old nuclear pulse propulsion scheme, with all the bad feelings that concept seems to spawn.
The other unpleasant fact of life is that the favorite "last-ditch defense" (a nuke detonated nearby to nudge it by radiation pressure and absorbed-heat thermal spalling) is simply not going to work very well on some of these, perhaps the bulk of them. If it's a gravitationally-bound pile of loose rubble, the nuke only disrupts and scatters the material, even though there's no blast wave in vacuum, because the applied forces exceed the local gravity forces.
You just turned a single bullet strike into a widespread shotgun blast, if you do this days or weeks before impact, maybe even months. One visit by astronauts to an NEO is simply not going to provide the experience we need to properly address this problem.
Asteroid defense is very good rationale for a very ambitious space program. But few understand the magnitude of the effort that is needed to mount an effective defense.
GW
Russel:
I took a quick look at the idea you mentioned (kill the orbital velocity while at orbit altitude, and free fall vertically). For a 200 km altitude circular orbit about Mars, the orbital velocity is 3455 m/s. That would be your delta-vee required to initiate the vertical fall.
A ballpark estimate of velocity at the surface would be a vertical fall at Mars gee constant through that same 200 km. That’s crude, because gravity isn’t constant, but it is ballpark since the drop is small compared to a Mars radius. I get 1220 m/s. You said about 1100, so I know we’re both in the ballpark.
There will be some retardation due to atmospheric drag as you approach the Martian surface, but not a lot, since the air is really thin there. So, you have to rocket-brake to kill the bulk of that terminal velocity. Just for a number, let’s say that delta vee will be somewhere around 1000 m/s. The total delta-vee required of the descent propulsion is then about 4455 m/s, for that scenario.
On the other hand, you could do a near-minimum deorbit burn, and see how much aerobraking you can get, which is the conventional reentry approach. From the same circular 200 km orbit, the surface-grazing transfer ellipse has an apoapsis velocity of 3405 m/s, thus requiring only a 50 m/s deorbit burn to put you on that trajectory.
As you descend along the transfer orbit, your velocity increases (conservation of energy). At the interface altitude of 135 km, I’m calculating 3469 m/s for the actual entry velocity, vectored 1.6 degrees below horizontal. And that’s the end of what standard vacuum orbital mechanics can tell you, because you are now inside the atmosphere. From there, I used my crude warhead entry approximation.
The final ballistic coefficient that I guessed for a 60 ton lander was 400 kg/sq.m. That plus 1.6 degrees trajectory depression below horizontal, and 3469 m/s at 135 km, together lead quickly to local Mach 3 end-of-hypersonics at around 700 m/s and 17 km altitude, still near 1.6 degree depression. That's too low to bother with a chute or ballute; you are only a minute or so from impact. That 700 m/s is the minimum velocity to be killed by rocket braking, but there’s also still a controlled descent to make to touchdown after you kill it. So, at least double it to somewhere around 1400 m/s. The min total delta vee for this kind of descent is thus somewhere around 1500 m/s.
Compare delta-vees: somewhere around 4400 to 4500 m/s for the velocity-kill / vertical drop scenario, versus somewhere in the ballpark of 1500 m/s for the conventional aero-braking entry. Shallow-entry aerobraking seems to have about half an order of magnitude advantage in reduced delta-vee requirements over the vertical-fall scenario. That same kind of effect is why all the capsules and spaceplanes we have ever flown here on Earth used shallow entry aerobraking. It’ll be no different on Mars, just the smaller numbers, similar to those we are contemplating here.
GW
Rusty Schweikart is not the only astronaut taking up the asteroid defense cause. There are several more. I don't know all the names, but I've seen Tom "Skywalker" Jones in print on this subject, too.
GW
Russel:
I don't have the tools to do all of what you describe. I did look at deorbit vs angle at entry (in a crude but decent pencil-and-paper way) by just calculating derivatives (slopes) off the elliptical vacuum path, evaluated at the "entry interface" altitude. The lower your orbit from which you deorbit, and the minimum deorbit delta-vee you think you can get away with, then the shallower your flight path angle at interface.
My numbers were picked in ignorance, but as educated guesses. I picked 200 km altitude from a circular orbit. I picked a surface-grazing transfer ellipse, which requires a 50 m/s deorbit delta-vee. At the "entry interface altitude" of 135 km as recommended by Justus and Braun, I got 1.6 degree depression angle of the flight path below horizontal. That really shallow angle is quite safe (no bounce-off) because all velocities are well below escape. And it gets you survivable end-of-hypersonics altitudes, even with ballistic capsules of very high ballistic coefficient. Best of all, you get this inherently, it is not a matter of careful precision control.
My entry model is a simple pencil-and-paper model that they used about 1956 to estimate non-lifting warhead reentry. In effect, figured as 2-D Cartesian (even though it's definitely not), the flight path that the warheads actually flew is very nearly constant flight path angle. The model equations give you a velocity vs altitude profile based upon a density scale height approximation, and a constant ballistic coefficient approximation. The model was given in the same Justus and Braun report I cited in my postings over on "exrocketman". The dynamics were pretty good, but the convection model as presented was pretty bad. I fixed that, and posted what I did.
That's what I've been using for the Mars lander problems. I ran some trade studies, then investigated a couple of ballistic coefficient extrapolations to high masses, before I was satisfied enough to run some lander designs. All this stuff is posted in a bunch of "exrocketman" postings.
I don't have any way to run a lifting model. But qualitatively, it's the same as a ballistic capsule with extra forces applied. The idea of decelerating earlier higher up to limit skin temperatures is real. You could run my 1956 warhead model at unrealistically-low ballistic coefficients and see the same phenomenon. If the pulse of deceleration gees is earlier in the entry trajectory, the convective heat rates are lower in the thinner air. The heat integrals over time are about the same though.
It's the lower heating rates that reduce skin temperatures. You still have to have a sink to store all the absorbed heat, however. Details like the actual thermal gradients achievable with this or that material, are very tough, extremely-compressible, heat transfer analyses, best done with appropriate correlations and models. NASA and the big aerospace companies have the best ones. Not me. All I can do is get "into the ballpark".
Flying a winged vehicle gets you a lot more hypersonic L/D potential (on the order of 1) than a tilting-attitude capsule (on the order of 0.1). Lifting bodies are in-between. The place where lift capability seems to get you more benefit is later in the trajectory, after peak deceleration, where you can get some cross range capability the capsule cannot get you. That's what the NASA (and related) reports seem to indicate. On the other hand, a winged vehicle with a very low wing loading gets you that early deceleration, and technologically seems easier to actually build than a low-ballistic coefficient capsule (for which ablative inflatables seem to be the only potentially-feasible route).
No one has ever done it, but a steel truss frame covered by a ceramic fabric, perhaps even a straight-wing design, might be that very-low-wing-loading reentry vehicle. It would resemble how we built Piper Cubs in the 1920's, adapted to a silica-fiber ceramic fabric covering. There are lots of technological problems, and I'm not sure I know what they all are, but I haven't come up with any show-stoppers yet. I do know that the skin has to be stood-off from the steel frame by low-conductivity stand-offs that are not ablatives (or you will have to recover the airframe for every flight).
I also know the interior must be fed with a sacrificial coolant flow (most likely steam) to absorb the heat absorbed into the vehicle, and to prevent hot gas through-flow into the interior, because the fabric is porous. Somewhere around 25-50% porosity, depending upon the weave. And structural strength (as always) is the killer. There cannot be high L/D early in the entry without tearing the wings off; plus a low wing-loading vehicle is inherently more fragile than a high wing-loading vehicle (more mass is more material available to be stronger).
A small model test article, perhaps thrown out from a de-orbiting capsule, might be a good final proof-of-concept test. I find this concept very intriguing. It might lead to a very simple, very reusable, and very inexpensive design for reentry vehicles, including really nice cross-range and subsonic flight characteristics.
GW
Hi Louis:
No, I never got to hobnob with any of the astronauts. I never worked at NASA, the companies I worked for were defense contractors. I'd like to have met them. Armstrong was a very retiring, very private man. Few ever got close to him. I did get to meet Rusty Schweikart and Dumitru Prunariu at an asteroid defense meeting 4 years ago. I've corresponded some with Tom "Skywalker" Jones, but that's about all.
I've looked at that photo of the "flower" several times now. To me, it's beginning to look like some sort of broken pieces of clear crystalline substance that have been eroded to really odd shapes by the wind and dust. There's at least one other protruding outcrop of this stuff, just not as prominent. What material might be clear among all that red and brown and black stuff, I dunno. I think they ought to laser it for the spectrometer and find out what it's made of. If we've learned anything, it's that appearances are very deceiving on Mars, which really is an alien place.
GW
Hi Louis:
Beats the hell out of me what it is.
But it surely will be an interesting tale as somebody figures this out. That's where the fun really is: watching 'em squirm over something unexpected.
GW
Very interesting. Very odd-looking. From what I have seen and read, it appears to those investigating it that it is stuck into the rocks, which rules out the "plastic trash from the rover" hypothesis. It will be very interesting to see how this plays out. I've seen lots of clear crystal minerals, but never something curved.
GW
I'm not a nuclear rocket engineer, or even a plain reactor engineer. I don't have hard numbers to throw out. But here are some thoughts about reactor radiation safety, coming from an old mechanical/aeronautical engineer with some knowledge of the subject coming from wide reading.
Using solid-core nuclear rockets as propulsion in anything resembling a safe manner is not a trivial issue, to be sure. I do think the applications of orbit-orbit transport, and planetary landing, end up addressing this risk entirely differently, though.
For the orbit-orbit transport, issues of artificial gravity should interact constructively with the need to provide radiation shielding from your reactor. There is a need to stage-off emptied propellant tanks after every “burn”, but if the design comprises a set of docked modules, it is easy to reconfigure into the same length “slender baton” shape at each stage-off. Spin the “baton” end-over-end for gravity: 56 m radius at 4 rpm is 1 full gee at a tolerable spin rate. (This is true even for chemically-powered designs.)
This reconfigurable “slender baton” shape not only maintains radius for artificial gravity at low rpm, it also maintains the much longer distance that is so very necessary for getting shielding benefits out of your remaining propellant tanks. Somewhere around 40 meters of propellant tank fluids and structures should be quite effective at shielding the crew from nuclear radiation, during or between “burns”.
The lander is a vastly different proposition. Compact as it has to be for landing stability, shielding “steady state” by distance with tanks and fluids is impossible. The alternative is tons of lead or concrete, etc, also very undesirable. But since the descent and ascent “burns” are brief (minutes only), there is no need to shield “steady state”. Using what little tankage and structures shielding benefit that there is, the crew need only endure brief intense exposures, integrating to a very modest accumulated dose, actually. But this does require that the crew evacuate to a surface shelter remote from the lander during the surface stay. And you keep your distance from them in orbit, too, except when in use.
The intensity of the exposures might be mitigated slightly by a shift to thorium reactors instead of uranium. This gets the worst-offender plutonium-239 out of the picture. But, fission leaks neutrons, no matter what. I like the thorium approach better than uranium, in part because of the slightly-reduced danger from shut-down cores, but mostly because it is a more plentiful fuel. But, I recognize that we have to start with what we know: uranium.
Dangers of radiation from a contained core after engine shutdown could be mitigated greatly by the open-cycle gas core concept, which is essentially an “empty steel can” between “burns”. Only induced radioactivity is still a problem, and that is far less intense, and decays far quicker. That’s why I’m such a fan of developing the gas core technology ASAP, although we still have to start with what we know, that being HEU solid core.
GW
Hi Koeng, Happy New Year!
I dunno where everyone went. Traveling for the holidays, I guess.
Updating what I said previously: it appears Curiosity found some chlorinated organics. This is just about what one would expect with perchlorates interacting with organics in the soil under harsh UV and in near-vacuum conditions. They (NASA) are still trying to figure out where the organics and the chlorine came from "for sure", a response to the Allan Hills meteorite debacle several years ago. Once that is done and they know "for sure" this stuff is of Martian origin (and I think that is what they will eventually find), then they will finally understand what the Viking probes actually found "for sure".
My best guess is that the organics in the soil really were mostly biologic in origin, but have been modified by the action of perchlorate (and other) evaporite salts from loss of surface water bodies around 3 billion years ago. Those water bodies acidified and salted-up as they evaporated away.
GW
I wasn't really thinking about fuel depots "along the way" in the sense of mid-trajectory. I was thinking more about "topping off your tanks" from the local ice when you actually make your stop. Although, if it's a stopping place that is often frequented, a fuel depot there makes sense.
GW
The following is about long-term sustainable interplanetary travel, not what we need for the initial mission to Mars or anywhere else beyond the moon. (Although, it would be nice to have for the initial mission to Mars.)
For long-term sustainable interplanetary travel, I'd look first for the most widely-available, highly-abundant volatile to use for propellant. What we're seeing from all the planetary science probes is: that answer is "water". It may not be pure, but ice is everywhere, and solid contaminants are really easy to separate with gravity, even if it is artificial (centrifugal force).
So, if we use the water, we also find it is really easy to ship anywhere, because we can ship it frozen with minimal vapor containment to prevent sublimation. An iceberg is not lost if you take a meteor hit; a remarkable safety feature. Just patch the hole in your vapor containment, which is holding at most about 6 mbar pressure. That containment might just be a big plastic bag. We do have to learn how to patch holes in plastic bags in-vacuo and in zero-gee. But it can be done. One way is to use an aluminized thermoplastic, and just heat bond it with a hand-held iron.
OK, so water makes the most practical propellant material we have. So, now how exactly do we use it? Solar or nuclear electrolysis is a known way to produce hydrogen and oxygen, which can be both rocket propellants and fuel cell reactants. We already know we're going to do this at some level, and how to do it. Known tinkertoys in-hand. But the energy cost to make the hydrogen and oxygen, store them successfully, etc, will never be trivial, no matter how the space commerce economy finally takes shape. You'd really like to use the water directly, as water, and just use your waste heat to melt the stored ice as you go.
Guys, that's a nuclear thermal rocket. A water-variant of the old NERVA. Everybody thinks the Isp will be far lower due to the molecular weight effect, and at the same core temperatures, it is. But water as steam will transfer a whole lot more heat than hydrogen. You can run the reactor core at a higher temperature and a whopping lot more generated power. I'd bet the finished Isp isn't as low as everybody fears, once the development is done. How would you like Isp 600+ at engine T/W > 5? I'd bet real money this could be done, within about 5 years of having resurrected the original LH2 NERVA, an item that itself should take at most about 5 years, if done by the right team.
The follow-on is a gas-core nuclear thermal rocket, which takes away the core temperature limitation, and eliminates the safety concerns associated with a "live" core inside an engine shell, between burns. Operated up to a modest power level, regenerative cooling is possible. How would you like 2000+ Isp at engine T/W > 30? Beyond that, you need a place to put the waste heat. A heavy radiator is probably needed. Would you like Isp 5000+ at engine T/W around 0.1? Still high enough for an impulsive burn for interplanetary travel. I'd bet we could have one of these working within about a decade of having the water-NERVA working.
High-Isp / high thrust nuclear propulsion with plain water as the propellant. Water which you can get nearly everywhere you go. Water which is easy to store and ship as minimally-enclosed icebergs. Sounds like a series of very smart technology development programs we should already be undertaking. Too bad no one is.
Sounds also like the government monopolies on nuclear power need to be broken, so that somebody actually motivated can go do this. Political anathema to some, I know. But it must be done.
Just how sustainable and low-cost do you want to be? Especially if you use a thorium reactor. Thorium is very likely "everywhere", too. It's very plentiful here, and on the moon. The probes should be looking for it "out there", shouldn't they? Too bad they're not. Not yet, anyway.
GW
In the paper I presented at the Dallas convention two years ago, I posited a transport vehicle from Earth to Mars and back, to be based in low orbit at each end of the voyage. Some or all of the tankage has to be jettisoned, depending upon selected transfer propulsion, but the engines and habitat module are recovered and reused on subsequent missions. That helps cut down costs.
Based from orbit like that, it is possible to send a single vehicle to multiple sites on Mars in the single mission, depending upon how many landers you send, and exactly how you send them there. That increases "bang-for-the-buck". Sending a single landing vehicle to any given site presents the “standard” risks that we already well-understand from Apollo. Given sufficiently powerful propulsion, these landers can be single stage reusable, even without refueling while on the surface. Otherwise, these are two-stage one-shot chemical vehicles, unless you can refuel them on the surface. Anyone can prove that by plugging in realistic numbers into the rocket equation.
If you add refueling while on the surface, so as to make single-stage reusable chemical propulsion feasible, there are two choices: (1) carry the fuel-making equipment with you, or (2) send it down separately to the same site. If you carry it with you, there are two issues to address: (1) your lander is necessarily much bigger and heavier, and (2) the fuel-making devices must work very fast, within the time frame of the surface stay, which is limited by the men, for any of a variety of very good reasons. (Long surface stays are not very realistic for a first mission, due to all the life support uncertainties.)
Plus, you are betting lives on the fuel-making gear working correctly, at that particular site, which might be quite different from “typical” Mars. Although, that last risk can be effectively eliminated by suitable development testing, which of course takes calendar time.
If you send it (the fuel-maker) down separately, that opens a whole host of other safety issues that I have not yet seen discussed very well. The most obvious one is the capability of actually landing multiple vehicles close together at the same site, not too far out of range of each other. This takes a radar transponder and a vehicle that is steerable during entry. These are things we already have (even capsules have been steerable since Gemini), but we have never actually carried out such a homed-in landing before. That’s another issue that can be effectively eliminated by suitable development testing, which again takes calendar time.
The other issues involve the achieved range between landed vehicles. If the return vehicle is too far from the fuel-maker, how does one transport the fuel from one to the other, when there is no fuel transportation infrastructure on Mars? By truck? By pipeline? By strung hoses? That last requires a very close range indeed between landed vehicles. The other two require equipment that raises lander vehicle size considerably; if you do that, you might as well carry the ascent propellant down with you.
Landing really close together (so that strung hoses are feasible) brings into play another very serious risk: rocket blast effects. Even a chemical rocket produces a very high velocity stream whose stagnation temperature is very high. These are very destructive plumes, and the forces they impose on impacted structures are very high (in effect the same size as the thrust force produced on the vehicle). You run the risks of puncturing the propellant tanks on your fuel-maker, and/or cooking-off the propellants with the heating of the jet blast washing all over it (that plume spreads widely at low backpressures).
BTW, the supersonic expansion that reduces gas temperatures is not “permanent”: as soon as the gas flow shocks down subsonic, its temperature is very nearly stagnation, and that’s the rocket chamber temperature. That’s what happens as soon as the plume strikes anything solid. The source temperature for heat transfer across the boundary layer is the recovery temperature, which is only a little lower than stagnation.
The other risk with close-range landings is the obvious collision risk. That can be handled by a human pilot taking manual control, as it was on the moon with Apollo. But, you have to budget descent propellant to handle that contingency. You cannot trim margins “to the bone” and still do that effectively. Minimalist mission plans never address things like this. That’s one big reason why I rarely believe the claims of practicality regarding anybody’s minimalist mission design approaches.
What I proposed in my Dallas convention paper was powering single-stage reusable landers with solid-core nuclear thermal engines (basically a resurrected NERVA), and avoiding entirely the making-return-propellant issue, by simply carrying it with you in a bigger, more capable vehicle. These same nuclear landers could push the entire landing propellant supply to Mars, separately from the manned ship.
Resurrecting NERVA for this purpose might well be a faster development time than any of the in-situ propellant technologies. I think NERVA could be resurrected by the “right team” in 5 years. I’d bet any of the in-situ propellant things will take longer. Fundamentally, it all boils down to how soon do we really want to go?
That brings up a very serious c caveat: I think the US government goal of “sometime in the 2030’s” is really code for “never”. Long development times mean this landing will not happen in our lifetimes, if it is to be done by them. If we humans are going to do this any time sooner, it has to be with tinkertoys we already have or can obtain very quickly. And it needs to be done by somebody non-governmental like a Spacex. Somebody actually motivated to go, and free enough of bureaucratic chains to go.
Pessimistic, I know. Sorry, but I’m a practical realist.
GW
Hi Russell:
Go visit XCOR's website and navigate down to see the products they offer. Their baseline product has been rocket engines for others while they develop their Lynx suborbital space plane. Their engines have every bit as good an Isp as anybody else's. Engine T/W is a little less, reflecting the extra material it takes to be robust and reliable over a long lifetime. They use a heat engine-driven piston pump on some models instead of a turbopump, which is driven off the waste heat in the regenerative cooling stream. Those might be a bit heavier, but I doubt it. I don't know the weight data either way.
Not all strong wings are flexible, particularly at high loadings, like high AOA or even broadside attitudes. The last airliners to have stiff wings were the Lockheed Electra-II models now serving as USN P-3's. The only real difference is ride quality: flexible is smoother in turbulence. Given that this was an early 1950's design still serving in the 2000's, those are extraordinarily strong, tough, old birds. Stiff wings may well be required at reentry conditions. The shuttle designers thought so. So did the X-37 designers.
As for a practical space plane, you just need a powered reentry vehicle small enough to shoot up there on an existing rocket. Like X-37 and a whole lot of other ideas dating back to about 1960. They even thought about shooting the X-15 into orbit before they figured out it could not survive reentry.
The difference with the "piper cub" idea is going for very low wing loading on a real winged vehicle. The structure has to be very strong to take the high-AOA to dead-broadside air loads at very high dynamic pressure (could be a few thousand pounds/sq.ft). That's a really strong wing indeed.
For all known materials, that means the structure has to stay cool. Most steels are losing it by 800-1000 F, even the high-alloy / super alloy steels are losing it well before 2000 F. All the aluminums are "butter" by 300-400 F. Titanium is similar to steel, except lighter. Unfortunately the useful alloys are not formable, and the brittleness of castings isn't really useful in main airframe structures, generally. Especially for spars and ribs - those need real toughness, and cast materials just don't have that.
The ceramic fiber cloth covering over a truss structure might really be a way to build this low wing-loading thing. Might need some carbon-carbon nose cap and leading edge pieces like shuttle, although those are pretty heavy. It does avoid the vulnerable fragile low density ceramic tiles shuttle used in favor of something with some toughness and flexibility.
I'm not at all sure how to provide the wetted-surface heat sink medium, since these fabrics are not gas tight. Maybe a limited-porosity bleed-through of sacrificial coolant (like steam) would work. An idea like this was proposed as one of the three experimental heat protection schemes they were going to try out experimentally on the old X-20 Dyna-Soar, also vintage 1960-something.
Any ceramic skin technique like that is going to need a low-conductivity stand-off between it and the supporting airframe structures inside. Otherwise the steel heats up close to the potential-2000 F skin temperatures, and it then loses way too much strength. Go see the data in Mil Handbook 5 to see the truth of what I am saying. Those standoffs cannot be ablatives, or you will be re-covering the craft after every flight.
A low density ceramic composite with reinforcing fibers in it might be the answer to the standoff problem. The stuff I made in 1984 was almost as low a density as NASA's fragile tile, accordingly it still held a measured 14,000 F/inch gradient in a steady-state test for me. It also withstood hours and hours of accumulated burn at flame temperatures up to 4000 F, corresponding to surface temperatures right at 3200 F (I did see a small bit of surface melting here and there). The real difference with NASA's tile was that it endured the pressure violence of rich blow-out instability in the combustor it protected, all the way up to that 3200 F surface temperature. All of this was far more demanding an environment than what NASA's tiles endured.
A lot of this ceramic composite and fabric-on-truss in reentry stuff is not yet "tinkertoys in-hand", ready for general use. But it could be made so fairly quickly, based on what I accomplished with these materials experimentally so long ago. Convincing skeptical minds will be harder than doing the actual design and testing.
GW
I'd heard about IRVE, too. That will lead somewhere, but it's a long way from a usable technology. Give it time.
Reducing heating at low ballistic coefficient by decelerating higher up is a known phenomenon. To do it with a spaceplane requires a wing loading not unlike a Piper Cub. That's about 8 pounds per sq. ft, or about 40 kg/sq.m. A capsule shape would have to have a ballistic coefficient in that same range. That's an awful long way away from where we are right now in construction techniques. However, that's a place the inflatables could take us, once developed and "wrung-out".
Interestingly enough, it just might be possible to build a spaceplane with a wing loading that low, by building it the same way a Piper Cub is built: steel tube truss frame, covered by ceramic fiber (instead of linen) fabric. There are a host of problems to solve, such as low-conductivity stand-offs between the fabric and the tubing, and a gaseous atmosphere inside to be at least part of the heat sink. Yet the fabric can easily survive reusably at 2300 F ( 1260 C), and doesn't melt in one-shot use (it does embrittle, though) at 3200 F (1760 C). This is the same alumino-silicate fire curtain cloth they have long used in modern aircraft engine nacelles. I used it myself as the reinforcement in a very low-density ceramic composite liner for a ramjet, about 3 decades ago.
The nose cap, windscreen, and leading edges might need a more sophisticated treatment, who knows? But it's an intriguing idea, and most of the tinkertoys are already in place to try it. It'll take someone really innovative like an XCOR or a Spacex to try it, because it's so counterintuitive, otherwise.
Speaking of XCOR, take a good close look at the engines they offer. Some of these have the same safety and lifetime characteristics as ordinary certified aircraft engines. We're talking thousands of burns and thousands of hours of burn, here. With minimal repair/refurbishment. That's precisely the kind of engine you need for any reusable spacecraft. I think you might be surprised to learn that the best of these, already used in real aircraft, do not use turbopumps as the propellant pumps. Another really good tinkertoy available, that.
GW
Myself, I'm leery of relying on inflatables ahead or behind a vehicle during entry. That is even more true here than Mars (the speeds are higher). Such things have flown before, but only very subsonically. Back in the late 50's, Goodyear even built something for the military that it called Inflate-a-Plane. It really flew, but only under 80 mph.
Ballutes have been tried supersonically (Mach 2-ish), with mixed results. They work better with power inflation than ram air inflation. But to my knowledge no inflatable has ever been tried hypersonically, where aeroheating gets somewhere between very serious and utterly extreme.
You have to worry about both the ballute and its towline. Especially the towline is subject to extreme heating where the vehicle's "bottle shock" closes upon the towline between it and the ballute. Shock impingement heating is so extreme a regime that no one in the business considers it a survivable regime under any circumstances. That's what nearly cut the tail off the X-15 on the scramjet pod flight that hit Mach 6.7.
Further, the trailing ballute is immersed in the vehicle's wake, where it sees less dynamic pressure BY FAR than the vehicle, and it sees incandescent wake gases already heated by passage of the vehicle. There are serious questions about how much decelerating drag you can get in such a trailing ballute approach in this hypersonic regime. Those experiments have never been done.
As for hypersonic aeroheating in general, it's less about the specific materials you choose, and more about controlling the rate of heat absorbed, and the size of the sink it can go into. Example: exposed Inconel-X skins on the X-15 worked just fine at Mach 6-ish, but only because the vehicle interior was a really large cool heat sink, even after cryo propellant exhaustion. Slow the conduction rate with a low conductivity material layer, and you can use a small heat sink. That's what they did on the shuttle.
Entry heat protection is always (and has always been) a truly transient phenomena. There are no steady-state solutions, no "magic" materials, and very likely never will be, not in the lifetime of anybody alive today, for sure.
With capsule-like manned landers and heavy cargo landers at Mars (no inflatables), you will always have a high ballistic coefficient: somewhere in the several hundred to a few thousand kg/sq.m range. If you restrict the entry interface speed to that from LMO (not direct interplanetary transfer), AND you restrict the entry angle to values under about 2 degrees (again, not generally possible from direct transfer, but inherent from LMO), then there is enough drag deceleration to slow you to local Mach 3 (about 6-700 m/s) at altitudes between 5 and 10 km from the surface in the "average" Mars atmosphere.
That's way too low to deploy any chute or ballute and get it open before you hit, much less any time for it to actually slow you down any at all. But it's high enough for direct rocket braking to set you down and never exceed decelerations larger than about one-something gees, under two for sure. That's quite practical, and all it requires is supersonic retro thrust. I've posted before about that technology, and how there is very good reason to believe it will work just fine and quite easily the very first time we attempt it.
Vehicles like this with storable or mild cryogenic propellants of "decent" density are well with technological reach without developing any "new" technologies. All the tinkertoys already exist. You could probably even do it with hydrogen, although voluminous tanks would be a serious packing problem for aerodynamic flight, just as they always have been. Build these vehicles tougher for multiple flights, and refuel them on the surface, and you have your reusable Mars lander shuttle.
Although, in a reusable vehicle, inert weights are simply going to be higher, because it literally takes more material to be stronger and tougher. Example: the most reusable rocket vehicle in all of history was the X-15, each of which flew more times with less maintenance and repair than any of the space shuttles. It's inert weight fraction was right at 40%, the shuttle's was lower. It had 1 fatal crash in 199 EXPERIMENTAL flights. The shuttle killed 2 crews in about 130 flights that were supposedly "routine" after the first 4, but turned out not to be.
I think the best of these reusable capsule-shaped Mars lander shuttles refuelable on the surface of Mars might be powered by a resurrected NERVA. Isp near 900 sec at engine T/W 3.6, just as we built them last in the early 70's. Put 4 small engines canted about 10 degrees firing through simple ports in the heat shield. Seal off that engine room space to stop the throughflow, and you can leave those ports wide open all through entry. Simple as that.
What I like about the reusable Mars shuttle capsule powered by NERVA is that it has the performance to work acceptably well as a reusable shuttle, even if you do not refuel it on the surface. That kind of capability held "in reserve" on the first mission is the "suspenders-and-belt" that can get them home alive without terminating the basic mission, even if the surface refueling fails to work. We've never actually produced any in-situ propellants, you know?
What I have just outlined (to me) suggests that the very first mission ought to stage from LMO, and it ought to carry enough lander propellant to carry out some of the mission, even if surface refueling fails. They ought to use NERVA-powered reusable landers to make that happen. We really ought to use NERVA for the Earth to Mars transfer, too. That's what it was developed for. If you do, the same NERVA engines that power the landers can push lander propellant to Mars. It also has the "oomph" to go visit Phobos and maybe even Deimos from LMO.
GW
Hi Josh:
Sure, the settling could have been accidental. But such incidents must be fairly frequent and numerous to emplant a population large enough to be genetically viable. I'd hazard the guess that the initial landing was accidental, but that he went home by hand-made boat and told what he found. Then more came deliberately, because it sounded like a good place to live.
I think these folks (who seem likely to be h. erectus on Flores) could work wood and fiber as well as the stone and fire we know about for that species. I think they talked to each other. Their presence on Flores suggests boat-building and navigation skills, at one level or another.
Did anyone see the forensic reconstruction of the "hobbit's" face? This one looks more recognizably human than some others I have seen before. It was in the science-section news on NBC news website recently. The 3-foot tall stature is unsurprising. Larger species tend to shrink when isolated on an island, while the little ones get larger. We've seen it before. Flore had little elephants and little people, but giant rats. Weird place.
GW
I guess my point is that we typically leave out too much when we assign "cognitive abilities" only to folks who leave cave paintings or sophisticated stone tools.
Hunter gatherers who build boats and voyage out of sight of a solid surface have "cognitive abilities". The Eskimos today, the Polynesians about 4000 years ago, whoever made it across an ice-covered Beringia around 12,000 years ago, and whoever made it to Australia about 40,000 years ago all did this. None left cave paintings, some but not all used sophisticated stone tools. The Aboriginees left rock art, yes, but used crude stone tools.
I know all were homo sapiens, yes, but none left the "standard indicators" for "cognitive abilities". Our lens is too narrow.
And whoever made it to Flores Island used only crude tools and left no art. Yet they built boats and sailed out of sight of land to get there. You do not found populations with a survivor-or-three clinging to a log, the genetic bottleneck is too tight. Those who made it to Flores may not have been homo sapiens. There is reason to suspect homo erectus, according to what I read in the professional journals. I don't know when whoever it was reached Flores, but from what I read, h. erectus dates back a very long time indeed: 1-2 million years. At least half a million years ago he was in China.
Again, our lens for "cognitive abilities" is too narrow. Much of the "evidence" is going to be indirect for stuff older than around a quarter million years, precisely because very little in the way of artifacts survives that long.
Just my opinion, of course. But "cognitive abilities" is hard to define, harder to recognize. One should be very careful saying this group has them and that group does not, based only on cave paintings and certain kinds of stone tools. My best guess is there is no "light switch" sudden change from ape to man, other than the punctuated-equilibrium effects that we know operate in evolution, on under-million-year timescales.
GW
Louis:
I don't seem to miss it very far by computing direct launch costs from published launch prices, and then multiplying by 3-to-5 for overall program costs. Somebody efficient like a Spacex corresponds closer to the 3, somebody not so efficient like a ULA corresponds to the 5.
A typical completely-government-run program (like NASA would usually do) would be closer to factor-10-higher-than-direct-launch, and that would be for their in-house launch price, which is factor 4-or-so larger than commercial launch costs.
I've got a curve for launch costs published over at "exrocketman", dated 5-26-12, titled "Revised, Expanded Launch Cost Data". It looks to be pretty close.
GW
The only problem with all of this is that we are looking at inherently cherry-picked data, the further back we look. The actual implements for living that would give you some sense of actual abilities, are not likely to survive for us to find. Most of these are going to be wood and fiber, not even bone or antler. Not everybody will use stone, or if they do, see the need to expand and extend a working stone technology.
Who's to say that Neanderthals didn't spent more time improving a tool kit of wood and fiber, than on the stone tools that we might find? Not all arrows need stone points. Fire-hardened sharpened wood is almost as lethal, depending upon what you're killing. There's been some reports they might have clothed or cloaked themselves in feathers. Yet it appears from the trash middens they did not eat birds.
The arctic peoples of today have almost exactly the boats needed to make their way over the ice from Siberia to Alaska, exposed land bridge or not, during the ice age. We know of these wood and hide boats because they have been seen in modern times. Who's to say that people 100,000 years ago (or much further back, why restrict this?) couldn't do exactly the same thing? Because no such traces have ever been found? Why would such ephemeral traces ever be found? The lack does not disprove possession. You cannot prove a negative.
Somebody reached Australia around 40,000 years ago. Yep, they were homo sapiens. They met and interbred with Neanderthals and Denisovans on the way there, because we can find traces of that in their DNA. How many migrations, why they migrated, how they lived along the way, all that may never be known. But they did go there. That took boats. There's too much deep water for there ever to have been a land bridge.
From what I read, there's some reason to believe homo erectus reached Flores Island. But that's not certain. Whoever reached that island did it a very long time ago, and would have needed boats to do it. That's across several miles of water too deep for a land bridge to have ever existed. We're talking real boats here, and some sort of navigation abilities. Survivors rafting on a log do not found viable populations.
What I see in this is indirect evidence of cognitive abilities more or less comparable to our own, going back a very long time, perhaps a million years. Evolution seems to operate in short bursts punctuating long periods of stasis. I suggest the burst that made us "smart" happened a very long time ago, and most likely long before we ever became what we today call homo sapiens.
GW
My impression of public opinion is that there might be some support for a base of some kind on the moon, if it can be tied to going to Mars. That's an easy enough connection to make, since a lot of the same equipment and techniques work on both the moon and Mars. But, once you actually have such a base or bases, two opportunities arise.
One is to expand the experimental scope of the base or bases. There's a lot of science to be done there, and, there's the possibility of open-plume testing of nuclear engines there, which might prove cheaper and more practical than closed/captured-plume nuclear engine testing down here. When you look at the expense of building plume-capture test facilities down here, shipping engine components and thrust stand parts to the moon doesn't look quite so expensive after all, even when we are restricted to rockets like Falcon-Heavy.
The other opportunity is to add the positive-economy things like mining or fuel manufacture to that base. This is a way to boot-strap into creating that infrastructure and economy that ensures a permanent presence on the moon. You'll simply never do it tax-supported as a direct colony proposal. You have to sneak it past all the nay-sayers, by piggy-backing it later onto something that they will let you do initially. The nay-sayers outnumber us, and thus generally out-vote us.
I have never seen a consortium of private interests that might take on doing real bases anywhere in space, much less actual colonies. There is only Spacex, and Musk's resources are too finite for him to go it alone. That's why he deals with NASA, to get paid for doing things he wants to do anyway.
That being the case, any moon missions (or missions to any other places), are going to be initially government-funded exploration voyages of one kind or another. That's not a bad thing, actually. That's the most successful of the voyaging models used 300-500 years ago here.
GW
I like the idea of heat shield on one end, engines and landing legs on the other. This does presume that the vehicle (and its heat shield) are intended to fly more than one time. It is still considered (by most) too conceptually difficult to fire an engine through a hole in a heat shield, although I disagree. The problem with a hole in a heat shield is throughflow. Stop that throughflow, there is no difficulty.
The idea of heat shield on one end and engines on the other presumes you can successfully flip end-for-end in supersonic flight without risking a breakup. Here on earth, that's not really possible in a flightweight aircraft. But the air on Mars is thinner, and the dynamic pressures are lower, so maybe it's possible there. This will demand really strong attitude control thrust/mass levels, and it will demand a compact and rather dense structure, in order not to be too fragile. That's inherently higher ballistic coefficient.
Point masses connected by a truss likely will not meet that aerostructural requirement, due to the length-squared factoring effect that dimension has on bending moments and stresses. Besides, any time you connect things with a truss instead of direct contact, you just added the weight of the truss to the inert weight fraction. This is why reentry shells have tended to assume conical to short, squat conical shapes. The ones with the lowest ballistic coefficients actually approach the form factor of a frisbee flown broadside.
I tend to think that a vehicle form which might successfully swap ends in supersonic flight would be roughly spherical to a short cylinder of L/D near 1. The heat shield and pressure shell structures get to do double duty conferring the structural strength necessary to resist the broadside air loads. Plus, the lower moments of inertia make thruster control and a fast flip easier. There is also the preference for a low cg position in the vehicle as it lands, for stability on rough terrain.
All in all, you might get a lower inert weight fraction by solving the problem of rocket plume through a hole in the heat shield. The same compact shapes can be used that we already know work. All you have to do is stop the throughflow by sealing the compartment containing the engine. A static gas column is a better insulator than any ablative ever made.
But, I dunno for sure. Neither idea has been seriously pursued.
GW
If microwave is the preferred way to transmit electricity without wires, could this be done with a maser instead of a noncoherent beam antenna? That way, the collateral damage potential and the transmission losses would be a lot less, because the beam spreading is massively reduced, like with a laser. I don't know anything about maser technology, really, except that they built one right before they built the first laser, about 1960-ish.
GW
OK, the genetics guys claim that the Micronesians and Australian Aboriginies seem to have the highest Neanderthal/Denisovan DNA content of all of us. As for the rest, we all seem to have a tad of Neanderthal DNA in our genes, excepting the San in Africa, who have almost none of it. That all suggests a whole series of migrations out of Africa long before homo sapiens left Africa and interbred/out-populated all of them. We are nowhere near untangling this complicated pattern, but we are now sure there is a pattern to untangle.
The Aboriginies reached Australia "they think" around 40,000-50,000 years ago. Even at the height of the glaciations, that required ocean voyaging. Fossil beaches indicate sea level stands as low as 450 feet below current levels, and as high as 380 feet above. Hundreds, not thousands, of feet. You compare that with current bathymetric depths in the thousands, and you can see where land bridges might or might not have been.
There's a lot of migrations that required ocean crossings, out of sight of land. Such as Australia. Not paddles and a log, but real boats. We have two precedents to look at: the kayak/umiak boats of the Arctic peoples, and the Polynesian's sailing canoes. Not a "tough" technology to master, but it does require cognitive abilities comparable to our own to craft good designs, and to solve challenging navigation problems.
My point is less about who went where and when, and a whole lot more about the evident fact that these early folk behaved more like us today than we have wanted to give them credit for. That's a prejudice thing. They did these feats in spite of the fact that some of them didn't look so very much like us (example: homo erectus).
There's some evolutionary reasons to believe that we lost our body hair developing the unique sweat-cooling system we have, running long distances all over the place as homo erectus. We've found their stone tools. Who's to say what else they had, that was made of wood, feathers, grass fiber, etc? Boats? Carts? Teepees? Would anything like that survive for us to find after a million years if they had it? So, how would we know?
Because of that, the evidence for cognitive abilities is inherently indirect. What kind of cognitive abilities were required for homo erectus to spread from Africa through Asia all the way into Indonesia? Same is true for the Neanderthals, who spread into Europe and Asia, during a very harsh ice age?
And then there's us, homo sapiens, who seems to have acquired more advanced technologies quicker than the others, and who seems to have swamped and absorbed the rest simply by out-reproducing them. How is not understood. But we do know we did it, because of the traces in the genes.
Very complicated story. It does raise questions about the definition of "species", too. Were these other humans different species, or not? Apparently, we did interbreed with them.
GW