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I think Quaoar's point was that the radiation does not reliably and predictably flow in one direction only at Ganymede. The risk appears to be (1) not fundamentally well-understood yet, and (2) variable enough not to be predictable in any reliable sense. Based on postings above, I think JoshNH4H would agree with that assessment.
Thus, at this time, it would seem wiser to pick a lower-risk site like Callisto for the first manned visits in Jovian space, while using probes and remote-observations while there to better understand the riskier sites like Ganymede. Although, technologically, we are not ready to fly that far manned yet.
By the time we are technologically ready to handle manned travel to Jupiter, we may also know a lot more about about protecting ourselves from the radiation. If that proves true, what looks very risky today would look far less risky by the time the mission is deemed feasible.
It is very important to understand something about what it is that you don't know, when you attempt something challenging, like manned travel to Jupiter. It's what you don't know that is most likely to kill you. The toughest part is recognizing that there is missing knowledge, so you can go after as much of it as you can, while getting ready to fly the mission.
GW
It's not about the tools or hardware built-to-drawing. It's about what you do with that hardware.
When you build something to someone else's plans, you are inherently dealing only with what was written down. Unfortunately, in most engineering work, especially in aerospace and rocketry, it's not all written down. It's about 40% art, passed on from older to younger guys in-person, on-the-job. That's never in the plans or documents, and for the most part was never written down precisely because no one wanted to pay for that activity. But, it almost invariably is critical information for success.
Early on, Spacex ran afoul of that little fact of life. Their track record with Falcon-9 is exemplary, but they definitely had some "teething troubles" learning how to fly supersonic staged vehicles successfully with their Falcon-1. They ended up consulting some of the older guys for some of that flight vehicle art (which is based on experiencing failures and how to overcome them), before encountering solid success. So far, that has stood them in good stead. Although, generally speaking, no one in their workforce is older. That could "bite" them again.
The same is true of Pratt and Whitney trying to build a copy of the RD-180 from the Russian plans. There's about 40% art missing from the files on the RD-180, and some of that will not overlap with the art that the P&W staff does have. That's because every specific design is different, each is its own case of technology application. The risk of problems is just inherently-and-actually quite high when you build from others' plans like that, unless there are very close ties between the two engineering groups.
The only reason the US Army didn't have too much trouble experimenting with captured German V-2 rockets in the late '40's and early '50's was that they had Von Braun and about 50-60% of his engineering group via Operation Paperclip. There was enough of the art embodied in that group to "get on with it" at White Sands without reverting all the way back to the failure rates initially experienced at Peenemunde.
In a nutshell, that's why it is unrealistic to expect a quick American replacement for the RD-180 in the Atlas-V.
GW
The actions of USAF, the White House, and that court, all demonstrate exactly what I have been saying:
BA = 1 / DC where BA = bureaucratic arrogance and DC = demonstrated competence; applies to any organization, public or private
HA >>>> H where H = number of horses and HA = number of horses' asses; applies to absolutely anything
GW
Thermal balance for a suit can be quite complicated to analyze. Sometimes simple experimentation is a better way to develop the suit. But, the physics should guide it, even if done as simple experimentation.
In vacuo, there are radiation exchanges and there is conduction (if on a surface or otherwise in physical contact with something). On an airless Jovian moon, there is the dark sky to which you radiate away your heat, and there are two sources irradiating you (the sun and Jupiter). Radiation exchange is "controlled" by color, really spectral reflectance without transparency. Most space suits are white because of this.
The conduction is a problem through shoes standing, and through the suit if seated or prone, for whatever reason. Outer planet moons are invariably very cold, so this requires thick, low-density insulation, and it does not have to be inside your pressure envelope.
When there is an atmosphere, you also have convection effects. Outer planet moons being cold, this is invariably a heat loss to the environment. The "cure" is the same as for conduction: very thick, low-density insulation. On Titan because of the cloudiness/haze/whatever-it-is, you won't see much heat sink sky, or the sun as a source, and Saturn is less of a source than Jupiter is. On Titan, it's therefore mostly conduction and convection.
The problem with the insulation, is that when you need it, you gotta have it. But when you don't, you will have trouble getting rid of waste heat, and then tend to overheat inside your suit. That leads to the need for portable "airconditioners", even in cold places. These are bulky and heavy. This is an inherent problem with all gas-balloon full pressure suits. It has always been an inherent problem with that design approach for suits.
Without evidence or experience to point at, I do believe the MCP suit might help with this problem. You sweat through your vacuum-protective underwear (a porous but tight garment or garments, best done as individual pieces for this-and-that parts of the body), cooling by evaporation, inside your loose outer clothing, which is layered and provides the insulation and the reflectance. Add layers as needed, take them off when not needed. Just like here at home. You can eliminate the "portable air conditioner" that way, and just get back to a simple oxygen backpack.
GW
This is good stuff, Quaoar. I saved a copy of the 1991-vintage NASA report. It mentions the droplet core reactor as one of several concepts.
That 1991 NASA report has the results of a panel who put together the outline of a program to focus on an improved NERVA and carry along R&D on some of the better advanced concepts. Their plan would have required a dedicated plume-capture test facility, probably in Nevada at the nuclear test site, and would have brought some version of NERVA on-line for flights in about 2 decades. Could have happened faster at higher funding.
It is worth noting that both the CIA and the KGB got interested in solid core NTR for a while, about that same time. I saw photos of mockups for both agencies not many years after. None of that ever led anywhere, though. There was a fear of a Soviet particle beam weapon with a nuclear energy source under test about that time, based on some fallout that floated over northern Europe from Russia, and a very odd-looking complex at Sary Shagan. It turned out to be their 1990's-vintage nuclear rocket testing.
Myself, I think all these concepts should be pursued at "best possible speed". What with the political fears of nuclear things, and the expense and complexity of a plume-capture test facility, testing experimentally on Earth is a poor option. You cannot test an engine free-falling in space, where every test is a vehicle flight test. I recommend building a nuke engine test facility on the moon. Right now, there is no better compelling reason to go back there. None of NASA's rocket (SLS) and capsule (Orion CEV) efforts are suitable for men-to-Mars (or even an asteroid in-situ "out there"), but they are suitable for men-to-the-moon.
GW
Thanks, Quaoar, that's one of the reports I never saw decades ago. I saw the Ragsdale Mars mission stuff, and I saw the United Aircraft reports on the stuff they did. A version of that spherical engine is what was proposed, back in about 1969, for the then-planned 1980's manned Mars mission. United Aircraft was of the opinion that 35:1 LH2:U flow ratio was as good as perfect containment, at the U burnup rates they were looking at.
Another version of that same spherical engine operated at higher power, but with a waste heat radiator because they thought regenerative cooling would not be adequate. Vehicle thrust to weight was somewhere between 0.01 and 0.1 gees, but the Isp was 6000 s, not just 2500 s. United Aircraft thought the tradeoff point for radiator-or-not was about 2000-2500 s Isp reactor power levels. Their thinking was even higher engine T/W ratios than NASA, without the radiator, perhaps 30:1.
Point is, this thing could have been ready for test flights back in the early to mid 1980's, if it hadn't all been killed by 1974.
GW
I think the answer to the question that is the title of this thread (Could we colonize Ganymede?) is "yes".
The other question is, of course, "should we?". That answer is probably "not until there's something identified there that we need".
That's quite separate from "should we explore Ganymede?", for which the answer is "yes". Always, and as soon as we are ready to take on the difficulties.
GW
Surely you do not expect logic and common sense from a government contracting effort with the government's favorite giant contractors? We haven't seen any common sense in that process for decades!
GW
The igniter for the SRB was essentially a "small" rocket motor about the size of a Phoenix missile motor. I doubt they changed that for the 5 segment motor.
Slots or not at which end, and wide vs narrow slots facing which way, are questions that involve compressible flow, stress-strain in propellant, and erosive-burning effects on the basic burn rate equation. If you squeeze-down on the flow by dumping too much into the stream too far down the motor, you get a huge velocity increase and a huge pressure drop at the aft end of the grain bore.
That pressure drop deforms the propellant charge such that it tries to move downstream, too. Typically, this acts to close off the available bore area downstream, aggravating the problem as strong positive feedback. The same squeeze-down conditions cause erosive burning, too, which is a second unstable but very strong positive feedback. The inevitable result is an overpressurization explosion. It happens very suddenly, on a scale of milliseconds. The net effect is effectively "plugging the nozzle with too much massflow", which converts your solid motor to a pipe bomb.
You have to analyze your design with more than standard ballistics to prevent this. It takes a combined compressible flow, solid ballistics, erosive burning effects, and grain deformation analysis, not to mention acoustic stability analysis. That process it usually quite iterative, and employs a team of engineering specialists to carry out. Been there and done that in the tactical motor business. We usually did the acoustic stability only if our first development experimental motor "sang". The other stuff was completely routine design for us, what the lawyers call due diligence.
Our colleagues in the big motor plant had engineers who knew what these things were, but they traditionally did not analyze their motors this way, as it is "expensive in terms of labor". However, blowing up AFRPL's stand was a lot more expensive, as Hercules found to its chagrin. They're now ATK, as is Thiokol across the valley. Because of the instability in the 5 segment motor, I see no signs that proper design analysis (due diligence) is being done as ATK, anymore than it was as Hercules or Thiokol.
BTW, I'd bet they're still doing that 3-O-ring joint on the 5-segment motor, that was the supposed "fix" for the 2-O-ring joint that failed and killed Challenger and its crew. Multiple O-rings might be a good design for a clean-gas system, but when mobilized solids are involved, that's a very bad design. That joint should have one and only one O-ring. Two or more is an invitation to failure. So is putting "pooky" of any kind in the insulation joint at the segment joint. Those two bad decisions, acting together with a cold-stiffened O-ring, are what really killed Challenger. It hasn't re-occurred, largely because they've never flown sub-freezing soaked-out again. The bad joint design risk is still there.
GW
The surface is airless, so far as I know. There's a lot more radiation in the vicinity of Jupiter, too. Any settlements would have to be heavily buried or far underground. One never knows what there might be that could interest us, so I cannot comment on the desirability of going there.
But because of the cold, the vacuum, and radiation, plus being in a massive planetary gravity well, going there would be quite difficult. The water is inside, not on the surface, so far as I know.
Other than simply exploring, at this point in time I don't see that much point to going there.
GW
Their thinking was that they wanted more length for more impulse, but wanted to use the same hardware and stacking techniques. One extra segment got them the impulse they wanted. Nobody thought about anything more than that.
As far as I know, or can tell, no one ever ran an acoustic stability analysis on it. There are programs available for that. We used them 20-30 years ago on the tactical motors. Stability was a real problem with reduced smoke propellants; usually the aluminized propellants gave us very little trouble. Adding aluminum cured the "singing" at the cost of smoke, quite often. SRB-type propellant is aluminized. Their problem is so bad that the aluminum doesn't help. But, it's still fundamentally unaddressed.
I'm not sure that a change in the length of one segment would help. I'm also not sure that it wouldn't. But, I'm also not sure that the first longitudinal mode is the source of the "singing" in the 5-segment SRB. Quite often in small motors, it was the first tangential mode. Sometimes a radial mode would offend. Neither of those responds to length changes.
Slots in the propellant can help with those other modes, sometimes. Just do not slot a forward segment. Always slot the aftmost. There's a whole sad story about that. No tactical motor guy would ever design forward slots, we all knew better from our design analyses and the testing and development experiences that confirmed them.
But the old big-motor Hercules guys did, and that's how they blew up AFRPL's biggest thrust stand. That was the billion-dollar write-off I wrote about somewhere just above.
GW
"HA >>> H"
Awww, ya just don't understand!
Outfits like Boeing and Lock-Mart view government contracts, especially R&D contracts as nothing but a very lucrative gravy train, and in the case of R&D, nothing ever has to really fly.
Uncle Sam's pig has a lot of juicy tits they've been sucking on, for a lot of decades. With all the consolidation, down to nothing but ULA, no one else is ever allowed a tit to suck on. That's because they bought the pig, and all the farmers and county agents overseeing it.
THAT'S why outfits like Spacex, Orbital, XCOR, and all the rest, have such a difficult time breaking into the business. It's very seriously rigged against them.
But, that being said, I am really cheering-on Spacex with its manned Dragon, and Sierra Nevada with its Dreamchaser. And I'm hoping that more enter the launch-men-to-LEO field soon.
As for large launch vehicles, Falcon-Heavy is the only thing I see on the horizon. SLS will never be anything but a single-purpose and very expensive government toy. If it ever actually flies at all.
SLS uses that same 5-segment SRB as its strap-ons, the same design I criticized roundly just above. If you thought a shuttle explosion was bad, wait till they trigger off an entire SLS with another SRB joint failure! Or that unaddressed 5-segment combustion instability.
GW
From what I see and hear, Spacex wants to use LOX-LCH4 in its new big engine. I think that's still just a rough paper design right now. They're too busy trying to ramp-up production to meet demand to work very hard on it. And their next big new thing is Falcon-Heavy. After that, the big engine gets prime-time attention.
Yet LOX-LCH4 does exist as a rocket engine right now, at least as an experimental R&D article. XCOR has been testing it. Yep, it can be made to work, and quite well, too. The real problem with anything new like that will be the other folks not doing it, especially the larger ones: they will raise questions about reliability until hell freezes over, because they aren't trying to make money on it themselves. Anything to defuse competition.
That same effect is at the heart of all the man-rating arguments over the Atlas-5 and Falcon-9, and for manned Dragon and Dreamchaser. Did you notice that no one seems to worried about man-rating Boeing's CST-100? Care to take a guess as to why?
GW
Long ago I worked for Hercules Aerospace before ATK bought them up, like it did our competitor Thiokol. I worked at a tactical motor plant, where propellant weights were usually 100's to a 1000 pounds. Our sister plant in Utah built the big motors comparable to the SRB's. Thiokol did build the SRB's, just across the valley.
We tactical guys had to do the full design analysis suite, and the full development testing suite, to assure that we would produce motors that burned stably and smoothly, and could withstand all the shake, rattle, and roll, and abuse testing, typical of military battle equipment. 1-in-a-million failure rates, in production lots of thousands to hundreds of thousands of motors.
Our colleagues at the big motor plant in Utah, and their competitors across the valley in Utah (and elsewhere), didn't do all that. It was "customary" in the big motor business to design things the same way they designed the Titanic: skip all the design analysis and development testing. Just scale up something you did before, and hope it works OK. They built 1's and 2's, to 10's and 20's, in their production lots.
That lazy big-motor process is exactly why Thiokol killed the Challenger crew with a bad joint design approved by an ignorant NASA (who remained blissfully ignorant of good solid motor joint design to the end of the shuttle program, even down to today), it is why Hercules had to take a billion-dollar write-off for blowing up an AFRPL test stand with an SRB-class strap-on motor, and it is why the 5-segment ATK SRB motor "sings" unstably at a low frequency, with an amplitude that could destroy the vehicle, or kill the crew even if the vehicle survives.
If the engineering were done with the due diligence typical of the tactical business, the big solids would have been way more reliable from the beginning, and all these issues would not have cropped up. But you don't make enormous windfalls off your business if you do your job correctly like that. And THAT was, is, and will always be, the real problem in the "big solid rocket business".
GW
Spacex Dragon was designed from the start to carry men. The Super Dracos are the launch escape system. They've been testing Super Dracos at McGregor, but to my knowledge, have never installed them in a Dragon. The idea was to get paid by NASA to do all that. If they got paid this year, I'd bet they could fly manned this year. It's NASA's penny-pinching schedule that drags all this out to 2017.
Boeing's CST-100 could probably also fly manned shortly afterward. It hadn't yet flown at all, while Dragon has., so I'd guess it'll take a bit longer, but not much. Again, the hangup is the same: waiting to get paid by NASA to do it.
The Dreamchaser spaceplane has flown a drop test or two, but nothing more so far. Spaceplanes are simply harder to do than capsules, so my guess is that, if money were no object, Dreamchaser might fly manned not this year, but sometime next year, a little behind Boeing.
Besides the vehicle testing, there is man-rating the launchers. Dragon rides Falcon-9, not man-rated. Both CST-100 and Dreamchaser ride Atlas-V, which is not man-rated. It is the bureaucracy of NASA man-rating that slows this down. In a pinch, that can be bypassed, as it was with Glenn's ride on the Mercury Atlas in 1962. But they will resist-to-the-end bypassing it.
The real problem with Atlas-V is the engines: they're Russian. Guess what gets embargoed next. They can be replaced with domestic engines, but that takes time.
GW
Not all solids have bad thrust oscillations, just that one did. The SRB as a 4-segment design did not oscillate (although there were some other mistakes in it).
When they went to the longer 5-segment design, they brought one of the many possible oscillation modes into resonance with some of the basic combustion noise and flow separation zones inside the motor. They never tried to identify and fix this, they just tried adding ballast as a dampener, which was nothing but a band aid.
That's just being too cheap, plain and simple. Not to mention risking other people's lives had it ever flown manned. I personally consider those things unethical, but that's just me.
GW
OK, back to water as a chemical propellant source. Here's a really oddball notion: why keep the water as liquid in tanks? For that portion of the mission's propellant that you are storing longer-term, just freeze it into a shaped iceberg, and keep it sheeted in metallized reflective plastic balloons for the 6 mbar vapor pressure to prevent sublimation at 0 C. That portion of the propellant source is thus modules at 99+% propellant mass fraction.
When the time comes, undock your iceberg, move it inside your processing tank (you only need the one), melt it, and electrolyze it. Liquify the gases. Use them as we have been discussing here. This is a great fit with a modular orbit-to-orbit transport design. It reduces inert weight, which greatly reduces total weight required of the design. It'll be a big effect, due to the logarithmic nature of the rocket equation.
So, your ship has a big hab module or modules, a big storage module or modules for life support supplies, some water tanks, a whole train of bagged icebergs, an engine module with both LH2-LOX and an oxygen ion thruster, an iceberg melt module, a gas liquification module, an electrolysis module, and a nice big solar array module or two. Arrange it in baton shape for spin to artificial gravity, and park some of the bagged icebergs around the hab for radiation shielding. You can carry landers, or send them separately. It'll just be a skinnier baton coming home, unless you find water you can mine and refine "out there". Impulsive chemical burns for departures and arrivals, low electric thrust to shorten the interplanetary transits.
I might even locate the electric thruster at the spin center, 90 degrees to the spin plane. I think I'll keep the LOX-LH2 engines at one end, for structural reasons, firing through the long axis of the stack when it is not spinning. Spin-up can be by big thrusters at the ends. Maybe that's an application of the lithium-aluminum stuff being addressed in the other thread. The fuel is easily storable for the long term. The H2O2 can be made "at need" as you go in the electrolysis module or in its own "peroxide plant module", since its storage lifetime is short. It was hypergolic, the very thing you need for thrusters, just more efficient than MMH-NTO. But you can always use MMH-NTO for thrusters.
Landers being larger in diameter than payload shrouds is not a problem. I posted about an excellent and easy LEO assembly facility for things like that on "exrocketman". It's got some MCP suit stuff in it as well, which we would need for the assembly facility and for any interplanetary mission. In point of fact, the orbit-to-orbit transport ought to have a similar facility as part of its design, for purposes of self-rescue and self-repair.
The idea is still forming, not gelled yet. Comments? Suggestions?
GW
Why not do it as solar-powered electrolysis between burns? And, "hybridize" by doing LOX-LH2 liquid rockets at 6:1, keeping the leftover O2 for both an oxygen electric thruster and life support makeup O2. Two kinds of engines: chemical and electric, one ultimate propellant source (water).
Do your departure and arrivals burns as impulsive (short-duration) LOX-LH2, to avoid the gravity losses of a long-extended electric "burn". That way you do not waste months spiraling out and in on straight electric propulsion.
In between, run the electric thruster continuosly over the long transit time to shorten it, at high Isp. LOX is far easier to keep in cryo storage than LH2.
The only downside I see to this would be the electric power. You'll need a much larger solar-electric rig to handle both the electrolysis rig and the electric thruster at the same time during the long transit.
I'm not sure about 40-day trips to Mars, but 60-90 days would be a lot better than 6-9 months.
Nicest thing is every bit of this is essentially off-the-shelf technology.
GW
With the consolidation of the US aerospace industry in recent decades, there were (until recently) only the giant conglomerates Boeing and Lockheed-Martin with any history of building launch vehicles. Them and the odd engine manufacturer not yet gobbled up. Together, they operate United Launch Alliance (ULA) as a government-mandated monopoly. The government's rules specifically make it extremely difficult for anybody else to compete. ULA lobby money made sure of that. Very typical of monopolies, and for over a century now.
Now there is Spacex, who found a slightly-easier back door through NASA and exploited it, with the aid of its founder's $billions. Without that money, Spacex could not have succeeded in a game rigged against it. Now it is time for them to participate in the military launch business too. Their rocket is at least as good as Atlas-V, and their price is getting ever lower, little by little.
I'm both sad and glad to hear of the suit to break the ULA monopoly. Sad because it was necessary to go to courts and give all that money to the lawyers instead of actually doing something with it. Glad because that monopoly needs breaking, or launch costs will never go down. One way or another, we-the-people have to pay those costs.
GW
If you insist on doing this in Antarctica, then the dry valleys are the best analog to Mars, and the weather conditions generally lack the extreme winds. It's plenty cold in the Antarctic winter to test heating. But, testing suits and other equipment against cold is just as well done in test chambers. We've done that since the first flights into space. There's a facility in Houston where they tested Apollo suits against hot/cold in vacuo.
As far as deserts go, you want a cold one, but not so isolated. Antarctica is difficult at best, logistically. The Atacama in Chile is cold enough in wintertime to provide a good field test. Outer Mongolia is another suitable place in winter. The Canadian archipelago is another place, although not so dry as a real desert. I don't know where all the suitable test sites are, but even China Lake in winter is cold enough.
GW
Why on Earth would we use a rocket to send a simulated Mars hab to Antarctica, when ships and/or aircraft can do that very same job at orders and orders of magnitude less money? And, why risk Antarctica, when almost any desert would do?
That being said, there are severe restrictions on when you can go to certain locations in Antarctica, no matter how you get there. Worst case is near South Pole Station on the Antarctic Plateau.
Darkness prevails 6 of 12 months. Winds can be at or above 100 knots quite frequently, with turbulence intensities near 30% (30 knot amplitude). And for pretty near 6 months out of 12, ambient temperatures are below -60 F, to as low as -110 F. Up in the atmosphere above the south pole, temperatures have been observed as low as -130 F, since the late 1940's. The ice is riddled with hidden crevasses, even when it has been graded smooth. Ungraded, it is fatally-rough for aircraft of any type.
Modern aircraft that use kerosene fuel with hydraulic-operated flight controls simply cannot cope with cold conditions like that, even ignoring the roughness and the crevasses, and the obscuring darkness. Both the fuel and the hydraulic fluid freeze solid in cold like that. 60 years ago, gasoline-fueled aircraft (specifically the DC-3) with mechanical flight controls could, and did, cope with it, but with extreme difficulty for the aircrews. High-power electrically-heated suits were required to survive. That's where the weather observations came from.
A better idea would be to update the 1945 design for JATO bottles. That would have a wide variety of applications in aviation generally.
GW
Maybe I was wrong about high-altitude performance of airships. Although, I doubt the ones that flew high carried much payload.
I'm not wrong about vulnerability to wind turbulence. There were 3 big USN airships just prior to WW2. All were about the same size as the Hindenberg (built by the same outfit, actually). All three were destroyed by the wind turbulence of storms, with many casualties. The wind turbulence around high mountains is much worse by far, being able to down or break-up modern fixed-wing aircraft. There's very good and lethally-important reasons jet airliners stay away from mountain peaks.
Speed: since the 1930's dirigibles and blimps have usually sized-out at cruise speeds in the 60 to 80 mph range, with max speed just about 80-90 mph. That's still true of today's blimps. That's not to say one of these couldn't be pushed faster, but you sacrifice payload for the bigger engine. With any kind of airship, weight is THE issue. These vehicles are typically substantially more fragile than fixed-wing aircraft or helicopters, just to be able to fly at all. I'd bet the high-altitude blimps are very slow and underpowered, but I'm not familiar with them, so I don't really know.
The needs of a mountain rescue craft are (1) substantial payload, (2) vertical landing/takeoff in very restricted and hostile spaces on extremely-rough ground, (3) structural robustness to resist very violent winds and extreme turbulence, and (4) speed enough to get there in time to do any good.
I submit that an airship's characteristics are not a good match with that list. A VTOL aircraft or helicopter is a far better match, but even these will have extreme troubles with rough ground and violent winds. Some sort of rocket might work, but will always be critically-short on payload because it has to carry so much propellant.
Such a craft will be a one-use design adapted to this kind of service and unsuitable for just about anything else. My guess is it would be a tilt-wing VTOL, somewhat along the lines of a really-beefed-up V-22, or a range-improved version of the much earlier XC-142. But, neither of those can/could actually touch down on the kind of rough ground typical of a high mountain, nor can/could either successfully hover in the very-common extreme winds and turbulence of that environment.
GW
F = m*Vex + (P-Pback)*Aex where m is the rocket massflow rate. Pback is the pressure surrounding the exit area. As this changes, thrust is directly affected.
Pback is now a rising function as you add both mass and heat to the cavern volume, when you fire your engine inside a sealed cavern. How many tons do you add in a 1 minute burn, at around 2000 K for a solid core NTR of several tons thrust? How big is your cavern?
m = (dP/dt)*V/R*T where T is the mass fraction mixed temperature, actually a transient here as well, but assumed constant in the formula given. dP/dt refers to the time dependence of Pback.
There's a 1st order model. V will have to be truly enormous (enormous = huge facility expense) for there to be a negligible change in Pback. Otherwise, you must correct test thrust over time for variable backpressure effects, a result one further step removed from what you are trying to measure with high confidence.
Once the backpressure rises above about 30% higher than expanded exit plane pressure, the nozzle flow separates "for sure", and all bets are completely off about what your thrust data mean. That 30% figure is a only rough-and-ready approximation. There is no real reliability to predicting nozzle flow separation.
Compare that to simply firing out in the open vacuum every single time on the moon, and not having to do anything to clean up the radioactive residues afterward, since Vex exceeds lunar escape.
GW
The only problem with an extreme gee gradient is fainting from reduced blood flow to the head, when sitting up or standing. Putting on a gee suit to counter this screws up the deal for experimenting with artificial gravity.
Large-radius artificial gee avoids this, as the gradient is essentially negligible. For 56 m at 4 rpm, I had a rather low number for the gradient, one not much different from sitting vs standing, in terms of cranial blood pressure change. And 4 rpm should be tolerable even to untrained civilians.
You get a much better experiment with a big rig, even if it has to be cable-connected, which is a real bitch to start and stop spinning. Sounds like something we should be building in LEO. Maybe something we should have built instead of the ISS that we did build.
Hindsight is always 20-20, ain't it?
GW
I think I already described Putin as wanting to rebuild the old Soviet Union/superpower, somewhere above. He is able to do this only because he has lots of supporters in this at home in Russia. The closest analog is Hitler's Germany in the late 1930's. Hitler had popular support, then, too. The parallel is eerie and disquieting.
I doubt NATO will stop this in the Ukraine or Moldavia, they're not of strategic interest. Without being stopped, Putin will only continue pushing. So, for now I predict it's appease-the-dictator and lose countries to him, until he grabs something you cannot tolerate losing.
Lithuania, Estonia, and Latvia are all part of NATO now. Estonia is where a major Soviet naval base was, and all that facility is still there. That'll likely be where the shooting war starts. Or maybe Poland. But, it's coming, sure as death and taxes. We've seen this movie before, many times.
GW