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The ISS is well worth it. I just wish they'd gone ahead with the medical centrifuge module. We could have gotten from it an answer to "how much gee is enough?" for the Mars trip.
GW
Bob Zubrin is a really smart man, and that plan (and its alternatives list) is a good one. Some of us, myself included, differ with some of the details. It is this debate which refines the mission and vehicle designs such that the astronauts (who are really test pilots at this stage of the game) are not killed.
I have concerns about months spent in a tight volume re: sanity, and that question also relates to the centrifugal artificial gravity Zubrin calls for. His proposal is two cable-connected modules.
I don't believe the bed-rest studies insofar as what level of gee is therapeutic. I do believe that they successfully show ill health effects if you don't get up and move around, even here at one gee on Earth. So how do you get up and move around inside a capsule that small? Your spin gravity won't be effective medically if you cannot get up and move around. That's a very important thing to remember about artificial gravity.
Sanity effects: A Dragon with two in it might resemble the confinement in a typical small Tokyo apartment. But, typical Tokyo apartment dwellers spend most of their day outside that apartment. These astronauts cannot do that. Unless, as Zubrin suggests, there is an additional inflatable module docked to the capsule. Where he and I differ is that I think it needs to be larger than the minimal thing you can hide in the unpressurized cargo space of a Dragon. One or two of Bigelow's modules might serve, though.
I don't much care for cable-connected modules as the way to spin for artificial gravity. This is because you have to stop the spin, untie things, and re-dock to a hard vehicle configuration for each and every mid-course or trajectory trim maneuver (and you WILL have them). That's too much trouble to carry out, and way too many opportunities for failure. Why not assemble your vehicle as one long string of docked modules, and just spin it head-over-heels as a rigid object? No EVA required to stop the spin and maneuver, just thruster firings. Much safer, not to mention easier.
Using ISRU for the return fuel means you need a system proven to work in-situ on Mars. It will need to be checked out on one of the unmanned probes before you send the men. But if you rely on small two-or-three docked-item assemblies from Falcon-Heavy launches, then ISRU return fuel is required. I see no proposals to get that checkout job done.
If it's more than two docked items launched by the same rocket, you'll need to do the docking in LEO before you depart, for better control and safety reasons. The volume and spin gravity issues may end up requiring orbital assembly instead of direct flight, precisely because 3-5 items may be required, even for a 2 or 3 man mission.
Zubrin is just about right regarding radiation. I pretty much agree. I'd prefer 20 cm of water to protect against X-class solar flare events, which is a bit more than he suggests.
Zubrin is dead-nuts-on right about making the attempt: you can't go to Mars unless you actually go. Risk is inevitably involved. But, remember, there's nothing more expensive than a dead crew. Those two things have to be balanced, quite carefully. Bad mission design and vehicle engineering has killed people for centuries. Space is no different, and a lot harsher and more hostile to life.
Let's just say I view most of the direct-flight scenarios with a jaundiced eye, for exactly the reasons listed above. Plus, the more minimalist you make your mission, the more likely it will fail. That's the history.
GW
I'd rather just solve the straightforward lander problem, than to try to develop ways and means of crash-landing stuff. Not everything is "hard" enough to crash and survive. We certainly are not. Most of our equipment isn't, either.
It's maybe low-level retro thrust during entry to raise the altitude at which you go "merely" supersonic, and then high-level retro thrust from there to touchdown. Maybe a supersonic chute or ballute might help reduce fuel burn a tad, maybe not. I'm not at all "scared" of making hypersonic retro thrust a good working tool. The only real issues are retro plume aerodynamic stability and holes in the heat shield through which to thrust. Both have solutions, just not yet tried.
GW
Titan:
Low gravity, surface "air" pressure about 1.5 x Earth, extremely cold (cryogenic, in fact) so the "air" densities are much larger there than here, low speeds of sound making high-speed flight difficult (supersonic = expensive). You don't need pressure domes, but you do need gas-tight, well-insulated buildings in which to live and work. EVA "suit" need only be an oxygen mask or helmet, and "super-duty" cold clothing. Probably actively-heated.
As for resources, there is water available as the local rocks and sand, and there are lakes of liquid methane. Both are slightly "polluted" by other compounds mixed in. The "air" itself is 97.7% nitrogen and 2.3% methane. Water, hydrocarbons, and nitrogen. We could do a lot of things with those materials in some sort of chemical plant. Especially with much of oit already in solid or liquid form.
The atmosphere is awfully thick geometrically and density-wise, since the aerobraking densities lie about 900 km up, but the gravity is low. It should not be that difficult a place to land on, or to take off from.
Could well be a future destination and eventual base or colony location. With solid/liquid resources like that, do we really want to terraform it?
GW
As for the airliner problem, you still have to make the decision on how fast you need to fly.
There's well-proven subsonic cruise. It will always be cheaper than anything supersonic. I think there will always be a market for that. Renewable replacements for petroleum jet fuel will make a lot of sense in that application, especially after we finish sucking all the petroleum and natural gas that we can reach out of the ground. Hydrogen has a storage volume problem, very serious in practical aircraft design. Renewables like biodiesel will have a production capacity limitation, although it is not today's limitations.
We're pretty sure based on the Concorde and a whole slew of fighters and bombers that we don't want to do supersonic cruise. Hypersonic would be even worse. Way too expensive, way too hard to do in a mass-producible commercial aircraft design. Although, there's a lot of supersonic transport fans that have always ignored those inconvenient realities. (Scramjet has missile applications, but I just don't see the utility of it for space launch or air transport. The characteristics are just too poor a match to the mission.)
That leaves the "relatively un-proven" modes of supersonic exoatmospheric skip glide, and supersonic/hypersonic exoatmospheric ballistic trajectories. The thruster-equipped SR-71's already proved skip-glide works, they're just not widely known. The real question is only how much savings the range extension confers to offset the super-high costs of supersonic flight, in a transport size. ICBM's long ago proved the practicality of ballistic trajectories, it's just that no one considered replacing the rockets and warheads with rockets and airplanes. That one could be "proven feasible" by something like the X-37, if they were to fly it that way. And they could.
We don't have a clear path to a nuclear air-breathing engine. We do have a clear path to a nuclear rocket.
I'd go for chemical propulsion in the skip-glider, very little different from what we did with the SR-71. You could build a real air turboramjet (100% max bypass, off the supersonic inlet ahead of the compressor face), or you could just put two kinds of engines on the plane: turbojets and ramjets. Either would work just fine. We're talking exit Mach in the 3-4 range. About the same as SR-71 and B-70.
For the ballistic bird, we're faced with staging if we go chemical. Typical aircraft are roughly half structure, and half payload plus fuel, anything less structure is too fragile, we've learned that over the last century of flight experiences. The most "reusable" way to stage chemical is the carrier airplane. But I don't see that as ever being inexpensive. Too many people, too much equipment. Dropping rocket stages is even worse. So, there's your commercial application for the nuclear rocket, if and only if you have the chutzpah to fly it in the atmosphere! One or the other of those old NTR designs has the Isp and T/W to support a single stage airplane leaving the atmosphere at about M6-10 and 45 degrees, coming back down 6,000-12,000 miles away.
Voila! Nuclear airliner.
GW
Biodiesel antifreeze:
I used ethyl tertiary butyl ether (ETBE) for that. Discovered it completely by accident. We were flight testing 20-30% biodiesel in Jet-A in a PT-6-equipped Beech KingAir. I was doing cold fuel screening tests on the ground, and put 0.5% by volume ETBE into a B-30 fuel blend. Jet-A by itself starts depositing waxes about -58 F. B-30 typically did that around -20F.
The B-30 with 0.5% ETBE went to -68 F with no sign of deposits at all. All the biodiesel-jet fuel blends showed a really ugly color change at about 28 or 29 F, but no deposits at all until somewhere under -10F. The more jet in the blend, the lower they would go, but all showed deposition earlier than plain jet fuel, except the ETBE stuff, which went way colder than even plain jet fuel.
I never looked at using ETBE as a trace additive in plain biodiesel in diesel engines. But someone should. You don't need a fancy expensive freezer to do the work, unless you plan on running the real ASTM tests. For "jake-leg" screening tests, I just used 4 gallons of ethanol in a 5 gallon bucket, plus roughly 15 pounds of dry ice. Set the bucket on styrofoam, wrap it with an army blanket or two, and in less than 15 minutes you have a cold bath at -110 F. You'll need a thermocouple thermometer, stirring sticks, and some clear glass beakers. Safety warning: if you drop a tool in the bucket, go in after it bare-handed, DO NOT WEAR GLOVES! You have 7-10 seconds immersion time before frostbite begins. The gloves will freeze to your hands, you cannot get them off in time to prevent frostbite from the cold gloves.
GW
Rune:
Your better search links paid off. I now have fairly good models for the atmospheres of Earth, Mars, and Titan, extending to altitudes over 100 km in all cases (about 900 km at Titan). These atmosphere models are now posted with plots and tables over at "exrocketman". That would be http://exrocketman.blogspot.com, in an article dated 6-30-12. There's a lot of text, 3 tables, and 18 figures over there.
These plus some data regarding entry conditions in one of those reports will enable me to run crude design sizings on a chemical one-way Mars lander, a chemical two-way Mars lander, and a nuclear two-way reusable Mars lander. I plan on doing these as "universal" designs, suitable for Mars, Titan, Mercury, our moon, and the airless moons of Jupiter and Saturn. Sort of a one-design-fits-all approach. This will take a while for me to do. Stay tuned .......
GW
Transition to ramjet? Depends.
For the SR-71, they would be bypassing a bunch of air and afterburning it, from about M2.5 on up for high thrust. At high fuel consumption, of course.
Thinking fixed geometry like the good missile guy that I was, there are two basic types of simple ramjets, a lower speed-range design, and a higher speed-range design.
The lower speed range is transonic or high subsonic to about Mach 2, or at most 2.5. These were the old "stovepipe" designs with the pitot or normal-shock inlets, a baffle flameholder, and a convergent only nozzle. Peak performance is about M1.5-ish. Min speed is well subsonic, limited by fuel consumption more than thrust. Looking slower, Isp starts looking worse than early-technology solid rocket somewhere around 300 mph (near 0.5 M). These were most successful if boosted to about Mach 1 before taking over on ramjet. If I were to do one of these today, I would do it as a dump combustor, not a baffle flameholder, especially if it had to fly at various speeds. Max speed is mainly limited by vehicle drag and low normal-shocked inlet recovery.
The high speed designs feature external-compression devices for inlets, such as the spikes on the SR-71. These can be axisymmetric, axisym-sectors, or 2-D ramps. There's all-external shock compression, or mixed external-internal compression. (All-internal really doesn't work in any practical sense.) Most of these today are dump combustors, although the early ones, like the RJ-43's on the old Bomarc, were baffle flameholders. The inlet design with its attached shock system inherently restricts you to Mach > 1.5 just for takeover, but internal engine pressures are correspondingly high enough for an always-choked exit, so these have very mild expansion bells. Throats are still very large, around 65% of the engine cross section. This kind of thing can be (and has been) flown to speeds as high as M6. Vehicle drag is the key limiting issue, followed closely by ionization instead of temperature rise in the combustor at speeds above M5-ish. (Recombination is not converted to velocity increase in a nozzle.) M5 is pretty easy to reach. M4 is a pretty common design cruise for missiles propelled this way.
GW
Yeah, Kelly Johnson let it slip at his retirement celebration from the old Lockheed "Skunk Works" that an attitude thruster-equipped SR-71 reached a one-million-foot altitude. That's 189 statute miles, 165 nautical miles, or 305 km.
He probably shouldn't have let that slip out, but he did. Not long after, we started hearing stuff about Mach 3 skip-gliding across the Pacific essentially unrefueled. You hear all kinds of things, now that the plane has been retired. Most of them aren't true, really, but the trend of these things is startling. That's why I do believe the skip-glide trans-Pacific thing.
But no faster than about 3.5 Mach, really, the J-58's are not true air turboramjets. About M3.8 on a very short transient is all the turbomachinery can stand, and even that shortens its life. They really have pushed it that fast. The air bypass to the afterburner is max 25%, and it comes off compressor stage 3 or 4, not the supersonic inlet. That's public info now. Long ago, that was classified.
Essentially what they did was add the same attitude thrusters to it that the X-15 had. Both were designed about the same time, and had similar strake-equipped fuselage shapes, just different wings and engines, aerodynamically. Like the X-15, slower exit Mach at near-vertical path angle leads to very high apogee and very short range, while higher exit Mach at lower path angle leads to a much longer range to re-entry. It's an energy-management thing before you leave the air.
GW
An alternative fuel for existing subsonic aircraft could be biodiesel-jet fuel blend. That's a drop-in fuel. I even know a good antifreeze agent for cold soak at high altitudes. I've been involved with flight tests of 20 and 30% biodiesel blends back about 1999. I think stiffer blends could be flown, if the higher viscosity and stickier surface tension characteristic don't screw up the metering and spray patterns. Biodiesel in existing turbines needs a thinner, and an anti-freeze agent.
For piston diesel, biodiesel a drop-in fuel, period. Viscosity resembles no. 2 diesel as-is. Just needs the antifreeze agent.
As I pointed out earlier, why screw around with supersonic airliners, or hypersonic airliners? It's easier to build a skip-glider, or even an antipodal rocket spaceplane. SR-71 equipped with attitude thrusters was successfully operated exoatmospheric, as a skip glider. Decades ago. Improved the range, it did, over straight supersonic cruise.
That choice would be true regardless of the propulsion scheme you choose to make it happen. Nuke rocket might be pretty good for an antipodal spaceplane transport. That's what Heinlein put in his science fiction about 1945-ish. Based on what we know about NERVA and the others, we really could do it from a technical standpoint.
GW
Hi Rune:
Two things. (1) you found better reports than I did in my simple google search. Thanks. I printed those out. I think I can revise my Mars atmosphere model to be good enough to serve, at least for rough feasibility calculations. Based on a quick read-through of your links, it appears the hypersonic deceleration at Mars is inadequate (you end up at M3 too low), and the available aero-decelerator supersonics are way short of adequate, for anything of a size and ballistic coefficient that we might really be interested in. I'm thinking retro thrust all the way down really is the answer. Low thrust hypersonics, and high thrust supersonic to touchdown. Aero-decelerators may or may not be feasible at all.
(2) this url you are talking about, could that be some sort of tag for the images I have been posting at exrocketman? If I could figure out what those are, could I use that link to make an illustration there appear here?
GW
I don't know about dilution gases, because there's not a lot of low total-pressure experience with them.
But, in a pure O2 atmosphere, there is a partial-pressure offset you have to take into account when deciding what level of pure O2 pressure is adequate, and for whom. The "biggie" is water vapor inside the wet lung passages: a constant 47 mmHg no matter the total pressure. The small one is CO2 in the exhalation, but it's negligible to first order.
I did these calculations a while back and posted them over at "exrocketman". Midoshi helped me figure out how to do this over a year ago, before the great crash. The stuff is already plotted. Pure O2 with the water vapor offsets included. The bar graph in the figure makes it perfectly clear how to do this.
For most of us flatlanders, 20-25% of an atmosphere of pure dry O2 supply inside the helmet is adequate. At 20%, it's no worse than flying up to 10,000 feet here without oxygen. 25% matches sea level. Somebody already accustomed to life in the open at around 15,000 feet in the mountains would do just fine on 15% of an atmosphere pure O2. Many of us could become accustomed to it, yes, but it takes time, and you have to handle the inevitable cases of "mountain sickness". They can never tolerate a p-total that low.
NASA's 1/3-atm O2 in the suits is needlessly high. But, decompressing from an oxygen-nitrogen atmosphere near 1 bar to 1/3 atm pure O2 is easier than to a lower pure-O2 P-setting. That's why they continue to require 1/3 atm or the compression equivalent. It would be just as easy to use a lower P-total a bit richer in O2, and thereby ease the decompression required going to 15-25% atm in a pure O2 suit. But they haven't done that, due to the inertia of tradition.
To find these calculations and graphs, go to http://exrocketman.blogspot.com, and use the navigation-by-date-and-title tool underneath my photo/profile. You are looking for the article dated 1-21-2011, titled "Fundamental Design Criteria for Alternative Space Suit Approaches". I was looking at mechanical counterpressure suits when I did this.
GW
Low-pressure pure O2 atmospheres were what we used in Mercury, Gemini, and Apollo, plus Skylab. It worked fine; you just have to be careful to use nonflammable materials. Ditch the petroleum plastics, for example.
NASA's standard was right at 1/3 atm (253 mmHg, 338 mbar) and still is, in space suits. However, you don't need that much. Around 20-25% of an atm (152-190 mmHg, 203-253 mbar) is just fine, as Paul Webb demonstrated with his elastic spacesuit mechanical-counterpressure garment back in 1969.
They stayed on Skylab 185 days in a 1/3-atm O2 atmosphere, and it worked just fine.
GW
ISRU cement is an unknown for a while yet. There's some ideas floating around, but cement as we know it here won't set there: too cold. "Icecrete" is one candidate, but you have to protect it from sublimation by a coating or by burial, and you have to insulate heavily to keep it from melting.
There's an underwater habitat-of-sorts idea floating around, too. In places on Mars where there is a buried glacier, you can just melt out a huge pond, and cover it with re-frozen pack ice and a regolith cover over that. If the pack ice is thick enough, the pressure in the water underneath may be high enough to support life in a wet suit and SCUBA, not a pressure suit. Takes about 6-7 meters of ice, I think.
In the pond, use lights to support photosynthesis in an aquaculture environment, and the waste heat from the lights keeps the pond from refreezing. There's no pressure dome; this could cover acres and acres, as big as desired. You import organic matter and organisms, and grow water plants and animals. Some could be fresh, others saline.
You could do this same under-ice thing in the trench you suggested, Louis. You just need a sealant of some kind to keep the liquid water from sinking into the subsurface geology.
GW
I predict there will some effort to make a "space race" among some of the participants, sort of a "who can be the first to do what the Americans did and now can no longer do" sort of thing. I hope the other spacefaring countries are not stupid enough to do it that way. There is no need to race, nor to do a "crash program" so very rapidly. What the Chinese are doing is the right thing. And they are being slow and careful enough to make it work right. I have no doubt they will go to the moon in a very few years.
There is a problem with a hostile power on the moon, if you cannot go there yourself, and international conditions are hostile enough to cause warfare. This is an old science fiction concept from the 1930's and 1940's, but it was also a very real fear during the buildup to the space race in the 1950's. A base on the moon with an appropriate "catapult" can throw guided rocks back at specific targets on Earth, because lunar escape velocity is so slow. The guidance need be no more sophisticated than a 1970's vintage missile to work. The rock hits atmosphere at Earth escape speed, and if monolithic, explodes with the force of a large nuke weapon on impact with the ground, not up in the air.
It is easy to shoot "down" at the Earth from a shallow gravity well, it is very hard to shoot back up out of our deep gravity well at the moon. Very asymmetric warfare situation. The moon really is a "high ground" in that sort of war scenario. You can bet your bottom monetary unit (whatever it is) that the US , Russian, Chinese, and several European countries are at least considering what a future like that might be, and how to forestall it. It's a risk as long as humans make war on each other. Hopefully, we all can keep it a low-probability risk. But that's another problem: the history of the 20th century makes that outlook look more than a little grim.
But, even that's no reason to "race" back to the moon. It's hard to shoot back up the moon, but not impossible these days, not like it was ca. 1960. A base there is not invulnerable anymore. The war is asymmetric, but not unwinnable. All the major spacefaring countries understand that. There's a couple of "wannabes" that might attempt such an ugly thing if they could, but fortunately, they can't. Not for a long while yet. No need to name them. Y'all know who they are.
That's why the form and detail of a space treaty is so important. You want to promote business and trade off Earth and with Earth, but you want to try to forestall any warfare, because it can be so very devastating here (or anywhere). That's not an easy thing to do, especially when the very best propulsion concepts we have, mostly involve nuclear stuff out there.
We still don't have the right treaty, or any mechanism to enforce it other than warfare.
GW
The energy cost of nuclear vs chemical is a big unknown, since a practical nuclear aircraft engine is still unknown. But, depending upon the thermodynamic cycle efficiency, it could be energetically cheaper. This might be true of an in-atmosphere cruiser, or a skip-glide exoatmosheric design, or even the antipodal rocket. We don't know.
The money cost depends upon that and the relative operating costs, which can be quite different. If oil gets expensive enough, nuclear air travel and nuclear or sail surface boat travel start looking much more attractive. The flier is fast. The boat is slow. So what is the difference in time worth? There is no one answer to that question.
But if some kind of emergency happened such that we need fast oceanic travel in a craft as re-usable as an ordinary jet plane, I'd look first at a winged nuclear rocket airplane on the ballistic trajectory outside the atmosphere. I'd start with the old NERVA technology, and maybe upgrade it later with some sort of gas-core nuclear light bulb engine.
The same thing could be done with chemical, of course, if oil were not a problem. Turbojet to ramjet would work well, leaving the atmosphere at M3 to M6, depending upon the range to be crossed. If you have a rocket too, then you can pull the exit trajectory up to about 45 degrees, and really extend the range without any skip-gliding. Mixed propulsion really makes a lot of sense for a thing like that. Transpacific in maybe 2-4 hours.
Skip-glide would be a bit faster trip, but more wasteful of fuel with multiple high-speed burns on each skip.
GW
Here's a notion for a quick habitat using something similar to a Bigelow inflatable. Have your equipment on a core structure to which the inflatable "shell" is already packaged around. This core has hard ends where the legs are. It sits on its side on those legs and inflates as an off-centered cylinder such that there is more volume above than below. The airlocks are also on the ends. You unpack your equipment inside, just mount nothing on the inflatable wall. Through something resembling an air mattress over it and inflate to not quite 7 mbar, as insulation and a meteroid shield. Throw an opaque tarp over that, and stake it all down. Voila: instant inflatable habitat, with all the gear already inside. Land it as a pallet and just set it up like a giant tent. There's probably a vertical-axis version, but stairs would be less convenient inside.
GW
Impaler: I dunno precisely how much a different entry orbit affects the heating drag. But, unless you're coming in at around escape velocity or more, it wouldn't be a lot different than any other entry. I used the surface circular orbit velocity as "representative" in my calculations: escape/square root of 2. From almost any closed orbit at low altitude the entry interface speed would be close to this figure.
From what I read, the heat shield on the Spacex Dragon is rated for a "free return" from Mars at some 50,000 ft/sec, beyond Earth escape velocity (about 36,000 fps) by quite a margin. Just a crude guess says such a shield might survive entry from LEO (about 25,000 fps) maybe 4 times before you ablate through it. KE proportional to speed squared, heating proportional to KE.
GW
All that I said in post no. 18 being what it is, I think the sense of Robert Dyck in post no. 1 is correct. We need a cheaper way to fly, one that is more or less independent of oil prices. If that's some nuclear engine, then so be it.
I'm not sure how this might be done at less than transoceanic ranges. But crossing ocean basins is flying across a fair fraction of the Earth's circumference. Once you're in that ballpark, an old science fiction idea rears its head as the best way to do things. Why not fly ballistic outside the Earth's atmosphere? The old "antipodes rocket" idea of Robert Heinlein. Actually, it makes good sense.
Once the range to be covered approaches half the Earth's circumference, the cost of accelerating high supersonic-to-hypersonic out of the atmosphere approaches the cost of cruising high subsonic over the same range. The cost of cruising supersonic over that same kind of range will never be as low, in my opinion, because of supersonic drag levels and supersonic aeroheating levels.
For a long range mission across an ocean basin, consider what it takes to reach Mach 3 to 6 on a short transient (minutes). Then look at sustained Mach 2 to 3 cruise (hours) vs that ballistic ascent into space. While you're coasting in vacuum, you can cool off by radiation. There is no drag. But cruising in the atmosphere at Mach 2 to 3 is very hot, and is done at very high drag.
Honestly, I have never understood why anyone would want to cruise supersonic in the atmosphere, when coasting in vacuum is so much easier to do. But, that's just me.
GW
Rune:
I used the data from the Glenn Research Center links you sent me to put together a sort of model of the Mars atmosphere for entry purposes. Extending the Glenn model to 100+ km altitudes is not very good, because the lapse rate is most likely wrong way up there. But the pressure is probably ballpark correct, and the density profile I got is conservatively too dense. Below around 28 km altitudes, it's probably not too bad a model.
I am still defeated by the notions of posting illustrations on this forum. I know those links you gave me are in English, but I do not share a dictionary with the writers of those instructions. So, I posted the model with the curves I got over at my "exrocketman" site: http://exrocketman.blogspot.com
All the guys:
This kind of model is a prerequisite to figuring out a landing scheme for landing large masses on Mars, either one-way or two-way. The bulk of the hypersonic deceleration should be taking place close to (or perhaps at, or even under) the 28 km I identified, and the chute supersonics and subsonics should be well below this altitude. At least it's a start.
I can rough out vehicles using pencil and paper from data like this, but it's just crude, order-of-magnitude stuff. Anything trustable in detail would require a trajectory code, which I don't have. Here's a first cut atmosphere model for those of you who might have such codes.
I'm going to dig out my pencil and solve this damned lander problem once and for all. One-way chemical, two-way chemical, and two-way nuclear thermal. This is going to take a while. Expect nothing quickly.
GW
Re: nuke B-36
It was the NB-36, not B-36N. This was a variant of an original 6-engine piston-prop design, not the intended design-that-didn't-work. There was a nuke turbojet engine design that they wanted to fly in it, but never did. They did fly a simple reactor and generated heat in flight. No real power output in usable form, though. That reactor flew inside a containment vessel tested to 500 mph impact against solid rock, on the rocket sled track at Holloman AFB, NM. The all-jet version of the B-36 came later, as the YB-60 8-engine prototype built and flown for the competition that resulted in the B-52, which in turn originally on paper was a turboprop resembling the Tupolev "Bear".
B-36 originally flew about 1945 or 1946 as a prototype with a single-tire main gear. Then B-36, B-36B, and B-36C flew configured with a 4-tire main gear truck (for safety's sake, since a blowout on the single tire design meant loss of aircraft and crew). All of these variants were 6 engine piston-prop, using the R-4360 4-row 28 cylinder radial engines of around 4000 SHP. From B-36D-on it was a 10-engine airplane: the same 6 piston-prop pushers, plus 4 early turbojets in two twin-engine pods, each out near the wingtip. B-36 is the ship with passages inside the wings out to the engines for in-flight maintenance.
I do not know which model of the B-36 was converted to NB-36 for the nuke flight tests. But I know they never actually flew the nuke turbojet. I don't know how much testing the nuke turbojet got on the ground, if any.
Re: nuke ramjet cruise missile
The Project Pluto cruise missile was a Mach 3 cruise, but very low altitude, ramjet. LTV Aerospace was airframe prime plus the inlet and nozzle guy, and AEC was to supply the reactor core. My father remembers this project, he was there when they did it, and told me what he knew. He didn't work on that one, though. Too busy designing the F-8 Crusader, among many other things. The shock wave alone was lethal to those on the ground, much less the spewed radiation. It would have killed more folks on the ground flying around, than with any warhead it could carry, and it carried a megaton-range fusion nuke.
This Pluto nuke ramjet thing got direct-connect tested on the ground at the Jackass Flats facility in Nevada on the old nuclear test site, adjacent to the nuclear rocket test facility. Some, but not all, of that gear is still out there, from both projects. Unlike NERVA, the Pluto guys were operating their reactor at a 10 deg F (5 deg C) margin above meltpoint for the reactor core supports. They had really bad erosion problems, never solved, unlike NERVA.
Re: earlier cruise missiles and jet aircraft
The V-1 Buzz Bomb of WW2 is often called the first cruise missile, but it was not. That distinction goes to a gyro-controlled pilotless biplane in WW1, which was Oliver Sperry's very first working automatic flight control and guidance system.
Gunther Diedrich at Argus Engine Works corrected some of the defects in Paul Schmidt's pulsejet tube to build the valved engine for that missile. He gave up work on a promising valveless pulsejet design to do that work for the Luftwaffe. The valve life averaged 45 minutes. Average flight time from France to London was 43 minutes. BTW, Luftwaffe test pilot Hanna Reisch (yep, a female!) was the first pilot to survive flight test flying the V-1. 6 died before her. She only died a few years ago. (I probably misspelled her name.)
Neither was the first jet engine to be flown a German turbojet in WW2. There was an Italian afterburning ducted fan that flew about 1939, but it was not the first, either. The very first jet powered flight took place in France in 1910 (yep, before WW1!!!), and it was accidental. Henri Coanda built two biplanes powered by an afterburning ducted fan, and accidentally took off and flew during a high speed taxi test. He crashed, but survived. The aircraft didn't. The surviving example still exists in a museum in France way outside Paris, BTW. It crashed because he had not yet trained himself to be a pilot.
We stand on the shoulders of the giants who preceded us. Sometimes it is really worthwhile to know who they were and what they did.
GW
Error correction: in post 121 above, I misremembered what I read about Curiosity's landing. The not-quite terminal velocity hanging on the chute approaching 2 km altitude from above is 100 m/s not 300 m/s. I have since calculated a soundspeed using the Glenn RC Mars atmosphere model, and it's right at 243 m/s from 2 km to the surface. That's 0.41 Mach for a terminal velocity at their chute mass loading, which is pretty near all that we can do. Compare that to terminal velocities in the 20-30 mph range for cargo and round personnel chutes here on Earth (0.026 to 0.039 Mach).
Rune: I'm glad you pointed me to that Glenn RC site. I have used it to generate profiles of temperature, pressure, density, and soundspeed to high altitudes on Mars. Below roughly 28 km, it looks pretty good. Above that, in the free-molecule flow regime (extreme low densities, too much mean free path length), the temperatures look anomalously low compared to Earthly profiles, which get very hot in the ionosphere. The soundspeeds look all wrong. I don't think I'd rely on that model for calculating entry drag and lift, the Mach numbers would look all wrong. Above 115 km, it predicts temperatures below absolute zero, so I cut that off as 4 deg K min.
Twinbeam: I think what we're talking about is not as different as it sounded at first. After having pored over that atmosphere model and using it to calculate Newtonian-flow stagnation pressures at circular-orbit velocity, I'm showing around 2 mbar pressure on the heatshield with vacuum behind, at beginning of entry, in the vicinity of 122-140 km altitude. I don't trust these numbers as accurate, but they are crudely representative.
Early in entry, while density is low, drag forces (and lift) are low. This lift and drag deficit could be made up with retro thrust through (or around) the heatshield, because flying tipped for lift points the rocket a bit downward, and that's what you were talking about. The only thing to "argue" about is how far to tip the heatshield.
Getting enough lift to flatten the entry trajectory up high has real benefits. It's best to do this as far from the surface as possible, for flight safety purposes.
Later, in mid entry trajectory, there's enough density and speed to fly fairly flat with little thrust at all.
Finally, late in the entry trajectory as Mach decreases under about 3 or 4, you have the density, but not the speed squared effect to have much drag or lift. Again, flying tipped for lift, using rocket retro thrust to make up the drag and lift deficits, could have real benefits. It keeps the trajectory flat and away from the surface. You'd like this to be around 10-20 km, maybe.
For one thing, you could pop the chutes at a lower, safer Mach, and do it higher up. That way, more of the descent could take place transonically to subsonically on the chute. You could use very little retro thrust, or even none. Save fuel.
Terminal velocities at reasonable chute mass loadings seem to be around half a Mach at 0-2 km on Mars. Heavily loaded chutes would still be supersonic. So chutes alone will never land anything of significant size on Mars. That's just plain rocket braking, from about 2 km or so altitude, at relatively high thrust.
Essentially, that's what Curiosity will do, except they are not making up deceleration deficits during entry or chute descent. Their final rocket braking scheme is a little complicated, but makes sense for a one-way probe of large mass.
GW
Answering TwinBeam in post number 117 above:
I think I understand what you were proposing. Using rocket thrust as lift to hold the entry vehicle in a flat trajectory long enough to slow down. Yes, that would work. However, if you have a blunt heatshield facing into the slipstream, and you tip the top edge forward a few degrees, you can generate a lift force comparable in magnitude to your drag force during the real hypersonics. That's lift without rocket thrust at all. It works down to around Mach 4-ish, with most any blunt shapes.
Depending upon whether we are entering at escape-class speeds, or orbital speeds, the Mach number is very definitely hypersonic. Here on Earth at orbital entry speeds, the initial hypersonic Mach number is around 25 as the vehicle grazes into sensible aero effects somewhere close to 90 km altitude. It was 36 coming back from the moon.
These are definitely some sort of free molecule slip flow aerodynamic conditions, not continuum flow, as the mean free path between air molecules up there is pretty close to 3 cm. I think most folks use some sort of modified Newtonian flow model, and an awful lot of correlations and Mollier diagrams to account for heat transfer and ionization effects.
From what I read about Curiosity, it enters at around 6 km/sec, "flies" on a tipped heat shield until the hypersonics are well over, presumably a little under Mach 3, pops a chute, and then sheds the heat shield. The chute takes it sort-of barely subsonic, but in that thin "air", the terminal velocity (drag = weight speed) is about 300 m/sec, which is close to speed of sound, unlike chutes here at home. From there it's rocket braking. They chose a skycrane rig to do the rocket braking.
As for lift during hypersonic braking, the tipped heat shield lift will be low initially, high up. Then in the middle of the deceleration deeper in the "air", there is lots of lift as the wind pressures are large at high speed and higher density. Late in the run, in the Mach 3-to-5 range, there is density, but not much speed, so lift forces are falling into insignificance again, and the trajectory inherently steepens. Rocket thrust lift would be advantageous here. And in the deceleration from below-Mach 4 to around-Mach 2.5, there is no aero lift, so rocket thrust lift could help there, too. Below Mach 2.5 is when you pop the chute.
GW
TwinBeam:
As I understand it, anything over a couple of tons mass cannot be slowed by sequential aerobraking enough to do a thrust touchdown, not without hitting dirt first. The "air" there is too thin to help enough for deceleration, but too thick to ignore in terms of entry aeroheating. That couple of tons is the entire vehicle mass at touchdown, not just the payload it contains.
A two or three ton limit is fine for small one-way probes, but they already smacked into the wall with the new Curiosity rover. That's what the complicated "skycrane" rig is supposed to address. Sure is a lot of stuff. Complicated stuff risks failure, and throwing away all that "skycrane" sure put some extra mass into what we had to shoot to Mars. I'd almost bet my idea both saves mass and reduces risk.
But any vehicle that might land 2-6 men is going to mass dozens of tons at touchdown, several to many dozen tons if it's not going to be a one-way trip. The dead-head payload crew cabin going back up might be about like a Spacex Dragon, and that's around 10 tons by itself. The ascent booster will be 10-20 times as big, with anything but nuke propulsion. That's around half a hundred to a hundred tons you have to land in the one vehicle. And we haven't included any sort of habitat or survival equipment, although that can be sent down one-way in separate smaller landers, given a beacon to home on. A 1-ton rover car will likely be close to the 2-ton lander limit, all by itself. It's hard to send down lots of tonnage if you're restricted to 2-3 total tons of landed mass per vehicle.
Anything with a blunt heat shield can be flown tipped slightly off-axis to generate a lift force, which can be used to adjust the trajectory to be whatever you need. No need for rocket thrust to do that lift job. We started using that with Gemini back in 1965. It works just fine. The lift force is comparable to the drag force, but on Mars supports 38% of the weight. You'll probably have to tip off-axis a bit more to compensate for the lower density better.
But, with Mars's too-thin "air", you're down to a small handful of km from the surface before the entry hypersonics are even over. You can pop a chute at about Mach 2.5-ish, but on Mars, if you're over the 2-3 ton limit, you'll strike before you can decelerate subsonic. Steepen the trajectory to hit denser "air" earlier, and you just hit the ground sooner. The trajectory has to be very shallow to work at all on Mars. It's already just about "flat horizontal". Plus, once you pop a chute, there's effectively no lift, and it steepens very quickly to near-vertical. That's just chutes. Ballutes would be no different.
So, to me, it appears there is a deficit in the aero-deceleration available at Mars. We need more, but it just ain't there to be had. No one can figure out how to make chutes and ballutes work during the hypersonics, so the only other option I see is adding a little rocket retro thrust. To my knowledge, no one has run the numbers for how much, but I bet it's low thrust during the hypersonics, and during the chute decel from supersonic to subsonic. Then throttle up and do a rocket touchdown.
GW
There's two things that haven't quite been done yet: (1) firing significant retro thrust into the oncoming slipstream at hypersonic speeds, and (2) firing significant retro thrust into the oncoming slipstream while hanging from a chute, at supersonic and transonic speeds. The Soyuz thing is like the battle tank: very subsonic.
Yet neither of these is particularly daunting, unless you are so over-bureaucratized as to attempt nothing that has not been done before. The key to firing engines through ports in a heat shield is no throughflow: a sealed engine compartment. Had shuttle Columbia's wing structure been sealed cell spaces inside, she would have brought her crew home safe in spite of the leading edge hole.
The key to retrofire while on a chute at Mach 2 is plume mixing with slipstream before it hits the chute, so that the chute is not damaged by hot gas. Not too much thrust, and stand the chute off well behind the vehicle on a heat-protected strap or cable. Easy enough.
Curiosity didn't really need a hovering skycrane. That kind of thing will never land men and habitats on Mars. Ridiculously big and complicated and wasteful.
GW