You are not logged in.
Spacenut:
I honestly don't know. I know there was a centrifuge module, and I know it was cancelled. Whether any hardware was actually built, I don't know.
GW
ETBE: I found that it simply did not absorb moisture itself, and neither did hydrocarbons, so mixtures didn't either. No real inhibition, just no tendency to accelerate moisture absorption. Ethanol does promote moisture absorption, but only up to about its azeotrope at 95% ethanol, 5% water. So it's not as bad an actor as "everybody" claims.
I've done gasoline-ethanol blends in on-the-road cars for about 6-7 years now, and I have found the problems "everybody cites" to be way overstated. I have found no problems at all (none !!!!!) up to about 35% ethanol in gasoline, in completely-unmodified engines, right down to fuel consumption, which is identical to 100% gasoline. This result I found in 3 completely-different vehicles in real (statistically-repeatable) road tests. Including what amounts to "cold weather" in central Texas (18 F = -8C). The only real problem occurs at about E-42, and it looks like late ignition timing. Which is unsurprising, since the book values for ignition delay time are longer for ethanol than gasoline.
Cold immersion at -110 F (-61 C): liquid pool immersion is quite a different environment than atmospheric, and it is not the same as "standard" cryogenic stuff. I was able to touch and pick up blocks of ethanol-wet dry ice with my naked hands, something absolutely impossible out in the dry air. Touching pieces of -30 F (-34 C) steel with bare fingers out in our atmosphere is something I would never, ever attempt. The behavior of surface skin moisture is completely different in those two scenarios (wet vs dry).
This is "school-of-hard-knocks knowledge" coming from a person who has actually worked outside more-or-less steady state at conditions as cold as -31 F (-35 C) and as hot as 130 F (54 C), or as hot as 300 F (149 C) if you count transients in jet blast. (Those jet blast transients are quite temporary, of course. No more than an hour at most.)
GW
The photo in the link was a gas balloon suit, not an MCP suit.
BTW, hi!! I haven't seen you in print here since before the great crash.
GW
Hi RobS:
Yep, your assessment is pretty close. I saw him in action at the 14th convention in Dallas last summer. I had a paper there myself.
Mine got classed as futuristic "Battlestar Galactica" without considering bang-for-the-buck. I was guessing around $50-100B to send 6 folks to Mars, making 16 serial week-long landings (3 folks each landing) +/- 30 degrees out-of-plane to the orbit of the crew vehicle. That's most of Mars.
Most of the more-minimalist mission plans I saw were around $5-50B to send 2-4 folks and make one single landing. Those sound more like "flag-and-footprints" than they do exploration, at least to me.
Mine did need NTR propulsion in the single-stage reusable landers. I used 3 landers for the 16 landings, with probably enough propellant left over for at least one visit to Phobos. That's why I keep saying a practical lander design is the key enabling element. The other tinkertoys already exist in one form or another.
I was hoping for gas core NTR to make the manned portion a fast trip, but the backup could be solid core NTR on a slowboat trajectory. Solid NTR is a tinkertoy we could resurrect pretty quickly. Turns out gas core is further off technologically than I had hoped (although it could be done). I got to talk to some of the original NERVA guys about this at that convention.
GW
OK, I'm not sure that anything of any size and utility to people on Mars can be truly "crash landed", meaning what they used to call a hard landing: no deceleration of any kind after the hypersonics are over. Mach 3 + impact pretty much dissociates matter as we know it, here or anywhere else. The "hypersonics" are typically over in the M 3-5 range.
On the other hand, those two small rovers used airbags to survive terminal-velocity parachute delivery. Unlike Earth, low-altitude terminal velocity on Mars with any practical chute is pretty close to local Mach 1. That kind of thing does not scale up very well, since mass grows as size cubed, while area (as for chutes) grows only as size squared.
Paraffin wax is a useful material, probably also to folks on Mars. Just like steel and a lot of other things. I really believe a M3+ impact is violent enough to vaporize even large quantities of it, based on kinetic impactor weapons tests I have seen. So, I think we might have to do better than supersonic crash landings.
On the other hand, there's lots of things that could survive a high-subsonic impact, even without an airbag. That might be a fruitful concept to explore. Surface sound speed on Mars is on the order of 240 m/sec, much slower than the 300+ here. Different gas, and much colder, too.
A 340-ish m/sec "terminal velocity" is very definitely still well supersonic on Mars. You do that with ribbon chutes or similar, not slow terminal velocity cargo chutes.
The Spacex Dragon set up with its Super-Draco thrusters could actually land stuff one-way on Mars. Those thrusters are canted 45 degrees or so, and exit from ports on the capsule's sides, not through the heat shield. My guess is that they plan on using small amounts of thrust during the hypersonics to put its end nearer 10 km altitude than 1 km, and then go to high thrust for deceleration-to-touchdown at reasonable net gee. Based on what I've read about Dragon, a wild guess for deliverable cargo mass this way might be 2-3 tons. But it'll require bigger thruster fuel tanks inside the capsule, for sure.
There's about a 30% drop in net performance (thrust and Isp) for rockets canted 45 degrees. It's the cosine factor effect. I do believe some cant angle is required to maintain hypersonic and supersonic plume stability so that attitude control is not upset by screwed-up unsteady aerodynamics. However, I also think that 10-15 degrees cant angle might serve as very stable, although that would require firing your thrusters through the heat shield during the hypersonics.
That through-the-heat-shield notion sounds very scary, because of what happened to shuttle Columbia's wing. But you have to remember, that structure was unsealed and admitted a whopping lot of throughflow through the hole in the leading edge. If on the other hand, the compartment containing your thruster is otherwise sealed to admit no throughflow, then there is no danger whether you are firing through the open port in the heat shield or not. The static gas column in the compartment is itself far better insulation and protection than any heat shield could ever be. The key to this concept is "static gas column". You leak, you die. But this can actually be done, and already was with the Gemini B heat shield test 4 decades ago.
Gemini B was a Gemini variant intended for the USAF MOL program that ended up being cancelled after 1 unmanned flight. There was a crew access hatch through the heat shield into the MOL station. It was a removable plug hatch cover. The gap around that plug hatch in the heat shield is exactly the static gas column effect I am talking about. It worked fine.
GW
A mechanical counterpressure rig looks even better. MIT is working on one for NASA, but it's not ready yet. Dr. Paul Webb had a working prototype of one back in 1969, though.
GW
Rxke:
Yep, cutting or avoiding government red tape is exactly why Spacex is looking for a private launch site eastward, but not at Cape Canaveral. One of the places they are considering is far south Texas, out over the Gulf of Mexico toward the Atlantic. I hope that goes through, it's not very far from the ground test station they already have in central Texas, only about 380 miles.
RobS & JonClarke:
One of the issues you've been addressing is how much more cost-effective Spacex has been vs NASA and its favorite contractor suite. There's actually two intertwined effects there: (1) government vs business efficiency, and (2) the tradeoff between being big enough to do the job vs being so big you are bloated and inefficient.
Regarding (1), I would say this: the smarts and the abilities do not lie in government labs, they lie in the contractors hired by those labs to actually do the job (pretty much true of everything, not just NASA). The folks in the government lab just need to be able to define the job, not do it. Typically, they can't even do that very well, that's where inappropriate requirements, inappropriate regulations, and excessive "red tape" come from.
Regarding (2), it doesn't take a ULA to be able to fly to Mars. Seeing things like Zubrin's minimalist mission plans is proof that it only takes outfits about like a Spacex. If a government lab is involved, it should more closely resemble the NASA of 1959 than the one we have today. I would also say that the more minimalist your mission design, the more likely it is to fail, vs the more like a "Battlestar Galactica" it is, the more likely it is never to be built for cost reasons. The one we could really do lies somewhere in between those extremes.
All:
I think the true limiting item for a manned Mars mission is a lander (or landers) capable of putting things on the surface in the 10-100 ton class. What's the point of going to all the trouble of sending people all that way, if you're not going to land? Gotta have a good lander. And you'll need a way back up, too.
Since mass more-or-less scales with dimension cubed, while (heat shield and/or lifting-surface) area more-or-less scales with dimension squared, the ballistic coefficients of vehicles in that size class are going to be much higher than anything flown to Mars before. The trend of reentry parameters in Mars's thin atmosphere with increasing ballistic coefficient is not good: you are still high supersonic when you come out of the heating phase, but at very low altitudes, even at shallow path angles. There's not time to slow down to touchdown, no matter how you do it.
I think that points toward augmenting the aero-deceleration during the hypersonics with low levels of retro thrust. That puts the end of the hypersonics at much higher altitudes, and gives you a selection of practical means to decelerate to touchdown. Retro thrust is involved no matter what, but parachutes may also be involved. Or not. That hasn't been decided yet.
There are ways to solve the very real design issues of hypersonic retro thrust. Too bad no one is yet working on them. We're going to need it.
GW
Impaler is right, launch costs are not the only costs in a complete program. How things are book-kept varies from outfit to outfit. But, most of the development and production costs plus an overhead and a profit are amortized into an anticipated production run, on almost any product you care to name, so they're not missing, either. So these things really are at least partly in the quoted launch prices per vehicle that we see. For a lean organization, I would expect the sum-of-launch-prices to be a larger fraction of an overall lower program price; for a bloated organization, a smaller fraction of a higher overall program price. We've seen that effect before, in all walks of life.
The point I was trying to make is that doing the same job today would be significantly cheaper, no matter who did it, and maybe a lot cheaper if the right entities were selected to do the job. The launch prices are but outer-limit "bounds" on what will really happen. Very poor bounds, but bounds, nonetheless.
Big is usually associated with bloated in most of our minds, but that is an oversimplification. You do have to be big enough to do the job: the local mom-and-pop grocery store cannot feed the entire city. On the other hand, too big is a problem, because bigness and inefficiency-of-bloat do really correlate. For example, no one today would ask Boeing to build and market a new Piper Cub. They would never be able to do it for a reasonable price. A small outfit could.
I would never expect today's NASA or its favorite contractor suite to come up with anything reasonably priced to go to Mars or anywhere else outside LEO as a manned mission. They're too big, too entrenched in doing things the same way they always did. Something reasonable is going to come from new kids on the block like Spacex, who are just barely big enough to do the job (as evidenced by the jobs they are already doing). We need more just like them.
I really thought some of those shuttle payloads were 25 tons. The launch price figure I'm pretty sure was $1.5B, which for a 25 metric ton payload, is $27K/lb. It's higher by far if you fly lightly-loaded, which is why it is wise to select the right launch vehicle for your payload. One-size-fits-all is a bad idea, until some sort of future warp drive makes launch dirt cheap. We're certainly a long way from that.
GW
Hi Louis:
What you say is true. However, things always change. The price to LEO via shuttle was about $27,000 per pound delivered, in 25 metric ton lots, higher price if smaller delivery. That's because each launch was $1.5B cost.
Right now with Atlas 5 and Delta 4, at their max capabilities of 25 and 20 metric tons, the unit price is near $2,500 per pound, and just about the same in 10-ton deliveries for Falcon-9. That $100B ISS would have cost nearer $10B if built today with these rockets, instead back then with shuttle. Same 10- to mostly-25 ton modules, too.
So we built it back then. Shuttle alone would have eaten up enough money to prevent manned missions beyond LEO. Shuttle plus ISS was just worse, but at least it was more serial expenditures than parallel. It was an experiment, one mandated years before, when Apollo was cancelled in the middle of the landings. Hindsight is always 20-20, foresight is not.
We sure do need that partial-gee-that-is-therapeutic answer. Those experiments have still never been done, not in 50 years of manned spaceflight. Hindsight again, but very clear.
BTW, during Gemini mid 1960's, the Mars mission was on the books for 1983. By the time of the Apollo landings, that had been pushed back to the 1987 opposition. The preferred launch vehicle was to be Saturn-5 with the S-IVB third stage replaced by a NERVA-powered nuclear upper stage. This would have doubled LEO delivery from over 100 metric tons to just under 250 tons. They were trying to find a way to do one-launch/one-mission, but it was becoming a little bit clearer that LEO assembly was required.
All that activity died when Nixon cancelled Apollo (and all manned spaceflight outside LEO !!!!!) by executive order in 1972. He had been talked into shuttle-then-space station as prerequisites to going to Mars, when they really were not. He just thought it would be cheaper, and (unlike LBJ) was no fan of NASA or manned space. Hindsight again.
GW
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