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If I have my facts stright, the Energia core was produced in Russia (the prosthetic limb factory) while the boosters were built in the Ukraine (where Zenit is buit right now.)
Moving production to Michoud would have its ups and downs. American labor is more expensive than Russian labor, and it would add to the cost if the booster cores had to be flown by M4 from Louisiana to Baikonour (and if a new assembly building had to be built.) However, Michoud has the skilled workforce and the equipment to do the job. Perhaps the same people will be working side-by-side on Energia and SDV.
I'm a bit curious as to the insulation on Energia's tanks. Is the rocket a double-wall design with insulation between the inner and outer tank walls? Or is it applied directly to the outer skin, as on the shuttle ET?
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Hmmm rebuilding the Energia booster using American facilities would be kinda pricey I imagine... but as long as it would cost less than a billion dollars a launch, it still might be more economical than developing an SDV or the folley of building a Mars ship with suped-up EELVs. Does anybody have any idea how much it would cost to build a 120 ton SDV?
I also can't help but be a tad concerned about Mars Direct's pretty tight mass and volume constraints... it would be nice(er?) to launch a larger Earth-Orbit-to-Mars-Orbit and back nuclear engined ship launched by 1-2 SDVs/Energias and stocked with a (fueled?) decent/acent vehicle, liquid hydrogen fuel for the NTR engines, and a crew of 6-8 (via OSPs) brought up by a few EELV HLVs. The Mars unmanned payload, replete with a larger multi-TransHab base, possibly a pressurized garage for the rover, and alot more equipment than you could cram into a direct flight for a much more extensive survey if a one-way megalander could be devised. The same NTR transfer stage could also be reuseable and launch a large number of small GPS/Telecom/Recon satelites when it reaches orbit with a minimum of hassle.
The uranium fuel in the nuclear engines would last for several flights probobly, give mass margin for a radiation shield or more Mars base equipment, and would be able to keep the whole trip short to minimize risk and maximize flight rate, not to mention give direct abort ability. An initial investment for the NTR stage and a few extra initial HLLV launches initially I would think would be worthwhile even if they did cost alot more than $120M a pop if future HLLVs would not be needed later per-trip.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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I know some people who are working on the Kepler prize ERV contest. For what they've told me, they think Zubrin was far too optimistic when he came up with Mars Direct, and that NASA had the better plan with the original Semi-Direct (DRM 1.) Somebody said that 80% of the delta-V for getting from Mars to earth will be spent getting to Mars orbit, hence NASA's insistence on an orbiting ERV that's fully fuelled. I can't say that the 80% number is accurate, but I'm sure that a Mars orbit rendezvous will still preserve most benefits of in-situ propellant production.
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The 80% figure is about right. Mars surface to low Mars orbit requires 4.1 km/sec. I suspect a Mars vehicle would be in an elliptical orbit, though; if it were roughly in a Deimos transfer orbit (apoapsis the same as Deimos's orbit) the delta-vee from the surface is 5.3 km/sec. Escape velocity is another 0.2 km/sec (5.5 total). Hohmann transfer to Earth requires another 0.9 km/sec (6.4 km/sec total). High speed transfer to Earth requires yet another 0.5 km/sec, I think (6.9 km/sec, total).
There's a website with all this: http://www.pma.caltech.edu/~chirata.deltav.html.
I have long wondered whether Zubrin's masses were too optimistic. In The Case for Mars he admits somewhere the NASA people thought so.
-- RobS
P.S.: Oh damn, the link is now dead. Maybe you can find it somewhere else, though.
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Yeah, the relativly small mass of all the gear sent in Mars Direct and the pretty small size with long durations in cramped quarters never apealed to me, and I think borderlines on "fantastic," especially considering how hard it would be to include good radiation shielding and signifigant science equipment & lab space. Though the 1980's $400Bn battleship was outlandish, the optimum mission should be somewhere inbetween there and Mars Direct.
Besides the cost of vehicle construction and development, alot of the costs come from the sheer mass needed to get there per mass of payload. I like the idea of a Mars-based fuel factory to gas up an acent vehicle and rovers which would save a good amount of mass, but I wonder if instead of hauling an acent vehicle, could the fuel instead be used to gas up the manned decent vehicle instead to return to orbit?
Next up... with a nuclear powerd engine complete with a boil-off reclimation system, build a ship to get a human mission to Mars and back again with areobraking to Mars and Earth orbit fast, preferably with enough power for a direct abort with the use of reserve fuel. Mass should not be skimped on the manned transfer stage which should have a water/polymer cosmic radiation shield and make each leg in eight months either way with extra fuel. Six months on ISS is already pushing it, and I hate to say it, but if spinning your spacecraft doesn't make the engineer in you cringe, it should. It would definatly be based on a big TransHab, and hopefully have room and a large enough lander for SIX, not four.
The cargo mission could instead be powerd by a giant Hall thruster run by nuclear reactors if there would be a substantial mass advantage compared to a duplicate NTR stage, and if it is practical make the stage reuseable. If not, and the mass to the surface of Mars were increased dramaticly, then I think it is an acceptable loss. The cargo lander could areobrake and enter Mars atmosphere with the same shield, and have a one-shot expendable engine to soft land with. The mass of this payload module should be substantial, with the hab suporting double the volume of Mars Direct at least, keep the pressurized rover and drilling rig, and carry the propellant factory.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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Hello RobertDyck,
Kudos on getting in touch with KBKhA Kosberg. I almost hate to ask, but do you think you could ask them about the total number of RD-0120s produced as opposed to those remaining at KBKhA, and if they have any information on the status of any of the engines not in their posession?
Assuming Humans to Mars is carried out as an international programme, which seems far more likely and achievable from a funding perspective, I don't see much point in producing Energia cores at Michoud. Lock Mart is in an excellent position to win major prime contracts for Transhab and/or ERV development and production. Such contracts will involve considerably greater expenditures that HLLV production. As NASA is likely to be the major funding source for the actual spacecraft, it makes more sense for the ESA to contract EADS to produce Energia HLLV cores at Les Mureaux.
I share the concerns expressed about Zubrin's Mars Direct mass margins, particularly with regard to the ERV. The easy way to make Mars Direct work (only two launchers per mission, no earth orbit assembly) without any new technology or cost increase is to replace Ares with a much more powerful Energia derived booster : Super Vulkan.
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After the 2002 Mars Society convention, I sent Gary Fisher a document with my idea for a mission plan. Gary works for Robert Zubrin's company. My document might be why the MS executive created the Kepler Prize Contest. (Hmm. It would be nice to think I had an impact.) My criticisms of Mars Direct were intended to be constructive, and my mission plan is just an attempt to update Dr. Zurbin's. Robert Zubrin used the latest technology available when he created the plan in 1990. Now we have space inflatables such as TransHAB, so I suggest using them. What I see as the weakest part of Mars Direct is the ERV. The HAB has artificial gravity and plenty of room during the trip to Mars, but the ERV doesn't. That means astronauts have to sit in a capsule for 6 months in zero-G. Experience on Mir and ISS has shown that most of the degenerative effects of zero-G can be alleviated by exercise, but 6 months in a capsule without either artificial gravity or exercise machines would be dangerous. I suggested a dedicated habitat for the Earth orbit to Mars orbit trip. The return from Mars orbit back to Earth orbit would use the same habitat. A dedicated space habitat can be designed for the unique conditions of space. I suggested a zero-G habitat with exercise equipment; you could argue for another design, but something is required to protect the health of astronauts during their return to Earth. My design would leave the interplanetary habitat parked in Earth orbit so it can be used for a second mission; it would be a reusable spacecraft. The Mars Ascent Vehicle would act as the Trans-Earth Injection stage so it could be propelled entirely with in-situ produced propellant. In this design the MAV would be expendable. The surface habitat would be transported collapsed and only inflated once on the surface of Mars. After the mission it would be left on Mars. As Robert Zubrin described, the surface habitats from each mission would accumulate and eventually form the core of a settlement.
Arguments for and against: 1) Surface rendezvous eliminates the risk of on-orbit rendezvous. I argue that orbital rendezvous was developed by Gemini and has been a mature technology since Apollo. 2) Surface rendezvous eliminates leaving the ERV unattended for 14 months (the surface stay). I argue that sending the return vehicle with astronauts eliminates the risk of leaving the ERV unattended on Mars surface for 16 months (time to produce propellant plus astronauts? trip to Mars). The ?unattended? argument sounds like six of one vs. half a dozen of the other.
I agree that Lockheed-Martin may not be the best choice to manufacture tanks for the Energia core module. If the goal is a second HLLV for redundancy (in case of another Shuttle accident) and competition (to keep the price down) then manufacturing the tanks at a single facility would defeat the point. It would be better to manufacture the Energia tanks somewhere in Europe or Asia, the best choice might be the Russian factory that produced them before. Let Michoud produce an SDV.
By the way, ILC Dover produced TransHAB for NASA. You can read about their idea for a Mars TransHAB here, but it looks like the space station TransHAB with the word "Mars" on the web page.
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Yes definatly. I didn't realize that the Mars Direct ERV was that small... it would definatly have to be pretty roomie, especially with the supplies it will need. There is also the psychological strain put on the crew, which would not have been on Earth for almost a year and probobly a little decondtioned still from the trip up from Earth.
I like this plan, though there are a couple of things that could be added to make it better... if the orbital transfer habitat were propelled by nuclear rockets fueled with LH2, which have about double the efficency of LOX/LH and tripple the efficency of LOX/Liquid CH4, then you don't need to refuel the hab module in Mars orbit at all. The reactor could power a condenser system that would prevent fuel boil-off for a lengthy stay on the surface.
So, since you don't need to haul up tons of fuel to the transfer hab, you could also make the Decent/Acent vehicle much smaller, since it would only have to carry the crew and a few hundred kilos of rocks and hard drives. In fact, if you could keep the down-mass of the decent vehicle low enough, you could skip the need for a huge Martian fuel factory and its LH2 feedstock entirely if storable acent fuel were sent with the cargo flight.
Since I love to "think big," I am imagining the transfer hab stage being launched by a pair of 120 tonne SDVs, one with a large version of TransHab with room for six and all the food, water, and life support gear needed for the whole journey, the water or polymer radiation sheild, the solar flare "storm vault," and perhaps the storable-propellant Mars Decent Vehicle. The other launch would be the nuclear propulsion stage with solid core thermal engines with bimodal power generation, and the "command module" with all the flight electronics/communications/Mars satelites etc. These would be mated up in LEO, and if nessesarry fueled with EELV HLVs, then tested by an OSP crew and finally manned by two more.
The cargo flight would be the huge "megalander," which could hopefully itself be pressurized for use as a "garage" or lab space, along with a small power reactor, acent vehicle fuel, science payload, and of course another fairly large Mars-only TransHab. This would be deliverd to Mars orbit by a second nuclear transfer stage, which would also be reuseable.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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The ILC Dover site is quite interesting. Yes, the web pages for the ISS transhab and the Mars transhab have the same diagram, so it appears that the exact same structure is conceived. I wish there were mass figures; has anyone seen mass figures for the transhab? The "Lawrence Livermore" page shows an ancestor of the transhab and gives masses for its components.
The ILC Dover site also shows a lunar inflatable, which is a cylinder laying on its side. This gives us an idea what a Mars surface inflatable could look like. I prefer an inflatable "tuna can" with hemispheric bottoms and tops that wold provide extra storage and the cylindrical central area being the habitat itself.
Regarding use of nuclear engines for the Mars mission, how do you plan to aerobrake them? I gathered that is a real engineering problem. If they are on booms far from the human-part of the vehicle, they can't be shielded well. Aerobraking large, bulky hydrogen storage tanks is another problem.
Regarding a "Mars Direct II" architecture, I think Zubrin has "political" reasons not to update it. You can't keep changing your architecture every few years; you are better off sticking to a basic design, selling it, THEN updating it if it has been accepted in principle. Besides, if Zubrin really has underestimated masses, as NASA seems to think he has, in another decade the estimates will be about right anyway, since technology will advance and allow lighter-mass vehicles to do the mission.
Of course, eventually you DO have to update your architecture if the technology has advanced so far that the old architecture is looking clunky. Transhab is an example of that. When you look at the latest drawings done through NASA of a Mars mission, they assume inflatables.
-- RobS
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A nuclear thermal engine can't sit on the end of a really long truss on the side, or easily on the back, since the reactor itself IS the rocket chaimber. It has to sit in the back, and pretty close to the crew section, so it would definatly use submarine-style doped polymer radiation shield. Somthing like this: http://spaceflight.nasa.gov/gallery....186.jpg
As far as areobreaking, with the kind of power that the NTR engines have, it might not be nessesarry at all. If not, or you want to maximize payload, perhaps an inflatable shield filled with foam instead of gas, like the crack-filler stuff that comes in cans from the hardware store, which would offer excelent insulation.. if it could handle the heat. It would be deployed in LEO before setting off for Mars. If they were light enough, you could even carry two, one for Mars and one for Earth given the differing atmosphere thickness.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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In most of the NTR schemes for getting to Mars, the crew and the reactor / engine are separated by the propellant tanks, which should provide sufficient distance and shielding. As for aerobraking, most NTR plans dispense with this and use a propulsive capture at Mars.
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Ernst Stuhlinger argues for an improved Saturn V to return humans to the moon
"The logical thing to do is build an improved Saturn V, and it could take advantage of lighter, improved materials," Stuhlinger said. "Von Braun and his coworkers thought all the time that Saturn V would be just one milestone along the path of space exploration. ... The Saturn V wasn't supposed to be the last large rocket."
A professor of mine suggested the same thing when I asked him about a possible lunar return. My feeling is that an updated Saturn V would be an entirely new animal, with lighter tankage and sigificantly redesigned (or even all-new) main engines. At least the SDV with 5-segment SRBs would use the shuttle facilities and keep the shuttle workforce employed. Likewise, a revived Energia would take advantage of the remants of its infrastructure. However, the Saturn V was the more capable booster, and it still had room for growth when production was terminated.
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As nice as it would be to revive Saturn or its even larger children dead on the drawing board, trying to restart these programs would be essentially a reverse-engineering, which is alot of trouble. More trouble than it would be to glue a pair or trio of RS-68's on the Shuttle Stack with its new boosters, which would then start to look alot like a small version of some of the solid-fuel Saturn derivitive rockets anyway. So, build this 120 tonne SDV today, return to the Moon, then work on a new megarocket if need be.
Also, we will need a way to get supplies up to the Moon and crews to and from every few months, so we will also need a smaller EELV HLV scale rocket that is cheap to fly, so we ought to build flyback LOX/RP1 boosters for Delta/Atlas.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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Reverse-engineering? I wouldn't say that would be the case with a revived Saturn. You'd keep it as close as possible, and make substitutions for materials and for parts that aren't available. When the blueprints are already available to you (don't believe the myth that the Saturn blueprints were destroyed,) it's far easier than disassembling something and copying it. And even in the case of reverse-engineering, it is doable--just as the Soviets did with the B-29.
I don't agree with reviving the Saturn, but I think the Saturn should be an inspiration for a third generation HLLV (the Saturn V being the 1st generation and Energia & SDV being the 2nd.)
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Replacing the materials, the engines, the control systems, etc on the old Saturn-V would seem to me to be too much trouble, and sub-optimal since the S5 was built with the old technology in mind, even if it wern't reverse engineered persay. Frankensteining old and new is usually not the best way to go, provided it works at all.
Now about using the same basic strategy... really big LOX/RP1 first stage (possibly reuseable) and multiple staging for various purposes does make sense when the time comes to replace a SDV type launcher with a minimum of trouble does make sense, but with the power offerd by SDV and the pre-exsisting infrastructure, that will be quite some time probobly.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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Regarding NTR and propulsive capture, the delta vee to capture into a high orbit around Mars is about 2 km/sec if the vehicle is flying to Mars in six months, and there would be another 2 km/sec delta-vee to fly back. Together, they are double the 4.3 km/sec delta-vee to fly to Mars in six months from low earth orbit. NTR gets roughly twice the specific impulse of hydrogen/oxygen propulsion. So there is very little advantage in terms of hauling a mass of fuel to orbit using NTR versus LOX/LH2, if the NTR is used for Mars capture and trans-Earth injection. If anything, there are several disadvantages to consider:
1. The entire mass of propellant has to be keep at liquid hydrogen temperatures, instead of 11% of the fuel, and has to be kept at that low temperature for 2+ years.
2. The NTR engines have to be used three times and can't fail, or you're in trouble. In contrast, aerobraking is fairly foolproof (unless you enter metric measurements into your computer instead of English ones!).
NTR for trans-Mars insertion has clear mass advantages; it doubles the throw weight to Mars of a heavy lifter. Couple that with aerobraking at Mars and a Mars ascent vehicle that can also propell the transhab back to Earth, and you maximize your mass savings. All the plans to use NTR were proposing throw-away NTR engines that would be used once and discarded into solar orbit.
-- RobS
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I wouldn't say that aerobraking is foolproof. In reference to Mars Climate Orbiter (which took a dive through Mars atmosphere because its software couldn't correct from English to metric units,) it was actually trying a propulsive capture. Aerobraking has been used to lower the apoapsis of spacecraft in orbit around Mars, but it has never been used to capture into Mars orbit. This is why I'm wary of aerobraking and why it should be demonstrated first before we start thinking about it for our Mars mission.
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Good point. I have been assuming that aerobraking will be demonstrated by the time of a human flight to Mars. The tricky part of aerobraking is that the density of the upper atmosphere of Mars varies from day to day because of solar and dust storm activity. But when modifying the orbits of various orbiters, NASA found it could predict the density pretty well. Aerobraking a large spacecraft with people on board requires dipping quite deep into the atmosphere, and the gee force goes pretty high for a short period of time. It has to be done just right or the vehicle will crash. But developing those skills should be cheaper than developing a whole new type of engine or hauling tonnes of fuel along. And it would probably be safer aerobraking a vehicle with a few tonnes of hydrogen feedstock than aerobaking something with nineteen times as much methane and oxygen, or other fuel.
-- RobS
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Areobraking into Mars orbit... and preferably into Earth orbit too... would be pretty darn nice if it could be pulled off reliably, but I have a few doubts. I think I would prefer to have a larger ship with bigger fuel tanks to dispense with it if weight did not become a huge issue, and you wouldn't have to bring a multi-tonne heat/areoshield or brace the structure as much. If not, then areobraking it is. The technique should be tested by the cargo flight before the manned flight is sent. Oh, and with a nuclear reactor, you have plenty of power to operate a fuel condenser to eliminate boiloff.
I also wonder how much fuel could be saved by moving the Mars ship out to a Lagrange before firing up its main engines with a trans-lunar tug? Pluuus, if its going to be a long time before we get really ready for a Mars flight, instead of using 1960's solid core rockets (~3000K), how about a GCNR (gas core) rocket? (25,000K)? Rough estimates put the specific impulse at six-or-seven times that of SSME.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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The document I have on NTR engines lists the open cycle GCNR with an expected Isp of 5,200 seconds, not 25,000. Or do you mean thrust? The same document lists expected thrust at 50,000 lbf (pounds force), and engine weight at 250,000 lb. Isp of the SSME is 455 in vacuum, so that is still 11.4 times the SSME. One problem with developing this engine is that unlike a solid core NTR, the exhaust is radioactive. That's fine in space but how do you test your development model? Static engine tests would require containment for the exhaust, and to prevent pressure backup as the exhaust fills the containment bottle, that bottle must be very big. The expected fuel for this rocket is uranium hexaflouride (UF6) with liquid hydrogen for propellant. Melting point for UF6 is only 64?C, which is also the tripple point. Boiling point is 56.5?C so its not hard to produce gas in the engine. It could be stored in solid form and sublimated by heating with an electric heating element to fill the reactor chamber. It would have to be stored in small pieces with neutron absorbers between them to prevent the radiation from reacting until its turned to gas. The health section of the hazmat document for this material states:
? Radiation presents minimal risk to transport workers, emergency response personnel, and the public during transportation accidents. Packaging durability increases as potential radiation and criticality hazards of the content increase.
? Chemical hazard greatly exceeds radiation hazard.
? Substance reacts with water and water vapor in air to form toxic and corrosive hydrogen fluoride gas and an extremely irritating and corrosive, white-colored, water-soluble residue.
? If inhaled, may be fatal.
? Direct contact causes burns to skin, eyes, and respiratory tract.
? Low-level radioactive material; very low radiation hazard to people.
? Runoff from control of cargo fire may cause low-level pollution.
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Sorry if I was not being clear, I was referring to the operating temperature of the engine. In rocket engines, nuclear engines imparticularly, the operating temperature large determines the propellant velocity, which determines the Isp. The operating temperature for a GCNR engine (open cycle) i've read of starts at around 25,000 Kelvin, and could go as high as 50,000K with similar Isp to what you have. Lots higher than the 3,000K limit of a solid-core NTR engine or typical SSME (lower Isp from higher gas mass), the big trouble is how to make the LH2 opaque to enough of the energy to prevent it from "shining" through the fuel and melting the engine and/or how to keep it cool while operating.
If such an engine with a little lower thrust (say 35,000 or 40,000lbs) could be kept to around 80-90 tons, that would make it practical to integrate into a single package that can ride on an SDV to orbit to stay within the "two SDV's per Mars ship" ease of construction envelope. This stage would also carry a Prometheous power reactor to operate the LH2 condenser. It could be fueled by EELV HLVs or if it were cheaper a 3rd SDV loaded with LH2.
Uranium Hexaflouride is nasty, but not unreasonably so. The radioactive effects are almost negligible. We have alot of experience with handling it in our fledgling nuclear power industry, and I think it would be easy to build a UF6 vapor "dispenser" like Zubrin's "Salt Water Rocket" graphite-straws fuel tanks but tiny instead.
As far as testing the thing, the solution is pretty simple... test it in the desert. It doesn't put off that much radioactive material compared to bomb testing. But, if that were totally unacceptable, then the easy thing to do is test it in a cave. Concrete over the cave face, bolt the engine with the nozzle inside, and fire her up...
So, one GCNR powerd stage on SDV #1, the manned Hab module (maybe) with the Mars lander on SDV#2, and 5-9 EELV HLV launches for fuel tanks, a pair of OSPs for crew, and perhaps one for the Mars lander. With the power of the GCNR, the trip could be made in only a few months perhaps and have enough fuel to return to Earth without having to gas up at Mars.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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Uranium U235 is such a low level radioactive isotope that you can handle uranium oxide with the plastic gloves you get with oven cleaner and a paper breathing mask. You don't want to inhale or ingest uranium powder because long term exposure could cause significant chance of getting cancer from the low level radiation. However, the fission by-products are highly radioactive.
Fission breaks U-235 into Barium Ba-144 and Krypton Kr-89 and 3 high speed neutrons. The neutrons will be absorbed; they are what cause the chain reaction. It's the Ba-144 and Kr-89 that are highly radioactive. Ba-144 has a half-life of 11.5 seconds, and Kr-89 is 3.15 minutes. Most of Ba-144 beta decays to Lanthanum La-144, but 3.60% decays by beta and neutron into La-143. La-144 goes through a series of beta decays to Neodymium Nd-144, which has a half-life of 2.29*10^15 years and alpha decays to Cerium Ce-140. La-143 goes through a series of beta decays to Neodymium Nd-143, which is stable. Kr-89 goes through a series of beta decays to Yttrium Y-89. Notice there is a lot of beta radiation produced, and some neutron and alpha radiation. But what if any of that absorbs a neutron? That would put it on another radioactive decay path.
It's many years since above ground nuclear bomb testing was allowed. The people who lived down wind were affected. You don't want to do that again, nuclear exhaust has to be carefully contained. One reason why Nerva was so successful was that they did test at Jackass flats. But Nerva contained its nuclear waste within its fuel capsules, only hot hydrogen was allowed to escape. An open cycle GCNR is another beast entirely.
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Another feature of an open cycle GCNR is that it's single use only. Once you vaporize the UF6 to get it going, you can control the reaction by surrounding the reaction chamber with control rods. Turn the rods one way they absorb neutrons, turn them the other way they reflect. When they reflect the reaction increases, when the absorb the reaction dies down. However, once the reaction goes completely cold how do you get the UF6 to condense into nice neat chunks with absorbers between them? How do you get it to condense directly onto the electric heating elements so you can start it again? I don't see how to give an open cycle GCNR a restart capability. You could use it for insertion into trans-Mars trajectory, but would have to jetison it after. That's another reason I favour ISPP for the return to Earth.
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The kinds of masses of fission fragments dumped into the air by a GCNR engine would still be pretty miniscule compared to the amounts that nuclear testing created and I question just how much people were injured from bomb testing either... The dose makes the poison, so to speak. Firing the engine into a porus stone cave system like Los Alamos thought about would work too.
And about it being re-startable, the secret is to simply not reuse the fuel at all, but rather to intentionally perturb or deflect the flow of fuel in such a manner that will eject the Uranium from the core out the nozzle. A "flushing" so to speak. The 6-12 fold increase in ISP is surely worth bringing a few more kilos of Uranium along.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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Whether the radioactive release into the atmosphere, or even into a cave, is scientifically or medically significant is almost irrelevant because it is emotionally significant, and that is what drives laws, policies, and what governments can do. No one would allow a big cave system become dosed with low-level radiation. They would worry about radioactivity leaking into the ground water, parts of the cave roof collapsing and releasing radiation into the air, etc. It might be easier testing the engine on the moon.
-- RobS
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