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Though i'm not sure I like the sound of the CEV- alot like the cramped, hard to land, throw away Apollo capsules but with modern engines and hardware- I am overjoyed that the ISS's days are numberd and the Shuttle along with it... thats five billion dollars a year, which if you add that up times ten years is a very sizeable sum indeed.
A space nuclear reactor's output depends largely on its efficency of conversion, the reactor that Dr. Zubrin had in mind probably wasn't the modern helium/turbine converter type, where a relativly small reactor like that could make hundreds of kilowatts, perhaps more... though they will need some more engineering done on them. Nuclear ion will clearly be the more robust technology in the end.
Speaking of nuclear reactors, you don't need to use a super-high-temp reactor to melt rock to make lava bricks or anything, since a focused blast of microwaves will sinter the metal-bearing Lunar soil into a hard material nicely. A microwave generator on a robot arm and means to pour sifted soil where you want it for easy automated construction of simple forms, just like a giant version of laser/polymer "3D printers" today that make solid objects from plastic.
Now about those ion engines... Although they have great efficency, they still don't provide enough thrust for time sensitive (human, living, cryogenic, radiation-sensitive, etc) payloads, and this isn't going to change. Ion engines have low thrust by their nature, and you can't stack them up without running into trouble with the mass of fuel and the power requirements. If you want to use chemical engines for the initial kick out from Earth that might get pretty heavy/pricey, especially being expendable, and you would have to spiral out from Mars for a long time if you didn't likewise carry another kick-stage, which would make the craft pretty heavy... unless you want to throw the whole thing away on each trip out. Oh yes, and since the ion drive has to operate continuously to work its magic, you can't spin the spacecraft on a cable or anything, and thats a big problem if an ion vehicle will take a while to get there.
My money is on hydrogen nuclear thermal
The US ought to use RD-68 engines for the SDV because they are actually in production unlike the RD-0120, whose factory has been essentially gutted. Russia doesn't have any other cryogenic engines of that scale. Frankly speaking, I don't think that international cooperation on a project of this magnetude is a good idea in the first place for lack of technical integration and all the comprimises in design that are sure to happen.
Well if Nasa isn't in the colonization business, it better learn to... the fantastic costs of getting even a small mass between worlds will, for the forseeable future, preclude any profit from a Mars venture without a massive subsidy which would dwarf the actual revenue of the venture. There will be no Martian trade, and even Lunar profitability is questionable.
If Nasa isn't looking ahead to building a real colony, say that crater would make a dandy dome wall, lets land there attitude, then a Mars venture will dwindle down to a small, mostly automated, perhaps not even perminantly staffed space cottages that nobody cares of... even if life is found on Mars, it will not be enough to sustain interest. Nasa should be going to prepare the way for settlers who would make it their business to prepare for more settlers, and worry about turning a profit later.
There is one good reason to go back to the Moon that can't be quantified easily... that Nasa would get practice, experience, and somthing it sorely lacks - confidance - in building manned space ships. I think that Nasa engineers would not sleep too soundly with their masterpiece, the ISS, if it didn't have that Soyuz attached. Its a long way from where we are now to risk putting astronauts in spacecraft for two years with no safety net. Technologicly speaking, no, the Moon does not make sense as a testing ground for a MarsDirect class mission, but it does get Nasa in the business of exploring again, and if the general tone of Bush's plan is any indication it screams "walk before you run."
Lunar ice could change this equation somewhat, and make a Lunar L1 fuel depot a good investment for future inner-solar-system development. I'm not sure where the idea that Lunar ice is somehow inaccessable compared to Martian regolith ice came from, you simply sift it a little then heat it up using a solar collector or reactor waste heat and it would readily sublimate off for easy collection. Either which way, humans will not be doing the digging on the Moon or Mars, but operating a "robot mining camp" will require real-time human oversight of some kind. Even if the camp doesn't require humans to perform maintenance, the robots will require oversight that cannot be provided over a multi-minute time lag.
Since alot of the money going into the Shuttle/ISS program is launch facilities, those probably will be on the hit list, especially since it is practical to build a Moon base with 25 ton class launchers... Dumping ISS into the sea or handing off to the Europeans puts a smile on my face though.
If Nasa has a ready supply of fuel, either sent from Earth or the Moon, to L1 then making a MarsDirect sized ship from EELV launches (say, 6, for around a billion total) is not impractical... one shot for the HAB/lander or ERV, one for various equipment & fuel, and one for the TMI propulsion stage... it isn't likly that Ares could be launched for under $400-500M a pop when you factor in the cost of the redesign and the big massive pad/crawler/VAB infrastructure anyway.
I still have a central, deep-seated fear though... that unless Nasa goes to Mars with the intent to stay reguardless of the science mission, then Mars will be a scientific curiosity like Antarctica, and not the next step for our species.
Do not underestimate the power of buracratic inertia and pride...
To "do Mars right," and not have it become Antarctica II where nobody really lives there, it will take that long to develop the nessesarry technology anyway.
What really should perk you up though, is Bush may order Nasa to drop the "we're going to finish no matter what" attitude and pull the plug on Shuttle/ISS (they really are the same program) earlier than planned, which alone would furnish big-time dollars for other uses.
Here we go again... No, its not, it doesn't in the slighest change my mind, but first a word about Bush I's SEI:
Bush I's nuclear-rocket powerd "Battlestar Galactica" Mars ship was a monster by any measure of the term; huge crew, hydroponics, multiple landers or fuel for them, loads of extra equipment, possibly rotating sections, and the biggie, enough fuel for a return trip via an Opposition orbital transit, which is a severe mass penalty when you are using low-temperature solid core NTR nuclear rockets as proposed. The $450Bn also included money to build at least one large space station to assemble the thing and a multitude of 20-50 ton launches to ferry the componets to orbit, and if memory serves, for a 2nd space station and a Moon base might be part of that figure?... $$$
Guess what? An advanced nuclear rocket tailored for Mars won't require that sort of undertaking, when better computer-modeling of physical extremes in nuclear engines and higher temperature resistant materials to build them with, which is not that far off. A GCNR engine is easily tripple the power of solid-core NTR rockets, perhaps as much as an order of magnetude more in a refined form: this alone turns Bush's nuclear battleship into somthing much smaller; every gram of fuel mass saved yeilds a nearly exponential decrease in total mass. Build the ship for the sole purpose of going to Mars efficently, no orbital assembly platform at all beyond perhaps a collapseable truss with a robot arm and a few solar pannels, and the cost falls even more. Finally, build the Ares SDV in some configuration, and build the pieces of the Mars ship to ride on it; lowering the number of launches needed by a factor of five or six with the improved SRBs. No no, a well designed and well managed GCNR/VASIMR Mars ship will not cost several hundred billion dollars, which is simply an impractical sum, any which way.
Well of course thats what the Russians told you that about the Energia, they have their pride to protect and old otherwise useless rockets to sell (which we would do too in the same position...). If you look at it from a mass-to-Mars point of view right now, then yes they are correct that at the moment it is the least expensive way. That doesn't mean its cheap(!), it only means its less bad and there are only enough for one mission without rebuilding the factory! They still do not carry enough mass for more than flags&footprints: 25 tons to Mars every 2.5 years each including lander is peanuts for anything long or short term, ISS would require five times that to do anything useful; you would be lucky to include a decent rover in the Mars Direct strategy... It really is that bad, especially when you consider that it can't withstand any weight-creep.
Areocapture and ballistic entry are absolutely two distinctly different orbital maneuvers, they are not in the slightest comparable: ballistic entry requires less trajectory accuracy, is straight through the atmosphere hence little concern for its maximum altitude or upper thickness, and is much easier to design in self-correcting aerodynamics. Aerocapture on the other hand, requires extreme trajectory accuracy; you are aiming for just the very outside rim of the atmosphere at exactly the right attitude and velocity with little margin for error: you must bleed off only just enough velocity, unlike entry where you want to get rid of all of it, hence is a vastly more exacting parameter. I doubt that self-correcting aerodynamics would be possible either. The Martian atmosphere, since it is so thin and the gravity weak, varies in maximum altitude often, especially with the solar wind hitting it, introducing another extremely dangerous variable... Real orbital aerocapture of the type you are thinking of has ever been accomplished, and you want to risk lives to it? Cargo maybe, but people no, propulsive capture offers obvious system safety benefits.
In order to do anything more than take pictures of flag-waving astronauts kicking up red sand, you will need a reuseable means of landing on Mars from Mars orbit... this is where the advanced composit materials come in, so that a "Martian DC-X" can be built that consumes a minimum of fuel per flight round trip, since it will likly be chemically powerd. Advanced high-temp materials for the insides of a GCNR or VASIMR engine, and maybe for a Scramjet SSTO cargo ferry, would substantially reduce risk and expense.
I won't go into the other safety or logistical failings due to MD's low-tech approach that you can read about in other threads, but I will summerize: no abort option, dangerously long transit times (6 months is long. Too long.), low practical flight rate, rare launch windows, landers for every payload, little mass margin for radiation shielding, no reuseability, and the best part is little room for improvement at all: MD is the plateu for today's tech.
I never said that we don't have the technology to go put man on Mars right now: we can build cryogenic engines, small nuclear reactors, inflatable habs, that much Dr. Zubrin is right about... what we don't have is suitable hardware on the shelf, which will be long/hard/expensive to become a flight-ready system, not that it is beyond current technology. HOWEVER, we do NOT have the technology to go for keeps and to stay, our rockets are still too weak, too unreliable, and too heavy to do more than plant a flag and make a little attempt to look for bugs... There is a threshold before exo-lunar travel becomes cheap and safe enough to really go out there, and current tech obviously cannot reach it. Wishes and foolish hopes to the contrary won't make that launch window any larger or the little MD "space cottage" any bigger.
Better to spend the money and time on developing advanced propulsion technology and building the SDV/Ares for tomorrow, instead of wasting it with a dead-end Mars sight-seeing trip what what is possible today.
Bush I's big megaspaceplan was not to put man on Mars. It was to put man in space as far as Mars, hence called the Space Exploration Initiative, and not "Mars Indirect".
Dr. Zubrin's estimates are almost comically low considering the fantastic cost that the ISS is chalking up and Shuttle's inflated price, and that isn't for hardware with a performance that nobody has yet matched and has to work the first time. No, there will need to be one more technological generation before travel outside Cis-Lunar space is really practical. Higher max temp materials, lighter weight composits, advanced nuclear systems (GCNR or light-weight VASIMR), higher reliability rockets & LSS systems, maybe a Scramjet "Shuttle III," and most of all skill in building spaceships.
The Helios airplane was neat and all, but with its very low thrust on the ground on Mars, I doubt it could do more than taxi under its own power in the Martian air, it needs far less thrust in "cruise" mode. The length of such a required runway, without a destructively powerful assist, would be prohibitive. Said runway would have to be pretty perfect, any Martian airplanes will be quite fragile, and even here on Earth they worry about anything bigger than gravel for the landing gear's sake. Using a helecopter to lift anything of mass is also impossible due to the same thin atmosphere, you would have better luck with a LOX/Methane or LOX/LH2 rocket-hopper.
Rocket power is also not an option, due to simple fuel capacity problems, reguardless of the fuel used. Reaction-mass engines of any type are not practical for long-range air travel. A jet engine on Earth gets its oxygen for free, cutting the fuel mass required to about 20% of what it would be just counting fuel mass when burning CH4, which doesn't take into account the LOX tanks or how much bigger the plane would be to carry it: a reduction in fuel needed yeilds a nonlinear return in saved mass. Liquid hydrogen needs much more oxygen by-mass than Methane per amount of reaction mass too, not to mention the need for bulky insulated tanks for LH, even if it were plentiful on Mars.
Further, no airplane designed for flight on Earth, save perhaps the Helios or some US Military UAVs, are even remotely compatible with a Mars mission even if the wings were radicly alterd and the structure composed of composit materials, especially not the Harrier fighter jet.
I still favor space-based reconisance whenever possible, yes it is harder to make a long-range camera and such, but to not have to mess with sending the mass down to Mars's surface, or building a finicky gossamer airplane, is a small price to pay... Just send more satelites based on spy sat technology, if one won't do, send MORE. They could also be multi-purpose, serving as a rudimentary GPS on Mars (Mars has no magnetic field, no compass!) and communications to pick up signals from low-powerd remote surface equipment.
Oh yes, and a solar powerd airplane would have to either fly constantly west faster than Mars rotates, or carry batteries to fly at night. Also remember, that Mars has a much thinner atmosphere per amount of gravity, so I have my doubts if any small airplane can fly effectivly.
A neat idea, but i'm going to have to weigh in against, I don't see the idea as having enough merit. Short-term (<1 martian day) scouting with an airplane that is too light to hold much useful equipment and too light to carry a good communications system to transmit it isn't worth it. You have to be able to transmit the pictures you take, which will prove hard to do with such a limited lifetime/weight constraint.
Any robot bulldozer equipment on Mars will have to pretty small and light, hence not able to do a great deal of digging. Using them early on to make a huge airstrip is not going to happen, either the airplanes must be dropped from the lander, launched by rocket/catapult, or verticle lift. Unless your vehicle can land verticly, this percludes reuseability.
I am also largely skeptical about the advantage and airplane has over simply sending another recon satelite with a spy-sat grade telescope/camera and a large fuel tank, which will last for years and capture multiple locations of interest, and could help serve as another Mars communications satelite for the gigabytes of data that will eventually need to be sent to the Mars base or Earth.
The aircraft I would like to see used on Mars is the good old' balloon. Fill it with hydrogen and make it rather large, perhaps putting some polymer solar pannels on top for power... Given the thin Martian atmosphere, I wonder if a propellar might help. An anchor made with an exotic polymer thread would probably also be a must-have; perhaps the anchor could even be used to haul up surface samples for analysis/collection?
I think the cathode tube isn't going to get you anywhere... basic rocket physics says that the momentum produced by a engine of any kind is equal to the momentum of the fuel that leaves the engine. Since momentum is the product of mass and velocity, the mass of the fuel per-molecule/atom is an important consideration in engine/fuel selection. Since a cathode ray tube fires electrons, the mass would be negligible, so your engine would produce essentially no thrust. Also, such a system would have an extremely hard time keeping electricly neuteral, which would monkeywrench any electric engine pretty fast if not compensated for.
Another concern about solar storm events is the lack of warning you would have on/near Mercury. Since the solar storm effect's main lethal blow doesn't travel at the speed of light, its possible to warn astronauts that its coming... the trouble is though, at the distance Mercury is at, such warning would be awfully short, maybe down to the seconds. The only way around this hazard would be to not live on the surface and to only leave the "bunker" for very short periods.
I'm going to have to learn tward a return to the Moon first... although most of the technology, the physics and engineering to make efficent rocket engines, good flight computers, long-term life support exsists, there is a pretty big chasem between being able to build an integrated, robust, operational system and just having some of the bits and pieces that resemble what you need. A trip to the Moon will force Nasa to do somthing that it hasn't done in a long time... build new manned space ships, which they will have to get good at before we ought to even to consider putting the lives of astronauts in them for a year or two with no abort option. And, sending out unmanned test flights first is of limited worth, since you aren't testing the ship with its most vital system "installed." Dr. Zubrin's estimation about the ease that we can regain this skill is woefully over-optimistic. It doesn't matter if most of the Moon hardware isn't useable on a Mars trip, it gets us back in the swing of things more easily because it is easier and safer to use current tech to go to the Moon than it is Mars.
Secondly. Simply because Mars is big and mysterious and would employ more planetary scientists is not a sufficent protection from flags/footprints end-of-mission syndrome OR somthing almost as bad, turning Mars into the new Antarctica, where only a few scientists go. The Moon had a pretty good grasp on the imaginations of people in the 60's too, and telling average people about all the cool rocks you find and the pretty pictures is not sufficent insurance for human colonization, the ultimate goal. Even finding microscopic life or past evidence thereof is not enough; people will get bored and will be little more impressed than the finding of bacteria in Antarctic ice.
Third: Although the technology exsists to go to Mars on a small scale, and in this respect just barely, the technology to go to stay on anything more than an little outpost scale does not. If we go now and set up a little research station that costs a decade and umpteen billions to build and operate operate, that is time, money, and political capital that is being wasted in reguards to the development of higher performance technology and hardware that can serve the science mission more effectively and for the REAL mission to Mars... To go and not come back
Too bad I can't put headlines on posts... "nuclear saves the day."
The way to get to the outter planets will unquestionably be by a high-ISP engine with reasonably high thrust. This rules out chemical, solar sail, electric-ion, and solid-core nuclear rockets... so, there are only a few options: Gas-core nuclear rockets, the Vasimr plasma rocket, the Orion nuclear pulse rocket, or maybe the nuclear salt-water rocket. The salt-water rocket poses control and temperature resistance concerns, the Orion pulse rocket has proliferation and control concerns, which will make them hard to build. Gas Core nuclear has much higher thrust than Vasimr, so your Discovery-style ship spends less time accelerating and decelerating, and you don't have to rely on the engine for long periods of firing time or lug along a multi-megawatt power reactor.
As for the Mercury rover, why not put RTGs and a solar shade on it? When the advanced dynamic ones become available, they will produce much more power per pound of plutonium fuel, so you don't have to worry about getting stuck in the dark.
Infra-red energy isn't hamperd by small amounts of dust in the air like visible wavelengths, though if your reciever is coverd in a layer of it there would be signifigant loss.
A laser would be an improvement, but still not all that great. I don't think there is any technology that can focus any sort of radiation down to a spot no more than 2-3m through any medium that would be light/compact and reliable. A large laser is a pretty hefty and short-lived device anyway. An optical laser fired through our atmosphere to the Moon will spread out to several miles. An IR laser would also be severely disrupted by accumulated dust on the reciever most likly.
It may one day be practical to build large satelites for fixed microwave transmission to very large recievers, but I still don't see this technology being preferably to an advanced dynamic RTG system for remote low power applications.
It isn't the efficency of the reciever thats the problem, the big issue is that the microwave "beam" will spread out too much at that distance, requiring a fairly large collector. Concepts for Earth-based recievers for industrial-scale solar satelites are measured in acres, not square centimeters.
The rovers only really go slow because of the time lag trying to control them. When we have people on the ground or in orbit, then they'll be covering serious distances, and keeping a large antenna pointed with precision would be difficult... which would be about the same time as you would have said miniature power satelite.
Solar power is also not all that efficent at Martian distances, and i'm skeptical that it would be easy to build a satelite that could transmit signifigant power without getting awfully heavy.
A clever idea, but not practical because of the size of rectenna required to soak up signifigant energy. And what happens if this gets coverd with iron-bearing Mars/Moon dust? Focusing the microwave beam that tightly is not going to happen, even if you could make your transmitter/recievers stay pointed at eachother as the rover bounces over rocks in the 1/3rd G gravity.
When somebody gets around to making a dynamic (moving generator) RTG, and after "Nuclear O'Keffee" gets comfortable telling the eco-wackies to take a long walk out a short airlock, then we won't have to worry about remote low-power sources.
Ah but the OSP project, unlike all of Nasa's other recent ideas, is not an end unto itself. If we are going to be sending ships to and from the Moon or Mars soon with a reuseable ship, which I hope that they do, you are going to need some means of getting people in and out of orbit easily and safely. Plus since the Moon is so close, OSP itself could be used as the manned section of a lander.
OSP need not be a completly stripped-down life raft of a space ship; OSP could be a robust space ship, just without the school-bus sized cargo bay and "space work truck" Shuttle hardware. A real ship with a cockpit and Shuttle-grade life support, not like the X-37 winged escape pod. A winged/lifting body ship with wheels is ideal since it can glide long ranges, having a hundred times more cross-range than Apollo, the re-entry is much more gentle at 1.5G and lower average heating plus a soft airplane landing instead of the 4G hard plunge with Soyuz. And if its built like HL-20, won't have to throw away the OMS module and heat shield on every flight, and its generally easier to handle a small airplane than a oddly weighted capsule.
If we need OSP or not really depends on what Nasa is doing for the next 20 years: if we are resolving to build a Mars or Moon ship in the near term, like 2015-2020 or earlier range, then building a good OSP makes sense, if Nasa intends to go anywhere later than this, then a manned Hyper-X/NASP SSTO spaceplane might be reliable and cheap enough, so OSP would be redundant. If this is the case, then the need for OSP hinges entirely on the ISS, and what Nasa intends to do with it, will it become a "mission sucess: we learned how to build big things in space! research? it wasn't ever supposed to be for research."?
Don't be so hasty to diss the OSP concept... I agree that the way that Nasa was setting about building it, telling LM/Boeing/Orbital et al to "come up with somthing right now" was a mistake but the idea of building a ship which can be done in the near-term with near-term technology soley as a people carrier is a good idea.
Why you might ask? The question begs... what else are you going to do? How else are you going to get people into space? Relying on Russia's too-small Soyuz system is a non-starter, since it isn't American and since its rough on the crew and recovery. Shuttle will hopefully be on its way out soon after ISS is finished. Putting people on a Shuttle-Derived vehicle Mars Direct style is a bad idea for a variety of reasons (man raiting mainly). And lastly, the supposed ultra-reliable hypersonic spaceplanes or "son of DC-X" are not going to happen for a long time. Private companies will hardly be getting orbital in this time frame.
Since it will be a long time before Hyper-X/NASP style planes become viable and its unlikly DC-X will be revisited or reach that reliability mark, we are stuck with the technology that we have today more-or-less of 1960's style rockets for our "next" manned ship. The very concept of putting people and cargo together is a great way to make your rocket unessesarrily heavy and expensive, like Shuttle turned out to be. Making a 20-ton-payload rocket to launch and recover one ton of human payload is rediculus, since 100% of that massive rocket should be >99% reliable and recoverable for that 5% of payload. This is probably the cheif reason, above and beyond any other, that the Shuttle design is a dismal failure. The Saturn-V system is a great way to beat Communists to the Moon, but is a horrible way to get people in and out of orbit.
So, since we do need a lifeboat/taxi on ISS in order to fully man it (unless Russia comes up with double the Soyuz flight rate), and we will need a new way in and out of orbit with reuseable Moon/Mars ships, a small and reuseable space taxi makes sense. Somthing like the HL-20 or DynaSoar that keep getting proposed and shelved in favor of quicker (Saturn) or bigger (Shuttle) systems, and hold a gun to Nasa's head to do it right. The only real reuseable alternative is to end manned space flight after Shuttle until Nasa's third-generation Hyper-X/NASP space plane becomes mature.
I also want to reiterate that Mars Direct and its children are also not safe enough for regular manned flights. The artifical gravity scheme is dangerous and heavy, all that redundant cable you use is by no means light-weight, and astronauts will have to do more than walk when they get there; 90 days ought to be the target zero-G limit... and what if your cable doesn't break cleanly?
Dr. Zubrin also underestimates the harm caused by cosmic radiation, which will easily exceed safe dosage without shielding during transit, even without a solar flare. I suspect that he has convienantly shrugged off this hazard to make MD more apealing to Congress. A heavy-duty radiation shield will be needed, more than food/water can provide, which will risk chemical being dangerously overweight and awfully close to the zero-G limit for NTR engines.
For the third time, i'd like to reiterate again how nice it would be to have mass headroom for any Mars ship design; you simply don't have to worry about making the thing so light, which in itself will save signifigant money plus give engineers and the science teams more flexability to make the Mars ship as sturdy and useful as possible. Stronger construction, more spare parts, more instruments, and of course bigger crews and mass for payloads. Chemical can't do this, solid-core NTR can't get it there fast, Ion still can't accelerate quickly enough... you need a super-engine.
MD and other ISPP-return-fuel methods that don't use a super-engine also have no abort option. With propulsive flight round-trip there should be plenty of fuel to simply turn right around if there is somthing wrong. A month into transit and a serious leak is discoverd in the LSS system or a crewman coming down with a virus that was incubating before launch? No problem, turn around and be back to Earth orbit in a week or two. The pure Zubrin MD couldn't even abort the second the Ares SRBs are lit. Use the ISPP to fuel extra lander flights.
Then there is this aerobraking stuff... I think is awfully dangerous because of all the "ifs" involved. With the Martian atmosphere being so thin and gravity being lower, it is harder to predict the depth of the atmosphere in any one spot at any time. The accuracy of the entry angle is so delicate that I have reservations that it can be done reliably. Aerobraking also limits the size of the ship that you can use, since the "hole" you have to punch must extend the length of your rocket, to prevent the superhot air from collapsing back and melting your tail. Your ship must be built strong enough to withstand the sudden deceleration forces, further increasing the mass for a given size instead of a relativly gentle propulsive capture. And best of all, it limits the maximum speed and hence the shortness of your flight. Its a neat trick, and could be used for cargo perhaps, but it will hold us back.
I disagree that the technology plays no role compared to politics in keeping a manned Mars mission from becoming Apollo-II. If it will cost a given amount and risk death a certain amount for one technology and a different amount for another technology, then it is a factor. There is a threshold, however hard it is to detect, when the difficulty of going to stay is too much of a sacrifice to pay.
A few 18 month missions is very much a flags & footprints if there is no provision for perminance afterward. Mars Direct and its immediate decendants could support a limited colony role with many expendable SDV/TMI/Lander shots if given unlimited funding and an unlimited supply of expendable astronaut-martyrs, but the question is can it comfortably achieve this threshold? No. Even with launch from high Earth orbit, it is still not enough and over the space of twenty years development costs begin to become quite small compared to operational costs, especially for throw-away hardware.
The #1 technology that determines if we are above or below the threshold is and will continue to be propulsion; the less trouble it takes to get from A to B, the easier it is to reach this threshold. Chemical/Aerobrake, NTR/Aerobrake, and the low-thrust SEP/NEP/(Aerobrake?) are unable to go to and from Mars fast enough with enough mass margin for manned travel of any scale. Can they get people there? Yes. Can they do it easily and safely enough for a colony? No. Of the available technologies, GCNR is the best super-engine: It does not require magnetic confinement because it doesn't reach an unmanageable temperature. You keep the engine from melting by spiking the LH fuel with a substance that absorbs the radiated energy, simple carbon black may work, so that it heats the fuel instead of the engine walls. The walls of the engine, cooled by a stream of liquid hydrogen (just like modern chemical engines), do not have to withstand a thermal flux that is far beyond what is achieveable today. CFD will make it easy to design the engine, which theoreticly speaking, is quite simple.
In relation to tether propulsion for unmanned cargo, yes I think GCNR is much easier than trying to mate a cargo (which must be devided up small and built strong, hence $$$) traveling tens of mach numbers at a exact instant in an exact spot and have the catchment mechanism latch on all coreographed perfictly. No "testing" is required to say that this would be very hard to do reliably. Spinning/despinning largely negates its efficency for space cargo and eliminates atmosphere-to-space transfer. Its not as "simple" as its "simple & cheap" label makes it out to be.
If it takes another ten or fifteen years to develop GCNR than MD so be it, as I can say with confidance that using Mars Direct or its children will take reasources away from building a real Mars ship. So we start with GCNR in mind, and use the money that would be spent on Zubrin's lander to build a DC-X style reuseable lander, and so on. This will help bring us closer to the colonization cost and technology threshold, a small price to pay in exchange for wasting billions on a pointless Zubrinesque mad dash to the Red Planet... The second someone puts a footprint in the red soil, its a race against time to build a colony before interest is lost and the plug is pulled, time that cannot be wasted by Mars Direct.
Nasa would be the obvious choice for building a GCNR engine, and the reasosn are as above stated. Speed, payload, launch window, robustness, etc...
That is correct, I am against the concept of tether propulsion, save perhaps a stationary space elevator, for the forseeable future. The concept is interesting and ingenuitive, but as the saying goes, the devil is in the details...
The engineering required to reliably mate with the tether end is not practical, and would require accuracy and precision beyond anything concieved. Landing on an aircraft carrier would be a walk in the park by comparison... timing/aiming a rocket stage to catch in the exact few-feet region at the exact fraction of a second needed? Or flying an airplane going Mach 10+ to hand off a cargo module to the end of the tether (which would also incur massive drag too)? The Delta II, arguably the most reliable and accurate rocket available, has a "good day" when it orbital insertion is a whole degree off. Course correction to try and mate up would likewise be too difficult to perform reliably unles the tether were stopped. I remain a little skeptical of aerobraking even, given the very percise attitude control required and the nonconstant thickness of Martian atmosphere.
The payloads that will be going to and from Mars will liky not be bulk payloads, since the local production of water, oxygen, food and of course the most valuble commodity in the universe - rocket fuel - will likly be easy to produce with ISPP plants, water ice/vapor harvesting, and maybe even ammonia synthesis. Deviding non-bulk payloads into small containers needed would also prove to be a severe economic, mass, and reliablility penalty. Even deviding bulk payloads would suffer from similar economic and packaging disadvantages.
The reliability of any tensioned cable in space has thus far been not good due to vibration and micrometeoroid impacts, and thus strengthening the cable would add alot of mass. I also have concerns about the G-forces that the payload (or for that matter, the tether ends) would be exposed to during the "tether boost" phase; delicate objects might simply be crushed from the force. Hardening any object against high forces also adds substantial mass penalty.
Although I could imagine such a system at the end of the century sending tanks of Ammonia or LH to Mars orbit, rockets of any type are clearly more reliable currently, especially ones with sufficent thrust for fast course corrections.
Mars Direct is a terrible arcitecture to set up a real more-or-less self sufficent colony if for no other reason than its too expendable, it is not intended for self-sufficency. It will take alot of payload down mass to do more than keep a few colonists alive initially until they can build things on their own. The MD style direct payload does save some on development costs, but we are hopefully going for a long time, so its worth it to go for the more powerful, reuseable technologies that are fairly close at hand even if they are not on the required scale for a colony. For the colony mission, we are not ready, but we aren't far off. The technology and arcitecture for the systems that are sufficent are close enough to warent skipping investment in MD's 1960s aproach entirely. Build colony-grade hardware, even if it is not colony-scale. GCNR or VASIMR engine technology, reuseable landers, and to start learning how to make rockets stronger instead of lighter.
Don't forget of those tons of payload you throw to Mars with MD, you still have to include a lander that reduces your down mass proportionally to actual cargo mass, and increases the expense of every flight; its not just the launcher! How much of that thirty or sixty tons is lander? Even if its not included, thats still another piece of hardware you have to throw away every trip, just like the HAB and ERV. MD can handle a small base in the short run, but the cost of tossing all that hardware will not outweigh the development and operations cost of better hardware in the longer run... Think of the sheer payload that a high-ISP engine could deliver to Mars Orbit if time were not a critical factor, an entire SDV worth or more maybe, especially if you didn't have to bring a lander. A reuseable lander is a must to compliment a super-engine rocket... And I think that Dr. Zubrin has a habit of being too optimistic about mass estimates. I would also like to add that using a large number of supposedly "fairly cheap" medium expendable launchers is not such a great idea. I think that it is quite possible to hold an SDV cost to around $500M per launch and at 120 tons a flight will put payload costs near that of Proton for example. Using a very large number of light launchers is impractical because of the mass penalty of having to break up your payload into many little containers (for liquids/gasses, many times the surface area of tank required), and the expense of docking them with the Mars ship outweighs a single SDV throw or a self-docking reuseable SSTO/TSTO medium shuttle.
A >3000Isp high-thrust engine simply revolutionizes space travel; it really does: A 100 day trip with a large and spacious GCNR or VASIMR ship could be built strong, safe, and for reuse with large design margins is a massive improvement compared to a 100-140 day trip with "just enough" 4-man margin-less throw away Mars Direct style design with save-every-gram gossamer construction. If you built a lighter weight TransHab style ship around such an engine instead, I imagine the 100 day "barrier" could be broken quite easily, with GCNR imparticularly. Six times the performance, with the possibility of a dozen times, compared to chemical is a huge huge improvement! Six times the performance, even if you cut that in half for mostly foregoing aerobraking compared to chemical/aerobrake, could get a lighter-than-SEI ship to Mars FAST. Thirty Days Fast.
And there is another ability that a very-high-power engine has that chemical can't match... launch window. A GCNR or high-end VASIMR is so powerful, that you could travel from Earth or Mars or vice-versa with a lighter ship a much greater portion of the time (maybe Any Time) and wouldn't have to wait for a year-and-a-half to come or go to minimize delta-V. The times that you just can't go to Mars or Earth are much longer with chemical or NTR. Further, without a large tug in Martian orbit, it would be hard to send a chemical powerd rocket back to Earth on a fast trajectory even with idealy aligned orbits, especially if you have to take a 25% Isp penalty if you use LOX/Methane chemical versus LOX/LH. A long flight back is unavoidable with MD, and would be alot of trouble to accomplish with chemical or simple NTR means in general. I still think three or four months of zero-G is pushing it, six months might be okay if you are coming home to a host of doctors and lesure for a while, but not six months to Mars to do heavy labor, rock climbing, etc in a space suit... I am concerned about the mass of a artifical gravity setup even if the safety concerns can be smoothed out (like what if vibration builds up in the cable?), heavy cable especially with multiple redundancy would weigh quite a bit, and Mars Direct cannot withstand the dreaded weight creep. I am also concerned about total radition dose during transit to and from Earth, and how MD still does not offer enough radiation shielding, which weighs quite a bit. Using their drinking water is a very practical idea, but we are talking a wall of water a foot thick or so, and a composit shield would only be a marginal reduction... either of which a GCNR engine wouldn't break a sweat pushing.
Not robust, not reuseable, not worth the investment. Now is too soon.