Reasons against Mars Direct

GCNRevenger · 2003-12-12 12:34:30

I would love to see the sky open for business... but, that simply is not going to happen to any degree because of the economics: there is no profit in space industry. Space tourism will remain the realm of the super-rich thrill seakers, space speciality chemicals are not worthwhile now and likly won't be with advances in Earth-side synthesis (we chemists are oft crafty people) and would be a nitche market anyway, satelites will become even more disposeable with cheap non-US launch services so there will be no business in fixing them given their rapid obsolecence, and mining of anything anywhere is futile because of the difficulty of getting it to Earth "en mass." Precious metals aren't precious if you flood the market with them. The only thing possibly of worth is He3...

There is no profit in space industry

Now about Mars... You cannot run a real mostly-self-sufficenct colony of any size with the Mars Direct arcitecture. It costs too much to get there since it is expendable, its flight rate is low, and is just not fast or safe enough to perform regular crew rotations. Hence, if we are going to go to set up a colony, MD is the last kind of ship you would want to make. So what if it takes another decade to design and build? If it takes another twenty years to make a system that can avoid a "Martian Apollo," then that is what it takes. MD with its puny engines could be made to work for the science mission, but the science mission will leave us stranded here, and no way can it support more than a "space camping trip." Avoiding a Martian Apollo is more important than getting to Mars fast.

So, if it takes us the better part of a decade to develop MD, and we use it for a decade (five flights two years apart) for all our Mars flights, even in the unlikly event that we do start building a colony-class ship the day after the first Hab touches down then it won't get us there for good for real any faster. Chances are, there will be another delay in building a new ship to get there which will stretch it even longer ("You already have MD, you need how many billion for this thing? What, you spent all that money on a system you can't use?"), and in any event all the trouble and investment we go through to make MD work will simply be thrown away when it has to be replaced.

The engine is the real enabling technology of all spaceflight. Modern rockets today are expensive to fly because they are limited by the performance of their fuel/engines, hence having to build a huge rocket that stands hundreds of feet high just to get a few tons into space. Thats the big reason why its hard, and when somthing is too unreasonably hard, it won't happen. The problem is the same for Mars ships, that it is only practical to do a real colony when it becomes easier to get there than with our technology today. Can we go to stay with our simple chemical engines and such today? Absolutely speaking, yes, practicly speaking, NO. So, new technologies will be needed... GCNR or VaSIMR are the only technologies likly able to robustly provide high thrust levels and high fuel efficency in a space-weight package in our lifetimes. Period.

So yes, I am fairly narrowly issue-minded, but the issues are often dependant on eachother. No super-rockets, no Mars colony.

GCNRevenger · 2003-12-12 12:51:38

Yeah, there are objections *grin*

Zubrin is a nuclear engineer, not a biologist for one... but say you want to add a water shield? Thats alot of tons that you have to push with little chemical engines, which will nessesitate more fuel, which requires more fuel to push the more fuel and tanks to carry it and so on... whats that phrase Nasa has, "terminal weight gain?" Adding stages in LEO is not the answer, chemical engines are still awfully puny.

Falcon-5 may not fly. Heck, Falcon-1 may not fly. If any of the super-cheap rockets fly, you still have to do orbital rendevous and orbital assembly, which is alot of trouble, as demonstrated by ISS. It is worth the extra expense to avoid large numbers of small launches.

Not too sure anybody figures how scarry it is engineering wise to spin your rocket on purpose. What if the cable is hit by a micrometeoroid and breaks? How about accuratly pointing comm dishes, star trackers, etc? Will months of the Coriolis effect make Astronauts unable to walk straight? You'd need a VERY long cable to negate its effects, adding more mass. What do you use for the counter-weight? A spent TMI stage you say? Well, you'll have to throw it away when you reach Mars orbit so you can perform retro-fire or aerobraking, eliminating reuseability. Segmenting your ship (Hab on one end, reactor on the other for example) is also a scarry proposition. And best of all... what if you can't stop the rotation reliably? ...its not worth it. Build the nuclear rocket first.

RobertDyck · 2003-12-12 13:20:03

As I said before, when gold mines were opened in the Yukon or California it did not make precious metals cheap. Any new mine will not make precious metals cheap, space is no different.

I am really tired of hearing the argument about 3He. As I have also said before, 3He will never be a viable commodity for power plants. Once you have technology to make a commercial fusion power plant work you will have technology for a double-deuterium reaction. Let me put it in more detail: The fusion fuel mixture with the most energy and lowest ignition temperature is deuterium-tritium, but tritium has a half-life of 12.32 years so it will never be found in nature, you can't store it for years, and it's radioactive. The fusion fuel mixture with the second highest energy and second lowest ignition temperature is deuterium-helium3. 3He is stable and can be found (in low concentration) in Lunar soil. The difference in energy released and ignition temperature is not much, so this is the fuel mixture Lunar advocates like to talk about. The fuel mixture with the lowest energy produced and highest ignition temperature is deuterium-deuterium, but again the difference in energy and ignition temperature isn't much. However, the big difference is waste product. Deuterium-tritium (2H-3H) produces helium4 (4He) and a neutron. Deuterium-helium3 (2H-3He) produces helium4 (4He) and a proton. But deuterium-deuterium (2H-2H) produces tritium (3H) and a proton about half the time, and helium3 (3He) and a neutron the rest of the time. That means a double-deuterium reactor produces its own tritium and helium3. That produced 3H and 3He can be fed back in as fuel. Now let?s look at the total reactions. To ensure we compare apples-to-apples, let?s look at what happens to 6 atoms of fuel. In a 2H-3H reactor you start with 3 atoms each of 2H and 3H resulting in 3 reactions. In a 2H-3He reactor you start with 3 atoms each of 2H and 3He also resulting in 3 reactions. In a 2H-2H reactor you start with 2 pairs of 2H, resulting in 2 2H-2H reactions and producing 1 3H atom and 1 3He atom. Then the secondary reactions start: the 3H atom reacts with one of the remaining 2H atoms, and the 3He atom reacts with the other one. The result is 4 reactions, not just 3. Those secondary reactions are the reason a double-deuterium reactor produces more energy for a given number of atoms of fuel. Furthermore, deuterium is lighter than either tritium or helium3 so you have an even greater energy per unit mass of fuel. This doesn?t work for a fusion bomb or fusion rocket since the waste from the primary reaction doesn?t stick around long enough for the secondary reaction to occur, but it does work for a power plant. The final nail in the coffin of 3He is that deuterium is extracted from water. One part in 7,000 of every drop of water on Earth is heavy water, and what makes it heavy is the hydrogen is the isotope called deuterium. You don?t need to go to the Moon for fusion fuel when you can get a much more valuable fuel from tap water.

Now can we talk about a mined commodity that actually has a chance of commercial viability; like rocket fuel, industrial metals to construct space stations, or precious metals?

Bill White · 2003-12-12 13:32:39

If we are going to set up a colony we needs lots and lots and lots of "one way to stay" supply missions, right?

Why do we need a re-useable vessel for one way to stay supply missions? In that case we want to Earth-Mars propulsive system to be as low cost and as expendable as possible, right?

Colonists? Large GCNR passenger ships? What do they carry on the return leg?

A GCNR tug to push passenger habitats? Well then you could use TransHab modules docked to a central keel and pushed by a GCNR positioned sufficiently far back. Actually once the trans-Mars insertion wsa accomplidhed, the GCNR could detach and return to LEO long before the colonists arrived at Mars.

Attached 4 or 6 TransHabs to a central hardpoint with tethers reeled in. Use the GCNR to get them up to speed and cut them loose for the coast to Mars and aerocapture. The GCNR separates, backs away and when at a safe distance uses its main engine to return to LEO.

Then spin up the Transhab array for simulated gravity and 36 colonists arrive several months later.

Its a time value of money thing. Having a big shiny expensive GCNR cruising to Mars and back may not be cost effective. High capital assets need to be engaged in productive work to justify the capital investment.

Thus, the R&D for a MarsDirect TransHab and centripetal simulated gravity is far from wasted. If we never learn to spin then nuclear propulsion will have a monopoly on crewed flight, not necessarily a good thing.

I see GCNR as being like the Iowa class BBs and Essex class CVs from WWII - - awesome fast powerful big ships and we needed 'em. But we also needed Liberty ships which also were an engineering marvel, but from a totally different perspective. Spinning TransHab modules could be the Liberty Ship for the settlement of Mars.

= = =

Also, how much MarsDirect R&D is really needed?

Ares rather than Energia so we can fly American? Okay, sure, but we still need Earth to LEO since GCNR won't ever set down on a planet, ever, right?

TransHab? Aerocapture technologies? Landers? Rovers? In situ fuel production? Technologies we will need whatever scenario we follow for Mars settlement.

RobertDyck · 2003-12-12 14:38:22

I made a very strong statement about 3He. Let?s look at the numbers to see if I know what I?m talking about. (I love the internet; you can look everything up while composing a message so you don?t have to memorize it all.) The energy is in units of Mega-electron-Volts.

2H + 2H ->3He = 3.27 MeV
2H + 2H ->3H = 4.03 MeV
2H + 3H ->4He = 17.59 MeV
2H + 3He->4He = 18.3 MeV

That means 3 deuterium-tritium reactions will yield a total energy of 52.77 MeV
3 deuterium-helium3 reactions will yield a total of 54.9 MeV
A mixture of 1 double-deuterium reaction producing tritium, 1 double-deuterium reaction producing helium3, 1 deuterium-tritium and 1 deuterium-helium3 will yield a total of 43.19 MeV.

Deuterium masses 2.014101779 grams per mole of atoms, tritium masses 3.01604927 grams per mole, and helium3 masses 3.01602931 grams per mole. A mole is 6.022 * 10^23 atoms. That means a deuterium-tritium reaction produces 21.0 * 10^23 MeV per gram. Deuterium-helium3 produces 21.9 * 10^23 MeV per gram, 21.5 * 10^23 MeV per gram.

1 erg = 6.24 * 10^5 MeV, and 10^15 ergs = 27.77778 kilowatt-hours. Thermal energy is the total energy produced, not all of which gets converted to electricity. The thermal energy of these reactions is: deuterium-tritium = 93.7 megawatt-hours per gram, deuterium-helium3 = 97.5 megawatt-hours per gram, double-deuterium = 95.8 megawatt-hours per gram.

This means the details are not quite as I stated. Deuterium-helium3 is a richer mixture than deuterium-tritium, so what I remember from high school isn?t quite right. Yup, I was studying beyond class material in high school and read about these reactions when taking grade 12 physics. I really have to keep looking things up to verify if I remember things correctly, or if there have been new discoveries. However, although the total energy produced by a double-deuterium reactor is slightly less than a deuterium-helium3 reactor, avoiding the expense of a trip to the Moon would make it worth while.

GCNRevenger · 2003-12-12 20:33:52

Yes, making a straight-deuterium fusion reactor should be the ultimate goal, but... a big but... if the temperatures required to reach sustained D-D fusion are unattainable, but D-He3 are, then He3 might be worth somthing if it looks like there will be a big "gap" between the feasability of D-D and D-He3. Otherwise, you are quite correct that He3 would be of little worth given the trouble to get it. Personally, I don't think that fission has nearly been given enough of a chance here on Earth, with the advent of HTGR and fuel recycling technology.

Back on topic... the #2 reason besides payload mass/fuel fraction of using an advanced nuclear engine is trip time: you simply don't have to spend as long in space. Let me repeat that: you don't have to spend as long in space. This is a considerably bigger advantage than seems to be given credit, that you simply don't NEED to bother with artifical gravity, or worrying about crew deconditioning, or with mental strain of living in a sardine can twenty million miles from anywhere. Plus, simply having to rely on your ship for a shorter amount of time decreases the risk of fatal failure proportionally. Earth orbit to Mars orbit in under 100 days is possible, maybe less. Your crew need not be hardend crack astronauts, and your ship is more reliable, two things MD or its children cannot hope to match versus GCNR or VASIMR.

That other advantage is also very, very easy to under-state too... that you don't have to cut your toothbrush handles off, or design everything out of Lithium alloy, or shave an extra inch off your radiation shield and whatnot when you have such a powerful engine; you can afford to make your ship strong and large, no aluminum tuna cans here. Estimates for a GCNR engine start at over SIX TIMES the Isp of LOX/LH chemical, and go up from there, maybe up to 6000sec... With that much power, you may not even need to aerobrake at all. Six months is just too long, and no chemical or <1,000sec Isp engines will improve on that.

The real thing is though, that these engines are not a distant horizon pipe dream, the VASIMR engine is nearly to prototype stage, and the concept of the GCNR is pretty well understood, there was even a little experimentation done if memory serves... With how long it will take Nasa to come up with the rest of the hardware, one of these super-engines will probably be almost ready... it took under a decade to develop the production NERVA engine with no computers, no previous nuclear experience, and without our more advanced materials of today. I doubt there will be enough reasources to go around to build/fly MD and build a GCNR/VASIMR ship at the same time... MD is close at hand technologicly, but the REAL way to get to Mars is not much futher either.

The justification for these sizeable projects is simple... you can't colonize Mars without them, even solid core nuclear rockets are not enough. Reuseability and payload mean everything, for example: launching 100 tons of cargo direct to Mars with a SDV would require 4-5 launches and don't forget the landers which cut deeply into your actual payload mass. Instead with a nuclear ion, or other high-isp reuseable tug you could ship many more tons of cargo to Mars Orbit per SDV and then ferry it down on LOX/Methane or LOX/LH reuseable landers. That is threshold technology, throwing 25 tons of hardware directly, less the lander mass, is not enough to sustain a Mars colony of any real size. Using an SDV and a tug to move 80 or 100 tons of cargo, and now you're talking...

Can you send stuff and people to Mars with chemical and solid NTR engines? Sure you can, but you can't send enough cargo or move people fast enough and safely enough to do more than plant a flag and look for bugs...

RobS · 2003-12-13 00:12:44

My feeling is that you are making a lot of assumptions, GCNRevenger. There is not that much difference between a 100 day flight and a 140 day flight in terms of deep-space duration. The latter is possible with aerobraking and existing engines. And with solar-ion or even solar-thermal to lift your cargo to almost escape (say, looping to the moon and back to Earth) with a chemical rocket to kick you the rest of the way (delta-vee 1/4 km/sec for a six month flight, I think), you get most of the advantages of speed and enough of the advantages of mass. If a SDV can put 120 tonnes into LEO, Zubrin says a solid core nuclear engine can send half of that to Mars. A solar thermal with chemical kick stage can send about 40% of the mass to Mars. Solar-ion with a chemical kick stage should be able to send about 60% of it to escape. Your VASMR or gaseous core won't do much better if you're also launching everything to such high velocities they can get to Mars in 100 days and have to fire the engine to decellerate at the other end. Sure, you could send more cargo at Hohmann speeds; but at a cost that probably exceeds two launches using ion/chemical technology (which is a proven technology; gas core engines are likely to cost really big bucks even after the really, really big developmental costs).

Sorry, I am not yet convinced of the advantages. I think Mars Direct is a lot more than the flags and footprints you think. Even with its technology, using chemical engines, one extra launch puts 27 tonnes of cargo on the surface; using solar-ion and chemical, which is easily achievable (Prometheus will develop the ion engines), the SDV could probably put 60 tonnes of cargo on Mars. Two of those would probably cost less than the gas core engine, when you include developmental costs. I don't know why a SDV with ion and chemical kick stage, putting 60 tonnes on Mars at a time, wouldn't be sufficient to build an outpost of several dozen people.

-- RobS

P.S.: I am inclined to think you overestimate the problems of creating artificial gee, too, though I am now less convinced of the need. The guys who came back from ISS over the summer had been in LEO six months, got hit by 8 gees during descent, and had to walk around outside because there was no rescue craft for a few hours. So maybe it isn't needed. But if it is, I've never heard of anyone who thought it was incredibly complicated to set up. If you are worried about a micrometeoroid cutting the cable to the counterweight, you use two or three cables. Using an expendible kick stage that costs maybe ten million bucks as a counterweight is not much of a cost issue. And if the cable gets cut, what would happen? Gravity would suddenly go away and you would have an extra ten miles an hour or so of delta vee in some random direction. Keeping your axis pointed toward the sun and Earth is done by spin-stabilized satellites all the time.

RobertDyck · 2003-12-13 14:26:40

I agree with Bill White, we can go now. Using many small launches with existing vehicles such as Proton instead of a few big ones was first proposed (I think) by RobS. Bill first proposed an experiment to test manoeuvring in tethered flight by using a Soyuz and Progress. I'm not convinced artificial gravity is necessary; nice but not necessary. Shannon Lucent walked off the Space Shuttle after 6 months on Mir, and the astronauts who returned from ISS via Soyuz after 6 months in space all demonstrate that existing exercise regimes do keep astronauts healthy long enough for the trip to Mars. As for radiation, part of Robert Zubrin's Mars Direct plan was to place sand bags filled with Mars regolith on the roof of the habitat. Now we know that hydrogen makes better radiation shielding than heavy metals, so bags of water could do the job. There is one new composite material that is supposed to be a better radiation shield; it's several sheets of polyethylene heavily impregnated with hydrogen. I could talk about life support, spacesuits, and other equipment, but the conclusion is the same: we're ready now, let's go!

Echus_Chasma · 2003-12-13 18:51:17

we're ready now, let's go!

Amen brother.

GCNRevenger · 2003-12-15 00:40:13

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.

Bill White · 2003-12-15 11:56:39

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.

Who will build this, and why?

By the way, we can do this right now as well. This would seem considerably cheaper for sending bulk supplies to a Mars colony that a nuclear powered rocket.

GCNR - do you oppose ALL tether experiments, period? Even proof of concept ideas?

GCNRevenger · 2003-12-15 14:30:58

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.

RobertDyck · 2003-12-15 16:18:40

My design for the first Mars mission included two habitats: one for transit from Earth orbit to Mars orbit, the other for the surface. The surface habitat would include just a seat for each astronaut during descent, all living space would be the inflatable. The space inflatable would include a micrometeoroid shield while the surface inflatable would not, it would have a dust/scuff/abrasion layer and sufficiently strong roof to hold regolith or water bags for radiation protection. The space inflatable would be designed for zero-G while the surface inflatable would be designed for gravity. The space habitat would have a reusable heatshield used for aerocapture at both Mars and Earth. It would be reusable.

It could use either a chemical TMI stage, which would be expended upon Trans-Mars Injection, or an advanced electric propulsion stage such as an ion engine or TAL hall thruster for solar-electric-propulsion (SEP), nuclear-electric-propulsion (NEP), or a VASIMR engine which is another form of NEP. If an advanced electric stage is used then it would be reusable, so it would remain with the space habitat all the way to Mars and back to Earth. Advanced electric propulsion would require raising the Earth orbit to almost escape velocity, then sending the astronauts to board just before they depart.

For return, use a Mars Ascent Vehicle that just carries astronauts in spacesuits and sample containers. The ascent vehicle would be the TEI stage, so it would be a single stage and clamp on, only to be jettisoned after Trans-Earth Injection. Eventually a Mars surface-to-orbit shuttle would be delivered to replace the ascent vehicle. It would deliver fuel for the advanced electric propulsion stage, so that same stage could be used to return to Earth. Fuel transfer on-orbit is an advanced technique that I would not use for the first mission, but the interplanetary vehicle could be built to support it. That Mars shuttle would also be used to carry down supplies from orbit, as well as astronauts up and down. Initially we want to build the base, so each lander would be a new habitat to expand the base.

This architecture is intended to avoid dropping onto Mars surface any mass that has to return to Earth. The MAV would use ISPP for chemical propulsion using Robert Zubrin?s technique for methane/LOX. Once a reliable method to harvest water from permafrost is developed, we could use LH2/LOX instead and not transport any hydrogen from Earth.

Radiation: Robert Zubrin?s idea is to store supplies in cabinets around a central radiation shelter. Your water and food is your radiation shield. You only need that shield if an unusual even occurs, such as a solar flare.

Artificial gravity: don?t. Use zero-G exercise machines enroute to Mars.

Getting back to Earth: Have an OSP rendezvous with the interplanetary vehicle in Earth orbit and carry the astronauts and supplies back to Earth. As emergency escape pods, carry 2 Soyuz descent modules. If they aren?t used they stay in space for the next trip to Mars.

This mission plan starts with entirely chemical propulsion, so it can be used now. It permits replacing propulsion modules as they become available. The interplanetary habitat is reusable immediately, and its propulsion module is reusable once developed. Strictly speaking, the Earth return vehicle is the Orbital Space Plane so it is not only reusable, it is the same vehicle shared with ISS.

Increasing speed to where aerocapture no longer works requires almost doubling the delta-V. That would require a lot more fuel. You would require an extremely efficient engine, not just a little better. I don?t think even VASIMR or a solid core NTR without aerocapture could achieve the total launch mass of chemical propulsion with aerocapture. VASIMR or a high-power ion engine combined with aerocapture would be ideal. Could a Gas-Core Nuclear Rocket do better? Perhaps, but no one has built even the first prototype so we don?t know if it has sufficient Isp to compensate for loss of aerocapture.

Let?s stuck to technology for which at least proof-of-concept hardware has been built.

atitarev · 2003-12-15 20:46:19

Haven't read the full thread but I think I should mention that record durations in weightlessness set by Russian cosmonauts exceed a year, not just 6 months. I don't remember exactly but I think Romanenko (?) spent from 13 to 18 months on Mir and came back safe and sound, only a little bit week. If someone needs exact info, I'll check later. He had no problems with health, only needed some adaptation to the normal G.

With specially designed exercises people should be able to get to Mars and be productive within a very short period. Even after a long plane flight people need rest. (When I flew from Moscow to Melbourne I had to sleep it off too). If the Mars mission takes place, it shouldn't be for a couple weeks stay, so the astronauts will have enough time to adapt.

Gennaro · 2003-12-15 23:21:06

Questions:

Radiation: Robert Zubrin?s idea is to store supplies in cabinets around a central radiation shelter. Your water and food is your radiation shield. You only need that shield if an unusual even occurs, such as a solar flare.

- I'm no expert and therefore I might as well ask: is it really safe consuming heavily radiated food and water?

Artificial gravity: don?t. Use zero-G exercise machines enroute to Mars.

- It seems artificial gravity suddenly has fallen out of fashion around here. But a sweaty gym in a tuna can? Just don't like the idea. Especially not using the same gritty place for the return trip. In relation to MD specifically, is it really so hard cutting a cable and regaining a stable projectory well before entering Mars injection, using little rocket puffs or something? We have ultra super duper computers but we can't work this out? If you want minimized coriolis, just extend the cable(s), give it a gentler swing, and if all else fails, give the crew some cyanide pills. Nothing is without risk.
Remember, there is no refuge on a Mars mission. You cannot just drop the pressure suit and go out to lose the smog and see the sun.
Don't make it harder than it has to be. At least, couldn't we actually test an arty grav system in orbit before going ahead ditching the concept?

Me thinks a fundamental obstacle exists in trying to press a technological culture, developed mainly to serve science goals, into serving human and settlement needs. If I created a Mars mission, I would begin by investigating three points: how to construct comfortable artificial g, how to deal with radiation protection and how to create the propulsion system to carry both.
It's amazing that after 50 years of the world's space programs, we have no straight answer to these questions.

RobertDyck · 2003-12-16 10:38:01

Yes, Russian cosmonauts have a longer duration record than American astronauts, but I noticed most Americans who post on this board like to cite American astronauts as examples, so I did as well to get the point across. And, sure, artificial gravity would be nice. However, I also stated that principle of excluding technology for which there hasn't been at least proof-of-concept hardware demonstrated. Manoeuvring in tethered flight hasn't been demonstrated yet, and there are no plans to do so. Sure I would like to see such a demonstration, but it hasn't been done yet. If I had my 'druthers I would also like to see development of a mini-magnetosphere into a radiation deflection system. That would require a nuclear reactor to power it, so obviously I would also like to see a space nuclear reactor. But none of this has been done yet, which is why I excluded it from the plan I described.

Bill White · 2003-12-16 10:55:14

I agree with Bill White, we can go now. Using many small launches with existing vehicles such as Proton instead of a few big ones was first proposed (I think) by RobS. Bill first proposed an experiment to test manoeuvring in tethered flight by using a Soyuz and Progress. I'm not convinced artificial gravity is necessary; nice but not necessary. Shannon Lucent walked off the Space Shuttle after 6 months on Mir, and the astronauts who returned from ISS via Soyuz after 6 months in space all demonstrate that existing exercise regimes do keep astronauts healthy long enough for the trip to Mars. As for radiation, part of Robert Zubrin's Mars Direct plan was to place sand bags filled with Mars regolith on the roof of the habitat. Now we know that hydrogen makes better radiation shielding than heavy metals, so bags of water could do the job. There is one new composite material that is supposed to be a better radiation shield; it's several sheets of polyethylene heavily impregnated with hydrogen. I could talk about life support, spacesuits, and other equipment, but the conclusion is the same: we're ready now, let's go!

I am genuinely worried that a MarsDirect might prove a once only mission, without any commitment to a follow on settlement. Yet this issue is one of political will not technology and a "been there done that" attitude might well apply to any first humans to Mars endeavor whether nuclear propelled or not.

Space rated reactors are essential and nuclear propulsion would be very, very useful, however, I see the call to "wait" for fancy new nuclear propulsion merely as an excuse to justify not doing MarsDirect now. A tactic to deflect criticism of NASA.

How to solve a potential "flags and footprints" dead end? This issue is not technological, its political, and therefore needs a political solution. One idea, perhaps, would be to start a First Steps endowment fund to facilitate discussion of planning for the first human birth on Mars.

IMHO - - a spacefaring species is one which can conceive, bear and raise its young at more than one celestial location. Nothing more and nothing less.

RobS · 2003-12-16 16:20:32

In the novel I have written based on Mars-24, called Mars Frontier (which Adrian started putting on the website in August, but we haven't heard from him since!) I assumed an interplanetary habitat that was capsule shaped, with a heat shield on the blunt bottom end that was six meters across, a vehicle that was four stories and 12 meters long. The interplanetary habitat could be permanently docked either to the Mars shuttle (which was similar in shape and a few meters longer, but greater in mass) or to another interplanetary habitat. The combination needed to be spun about 4 to 5 rotations per minute to produce Martian gravity. I don't remember which rate. This is at the high end of what is recommended, because of Coriolis. But my guess is that the vast majority of people are very adaptable and can adjust to it. After all, sailors have dealt with far worse for thousands of years; they get seasick for a few days or a week, then adjust to constant pitching and motion. I suspect a small wooden boat in a week-long hurricane, or a small wooden boat crossing the Atlantic for three months with two dozen hungry sailors, is far more unpleasant than a spinning habitat for four to six months! I suppose it is possible there is something in this arrangement that the vast majority of people can't adapt to, but the evidence feels to me to be stacked to the contrary.

Regarding the momentum needed for a rotating vehicle, it is not very much; the rotation velocity is just ten or twenty miles per hour. If the cable were to break cleanly, the vehicle would not be "thrown" very fast or very far. You can figure this out pretty easily. Take the example above of two interplanetary habitats rotating at 4 revolutions per minute. Each revolution sweeps through a circumference equal to the diameter (12 meters times 2 =24) times pi or 75 meters. In one minute it does this four times, or 300 meters per minute, which is 5 meters per second. That equals 18 kilometers per hour or almost 12 miles per hour. If you want less Coriolis you make the spin arm much longer and you do have to move at a faster speed, but still less than 100 miles per hour.

The big problem with spinning two docked vehicles is that the docking collar has to be pretty strong; each interplanetary habitat masses about 20 tonnes, which in Martian gravity weighs 8 tonnes. So the docking collar has to be anchored into the structure of each vehicle and able to hold them together against that sort of pull. That is not a complicated engineering problem, though it may require additional mass.

I hope they do some studies in low earth orbit on rotation to make gravity. It is a shame they haven't done it yet.

-- RobS

RobS · 2003-12-16 16:31:47

Regarding the issue of gas-core and VASMR "versus" tethers and aerobraking. I wonder about this:

GCNRevenger says: "The engineering required to reliably mate with the tether end is not practical, and would require accuracy and precision beyond anything concieved. . . . . I remain a little skeptical of aerobraking even, given the very percise attitude control required and the nonconstant thickness of Martian atmosphere."

But surely, GCNRevenger, don't you think that gas-core nuclear has far more serious technical obstacles than these? It is easy to understand the idea that mating a payload carried by a hypersonic vehicle to a rapidly moving tether would be complicated. But it seems to me there has been more thought devoted to this problem, and it is probably easier to solve, than the problems of containing with magnetic fields a spinning vortex of gaseous uranium that will vaporize the thrust chamber in seconds while superheated hydrogen rips past it at 100,000 kilometers per hour? This is not to say that gas-core is impossible. It probably is possible. But I would need to be convinced that it is simpler than a tether! Certainly aerobraking, which has been used already, is a demonstrated technology.

-- RobS

atitarev · 2003-12-16 18:00:41

My apologies. Yury Romanenko spent 326 days in space. So it was less than a year. (Read this.) Still it's a long time and is about the average time needed to get to Mars.

I agree with Bill White. Let's concentrate on the exercises, maybe special vitamins. I can see the tethered spacecrafts only in the far future when we fly to the Jovian moons or farther out or maybe by the time there is a human mission there, we will have faster engines.

rpcyan · 2003-12-16 21:57:43

MD is far from a "flag and footpints" missions. 18 months of surface time! To prevent a Mars mission from suffering the same fate as Apollo isn't a technological one, its political. I think the best way to keep the momentum going is to make it an international program. Then, if you stop, the whole world is angry with you, and you look weak. That can be a very strong political motivator - who could imagine the U.S. talking about returning to the moon if China didn't mention it?

There's a lot of ways to improve upon MD with existing technology - remember MD was written in the early 90s. Concepts like not putting anything on the surface you don't have you and not sending anything to Mars you don't have to save you a lot in the long run. I particularly like the idea of using the OSP for Earth reentry - why send an Earth reentry system all the way to Mars and back?

We also need to consider not providing return trips. Settlement, on a managable scale, needs to occur before the science, otherwise it cannot be sustained. You need a substantial amount of mass to send all this gear to Mars just for a return trip - the dru mass of the ERV by Zubrin's conservative numbers is 20 tons (excluding things used for surface operations). Imagine the supplies and infrastructure that could provide. For example, it could provide the hardware to construct a dome used to grow food in a CO2 rich environment. It could provide a few pressurized rovers.

We can go now, but do we have to return? Having one-way trips accelerates everything - settlement schedule, science capability, biological studies, the first children born on Mars. I see little reason to return crews after the first mission.

Josh Cryer · 2003-12-17 12:57:00

rpcyan, totally agree with you there! Even if it means having to ship shipments of food/supplies every year or something (mind you, a food only shipment could last a few people many years)! Sure, self-suffiency is really great and all that, and I've always wanted something along those lines (robots, or other kinds of machines which can help us build our colonies), but that requires technology we don't have just yet (which I say is increasingly within our reach though). We can definitely go now for a relatively low cost, and we can stay there for an appreciable amount of time; indeed, enough time perhaps to actually develop the very technologies that will push along colonization technologies (Zubrin's theory!).

The main problem is public interest, though. Always has been, always will be.

Rxke · 2003-12-17 13:27:36

One way... Not going to happen unless it's a crew of at least like 50 people... imagine a 6-10 crew getting old... Who's gonna care for them?
Of course if you'd launch 10-20 people every 2 years, things would change.

Me, i'd love to live AND die on Mars, ever since being a child. Even with low-cost mass transport in a far off future, they'd never allow me, for that would be considered 'nuts' on their psy profiles

... but could they find 'normal, stable' people willing to go one-way today? A lot of people would see it as a kind of suicide misin (it is not, of course...)

Bill White · 2003-12-17 16:48:08

I agree with Bill White, we can go now. Using many small launches with existing vehicles such as Proton instead of a few big ones was first proposed (I think) by RobS. Bill first proposed an experiment to test manoeuvring in tethered flight by using a Soyuz and Progress. I'm not convinced artificial gravity is necessary; nice but not necessary. Shannon Lucent walked off the Space Shuttle after 6 months on Mir, and the astronauts who returned from ISS via Soyuz after 6 months in space all demonstrate that existing exercise regimes do keep astronauts healthy long enough for the trip to Mars. As for radiation, part of Robert Zubrin's Mars Direct plan was to place sand bags filled with Mars regolith on the roof of the habitat. Now we know that hydrogen makes better radiation shielding than heavy metals, so bags of water could do the job. There is one new composite material that is supposed to be a better radiation shield; it's several sheets of polyethylene heavily impregnated with hydrogen. I could talk about life support, spacesuits, and other equipment, but the conclusion is the same: we're ready now, let's go!

While going "google" on Robert's embedded link I found this:

http://www.space.com/science....16.html

Rad shield Mars bricks.

Way cool!

Enyo · 2003-12-17 18:18:39

MD is far from a "flag and footpints" missions. 18 months of surface time! To prevent a Mars mission from suffering the same fate as Apollo isn't a technological one, its political. I think the best way to keep the momentum going is to make it an international program. Then, if you stop, the whole world is angry with you, and you look weak. That can be a very strong political motivator - who could imagine the U.S. talking about returning to the moon if China didn't mention it?

This is very true. The thing people seem willing to forget is Apollo created an infrastructure capable of may things beyond sending people to the Moon. Yet we abandoned it! This for the promise of completely reusability system which would cost less and what we got was the STS. Politics will change anything designed to something less functional. We must work against this trend and educate people so they also fight it.

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