Elon Musk says he will put millions of people on Mars.

[GIThruster]

Well, I'm not especially unfavorable toward the idea of cracking water in space and using it for propellant, for either chemical or nuclear rocket, but I do think it's silly to talk about these ultra-huge structures that have to have thrusters attached and pretend they don't endure serious stress. If you're going to have propellants onboard, maybe the simplest thing is have many thousands of tiny Hall thrusters, but certainly you'll need thrusters. Stuff doesn't just stay put in space. Especially with a very large surface area, the thing is certainly going to act like a solar sail.

I watched a vid the other day where Musk was asked about using nuclear rockets because he had discounted the idea and those in his audience started arguing that is was a viable option. He backed off his position and agreed to look at the issue more carefully in the future.

If you use steam for a nuclear rocket, you will likely lose your ultra-high Isp, and any advantage to going nuclear, but I'm not certain of that. One of the nice things about TRITON is you have the energy density of both the Nuke and the propellants.

[ladajo]

GIT,
I agree with your point about large surface area to low mass structures, as it is a known phenomena. I do also think that if you use a bunch of smaller ones and help limit the sizes, it would reduce the problem. Again, I accept that this introduces more complexity to raw material and product logistics.

[kunkmiester]

http://www.strout.net/info/science/delta-v/
Interesting math widget thing. Be nice if you could go backwards and find fuel from delta V though.

Moon mission needs at least 8200 m/s? Apollo stack was pretty small, but they weren't bringing everything back with them, so the return trip was less energetic.

If you're looking at bringing the whole thing back though, it seems much harder. It might be best for the first mission to leave a bigelow in lunar orbit with the lander, even make that a separate launch, and do the same with the Dragon for the crew--just the transfer vehicle and crew and fuel move. This would much reduce the delta v needed for each mission.

Separating into three launches gives other advantages--two landers could be launched, providing greater flexibility and reliability at the moon end, and I suppose the Dragon could even be docked at the ISS for various reasons you'd want to do that.

[KitemanSA]

Well, I'm not especially unfavorable toward the idea of cracking water in space and using it for propellant, for either chemical or nuclear rocket, but I do think it's silly to talk about these ultra-huge structures that have to have thrusters attached and pretend they don't endure serious stress. If you're going to have propellants onboard, maybe the simplest thing is have many thousands of tiny Hall thrusters, but certainly you'll need thrusters. Stuff doesn't just stay put in space. Especially with a very large surface area, the thing is certainly going to act like a solar sail.

You are totally out of it. NOONE has been talking about putting thrusters onto the mirrors (other than minute station keeping type things, and those may indeed be thousands of hall thrusters). The cracking is to produce propellant for a Mars transfer rocket. Conceptually, very large very light static mirror, mobile receiver/cracker unit, propellant transfer tanks.

But again, with Nuke rockets, the cracking stage may not be necessary.

[williatw]

watched a vid the other day where Musk was asked about using nuclear rockets because he had discounted the idea and those in his audience started arguing that is was a viable option. He backed off his position and agreed to look at the issue more carefully in the future. If you use steam for a nuclear rocket, you will likely lose your ultra-high Isp, and any advantage to going nuclear, but I'm not certain of that. One of the nice things about TRITON is you have the energy density of both the Nuke and the propellants.

You wouldn't happen to have the link handy would you? Would love to see Musk get on board with NTR like the TRITON. Like you I love the idea of it being "trimodal", though I think if your going to mars you want the extra specific impulse of using H2 instead of H2O. If I recall the exhaust temperature of a solid core NTR is actually lower than a H2/LOX chemical rocket. The reason the SI is higher (800-1000) vs 450 for a chemical rocket is because the exhaust particle is lighter. For a chemical rocket it is obviously H20 molecular wt. 18 vs H2(2). The lighter particle is more energetic, therefore higher exhaust velocity even though the temp. is lower. Obviously therefore if you used water through the NTR the specific Impulse would likely be lower than for a LOX/H2 chemical rocket.

[Aero]

watched a vid the other day where Musk was asked about using nuclear rockets because he had discounted the idea and those in his audience started arguing that is was a viable option. He backed off his position and agreed to look at the issue more carefully in the future. If you use steam for a nuclear rocket, you will likely lose your ultra-high Isp, and any advantage to going nuclear, but I'm not certain of that. One of the nice things about TRITON is you have the energy density of both the Nuke and the propellants.

You wouldn't happen to have the link handy would you? Would love to see Musk get on board with NTR like the TRITON. Like you I love the idea of it being "trimodal", though I think if your going to mars you want the extra specific impulse of using H2 instead of H2O. If I recall the exhaust temperature of a solid core NTR is actually lower than a H2/LOX chemical rocket. The reason the SI is higher (800-1000) vs 450 for a chemical rocket is because the exhaust particle is lighter.

That is reasonably accurate.

For a chemical rocket it is obviously H20 molecular wt. 18 vs H2(2).

You're overlooking dissociation here. At LOX/LH2 engine temperatures, (SSME is a prime example), the molecular wt. becomes (O + H + H)/3 or about 6. (Ignoring various mixture ratios.) About, because even the molecules start to loose electrons at those temperatures. Same holds for H2, but I've never looked at H2 at NTR temperatures so I don't know to what extent of dissociation occurs. I speculate that it is like (H + H)/2, or 1.

The lighter particle is more energetic, therefore higher exhaust velocity even though the temp. is lower.

Still reasonably accurate using my numbers.

Obviously therefore if you used water through the NTR the specific Impulse would likely be lower than for a LOX/O2 chemical rocket.

I think you intended to write LOX/LH2.
Your conclusion is not so obvious unless you plan to use an engine designed to use hydrogen as the reaction mass, simply by filling the hydrogen tank with water. But nobody would do that. A nuclear engine using water as the reaction mass would be designed to take advantage of the characteristics of water. Its ease of pumping to very high pressure and its very good heat capacity for cooling the engine, not to mention its much higher density allows much smaller fuel tanks for the same amount of reaction mass, and the apparent wide spread available of water or ice throughout the solar system.

Of course there are difficulties in making such and engine compete with the efficency of an LOX/LH2 engine. You need to bring the temperature of the heating chamber up substantially to dissociate water and so create those lighter, more energetic exhaust particles. Can it be done? I don't know that it has ever been seriously studied.

[Skipjack]

There was some powerpoint presentation of SpaceX that showed NTR propulsion and nuclear electric as well, IIRC.
I think nuclear is a must, if you want to build a sustainable transport architecture for mars, even for the moon.

[Skipjack]

Btw, this obsession with a small difference in Isp for an in space propulsion system is somewhat odd. If transport to LEO is cheap enough that the added mass for the extra fuel and tanks, which both by themselves are cheap is easily covered, then why bother with it? The lower Isp is usually compensated a little bit by higher thrust. If you want to go extreme Isp, you probably have to go electric anyway. There is still room for research and improvements there. Just look at the work Slough is doing with ELF thrusters!
Fact is that cheaper access to orbit beats larger payloads to orbit in terms of cost, if you are able to adjust your mission profile accordingly.

[HopDavid]

Station keeping is trivial. You don't.

There will be perturbations. Satellite sans station keeping is a fantasy.

I suspect tidal forces may keep it aligned axially.

In LEO, by far the dominant tidal forces are earth's. Thank you for demonstrating the axis of the paraboloid will almost never be aligned towards the sun.

Even unfurling it is trivial, you spin it and the unfurl is after the spin is begun.

Not demonstrated in a 40 acre paraboloid.

HopDavid long ago showed he passed the point where skepticism was productive, and instead is merely a sabot pitching pathology

Space advocacy groups have gained a reputation for being short on math and rigor and long on wishful thinking. And rightly so.

I am trying to prod people to do more cites and math.

And yes, hydrogen can be split effectively from water by heat and reactive surfaces, it is for example done in every biogas system on the planet, the feedstocks never being perfectly dry.

For the umpteenth time, what is the mass of this system? How many thermal joules does it take to crack a kg of water?

[HopDavid]

You are totally out of it.

More math and cites, less ad hominem, please.

NOONE has been talking about putting thrusters onto the mirrors

Wrong. I HAVE been talking about putting thrusters on the mirrors.

Without them, station keeping is not possible.

As TDPerk mentioned, earth's tidal influence would probably tend to pull the long axis of the mirror towards earth's center. (the long axis would more likely be through the mirror's diameter rather than through the paraboloid's axis.) But in either case, the mirror wouldn't be aligned towards the sun.

There are other influences but that alone is more than sufficient to demonstrate a mirror sans station keeping is utter ridiculous.

(other than minute station keeping type things, and those may indeed be thousands of hall thrusters).

What's the mass of these thousands of hall thrusters?

[hanelyp]

At LOX/LH2 engine temperatures, (SSME is a prime example), the molecular wt. becomes (O + H + H)/3 or about 6.

Since the energy for a LOX/LH2 rocket comes from the combustion of the propellant, any dissociation reduces the thermal energy. There is some dissociation at the combustion temperatures of H2/O2, but nowhere near 100%.

[hanelyp]

This talk of thermal cracking of water into H2/O2 has me thinking of solar thermal rockets. Unlikely to get the high thrust of chemical combustion, but exhaust velocity is limited chiefly by thermal issues in the thrust chamber. Add some radiant heating inside the expansion nozzle and a fair margin over combustion exhaust velocity should be possible.

[GIThruster]

watched a vid the other day where Musk was asked about using nuclear rockets because he had discounted the idea and those in his audience started arguing that is was a viable option. He backed off his position and agreed to look at the issue more carefully in the future. If you use steam for a nuclear rocket, you will likely lose your ultra-high Isp, and any advantage to going nuclear, but I'm not certain of that. One of the nice things about TRITON is you have the energy density of both the Nuke and the propellants.

You wouldn't happen to have the link handy would you? Would love to see Musk get on board with NTR like the TRITON. Like you I love the idea of it being "trimodal", though I think if your going to mars you want the extra specific impulse of using H2 instead of H2O. If I recall the exhaust temperature of a solid core NTR is actually lower than a H2/LOX chemical rocket. The reason the SI is higher (800-1000) vs 450 for a chemical rocket is because the exhaust particle is lighter. For a chemical rocket it is obviously H20 molecular wt. 18 vs H2(2). The lighter particle is more energetic, therefore higher exhaust velocity even though the temp. is lower. Obviously therefore if you used water through the NTR the specific Impulse would likely be lower than for a LOX/H2 chemical rocket.

http://www.youtube.com/watch?v=PGSbQapl ... re=related

I'd recommend watching all 4 parts, but here's the one I was thinking about. I think it demonstrates that at least in public settings, Elon is a remarkably sensible and open guy.

[KitemanSA]

You are totally out of it.

More math and cites, less ad hominem, please.

Sorry. It wasn't intended as a personal attack so much as a description of the arguement. His argument is disconnected = he is out of it.

NOONE has been talking about putting thrusters onto the mirrors

Wrong. I HAVE been talking about putting thrusters on the mirrors.

Without them, station keeping is not possible.

That is what I said in the completion of the sentance. Duhh.

As TDPerk mentioned, earth's tidal influence would probably tend to pull the long axis of the mirror towards earth's center. (the long axis would more likely be through the mirror's diameter rather than through the paraboloid's axis.) But in either case, the mirror wouldn't be aligned towards the sun.

Which is why I recommended parking it on L4 or L5.

There are other influences but that alone is more than sufficient to demonstrate a mirror sans station keeping is utter ridiculous.

From the excized portion of my sentance above, it is obvious that I have not been assuming san-station keeping. I just know enough about cis-Lunar/circum Lunar space to have a decent idea where to put things.

(other than minute station keeping type things, and those may indeed be thousands of hall thrusters).

What's the mass of these thousands of hall thrusters?

Oh, you DID read it. So why the disconnection above? But to answer your question... obviously less than the mass of fewer larger thrusters of equivalent capability or someone has made a design error.

Personally, I would envision a pair of magnetically coupled counter rotating rings with a pair of film sheets (CP-1?) "stretched" between them. The films would be sealed together around the edges with one metalized to create a mirror. Slight electro-static charges would press the mirrors apart and the degree of charge/rotation would define the curvature. Alternatively, a slight amount of gas introduced between the sheets might also do the trick. Place them on L4 or L5 (or both) with an appropriate degree of rotation about the axis of symmetry, and a MUCH smaller degree of rotation about the solar north polar axis, and "viola".

But this only applies if someone INSISTS on chemical rockets. Not my prefered method, but if you MUST...

Oh, and by the way, I suspect it would be easier and cheaper with acceptable losses to just store the fuels cryogenically. But if not, and you must...

[HopDavid]

"Where did you intend to get the water?"

For one, the moon has lots and in a shallow well.

I agree.

But some thread history is in order:



If you want to argue lunar propellant can aid Mars missions, fine. But you're missing my point.

The point of my arguments is that the moon is a more plausible near term goal.