Speculative - cislunar transport infrastructure, 2054

Ahh, now your assumptions are becoming clear. That vehicle has an acceleration on the order of 1/1000 of a G. The paper mentions taking several days just to spiral up from LEO to escape Earth's gravity. Clearly not good for lunar transport. If they could get the acceleration up to 1/10 G, it could get to the Moon in half a day. And at 1 G the trip time is only 3 1/2 hours. Clearly that performance is far beyond the proposed vehicle.

OK, that is clear. The practical limit there is how many Gees the passengers want to experience. Accelerating half way at 1 G and decelerating half way at 1 G, the trip to the Moon takes about 3 1/2 hours and the top speed at the half way point would be about 63 km/sec. The other practical limit is, does a powerful enough engine exist for such acceleration. Accelerations on the order of 1 G are far, far beyond what Centrillium is considering. I don't think stresses or precision would be any problem at all until much higher accelerations.

Chemical rockets? Fusion? I say go with good old fission!

For the LEO to moon section, a simple system, (and achievable with today's technology), would be nuclear fission rockets using water as the reaction mass.

The water would be extracted from ice or rock on the moon, and/or near earth asteroids, much cheaper (in deltaV) than bringing it up from the Earth. The power and equipment needed to extract water is orders of magnitude less than that needed to electrolize it into Hydrogen & Oxygen, then liquify each gas.

These rockets would not have huge specific impulses, but you don't need that for a moon to LEO / LEO to moon ferry. The same rockets could also move water to LEO, Lagrange points, or even Mars orbit for refueling of other fission/water rockets. Slow, unmanned trips using the inter-planetary super-highway, would maximize cargo payloads.

Where you do need higher Isp, then you can breakdown the water to (2)H2 & O2, for chemical or nuclear/H2 rockets.

In that case ordinary LH2/LOX engines of today would be the way to go. No sense in dealing with a reactor if you can just burn the stuff back into water for comparable ISP. If LH2 is available on the Moon, then the energy requirements are pretty evenly split between an Earth based shuttle to LEO and a Moon based shuttle from the lunar surface to LEO and back. DeltaV from the Earth to LEO is 9.7 km/s and the total DeltaV from the lunar surface to LEO and back is 11 km/s, assuming aerobraking into LEO. (EDIT) Oops, with the aerobraking it is only 7.8. It would be 11 with no aerobraking. (/EDIT) It would be like the depiction in "2001, a Space Odyssey" where the winged shuttle transfers passengers to the Moon ship at a space station in Earth orbit. The energy requirements could be more evenly split (EDIT assuming no aerobraking /EDIT) by doing the transfer in a higher Earth orbit, but that would be in the Van Allen belts. By the way, the pictures of the space station in 2001 do make it look like a very high orbit, not like the ISS at all. (EDIT, picture added /EDIT)


Now, if you use only LH2 in the nuclear thermal engine, the higher ISP might make it worth the trouble to deal with a reactor. I say might because we don't have any real operational experience with nuclear thermal rockets, especially ones that have to be stopped and started several times, throttled deeply (for landing on the Moon), and reused for many flights. (The lunar vehicle will be reusable, not expendable, right?)

True, Peter, and LH2/LOX drives don't have the political connotations of a nuclear rocket.

Unlike Orion, an NSWR could give you steady one-g thrust for hours. It would be hot though--in a lot of ways. But it would also be very simple.

Nukes are simply not an option. They're simply not feasible from the viewpoint of politics or public opinion. Technically, they're great. But you have to remember the NIMBY factor: Not In My Back Yard.

Shoot, attaching a fission reactor to a fusion drive as a power source is touch-and-go, at best.

Depends on whose public get to have a opinion. I don't think the Chinese public opinion will worry their government over-much.

Besides, a fission reactor that never comes closer to Earth than high LEO, is not as big an issue. Even better if you can eventually mine & enrich the uranium on an asteroid. ( Does the moon have any extractable uranium? )

The advantage of not processing water into LH2 & LOX, is HUGELY less infra-structure & power use on the moon, or anywhere else you want to refuel. Go to Saturn, refuel in the rings using equipment you take with you, and a little power from a secondary generator attached to your drive pile.

[Edit] Ok, yes enrichment facilities would need much more infra-structure, than LH2 & LOX plants, but you would only need one off-planet, and on an asteroid, environmental pollution wouldn't matter.

It is to the anti-nuclear activists. The protested Cassini because of the Earth flyby, not only the launch.

The only reason I said LH2/LOX was just as good as NTR is that you were talking about a water NTR which would not have a very high ISP. A LH2 NTR or SWNR is a totally different story.

There is a variant of the NTR that hasn't been mentioned here yet but which could certainly be worth exploring. It's still a paper project, but it seems to be possible to boost a LH2 NTR by injecting LOX into the NTR's expansion nozzle. Going that way gives you three modes of operation:

1) drive off / reactor provides electrical power only
2) drive on / hydrogen reaction mass only
3) drive on / oxygen augmentation

This has the added advantage that if you size maximum thrust requirements to the augmented mode, you can make your reactor smaller and/or improve safety. Overall ship size should also go down because wings etc don't have to be as big.

The disadvantage is that you have to lug (heavy) LOX along.

Astronautix and Nuclearspace have articles on the principle.

Cheers,
ErikM

The anti-nuclear activists oppose even the small RTGs on Cassini and the recently launched Pluto mission. They will not sit still for any quantity of radioactive material on a space vehicle, no matter how safe.

As to the paper only study nature of that engine, that applies to all of the nuclear engines, except NERVA.

Sorry for taking a while to get back on this thread.



If they were rational, they'd have less of a problem with nuclear reactors than RTGs. RTGs are essentially blocks of highly radioactive plutonium, which really is quite nasty stuff (although I've no problem with RTG-powered space probes). A fission reactor would launch into space containing nothing nastier than enriched uranium, which is actually less radioactive than the uranium ore that's mined out of the ground. It would only start to become dangerously radioactive once it was activated, which would take place in a high orbit.

Of course, these people aren't rational...



Yes - you've basically just described Chang-Diaz's VASIMR concept. Doing this will not allow higher accelerations, however. The acceleration of a fusion driven ship is limited by the specific power of the combined drive system, which is mainly limited by cooling. More power means more radiator mass, so it really isn't possible for such a ship to accelerate quickly. Chemical and nuclear thermal rockets are able to do without radiators because the propellant carries most of the waste heat off into space, but this approach really limits your Isp. In short - the high thrust, high impulse 'fusion torch' of science fiction would be almost impossible to engineer in reality.

Oh - last time I posted, I didn't thank nacnud for his links to the MXER page. That looks really cool. I need to teach myself about orbital mechanics so that I can really understand how it works, but from what I've read, I should think that these would certainly be preferred to rockets for regular journies to the Moon or Mars.

These kinds of tether schemes are much more credible than things like the space-elevator.

But Cassinin still flew. Anything will be protested, but only radio-active exhaust released into the atmosphere should be impossible.

[wish] Wouldn't it be great if we could build just one really big Orion? Put a hundred thousand tonnes of cargo into LEO in one launch. Then send the ship itself on a grand tour of the Solar System ( 300+ km/s delta-V ) Refuel it every year or 2 with a couple of HLLV launches. I call this concept 'One Time Orion' (Interestingly, it would only increase the radio-activity in the Earth's atmosphere by about 1%) [/wish]


True, at 1100 Kelvin (about the limit for non-corrosive steam) Isp is around 200. If you could find better materials, that might be improved, but it's not essential. 200 Isp is workable for a fairly simple and reliable engine and a relatively high density propellant. A mass ratio of up to 12 (91% propellant) should be achievable, giving 5000 m/s delta-V.

The key is to produce the propellant easily and in quantity on the moon, or asteroid, or wherever, and move it to refueling points in lunar orbit, then Lagrange points and LEO, using unmanned water NTR rockets in low energy transfer orbits. Then every trip is between fuel depots, less than 5000 km/s apart. A lot would be less than 3000, allowing much better mass ratios.

Is Cassini using radioactive Uranium? Or is it using Plutonium and is Plutonium produced out of enriched Uranium?

My question i: Is it based on the same process which is subject to the debate with Iran currently? If yes what then about enrichtment of Uranium in space, at the moon or the like?

This is valid for Cassini-kind spacecrafts only but not for Orion or Daedalus of course.



Dipül.-Volökswirt (bdvb) Augustin (Political Economist)