shuttle to the moon?

[ka9q]

Nitrogen's a bit harder to come by than carbon.

Good point.

Perhaps you could scoop it from Earth's atmosphere

How practical is it to 'scoop' an atmospheric gas during an aerobraking pass? I'm also thinking of all that hydrogen at Jupiter. That would be a much faster pass than earth, though.

I think reactors are better put to stationary uses doing things like producing propellant.

What's wrong with nuclear thermal propulsion? You see how much bigger the I_sp figures are. And they can be increased further if we can figure out how to run at higher temperatures. Chemical propellants, otoh, have characteristic I_sps that are upper limits. You can't improve them without changing them or adding extra external energy.

Nuclear reactors on the lunar surface, though necessary, will need some pretty big heat sinks. There are no rivers or oceans for cooling on the moon, so large radiators will be required. How large depends on the required temperature at the radiator outlet as well as the rejected heat power.

The neat thing about nuclear thermal propulsion is that while it's a heat engine, it doesn't require a radiator -- the exhaust itself carries away the waste heat. The artist sketches I saw for the Jupiter Icy Moons Orbiter showed some very large radiators made necessary by the use of nuclear-electric power for its ion engines. Ion propulsion is appropriate for that mission because of the very high I_sp required and the long mission times available, but it does demonstrate a major difference between thermal and electric propulsion.

[raven]

I would think it would still need radiators to a degree as the reactor can not be completely turned off, I think, and you are not thrusting all the time.

[ka9q]

Point taken. It would depend on the design of the reactor -- how much it could radiate by itself, how high a temperature it could withstand, and the decay heat curve. As I understand it, a typical thermal reactor in steady-state operation produces about 6.5% of its heat from the decay of fission products. This drops rapidly after shutdown: to 1.5% after 1 hour, 0.4% after a day, and 0.2% after a week. One might simply ramp down the power instead of shutting down abruptly, if this could be done with efficient use of propellant and without poisoning the reactor from xenon buildup.

Anybody heard of a 'fission product rocket'? It's an intriguing idea; the fission products are your propellant, accelerated to extremely high velocity by mutual electrostatic repulsion as the fissile nucleus flies apart. (Most of the energy in fission is released this way.) You use very thin fuel layers so most escape the fuel, and magnetic fields guide them out the back. (The fission products are positively charged ions.)

I_sp of 100,000 to 1,000,000 s (!!) are supposedly possible without any major show-stoppers, but the analysis is pretty preliminary.

Obviously this is something you wouldn't run anywhere near the earth, but I don't see why it couldn't be used on an interplanetary mission. Or even an interstellar one; one could achieve 10% of c and make it to Alpha Centauri in a century...

[Bob B.]

Nitrogen's a bit harder to come by than carbon.

Just to clarify something … the compounds on my list in post #42 are those that I took a look at a couple years ago when I did some studies on NTR propulsion.  I’m not proposing that any of these are available on the Moon, though some may be.

Sooting could be solved by the addition of oxygen. This could be a brief procedure done while shutting down to prevent accumulation or burn off any deposits that have formed, as well as something done when higher thrust is needed. Lift off on LOX-augmented CH4, burn to your desired orbit on plain CH4, and use a brief shot of LOX on shutdown to clean the engine out.

Interesting.  Thanks.

I favor just using LOX/CH4, though...I think reactors are better put to stationary uses doing things like producing propellant.

I tend to agree that LOX/CH4 may be the best option.  And if someone really wanted to use a NTR, the methane could be used in that application as well.

[Bob B.]

Okay, then what about ammonia?

I found that the compounds near the bottom of my list – ammonia, ethanol, MMH and UDMH – all produce specific impulses close to that of LOX/LH2.  In that case, and assuming LOX/LH2 is available, I’d rather just stick with chemical propulsion.  However, if ammonia can be found in significant quantities, I think it would be a viable fuel in an NTR with an ISP around 400 s. 

Nitrogen, assuming it doesn't dissociate, is pretty inert.

There’s negligible nitrogen dissociation at the temperatures we’re talking about.

[cjameshuff]

How practical is it to 'scoop' an atmospheric gas during an aerobraking pass? I'm also thinking of all that hydrogen at Jupiter. That would be a much faster pass than earth, though.

Difficult, at best. If you have even partially reusable launch vehicles, it might be better to launch loads of propellant.
Jupiter's surrounded by icy bodies with all the hydrogen you could want. If you're scooping something from Jupiter, helium's a lot more likely. (though you might use the hydrogen as propellant to restore your orbit after each pass)

What's wrong with nuclear thermal propulsion? You see how much bigger the I_sp figures are. And they can be increased further if we can figure out how to run at higher temperatures. Chemical propellants, otoh, have characteristic I_sps that are upper limits. You can't improve them without changing them or adding extra external energy.

They aren't really that much better (a round twice the specific impulse of LO2/LOX when using LH2 propellant), and they need reactors, which are heavy, complex, need neutron shielding, must be used with care anywhere a human might go, bring in operational complexities due to their startup/shutdown behavior, and require fissile materials that must be imported from Earth.

A nuclear thermal rocket is also going to wear out long before a similar power reactor, and will sit idle most of the time coasting far away from any applications that would benefit from a nuclear reactor's power supply. Stationary applications seem a much more efficient use of fissile material to me.

Nuclear reactors on the lunar surface, though necessary, will need some pretty big heat sinks. There are no rivers or oceans for cooling on the moon, so large radiators will be required. How large depends on the required temperature at the radiator outlet as well as the rejected heat power.

Stationary applications can afford big radiators. I've also had ideas of lunar or Mars applications using the cold side of a power-producing heat engine as a heat source for cooking out polar volatiles, possibly in-place.

[raven]

@ka9q
Yeah, I would consider that under 'Only to be used in case of solar system emergency'. Seriously, it's like someone looked at Orion and thought "Hmm, how can we make this have more fallout?"
Actually, I bet the thought was 'How to make more efficient use of those massive energies,' but the results are definitely in the category of 'Don't point that at me!'

[cjameshuff]

Yeah, I would consider that under 'Only to be used in case of solar system emergency'. Seriously, it's like someone looked at Orion and thought "Hmm, how can we make this have more fallout?"
Actually, I bet the thought was 'How to make more efficient use of those massive energies,' but the results are definitely in the category of 'Don't point that at me!'

Standing in rocket exhaust is rarely healthy, and in this case the neutron flux from standing next to an open and operating nuclear reactor would probably be more worrying. There is no fallout, the exhaust consists of a sparse spray of moderately high energy particles that depart the solar system on essentially a straight line trajectory. The fission fragment rocket is essentially an ion drive that avoids the electric intermediate stage, allowing very high exhaust velocities and efficiencies.

[raven]

Ok, maybe I am thinking of something different then.

[ka9q]

the neutron flux from standing next to an open and operating nuclear reactor would probably be more worrying.

I think gamma would be the real problem since it takes a lot of mass to shield. It's important to conserve neutrons in a reactor like this so it would be surrounded by some high efficiency neutron reflectors.

There is no fallout, the exhaust consists of a sparse spray of moderately high energy particles that depart the solar system on essentially a straight line trajectory.

Right, you beat me to this point. I don't know what effect the sun's magnetic field would have on them, but probably not much.

As the hoaxers are constantly reminding us, the solar system is already filled with high energy charged particles, i.e., radiation, and I doubt we could change that very much. The solar system is pretty big, and even detonating a nuclear warhead, as long as it's well away from planets and active spacecraft, would probably not have much long-term effect.

[cjameshuff]

I think gamma would be the real problem since it takes a lot of mass to shield. It's important to conserve neutrons in a reactor like this so it would be surrounded by some high efficiency neutron reflectors.

If you're hanging around in the exhaust, there probably isn't any shielding at all between you and the reactor. The neutron reflectors would be reflecting neutrons at you from behind the wires/plates/suspended dust comprising the reactor fuel elements.

Even then, if you want to commit suicide, you're better off with a large chemical rocket. As for environmental concerns, you might avoid operating them in the area of magnetically-shielded habitats, but it's otherwise hard to see how they'd be a problem.

[Grashtel]

Ok, maybe I am thinking of something different then.

A nuclear salt-water rocket maybe?  I have occasional suspicious that it was created to make nuclear pulse propulsion look like a sane option.

[ka9q]

If you're hanging around in the exhaust, there probably isn't any shielding at all between you and the reactor.

I'm not talking about the gamma (or other radiation) from the decay of the fission products long after they're gone from the engine but the 3.5% of fission energy that comes out as prompt gamma before the fission products leave. Without sufficient shielding that would be a problem for a crew (or sensitive electronics) forward of the engine.

Even then, if you want to commit suicide, you're better off with a large chemical rocket. As for environmental concerns, you might avoid operating them in the area of magnetically-shielded habitats, but it's otherwise hard to see how they'd be a problem.

My concern is trapping in the earth's magnetosphere. VAB protons can go upwards of hundreds of MeV, and about 169 MeV of the the total of 202 MeV from a U-235 fission appears as kinetic energy of the fission fragments so I wouldn't assume without some analysis that they'd just fly right out of the solar system.

I wouldn't even assume that they'd do this in interplanetary space as you have to consider the sun's magnetic field. On the other hand, interplanetary space is a lot bigger than the earth's magnetosphere.

As in any ion engine you'd also have to eject the electrons originally in the fissioned fuel to avoid building up a high negative charge. I doubt they'd find the positively charged fission products and form neutral atoms, so they too would be deflected by any ambient magnetic fields.

Come to think of it, what happens if you fire a neutral atom fast enough through a transverse magnetic field? Do the Lorentz forces rip the electrons off and send them and the nuclei in opposite directions, a sort of super Hall effect?

[ka9q]

A nuclear salt-water rocket maybe?  I have occasional suspicious that it was created to make nuclear pulse propulsion look like a sane option.

I think this could actually be cleaner, in terms of total radioactive waste released per unit impulse, than the fission product rocket. The extremely high I_sp of the fission product rocket requires an extremely high ratio of reactor power to thrust, and the fission waste production rate scales directly with reactor power.

The nuclear salt-water rocket has a much lower I_sp (though still extremely high compared to chemical and even nuclear thermal rockets) so its ratio of reactor power to thrust is much lower. This means less ejected fission waste per unit impulse.

On the other hand, lower I_sp requires a greater propellant mass for the same impulse, which means more impulse is required just to move the extra unused propellant, which would increase total waste output. You'd have to compare the figures for a given total mission impulse requirement.

Is Orion so bad compared to either of these? In each case you release all the byproducts of a nuclear reaction, and while the fission product rocket and the nuclear salt-water rocket both attain 100% of their energy from fission, Orion at least makes it possible to generate a good fraction of that from fusion. (Note that most so-called "hydrogen" bombs still generate most of their yield from fission; bombs for Orion would have to maximize their fusion/fission ratios.)

OTOH, the extremely high peak temperatures in a thermonuclear detonation mean that Orion would generate extremely bright pulses of X and far UV radiation not associated with the other two methods. These pulses might cause problems, e.g., unwanted ionization of the upper atmospheres of nearby planets, damage to solar arrays and sensitive astronomical sensors in spacecraft, etc.

[Count Zero]

Here's a film of Orion subscale tests done in the late '50s.  If you want to cut to the chase, go to the 9-minute mark.