Nontoxic Rockets?

[Tallor]

Quick question. Are there nuclear-powered rockets that don't leave a trail of fallout?

[vicky_molokh]

The lightbulb?

[Bruno]

The bigger your rocket, the easier it is to set that sort of thing up.

You could "easily" set up a portable nuclear reactor like from a nuclear submarine, and then instead of using your steam (which is totally not radioactive, btw) to turn turbines, you use it as reaction mass. The big problem with current technology is getting a sufficient reactor into orbit - and surviving the vibrations.

I'm pretty sure a steam rocket doesn't have the impulse whatever-it-is to get out of the gravity well, but once you're up you can certainly tootle around on it.

I'm sure you could heat some other reaction mass instead of water - water just happens to be safe to handle and easy to find.

[Tallor]

Originally Posted by Bruno

...
I'm pretty sure a steam rocket doesn't have the impulse whatever-it-is to get out of the gravity well, but once you're up you can certainly tootle around on it.
...

Agh I forgot to mention! It doesn't need to be an orbital rocket. Just something that can produce similar thrust to a turbojet engine.

[malloyd]

Originally Posted by Tallor

Agh I forgot to mention! It doesn't need to be an orbital rocket. Just something that can produce similar thrust to a turbojet engine.

In the limit you could do that with an electric motor driven propeller and a nuclear reactor somewhere entirely elsewhere on the vehicle.

The reason nuclear engines tend to be radioactivity hazards is just weight. If you want to guarantee no radioactive material in the exhaust, just use something that can't be neutron activated for reaction mass and never let it touch the core. That requires a heat transfer loop to carry energy from the core to the exhaust, which will be a good deal heavier than the core is, but maybe that's OK. If you want it to emit no radiation in any direction, you need to shield it in all directions, which requires more shield than if you are willing to let radiation leak in some of them. If you worry about eventual release of spent fuel you can seal the whole thing a mile of concrete when you build it, use a mile long heat exchanger, and drop the moonlet into the sun once the fuel burns low. It's a tradeoff of how much hazard you are willing to live with against how heavy and expensive the engine is allowed to be.

[clu2415]

Check out Project Pluto. Launch the rocket with boosters, then have the reactor turn on at high enough altitude that the radiation won't affect the ground.

Generally, fallout is caused when a nuclear explosion picks up dirt and dust and makes it radioactive. A nuclear rocket in flight will irradiate any dust it picks up, which is a lot less than a nuclear explosion in contact with dirt.

[Fred Brackin]

Originally Posted by Tallor

Agh I forgot to mention! It doesn't need to be an orbital rocket. Just something that can produce similar thrust to a turbojet engine.

Not an option. Masses of uranium sufficient to produce a controlled chain reaction plus damper rods and shielding weigh lots more than titanium fans and rotors.

However, Bruno has described the basic Nuclear Thermal Rocket from Spaceships although she was using the water option rather than the default L. Hydrogen.

These stats conform decently to the historical NERVA engine from the 60s. Non-radioactivity of exhaust was the norm. Radiation release normally occurred only when one was blown up. They actually did blow one up as a test to see how bad the radiation problems was. They were more free to do that sort of thing in the old days. :)

As suggested, you're not going to get an SSTO out of such an engine but it has potential as an upper stage. The secret there is you use NERVA after you're not worried about falling back to Earth. Two minutes is normal fr a liquid fuel rocket engine. The Shuttle's engines were able to go 8 minutes but that's one of the reasons they were so complicated and expensive.

They tested NERVA from 30 to 45 minutes and longer might well be possible. So if you can get by one 1/10th the thrusts (and hence engine weight) you can make up for it by burning 10x as long and the Delta-V will come out the same. This ameliorates the heavier engine and lets you benefit from the higher Isp and Delta-V per fuel tank in Spaceships terms.

[malloyd]

Originally Posted by Fred Brackin

Not an option. Masses of uranium sufficient to produce a controlled chain reaction plus damper rods and shielding weigh lots more than titanium fans and rotors.

But most of that is the shielding. It's been possible to design a nuclear jet engine for 60-some years now, as long as you don't care it'll deliver a lethal dose of radiation to anybody who spends a few seconds within an aircraft length of it when you use it, and for a few years afterward. Obviously this is a rotten choice for a commercial aircraft. If you are building an engine for a missile to deliver (or intercept) a nuclear warhead, that might be acceptable.

[Anaraxes]

Originally Posted by Tallor

similar thrust to a turbojet engine.

Originally Posted by Fred Brackin

Not an option.

Not that far off, actually. There are solid-core nuclear thermal rocket designs with a 7:1 thrust-weight ratio. The SR-71 and Concorde engines were about 5:1, while the Pratt & Whitney F119 in the F-22 reaches almost 8:1.

Chemical rocket engines are roughly ten times higher.

[Johnny1A.2]

Originally Posted by Bruno

The bigger your rocket, the easier it is to set that sort of thing up.

You could "easily" set up a portable nuclear reactor like from a nuclear submarine, and then instead of using your steam (which is totally not radioactive, btw) to turn turbines, you use it as reaction mass. The big problem with current technology is getting a sufficient reactor into orbit - and surviving the vibrations.

On small nit, you probably couldn't use a true submarine reactor for an orbital system, because my understanding is that such naval reactors depend on gravity to function properly, so they'd have trouble in orbit.