Out of the Cradle

[galantz]

I think it would be fun to get the feedback from people regarding nuclear propulsion! The performance benefits, and with modern technology the potential for radioactive contamination seems significantly reduced...

I'm wondering how long politics and media terror will stay in the way?

[Rob]

It's been interesting watching the preparations for New Horizons and contrasting that with the run-up to the Cassini launch. With the exception of the usual suspects like Gagnon, the 'nuclear disaster' hysteria just isn't there.

I think the time is approaching, both on the ground and in space, when we can get over our fear of the nuclear bogeyman, and approach the risks and benefits of nuclear technology rationally.

But then, I'm also an incurable optimist

In terms of propulsion, were you thinking nuclear-electric, or nuclear-thermal rocket engines? My thoughts at the moment are that the first new reactors that go into space will be purely for power generation, probably one emplaced on the Moon as a tech demo for a Mars ISRU power source - if NASA manages to get VSE beyond low earth orbit. Judging by the fate of JIMO, I think that nuclear propulsion tech will be on the back burner for a few more years yet.

But then, say we could get propellants fairly cheaply with robotic missions to NEOs (SpaceDev's Jim Benson is a big proponent of this) - if chemical propellants in LEO were cheap, then most problems could be solved just by adding more of them, rather than introducing the added complexity of nuclear tech.

Of course, cheap prop in LEO is a bit like snow in the Sahara desert right now.
_________________
Rob Wilson

"The Earth is the cradle of mankind, but one cannot remain in the cradle forever."

[Kelly Starks]

[halman]

Nuclear power is an attractive source of energy for spaceship propulsion, because it is capable of much longer operation than chemical power. But there are a few problems inherent in nuclear propulsion.

The first one is that a fission reaction generates a great deal of heat, as well as emitting radiation. How is this a problem? Turning heat into push is not easy, unless the most dircet method is used, which involves pumping a working fluid, preferably rich in hydrogen, through a reactor core, where it absorbs heat, and expands. The expansion is done in a chamber with a specially shaped nozzle, which maximizes the expansion of the working fuel. The exhuast of this type of motor would be fairly radioactive, so it could not be used in atmosphere.

To generate electricity with nuclear power means converting heat into mechanical energy, which is then used to turn a generator. In the past, on Earth, this has been done with water, which turns to steam when heated, which spins a turbine. The water is not exposed to the nuclear core itself, but instead to primary coolant, which transfers its energy to the secondary coolant, which is what spins the turbine. Such a conversion process requires a lot of massive components, which reduces the effiency of this type of conversion for propulsion.

Another problem with nuclear power is that the reactor emits a lot of radition, which is currently sheilded by thick slabs of dense, heavy material. Such an approach on a spacecraft is going to seriously reduce the effiency of the system. One answer is to build the spacecraft so that the nuclear reactor is located as far from the crew area as possible, like on a boom, which sticks a long way out from the crew portion of the craft. Some sheilding would probably still have to be used, at least on the area faceing the crew section.

So the use of nuclear power is going to take a while to adapt to outer space. But it is almost certain to occur, at some point, because chemical power so limited.