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Sunday, November 30, 2008


Quite a lot of designs are in process for orbital spaceships & traditional chemical rockets to get us to the Moon. Most of the immediate future will be in the Earth-Moon system involving zero-G production & solar power & Earth monitoring satellites. Nonetheless there is a lot of space beyond that. To get to Mars for a practical cost & to get beyond that at all we will need atomic rockets.

There are a lot of possible forms of atomic rocket propulsion & obviously we already use it for nuclear propulsion in submarines. The problem with chemical rockets is that we have to fetch up an enormous quantity of fuel from Earth. Just as, with a 3 stage rocket, each stage launches progressively less so the payload of a rocket to Mars would be but a fraction of what was put into orbit. On the other hand once an atomic rocket has been assembled it can use a non-reactive material, like Moon metals, as fuel & rather less of it.

The wikipedia article on nuclear propulsion lists several possibilities Antimatter catalyzed nuclear pulse propulsion
Bussard ramjet
Fission-fragment rocket
Fission sail
Fusion rocket
Gas core reactor rocket
Nuclear electric rocket
Nuclear photonic rocket
Nuclear pulse propulsion
Nuclear salt-water rocket
Nuclear thermal rocket
Radioisotope rocket

of which the nuclear electric looks to me like the front runner for manned exploration in the near future.

In a nuclear electric rocket, nuclear thermal energy is changed into electrical energy that is used to power one of the electrical propulsion technologies. So technically the powerplant is nuclear, not the propulsion system, but the terminology is standard. A number of heat-to-electricity schemes have been proposed.

One of the more practical schemes is a variant of a pebble bed reactor. It would use a high mass-flow nitrogen coolant near normal atmospheric pressures. This leverages highly developed conventional gas turbine technologies. The fuel for this reactor would be highly enriched, and encapsulated in low-boron graphite balls probably 5-10 cm in diameter. The graphite serves to slow, or moderate, the neutrons.

This style of reactor can be designed to be inherently safe. As it heats, the graphite expands, separating the fuel and reducing the reactor's criticality. This property can simplify the operating controls to a single valve throttling the turbine.

While I am not overly worried by small amounts of radioactivity a long way away, being convinced of the hormesis effect, a system which does not involve release of any material which has been in direct contact with anything radioactive is as good as you get. While the parts, particularly the engine, would be manufactured here the ship itself would be put together in orbit & would never land.

The electric propulsion means firing charged particles (electrons, protons) out the back to provide propulsion. Depending on the speed at which it accelerates them it will not need much mass to throw. It will probably not produce a great acceleration but a small acceleration goes a very long way when there is no friction or countervailing force. The speed to which the particles are accelerated will be proportional to the efficiency of the electromagnetic accelerater hence the length in the picture shown. 1/100th of a G for 5 hours is the same as 1 1/2 G for 2 minutes so such a ship could get explore a long way on a trip of a few months. Since distance covered is a function of acceleration times the aquare of the time even a very small acceleration can suffice to get a long way if you can run voyages for as long as in the age of sail. This makes the entire solar system more colonisable than Australia was in the time of Captain Cook.

We obviously can't start building it yet but an X-Prize Foundation could start producing prizes for designing the sort of engine that will, in time, do it.

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