Profac, PRopulsive Fluid ACcumulator, was described by its inventor, Sterge Demetriades, in the pages of the British Interplanetary Society's Journal as long ago as 1959. In this concept, a nuclear electric vehicle would orbit in the Earth's atmosphere - only 75 miles (120km) up - scooping up the rarefied air, separating out the oxygen and using the residual nitrogen in an electric propulsion thruster to make up the drag losses caused by the reaction of the tenuous atmosphere on the vehicle. A 10Mw reactor could provide enough oxygen every 20-30 days to launch 15 tons of payload into lunar orbit for the cost of a single Space Shuttle launch. On paper, Profac wins over all other proposed nuclear transport systems simply because it does not have to move the huge mass of the nuclear reactor to and from the Moon with each payload launch. With a system like this the cost of putting cargo on the Moon might approach the $54/lb ($1,000/kg) mark by the year 2000.

Safety is perhaps the biggest concern with Profac. Nobody is particularly happy at the moment with the prospect of a live nuclear reactor orbiting actually within the fringes of the atmosphere, especially after the Cosmos 954 accident in Canada. But Profac would be a far more advanced system than current satellites. The decay time of the orbiting vehicle is fairly long. If the nuclear propulsion system failed, then the system would burn some of its accumulated oxygen with a suitable stored fuel in a conventional rocket to put it into a higher orbit until maintenance could be arranged. After all, it would be unacceptable to have a system which was thrown away because a simple fuse blew at a critical moment.

As a footnote, while the idea seems to have been American, the Russians seem to have been doing something towards it. At the end of the 1960's they tested an "air breathing plasma engine" suitable for making up drag losses at low altitude aboard a test rocket called Yantar. Reports are sketchy, but Yantar appears to have had just about the exhaust velocity required for a Profac engine.

Orbiting in the outer fringes of Earth's atmosphere, at an altitude of perhaps 74.5 miles (120km), the unit scoops up the residual air, compresses and cools it by both ramjet compression and more conventional compressors, separates out the liquid oxygen and uses leftover nitrogen in a nuclear electric propulsion system to make up for the atmospheric drag. Most of the outside of the vehicle is a radiator area, both for waste heat from the powerplant and heat from the compressors and liquefier. A compact nuclear plant is required because the large area of solar array - preferred for further out in space - would add unacceptably to the total drag. However, the unit would not be totally dependent upon the nuclear powerplant, and would have a more conventional rocket system to boost it into a higher orbit for emergencies and unloading.

In routine operations, Space Tugs would obtain their supply of liquid oxygen from the Profac vehicle. The economics of the Profac system are strongly dependent on the power per unit mass of the powerplant. However, it appears possible to envisage a unit that could generate its own mass in liquid oxygen in 30 to 40 days of operating time.

Profac Cutaway1. Auxiliary propellant tanks
2. Liquid oxygen tank
3. Liquefaction and separation unit
4. Nuclear reactor
5. Air scoop
6. Radiators for liquefier
7. Radiators for reactor
8. Boost engines
9. Electric thrusters
10. Docking ring