Nuclear-Electric (Ion) Drives


While photovoltaic (PV) solar cell arrays can provide a cheap, virtually limitless source of electrical power for spacecraft operating within the inner solar system, they quickly loose their efficiency beyond the orbit of Mars. For space colonies in fixed orbits this isn’t so much of a problem, since really large PV arrays and giant light-concentrating mirrors can compensate for the reduced sunlight. However, a spacecraft operating in the “Deep Beyond” suffers from increased mass/decreased performance even as PV arrays become massively huge. One solution is a power source that doesn’t require solar energy: an on-board nuclear reactor. Nuclear power can provide ample electrical power for life-support and other spacecraft systems on the often years-duration voyages to the outer planets.

The spacecraft pictured above and below is one example of a nuclear-powered vessel – in this case, one using a nuclear-powered electron bombardment thruster (a.k.a. "ion drive"). A small nuclear reactor is mounted at the end of a long boom that extends behind the vessel. Waste heat is handled by large, triangular radiators mounted along the sides of the boom and by smaller vanes at the end of the nuke. The crew occupies the shielded forward section of the spacecraft hull, with payload space and propellant tanks located aft. Since space is already full of radiation (including some rather nasty rads from solar flares), the spacecraft’s shielding easily protects the crew from the comparatively miniscule amount put out by the nuke. The nuke is coupled with a compact Brighton-cycle or similar closed-system turbine to generate electricity, which in turn powers a pair of high-energy electron bombardment thrusters mounted on arms extending from either side of the main hull. Conventional auxiliary thrusters are also mounted on the hull to assist in maneuvering and docking.

Firefly with drives litHow it works: During electron bombardment thruster operation, heavy inert gas (typically xenon or argon) from the propellant tanks enters a discharge chamber at a controlled rate. A hot cathode (negative electrode) at the center of the chamber emits electrons, which are attracted to a cylindrical anode (positive electrode) around the walls of the chamber. Some of the electrons collide with and ionize atoms of the propellant, creating positively-charged ions. The ions are then drawn toward a high-voltage electric field set up between two closely-spaced grids at the downstream end of the chamber. The grids contain numerous tiny lined-up holes so that they serve as porous electrodes. The ions are drawn through the first grid (the screen grid) are accelerated in the narrow gap between the first and second grid (the accelerator grid) and then pass through the second grid as a fast-moving ion beam to produce thrust. On the downstream side of the accelerator grid, electrons are injected back into the beam before it is expelled in order that the spacecraft remains electrically neutral. Ion propulsion provides much lower thrust levels than conventional thermo-chemical propulsion does, but much higher specific impulse (>3,600 sec). Although an ion drive must thrust for a longer period to produce a desired change in trajectory or velocity, the drive can operate continuously for weeks using relatively little propellant. You can read more about xenon-ion propulsion at this link: A diagram of an electron-bombardment ion drive is shown below:


NOTICE: The above image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. This applies worldwide.

About the model: The 1/200 scale model of a Russian-built "CBeTJIR4OK" - type spacecraft (which would supposedly spell “Firefly” in Russian if I had the correct font face) was bashed together from pieces of an Armageddon Space Station kit and scrap plastic. This “hard s-f” model measures about seven-and-a-half inches long and was inspired by some actual conceptual designs for nuclear-electric powered spacecraft, including NASA’s Project Prometheus. Coincidentally, the model vaguely resembles the fantasy spaceship Serenity from Joss Whedon’s cancelled television series, Firefly. The "CBeTJIR4OK" - type is a scaled-up version of the TKS spacecraft design originally developed for the canceled Almaz military orbital station project. The spacecraft has an overall length of 27.4 meters (90 ft) and a maximum hull diameter of 4.35 meters (14.3 ft).



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