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A cross-section of a toroidal gas-core fission rocket engine.

Solid-Core Fission Rockets
Tech Level: 10
Gas-Core Fission Rockets
Tech Level: 12

Fission Rockets are also referred to as Nuclear Thermal Rockets.

Nuclear-powered rockets had been studied in one form or another for over 50 years, going back to musings of the scientists struggling to build the first atomic bombs at Los Alamos. A test program called NERVA (Nuclear Engine Rocket Vehicle Applications) tested the solid-core concept in the 1960ís. Ronald Reaganís SDI program tested the gas-core and liquid-core concepts in the 1980s, and the Space Exploration Conference held by the first Bush Administration in the early 1990s refined many of the concepts in the context of a possible Mars mission.

Fission rockets could have been built and flown decades ago, but they have one fairly huge disadvantage: their exhaust is extremely radioactive. Their use or testing in Earthís atmosphere would be politically controversial at best and precipitate environmental disasters at worst. Open-air tests like those conducted under the NERVA program would be unthinkable in todayís political climate. Thus while it is possible to use fission rockets for orbital travel as well as near-Earth operations, it is far more likely that they would be employed only on deep space and interplanetary missions.

Also, in flight, heavy shielding would be needed to protect the crews and other operations systems from the engineís radiation.

Tech Level: 11
Diagram of a particle bed fission rocket.

A solid-core fission rocket is fairly simple in concept. A reactive fluid, typically hydrogen, is pumped through narrow channels in an active nuclear reactor, which heats the hydrogen into a high-energy plasma and is then ejected from the ship. Solid-core fission rockets achieved specific impulses of 850 seconds during the NERVA tests, compared to the 450 second specific impulses for conventional chemical rockets.

Over a dozen different configurations and schemes for solid-core rockets beyond the NERVA designs exist, based primarily variations of existing nuclear reactors. The most promising of these is the so-called Particle Bed Reactor(PBR), a concept still actively being pursued by the US Department of Defense as a space-based power source for missile defense.

Basically, nuclear fuel particles are suspended in a solid medium between two porous rotating cylinders or drums called frits. The outer cold cylinder is made of stainless steel; the inner hot cylinder is made of Tungsten and Rhenium. The power output of the reactor can be controlled by the rates of spin on both the inner and outer cylinders. PBRís are considered more desirable for fission rockets than other solid-core designs primarily because they can operate at lower temperatures but without the loss of efficiency because of their greater surface area to allow heat transfer. Also, they require much less fuel-injection pressure of other solid-core schemes, greatly reducing the potential stress on the engine.

Hydrogen fuel is pumped into the reactor in a inner radial direction. PBR fission rocket engines are estimated to have specific impulses of about 1300.

Tech Level: 12

One of the limitations of solid-core schemes is that they can only get so hot before they melt down, limiting the amount of energy they can impart to the exhaust plasma. One way around this is to eliminate the solid core and replace it with a liquid or gaseous one. Liquid-core reactors have at times been proposed by various proponents, but most designers seem content to bypass them in favor of more efficient and powerful gas-core models.

Like solid-core fission rockets, there are numerous gas-core concepts based on different configurations and assumptions. The design currently showing the most promise, and is actively being pursued at the Los Alamos National Laboratory, uses a toroidal vortex of uranium plasma. The fuel is shot through the center of the spinning toroid, where it is superheated and most of it is ejected out of the spacecraft. However, a significant portion of the fuel is diverted to recirculate around the uranium plasma toroid, keeping the gaseous core compressed enough to remain in a critical (ie, undergoing nuclear fission) state. Fission rockets using this scheme are thought to have specific impulses in the range fo 3000 to 5000 seconds.

However, several design problems have arisen, including how to keep the walls containing the reactor from melting down from the extreme heat, and how to efficiently inject more uranium plasma into the toroidal core once the engine is engaged to make up losses. Research is continuing.


A very in-depth discussion of the evolution of thought regarding nuclear power and propulsion in space:


An article on using gas-core rockets on a manned mission to Mars:

A brief overview on Nuclear Rocket technologies:

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