Fusion rocket

From Academic Kids

A fusion rocket is a rocket that uses a fusion reaction to power it. There is currently no known method of producing a self-sustaining artificial fusion reaction, although the theoretical power can be calculated, and the nuclear reaction has been verified in laboratory experiments and nuclear weapons. The advantage over a fission rocket is that less radiation is produced (depending on the fusion reaction), which requires less shielding, and that there is greater energy density in the fuel. Of the speculative spacecraft propulsion systems that have been proposed, pulse-fission or pulse-fusion may already be technically achievable (see the Project Orion and Project Daedalus studies) and a direct fusion system is also likely to be feasible in the medium term as steady progress is being made towards self-sustaining fusion reactions.

For space based application there are several configurations for a fusion drive.

Contents

Power generation

Many spacecraft propulsion methods require an input of electric power to run but are highly efficient. In some cases their maximum thrust is limited by the amount of power that can be generated (for example, a mass driver). An electric generator that ran on fusion power could be installed purely to drive such a ship.

Magnetic confinement

One would mirror a terrestrial fusion reactor like a tokamak. The fusion plasma would be vented to provide thrust. Currently tokamaks weigh about the same as an oil tanker, so the thrust to weight ratio would seem unacceptable. However, one could possibly design such a system based on a VASIMR engine modified to induce fusion in the plasma.

Inertial electrostatic confinement

One of the few ways controlled fusion can currently be achieved is in the Farnsworth-Hirsch Fusor. This machine operates with a rarefied plasma far from thermodynamic equilibrium. Rather than being forced to collide by high temperatures and pressures, the ions are accelerated towards the center by electrostatic fields. In an article ([1] (http://torsatron.tripod.com/fusor/fusor.html)) in the Analog Science Fiction and Science Fact publication, Tom Ligon suggested that the fusor might make the basis for a highly effective fusion rocket. It was also featured in this role in the science fiction novel The Wreck of the River of Stars, by Michael Flynn.

Unfortunately, Dr. Todd Rider has shown [2] (http://theses.mit.edu/Dienst/UI/2.0/Composite/0018.mit.theses/1995-130/1?nsections=13) that any non-equilibrium fusion system, such as the Farnsworth-Hirsch fusor, must produce X-rays due to bremsstrahlung which will carry away many times the energy that is released by fusion. Such a system must therefore be supplied with a great deal of input energy. With currently imaginable technology it is infeasible to capture the energy carried away by these X-rays with sufficient efficiency to obtain a net release of energy from such a reactor. These X-rays are also unsuitable for propulsion, so it appears that non-equilibrium fusion systems are not candidates for fusion rockets.

Inertial confinement

Another proposed configuration would resemble an orion drive using inertial confinement fusion. A small pellet of fusion fuel (with a diameter of a couple of millimeters) would be ignited by an electron beam, a laser or even a tiny amount of antimatter. A magnetic field would form the pusher plate. The preferred fuels would have an aneutronic fusion reaction to minimize radiation. Most researchers have focused on a combination of Helium-3 and Deuterium. Helium-3 is very rare on Earth and might have to be obtained from the Moon or a gas giant.

Fusion-based bombs could (and almost certainly would) be used in an Orion design, improving efficiency substantially while a pellet based inertial fusion approach has been extensively studied by the British Interplanetary Society during their "Daedalus" interstellar probe design study in the 1970s.

Antimatter catalyzed nuclear pulse propulsion would use tiny quantities of antimatter to catalyze a fission and fusion reaction, allowing much smaller fusion explosions to be created.

See also

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