Talk:Fusion rocket

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[edit] IEC

Look, I know the Farnsworth-Hirsch Fusor is cool, but it's not going to yield a useful fusion drive (Analog article notwithstanding). To build a useful engine you need two things: energy and reaction mass. Ideally you'd combine the two. The goal of a fusion drive is essentially to extract energy from the fusion reaction; this is impossible with the fusor. Now, when I say extract above, what I actually mean is "use it to accelerate reaction mass". So if the problem were that various high-energy ions went flying out, we could direct them backward and get acceleration. But what comes out are omnidirectional X-rays. There's no way to usefully direct X-rays, and they're not much good at propelling you. The problem is that if you were to try to use a fusor as an engine, you'd have to take along some other energy source (possibly a fission reactor) to power it: the vast majority of the energy you put into the fusor comes right back out as X-rays, regardless of whether fusion occurs or not. --Andrew 06:26, Dec 31, 2004 (UTC)

I'm taking out the old text and putting in an explanation. --Andrew 06:47, Dec 31, 2004 (UTC)

The most practical approach might eventually be a Farnsworth-Hirsch Fusor. A fusor uses inertial electrostatic confinement. Since an electron volt equals 11,604 degrees, electrostatic confinement can and has achieved fusion in large vacuum tubes. The reactors still have not broken even, but the problems may be solvable.

Fusors have three important advantages:

1. They can react fuels that no other design could. For example, they might be able to fuse protons and Boron 11. This reaction produces neither gamma rays nor neutrons.
2. They are mostly vacuum, and therefore very light weight, suitable for vehicles.
3. They might be able to generate electric power directly. The ionized reaction products would be permitted to fly from the reaction site through a high voltage field of several million volts, and then hit a grid. This would create a small current at several million volts. This power could either be used directly, or pulsed, and used to operate a transformer to get more normal voltages.

Unfortunately, Dr. Todd Rider has shown [1] 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; with currently imaginable technology it is infeasible to capture the energy from these X-rays with sufficient energy to obtain a net release of energy from such a reactor.

Of course, in a propulsion system the goal is not to 'capture' the fusion products but rather to direct the 'flow' of these products out of the 'exhaust' with a velocity that is orders of magnitude greater that that achieved by chemical rockets. In such a system, the issue of 'break-even' is largely irrelevant, as propulsion is proportional to 'Mass times Velocity squared' (see Spacecraft propulsion ) - which means more is to be gained by inceasing exhaust Velocity than be increasing the exhaust Mass (hence ION Engines / ION Drives, see Ion thruster )

[edit] Thrust with neutrons

The article currently states that it would be "very difficult" to use the D-T reaction directly for thrust. Is it really that hard? Take the worst case that you have to fly a D-T tokamak, can't you just leave off the bottom half of the blanket? Then half the neutrons would stream out the bottom, the other half would be absorbed by the remaining part of the blanket, providing thrust. The energy of the absorbed neutrons could also be converted to electricity, but it might be better to just directly heat a gas to use for additional thrust. (I am assuming that you are not trying to breed tritium from lithium on board!) --Art Carlson 09:52, 22 September 2006 (UTC)