Space propulsion lecture questions.

[theCandyman]

Today I attended a lecture by guest speaker Dr. Terry Kammash. He was speaking about the possibilities of space propulsion, and fusion was mentioned. As fusion is one of the subjects that interestes me the most, I was eager to hear about it. However, he seemed to treat fusion as though we are capable of net gain and after the seminar I asked him why not use the energy used to create the conditions for fusion to power the rocket. I am a bit confused, but I think he said something along the lines of "terrestrial conditions varying from space" and mentioned a factor called Q. This Q is slightly greater than one for space reactors, but if fusion was to be considered a viable power source it has to be 20 or 30 times greater.

Can science (or has it already) achieve net gain from fusion in space? What is this Q referring to exactly?

 

[hitssquad]

Q is the ratio of power out to power in. Power breakeven would be a Q of 1.0.
http://www.faqs.org/faqs/fusion-faq/glossary/q/

 

[Morbius]

Can science (or has it already) achieve net gain from fusion in space? What is this Q referring to exactly?

Candyman,

No - we have not yet been able to get a net gain in fusion - neither on Earth
nor in space. The conditions required for fusion do not depend on location,
nor the strength of the gravitational field.

I'm not sure why Dr. Terry Kammash, Professor Emeritus of Nuclear
Engineering at the University of Michigan; would leave you with a false
impression.

Dr. Gregory Greenman
Physicist

 

[Astronuc]

On Earth, one does not have to worry about the 'mass' of a fusion reactor, although cost is certainly an important issue. Both magnetic confinement and inertial confinement systems require massive structures to support the fusion process.

In space, every gram counts, because that is mass that has to be accelerated by the thrust.

IIRC, at the moment, we have not achieved a 'sustained' scientific breakeven, Q=1.0, or at least not for a prolonged period. One not only has to look at Q = (power out)/(power in), but also Q = (energy out)/(energy in), where energy is power integrated over time.

See - http://www.fusion-eur.org/fusion_cd/jet.htm

For spacecraft propulsion, one would like as high a Q as possible, and most likely one would want an aneutronic reaction based system, or at least a reaction with minimal neutron production.

Remember, fusion energy, like fission energy, is simply stored energy. The objective is to develop a system that realizes that energy in a 'useful' manner.