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 intrestes 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 refering to exactly?