Reply To: reference to electric field propulsion?

[KeithPickering]

dennisp wrote: Being that focus fusion is already a rocket, I’m wondering: what is the exhaust velocity?

Beyond that, has anyone figured specific impulse and thrust? Would it make sense to simply mix additional reaction mass into the exhaust stream for a higher-thrust application?

I compute it as follows:
1. The energy released in a single reaction (p+11B -> 3He) is 8.7 MeV, or 1.3939e-12 Joules.
2. I will assume here that ALL of this energy is kinetic energy carried away by the He ions. In actual fact, they tend to run into electrons in the plasma (which slows them down) and that energy is emitted by the electrons as x-rays. But let’s assume best-case scenario for now.
3. The mass of a 4He nucleus (2 protons, 2 neutrons) is 6.7e-27 kg, and three of those have a combined mass of 2.01e-26 kg.
4. Since e = 1/2 m v², we can compute v = √ (2e / m)
5. From this, the velocity of the three helium nuclei, in the best case scenario, would be 11781 km per second, about 4% of c.

Specific impulse and thrust depend on how much and how rapidly fuel is burned. I don’t believe we know the answers to those questions yet.