Reply To: EmDrive + Focus Fusion = Space Access for all?

[belbear]

Tulse wrote:
Thanks for clarifying that. I wonder if instead of a heat exchanger if something like a plasma torch would be more efficient, as it would use the FF electricity directly.

A plasma torch (some sort of spark inside the airstream) looks possible, but then only electricity can be used as input, not directly fusion energy. And I have no clue how sparks behave in a hypersonic airflow. Could be as problematic to keep the spark firing as combustion is. Electric discharges are doused in high-power high-voltage switches, using a jet of inert gas such as argon to “blow out” the spark like a candle flame. In a plasma welding torch, a relatively slow flow of gas is used.

I was rather dreaming of using fusion energy even more directly than electricity in a two-stage fusion engine.
My idea is to use an “externally powered” thrust-FF reactor in the engine nacelle, whose input capacitors are charged by the first-stage, ordinary electricity-producing FF unit inside the hull.
ALL of the energy output from this thrust-FF (ion beam + X-rays + electrode cooling) would then be converted directly into heat for engine thrust by releasing all them rays into a tungsten heatsink, thus multiplying the input energy directly by the achievable fusion Q-factor and greatly reducing the amount of on-board electric power needed.
Actually this second-stage FF reactor is like an over-unity plasma gun, but since FF needs a pure hydrogen-boron gas fill to work instead of air, heat needs to be exchanged.
Maybe hare-brained but I like the idea.

Tulse wrote:
I’m also still not clear on the power required, however. What kind of power density does one have to reach to match that of hydrocarbon fuel? Is such density practically achievable?

I’m not sure. The power of reaction engines is measured in thrust force (kilograms, pounds..), which is the punch you can expect, and specific impulse (in seconds), a measure for how efficiently your input energy is converted into that thrust. My guess is that specific impulse for fusion jets can be assumed equal to that of conventional jet engines, but I would need to look into how to translate electric or thermal kilowatts into thrust.
As for the power density of the actual borane fuel, so little of it will be needed to reach orbit (a few grams) that its mass is neglectable against tonnes of engine and airframe mass. When dealing with a fusion engine, the mass of fuel needed is totally irrelevant, unless you intend reaching a sizeable portion of light speed.

Tulse wrote:
I also wonder if FF won’t be far more useful in deep space, where low thrust electric propulsion systems are more practical, and where one might not need a lot of the support gear on needs in an atmosphere. For example, I presume it wouldn’t be necessary to have a vacuum pump to keep the reaction chamber evacuated in space, or to have the reaction chamber heavily reinforced to fight against implosion from atmospheric pressure. I would think a space-based FF reactor could be much lighter and smaller in space. (The one issue that might be more of a problem is dealing with the waste heat, as it it more difficult to radiate it away in a vacuum.) I would also think that the inherent simplicity of the system would be attractive, relative to fission reactors that require complex systems to turn their heat into electricity.

Of course it will. But the launcher I propose is not intended to leave the atmosphere, maybe ideally to fly into low orbit. A real deep-space engine is to be carried up as a payload.
As I mentioned in one of my very first posts here, a space-optimized FF engine could take us to the stars by beaming the alpha ion beam directly into space as reaction mass. (charge-neutralized by electron emissions, of course)
X-rays can be recuperated into electricity as usual and the eventual energy deficit of the direct-beam FF engine should be filled with energy from a FF electricity generator (also space-optimized) or by partly tapping into the beam energy. You indeed don’t need a vacuum pump, unless you find the loss of unfused reactants (especially boron) unacceptable. In that case you do need to pump them off and recycle them. Space vacuum can help with that, though.
Radiating waste heat into deep space isn’t so difficult, on condition you shield your radiator from the sun. After all, space shows us a 3 Kelvin black-body and that’s really cold.