Reply To: General thought on old coal mines.

[vansig]

zapkitty wrote:
currently projected FF units… even vansig’s massively ganged units… will not be able to compete economically with chemically fueled supersonic military combat units. And there still isn’t much of a market for supersonic cargo flights.
[…]
In seeking ways to transfer energy from a notional array of FF units to the airstream vansig has proposed laser heating using a terahertz beam.

I have proposed using the alpha beam directly from the FF array directed through a plasma window into the airstream.

As a baseline Lerner-hakase proposed using a relativistic electron beam (REB) for heating propellant.

The first thing that must be addressed is the efficiency of the heating. A Thz laser is not very efficient. A free-electron laser tailored to operate in the precise Thz-range frequency desired might be as much as 40% percent efficient in transferring energy from its electron beam to its laser output…

… but then how are you ever going to catch up to the efficiency of just using a straight REB for heating? Not so good.
[…]
So it seems to me that the question now goes back to direct alpha heating vs REB heating… but did I miss anything?

No market for supersonic cargo flights? there goes my delivery service, then. (“world-wide 90-minutes or it’s free”) 😉

Re efficiency.. that’s actually not as important as thrust:weight ratio, heat management, specific power, overall energy density, and specific impulse.
Nuclear wins hands-down on overall energy density.. even if it’s a larger reactor. Everything else is a matter of engineering.

greater efficiency does help these other things; but hypersonic speeds are already like exposing yourself to high temperatures.
7.8 km/s through atmosphere is about like having a 7800 kelvin head-wind; i’m glad it will be at low pressure.