Reply To: General thought on old coal mines.

[zapkitty]

vansig wrote:

Query: what does the THz laser offer that REB or direct alpha heating doesn’t offer?

It *might* act at tunable distances and temperatures; and it *might* be possible to select wavelengths for strong absorption in propellant without absorbing strongly in tube walls.

for comparison,
if i recall correctly, VASIMR uses microwaves to heat propellant to plasma temperatures, at which point it accelerates the plasma magnetically. presently, VASIMR sizes are too bulky for a decent enough thrust/weight ratio for surface lift-off, *and* must be operated in vacuum; but what are the ultimate consequences of scaling it up, if amount of electrical energy required is not a factor and the propellant is atmospheric air?

this paper shows that the deepest absorption line for air is near 4.4 to 4.6 THz, with .001% transmission over a distance of 80cm; seems just right, but this would have to be studied at high temperatures also:
— http://act.nict.go.jp/thz/en/2/research2_e.html

… but that’s for em radiation, not particle beams.

As it turns out, most references to DPF propulsion assume it’s a vacuum-only thruster. Where REBs are mentioned it’s for the atmospheric stage of flight in the form of augmenting chemically-powered jets and heating additional propellant.

The odd thing seems to be that the plasma window and direct alpha heating are not mentioned even though some of the papers are as late as 2005… but then they were after high-performance military vehicles and the plasma window’s 9 atmosphere limit doesn’t lend itself to jackrabbit starts…