Alternative Rocket Designs/Ideas

[Whiskey144]

So, I was thinking about the nuke thermal-derivative LANTR, which is basically a solid-core NTR with a LOX injection system for what is effectively an afterburner effect.

And I got to thinking, what if you could do something similar with a fusion reactor? The idea is simple: replace the fission reactor with a fusion one, employing a torus-shaped containment vessel. Would this, at least theoretically, work? What are some other materials which could be used for reheat capability in similar designs?

As a side note, could one potentially throttle down an ion-type thruster to increase mass flow at the cost of exhaust velocity?

[someone_else]

AFAIK, most if not all fusion designs use fusion to heat up hydrogen propellant, which is... well just dumb propellant not involved in any chemical reaction. Fusion fuel is whatever can fuse in that design, but it's too fucking little to provide any thrust on its own (it is supposed to be therer to be fused and produce power, not to be wasted as dumb reaction mass).

It's the same for the inertially-confined concepts. The pellet's are 99% or so dumb cheap hydrogen propellant that does not fuse at all.

You could theoretically inject LOX and do an afterburner in the throus kinds of fusion engines. The issue is that fusion designs work with plasma, so they use magnetic nozzles. To work with a chemical reaction I think you need a physical nozzle, which won't survive a second under the plasma of standard operation.

Besides, I really doubt you get a real increase in performance, since any engine slightly practical for interplanetary does accelerate TINY amounts of mass to HUGE speeds, while chemical rockets (or the afterburner of the LANTR) relies on big amounts of mass at relatively so-so speeds.

Fusion concepts can get more thrust by just using more propellant (within reason) and keeping constant the rate of fusion.
More propellant in same time means it will get heated less, so it will have a lower exhaust speed, but of course it's more mass so it gives more thrust.

I think you're far better off adding chemical rockets with LOX tanks and divert to them some of the fusion engine's LH2 if you want an "afterburner" effect. You have 2-3 minutes at most and you risk blowing through your whole interplanetary propellant supply (yay, slingshot to outer space!), but it may be worth it. Or not.

As a side note, could one potentially throttle down an ion-type thruster to increase mass flow at the cost of exhaust velocity?

Ion-trusters cannot be throttled down. You can add propellant at the end (with crappy efficiency), but the engine itself cannot handle any real amount of mass due to design limitations.

VASIMR isn't an ion-thruster, since that is working with magnetic fields and not electric fields (and quite a bit more mass of hotter plasma so it has far better thrust if it wants), but even then it does have its limits (it can only work with plasma, so exhaust speeds won't go lower than 50-ish km/s).
Assuming it has some kind of power plant able to keep it online, which isn't that easy.