Originally posted by TheEngineer:
What would You think about a kind of fusion ramjet, meaning turbines collect air, mixed up with the fusion stream and this mix is used as reaction mass ?
Have you ever seen the TV show
Firefly? I thought it was very well done and - better yet - it just
screamed "Traveller" at me.
Too bad it was cancelled after the first season.
What you're suggesting seemed to be the principle behind the thruster pods on the ships in that show.
The principle of using more mass at a lower velocity to get the same thrust (thrust equals mass flow rate times exhaust velocity) with less power (exhaust kinetic energy 'power' equals one-half times the mass flow rate times the exhaust velocity squared) is why jet aircraft today use turbofan engines instead of turbojet engines. There's no reason I can think of why you couldn't take advantage of the same thing with a fusion rocket. In space, you want to maximize exhaust velocity to minimize propellant consumption. But if a big part of your propellant is essentially free from the atmosphere, it should reduce fuel consumption for the same thrust.
I've seen schematic conceptual designs of hybrid rockets which are essentially ramjets in atmosphere and rockets outside atmosphere. The really big bonus there is that you don't have to carry oxidizer for the portion of your flight in the atmosphere - which doesn't apply to a fusion rocket.
The possible drawbacks I see are 1) that you have a much more complex system if you include air intakes and 2) the innards of the fusion rocket, which normally only sees hydrogen, may not take well to the oxidizing quality of air. Both are practical matters, though, not show-stoppers in principle.
