>>13513910>if they fly through the ionosphere really fase can they generate enough power with MHDs to power ion thrusters for it to be worth it?Okay my thoughts on that idea are as follows.
From what I gather the air comes in and is shock heated to ionized plasma. This plasma flows into a MPD thruster where strong electric currents cause the plasma to accelerate out of the back of the thruster.
Total drag on the vehicle is a function of the velocity of the air stream and the area of the vehicle with respect to the air stream. Basically, bigger, faster vehicle lower down means more drag force overall.
Meanwhile, total thrust is a function of how ionized the air is by the time it enters the MPD thruster, and of course how much power you are running through that thruster in order to push the plasma. Basically, faster plasma, more electricity supply, equals higher thrust.
This concept only starts to do any work at all once the total vehicle drag force is less than the total thrust output. The things I mentioned above are going to favor very high flight velocities, very high power electric currents, and vehicles that consist almost entirely of thruster hardware (because any hardware that is being hit by air that is not contributing to the amount of ionized gas flowing into the thruster is pure detriment to performance). The minimum flight speed of a vehicle like this would be extremely high. The minimum flight altitude would also likely be very high, given that electrical power supply is likely the limiting factor in any setup and above a certain air density proportional to electric power the thruster will simply be swamped, like trying to run a jet engine underwater.
Overall, I would rate this propulsion system as low feasibility (waiting on significant advances in battery technology, maybe even needing compact nuclear energy) and low utility for space launch (chemical rockets are the GOAT and will never be beaten for low Earth orbit launch).
