In theory, shouldn't we be able to deflect the plasma generated during reentry with an EM field? I get that active generation would have all sorts of complexity/power/weight costs. But could it be possible to engineer some type of material which can passively convert thermal energy to a powerful enough EM field that would envelop the spacecraft and work as a "deflector dish"? I just feel like there has to be a better solution to reentry than ablation. It's the main thing that sticks out in my mind which kills true reusability.
Absolutely, and NASA is currently investigating this. There is the idea of magnetoshell aerocapture[0][1], which is to create a shell of magnetized plasma around the spacecraft which can be used to slow down the atmosphere. There are a couple reasons why this is attractive. For one, you can modulate the size of the magnetized plasma shell and how much drag you make. Aerocapture, that is getting into orbit via dissipating momentum with planetary atmosphere, is a very risky maneuver because things like solar flares can change the height of the atmosphere causing there to be too much(crashing into the planet) or too little drag(flying off into space). Being able to change the size of the shell fixes this problem. The other is that this scales better than traditional aerocapture methods. The authors show in [0] how this could enable 2 tons of asteroid to be delivered to earth orbit, faster delivery of payloads to Mars orbit, and more economical delivery of payloads to orbit around outer planets like Neptune. There's a lot of promise for this technology and it's currently being developed for a cubesat demonstration mission[2].
This does not harvest any energy though, however, a separate NASA project did investigate the feasibility of generating power and capturing Mars' CO2 atmosphere to make O2 oxidizer through aerocapture[3]
> passively convert thermal energy to a powerful enough EM field
This is much easier said than done. If such a conversion were simple, we would not use turbines in nuclear or geothermal plants. You might be interested in looking into thermocouples [1], however.
Also of note, any such conversion must by nature exploit an energy gradient, which means any energy spent redirecting particles in such a manner would heat up the interior of the spacecraft by an equivalent amount.
perhaps instead of trying to capture the energy: a (set of) heat pipes to the colder side of the craft?
if the temperature difference between hot and cold side is too large one might need multiple heat pipes in series each operating at their own temperature. The hot side heat pipe might only act as a heat pipe as it nears its operating temperature, i.e. it might be solid during travel, then melt during the first phase of entry, and only reach both vapour and liquid phase -thus enabling it to work as a heat pipe- during the hottest part of re-entry
right the temperature on the back side would be higher than on current craft, but the backside would be able to radiate heat no?
alternative idea:
the solid aborbs heat (cools the shield) to melt, then absorbs heat to warm up and boil, and the boiling gas is used as retro-firing rockets, any decrease in momentum due to the retro-rockets is less future heat absorbed (since the craft is slowing down faster)
If I understand correctly the main energy transfer mechanism to the heat shield is thermal radiation, not conduction or kinetic effects. The plasma is separated from the vehicle by a boundary layer. So it doesn’t help that the really hot stuff isn’t directly touching the heat shield.
It has to become plasma before it's susceptible to EM deflection. So you'd still have to heat it to 4000 degrees through friction. And so you'd still need a heat shield.
