Magnetic fields and reentry

[Idobox]

For the past few days, I've been trying to figure out what would happen if you put a strong electromagnet on a capsule during reentry, and if that could somehow be useful. The thing is, I'm not an expert in plasmas or aerodynamics, so I come here to share my ideas and see what you have to say about them.

My first thought was that it would deflect charged particles found in the exosphere, which would cause drag, and if managed properly, lift.

When the atmosphere starts to get thicker, it could either deflect air, or cause eddy currents, in both case causing some draft by "catching" more air.

All that could help ditch some velocity in the early phase of the deorbit, making the rest easier.

But it's mostly when you start to have plasma generated by compression heating that it could be very useful, by confining it and reducing the need for heat shield.

If you orient the magnet perpendicularly to velocity, the plasma will have a lot more difficulty reaching the heatshield, limiting heat transfer, and will be pushed aside where it will cause more drag. The main problem I see with this is that some plasma might reach the hull from the side by following the field lines if it's not all blown away by the wind.

If you orient the magnet along the velocity, the plasma will tend to get stuck, not being able to escape by the sides. It sounds like you could have a bubble of hot plasma protecting your ship from superheated plasma. And the funnel shape of the field could potentially also increase drag significantly.

And in both cases, eddy currents in the ionised atmosphere would cause significant drag, heating air that doesn't touch the capsule. And that's the main idea: you want to get rid of your kinetic energy while not getting to hot. Right now, the only way we've found is to slam into air and put a heatshield, but magnetic fields allow you to dump your energy into air you're not touching.

I quickly googled, and only found people trying to mitigate the plasma sheath for radio-communication (plasma reflects radio waves, so there is a communication black-out during reentry), which at least tells me it can be done on a small scale at least.

[K^2]

Most of the plasma is generated in ionosphere, while most of the braking happens much lower. It'll help, but it's cheaper and lighter to just add a bigger heatshield.

[Bill Phil]

Since it's only a late mission addition, that's also expensive, this won't happen for a while. Though maybe things like Skylon could use it...

[Starman4308]

Most of the plasma is generated in ionosphere, while most of the braking happens much lower. It'll help, but it's cheaper and lighter to just add a bigger heatshield.

Plus, given the experiences I've had with DRE and FAR, you don't want to brake too much in very high atmosphere. If you do, you will lose just enough velocity to send you screaming towards lower atmosphere: what you really want to do is "skip*" off mid-atmosphere, and maintain a 3-4G deceleration for as long as possible.

*Not to be confused with a full skip reentry: what I aim for is to reach 0 vertical velocity (and potentially start going back up for a bit) in mid-atmosphere, where it's thick enough for large amounts of braking, but not so thick as to cause fatal G-forces or strip off my heatshield. If I can get 2-3 minutes at steady 3-4G deceleration, I've just about nailed my preferred reentry profile.

[Idobox]

Most of the plasma is generated in ionosphere, while most of the braking happens much lower. It'll help, but it's cheaper and lighter to just add a bigger heatshield.

That's because you can only hit little bit of thin atmosphere with your capsule while in the ionosphere. If a magnetic field gives you 10 or 20 times the drag, you'll bleed off a lot more velocity before reaching thicker atmosphere, saving on heatshield mass. And if you somehow manage to get some lift, you can stay there for a longer time, bleeding even more energy at low temperatures.

For single use craft, it will never be cheaper than a "dumb" ablative heatshield that can absorb an impressive quantity of energy per kg, but for reusable craft it might be an option.

Also, there is the case of planetary missions. NASA is working on inflatable heatshields for Mars because they want larger ones than what can fit inside a rocket.

Plus, given the experiences I've had with DRE and FAR, you don't want to brake too much in very high atmosphere. If you do, you will lose just enough velocity to send you screaming towards lower atmosphere: what you really want to do is "skip*" off mid-atmosphere, and maintain a 3-4G deceleration for as long as possible.

The reentry profile depends on the type of heatshield you have, and your payload. If you can survive lots of Gs, it's apparently preferable to choose a steeper descent and an ablative material that is a better insulator. I'm not sure how it works, I guess more energy is dumped in the atmosphere and less in the heatshield if you work with higher deceleration and temperature.

All that to say that if you have a very different mechanism for braking, the optimal will be different. Also, the drag will also be larger in the lower atmosphere, and there might be some lift, so it's not directly comparable.

[Nuke]

how are you going to power a significantly large field? look at how much power fusion researchers regularly pump through coils to contain plasmas. i dont think you could get that on a space craft.

Edited December 3, 2014 by Nuke