Comments on: MHD Aerobraking and Thermal Protection Part I: Introduction

By: Jonathan Goff

Jonathan Goff — Thu, 01 Apr 2010 18:05:42 +0000

theCase,
The point wasn’t trying to reduce the mass of an orbital reentry vehicle, so much as to decrease the maintenance cost of a reusable orbital vehicle’s TPS. Right now TPS work is the largest chunk of shuttle refurbishment, and having something that was more reusable, more robust, and more testable in advance would be worth it even if you had to pay a weight penalty. Think about it, right now for TPS systems, there is never a backup. The system either works or you get an event like Columbia. With something like this you might be able to do a backup, where if the MHD TPS works out, the maintenance is really low. But if there is a failure in the MHD TPS system, you can have a backup TPS that at least saves the vehicle and cargo/crew, but requires more rework.

But while the reentry TPS application is interesting, I think the aerobraking possibilities are where this concept really shines.

~Jon

By: theCase

theCase — Thu, 01 Apr 2010 17:46:54 +0000

If the desired end result is a reduction in mass boosted to orbit, then I’m skeptical as to large the savings would be:

1) you still need a “container” of some sort to hold whatever you’re returning to earth.
2). The equipment to generate the field takes up space (coils) and has mass
3) the energy source for said coils takes up space and has mass (batteries, APU, capacitor, etc)
4) you just might want some backup device, (secondary power source, coils, or an ablative heat shield).

Given these, I question the efficacy of such a proposal.

By: Roga

Roga — Sun, 14 Feb 2010 21:29:00 +0000

@A_M_Swallow:

Yeah, the first thing I thought when I heard about LOX cooled magnets was LOX, sparking potential, and just about anything = fire or explosion. But Nitrogen is just as good – still useful for transpiration and breathing after it is done cooling the magnet.

The fact that you don’t need a power source during re-entry opens up a lot of options, especially if you can charge it up in space. The strong magnetic environment would make lots of expensive testing necessary – checking effects on paramagnetic and diamagnetic propellants, inductance in moving parts, hall currents in just about everything, and what to do with the energy in case cooling is lost and the field collapse back into the coil.

Another nice feature is that it is not necessarily a safety-critical component. If you design the magnet to allow for reusability, it is very likely that you can also design the shield material so that it still protects ablatively in case of failure. You can’t reuse the shield, but everyone gets back home. A solid ceramic shield with a big magnet behind it certainly inspires more confidence that a precision-machined carbon or metal transpiration shield with lots of holes in it and plumbing behind it.

By: A_M_Swallow

A_M_Swallow — Sat, 13 Feb 2010 04:21:17 +0000

If LOX causes things to oxidise alternatives include nitrogen and argon.

By: robertross

robertross — Sat, 13 Feb 2010 02:52:33 +0000

This is truly fascinating stuff. This has some fantastic applications. Of course, if you’re that worried about the brittleness and cost of high temperature superconductors, just use ones that operate at LOX temperatures and cool them through a loop. The coolant you carry is also propellant/breathing air that you bring with you anyway, though it has a mass penalty, but until the technology is more highly refined, you can do testing & missions on this basis for now. Wth the special properties of LOX & magnetism, you could even bathe the magnet in LOX and it is rotected from the heat of the outer hull as it transfers through.

On top of all that, you could have your radiation shield as part of all this (using magentism). Upon re-entry, you no longer need shielding, so the LOX gets used for that instead.

By: Tim

Tim — Fri, 12 Feb 2010 11:46:50 +0000

Actually Ruediger, if the working fluid is vented overboard, you don’t need a heat sink, as the waste heat is carried away in the exhaust. After all, car engines don’t have cooling towers.

By: Ruediger Klaehn

Ruediger Klaehn — Fri, 12 Feb 2010 10:08:10 +0000

Re: Tim

The heat source does not do you any good as long as you do not have a heat sink. And if you had a heat sink for large amounts of heat, the whole reentry problem would be trivial. Radiating the heat away does not work for the amount of power involved in reentry if your heat sink has a reasonably low temperature. If you drop fluid overboard you might as well use water evaporation cooling and be done with it.

But there might be a clever way to use the interaction of the plasma with a weak initial magnetic field to increase the magnetic field. Basically like a self-exciting generator.

By: Tim

Tim — Fri, 12 Feb 2010 08:07:15 +0000

“it can possibly also produce lift as well as control torques without the need for aero control surfaces.” Would that mean you could add this system to an existing capsule? i.e design a capsule in such a way that you could begin operations with simple balistic reentry and then add a system like this later?
I wonder if you could use the heat of reentry to power the electromagnets? I’m thinking you could mount a heat exchanger just behind the heat shield (where I would imagine it is still fairly hot) and route the fluid through a turbine connected to a generator, and then vent it overboard. Of course the working fluid would add further to the weight penalty.

By: Danny

Danny — Thu, 11 Feb 2010 23:59:46 +0000

I forgot to add that the temperature of HTSCs isn’t really increasing. There are reports of higher critical temperatures for higher pressures, but unless you want to hold your superconducting magnet in your spacecraft at 10 kbar to get a 10 K shift in transition temperature, we’re going to need a completely new type of superconductor. Cuprate superconductors are notoriously brittle, so the amount of mechanical stress they can handle in re-entry puts some practical limitations on the how much aerobraking you can get out of this setup.

By: Danny

Danny — Thu, 11 Feb 2010 23:44:32 +0000

All you need to have a trapped-field superconductor to have the external magnetic field present at the time you cool the superconductor past its critical temperature. It probably wouldn’t be too hard to have a capacitor bank be charged by a solar panel, and then have your capacitor bank provide the needed current while you dump the coolant from storage, immediately prior to de-orbit.

You’re right that the charged particles in the plasma will move in such a way as to create an opposing magnetic field. That effectively pushes them away, and they still interact with neutral particles in the same manner as moving particles pushing against each other in a fluid. So, yes, you’re understanding correctly.