In this installment, I want to dig a lot deeper into the mechanics of how one might maximize the utility of MHD effects for orbital reentry. But first, I wanted to spend a few seconds discussing what is important for an RLV TPS system.
RLV Thermal Protection Systems
Protection from the harsh heating environment caused by atmospheric reentry is one of the biggest challenges for reusable vehicles–far more difficult than the often harped-on rocket equation or the “inefficiency of chemical propulsion”. The problem isn’t even the weight of the thermal protection system as much as it is the maintenance requirements. Ideally you’d like a TPS solution that requires very little maintenance, and can be “tested” easily and quickly on the ground before flight, even if it cost you a little extra weight. You’d also prefer something that was relatively simple operationally to use, with a minimum number of failure modes. MHD thermal protection seems like an interesting match for these requirements. I should note however that there are other promising ideas out there such as transpiration cooling that might also work on their own or in combination with MHD thermal protection, but they are beyond the scope of this blog post.
Some Take-Aways from the Literature on MHD Reentry TPS
There have been several interesting papers on this topic, including the JS&R article “Experiment on Drag Enhancement for a Blunt Body with Electrodynamic Heat Shield” that got me thinking about this more seriously, a second JS&R article that goes into experimental proof of the heat flux reduction “Experimental Veriﬁcation of Heat-Flux Mitigation by Electromagnetic Fields in Partially-Ionized-Argon Flows”, and another JS&R article from a year and a half ago “Numerical Analysis of Reentry Trajectory Coupled with Magnetohydrodynamics Flow Control” that I’ll be leaning on pretty heavily for this discussion. You can purchase the articles from AIAA, or if you already have a subscription to the Journal of Spacecraft and Rockets, you can read them for free.
I’ll briefly summarize some of my takeaways before going into my thoughts on how to move things forward from there:
- Both analytically and experimentally, magnetic reentry TPS appears to provide large reductions in both peak heating and in total heat load. The third paper above suggested a 30% reduction in peak heat load and a 40% reduction in total heat load for ballistic reentries. Under the conditions tested in the second paper, heat reductions up to 85% were shown.
- The magnetic braking effects dominate aerodynamic braking effects at high altitudes. This is mostly due to lower density meaning that atmospheric drag is fairly low, while also lower density means that Joule heating caused by the currents (the loop marked “J” in the previous post) induced by the magnetic fields increases the electrical conductivity more effectively than at lower altitudes.
- The more deceleration that can be done high up in the atmosphere, the lower the peak heating and the lower the total heat load. The heat flux is roughly proportional to the cube of the velocity.
- The heat flux reduction from this scheme is dominated by the increased shock layer thickness at high altitudes, and at lower altitudes is dominated by the much lower velocity by the time you get there by getting extra braking high up.
- Conductivity of the plasma is one of the keys to making this work. The conductivity in these cases was entirely due to the temperature in the plasma–higher velocities lead to higher temperatures, and Joule heating also leads to higher temperatures. As velocities slow down, conductivity drops, as does the effectiveness of the braking system. Below about Mach 12, the only way to keep the flow ionized enough to control magnetically is to add energy via some mechanism.
- Because of the large induced currents, this idea only works if the heat shield is an electrical insulator. If it is a conductor, you’ll just generate hall currents in the heat shield which will null out a lot of the benefit of the approach.
Thoughts on Maximizing the Effectiveness of MHD Reentry TPS
Based on these takeaways, and the discussion in the last post, I’ve come up with a few ideas for how to maximize the effectiveness of an MHD heat shield.
- Use a lifting reentry. Just as it is possible to offset the CG of a reentry body to generate some aerodynamic lift, it may also be possible to locate and orient the magnet in a way to create both lift and drag. If you do a force balance on a body in a circular orbit, the downward gravitational force is exactly balanced out by a fictitious centrifugal force due to your forward velocity. As you decelerate though, that centrifugal force component decreases, but by using lift, you can counteract some of that gravitational force. This allows you to stay up at a higher altitude longer, which allows you to do more of your deceleration in the lower density air. This is already used by all manned space capsules as well as the shuttle in order to keep reentry decelerations to a reasonably low level, and also to reduce the peak heating. This is even more beneficial for magnetic braking concepts, because you can do more of your deceleration at a point where the magnetic effects dominate, electrical conductivities are high, and heat fluxes are low.
- Use as strong of a magnet as you can reasonably work with. While there are diminishing returns according to all of these papers, a stronger magnet does help provide more deceleration and shoves the boundary layer away further.
- Use an alkali seed. As velocities decrease, it gets harder and harder to maintain the electrical conductivity in the plasma at a high enough value to maintain useful levels of Lorentz interaction. This is similar to the challenge with MHD electric generators. In order to keep the conductivity high, injecting an alkali metal into the stream can help. Alkali metals, particularly Potassium and Cesium have very low ionization energies compared to air. In a weakly ionized plasma, most of the atoms are actually not atomized–almost all of the conductivity is provided by the small number of atoms that are. So, a little bit of seeding can go a long way. This helps you keep your magnetic deceleration forces high even as altitude and velocity drop. The other nice thing about seeding, is that depending on what the fluid is, it might also cut down on the radiative heat transfer from the hot shock layer back to the heat shield.
- Heat the plasma. This may sound counterintuitive, but you might actually get better thermal protection if you start heating the plasma once you get to a certain point. Below Mach 12, even with seeding, there just isn’t enough heat rise caused by the shock layer to keep the plasma sufficiently ionized. But, it is actually possible via several different means to dump a bit of energy back into the shock layer to push the gas back into an ionized state. It’s unclear at this point if this is worth doing, but if the system is light and simple enough it might be worth considering. As it is, you’ll have a lot of stored energy in the superconducting magnet, and you probably want to dump that somehow before landing–using it to keep the incoming air ionized a bit longer to get a little more deceleration before you hit the thick air might be worth it.
All told, you’re still going to need some sort of thermal protection for the last bit of deceleration, but the heat loads and max temperatures are so much lower if you can dump say half the reentry velocity while you’re still high up, that the problem becomes a lot easier to deal with. If you could only get down to Mach 12 with this system, that would cut the peak and total heat loads by at least a factor of 8x. The heat fluxes at this point would be low enough that you wouldn’t need ablative materials, and could probably use a ceramic tough enough that it was low maintenance.
Anyhow, the key questions I have at this point are: a) what sort of effective “L/D” ratio can you get by varying the location and orientation of the magnet, b) how much does seeding help, c) how long can you stay up in the high altitudes, d) what is the maximum amount of velocity decrease you can provide via this method, e) how strong of a magnet could you reasonably hold on an RLV, f) how does the strong magnetic field interact with the operation of the RLV itself–what does it do to solenoid valves, electric actuators, etc. and is there a way to shield against these issues?
In the next segments, I’m going to talk about another, possibly even more interesting application of this concept, as well as some thoughts on how we can reduce this technology to practice.
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