MHD Aerobraking and Thermal Protection Part II: Atmospheric Reentry for RLVs

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:

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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.

  1. 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.
  2. 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.
  3. 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.
  4. 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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Jonathan Goff

Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
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19 Responses to MHD Aerobraking and Thermal Protection Part II: Atmospheric Reentry for RLVs

  1. Hmm, interesting, a suborbital flight up to 1000km or so should give you a nice Mach 12 reentry.. and still give you zero downrange.. probably a bit out of your capabilities for a little while but the same hardware you’re using for MHD experiments could be used for Van Allen belt experiments, so it might pay for itself.

  2. Eric Collins says:

    CFD with coupled MHD is still a relatively new field of study. The numerical solver capabilities are beginning to mature, but the validation of these models is lagging somewhat due to lack of experimental data to compare against. They’ve only recently started using the Shuttle TPS as an experimental testbed to begin collecting data on the onset of turbulence (boundary layer transition) at rentry conditions. As the previous commenter suggests, the study of electromagnetic interactions with plasmas in the space and reentry environments would seem to be an ideal candidate for the new sub-orbital research vehicles.

    The biggest questions in my mind are: How much magnetic field are you gong to need in order to get a significant lift or drag effect? and, Will it be feasible to carry the magnet and power supply on board?

    I don’t know how you plan on seeding the plasma without having some form of probe extending out to the shock front – at which point you have to start worrying about how to manage the TPS for the probe or else just make the probe ablative and make that your delivery system. As for reionizing the gas, there are a few possibilities. Unfortunately, they will almost all be most effective at higher altitudes where the density of the plasma is lower.

    The main suggestions that I would make at this point would be to consider allowing the plasma to pass through the center of your magnetic field generator. That is where the magnetic field strength is the greatest, so you will get your largest interaction there. I have in mind an RLV configuration that uses something like a ramjet/scramjet configuration during launch. During reentry, the combustion chamber does double duty as a flow path for magnetically constricted plasma. It should be possible to extract a very large amount of energy from this flow path as well. In addition to being able to apply that energy back to the TPS, such a system will most likely also increase the effective drag due to the fact that you are decelerating the plasma relative to the vehicle. (Then there are the problems associated with electrical heating caused by trying to shunt that much current around. But hey, that’s another post altogether.)

  3. jstults says:

    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.

    That brings up the interesting failure mode of getting some of your intended acceleration up high, then your MHD hardware cuts out half-way through, and you plummet to a fiery death because the TPS can’t handle the extra velocity that you are now carrying down low.

  4. jv says:

    Wouldn’t such TPS be ideal for areocapture into LEO as the velocity always stays very high?

  5. jsuros says:

    Jonathan,

    Very interesting series. Had you thought of building an MHD system into a ballute to mitigate magnetic effects on the return vehicle?

    You mention mach 12 as a rough low end speed for getting useful work from this system, but how about the high end? Would this be useful for return to LEO scenarios from the moon or farther out? I like the ability to “dial up” your ballistic coefficient at will. I couldn’t help but wonder if the ability to generate lift means you could also generate planetward (downward) forces, to keep a vehicle in the atmosphere and decelerating for a longer period of time.

  6. Elmar Moelzer says:

    I like, it really like it. I think there is a lot of potential in this and I really hope that NASA will invest some money into doing research on this. This is IMHO exactly what NASA should be doing. Not building LVs, but researching technology like this with all variations, etc. This could then be an enabling technology for RLVs.

  7. Rocketeer says:

    “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…”

    …suggests to me that MHD aerobraking gains for *Martian* atmospheric entry in particular would be significant (because the current mechanical methods are at the raw edge of what’s achievable — see “seven minutes of terror” 😉 ).

  8. Martijn Meijering says:

    How does this compare to lifting bodies with metallic TPS? Aren’t they supposed to be low maintenance too? Could that and MHD braking be combined? Would it make sense to do so?

  9. Tim says:

    I wonder if the burnt off material from an ablative shield could provide the alkali seeding material? Or would it be released behind the shock and too close to the Vehicle itself to be useful?
    With regard to Eric’s suggestion of a probe to release the seeding material, could such a prode also act as a drag resistant aerospike, or would reentry be too intense an environment for that? I suspect you would need a fairly advanced TPS for the spike itself, but it would be a much smaller component than the entire behicle.
    http://en.wikipedia.org/wiki/Drag-resistant_aerospike
    Finally, the failure mode jstults brings up could possibly be avoided by using a sheild that is normally non ablative, but can burn off if things get too hot.

  10. Jon,
    You might be interested in this
    http://www.flightglobal.com/articles/2009/11/24/335327/magnetic-heat-shield-test-could-use-russian-launcher.html

    in a few years ESA and the Russians could be testing something similar. Rob

  11. Adam Greenwood says:

    One of the attractions of sub-orbital is that it gives you a cheap way to test and prove out new technologies. That doesn’t seem to be the case with MDH, though, or am I mistaken?

  12. Adam Greenwood says:

    Err, MHD

  13. Jonathan Goff Jonathan Goff says:

    Jv,
    Shhh…I’m getting there.

    ~Jon

  14. Jonathan Goff Jonathan Goff says:

    Rob in 10,
    Yeah, that was one of the media reports I mentioned in my first part. Very vague on details, but yes the two active groups in the field right now are the Germans and the Japanese. The Germans are planning on flying something on a Russian sounding rocket, and the Japanese are planning on flying something on their own sounding rocket–one of the key JAXA guys is presenting at the SARG conference this week.

    ~Jon

  15. Tom Billings says:

    Jon, Good to see you doing so much good work!

    Reading your article, something tingled in the back of my brain for 4 days. Then I remembered what it was that might affect what you are describing. 3 NIAC studies, by Dr. Winglee, at the Un. of Washington,
    (downloadable in pdf at, http://www.niac.usra.edu/studies/372Winglee.html, and http://www.niac.usra.edu/studies/1012Winglee.html), and looked at what he called, “mini-magnetosphere plasma propulsion”, and in it was the basis of what might benefit an MHD re-entry system. He was to beam energy using a plasma. To do that the plasma sustains its shape over long distances, *pushing*aside*other*plasmas* to do it. Well, you need to push aside a plasma, and you need to do it with a magnet that has as little mass as possible.

    He noted in the conclusions of the 2001 study presentation that :

    “These results are all strong indicators that the inflation of a mini-magnetosphere can be achieved with existing technology. The closed magnetic field geometry of M2P2 provides an efficient means for deflection external plasma winds at very much larger distances than can be accomplished by a magnet alone.These results are all strong indicators that the inflation of a mini-magnetosphere can be achieved with existing technology. The closed magnetic field geometry of M2P2 provides an efficient means for deflection external plasma winds at very much larger distances than can be accomplished by a magnet alone.”

    At his presentation in Seattle, I asked if this could also broaden the ability of a magnetic field to shield a vehicle on the surface of the Moon, and he replied it could, out to several kilometers. That is the distance range needed to bend away charged particulate radiation coming in to the surface. That stayed with me, but also that pushing aside other plasmas part.

    That was what I was thinking about when I read your article. I don’t suggest that Dr. Winglee’s M2P2 work would help to push aside a shockwave plasma to kilometers distance as the atmosphere got hugely denser, but right at the highest altitudes, it might. That would enhance drag at those altitudes, and nearly obviate heat loads. As the altitude drops, and density soars. the field generated by the M2P2 plasma, and the M2P2 plasma itself, would get squished in towards the vehicle, sending heat transfer soaring. But this may well help in the upper altitude regime. How far down into the atmosphere you’d have to go before benefit is gone would have to be found in simulations and field tests.

    Admittedly, it has the complication of sustaining 2 plasmas, instead of one. That may be ameliorated, however, if the seeding of the plasma to be pushed away were done by the M2P2 plasma itself. No reason it cannot be made of easily ionized materials, is there? I hope this helps the concept more than it diverts attention.

    Regards,

    Tom Billings

  16. Eric Collins says:

    Hmm.. I need to go back and look closer at Dr. Winglee’s research. I remember thinking at the time that I was unsure how injecting plasma into a magnetic field could be made to enhance the properties of the field. In my experience, when charged particles move in an external magnetic field they usually generate their own magnetic fields in such a way as to diminish the strength of the external field. This effect can be seen in the Earth’s magnetic field as it responds to changes in the density and energy of the solar wind.

  17. johnhare john hare says:

    Is there any chance that such a shield could be static tested inside a rocket exhaust?

  18. Vladislaw says:

    Rutherford Appleton Laboratory is doing some work on magnetics also, according to this article, but for shielding, maybe something to look at also?

    http://www.sciencedaily.com/releases/2007/04/070419113601.htm

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