MHD Aerobraking and Thermal Protection Part I: Introduction

I’ve been meaning to write for a while about a rather fascinating, but not very well known, area of research that I think might have significant implications for several areas of space transportation. The research I am referring to is focused on exploiting Magneto-hydrodynamic forces to manipulate weakly-ionized plasmas caused by hypersonic flight in rarefied flows–ie using magnets to shove around the hot flamey stuff caused by slamming into the thin air above us at crazy-high speeds. I’m going to be a tease, and not go into some of the ramifications until later posts in this series, but for now I want to give a bit more of an explanation than I’ve found available in the popular press so far.

Oh, and one small caveat before I jump in–while I think there’s some real potential here, electromagnetics is a topic that I’m truly awful at. I’ve never had another class, including a PhD level turbulent fluid dynamics class that made me feel like such a brow-dragging neanderthal as my Physics 122 class on Electromagnetism. This may be yet another niche technology that while somewhat interesting, ends up not being all that useful. But it looks at least possible that this may become a game changing technology in many space transportation fields. Without further ado, let’s go over some of the basics.

Some Background on MHD Aerobraking and Thermal Protection
The basic concept behind MHD Thermal Protection is that during hypersonic flight, above about Mach 12, the shockwave formed in front of a blunt-bodied vehicle reaches a high enough temperature to form a weakly ionized plasma that is conductive enough to be manipulated by strong magnetic fields. A powerful magnet near the leading part of the vehicle interacts with charged particles in the plasma via the Lorentz force. This force bends the trajectory of charged particles, creates large hall currents, which if I’m understanding correctly repel the magnetic field. These charged particles also impact with the uncharged gas particles nearby (the plasma is only “weakly ionized”) transmitting these forces to them as well. Here’s an interesting diagram I’ll reference from one of the papers I’ll talk about more later (“Trajectory Analysis of Electromagnetic Aerobraking Flight Based on Rarefied Flow Analysis” by Otsu, Katsurayama, and Abe–well worth the $28):

Figure 1 (from Otsu et al): Schematic View of the Flow Around a Vehicle With Applied Magnetic Field and Induced Current

Figure 1 (from Otsu et al): Schematic View of the Flow Around a Vehicle With Applied Magnetic Field and Induced Current

If the magnet is strong enough, this leads to two interesting effects–first, the distance from the vehicle to the bow shock increases (I think the plasma density between the bow shock and the vehicle also decreases, but I’m less sure about that). This can significantly reduce the heat transferred into the vehicle for a given velocity and altitude. The other big effect is that the Lorentz forces create forces that can produce drag or lift. At high altitudes these Lorentz forces can greatly augment the aerodynamic drag forces, effectively making it as though the vehicle had a much lower ballistic coefficient. It should be noted that this force is electrically controllable. In fact, depending on the sophistication of the magnetic apparatus and its location within and orientation with respect to the vehicle, it can possibly also produce lift as well as control torques without the need for aero control surfaces.

Both of these help from a reentry thermal standpoint, because by the time you hit the denser air, where the heating is the highest, you’re going a lot slower than you would’ve been otherwise, and a lot of that earlier braking is done at much lower heating loads than would have been the case without the electromagnetic effects.

Several of the papers I’ve read introduce an interaction parameter term, Q, that relates the relative strength of the Lorentz forces to drag forces. The relationship takes the form:

Equation 1 (from Otsu et al)

Equation 1 (from Otsu et al)

Sigma is the conductivity of the weakly ionized plasma, B is the magnetic field strength, L is a reference length (I think related to the magnet configuration), rho is atmospheric density, and V is velocity. As you can see, for a given magnet, the drag forces start dominating as the conductivity drops and as the atmospheric density increases. Atmospheric density increases dramatically as you descend from orbit, so for a reentry application, you get most of your benefit from the first little bit of descent.

We’ll go more into some of these ramifications starting in my next installment.

The following two tabs change content below.
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.
This entry was posted in Launch Vehicles, Lunar Exploration and Development, MHD Aerobraking and TPS, Space Transportation, Technology. Bookmark the permalink.

31 Responses to MHD Aerobraking and Thermal Protection Part I: Introduction

  1. Ya know how every now and then you hear people saying that rocket science is stuck in the 60s? This is the kind of stuff they’re saying we’re missing. In a recent The Space Show (http://thespaceshow.com/detail.asp?q=1297) John Powell was once again asked how his orbital blimp could possibly work and could he write a paper explaining it. For perhaps the first time he had an answer “I won’t do it” .. his argument was that by the time he gets to being in the situation where he is ready to design and build an orbital blimp the active drag reduction research will have moved on.. so why waste time today talking about what might be possible then? Not to mention that this is the same guy who got in trouble hinting at classified active drag reduction research, that has now been published by Japanese researchers, so its really not in his interest to tell anyone how he thinks he’s going to do it. This attracts some amount of “fraud” and “crackpot” accusations but, frankly, Powell’s experience speaks for itself on that.

  2. Pingback: Magneto-Hydrodynamic Shields for Atmospheric De-orbiting | Junior Ganymede

  3. kert says:

    The question is, where do you get that much electric current to sustain strong enough magnetic fields. Also, coils and magnets tend to be heavy.

  4. Jonathan Goff Jonathan Goff says:

    Kert,
    I think most of the proposals were looking at superconducting electromagnets. I’m not sure what they had for power sources in mind. The good news is, is that this is a field with lots of money being poured into it, so the magnets are getting stronger, and higher temperature every year. All that said, yeah I don’t have a good feeling for that side of the problem, and have no idea how heavy the hardware would be. Most of the advocates claim it would be weight competitive with existing TPS systems though.

    ~Jon

  5. Rob says:

    hmm, interesting – isn’t the plasma conductivity also a function of the velocity? At first I thought this might be like a force field that effectively would give a large rarefied boundary layer between the vehicle TPS and the atmosphere, but after reading all the way through it sounds like a way to pick up some drag before you get into the denser part of the atmosphere, more like a low atmospheric density parachute, is that correct?

    I guess the assumption is that the weight required for the system is less than what you’d need for more conventional propulsive braking?

  6. Elmar Moelzer says:

    I was advocating some research on this stuff years ago. It was hitting on deaf ears mostly. So I am really glad to see that happening finally. I think that this is mostly a matter of balance. If a system like that can reduce the total heat load just enough so you can employ a more lightweight and less maintenance intensive, “normal” TPS, you have already won a lot. So maybe you dont need that much power since it would not have to be running on full power for that long. Just long enough. That is my thinking anyway. Anyone with the brains care to do some calcs?

  7. Chris (Robotbeat) says:

    Since this reentry only lasts for a few minutes, the current could be generated by a li-po, fuel-cell, supercapacitor, or APU of some sort.

    BTW, do I read that correctly that the effective drag increases with decreasing velocity? Or does the more robust ionization of higher-velocities counter-act this to some extent?

  8. Bartosz says:

    Jon, would it be possible/useful at all for application of this technique to try to ionize the gasses a bit more – by means of some sort of e.g. x-rays radiators, etc? I’m sorry for this stupid question, I didn’t have anything to do with physics for quite a long time.
    Best regards,
    Bartosz

  9. Ruediger Klaehn says:

    I tried to post a few hours ago, but apparently it got swallowed by the tubes.

    Regarding power supply: if you can accept a large magnetic field during launch, it is not necessary to bring a power source. Simply create a large magnetic field using a normal elecromagnet on the ground, and then “freeze it” in place using a superconducting magnetic ring. This is how it is done in the dipole fusion experiment LDX.

    Superconducting magnetic dipoles in space are a very interesting subject in many respects:

    First of all, there is the concept of using an artificial magnetosphere as a magnetic sail (known as M2P2).

    Then there is the idea of using a large magnetic field as a shield against charged particle radiation from the sun (solar flares etc.).

    As mentioned above, there is also the very promising concept of using a magnetic dipole ring for plasma confinement for nuclear fusion. On earth, this concept suffers from the fact that it requires relatively large vacuum vessels. But in space, the vacuum comes for free.

    And finally there are the interesting interactions between strong magnetic fields and plasma during reentry mentioned here.

    This would be a very nice technology development mission: launch an instrumented superconducting dipole ring into GTO. Try to create an artificial magnetosphere. Study the interaction with the solar wind and with the earth magnetic field. Do some plasma confinement experiments. And finally study the plasma dynamics during reentry.

  10. Doug says:

    MHD can generate its own electrical power. Russians are building a flight demo of magnetic TPS. Sadly this way beyond what our private spacers are interested in. they just want to keep it simple KISS 60’s stuff its cheap, easy and now NASA pays them to keep it that way…FLEX.

  11. Tom D says:

    I read a book in the mid 80s by Leik Myrabo and Dean Ing that speculated quite a bit about using beamed power and MHD for aircraft and spacecraft propulsion. I haven’t found the reference for it, but “Lightcraft Flight Handbook LTI-20” by Leik Myrabo and John Lewis is almost certainly related. Myrabo has advocated using lasers and masers from external sources for ionizing gasses and powering rockets and jets of all kinds. His arguments and USAF-funded demos look very promising. The use of MHD Jon describes here is new to me though.

  12. Ruediger Klaehn says:

    Re: Doug

    I think you’re the first person that ever criticzed private companies for being not ambitious enough. Spacex is planning to launch a reusable capsule on a reusable launch vehicle, and you criticize it as 60’s stuff? Then what is Ares-1? Neolithic?

  13. Tom D says:

    Doug, I’d throttle back a bit on the cynicism if I were you. A little bit may be fun, but a steady diet of it just eats you up. I’ve been closely following NASA and commercial space for 30 years. The settlement of space has advanced much less than I hoped in junior high, but I’m not giving up yet. Good things are still happening with space and, more importantly, good people are still trying in private enterprise and in government. There will certainly be plenty of failures along the way, but that is life. Take care.

  14. Ruediger Klaehn says:

    It seems that there is a device called a magnetic flux pump that would allow you to gradually create a magnetic field in a superconducting ring. So you could launch the superconducting ring and then gradually build up a large magnetic field using a small power source such as a solar array.

  15. Ed H says:

    Re: 11

    Probably “The Future of Flight”, by Myrabo and ???

  16. Robotbeat says:

    Another application of superconducting magnets in space is that a strong enough magnetic field gradient can serve as gravity by the effect of the paramagnetic property of water in our bodies. I’ve seen pictures of mice floating in a strong magnetic field on the internet somewhere…

  17. Doug says:

    Tom,

    I’ll gladly thottle back when Bolden puts the path to FLEX, some accountability and teeth to it. Critics are all over him for pulling the plug without a path or some sort of goal to define FLEX. I’ll keep pounding FLEX until I see some sort of indication that it has even the slightest hint of project structure to it. Then I’ll throw everything I’ve got behind ULA and Space DEV “Dream Chaser”. Because they have the rockets Atlas and Delta and the vision to at least go beyond the capsule paradigm. Then on to Bigelow get it going and dump that ISS relic, then VASMIR I want to see something in black and white. FLEX as it is sounds like a pud-knockers gold mine of random KAOS. I’m all for commercial but I’m not going to pussy out on them I expect innovation like what is discussed above, accountability and results. This is it commercial your up to bat now show me what you got. Enough with the hype shut up and do it.

  18. @Doug

    Maybe if you knew what The Flexible Path To Mars was even called you might have a clue what it is about. Just because you’re ignorant doesn’t mean everyone else is. Now please, shut up and go do your homework.

  19. Danny says:

    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.

  20. Danny says:

    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.

  21. Tim says:

    “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.

  22. Ruediger Klaehn says:

    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.

  23. Tim says:

    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.

  24. robertross says:

    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.

  25. A_M_Swallow says:

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

  26. Roga says:

    @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.

  27. theCase says:

    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.

  28. Jonathan Goff Jonathan Goff says:

    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

  29. Doug Gard says:

    MHD can extract its own power from the plasma field around it. In addition if the vehicle has super cooled hydrogen, methane, propane on board as a fuel the temp difference can also produce its own source of power. In fact a super cooled fuel vehicle only needs battery for back up once tanked up the power is on and once accelerated to high mach hyper speed MHD takes over. TEG tech has already been applied in automotive systems allowing them to recharge the battery without a power parasite alternator. I know I work for the company that designed it. ATEG heat/exchangers.
    MHD can also be used to alter the flow an compress incoming air inside an inlet system. And yes the China and the Ruskies have already demo’d this in actual flight demo. We are way behind and getting more so every-day…..WAKE UP PRIVATE SPACE LET GO OF THE 20th CENTURY COLD WAR ERA and toy rocket hobby based NOX rocket TECH. Time to break the old stuck in the mud 1960’s NASA paradigm.

  30. You need a power-plant; Technology Submission – State of the Art – Novel InFlow Tech – Featured Project Development;|/ ·1; Rotary-Turbo-InFlow Tech / – GEARTURBINE PROJECT Have the similar basic system of the Aeolipilie Heron Steam Turbine device from Alexandria 10-70 AD * With Retrodynamic = DextroRPM VS LevoInFlow + Ying Yang Way Power Type – Non Waste Looses *8X/Y Thermodynamic CYCLE Way Steps. 4 Turbos, Higher efficient percent. No blade erosion by sand & very low heat target signature Pat:197187IMPI MX Dic1991 Atypical Motor Engine Type. |/·2; Imploturbocompressor; One Moving Part System Excellence Design – The InFlow Interaction comes from Macro-Flow and goes to Micro-Flow by Implossion – Only One Compression Step; Inflow, Compression and outflow at one simple circular dynamic motion / New Concept. To see a Imploturbocompressor animation, is possible on a simple way, just to check an Hurricane Satellite view, and is the same implo inflow way nature.

  31. Pingback: El “ultimo acto” de la misión Venus Express arroja luz sobre la atmósfera polar de Venus – Asociación de Aficionados a la Astronomía del Uruguay

Leave a Reply

Your email address will not be published. Required fields are marked *