Selenian Boondocks had three articles about MHD Aerobraking and thermal Protection.
The benefits should be
* reduction of the heat generated for re-entry into earth atmosphere so that there is up to an 85% reduction in heat generated which can enable a low maintenance reusable heat shield
* reduction in aerobraking time against any atmosphere or magnetosphere for reduction in mass compared to retrorocket braking and far faster aerobraking from months to a few days
* savings in mass of systems that need to be launched for various space missions
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
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
Journal of Space and Rockets articles provide the basis
* “Experiment on Drag Enhancement for a Blunt Body with Electrodynamic Heat Shield”
* experimental proof of the heat flux reduction “Experimental Veriﬁcation of Heat-Flux Mitigation by Electromagnetic Fields in Partially-Ionized-Argon Flows”
* “Numerical Analysis of Reentry Trajectory Coupled with Magnetohydrodynamics Flow Control”
Both analytically and experimentally, magnetic reentry thermal protection 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.
How to maximize the effectiveness of an MHD heat shield by Selenian Boondocks:
1. Use a lifting reentry.
2. Use as strong of a magnet as you can reasonably work with. A a stronger magnet provides more deceleration and shoves the boundary layer away further.
3. Use an alkali seed. (injecting an alkali metal into the reentry plasma in front of the craft)
4. Maybe heat the re-entry plasma below Mach 12
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.
one of the key takeaways was that the enhanced braking and thermal protection provided by strong magnetic fields was strongest at high altitudes where atmospheric density was lowest. At high altitudes, the ambient atmospheric density is low, but Joule heating caused by the interactions between ions in the shock layer and the superconducting magnet keeps the electrical conductivity of the plasma in the shock layer high. Also, for aerobraking or aerocapture short of reentry, by definition you are both always at a speed and altitude high enough that you don’t have to worry about the shock layer losing sufficient conductivity for MHD effects to dominate aerodynamic drag effects. The magnetic interaction parameter (Qmhd) introduced in my first post in this series can easily be in the 250-1000+ range at high altitudes compared to down in the 5-50 range you might see during atmospheric reentry. For example, the paper I cited in my first article (Otsu et al) showed that for a vehicle coming back from a GTO-like orbit, you could cut the return time by 70% with a 0.1T magnet, which is about 5x weaker than the magnet assumed for most of the reentry magnetic TPS studies. While magnetic effects may be helpful for reentry, they truly come into their own for aerobraking and aerocapture.
For most previous Mars and Venus aerobraking missions, velocity changes in the 1-1.2km/s range have taken between 70-150 days, over several hundred passes. While this is fine for unmanned missions, it’s harder to do for manned missions, where radiation concerns make you want to minimize your time spent in-transit.
With an effective total drag 4x higher at a given altitude combined with being able to go to a lower periapsis, you get bare minimum a 8x reduction in total aerobraking time compared to the non-magnetic case.
If you increased the magnetic field from 0.1 to 0.5T (similar to what was being suggested for the reentry studies done by Fujino et al and some of the others), you could maintain a Qmhd of 250 even if you increased the local density by a factor of 25. At Qmhd of 250, the effective drag coefficient is about 3x higher than the non magnetic version. That would give up to a 75x reduction in aerobraking time compared to the non-magnetic case.
Magnetic Aerobraking as an alternative to propulsive retrobraking for Centaur-derived cislunar tanker vehicles.
While a Centaur stage actually can do a lunar round trip fully propulsively, with at least some payload delivered to the Moon, the “gearing ratio” (initial mass in LEO compared to payload delivered to LUNO or the Lunar Surface) was pretty pathetic. Just to use some ballpark numbers, without digging up my more precise calculations, I’m getting around 8000lbs payload to LUNO if you drop it off in orbit and the Centaur only returns to earth, dropping to only 2500lb if the Centaur has to haul the payload all the way there and all the way back propulsively. However, if you could do 3km/s worth of aerobraking (assuming about 1200m/s worth of burns between the Trans-Earth Injection burn and any periapsis raising maneuvers, including the final circularization), all of the sudden you’re talking about almost 20,000lb of payload on the dropoff mission, and about 13000lb on the round-trip maneuver. Depending on how massive and expensive the aerobraking system weighs, it makes a massive difference in the performance of a reusable cis-lunar architecture
The technique uses repeated applications of weaker fields, making it possible to generate more powerful fields without an intrinsically strong magnet. The key is to create a pulse of magnetic flux that travels across the puck in a single direction, creating a consistent electric field in the material. Coombs says that other magnetisation techniques, such as repeatedly applying a uniform magnetic field, induce an electric field that is countered when the field is removed. The technique could let a 240kg magnet replace a current design weighing 3t in magnetic-resonance imaging scanners and cut the energy demand for cooling from 10GJ to 338MJ.
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
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