A magnetic field is capable of exerting a subtle yet discernible pull. This implies that it may be employed to decelerate spacecraft prior to their re-entry into Earth’s atmosphere. Researchers indicate that the most perilous phase of the re-entry procedure can be significantly refined to ensure a smoother transition.
Challenges in safeguarding against thermal loads
According to the website phys.org, researchers at Tokyo Metropolitan University have developed a new system for testing magnetohydrodynamic aerobraking for spacecraft re-entering the atmosphere. Their platform generates powerful magnetic fields using a strong electromagnet when a miniature craft is struck by a shock wave traveling at a speed of over seven kilometers per second. The magnets have achieved significantly higher field strengths than previous experiments using permanent magnets, and this is an important step toward testing actual spacecraft in the atmosphere.
When spacecraft reenter the atmosphere, they are hit by shock waves exceeding several kilometers per second that heat the air at the vessel surface to several thousand degrees. To counter this intense heating, current technologies use heat-resistant tiles and sacrificial material that help dissipate heat and protect the craft.
While reliable, this approach has serious limitations, increasing weight, surface wear, cost and repair times. This is especially limiting as demand increases for reusable vessels.
Electromagnetic field deceleration technology
A promising technology for overcoming these challenges is magnetohydrodynamic aerobraking (MHD). By applying a magnetic field to the weakly ionized plasma at the shock wave, the ultrahot shock layer can be expanded and pushed away from the craft surface. Not only does this reduce the flow of heat into the vessel, but it can increase aerodynamic drag, slowing the craft down. While previous work strongly supports this method, testing such systems is a major challenge. Experiments usually involve putting a permanent magnet inside a small test model and hitting it with a shock wave, but this design makes it difficult to systematically test different field strengths and shapes.
To enable engineers to test a wider range of magnetic fields, a team led by Associate Professor Kohei Shimamura of Tokyo Metropolitan University has engineered a new system using a powerful electromagnet mounted inside a small model. The electromagnet is formed by a customizable set of coils and powered by a pulse-forming network (PFN), which hits it with an intense pulse of current, generating a strong field for a short period of time.
Precise synchronization of the magnetic field with the duration of the shock wave
In a test, the model is hit by a shock wave traveling at over seven kilometers per second (4.3 miles per second) for tens of microseconds in a hypersonic expansion tube, a ground-based facility for testing aircraft and spacecraft in extreme environments. The team designed the system to track the arrival of the shock wave and precisely synchronize the magnetic field to its duration, reaching field strengths significantly exceeding those possible with a permanent neodymium magnet. A high-speed camera was also synchronized to the shock wave to record the light given off by the heated shock wave layer (or “self-emission” layer).
To see it at work, the team designed two different models, each with coil configurations specifically tailored to its shape. They confirmed that fields of 1.24 and 1.58 tesla were created, with the latter more than double the field strength of conventional neodymium magnets. The self-emission layer was also observed to be more than 15% thicker with the field on.
The team’s work is a vital step toward planned tests of real reentry experiments and the development of a core technology for any future space mission involving reentry into an atmosphere.
