MTL Annual Research Report 2011

One of the most fundamental technical problems concerning spacecraft design is preparing the vehicle to survive the extreme conditions encountered during reentry into the Earth’s atmosphere ^[1] .  When a hypersonic vehicle travels through the atmosphere, a high-density, low-temperature plasma sheath forms around it ^[2] .  The reentry plasma sheath affects heat transfer to the spacecraft, aerodynamics, and perhaps most notably, communications.  A communications blackout is a major threat, bringing about a complete loss of RF signal strength between the reentry vehicle and the ground.  A thorough knowledge of reentry plasma sheath properties is needed to effectively develop systems capable of maintaining communications during reentry.  However, the reentry plasma sheath occurs due to processes that are not well understood.  Furthermore, the conditions of the plasma sheath rapidly change throughout reentry, which introduces additional complications.  Analytical approaches alone are not sufficient to gain a complete understanding of the plasma sheath.  Therefore, instrumentation must be developed to measure properties of the plasma sheath during reentry ^[3] .

We propose a novel approach to reentry plasma diagnostics, utilizing planar arrays of MEMS Langmuir probes to perform real-time measurements of the electron temperature and number density of the reentry plasma sheath.  The MEMS Langmuir probes, shown in Figure 1, consist of an array metallic vias in a high temperature-resistant dielectric substrate, which can be blended onto the outer surface of a reentry vehicle (i.e., as a sensorial skin).  Figure 2 shows one of the early prototypes we made as proof of concept of the device process flow.  The MEMS Langmuir probes are made using electroplated gold and an ultrasonic drilled Pyrex substrate.  The performance of the MEMS probes will be validated experimentally in laboratory plasmas similar to those encountered by spacecraft during reentry.