As NASA and other space agencies send space vehicles on missions that bring them closer and closer to the sun, one problem is finding improved heat shields. Recently, a group from Johns Hopkins University announced it has some innovations to offer for near-solar environments, using a shield with an alumina optical surface. Their research is published in ACerS’s International Journal of Applied Ceramic Technology. The group, Don E. King, David G. Drewry, Jennifer L. Sample, Dale E. Clemons, Keith S. Caruso, Ken A. Potocki, Douglas A. Eng, Douglas S. Mehoke, Michael P. Mattix, Michael E. Thomas and Dennis C. Nagle, comes from JHU’s Applied Physics Lab and Whiting School of Engineering. Heat shields for these types of missions are typically conceived of having two parts: a primary conical shield and a secondary shield made of a solid disk. The cone shape of the primary shield serves two purposes. It presents a relatively small cross section to the radiating body, i.e., the sun, and a large surface area to reradiate the heat. The secondary shield provides a barrier between the primary shield and the instruments being carried by the space probe. The JHU group looked at what happens if the primary shield is designed to reflect much of the radiation (as compared to just absorbing and reradiating it). The material of choice was alumina (Al2O3) on carbon – carbon applied via a plasma-spray process. The authors conclude that the alumina-carbon system provides a nice substrate – one that is 12 percent cooler – that “is highly reflective in the visible band yet highly emitting in the IR band (both very desirable characteristics for near-solar spacecraft). Use of the Al2O3 coating on the Solar Probe spacecraft is predicted to reduce the overall equilibrium temperature of the heat shield.” They say there should be three clear benefits:
- Less contamination of the instruments from material coming from the carbon-carbon shield,
- Lower temperatures permit more materials choices for the instruments, themselves, and
- Less overall mass for the spacecraft because the secondary shield can be thinner.