Hypersonic RF Blackout Using STK and Ansys Fluent

Reentering space capsules routinely travel at five times the speed of sound, often defined as hypersonic speed. There is much interest in the concept of hypersonic airliners, which could take you from New York to Paris in 30 minutes.

Hypersonic vehicles experience strong shock and intense aerodynamic heating. The temperature around the vehicle is enough (thousands of degrees Kelvin) to cause dissociation of the surrounding air and generate plasma. The plasma enveloping the vehicle poses challenges for radio frequency (RF) communication. If the RF link frequency is lower than the plasma frequency, the signal does not propagate through the plasma and a “blackout” happens.

Simulating a hypersonic blackout is challenging due to the multidomain nature of the problem; it involves trajectory and RF modeling, access computation, and aerothermodynamic analysis. Turns out, it is exactly these types of demanding mission scenarios where STK shines. By combining STK’s powerful multidomain trajectory propagators and access algorithms with high fidelity aerothermodynamic simulations in Ansys Fluent, we can build an entire mission composed of physics-based models tied in a digital thread. STK’s flexibility to interact with other applications enables us to adjust the level of fidelity in our simulations.

Here we model a capsule re-entering at Mach 20 and transmitting data to a ship over a 2.4 GHz S-band link. The antenna pattern for the capsule is generated in Ansys HFSS and loaded into STK as a .ffd file.

For every scenario time step, airflow around the capsule is simulated in Fluent. An 11 species, 20 reaction non-equilibrium model simulates the chemical reaction in the air. We also use Fluent to simulate the plasma density distribution around the vehicle. An external script computes the RF attenuation due to plasma and reports the results back to STK. The entire solution workflow is automated in Ansys ModelCenter. The result plot below shows the blackout interval (black) and the signal cutoff frequency (red) below which a blackout will occur.

The report snippet below shows the signal attenuation caused by the plasma.

Watch our webinar about simulating a complex hypersonic mission in STK, which involves trajectory design and sensors, here.