Supersonic aerodynamics

I was checking my feed for the BBC’s news stories today when I came across a story about an experimental car designed to go 1,000 MPH called the Bloodhound SSC. The article mentioned the car that set the current the world-record speed of 763 MPH in 1997, known the Thrust SuperSonic Car.

I searched for this car but didn’t find an entry for it on Wikipedia. However, I did find a fascinating article on the topic of supersonic. I know a little about aerodynamics from the pilot ground school I took 15 years ago but I had no idea how much different supersonic flight is from subsonic flight.

For instance, from the Wikipedia page:

Intake design was also a major issue. Normal jet engines can only ingest subsonic air, so for supersonic operation the air has to be slowed down. Ramps or cones in the intake are used to create shock waves that slows the airflow before it reaches the engine. Doing so removes energy from the airflow, causing drag. The key to reducing this drag is to use multiple small oblique shock waves, but this was difficult because the angle they make inside the intake changes with Mach number. In order to efficiently operate across a range of speeds, the shock waves have to be “tuned.”

Here’s another great entry about wave drag:

Wave drag is caused by the formation of shock waves around the aircraft. Shock waves radiate away a considerable amount of energy, energy that is experienced by the aircraft as drag. Although shock waves are typically associated with supersonic flow, they can form at much lower speeds at areas on the aircraft where local airflow accelerates to supersonic speeds. The effect is typically seen at transonic speeds above about Mach 0.8, but it is possible to notice the problem at any speed over that of the critical Mach of that aircraft’s wing. The magnitude of the rise in drag is impressive, typically peaking at about four times the normal subsonic drag. It is so powerful that it was thought for some time that engines would not be able to provide enough power to easily overcome the effect, which led to the concept of a “sound barrier”.

Who knew?