Bloodhound video: Managing the aerodynamics of supersonic shockwaves

Watch aerodynamicist Dr Ben Evans, University of Swansea, as he explains more about Bloodhound, supersonic shockwaves and aerodynamics. Or read the transcript.

“Bloodhound SSC is designed to travel faster than the speed of sound, so its aerodynamics must be intricately managed to ensure the car is safe and stable.

As the car accelerates, its nose generates the first shockwave, called the ‘bow shock’. Air, which has not had time to move around the car, suddenly compresses causing a pressure increase and a temperature rise. At top speed the temperature of the nose will hit about 120 degrees centigrade.

The next shockwaves form ahead of the front wheels. Compared to Thrust SSC, Bloodhound is a slender design. Thrust’s width caused very strong shockwaves, which fluidised the surface that the car was running across. Bloodhound’s streamlined design minimises the shockwaves and it is predicted that those extreme effects will not recur.

Further along the body of the car, air flows around the cockpit canopy leading up to the jet intake. Ideally, the intake to the jet engine should be at 500-600 mph. However, around the car in the freestream the airflow is up to 1000 mph. The car’s geometry and the design of the cockpit canopy generate shockwaves which decelerate the flow from 1000 to about 500 mph in just a 1.5metre section.

Air then reaches the rear wheels, which are rotating at 10,000 revolutions per minute. As the wheel rotates, the top point is travelling forward at 2000 mph, but as the wheel rim connects with the ground, instantaneously the bottom of the wheel is stationary. This makes the wheels simultaneously the fastest and the slowest element of the car. It is expected that there will be some slippage between the wheel itself and the ground, which begs the question: how will an accurate reading of the car’s speed be taken?

There are two ways to measure Bloodhound’s speed. A Pitot tube is mounted in the car’s fin, from which the pressure of the air forced into the tube can be read. From the measurement of that pressure you can calculate how fast the car is moving. However, weather conditions can affect the pressure reading by about 20-30 mph.

The most accurate way to measure Bloodhound’s speed across the ground is by GPS. The Rolex speedometer inside the cockpit is GPS-controlled, though this system provides far greater accuracy than a GPS found on a mobile phone. It will provide the accuracy needed, as Bloodhound covers a mile every 3.5 seconds at 1000 miles per hour.”