Aerodynamic characteristics of a supersonic car

An engineer working on the Bloodhound SSC (supersonic car) project has published a paper on the aerodynamic characteristics of travelling at 1000 mph (1609 km/h), increasing the current land speed record (LSR) by over 30%. The paper can be found in the Journal of Automobile Engineering.

The jet- and rocket-powered car has an engine which produces more than 135,000 horsepower; a slender body of approximately 14 m long, with two front wheels within the body and two rear wheels mounted externally within wheel fairings; and a weight of over 7 tonnes. It is a mix of car and aircraft technology, with the front half being a carbon fibre monocoque like a racing car and the back half being a metallic framework and panels like an aircraft.

Bloodhound computer image. Credit: Swansea University.

Simulations conducted by Dr Ben Evans and his student Chris Rose, both from the Swansea University College of Engineering, have looked at how the car will cope with the supersonic rolling ground, rotating wheels and resulting shock waves in close proximity to the test surface at the record attempt site in Hakskeen Pan, South Africa. It is here where the car will make high-speed test runs of up to 800 mph (1287 km/h) in 2015, with the full 1000 mph attempt scheduled for 2016.

In order for a ground vehicle to travel at over 1000 mph (approx Mach 1.3), the designers are said to have created the most advanced fusion of space, aeronautical and Formula 1 engineering ever attempted. They must combat the aerodynamic challenges associated with a land-based vehicle travelling at transonic speeds, with drag minimisation and vertical aerodynamic force control of paramount importance for a safe record attempt over a distance of only 12 miles (19 km).

“This is a complex aerodynamic problem because of the supersonic rolling ground, the rotating wheels and the shock waves in close proximity to the ground,” the authors said.

Computational fluid dynamics has been chosen as the primary tool to guide the aerodynamic design of the vehicle. Dr Evans and Rose are developing models of the aerodynamic flows that Bloodhound will experience - models which “have already influenced significant design aspects of Bloodhound including the front wheel configuration, the shape of the nose, the jet engine intake shaping, rear wheel fairings and wing shape and size”, according to Dr Evans.

“The computational fluid dynamics predictions indicate that the current design has a benign lift distribution across the whole Mach range of interest and a sufficiently low drag coefficient,” the authors noted. “It also indicates that the fin is sized appropriately to achieve the static margin requirements for directional stability.”

The authors acknowledge that there are “still questions to be answered regarding the accuracy of the model”, which will require refinement in parallel with vehicle testing. However, they ultimately conclude: “The aerodynamic characteristics of the final vehicle shape … indicate that the Bloodhound SSC vehicle is aerodynamically capable of safely achieving its target objective of a 1000 miles/h LSR.”