Bloodhound LSR – Braking systems
BLOODHOUND SSC has three primary braking systems: airbrakes, parachutes and wheel brakes. These will be used one-by-one to slow the car down from its top speed of over 1,000mph, taking advantage of the inherent benefits of each one:
- 1000mph: close the throttle
- 800mph: start to deploy the airbrake
- 650mph: deploy first parachute
- 400mph: deploy a second chute if required
- 200mph: apply the wheel brakes.
Airbrakes
The airbrakes in Bloodhound LSR are located either side of the Car in front of the rear wheels. They are made from carbon fibre and will experience around 5 tonnes of load at speeds of up to 800mph.
One of the prime advantages of airbrakes is that they are mechanically straightforward to operate. But the problem with using them in Bloodhound is that in order to stop the Car in the available distance of 5.5 miles (8.8km), they must virtually double the cross-sectional area and drag of the car, and that represents a significant technical challenge.
The result is that Bloodhound’s airbrakes are the biggest ever seen in land speed racing.
How they work
The airbrake doors need to be moved out into the airflow at a very specific rate in order to maintain the G force on the Car at just under 3g – which is the equivalent of slowing it down by 60mph every second.
Each door is moved by a separate huge hydraulic ram. However, it’s also vital that they open symmetrically, so the twin hydraulic pistons drive a single slider plate, which in turn is connected to both airbrake doors so that both move together.
For safety, each door also has its own separate accumulator that acts as a form of backup in case the main hydraulics fail.
Perforations in the airbrakes help break up the airflow, which will reduce the trauma on the rear wheel assemblies.
Parachutes
Bloodhound LSR will be using the same parachutes as Thrust SSC as, not surprisingly, we need about the same amount of drag at the same speed.
The chute system is designed to be simple and reliable. A button on the steering wheel will pull a pin from the chute pack, allowing a large steel spring to force a small drogue chute out into the 650mph airflow. The drogue will then pull the 2 metre diameter brake chute out, together with the line (or ‘strop’).
The chute generates 9 tonnes of drag, while the strop is rated to over 21 tonnes to give us a good safety margin against unexpected ‘snatch’ loads.
Dealing with drag
A 2 metre diameter ‘ring slot’ chute will give us 90kN (9 tonnes) of drag at Mach 0.9 (670mph).
To keep the chutes away from the turbulence immediately behind the Car, we need a 20 metre line, or ‘strop’, to attach the chute to the Car. With a 9 tonne working load, plus opening shocks of up to 50% above that, plus a 50% safety factor, that left us looking for a strop which would take an amazing 20 tonnes or so!
Wheel brakes
The wheel brakes on Bloodhound LSR are the final step in slowing the car to a halt after its record-breaking runs. What makes them unusual is that when the Car is being driven on those record-breaking runs the discs will be made from steel, rather than the usual carbon discs used on cars and planes.
Bloodhound’s aerodynamicist and performance expert, Ron Ayers, calculated that when slowing down about 52% of the Car’s energy will be absorbed by aerodynamic drag, 36% will be absorbed by the airbrakes (and/or drag chutes) and 11% will be dissipated by the vehicle’s rolling resistance. Only 1% needs to be absorbed by the Car’s wheel brakes.
Nevertheless, the engineering challenge was to provide wheel brakes that could survive spinning at over 10,000rpm, even though they would only be used from about 200mph downwards. In addition, Bloodhound’s wheels will have very little grip on the desert surface, so although the surface will be dry when the Car runs, the wheel brakes will be doing a job that’s more akin to trying to stop a car on a very wet road.
An additional requirement is the need to stop as close as possible to the Bloodhound team at the end of the first run for refuelling and replacement of any parts. With such a short time allowed in the world land speed record rules to do both runs, driver Andy Green needs the brakes slow the car down controllably, so that he can turn it around and then stop the car exactly where he wants to stop it – ie right next to the crew – not just bring it to a halt randomly somewhere at the end of the desert track.