by Andy Green
Setting a World Land Speed Record (LSR) requires 2 runs through the measured mile (and kilometre) in opposite directions within one hour. The quickest and simplest way to achieve this is to stop the Car, after the first run, at the point where we want to start the second run. Put another way, with the measured mile in the middle of the track, if we make the acceleration distance and deceleration distances the same then we won’t waste any time having to tow the Car to the start point for the return run.
Our early simulation work suggests that we will be able to get BLOODHOUND SSC up to 1000 mph in 4.5 miles. This leaves us with a problem – we need to stop the Car (still weighing over 5 tonnes empty) from 1000 mph in just 4.5 miles. And just to make this a bit harder, the vehicle has been designed to be as low-drag as possible.
Up to about 250 mph, ‘conventional’ car disc brakes can be used. These discs will have to endure the same spin speeds of over 10 000 RPM that the wheels will experience, with similarly massive stresses, so they are somewhat different to ‘normal’ car brakes. The BLOODHOUND SSC solution is likely to be multi-plate carbon discs, using aircraft-style circular stators (effectively large circular ‘pads’ to sandwich the disc under braking).
Above 250 mph, however, we have to lose a lot of energy in a short space of time. We considered exotic solutions such as retro-rockets, but they have a number of drawbacks (they would de-stabilise the vehicle, would require a blow-off cover that could then come back and hit the vehicle, could misfire and push the Car sideways, etc. None of these things are good). We rapidly settled on 2 robust and independent solutions – airbrakes and drag parachutes.
Airbrakes are a common feature of aircraft, to slow them down from high speed, and are mechanically simple to operate. They have also been around for a long time in the Land Speed Record world.
Left: George Eyston used airbrakes to slow his massive 7-ton Thunderbolt from over 350 mph in the 1930s
Right : A Tornado F3’s airbrakes extend on either side of the fin.
The problem with airbrakes is that to stop BLOODHOUND SSC in the required distance, the airbrakes would virtually have to double the cross-sectional area (and therefore drag) of the Car, which is technically quite challenging! Hence we are including the largest airbrakes that we can, to minimise our reliance on parachutes and to reduce the length of the safety overrun at the ends of the track, in case the parachutes fail.
Brake parachutes have also been around for a long time, in both aerospace and vehicle applications. Everything from Thrust SSC to the Space Shuttle has used them to slow down.
BLOODHOUND SSC’s parachute design is being finalised on the basis of the detailed performance figures for the Car, which are in turn based on the Computational Fluid Dynamics predictions of aerodynamic drag. However, in general terms, we expect to be deploying the cutes at anything up to 600 mph, which will add a drag load of anything up to 6 tonnes to the car. This will add another 1 ‘g’ to the car’s deceleration – in other words, it will reduce the speed by an extra 20 mph every second.
The problem with both the airbrake and parachute solutions is that they rely on aerodynamic drag, which changes with speed. Indeed, aero drag is proportional to the square of the speed – the drag at 800 mph is 4 times as high as the drag at 400 mph, which is 4 times as great as the drag at 200 mph. Hence an airbrake that works well at 800 mph will be ineffective at lower speeds. To optimise the Car’s deceleration, and thus stop it in 4.5 miles, we need to increase the effectiveness of the airbrakes and/or deploy extra parachutes as we slow down.
So how will we use these ideas together? The following sequence is an indication of how we hope to stop the Car:
- 1000 mph: close the throttle – deceleration rate is 3’g’ initially, then falls off rapidly
- 800 mph: start to deploy the airbrake, gradually increasing its area to try and maintain 3’g’ deceleration through the transonic region (800 down to 650 mph)
- Below 600 mph: deploy a ‘chute to increase the deceleration rate back up to 3’g’.
- Below 400 mph: deploy a second ‘chute if required.
- Below 250 mph: apply the wheel brakes as required to stop at the end of the track, ready for the turn round.
And if something goes wrong? If we need to stop in a hurry, then using all of these systems in quick succession (airbrakes at 800, both ‘chutes at 600/400 and then wheel brakes at 250) will stop the Car in well under 4.5 miles. If the airbrakes fail, then the ‘chutes are being designed to stop the Car within 4.5 miles by themselves. And if the ‘chutes both fail (which qualifies as really unlucky, but does happen – twice in Thrust SSC’s case) then the airbrakes will do what they can and the Car will have to use the track overrun. The larger the airbrakes that we can fit, the smaller the overrun required.
If this seems like a lot of complication, it’s necessary if we are to run BLOODHOUND SSC as safely as we intend to. For example, Craig Breedlove’s remarkable record run in 1965 almost ended in disaster when both of his ‘chutes failed. Spirit of America overshot at Bonneville and ended up nose-down in a brine pond, where Craig had to swim for survival. – we need to learn from his experience and make sure that we don’t repeat it!
A final thought on braking. Having a design where one of our big problems is having too much speed, and therefore having trouble slowing down, is a really nice problem to have for a Land Speed Record Car!