The Bloodhound project is back on track. The British team developing a car capable of reaching 1600 kmph is in South Africa for several weeks of high-speed testing on the dry desert track at Hakskeenpan in the Northern Cape. This is key to preparing for an attempt at a new 1200+ kmph land speed record next year. By the end of 2019, Bloodhound aims to demonstrate speeds above 800 kmph. The next step is to break the existing world land speed record of 1228 kmph.
The Bloodhound is powered by a rocket bolted to a Eurofighter-Typhoon jet engine. Commander Andy Green, who is to doing the driving, says that the team is fitting high-speed metal wheels, brake parachutes, pressure sensors and wheel fairings ready for speeds well in excess of 800 kmph in order to test the aerodynamics.
The first thing to test is the high speed desert wheels. Each weighs 95 kg and is forged from solid aluminium. At 1600 kmph, a wheel experiences 50 000 times the force of gravity tearing the rim apart, so it has to be solid metal; nothing else will cope with the extreme loads.
We don’t know how these wheels will behave on the desert surface. Metal rims running on the hard mud surface of Hakskeenpan will have very little grip due to friction,” he explains. “Normal cars rely on tyre grip for their stability and safety but 50 000 g would destroy any rubber tyre, so we are working with the unusual and poorly understood dynamics of solid metal wheels. We have given them some lateral grip on the desert surface by making them with a shallow V profile.”
As the car runs along the track, the wheels cut ruts in the mud surface, providing the sideways grip needed. Unfortunately, the faster the car goes, the shallower the ruts become. At supersonic speeds the wheels will be making tracks less than 5 mm deep, which will provide almost no sideways grip. However, as the aerodynamic grip will be huge, the car will get its directional stability from the supersonic airflow. This should give it some very lively steering at high speeds, with the front wheels acting like rudders in the supersonic airflow, producing very rapid steering responses.
The bad news is that as the car accelerates, the mechanical wheel grip goes down quickly, but the aerodynamic forces (which depend on the square of the speed) build up more slowly. This means that at medium speeds between 500 and 800 kmph there is very little surface grip from the wheels and very little aerodynamic response. “Just to make things more complicated, we also need to assess the lateral stability as we increase the speed, so I need to control the car and try to measure its stability, all at the same time. Luckily, I love a challenge,” he says.
Another key thing is how to stop the car from high speeds. “We’re using airbrakes and two separate brake parachutes, any one of which can stop the car by itself, to give us plenty of fail-safe. Going faster is optional, but slowing down is compulsory, so we have to get this right every time,” he adds. “At the maximum deployment speed of around 1100 kmph, the chute will produce a drag force of 9 tons, with an opening shock load 12 or 13 tons. This is a very rapid and violent process, which is why it is vital to test the whole system this year, before we go supersonic next year.”
The airbrakes are a more civilised way of slowing down. Large perforated panels open up on either side of the bodywork towards the rear of the car, gradually increasing the drag in a way that doesn’t try to tear the driver’s eyeballs out. The downside is that the airbrakes will produce a huge amount of turbulence and vibration at the back of the car, which needs to be measured to make sure it won’t break anything.
The next key area is the aerodynamics. This is such a complex subject that the detailed measurements still have to be done by building parts and testing them. This year’s testing will validate the computer modelling and yield very accurate drag figures. To help with this process, there are 200 pressure sensors fitted to the car to confirm the exact pressures across the bodywork, with and without the airbrakes. This is key to making sure the right size of rocket is fitted next year when Bloodhound takes aim at 800 kmph and beyond.
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