‘The greatest car race on God’s earth.’ That’s how impassive yet inspiring Richard Noble, holder of the current world land speed record (633mph in Thrust 2, 1983), describes the contest between his Thrust SSC and Craig Breedlove’s Spirit of America – to be the first to go supersonic on land. He’s not wrong.
And by the time you read this, I for one am convinced that Thrust will either have won, or be well on the way to winning this quite extraordinary challenge. For when RAF Squadron Leader Andy Green, pilot of this incredible machine, here’s the message, ‘Thrust SSC, clear to roll’, he will be at the ‘wheel’ of the most powerful car ever created.
The technical data are awesome. Noble: ‘Thrust SSC is 54ft long, 12ft wide and weighs 10tonnes. It’s built on a steel frame with titanium and carbon fibre shell and runs on solid aluminium wheels. It’s powered by two ex-Phantom Rolls Royce Spey jet engines with afterburners delivering 110,000hp. That’s three times my Thrust 2 – equivalent to the combined output of 140 Formula 1 cars or three Royal Navy frigates!’
That horse power generates maximum acceleration of 1.3g – burning four gallons of aviation fuel a second.
Ron Ayers, sprightly chief aerodynamicist and research co-ordinator for the SSC project, says: ‘SSC delivers 20tonnes of thrust and we’ve designed it to achieve 850mph. From 0-800mph and back down to 0 will take 11miles – and just 90 seconds!’
He adds: ‘There is nothing remotely puny about this car!’
According to Green – clipped, dry, and with 10 years of flying Phantoms and Tornadoes – SSC will start gently: ‘so as not to suck in too much desert! At 100mph I engage full power; at 150mph I light the afterburners. 0-200mph takes 10 seconds; 200 to 400 takes eight; top speed is … when I take my foot off!
Just 12 seconds to go
‘When we reached 540mph on the Al Jafra desert in June, acceleration was still excellent. I’m sure we were five seconds away from Richard Noble’s record and just 12 from going supersonic. How much faster she’ll go … we’ll soon see.’
Some time in the six weeks allotted from September 2nd at Black Rock desert, Nevada, Thrust SSC is going to do it. They and I are as certain as anyone can be about something as yet untried. And this is the point; untried, yes; but untested, no. Green makes the point: ‘This is a safe car designed to go very fast. It’s not a fast car made safe’.
And this is where the world of sensing, instrumentation and control comes in. Ayers: ‘When first approached, my view was not only that supersonic travel on land is very dangerous; it could be impossible. The real problem is keeping any car on the ground; the aerodynamic forces generated by the air flow under it will be enormous. And then there’s the litterally huge shock waves to withstand as the car moves up beyond Mach 0.7.
‘While there’s a lot of data on sub and supersonic movement; very little was known about the transonic region from Mach 0.7 to 1.1. So there was no theory to go on – and we couldn’t construct a conveyor belt capable of 800mph in a wind tunnel!’
So although Ayers was confident of his car shape (forward centre of gravity; wide laterally symmetrical weight distribution; and rear, near-centreline steering), he had plenty of misgivings. And hence his use of simulation via Computational Fluid Dynamics on Cray supercomputers. In fact, Cray gave the project over £1/4 million pounds worth of supercomputer time to predict pressures and shock waves around the car as it moves through the transonic region!
Ayers: ‘And it looked good; theoretically, the design would hug the ground. But I couldn’t risk just trusting simulation. So in June 1994 we took an instrumented model of the car to the Pendine rocket centre, and fixed it on the front of their rail-mounted sledge.’
The idea was to validate the test data by blasting the model up to supersonic speeds just above ground – with real time data from Kulite pressure sensors telemetered to a data recorder. In fact, air-to-ground strafing rockets provided the power.
‘They took our model from 0 to 820mph in 0.8 seconds, pulling 50g acceleration!’, says Ayers. ‘But the sensors are very fast acting and the data proved the simulation results, showing that SSC would stay on the ground – and be controllable.’
Research and development for safety and controllability has covered about every aspect of cutting-edge engineering. Calculations on the front wheels, for example, showed radial stresses at the design speed limit of 35,000g. So CAE was used to develop a pretty unorthodox solid aluminium forged design. Then there’s the combined passive and active suspension and it’s computer-controlled safety systems.
Jerry Bliss, systems manager, young, enthusiastic and utterly committed, takes up the story. ‘Thrust SSC has quite a large real time data system. There’s 121 on-board sensors and two on-board computers – one handling safety and the active control systems, the other high speed data collection and telemetry. There’s also two ports on the car for plug-in laptops.
‘H&B sensors monitor brake and wheel bearing temperatures and we also monitor bearing condition acoustic signals – information we need to know very fast! In the nose cone and under and over the car body and one of the engines are Kulite pressure sensors monitoring air flow pressures. There’s also a big strain gauge array from Measurements Group throughout the body, monitoring real time forces.
Sensing and control at 1kHz
‘Then on the jet engines we have thrust output monitors from NPL looking for “flame-out” – if that happens, the instantaneous asymmetric thrust could easily spin or roll the car. So this is the only system where the computer overrides the driver. If measured thrust variance goes beyond threshold, the computer instantly shuts down the engines, jacks up the car rear end and deploys the parachutes. The first Andy will know is a light in the cockpit!
‘There’s also load cells on the wheels and LVDTs on the steering and active suspension controls.’
Actually, the LVDTs are classic, high speed safety-critical stuff. Six were provided free by Solartron – four measuring wheel position and two on the active suspension.
As the car goes transonic, air moves at supersonic speeds over some of the surfaces, causing rapidly-increasing shocks which, close to the ground, could wreak havoc with stability. So two active struts (hydraulic rams) control car pitch in fast, closed-loop mode. The LVDTs monitor how far the struts are open, signal the on-board computer, which in turn signals Moog servo valves to cancel error. The loops control six tonnes of force to 1mm accuracy 1,000 times a second!
Not only do they thus handle the car’s ride in just about the most extreme conditions you can imagine; they’ve also provided Mb of test data for the design team to understand, diagnose and refine the suspension. For example, in the last Jordan run, on-board video cameras showed the rear steering wheels “shimmying” from side to side. Analysis of the LVDT data pointed to the caster angle of the rear wheels, also suggesting softer suspension.
Bliss says it all: ‘Without the LVDTs we wouldn’t be able to run the active suspension system – key to the car’s safety at speed.’
Sponsorship from the world of C&I
Thrust SSC is an amazing story, not just of technology and engineering, but also of team spirit and endeavour. It involves sponsorship and support from over 200 companies to the tune of millions of pounds of equipment, services, expertise and time.