Saturday, October 06, 2007

Andy Green and Thrust SSC

It's now 10 years since Andy Green broke the world land-speed record, and the sound barrier, in Richard Noble's Thrust SSC at Black Rock Desert, Nevada. Erstwhile fighter-pilot Green is a remarkable individual, and this month's Motorsport magazine carries a gripping account of the difficulties (those are 'challenges', for younger readers) he faced.

We went back to Jordan in May 1997...We started to use some quite high-end acceleration, and on the first full afterburner run I'm taking it nice and steady, 200, 250, 300, max burner, and suddenly it's going all over the place. I just cannot keep it in a straight line.

Driving a normal car, if it diverges, you respond to correct it. But I was getting into what in an aircraft you'd call a pilot-induced oscillation. The vehicle diverges, you put in a response through the steering wheel, it responds, you put in another, but at that point the vehicle is reversing its response, so you get a double input. You and the vehicle are working out of phase. Every time there's the slightest input, say from irregularity in the desert surface or a crosswind, you've got to correct it. But if you start steerting at the frequency of the car it gets worse, not better. I got out of the car that day and I said to myself, I think this project has just finished. The car's unstable. I think we're stuffed.

I didn't feel I could talk to Richard or anyone else about it, so I spent the night thinking it through. And I came up with the conviction that I was simply going to have to bully the car into being stable, go very high frequency with my steering movements to keep it under control, coupled with lower frequency inputs to steer it. I'd have to steer it at two separate frequencies, simultaneously. So that's what I did, and it worked. I'd found a way to drive the thing. The downside was that I was never going to enjoy it. Imagine taking all the hairiest bits out of a three-hour sortie in a Tornado, and doing them all in two minutes. It was the hardest thing I've ever done: like trying to balance the point of a pencil on the end of your finger.

The team moved to Black Rock Desert in September 1997, and on October 15th set a record of 763.035mph, Mach 1.02

We had active suspension because every time the aerodynamic neutral point moves you have to alter the pitch of the car to compensate. And plus or minus one degree of pitch on the car equals plus or minus 10 tons of load on the front wheels. Running the active fully up through the trans-sonic region increased the load on the front wheels by almost 10 tons. The wheels started to plough into the desert.

We were generating some very powerful shock waves in front of the car. We found the wheels were actually going round less fast than the ground speed: we were ploughing the surface in front of the car. The shock wave was digging up the desert, so the four individual wheel tracks disappeared, and we started to get a single ploughed trace 12 feet wide.

Things change so much between Mach 0.8 and Mach 1. When the shock waves start to form, depending on the shape of your vehicle, you've got a mixture of sub-sonic and supersonic flow. If you're doing Mach 0.95 and the air accelerates locally by another five percent, over the cockpit or the curve of a wheel arch, it will form its own little shock wave there. In the trans-sonic period its doing that in various places around and underneath the car. The pressure difference where the air goes from sub-sonic to supersonic is so great that if it happens underneath the car it can lift it off the ground.

And the car may be fractionally different in shape from one side to the other. It's a hand-built car. We measured it as accurately as we could, but the tiniest difference, the thickness of a few coats of paint, can make the shock waves form earlier on one side. It happens with aircraft when you take them supersonic, but tiny corrections with the controls can fix that. With the car, it's the wheels that have to take the differences in load, and you start to realise the magnitude of the forces involved when a tiny difference can translate to an extra ton of load on one of the front wheels. Once you get well over Mach 1, life gets much easier. It's getting there that's the challenge.

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