Attacking the Sound Barrier on land is one of the last great adventures left to man. Technologically and physically it is a tremendous joust with the unknown.
In 1947 Charles 'Chuck' Yeager became the first man to fly at supersonic speed. In 1953 Edmund Hillary and Sherpa Tenzing Norgay scaled Mount Everest, the world's highest peak. And in 1969 Neil Armstrong took that 'small step for man but giant leap for mankind' when he first set foot on the surface of the moon. Now small teams of far sighted men across the globe are set to probe new frontiers of technology by confronting the challenge of supersonic speed at ground level.
Supersonic vehicles need not only enormous power to overcome the dramatic rise in drag, but impeccable stability in an air flow regime that is itself unstable. Many times since the Sixties contenders have spoken of their plans to exceed the speed of sound with landbound projectiles. None, so far, has succeeded. When Gary Gabelich set his 622mph (1001 kph) record in 1970 the tops of his Blue Flame's tyres were travelling at supersonic speed; and when Richard Noble set the current 633.468mph (1019.44kph) record in the Castrol- sponsored Thrust 2 in 1983, airflow over its cockpit screens was also supersonic. But nobody has yet gone fully through the sonic range. The problems involved have proved insuperable, a barrier in themselves.
As a car accelerates, the force of gravity becomes less and less able to keep it firmly in contact with the ground, thanks to aerodynamic lift as air flows over and beneath it. That's why racing cars use the wings that have become so familiar. When that 'car' is powered by jet or rocket engines and seeks its absolute maximum speed, the problem becomes ever more acute. Without very finely tuned aerodynamics, the vehicle will simply take off even at subsonic speeds, but since it is neither designed nor intended for flight, it will only fly momentarily before crashing back to the ground. The forces involved - around forty times gravity at speeds around 800mph (1287kph) - would destroy it.
Air, like water, is a fluid that can be compressed. It is made up of innumerable tiny particles which are primarily molecules of oxygen and nitrogen. These travel at random in all directions and are continuously colliding with one another some five thousand million times every second. This generates both energy and pressure. Though these erratic particles travel at 1200mph (l93lkph), their mean speed is less. At sea level temperature conditions this is 760mph (1223kph), and this is known as the speed of sound or Mach 1, after the Austrian scientist Ernst Mach who made the first calculations of supersonic airflow in the 1870s. It can vary according to altitude and temperature.
When a vehicle travels at subsonic speed, where all airflow is below the speed of sound, it creates pressure waves because of the air's compressibility. These move away from it, and gradually get weaker As it moves into the transonic range, however, where some but not all parts of the airflow are reaching supersonic speed, it begins to catch up with the pressure waves moving ahead of it. When it reaches the speed of sound it is travelling as fast as those pressure waves and instead of being pushed away they fan out to form what is called a Mach wave.
In the case of an aeroplane in supersonic flight, the air waves are distorted into a cone shape and when the outer edges of this cone fan out and hit the ground, the sudden increase in air pressure is detectable as the famous sonic boom. There may be one or two, depending how close the shock waves are together.
What nobody yet knows with any degree of certainty is how a car will react when it creates a sonic boom. At 30,OOOft (9144m) there is plenty of room for a plane's shock waves to dissipate. At ground level there will be no such margin. What effect this will have remains to be seen. Will the shock waves be so severe that they try to lift the vehicle as they bounce back from the ground? Might they be strong enough to damage its structure? Might they actually prove fatal to a projectile and its pilot?
The risks may yet have to be quantified, but they are known to all of the teams currently challenging the Sound Barrier on land. The American contender Craig Breedlove, who was the first man to 500 (804) and 600 mph (966kph), speaks freely of his feelings. 'Of course you feel a little fear It's your safety valve. You need to control it, but if you are not a little bit afraid of this thing, you're not playing with a full deck. You are essentially a test pilot, and you build up your speed a little bit at a time. One step after the other.'
Can men in cars really go fast enough - and with sufficient safety margin - to break the Sound Barrier? Richard Noble, Ron Ayers and Andy Green of the Castrol-supported Thrust SSC Team have no doubt at all. Noble says: 'Early in the programme we had done extremely complex computational fluid dynamics to study the effects of airflow on the model of Thrust SSC, and we had then done our testThg with a model on a rocket sled travelling at Mach 1.1 at the Proof and Experimental Establishment at Pendine.
We then reached the critical poiht at which we had to compare the two sets of figures to see if they agreed. They did! They were an exact match. That meant that each set bore out the evidence of the other and from that point onwards Ron and I were confident that we knew exactly what happens to the airflow over and beneath a car travelling at supersonic speed. And from that moment an attempt on the Sound Barrier on land truly became feasible.'
The rotational speeds of a car's wheels, and the pure geography of the sites available may well prove to be the only real limiting factors in the continuing quest for speed on land. Certainly, Man's spirit, courage and technological ingenuity know no bounds.
 |
 |
 |
||
| Sponsored by | This site best viewed with Microsoft Internet Explorer 3 | |||
 |
 |
 |
||