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Driving to Mach 1--"Jetmobiles" try to go supersonic

American and British teams face off in the desert to be the first to drive at the speed of sound



Image courtesy of SSC Programme Ltd

(Editor's note: This article originally appeared in the October 1997 issue of Scientific American magazine. We are posting it because of some related news.)

Before Chuck Yeager broke the sound barrier in 1947 in the X-1 experimental plane, engineers had predicted that the buffeting produced by supersonic shock waves might tear apart his sleek craft. As drivers—one might call them pilots—of two custom-made supersonic cars recently prepared to punch through Mach 1, the engineering community voiced similar concerns, perhaps this time with more reason. "Anything that upsets a vehicle at 600 miles [around 965 kilometers] per hour or more puts it in a regime you don't want to be in," comments Make McDermott, a professor of mechanical engineering at Texas A&M University. "Aerodynamic forces make a ground vehicle not a ground vehicle. They make it want to fly."

As this issue goes to press, the most serious attempt ever to break the speed of sound in a ground-based vehicle is scheduled to take place this year in September and early October in the Black Rock Desert northeast of Reno, Nev., the largest dry lake bed in America. The push toward Mach 1—the speed of sound, which is around 750 mph at the temperatures encountered at Black Rock—promises to be a dramatic face-off between two teams that have at different times claimed ownership of the title of fastest car on earth.

One contender is Craig Breedlove, the 60-year-old driver of a "jetmobile" called the Spirit of America. Breedlove captured the record five times between 1963 and 1970. The other team is headed by Richard Noble, the now 51-year-old driver of the British vehicle that achieved the current record of 633 mph in 1983. Although Noble is overseeing the effort, his car, Thrust SSC, will be driven by Royal Air Force fighter pilot Andy Green.

The contest is not a drag race in which both cars compete simultaneously. The two teams will share the desert, making separate runs at gradually increasing speeds. Even if they do not break the sound barrier, they could still best Noble's 1983 record or the 700-mph mark.

These teams are not the only ones in the world trying to break the 1983 record. But the intensive engineering and expense that have gone into both their vehicles make them the only candidates that can expect to approach anywhere near the speed of sound.

Unofficially, the sound barrier may already have been broken. In 1979 stuntman Stan Barrett claims to have piloted a rocket-powered car, the Budweiser Rocket, to a speed of nearly 740 mph.

But if the car did go that fast—which Breedlove and others hotly debate—it achieved that speed going only in one direction. The International Automobile Federation, the Paris-based organization that certifies these records, requires that a vehicle must average a record-breaking speed over a measured mile during two runs going in opposite directions, each drive within an hour of the other. In the Black Rock Desert, the cars will move along a 15-mile flat. They will accelerate for nearly five miles, move through a measured mile in about five seconds in the middle of the course, then slow down for five miles by cutting power and releasing parachutes before applying brakes at speeds below 300 mph. Then they will turn around and go back the same way.

How does one build a car to drive to Mach 1? Both Breedlove's and Noble's teams have chosen jet engines originally used on fighter aircraft. But choosing to strap a driver to a jet engine turns out to be one of the simplest design decisions. Keeping the driver alive is another matter. Little is known about what happens to a car when it reaches the speed of sound. In an airplane the shock wave that occurs when the vehicle nears Mach 1 attenuates in the surrounding air, as Yeager learned. When a car approaches the speed of sound, the boundary between air flowing at supersonic and subsonic speeds creates shock waves between the vehicle and the ground that could initiate a potentially fatal back flip or side roll.

Jet cars have already demonstrated their perils. In an attempt to achieve a record last year in the Spirit of America, Breedlove veered out of control at an unofficial 677 mph and damaged the rear wheels. The British team has also experienced travails because of stresses on the car's frame. Thrust SSC sustained damage this past July when a rear suspension bracket failed during a 540-mph-plus test run in Jordan's Al Jafr Desert.



Airplanes without Wings
The competitors have adopted divergent design approaches. Breedlove has tried to reduce the car's frontal cross section to lessen drag from an oncoming wind of 750 mph or more. The Spirit of America weighs 4.5 tons and is 44 feet long and 8.5 feet across at its widest point, the span between the back wheels. That width is almost four feet less than its British competitor. The elliptical shape of the front of the body is intended to let destabilizing shock waves underneath the car escape to the sides. In addition, the front part of the body sits only one inch off the ground to reduce the area in which pressure can build up. A larger clearance of 18 inches in the rear allows pressure waves to escape from the back.

The front wheels are three aluminum disks that spin on a single axle, each separated by a tenth of an inch, a configuration designed to increase the inertial forces that prevent yaw—side-to-side movement. The wheels themselves are wound around their circumference with graphite fibers capped with fiberglass. These high-performance tires can protect the outer wheel rims, which may be subject to 35,000 times the force of gravity. "Should a wheel hit a rock when highly stressed, you don't have to be a rocket scientist to figure out that it could fracture from the outer periphery to the hub, and you'd have catastrophic failure," Breedlove says.

Each rear wheel extends out a few feet from the body of the car. This choice of design tends to move forward the center of mass (center of gravity), thereby enhancing stability. "It's like handles on a wheelbarrow," Breedlove says. "The longer you make the handles, the easier it is to pick up the load and the more weight goes on the front wheel of the wheelbarrow." The rear axles are encased in a flattened, horizontal winglike structure called a fairing. Fins are attached to the far edges of each fairing to guard against yaw forces.

Since his accident last year, Breedlove has refined the aerodynamic shape of the fairing so that airflow speeds up on top while slowing down on the bottom. This design change is intended to prevent the air underneath from becoming supersonic and generating shock waves. Taking this step also required adding a set of flaps—wing surfaces that can be set to prevent the wing from lifting the car from the ground.

Stability First
Taking a different tack, the Thrust SSC team first determined the most stable design needed to reach Mach 1 safely. Only then did it decide on power and drag reduction measures. In contrast to Breedlove's penchant for trial-and-error development, team aerodynamicist Ronald F. Ayers, who once designed guided missiles, relied heavily on supercomputer simulation and supersonic tests with a two-foot-long model on a sled that was propelled by rocket fuel along a track.



From Ayers's technical analyses, the Thrust SSC emerged as a larger, bulkier vehicle than Spirit of America, weighing seven tons and measuring 54 feet in length. The team went to great pains to keep stable the vehicle's pitch—the slant of the nose above or below the vehicle's horizontal axis. "Too much nose up, and you take off like an airplane," Ayers says. "Too much nose down, and you bury yourself in the desert."

The difference between becoming a Patriot missile or a miner's drill is an angle of only a degree or so. That angle also changes at supersonic speeds. The car incorporates an active suspension that can make the necessary adjustments in pitch as the vehicle nears the sound barrier. Strain gauges measure the load on the wheels and relay this information to an onboard computer. Hydraulic jacks at the rear of the car can then adjust attitude automatically. Between runs, the angle of a horizontal stabilizer at the back of the car can also be adjusted to ensure that the rear wheels remain firmly on the ground.

The Thrust SSC's front wheels hide inside the engine cowling, the covering that houses the engine, which reduces the cross-sectional area that faces into the wind. Placing the engines on the side and maintaining a wide front wheelbase moves the center of gravity farther forward than that of Spirit of America, a measure intended to keep the nose in position. Setting forward the center of gravity counteracts the tendency of the car to lurch into a spin. Increased thrust from two engines, Ayers says, compensates for added weight and the additional drag produced by the wider front profile. "There's no weight limit for this class of car," he quips. Unlike Breedlove, the Thrust SSC team uses forged aluminum wheels that turn without tires. The team hopes the desert surface will be soft enough to make up for the absence of tires on the wheels.

Driver Green sits in a cockpit placed at midsection between the two engines, allowing him to gain a better feel for side-to-side movement of the car. Green steers the two rear wheels, which are tucked toward the back of the body's underside to avoid interference with the jet exhausts. One rear wheel is placed slightly behind and to one side of the other, avoiding a drag-producing bulge in the rear section that would have occurred if the wheels had been placed parallel to each other. The front wheels are fixed in place: avoiding a front steering mechanism reduces drag.

Lessons learned in building a supersonic car may have scant value beyond an entry in the Guinness Book of World Records and thrills for those who make and drive the cars. "I am convinced that taking part in the world land-speed record is the most exciting thing you can do on God's earth," Noble says, expressing the missionary zeal that his team brings to the task. Practical spin-offs of running a car at these speeds are at best conjectural. Breedlove notes the possibilities for the tire technology. But when asked about where this type of graphite tire might be useful, Breedlove ponders for a moment and then replies, "I have no idea. My mission is to get the landspeed record. I'm not moved by much else. I think Kennedy wanted to go to the moon. He didn't care about spinoffs from it. He just wanted to beat the Russians."

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