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Speed duel in the desert
October 7, 1996
11 Min Read
It's said that power tends to corrupt, and absolute power corrupts ab-solutely. So, what does 150,000 horsepower do? It gets you Spirit of America and Thrust SSC--two cars designed to blow through the sound barrier and leave the current Land Speed Record tumbling in their wake.
Should all go as planned, the vehicles will meet in the southwestern desert this fall and duel for the title of world's fastest car. At stake is nothing except ego, national pride, claims of engineering prowess, and the possible distinction of having designed the first car to exceed 800 mph.
Each is a mechanical masterpiece, a four-wheeled Olympian that combines strength, power, precision, and finesse. The vehicles exploit advancements in materials, sensors, electronics, hydraulics, computer analysis, and construction techniques that previous car designers could only dream of. Under their streamlined skins lie active hydraulic suspensions, ultra-high-speed solid wheels, gasoline-fueled jet engines, and, most importantly, the deft touch of designers possessed of decades of experience in designing land-speed-record cars.
For both Craig Breedlove and Richard Noble, way too fast has never been fast enough. Breedlove, 59, already an American legend, held the World Land Speed Record (WLSR) five times and is the first person to drive faster than 400, 500, and 600 mph. Noble, 50, an Officer of the British Empire, holds the current WLSR of 633 mph, set while piloting his Thrust 2 jet-powered car at Black Rock Desert, NV, in 1983. Obviously, neither can be accused of driving more slowly as he ages.
Yet, campaigning a land-speed-record car isn't something to be approached casually. In the past 97 years, the names of numerous drivers have been etched into tombstones instead of the record book. Already, Guinness Book of World Records credits Breedlove with the sobering distinction of having created the world's longest skid marks--six miles--formed during a record-attempt gone bad, in 1964. Bowing to the demands of simply designing and funding the project, Noble has passed the car keys to Andy Green, an RAF pilot who will actually drive the Thrust SSC.
Handed the exact same problem--in this case, how to build the fastest possible car--two engineers will often arrive at completely different solutions. Thus, the designers of Spirit of America and Thrust SSC agree on some things--jet-engine propulsion, for instance--and they disagree on many others--such as number of engines, construction of wheels, types of brakes, and even which end steers the car. Here's a closer look at each vehicle and the engineering that may propel one or both of them into the record book.
Spirit of America
When asked about his design philosophy, Breedlove responds, "to build and package the car as tightly as possible around the jet engine." Not a schooled engineer--but arguably a brilliant natural designer--Breedlove penned the basic shape by drawing on 30 years of experience with past record-setting vehicles.
Spirit of America, housing a single turbojet, is the simpler of the two vehicles. Proportioned like a dart, the car's cylindrical body measures 47-ft-long and just 8.3-ft-wide. It consists of a welded truss frame made from Ryerson RyStar, a low-carbon, high-strength-steel tubing. Over the top of the frame lies a stressed skin of 0.080-inch-thick Alcoa 3003 aluminum attached with Monadnock's Airloc fasteners.
The snug cockpit prevented the application of this same construction method. Instead, engineers specified a composite construction formed from a Baltek balsa-wood core sandwiched between layers of carbon fiber (from Hexcel). An outer layer of tough DuPont KevlarTM protects the cockpit from kicked up debris, such as stones, while a GE Plastics' LexanTM polycarbonate windshield supplies the outward view.
Breedlove eliminated the high rear stabilizer synonymous with his past designs. Instead he mounted the rear wheels outboard and aft of the main body. Cantilevered on pylons, the wheels are housed in composite vertical fairings, also from Hexcel, that assume the role of stabilizers. Their positioning also moves the center-of-gravity forward and the center-of-pressure aft, important qualities for static stability.
Power is provided by a General Electric J79 engine that pumps out 22,650 lbs of thrust running on Shell premium unleaded gasoline. This compares to its normal duty in the F-4 Phantom fighter where it produces just 16,800 lbs of thrust from jet fuel. What modifications were required to accommodate unleaded gas? "None, really," says Breedlove. "We tweaked a few adjustment screws, and that's it."
It's drag, stupid. He chose the body's cylindrical shape to minimize interference between the bottom of the car and the ground. Spirit of America is designed to pass through the speed of sound (about 750 mph) and, theoretically, a shock wave constrained beneath the vehicle could lift it off the ground.
The shape has other advantages. Though angled down slightly, it shouldn't generate too much natural downforce. Air drag normally exceeds rolling resistance by a large margin. But, Breed-love notes, a poor design could result in unwanted downforce at 700-800 mph of 10,000 or 20,000 lbs, sending the rolling resistance soaring.
"My first car was cylindrical as well," he says. "Its low drag enabled the car to average 539 mph through the mile with only 4,200 lbs of thrust. This car has probably 25% less drag and five times the thrust."
Greatest challenge. The design of the wheels posed possibly the greatest engineering challenge. Measuring 36.5-inches in diameter, they must withstand more than 7,000 rpm. At this speed, the tension load at the periphery of the rim could easily cause catastrophic failure. So Breedlove created a multi-piece wheel with a "tire" made from filament-wound carbon/glass composite in a rubberized epoxy matrix. An Alcoa 6013 aluminum billet hub attaches to an aluminum disk spin-formed by Center Line Performance Wheels. Each wheel turns on Timken tapered roller bearings protected with seals from Chicago Rawhide. Special bolts from Holo Krome hold the entire assembly together.
"We made the tire into a reinforcement hoop," Breedlove explains. "The wheel expands into the hoop, actually putting mild compression loads into the disk." In tests, the wheels have survived more than 8,000 rpm.
Two of the wheels, mounted in tandem, lie just behind the cockpit. They attach to a coil-over hydraulic shock with just 1.5-inch jounce and 1.0-inch rebound. The rear wheels rely on the natural flex of the long, cantilevered body and wheel struts for their suspension. Strain gauges at the rear axles and load washers under the front-suspension springs measure the down- or up-force on the car. After gathering data on test runs, Breedlove plans to fine tune the suspension loading by trimming two small airfoils located on either side of the forward fuselage and aluminum plates on the rear-wheel struts.
An unusual braking system consists of a steel ski mounted behind the front wheels that is pressed against the surface of the desert. Breedlove refers to it whimsically as the "Fred Flintstone" brake. Parachutes produced by Syndex Aerospace form the primary means of slowing the car, and the ski is intended only for speeds of less than about 60-mph.
The team has scheduled time at both Black Rock Desert, NV, and Bonneville, UT, this year to conduct tests and--ultimately--top-speed runs. Breedlove expressed nothing but respect for the capabilities of his competitor, Noble, but still managed a good natured jab now and then. "I tease Richard about the two engines," he says, "I'll ask them, 'what did you do, double the power and triple the drag?' I think our lower-drag design will win."
Nothing defines the design of the Thrust SSC more than its most dominating feature: two engines. Strapped on either side of the slender fuselage, the pair of Rolls-Royce Spey 205s produce 25,000 lbs of thrust each, more than twice that of the Spirit of America's single J79.
Ron Ayers, aerodynamic engineer on the project, arrived at the twin-jet configuration after weeks of trying to position the cockpit in an acceptable location around a single engine. For safety reasons, he was unwilling to place the driver at the nose of the car--Breedlove's solution. And Noble's previous car, Thrust 2, with its engine to one side of the cockpit, had unacceptably high drag. But it was while dickering with the along-side concept that Ayers surmised, why not an engine on both sides of the cockpit? And Thrust SSC was born.
With the freedom to position the engines well forward, he obtained a desirable center-of-gravity. Wheels mounted inside the nacelles yielded a wide, stable track. And the 54-ft-long chassis with a rear stabilizer placed the center of pressure well aft.
Thrust SSC will have to contend with the unknowns of passing through the sound barrier. At that speed, the aerodynamic forces will greatly exceed that of gravity. Should the nose pitch up just 0.5-degree higher than optimum, the car will take off. "The sound barrier is, in fact, the problem, now isn't it?" says Ayers. "What happens in the transonic region is very uncertain, since there is no transonic theory of aerodynamics."
To confirm the soundness of his basic design, he turned to CDR Limited of Swansea University. Researchers there crunched six hours of time on a Cray 92 supercomputer conducting computational fluid dynamics studies with their own Flite CFD program. Thir-teen high-speed tests followed in which a scale model of the car was mounted to a rocket sled and, while passing just 1-mm off a simulated ground surface, propelled to 820 mph. "We got brilliant correlation between the models and the tests," beams Ayers.
Thrust SSC's primary structure consists of a thin-walled, square-section steel tubing welded into a space frame, and a carbon-composite nose. To the frame are attached aluminum body panels from British Alcan along most of the car, and titanium sheet from IMI Titanium aft of the engines to withstand the 300 degrees C exhaust and 175-dB sound-pressure levels.
Engineers made liberal use of Perma-bond structural epoxies and acrylic adhesives in both these areas to stiffen the chassis and attach panels that would rip free with rivets alone. Adhesives also bond the series of aluminum hoops that form the engine casing.
Active suspension, tandem steering. More than 147 onboard sensors monitor everything from thrust imbalance to airspeed. In particular, strain gauges from the Measurements Group sense the loads on both the front and rear suspensions 500 times a second. This information feeds to a computer that controls the Active Ride Suspension (ARS), a hydraulic system at the rear wheels designed by Sterling Hydraulics that trims the car's attitude to maintain constant downforce. "We've essentially turned the car into a ground-hugging servo mechanism," says Ayers. Filters, supplied by Pall Industrial Hy-draulics, keep the fine, abrasive desert dust out of both the hydraulic fluid and fuel systems.
Should the nose lift off suddenly, a safety system triggers aero-dynamic surfaces to respond in 10 msec. The system applies about three tons of downforce to the nose, preventing a back flip, and permitting the hydraulic sus-pension to catch up.
As with Spirit of America, wheel design proved especially critical. Glynne Bow-sher, mechanical engineer, drew on his experience with Thrust 2 to create 34-inch-diameter solid disks forged from L27 aluminum alloy. Each weighs more than 160 kg (352 lbs) and runs on SKF hybrid angular-contact bearings with steel rings and ceramic balls. Non-vented, carbon-disk brakes, 15 inches in diameter, will bring the nearly nine-ton car to a stop from 300 mph. They have been tested to more than 10,000 rpm.
To minimize frontal area, Bowsher fixed the front wheels and steers the rears with an unusual staggered tandem arrangement. This was necessitated by the confines of the aft chassis, which measures just 26 inches wide. "It was obvious that you couldn't fit two active wishbone suspensions side-by-side in that space," he says.
Instead, he placed the left rear wheel behind and to the side of the right rear one. Each requires its own gearbox, since the steering ratios are unique. Past about 400 mph, the effect of the vertical stabilizer renders the steering ineffective, and the car tracks straight on its own.
Ayers, Bowsher, and Noble each emphasized the importance of safety to the project. Some of the car's critical systems have quintuple redundancy. Says Ayers, "It's a very unforgiving exercise. You can't make one mistake; you can't eject the driver--not at 800 mph; you've got to make sure you do not have accidents."
Fast as they are, will either Spirit of America or Thrust SSC set the "ultimate" speed record? Noble thinks not, basing his guess on studies of the human body's acceleration limits. "I guess one can draw conclusions that you can work up to an 8g acceleration and hang on tight for around a 20g deceleration! With the new LSR cars approaching 3g acceleration, you can see that there is a long, long way to go."
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