Taylor Made: Superfast
Last month at Oshkosh, I was grateful to once again have the opportunity to see warbirds and fighter jets and aerobatic planes tear up the sky during the daytime and evening airshows. Whenever I get to wander the grounds, I am always keeping an eye out for samples of innovative design (they’re not hard to find in a place like that). Ironically, one thing that caught my attention this time around never actually left the ground.
Before watching the Shockwave, a turbojet-powered tractor unit, race down the runway at KOSH, I was unaware that we have a full-sized jet-powered truck category in the world records. Apparently we do, and the Shockwave currently holds that record at 376 mph. When it raced at Oshkosh, a MiG-17 followed it down the runway at half speed. The Shockwave is a 1985 Peterbilt semi, but the only thing stock about it is the cab itself.
The turbojet is commonly considered the simplest type of jet engine because it follows a simple combustion cycle: air comes in through the inlet, then gets compressed and mixed with fuel. The dense mixture is combusted, after which exhaust is expelled through the outlet, producing thrust. I’ll take this opportunity to point out that to make more power, a turbojet needs to burn more fuel; performance does not equal fuel economy in this case. Balancing good performance with decent fuel economy has become ubiquitous with automotive design, but in aviation, design considerations are slightly different. Performance is more highly valued because failure in a plane has the added dimension of being suspended above ground. To put things in perspective, the Boeing 777 series runs GE90 high-bypass turbo fan engines. The GE90 series comprises the world’s most powerful jet engine to date. The 777-300ER has a range of 8,555 nautical miles and max fuel capacity of 47,890 gallons.
The Shockwave’s engines are mounted in a pyramid configuration with a slight offset from zero-degree angle of attack to direct thrust downward, keeping the 6,800-pound truck from lifting off. The engines produce 36,000 horsepower and are outfitted so that diesel fuel can be dumped directly into the hot outlet to produce flames of exhaust. The twin exhaust stacks are similarly outfitted to produce flames out the top, so that the truck can go into afterburner mode and produce a show of flames. In this mode, the excess combusting fuel explodes in large, loud bursts, giving the reason for the truck’s name. The truck features lots of design considerations that normally wouldn’t be necessary; for example, treads are shaved off its tires prior to racing so that it doesn’t shed rubber as it accelerates. Peterbilt semis were not designed to race jet planes, but with careful attention to detail (and tolerances, our favorite), this one has been successfully converted. The moral of this story is, you never know when your design may have potential to excel in an application other than the one you were aiming for.
Photo at top: The Shockwave is part of a series of jet-powered trucks (photo credit shockwavejettruck.com).