High-speed photography shows the shock waves produced by the rapid acceleration of the 4-stage rocket sled.
It might make the ultimate amusement park ride, if anyone could survive. Hitting hypersonic speeds of Mach 8.5–that’s 6416 mph, in civilian terms–a 4-stage rocket sled took just 6.04 seconds to blast the more than 3-mile length of track at Holloman Air Force Base in New Mexico.
The test broke a 20-year-old standing land speed record. But at a cost of $750,000, the objective of the run was to do a lot more than get into the Guinness Book of Records.”We’re fighting tomorrow’s wars today,” says Lt. Col. Russ Kurtz, Operations Director at the 846th Test Squadron, after the successful test at the upgraded Holloman High-Speed Test Track (HHSTT). The rocket sled carried a 192-pound instrument package designed to simulate the warhead of a ballistic missile defense system. The only ground test facility capable of punching payloads up to that speed, the HHSTT is a key part of the U.S. Air Force missile defense warhead test program, and is being used to simulate full-scale intercepts of enemy rockets.
Rocket sleds have been used for a variety of research purposes over the years, both manned and unmanned. They helped in the development of aircraft ejection seats, and tested the limits of human endurance during rapid acceleration. Holloman has been a center of rocket sled research since 1950, and held the previous land speed record set in October 1982 when it blasted a 25-pound payload to a speed of 6119 mph.
Streaking across the New Mexico desert, the rocket sled lights the sky.
Sending a sled along a rail track at hypersonic speeds–more than five times faster than the speed of sound–isn’t easy. When you’re traveling the equivalent of 31 football fields per second, there’s no room for error. A 1994 run for the record failed when metal “slippers” holding the sled to the track delaminated, requiring the sled to be redesigned.
The April test marked the culmination of a 5-year, $20 million Hypersonic Upgrade Program that resulted in an improved rocket sled, new propulsion systems and precise alignment of the rail system. The result is what the Air Force calls “the longest, straightest, fastest track in the world.”
A critical difference between then and now: In 1982, the HHSTT was a monorail system. The sled rode on a single slipper–wheels wouldn’t survive such speeds–and created a lot of vibration. The upgraded track is a narrow-gauge track, and the sled rides on a pair of slippers. “That reduced the vibration fourfold,” explains Lt. Col. James Jolliffe, 846th Test Squadron Commander, “producing a smoother ride,” and making it less likely the sled would jump its track.
Vibration encountered at high speeds ruled out the use of wheels. The sled travels on a pair of pads called “slippers.”
The new sled operates much like a rocket launching into orbit. It’s powered by 13 separate motors organized in four stages, each attached to its own part of the sled. As each stage burns out, it detaches, while the next stage ignites and pushes the sled to an even higher speed.
The final two stages each use single Super Roadrunner, or SRR, rocket motors. Designed specifically for the HHSTT, they weigh in at a mere 1100 pounds apiece. Yet, during burns of just 1.4 seconds, each produces a total of 228,000 pounds of thrust.
Of the 17,014 ft. traveled by the sled, 11,000 ft. was through a 184-in.-dia. tube filled with lightweight helium gas, simulating flight through the upper atmosphere, where friction is sharply reduced. The sled accelerated “like a jet hitting its afterburners,” according to Capt. Steve Georgian, 846th TS Acquisitions Management Chief.
A Navy jet shot from a carrier by a steam catapult might reach 8 or 9 g’s, about as much force as a human can take before blacking out. The unmanned, bullet-shaped payload on Holloman’s rocket sled reached 157 g’s, or 157 times the force of gravity. The Air Force estimates that when the 192-pound payload slammed into its target, it carried the energy of a car hitting a brick wall at 2020 mph. The sled itself slides beneath the target, so it can be reused.
Jolliffe calls Holloman’s missile defense tests a graduation step between computer models and simulation, and a test flight. It’s certainly a lot cheaper than actually launching a missile defense test. The Pentagon expects to spend just over $10 million on 16 hypersonic sled tests. The tests supposedly will eliminate the need for four missile launches, which would ring up a $300 million bill.
It took the Air Force more than two decades to best its old land speed record. But it’s not likely to wait that long to do it again. The track can reach Mach 9, and Holloman officials acknowledge Mach 10 is a possibility though they won’t say when they expect to get there. They’re also planning to upgrade the HHSTT to carry even heavier payloads.