Blazing like a meteor through the sky at nearly Mach 10 on Nov. 16, 2004, NASA's third X-43A scramjet flight looked as easy as the second flight did months earlier, but looks are deceiving.
Image right: Infrared image of the world-record Mach 6.8 flight of the second X-43A scramjet on March 27, 2004. U.S. Army photo.
This high-risk flight, following the March 2004 world record-smashing Mach 6.8 flight, demanded that no details be overlooked, no matter how small. When the second X-43A research vehicle flew at Mach 6.8, or nearly seven times the speed of sound (the speed of sound is about 760 mph at sea level), the friction-generated 2600-degree Fahrenheit temperature on the leading edges of the vehicle’s horizontal tails was more than enough to melt unprotected metal. Carbon-carbon thermal protection material kept them cool enough to withstand the searing heat. This is a challenge for even the most advanced thermal protection materials.
As the final X-43A flew, blistering temperatures created by the nearly Mach 10 (7000 mph) speed were in the neighborhood of 3600 degrees, the hotspot this time being the nose of the vehicle. The heat distribution was different this time around due to material differences. For further protection, Vehicle 3 had additional thermal coatings on the horizontal tails’ carbon-carbon leading edges.
Another change in preparation for the third flight was the fact that Vehicle 3’s vertical tails were solid, as opposed to the ribbed structure construction used on Vehicles 1 and 2. Carbon-carbon leading edges were added to the vertical tails as well.
The separation of the research vehicle from the booster was performed at a higher speed than the Mach 7 flight, but dynamic pressure was lower due to the planned higher separation altitude this time.
An important product of flight research is data collection, and one of the prime data objectives for the Hyper-X program was validation of scramjet ground predictions. Prior to the Mach 7 flight, engineers were able to use hypersonic wind tunnel data for risk reduction tests. However, they couldn’t do this in preparation for the Mach 10 flight, as fewer ground test facilities were available.
“One of the more significant challenges we faced in preparing for the Mach 10 flight was the reduced amount of ground test data,” said Laurie Marshall, NASA Dryden Flight Research Center’s X-43A Vehicle 3 chief engineer. “For Flight 2 we were able to do more wind tunnel work than we could for Flight 3. In some cases the same tests couldn't be repeated; the facilities and capabilities just aren't there. So having to design a vehicle and engine that could survive the environment and complete the mission successfully without some of that data was a challenge,” Marshall said.
Another of the exciting things about the Mach 10 flight of the X-43A was that NASA gathered data that has never been obtained before.
“That's why we do this, that's why we fly,” Marshall said. “The research data that we obtained with this flight can’t be obtained on the ground.”
NASA Dryden Flight Research Center