NASA awarded Lockheed Martin a modest $892,292 earlier this month to study the feasibility of developing an unmanned hypersonic spy plane called the SR-72. This superfast recon drone, first teased in November 2013, would fly at speeds of Mach 6.0, or 4,500 mph. That’s almost double the speed of the Lockheed SR-71 Blackbird, which made its first flight 50 years ago.
Neither Lockheed Martin Skunk Works nor NASA Glenn Research Center officials are talking about the recent award. But a Lockheed Martin website notes that the company has been working with Aerojet Rocketdyne to find a way to integrate a turbine engine, which would get the plane up to Mach 3, with a supersonic ramjet engine, or scramjet, to push it to Mach 6.
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Scramjets are, structurally at least, pretty simple. There are no moving parts—just an inlet, fuel injector, flame holder, and nozzle. One the vehicle reaches a sufficiently high speed, the scramjet compresses the supersonic air flow entering the inlet, mixing it with propellant and igniting everything within a few milliseconds. Most of the hot exhaust leaving the nozzle comes from the surrounding air and not the fuel carried on board the plane, which is why these engines are called air-breathers.
The military has been experimenting with such designs for years now. Between 2010 and 2013, the U.S. Air Force tested a hypersonic scramjet concept called the X-51A WaveRider. Mounted on a rocket that was fired from the wing of a B-52, the cruise missile-shaped Waverider would separate from the rocket and accelerate up to Mach 5.1. Because such great speeds generate a lot of heat, the Waverider had a tungsten nose and an Inconel nickel-chromium alloy engine.
Unfortunately, the Waverider did not perform well during a series of three flight tests over the Pacific Ocean, the last one resulting in the aircraft spinning out of control and disintegrating 15 seconds after separating from the rocket. Still, the Air Force intends to take what it can from that experience to develop the new High Speed Strike Weapon program.
According to the Lockheed website, the SR-72’s design will take into account lessons from other attempts at supersonic flight, namely the rocket-launched Falcon Hypersonic Technology Vehicle 2 (HTV-2) program. Developed by DARPA, the HTV-2 was part of what it called the Conventional Prompt Global Strike effort—a mission to build something that could deliver a conventional weapon on any target on Earth within an hour.
Like the Waverider, the HTV-2 didn’t fare well in its test launches. In the last one, in August 2011, the HTV-2 hit Mach 20 (13,000 mph), generating a surface temperature of 3,500 degrees Fahrenheit that pulled apart the skin of the unmanned plane and forced the onboard fight safety system to detonate it over the Pacific Ocean.
Remarking on the 2011 test, U.S. Air Force Major Chris Schulz, the DARPA HTV-2 program manager, said, “We know how to insert the aircraft into atmospheric hypersonic flight. We do not yet know how to achieve the desired control during the aerodynamic phase of flight.” But he expressed confidence the issue would be overcome. Lockheed believes it can develop its SR-72 by 2030.