You have to love optimists. Despite premature ends to both attempts to fly a hypersonic glider, the U.S. Defense Advanced Research Projects Agency wants to try again. Darpa’s new Integrated Hypersonics (IH) program looks a lot like its unsuccessful Falcon Hypersonic Test Vehicle 2 (HTV-2) project, with a few tweaks—including the addition of a rocket motor to increase range and maneuverability.
HTV-2 was a rocket-launched unmanned glider intended to demonstrate long-duration hypersonic flight for prompt global strike by flying across the Pacific at Mach 20. But both flights ended after about 9 min. The April 2010 failure was blamed on excessive yaw-roll coupling, which overpowered the flight controls. The August 2011 failure was blamed on unexpected aeroshell degradation.
According to Darpa's draft broad area announcement (BAA), the Integrated Hypersonics program is aimed at demonstrating technology for transatmospheric vehicles (TAV) or hypersonic X-planes. The agency defines a TAV as a rocket-powered reusuable launch vehicle able to reach distant locations in minutes, and a hypersonic X-plane as a rocket-augmented recoverable suborbital vehicle able to fly for up to 2 hr, in the atmosphere.
The IH program is planned in two phases. A 12-30-month, $40-70 million technology-development track followed by a hypersonic X-plane (HX) track leading to a flight demonstration in 2016.
Darpa says it is looking for a 2,500-5,000-lb. flight vehicle that can be ground- or air-launched to fly hypersonically for 2 hr., with a down-range capability of greater than 20,000 mi. and cross-range capability of more than 10,000 mi.—either a single maneuver or a combination of mid-course, near-terminal and terminal maneuvers.
Although the draft BAA makes it clear that Darpa is interested in a non-axisymmetric, high lift/drag vehicle like Lockheed Martin's arrowhead-shaped HTV-2, and not a simpler biconic shape like Boeing's earlier AMaRV, there are some interesting differences between IH and HTV-2.
The biggest is the addition of a propulsion technology area. Others include making the vehicle air-recoverable, so that the thermal protection system can be examined after flight. And the creation of an open-source forum—the Intellectual Property Commons—to enable program participants to share hypersonic technology data.
HTV-2 was a glider, launched from Vandenberg AFB, Calif., toward Kwajalein Atoll on a Minotaur IV Lite booster. And even though the Minotaur maneuvered to burn off excess energy, it still took the HTV-2 too high, requiring the vehicle on release to execute a re-entry maneuver before it could begin its glide. It was shortly after this maneuver, on both flights, that the HTV-2 was lost.
For IH, Darpa is looking at propulsion in two ways. One is during launch, to be able to insert a hypersonic glide vehicle (HGV) into a long-range, horizontal flight path at the edge of the atmosphere with sufficient energy to meet the range and maneuverability requirements. This would avoid the trauma of re-entry. The other is during cruise, using a rocket to maintain high velocity, maneuver and reboost to high Mach.
The draft BAA includes design reference mission, which shows how propulsion would be used:
Compare this with the planned HTV-2 flight path:
Maneuverability is a key aspect of IH, and explains the focus on an HTV-2-like vehicle. According to Darpa: "The aerodynamic maneuvering capabilities of high L/D HGV vehicles enable changes to flight trajectories to significantly reduce over flight issues, significant cross-range divert capability for in-flight retasking, and flight profiles which can mask the vehicles intended destination."