Hypersonic X-Plane (HX) - DARPA Tries Again


If at first you don't succeed, make it harder then try again. That seems to be the premise behind the US Defense Advanced Research Projects Agency's new Integrated Hypersonics (IH) program.

IH is a follow-on to DARPA's Falcon program, which failed in two attempts to fly the HTV-2 hypersonic glider. So much of a follow-on, in fact, that the IH program is to begin with a third flight of the HTV-2.

The new program is planned to culminate in 2017 with the flight of the Hypersonic X-Plane (HX), to demonstrate technology not only for a prompt global strike weapon, but potentially hypersonic aircraft missions "such as time-critical, survivable transport", says DARPA.

Notional HX concept. (DARPA)

DARPA's briefing to industry, on Aug. 14, didn't mention the ill-fated HTV-2 by name, but did make clear that "Flight 1" in the IH program will "Conduct [a] full-scale baseline test of [a] modified vehicle incorporating lessons learned from previous flights in aerodynamics, controls, and thermal design." The goal is a 4,000nm flight at Mach 17-20. So HTV-2 Flight 3 it is.

The IH briefing makes clear the lessons DARPA has learned from the failures of the HTV-2 flights in April 2010 and August 2011, both of which ended about 9 minutes into the flight as the vehicle re-entered the atmosphere after its rocket launch and pulled up at Mach 20 to begin its glide across the Pacific.

Several things are different about IH, some to fix what went wrong with HTV-2 and some to make the program more technically challenging/rewarding and justify spending money on what could be viewed as a thinly disguised effort to fly the HTV-2 one more time.

HTV-2 (Concept: DARPA)

First, what went wrong with HTV-2? On both flights, the hypersonic glider was launched from Vandenberg AFB on  a Minotaur IV, which maneuvered to reduce the energy imparted but still boosted the HTV-2 on a ballistic trajectory out of the atmosphere, from where the vehicle had to re-enter then pull up into a glide.

On Flight 1, higher-than-predicted yaw coupled into roll and exceeded the control authority of the glider's body flaps. The solution was to adjust the center of gravity, reduce the angle of attack and use the reaction-control system to augment the flaps. And these fixes appear to have worked on Flight 2, where something else went wrong.

On Flight 2, the HTV-2 had established stable flight at Mach 20 when a series of shocks caused the autonomous flight safety system to terminate the flight. Investigators concluded that pieces of the vehicle's skin peeled away in the intense heat, creating strong shocks that caused the vehicle to roll abruptly. The enhanced control system righted the vehicle after several shockwave-induced rolls, but eventually the disturbances became too severe.

So what does that mean for IH?. First,  DARPA wants to increase the aerodynamic and control performance of the vehicle. It is aiming for a 20% higher hypersonic lift-to-drag ratio (L/D), to extend range, and twice the control authority. Second, it wants to improve the thermal protection system, aiming for a 25% higher margin. DARPA wants to combine the high L/D of the sharp-edged HTV-2 with the robust control of Sandia's Swerve re-entry vehicle or the X-37B winged spaceplane.

HX vs HTV-2 boost profiles. (DARPA)

But, more importantly, it wants to change the way the vehicle is launched (above), from a ballistic trajectory (blue) to a direct insertion (green), to reduce the heating on the aeroshell. One way to do that is to add a rocket to the vehicle, to augment the launch vehicle and insert the HX into its cruise with a shallow flight-path angle, avoiding the high apogee and subsequent pull-up from a low perigee that marked the ICBM-like HTV-2 launches.

The same multi-burn propulsion system would then be used periodically during flight to reboost the vehicle, to extend its cruise range. For HX, DARPA is aiming for a downrange of more than 20,000nm at Mach 20-25, and a cross-range capability of more than 10,000nm, in a single maneuver or a series of mid-course and terminal maneuvers.

HX boost-glide flightpath (DARPA)

And, finally, DARPA wants to both improve the telemetry and instrumentation and air-recover the HX, so it can get a better idea of what is going on aerodynamically and thermally at hypersonic speeds and so it can examine the thermal protection system after flight to see how it performed.

The agency wants to be able to measure the temperature gradients on the vehicle's skin and to measure the surface recession as the thermal protection ablates (burns away) in temperatures exceeding 2,900°F. And it wants to trasmit the telemetry in flight at Mach 20 via commercial satellite at data rates up to 10Mbps.

HX recovery concept (DARPA)

Under the IH program, DARPA plans to award contracts for ground-based and subscale flight tests in five technical areas from aero-configuration to propulsion. Up to $40 million is available of the 12-month base phase and another $30 million for an 18-month option phase that would include three subscale flights in the second quarter of fiscal 2015.

The plan is to bring the technologies developed and demonstrated under these individual contracts together in the HX vehicle, with a 20,000nm-range, Mach 20-25 test flight planned for the first quarter on fiscal 2017.

What can be said is that HX is not as ambitious as DARPA's once-planned follow-on the HTV-2, the HTV-3X Blackswift, which was to be capable of taking off from a runway on turbojet power, accelerating to Mach 6 on scramjet propulsion, maneuvering and landing back on a runway.

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