DARPA has cancelled the Blackswift reusable hypersonic testbed program. Not surprising, really, after Congress cut its Fiscal 2009 budget from $120 million to just $10 million - slicing $60 million from DARPA's budget request and eliminating all $50 million the U.S. Air Force sought for its share of the program. (You can read the story here.)

I'm not sure what I think about this. Disappointed, certainly, but not surprised. Congress was skeptical of Blackswift's technical feasibility and operational utility. I always had the uncomfortable feeling the research agency was trying to run before it could walk - tackling the "DARPA-hard" challenge of reusable hypersonics before it had some of the enabling technologies firmly in place.

Blackswift was the outgrowth of the agency's Falcon program, which continues. Falcon started out trying to develop three things: a small launch vehicle (SLV), a common aero vehicle (CAV) and a hypersonic cruise vehicle (HCV). The SLV was a small expendable booster; the CAV was a conventional-warhead re-entry vehicle; and the HCV was a powered, reusable, unmanned hypersonic strike platform.

The idea was the SLV would be used to launch unpowered hypersonic test vehicles (HTVs), which would demonstrate aerodynamic and structural technologies for both the CAV and the HCV. An expendable SLV carrying the CAV would provide a near-term prompt global strike capability. The reusable HCV deploying multiple CAVs would provide a far-term capability.

Congress intervened and instructed DARPA to stop work on the CAV, but it continued supporting Air Launch and SpaceX as they developed small launchers. And it continued funding Lockheed Martin's Skunk Works as it developed the HTV. But work on the initial HTV-1 was halted after it proved unproducible, and attention switched to the more-advanced HTV-2, two of which are scheduled to fly in 2009.

While all this was going on, NASA abandoned aeronautics in favor of exploring space and cancelled its hypersonics work, which included plans to fly combined-cycle propulsion demonstrators. Falcon was intended to involve only unpowered demonstrators, but DARPA stepped into the gap and under the Falcon program Lockheed began work on the HTV-3X - a reusable, powered hypersonic testbed.

The Skunk Works completed conceptual design of the HTV-3X, and also conducted subscale ground tests of its turbine-based combined-cycle propulsion system under the Facet program. This combined a small high-Mach turbojet with a dual-mode ramjet/scramjet, the two sharing an axisymmetrical inlet and nozzle. The propulsion system was designed to power the demonstrator from takeoff to a Mach 6 cruise and back to the runway.

Because of the funding and time required to build a flying demonstrator, HTV-3X became a separate program, renamed Blackswift. DARPA conducted a competition, but it's believed the only bidder was a "national" team formed by Lockheed and Boeing. Source selection was still under way when Blackswift was cancelled in early October, and DARPA is not revealing who the bidders were.

What does this mean for hypersonics? It's a setback, for sure. Without Blackswift, the hopes for a breakthrough in high-speed flight fall firmly on the shoulders of the AFRL/DARPA X-51A WaveRider program. The Boeing X-51A is scheduled to fly about a year from now, to demonstrate the ability to accelerate from Mach 4.5 to Mach 7 under the power of a hydrocarbon-fuelled, fuel-cooled scramjet.

But the X-51 is not reusable. If it succeeds it is most likely to spin off a hypersonic missile, not the reusable vehicle envisioned by Blackswift. In fact the gap in technology between the reusable, combined-cycle Blackswift and the X-51, which can trace its heritage back almost 20 years to the X-30 National Aero-Space Plane, was one reason for Congress' skepticism.

But perhaps X-51 - and the unpowered HTV-2s - are steps we must take before anyone will take a vehicle like Blackswift seriously. Because, as DARPA program manager Steve Walker says, we will one day have to build a vehicle just like Blackswift if we are ever to make air-breathing hypersonics a reality.