DENVER — First flight of the X-51A scramjet demonstrator is now on track for early December while captive carriage tests on the NASA-operated B-52H mothership at Edwards Air Force Base, Calif., are set to begin in October.

A joint effort by the U.S. Air Force, Defense Advanced Research Projects Agency (DARPA), Pratt & Whitney Rocketdyne, and Boeing, the hypersonic vehicle is designed to be the first air-breathing craft to demonstrate sustained speeds in excess of Mach 4 using a “logistically friendly” hydrocarbon fuel.

The initial vehicle is the first of four X-51As to be launched by April 2010.

First flight, targeted for December 2008 under the original schedule before budget cuts in 2005, was later reset for early in the fourth quarter of 2009. But integration issues with the B-52H mothership, along with logistic delays, pushed the first flight target toward late November.

The target date has now been moved to Dec. 3 to take advantage of aircraft availability and avoid crewing complications around the late November holiday period in the U.S., program officials say.

Although current plans are to fly four identical flight profiles, Air Force Research Laboratory X-51A Program Manager Charles Brink says senior Air Force officials have recently been briefed on potential tactical demonstrations with the final pair of flights, should the first two go as planned. Speaking at the American Institute of Aeronautics and Astronautics 45th Joint Propulsion Conference here, Brink says the scenario, which includes modifications to the guidance and control system, would entail additional costs and include “standing down the schedule and revising the timetable.”

Brink says if more X-51 flights are funded beyond the current sustained hypersonic demonstrations, future potential goals could include longer-duration flights and slower scramjet-cycle initiation speeds. “The lower it can go, the lower the stress on a turbine,” he adds, referring to potential combined cycle-weapon developments.

The static test vehicle has completed initial “risk-abatement” ground tests at Edwards and is now being moved back to Boeing’s Palmdale, Calif., facility for refurbishment and installation of a thermal protection system. Once completed, the revamped vehicle will become FTV-4, the final X-51A to be launched next year.

The first flight vehicle, FTV-1, is nearing completion at Palmdale and is expected to be moved to Edwards “within weeks” in preparation for the start of captive-carriage flights on the B-52 attached to a MAU-12 stores rack, Boeing X-51A Program Manager Joe Vogel says.

On the first flight, the B-52 will drop the 3,942-pound “stack” consisting of the 25-foot long X-51 cruiser and its modified ATACMS missile rocket booster, at around 50,000 feet and Mach 0.8. The booster will accelerate the stack to Mach 4.6-4.8. During the boost, air will flow through the cruiser’s engine, exiting via a ducted interstage, warming the scramjet and its circulating fuel.

At the top of the boost phase, around 60,000 feet, the vehicle’s own guidance control unit and scramjet digital controller will command the X-51A to roll inverted, placing the inlet uppermost and giving the vehicle a positive angle-of-attack.

At booster burnout and separation, the vehicle will briefly coast before ethylene is injected at 4,500 psi to light off the engine.

Heated by the burning ethylene, fuel will be introduced and the two flows combusted simultaneously until thermal equilibrium is achieved and the vehicle can accelerate on JP-7 alone.

Rolling upright, the X-51A is expected to cover almost 400 miles in five minutes and reach an altitude of around 100,000 feet before the fuel is exhausted and the vehicle ditches in the Pacific test range off southern California.

Photo: USAF