WASHINGTON — An experimental robotic space plane was dropped from high altitude April 7, but after successfully maneuvering autonomously to the designated airport, the vehicle had difficulty upon landing and rolled off the runway.

The X-37 Approach and Landing Test Vehicle (ALTV), a project of the Pentagon’s Defense Advanced Research Projects Agency (DARPA) and the Space and Intelligence division of the Boeing Company, with some support from NASA, took off from the Mojave, Calif., airport at 6:30 a.m. PDT attached to the underside of its carrier aircraft, the White Knight.

The test vehicle was released at 7:28 a.m. PDT at an altitude of 11.28 kilometers and conducted its flight within the air space of the Edwards Air Force Base test range.

Boeing spokesman Joe Tedino, said the ALTV successfully executed its autonomous landing profile, “but the vehicle experienced an anomaly after touchdown at Edwards Air Force Base, Calif., and departed the end of the runway. The ALTV flight team is assessing the situation and reviewing test data. No further information is available at this time.”

After its release from the White Kni ght, the ALTV touched down on runway 22 at Edwards at 7:31 a.m. PDT.

“ALTV’s autonomous landing sequence and initial touch-down were flawless and fully according to plan,” said DARPA spokeswoman Jan Walker, “but ALTV did not stop in the distance expected and rolled off the end of the runway. ALTV’s steering was nominal for the full length of the runway.”

“All flight data has been recovered from ALTV. There was minor damage to ALTV — the nose landing gear is heavily damaged but the main landing gear and aircraft appear structurally intact,” Walker said in an April 7 statement.

White Knight is operated by Scaled Composites of Mojave, Calif. , the pioneering firm that used the White Knight to carry SpaceShipOne to release altitude during the company’s successful quest to win the $10 million Ansari X Prize with back-to-back piloted suborbital flights in 2004.

The White Knight/X-37 mated combination had undergone a series of taxi and flight hops in preparation for the first release of the vehicle April 7. Several attempts to conduct that first drop test were plagued by bad weather, as well as telemetry issues between the vehicle and ground controllers.

In the late 1990s, the X-37 was a NASA-sponsored project — part of a planned series of flight demonstrators dubbed Future X.

At that time, the Boeing-built X-37 was advertised as an unpiloted, autonomously operated vehicle designed to conduct on-orbit operations and collect test data in the Mach 25 (re-entry) region of flight.

Early plans for the X-37 called for it to be ferried to orbit by the space shuttle or an expendable launch vehicle. Once free in Earth orbit, the craft would remain in space for up to 21 days, carrying out a variety of experiments before re-entering the atmosphere and landing on a conventional runway.

Those plans were eventually cancel ed, with NASA transferring its X-37 technology demonstration program to DARPA in late 2004.

NASA determined that the X-37 ALTV did not meet the goals of the Vision for Space Exploration and as a result transferred the program to DARPA. NASA’s only role in the drop tests is as a technical advisor, despite the large NASA logo on the vehicle.

Leonard David has been reporting on space activities for nearly 50 years. He is the 2010 winner of the prestigious National Space Club Press Award and recently co-authored with Apollo 11’s Buzz Aldrin the book “Mission to Mars — My Vision for Space...