A privately built rocket nicknamed Xombie suffered an apparent engine leak Sept. 16 that stalled its attempt to win $150,000 in a NASA contest to fly mock Moon landers.

Built by Masten Space Systems in Mojave, Calif., the Xombie rocket lifted off at the Mojave Air and Space Port and successfully flew to a nearby landing pad, but failed to make the required return trip during a flight for the Northrop Grumman Lunar Lander Challenge. A small leak detected in the rocket’s engine chamber after landing was cited as the cause, contest organizers said.

“There was some wear that they were unhappy with on their regeneratively cooled engine,” said Will Pomerantz, senior director of space prizes for the X Prize Foundation that manages the challenge for NASA. “It was something where they did not feel comfortable flying again with the vehicle in that state.”

The Lunar Lander Challenge is part of NASA’s Centennial Challenges program to offer cash prizes for successful feats of engineering. For the Lunar Lander Challenge, NASA is offering a total of $2 million in prize money for successful flight demonstrations of vehicles capable of hopping from one launch pad to another.

The Masten team now hopes to try again with its Xombie rocket — officially named XA-0.1B — during one of two more chances in October. The Xombie rocket will have to start the round trip from scratch, despite making it halfway in its first attempt  and landing within 20 centimeters of its target.

“They’ll be on a new clock,” Pomerantz said.

The Xombie rocket’s next chance to fly is a two-day window that opens on Oct. 7. The rocket can make a third round of flights Oct. 28 and 29, Pomerantz said.

In addition to Masten Space Systems, at least two other teams are competing in the Lunar Lander Challenge this year. The teams have until Oct. 31 to vie for the Level 1 and Level 2 prizes. The third team — a father-son duo called Unreasonable Rocket — is slated to go last at the end of October in Cantil, Calif.

Level 1 of the competition, which Masten tried to qualify for Sept. 16, requires vehicles to fly for at least 90 seconds and fly a round trip between two different launch pads. The team Armadillo Aerospace of Mesquite, Texas, won the $350,000 first prize for Level 1 in October 2008, leaving the $150,000 second prize up for grabs.

Level 2 requires competitors to fly vehicles on a round trip of at least 180 seconds in duration and land on a simulated lunar surface. First place awards $1 million, with $500,000 available for the runner-up.

On Sept. 12, Armadillo Aerospace successfully qualified for the Level 2 competition with its Scorpius lander that flew in Caddus Mills, Texas, near Dallas. If another team does not qualify for the Level 2 prize and land more accurately than Scorpius, Armadillo will take home first prize.

“It is now guaranteed that we will definitely give away at least $1 million,” Pomerantz said.

Pomerantz said that since its inception the different teams competing in the Lunar Lander Challenge have collectively spent between $15 million and $20 million for the $2 million purse.

If any prize money is not won during this year’s Lunar Lander Challenge, it will be available during one last competition in 2010 before expiring, he added.