The rocket engine behind the privately built, piloted spacecraft SpaceShipOne will carry more propellant and burn longer during a Wednesday space shot, but has not undergone any serious modifications since its historic June flight, according to its developer.

"We're not changing anything," said aviation pioneer Burt Rutan, who developed the spacecraft with his Mojave, California firm Scaled Composites, in a telephone interview to clarify an earlier story. "It's the same motor and the same tank."

SpaceShipOne's engine is a hybrid rocket motor that burns liquid nitrous oxide, also known as laughing gas, as an oxidizer and a solid fuel material found in rubber tires. The motor successfully propelled the piloted spacecraft to suborbital space during a successful June 21 flight with civilian astronaut Mike Melvill at the controls.

The spacecraft is one of about two dozen entries in the international Ansari X Prize competition to build and launch a reusable, three-person spacecraft to a suborbital altitude of 62 miles (100 kilometers). The first team to do so twice in 14 days wins $10 million.

During the Sept. 29 flight, the rocket will carry about 150 more pounds of oxidizer and 10 extra pounds of rubber fuel than that used in SpaceShipOne's first space shot. The launch is the first X Prize-qualifying flights for the spacecraft.

"Essentially, what we've done is added propellant to get more performance," Rutan said, adding the extra fuel and oxidizer should allow SpaceShipOne to fly a heavier weight, equivalent to two additional human passengers, to the target altitude. "We have not been using this time scrambling to make modifications."

The development of SpaceShipOne's engine has been an in-house effort by Scaled Composites, though some components were developed and supplied by four separate vendors, including Thiokol, AAE Aerospace, Environmental Aeroscience Corp and SpaceDev.

Rutan said SpaceShipOne engineers are not squeezing every ounce of performance out of the spacecraft's engine, but are looking forward to this week's launch, since many project team members are contributing small items to ride aboard the spacecraft as ballast.

"Our goal is to have an operable suborbital spacecraft at least as safe as the early airliners," he said, adding that SpaceShipOne's hybrid engine is a safety breakthrough. "If we're going to have space tourism, we need greater safety for any manned spacecraft."