Virgin Galactic says it's switching from a rubber-based solid fuel to a plastic-based fuel for the motor that's designed to power its SpaceShipTwo rocket plane to the edge of space.

"We made the decision to go with a polyamide, which is a fancy way of saying a type of plastic," Virgin Galactic CEO George Whitesides told NBC News on Friday. His statement came after months of reports indicating that an alternative to the rubber-based fuel, known as HTPB, was being used in some rocket test firings at the Mojave Air and Space Port in California.

The rubber-based solid fuel and nitrous oxide were the propellants for SpaceShipOne, the smaller-scale rocket plane that won the $10 million Ansari X Prize for private spaceflight a decade ago. Whitesides said Virgin Galactic and its development partner, Scaled Composites, have been testing both HTPB and the polyamide-based fuel grain for years.

"Frankly, we had good performance from both of them, but as we look for the final range of test flights, we decided to go with the polyamide grain," he said. The plastic-based fuel showed better performance by several measures, including the capability to send SpaceShipTwo higher, he said.

How high will it go?

In recent weeks, some reports have questioned how high SpaceShipTwo would be able to take its passengers, who are paying as much as $250,000 per seat to go into space. The internationally accepted space boundary is 100 kilometers (62 miles), but NASA and the U.S. military have given astronaut wings to pilots who go beyond 50 miles (80 kilometers) in altitude.

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Last week Whitesides said that Virgin Galactic was aiming for 62 miles, but added that the company would grant spaceflier status to anyone going past the 50-mile mark.

Virgin Galactic's more than 700 customers were alerted to the fuel switch on Friday. "The customer base is great," Whitesides said. "They are a group of people who understand that this is a hard thing to accomplish, but they have a lot of trust in this team."

He didn't expect the switch to the plastic fuel to create much of a delay in SpaceShipTwo's flight test schedule. The third and most recent powered flight took place in January, and sent the rocket plane to an altitude of 71,000 feet (13.4 miles, or 21.6 kilometers).

"It basically is the same cartridge," Whitesides said of the hybrid rocket motor. "You just plug it in, and you connect the plumbing in a slightly different way."

The road ahead

Polyamides are a class of plastics that include nylon and Kevlar. The fuel to be used for SpaceShipTwo's rocket is a thermoplastic, meaning that it becomes pliable above a specific temperature.

Whitesides said he expected Scaled Composites and the firm that helped develop SpaceShipTwo's motor, Sierra Nevada Corp., to continue playing a role in rocket development — along with Virgin Galactic and its production arm, The Spaceship Company.

"We're thinking through the respective balance of responsibilities of the three parties as we proceed toward final test flights and commercial service," he said in an email.

The polyamide-fueled motor has already been tested at full duration on the ground, including a burn that went on for about a minute on May 8, Whitesides said. That's significantly longer than January's in-flight firing of 20 seconds.

"We're going to be shooting for increased burn duration on the next flight, when we get there," Whitesides said. "We're still shooting for summertime flights, but we have a bit of work to do."

The plan still calls for Richard Branson, Virgin Galactic's billionaire founder, to fly on SpaceShipTwo by the end of the year. However, Whitesides cautioned that the schedule depended on how the flight test program proceeded. Full-fledged commercial operations would follow Branson's flight, which is due to be televised live on NBC's TODAY show.

NBCUniversal has established a multi-platform partnership with Virgin Galactic to track the development of SpaceShipTwo and televise its inaugural commercial spaceflight. Virgin Galactic is owned by Branson's Virgin Group and Abu Dhabi's aabar Investments PJS.