Vulcan Aerospace and Orbital ATK are partnering to launch satellites using Orbital’s Pegasus rocket slung underneath Vulcan’s giant Stratolaunch aircraft.

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After two previous efforts fell through, Paul Allen’s Vulcan Aerospace has secured a new rocket to launch from the giant Stratolaunch airplane it’s building in the Mojave Desert.

Or rather, it’s an old rocket — and one considerably smaller than originally planned.

Instead of developing a customized rocket, Vulcan said Thursday it will initially go with an existing rocket that first flew in 1990 and is specifically designed for launching from underneath an airplane.

It’s a more conservative approach, aimed at making the economics of the Stratolaunch project work.

Industry experts have questioned the business case of the entire Stratolaunch project, some fearing that the aircraft Vulcan is building — it’s nicknamed the Roc, after a giant mythical bird — is simply too large. The massive aircraft taking shape in the Mojave has twin fuselages and six re-purposed Boeing 747 engines.

Vulcan originally announced Stratolaunch five years ago with plans to use a 120-foot-long derivative of SpaceX’s Falcon 9 rocket to launch payloads of up to 10,000 pounds from underneath its aircraft.

When the economics of the SpaceX partnership didn’t work, that option was dropped and Vulcan turned to Dulles, Va.,-based aerospace manufacturer Orbital ATK to develop a new rocket. But that, too, was abandoned as once again the business case didn’t pan out.

The new deal with Orbital instead uses its long-established and much smaller Pegasus rocket.

Orbital has mostly used a converted Lockheed L-1011 airliner as the vehicle to carry the Pegasus aloft. Over more than two-and-a-half decades it has launched 42 Pegasus missions, with three failures.

Once the carrier plane has reached the right position, orientation and speed, it drops the rocket, which then ignites its engines and takes off for low earth orbit.

Stratolaunch will take over the role of that aging L-1011, which in June was parked in Mojave near Vulcan’s hangar.

The Pegasus, 58 feet long and four feet in diameter, can carry a payload of up to 1,000 pounds.

Vulcan spokesman Steve Lombardi said the shift to this smaller rocket was made because “the market is continuing to move toward mini-satellites and smaller payloads.”

So why does it take this unique airplane, the size of two 747s linked by the largest wing ever built, to launch them?

Last December, Richard Branson’s Virgin Galactic unveiled a single converted 747, dubbed LauncherOne, as its proposed vehicle to launch rockets from the air.

Lombardi said the massive size of the Stratolaunch aircraft provides the flexibility to send up larger payloads later. “We hope to do that in future,” he said.

And he pointed out that Stratolaunch will be capable of carrying up to three Pegasus rockets at once.

While carrying three rockets on one mission could lower the launch cost, dropping them safely so that the first to launch doesn’t damage the others will be an engineering challenge.

“The first time we carry a Pegasus up it probably won’t be a cluster of three,” Lombardi said. “It’ll take some time to get there.”

Lombardi said Vulcan may later announce other rocket partners besides Orbital. And he said it’s “too early” to announce any customers who want to use the system to launch their satellites.

He said the Roc is still under construction and now has its second tail attached. Mechanics are putting in “a lot of finishing touches,” he said. Ground tests could start “in the coming months” with flight tests much later.

Vulcan still aims to be launching rockets from the Roc “by the end of the decade,” he said