The X-15B: The Spaceplane That Wasn't


One of the biggest stories last week was Virgin Galactic’s successful flight of SpaceShipTwo. The suborbital spaceplane was released from its mothership, WhiteKnightTwo, at 46,000 feet at which point pilots Mark Stucky and Mike Alsbury ignited its rocket motor, gained an additional 10,000 feet of altitude, and accelerated for a 16 second supersonic flight before gliding to a smooth runway landing.

Reactions online are celebratory, heralding this flight path as an amazingly futuristic model of spaceflight. But it’s actually a very old flight path flown by the X-15. And while it never went into orbit, the X-15 did take some of the nation’s first steps into space. Unfortunately its orbital successor, the X-15B, never got to take the next step.

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The X-15 was heralded by many as the world’s first space plane. The tiny aircraft was launched from underneath the wing of a B-52 bomber at around 45,000 feet at which point the pilot would light its main engine, climb in a high arc as he burned through all his fuel, then made an unpowered landing on the dry lakebed at Edwards Air Force Base in California. Following these arcing flight paths, X-15 reached a peak altitude of 67 miles and a top speed of Mach 6.7 on two separate flights; it’s a flight profile identical to the SpaceshipTwo’s.

The X-15’s story usually ends there, with these high altitude supersonic flights. But its designers wanted to send it further. Namely, into orbit.

In February 1956, the U.S. Air Force’s Air Research and Development Command (ARDC) started working on a follow-up program to the X-15. Called Project 7969, it was designed to fast track the launch and recovery of a manned spacecraft. The push to get a man in space took on a sense of urgency after the Soviet Union launched Sputnik in October 1957.  Proposals from aviation companies started pouring in to the Air Force, so many that by the end of January 1958, a joint committee from the Air Force and the National Advisory Committee for Aeronauts met to evaluate the proposals.

Among the submissions was an orbital version of the X-15 proposed by its builder, North American Aviation. This first proposal was for a “stripped” version of the X-15 with an empty weight of just 9,900 pounds; the suborbital version that flew at Edwards weighed 14,600 pounds. It would be launched on a two-stage booster — the first stage would be a cluster of three Navaho missiles and the second stage a single Navaho — and the X-15’s own XLR-99 engine would serve as a third stage. With this configuration, the orbital X-15 could reach an apogee of 400,000 feet and a perigee of 250,000 feet.

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Its low perigee meant the orbital X-15 wouldn’t need retrorockets to start its descent back towards the Earth; it would return naturally after one orbit. To survive the all-important atmospheric reentry, the orbital X-15 would be have ablative (a material that would burn away when heated) beryllium oxide on its leading edges, a René 41 alloy shingle heat shield, and an overall thick body made of Inconel X that could withstand the high temperatures. But unlike the  suborbital version, the orbital X-15 wouldn’t land. The pilot would eject and land by parachute just before ditching the X-15 in the Gulf of Mexico. The aircraft wouldn’t be recovered. North American anticipated launching a man into orbit on this cobbled together spaceplane within 30 months for a mere $120 million.

North American took this preliminary plan a step further to create the X-15B, a larger model of the aircraft with room for a second pilot. This version used two Navaho boosters as the first stage, a single Navaho as the second stage, and the spaceplane’s Rocketdyne-built XLR105 Atlas sustainer engine as the third stage.

Its proposed flight plan was simple. The first stage would drop away and the second stage would fire eighty seconds after launch. At 400,000 feet, the second stage would burn out and the X-15B would continue ascending under its own power. The vehicle would eventually speed up to 18,000 mph, fast enough for three orbits before the pilot would fire his XLR105 engine to reenter the atmosphere and fly the X-15B to a runway landing at Edwards just like the traditional aircraft.

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The X-15B was the proposal North American packaged and took to Washington as a bid to get the first American in space. But there were 421 other bids coming to the nation’s capital at the same time, many of which used the much simpler capsules. The X-15B faded quietly into the background. North American retooled the proposal at the end of 1959, suggesting an X-15B launch on a Saturn I rocket. It could carry a two-man crew hundreds of miles from the Earth into a sustained orbit. But again, the proposal went nowhere; NASA was busy with its Mercury Program and the Air Force had enough of its own flight projects to stay busy. The X-15B again faded into the background.

Image: The X-15 in flight after being dropped by a B-52 bomber. The orbital version, the X-15B, never made it off the drawing board. Credit: NASA

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