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X-15
X-15 pioneered some major new design advances, but also had a number of weaknesses. (credit: NASA/DFRC)

X-15 backgrounder

The X-15 was a research aircraft jointly operated by NASA and the Air Force from 1959 to 1968. It obtained altitude and speed records for winged craft achieving 107,900 meters in August 1963 and 7,300 kmph in October 1967.

In a sense, X-15 is the road not taken by the US space program. X-15 showed promising potential for routine suborbital space access by the end of the program’s 199 flights. Followon programs did not pursue this track, but instead turned to expendable rockets, space capsules, and the space shuttle. With SpaceShipOne’s successful flight, it makes sense to revisit the X-15 program, and its strengths and weaknesses.

X-15 had some extraordinary design advances over previous air and spacecraft. It made use of novel materials such as Inconel-X and novel avionics many years before fly-by-wire systems were routine. X-15 rocket engines allowed high-altitude and high-speed research to be conducted.

X-15 also had weaknesses. X-15 was air launched from a B-52 to get the utmost in velocity and altitude, but this made engine no-lights quite dangerous. Similarly, X-15 had skids and not landing wheels which made control upon landing somewhat less than routine. Ground handling and air launching were also complicated by ammonia fuel for the engines and peroxide fuel for the APUs. The greatest danger may have been X-15 had difficulty re-entering the atmosphere due to its configuration. SpaceShipOne’s feather mode, in contrast, does not require careful management of orientation. The dangers inherent in the program and the design resulted in many emergency landings including one that killed test pilot Mike Adams.

The X-15 is the road not taken by the US space program

X-15 first and foremost was a research program. It had a great deal of resources to counter the perceived threat to US international prestige posed by the Russian Sputnik satellite. While these resources were a blessing, they also proved unsustainable as Apollo became the new vehicle for prestige and Vietnam War ate up more and more money.

The X-15 project was also more expensive than the private space programs getting underway today. At $300 million in 1961 dollars—$1.5 billion in today’s dollars—it had vigorous backing. Today’s programs at Scaled and Rocketplane have 1-2% as much funding reported or about $20-$30 million each in today’s dollars. XCOR has also reported that it can produce a winged suborbital aircraft, the Xerus, for cost levels in that ballpark. These programs are also meant to be profitable, with Paul Allen breaking even possibly by SpaceShipOne’s 3rd spaceflight, winning the Ansari X Prize and increasing viewership for the Vulcan Video documentary. If the documentary alone pays for the creation of an X-15 style rocket program only four decades later, that bodes well for future documentaries of recreations of more ambitious programs (See “Space as Entertainment”, The Space Review, May 17, 2004).

Granted, some of the cost difference between SpaceShipOne, Xerus, and XP versus the X-15 is because the current planes are not trying for speed records, only high altitude. Nevertheless, cost and commercial operation may be the primary differences between today’s programs and the X-15 program although composites, avionics, design, propulsion, navigation, telemetry, ferrying, and control have all changed in the last 40 years.

As today’s private space programs pick up where the X-15 program left off, it is interesting to look back at the X-15.


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