A cutaway diagram of the X-15B. While the shape of the plane was the same as the original, a large number of changes would have been necessary to send it into orbit. Space was carved out for a second crew member, who also had a canopy of his own. A small payload bay was put directly behind him. Image from the NASA publication SATURN/X-15 Flight Research Program Report. Click for a larger view.
What it was: A North American Aviation proposal to build an orbital spaceplane based on the same aerodynamic shape as the X-15 and boosted by leftovers from the cancelled Navaho cruise missile program or a Saturn. Initially proposed as a single-orbit craft in 1956-57, in the short period post-Sputnik I but before the responsibility for manned space exploration was given to NASA it was one of three ideas seriously considered by the US Air Force to get a man into space before the Soviet Union.
Details: In February 1956 the US Air Force’s Air Research and Development Command (ARDC) began work on a follow-up to the X-15 research craft, Project 7969, with a specific goal of putting a man into space. As this was pre-Sputnik I progress was leisurely, but before long there were two main lines of approach being studied: the Manned Ballistic Rocket Research System, which would work on a ballistic capsule (probably to be launched on an Atlas missile), and the Manned Glide Rocket Research System, which was to work on a rocket-boosted spaceplane. There were no funds for any of this research, so contractors were enlisted to work on it themselves and were paid with the hope that it might lead to lucrative contracts
North American Aviation was one of those that responded to the Glide Rocket specifications. Their work on the X-15 was already well underway, so they suggested an interesting shortcut to getting something into orbit. The aerodynamic properties of the X-15’s shape were already understood from wind tunnel work, and within a few years there would be a plethora of data from the actual flights it would make, so why not build on that? The X-15B would need no further aerodynamic testing—NAA could proceed right to redesigning the craft for orbit.
Once built, this iteration of the X-15B would be launched on a bundle of four G-38 Navaho missiles—also a product of NAA. The X-15B would make a single orbit (and no more: while its apogee was 120 kilometers its perigee was only 75, and that would force re-entry whether they wanted to stay up for a second orbit or not), and then the pilot would parachute to safety while his craft crashed into the Gulf of Mexico. Total cost? US$120 million over thirty months.
While not entirely keen on the specifics of this mission, the Air Force realized that there might be something there worth investigating. Work on the X-20 Dyna-Soar had already started but it was still years away from flight. Why not slot the X-15B into the space between the end of the regular X-15 flights and the start of the X-20’s in order to slowly build up to the latter?
The level of the Air Force’s interest rose radically in the months following Sputnik I. Project 7969 morphed into Man in Space Soonest (MISS), and proposals were once again solicited. There were eleven in all, but three were picked as likeliest to succeed: one of the ballistic capsules proposed by six of the defense contractors, accelerating the Dyna-Soar, and building the X-15B.
The general consensus, though, was that a ballistic capsule was the way to go. Man in Space Soonest was an entirely political exercise: beat the Russians to the punch by getting a man into space as quickly as possible. Whether there was any more practical reason for him to be up there was irrelevant. The Dyna-Soar and X-15B would have uses like reconnaissance and bombing, but that didn’t matter in context. The former would take too long no matter how many resources were thrown at it, and the X-15B suffered from the problem that it needed to be piloted: learning if an astronaut could function as a pilot in the conditions of space would take extra time. In a ballistic capsule, he’d just have to sit there.
Their preference was confirmed in October 1958 when MISS was absorbed by the newly created NASA. It became Project Mercury, and NASA focused on its Mercury capsule.
That didn’t mean that the X-15B would not be built at all, though. Returning to the idea that it could fit between the regular X-15 (which was now NASA’s baby) and the X-20, NAA continued to pitch it to the new agency. In the past this phase of the X-15B’s existence has been overlooked, mostly because it was classified Secret in 1959 and dropped out of sight. NAA’s final 1961 reports to NASA did come out a few years ago though, and gives us a clear idea of how it might have flown.
Diagram of the unusual materials needed for the X-15B’s structure so that it could resist re-entry temperatures. The skin would have been even more exotic. Click for a larger view.
Like the original X-15B proposal, the next iteration of the craft would have had the same shape as the plain X-15. The biggest difference between the two planes was the materials used to make them. While the X-15 could get by with aluminum and steel for its frame, the X-15B’s orbital re-entry would have caused it to deal with much higher temperatures: as high as 2700 Celsius on the leading wing edges. Regular structural metals would sag and melt when subjected to that.
As a result the nickel alloy Inconel X, which was used only for the heat-resistant skin of the X-15, was restricted to the area of the plane’s frame behind the nose and above the belly. Those other two areas, as well as the wings and tail fins, had to use more exotic refractory materials like graphite, molybdenum, and beryllium oxide. Where the heat was worst, NAA planned on using thorium(IV) oxide, which means that—like its predecessor the Douglas Model 684—the X-15B would have been somewhat radioactive. Unlike the Douglas plane only structural elements for the tail fin edges and the roots of the wing edges would have used the stuff, not the whole skin.
The skin of the plane would have been even more exotic, including molybdenum, tungsten, the René-41 alloy used for the shell of the Mercury capsule, and niobium. The last of these was a particularly interesting call as at the time the Dyna-Soar was also looking as if it were going to made out of the same metal, and together they would have used up more than the world’s annual production of the metal for a few years.
Inside the plane was rather different too. Space was carved out for a second crew-member, who was also given a canopy, while instrumentation was stuffed into an area behind him (in the X-15 you could have instrumentation or a second crew, not both, and if you went with the latter he couldn’t see out). Behind that the X-15B’s oxidizer tank could be accessed through a payload door on the top of the craft; the tank could either take up the whole space for missions where more fuel was needed, or a smaller tank could be fitted and part of the space be used for a small satellite payload.
The extra space for this arrangement was available because while the X-15B was to have the same shape as the X-15, it would have been somewhat bigger. Oddly enough, how much bigger is not clear—two figures are 57’ 4” and 52’ 4”, and one presumably incorrect NAA illustration even shows it as taking up approximately 42 feet of a Saturn/X-15B stack even though the original X-15 was seven feet longer than that. However much it was, this extra space would also have been used for one more purpose: instead of bringing oxygen bottles as was done on the short flights of the basic X-15 or as would have been done for the earlier single-orbit X-15B, the final X-15B plan had a regenerative oxygen system.
Saturn/X-15 stack for the first mission. In later missions to higher altitudes, the S-IV would be swapped out for the more-powerful S-II. Click for a larger view.
They couldn’t carry bottles because the weight of them needed for the final plan’s missions was too great: even the first X-15B launch was going for 32 orbits over the course of two days (thus giving the 90-minute orbit that was ubiquitous in early space mission designs). The craft would be perched on top of two-thirds of a Saturn V rocket stack and would be launched to 150 kilometers. For the first mission the second stage would be deleted, but for later, higher missions would have taken out the smaller third stage instead. Using equipment stowed in the payload bay, they would engage in some optical and radio telescopy, measure the Earth’s gravitational field, and study the planet’s surface with infrared and radar.
Once their two-day mission was done, the crew would retro-fire the X-15B’s rockets to get them down to 105 kilometers where the residual atmosphere would cause them to re-enter. Though they’d have to bleed off 28,000 kilometers per hour of speed, before too long they’d be back to velocities more typical of the original X-15 and glide back to base like that more-staid craft did. Unlike the original X-15B proposal, this version of the craft was almost fully recoverable—no parachutes and dips into the Gulf of Mexico now. The only blemish was the need to jettison its ventral tail fin so that the plane’s aft landing skid could touch the landing strip.
North American Aviation promised that if they started work in December 1959, the first manned orbital flight in the X-15B would take place in June 1964.
What happened to make it fail: It turned out that the time-saving of re-using the X-15’s shape wasn’t nearly enough to turn the X-15B into a useful program. Once NAA worked it all out and came up with that target date of June 1964, it meant that the X-15B wasn’t going to fly until nearly two years after the X-20 Dyna-Soar was supposed to make its first suborbital flight. The idea of using it to build up to the X-20 was a complete impossibility.
What was necessary for it to succeed: At first blush it seems like the X-15B might have had room to fit into the gap between the X-15 and the X-20 anyway, once delays are worked in The Dyna-Soar didn’t fly in mid-1962 like it was supposed to, and didn’t even come close. When it was cancelled in December of 1963, Boeing hadn’t even begun building it, and the first flight had been pushed back until January 1, 1966.
The X-15B still wouldn’t have been able to catch up. What was delaying the X-20 was the work needed to develop the materials for its frame and skin; exactly the same work would have been needed for the X-15B; its worked-out aerodynamics and the time saved there were irrelevant to the problem, so all the real-world delays that affected the more advanced craft would have affected its hypothetical predecessor too.
Ultimately the X-15B was an interesting idea, but it couldn’t fill any of the niches it was supposed to fit—first orbital craft or interim test-bed—and so there was no reason for it to get built.
It did give history one interesting legacy, though. Astronaut selection for MISS was completed on June 25, 1958. Nine astronauts were selected and several would be pilots for the vanilla X-15 instead (one, Joe Walker, would reach space that way), and a couple would be selected for the X-20. But more important from a historical standpoint is the one who moved on to NASA from MISS via the X-15: Neil Armstrong.