The facts, fiction, and body of knowledge the X-15 generated is enormous. School kids should be taught about it as one of the great things the US should be proud of. In fact, I do bring it up with them, but (contrary to what you may have heard) I do have some sympathy and I suspect the readers of this blog may well glaze over in the same way those 10 year-olds do when your truly bangs on about air density and friction heating for too long. Consequently, I’m going to cover the X-15 in a couple of posts.
The X-1 set the standard and expectations for the X-planes that followed. Some lived up to their historic predecessor, some didn’t. The X-15 has both feet firmly in the “did” camp. Its iconic shape is probably even better known than the X-1’s, and if you see it hanging from the Smithsonian ceiling, you can’t help but be impressed by its combination of grace, strength and brute force. Its looks, achievements, and reputation, inspired generations of science fiction.
The X-15 started life in the mid-fifties when NACA got carried away with it’s (and the USAF’s) mach busting success, and contracted North American Aviation for a hypersonic research aircraft. Hypersonic in this context means able to travel several times the speed of sound. In science fiction it can mean anything from being able to travel under water, to fast enough to make stars look like long streaks of light. Ah, special effects.
North American already had numerous successes under its belt, but even so, the X-15 was no picnic to design. The aircraft was intended to reach speeds in the range mach 3 to 6, well beyond any other manned vehicle. This would heat the surface of the aircraft beyond the point at which common metals become soft, and no one wants a soft aircraft. The control of the aircraft at hypersonic speed was beyond what could be investigated in wind tunnels, making the development of the control surfaces a fifty-fifty split of science and faith. The balance of weight and thrust was an ever present issue, and then there was the need to carry thousands of pounds of highly reactive liquids and force them into the engine at the rate of hundreds of pounds a second, all while under high-g acceleration.
The surface temperature issues were addressed by the use of a nickel alloy called Inconel-X and later in the program an ablative coating (one that vaporizes and carries away the heat in the vaporized material). Inconel-X is significantly heavier than titanium and aluminum, so it had to be used judiciously in order to keep the thrust-to-weight ratio high enough to reach the speed goals.
The stability issue was tackled in several ways. One of the characteristics of the X-15 is its thick rear fins (see upper fin below).
At lower altitudes these were a significant drag, but at higher altitudes where the air is very thin, their size was needed to give enough control to keep the aircraft stable. The lower rear fin was actually so large that it had to be jettisoned for the aircraft to land (and obviously isn’t present in the picture above).
Controlling the X-15 at extreme altitudes required more than control surfaces, even at the X-15’s high speeds the amount of air impinging on the control surfaces wasn’t enough to generate significant force. A Reaction Control System (RCS) was installed that consisted of small rockets mounted pointing sideways, up, and down. These were controllable in small bursts and used to point the speeding aircraft in the desired direction. The fuel they employed was hydrogen peroxide, which was catalytically decomposed to oxygen and high temperature water vapor – which leads to the joke that the X-15 was steam powered.
On top of the airframe challenges, only a rocket engine would be capable of the thrust required to achieve hypersonic speeds. Reaction Motors was contracted for the 57,000lbf XLR-99, an engine some 15 times more powerful than the XLR-11 they produced a decade earlier for the X-1. The XLR-99 was the first man-rated, throttable rocket engine. It combined ammonia and liquid oxygen at the rate of 200lb/second by use of a high-speed (steam powered) turbopump. The aircraft’s entire 15, 000lb fuel supply could be burnt in 80 seconds. As shown below, the fuel and oxidizer took up two thirds the volume of the X-15.
If that wasn’t a challenge enough, the rocket had to work from the oxygen rich earth’s atmosphere to virtual vacuum without going out.
The X-15’s pilot wore a pressure suit and breathed through an oxygen supply because the cockpit was pressurized with nitrogen and the fuselage with helium. Here’s Captain Joe Engle sporting the USAF’s designer pressure suit wear (and, it has to be said, a bit of a Vulcan hair cut).
These inert gasses helped stop bad things happening in the extremes of temperature and pressure (every rocket powered aircraft is part controlled burn and part bomb, so it’s best to minimize the chances for detonation). The pressure suit was also for protection in the event the pilot had to eject. Yep, the X-15 had an ejection seat. No pilot ever used the seat, but it was designed to operate up to a staggering Mach 4 and 120,000ft. I think even Joe Kittinger would have paused for a moment before pulling the handle at that speed!
Life in the cockpit wasn’t easy. The pilot had a very restricted view through two small slot windows, and given the aircraft’s main operating regime (blue-black sky with no visual cues), he was forced to depend almost entirely on his instruments. Here’s Neil Armstrong demonstrating how small the cockpit was – his head fits into the triangular wedge formed by the two windscreen in the canopy above his head.
There were three joysticks: one for traditional control surface maneuvers, one for the RCS, and one for high-g maneuvers.
Later aircraft had an improved flight control system that enabled the pilot to operate just one joystick and let the computers determine the optimal response depending on the flight conditions. During high-g decelerations the pilot used a flip-down forward headrest to alleviate strain on his neck (bear in mind the suit and helmet were heavy, and several time heavier under g’s.).
The aircraft’s operational profile was
- Carrier aircraft drop (at 8.5 miles high and 500 mph).
- Acceleration using the rocket engine until burn out.
- Ballistic (ie unpowered) flight into upper atmosphere/space (depending on the mission).
- Glide to landing.
As you can see the XLR-99 engine only operated for a tiny part of the overall flight (about a minute and a half). The rest of the time the aircraft was either trading speed for increased altitude, or gliding back down to landing. This is a scenario familiar to almost any rocket pilot back to the X-1 and even early German rocket propelled aircraft. On glide only flights, the X-15 dropped rapidly and had to glide immediately to a landing. To put rapidly in context, in a normal landing the X-15 would descend 30, 000ft (five miles) in a little over 2 minutes, whereas a commercial aircraft would generally take 20 to 30 minutes.
Over a four-year period all these challenges were tackled, the aircraft was built, the procedures developed, and pilots trained – a staggering achievement. The first pilot was Scott Crossfield, a brilliant pilot/engineer who was heavily involved with the design and development. So on June 8th 1959, there was no better person to have in the cockpit when the explosive bolts fired and the craft dropped from its carrier aircraft. Scott glided the X-15 to a perfect 300mph landing at Edwards AFB.
It was less than a dozen years since Chuck Yeager and the X-1 had broken the feared, but mythical, sound barrier in the clear skies over Muroc Field, and the X-1’s spiritual successor was chomping at the bit to set its own records over the same desert. Over the next ten years, it would.