I’ve recently delved back into the X-15 again. But instead of focussing on the aircraft and its role in America’s move into space, I’ve been looking into the structure of the program as a whole. I was surprised, though I shouldn’t have been, at the manpower involved each flight. Like the impressive number of men on hand to recover a single Mercury astronaut, each X-15 flight had a substantial crew both in the air and on the ground at multiple points – another similarity shared with Mercury recover efforts. (Left, workers secure the X-15 after landing.)
In two previous posts, I’ve looked at the dual nature of the X-15. It was at once a cutting edge research aircraft as well as a precursor to orbiting space planes; the space shuttle’s roots in the X-15 is a connection I’ve previously pointed to. A closer look at the test program reveals just how complicated flying the unique vehicle was. During a single flight, the X-15 acted like a traditional jet, a spaceplane, and a glider. It accelerated to speeds upwards of Mach 5 in a minute of powered flight before landing without power on the dry lakebed at Edwards Air Force Base in California. Each flight lasted on average less than ten minutes. A successful flight demanded a lot happen in a very short time span.
Each flight followed the same basic profile. The X-15 began strapped underneath the wing of a B-52 bomber. While the two aircraft were mated, the X-15’s pilot had little to do but check his instruments and wait for the B-52 to ascend. After usually half an hour when the B-52 reach an altitude around 35,000 feet – close to the average cruise height of a commercial flight across the United States – the pilot released the X-15. (Left, a schematic of both an altitude flight path and a speed flight path. Launch and emergency lakes are noted on the map below the flight line.)
Upon separation, the X-15 pilot lit his single engine and began the short powered portion of the flight, which was measured in seconds. Here the flight path changed to reflect an individual flight’s goal. If the flight was to achieve a peak altitude, the pilot burned his engine as long as possible on an upwards trajectory into the nearly airless upper limits of the atmosphere. If the flight was aiming to achieve or break a speed record, the pilot burned his engine on a more gentle incline. More lateral motion meant the aircraft was not fighting gravity and the energy of the engine’s burn was more effectively transferred into speed.
In either case, once the engine burn elapsed, the pilot shut it down and physics took over; momentum carried the aircraft throughout the rest of the flight. The X-15 rose over the top of its parabolic arc before gravity took over and the aircraft began its unpowered descent. Tracing out circles over the landing lake at Edwards (although he usually only had time for one), the pilot slowed his airspeed to make a soft and controlled landing on the dry lakebed – soft and controlled at close to 200 miles per hour.
The X-15 landed on skids rather than wheels. Akin to metal skis, the skids slowed the aircraft faster than traditional brakes at the cost of directional control. This landing method was also only effective on the dry lakebed; the skids couldn’t dig into a cement runway for increased drag the way they could into dried mud. There was no guarantee an X-15 – or the pilot – could survive a skid landing on a paved runway. (Right, the X-15 lands at Edwards on skids. The wheels in the nose gear gave the pilot some control, but only if his speed was in excess of 200 miles per hour. 1961.)
The tripartite control of the X-15 was achieved through different control methods and different environments. The powered ascent relied on traditional flight controls. In the nearly airless upper atmospheric flight, small hydrogen peroxide rockets in the aircraft’s nose expelled steam, allowing the pilot to adjust his orientation orientation in the absence of an atmosphere. The unpowered descent turned the X-15 into a glider; the aircraft produced sufficient lift, allowing the pilot to control of the falling aircraft.
To say that the unpowered descent was controlled is a bit of a misnomer. The pilot could guide the X-15 and take certain measures to slow its speed, but without any power he only had one chance to land. The aircraft was also a fairly lousy glider. Typical gliders glide well because their wingspan is greater than the length of their body. This increases the lateral surface area and makes the vehicle fall slower than its lateral motion. The pilot has more time to guide the glider to a soft landing. The X-15 had the opposite proportions. It’s 22-foot wingspan was less than half of its 50-foot length. As such, it produced only a small amount of lift – it fell almost as fast as it traveled forward. One of the engineers who worked on the program half joked than only a stone falls at a steeper angle than the X-15. This is why the pilot could typically only trace one circle over his landing lakebed to slow his speed.
The X-15’s limited lift and consequent limited controllability in landing necessitated a very specific flight path. The aircraft needed to be within safe landing distance from a suitable lakebed at all times. But limiting the flight path to the airspace above Edwards was not an option. If the aircraft was going to ascend to the upper atmosphere at hypersonic speeds, the pilot would need more room. To give the pilot the space required to complete the X-15’s flight profile, the B-52 began its flight from a different lake.
The launch lake had to be far enough away to give the pilot ample room for his flight profile but also situated such that he had options for an emergency landing. To this end, a series of lakes were selected throughout Nevada and into California located north and west of Edwards – south and east lay Mexico and the Pacific Ocean respecitvely. The most commonly used launch lakes were Mud, Delamar, Hidden Hills, Smith Ranch, Silver, Railroad Valley, Rosamond, and Cuddeback. Each is located such that multiple lakebeds suitable for landings lay between it and Edwards. (A B-52 launches an X-15.)
Emergency landing lakes had to meet certain criteria. The ground had to be solid enough to hold the X-15 and emergency vehicles. To test its strength, NASA pilots flew aircraft down to ground level over the lakebed with their wheels down. They then inspected the depth of the wheel tracks. If they weren’t too deep, they could assume the lakebed was firm enough to hold the weight of an X-15 and any emergency vehicles without them all getting stuck.
If the lakebed was too moist to support the aircraft, the pilots flying the reconnaissance mission just hoped to get out without lodging their aircraft in the mud. But even skilled pilots couldn’t managed a safe takeoff from a muddy lakebed. On one flight to check the strength of a lakebed, Neil Armstrong and Chuck Yeager flew together. Armstrong lowered the wheels to the lakebed, but the ground proved to be too muddy for a clean pass and the two pilots got stuck in the mud. It was the only time the two pilots flew together.
Another challenge of the long flight path of the X-15 was communication. Ground crews would need to know what was happening to the aircraft even when it was hundred of miles away at its launch point. To enable communication with the X-15 throughout a flight, tracking stations were set up in Beatty, Nevada and Ely, Nevada in addition to the site at Edwards in California. These sites, nearly 200 miles apart, covered almost 400 miles on the ground and established a communications network that put NASA in contact with its aircraft anywhere over Nevada and the south of California.
But what if the pilot lost contact with Edwards during a flight and crashed, effectively disappearing? To maintain visual contact with the X-15, support crews in aircraft were stationed at various points across the aircraft’s flight path. The aircraft, referred to as chase planes, were on hand to keep visual contact with the X-15 or else be in the vicinity to look for a crashed aircraft. (Left, the result of a crash at Mud Lake after engine failure forced pilot Jack McKay to make an emergency landing. The pilot survived with serious injuries. November 1962.)
Each flight made use of multiple chase planes, typically between three and five. The first plane was a T-38 that followed the B-52 during launch. This was the only aircraft NASA had in its fleet that could fly in close formation with the B-52 and therefore get a good visual look at the X-15 before launch. In the air, the second chase plane caught up with the B-52 before it released the X-15. This time it was an F-104. Its pilot was charged with visually confirming the engine’s burn and the pilot’s angle of ascent after launch.
The X-15, however, travelled too fast for a single F-104 to follow it. The rest of the flight therefore required multiple chase planes to be on hand – all F-104s. As the X-15 progressed on its flight path and made its way from the launch lake to Edwards, it caught up with a second and third F-104 that waited in holding patterns at set locations along the flight path. These aircrafts offered the same visual confirmation of flight stages and assistance in emergency landing as the first. (An F-104 lands in formation with an X-15.)
Visual aid was necessary for the X-15 pilot. The small aircraft’s cockpit window afforded the pilot a limited view. He could not see the horizon unless the aircraft’s nose was pitched down, and he had no view of his body or small wings. This left the pilot without any visual reference and no way to check for any external problems. The X-15 pilots thus relied on the chase planes to alert them to any problems and confirm their angle of ascent was correct. The pilots could, for example, receive a nasty shock when he saw the Pacific Ocean directly in front of him when he expected to be over Nevada. The sight was a trick on the eye brought on by the Earth’s curvature that could derail a calm pilot. Milton Thompson thought he had missed his angle of ascent and traveled hundreds of miles further than expected when he first saw the ocean during an X-15 flight.
The F-104 was a popular chase plane because of its handling during landing. An unpowered landing in an F-104 was the closest approximation to a landing in the X-15 – this was how X-15 pilots learned to land the research aircraft. It also meant that a pilot in an F-104 could land almost as quickly as the X-15 in an emergency, a vital capacity for any support crew.
The X-15’s support network wasn’t limited to the air, the pilot also had forces on the ground. Ambulances a fire trucks followed the B-52 down the runway during takeoff just in case an emergency developed during take off. The same crews were on hand during landing; pilots typically saw hordes of men and vehicles racing towards the small airplane before descending upon him as soon as he opened the canopy. The emergency crews were as eager to celebrate a successful flight as they were to assist an injured pilot. (Right, ground crewmen arrive at the X-15’s cockpit after Neil Armstrong lands. 1960.)
The support on hand for an X-15 flight was substantial, but also expected. Unlike the Mercury flights, the fate of the X-15 lay solely in the hand of its pilot; there was no preprogrammed flight path and the control centre at Edwards had no way to remotely adjust the aircraft’s path. With an experimental flight at the hand of fallible (though highly skilled) men, there were ample ways for a flight to turn sour. There were fatalities and non-fatal crashes as part of the program, but in general the program was a great success. Interestingly, the support forces turned out to be less necessary than anticipated. In a review of chase plane activity, Milton Thompson found that in 20 years of experimental flying only one would-be fatality had been prevented because of a chase plane’s presence.
During the X-15 program, pilots in the air offered great support and helpful advice, but few pertinent or life-saving pieces of information were passed on the pilot. Still, many X-15 pilots appreciated feeling like someone was with them during their record-breaking and usually dangerous flights.
Suggested Reading/Selected Sources
Milton Thompson At the Edge of Space. Smithsonian, 1992.
Richard Tregaskis X-15 Diary: The Story of America’s First Space Ship. Bison, 2004.
vintagespace.wordpress.com 