The X-15 had redundant (two) APUs since hydraulic and electrical power were essential to control and to fly the aircraft. The APUs were started about 12 minutes before launch to provide power during checkout of the various systems in the X-15 prior to launch. Prior to starting the APUs, the B-52 supplied electrical power to the X-15. This external power from the B-52 allowed certain X-15 systems to be active at all times during captive flight. The APUs ran continuously from 12 minutes prior to launch until the pilot shut them down after landing. Each APU had its own peroxide tank which could also be replenished with any leftover peroxide from the main engine turbo pump tank. We had enough peroxide for 45 minutes of APU operation under normal circumstances. During the X-15 flight program, we had many problems with the APUs, but we did not lose an airplane due to APU problems. We came close, however, on at least two occasions.
The liquid oxygen (LOX) tank was located behind the APU compartment. Liquid oxygen is quite cold, as you may remember from demonstrations in high school. We could always tell how much liquid oxygen was in the tank by the frost on the outer skin of the fuselage. It was always comforting to know that if we crashed in the X-15, we would not suffer very long. The liquid oxygen from a ruptured tank would freeze dry us in seconds.
Behind the LOX tank there was a small equipment compartment that also contained a large spherical helium tank. This helium was used for tank pressurization and pneumatic valve operation.
The ammonia tank was behind this compartment. Behind the ammonia tank there was a compartment for the large engine peroxide tank. This tank contained about 800 pounds (about 80 gallons) of peroxide to run the engine turbopump which pumped both LOX and ammonia into the rocket engine. The engine turbopump was similar in concept to the APU—a steam turbine driven by decomposed peroxide. It pumped roughly 30 gallons of LOX and ammonia every second into the rocket engine combustion chamber at 600 psi. It was a pretty potent pump.
Behind the engine peroxide tank compartment was the engine compartment. The LR-99 engine was approximately 7 feet long, 2 feet of that being the nozzle. The engine weighed approximately 910 pounds. It produced 60,000 pounds of thrust which gave the airplane an impressive thrust-to-weight ratio. Liquid ammonia running through tubes on the inner surface cooled the engine nozzle. This inner surface was also coated with a ceramic material that protected the metal from the searing heat of combustion. This ceramic coating had to be repaired quite often due to the erosion caused by the tremendous heat and gas velocities in the rocket plume.
The landing gear on the aircraft was unusual. The nose gear was rather conventional with a strut and dual wheels. It folded forward and was stowed internally forward of the cockpit. The main gear was unconventional. It consisted of metal skids mounted on struts at the rear of the aircraft. The main gear struts also folded forward, but they were stowed externally along the lower side of the fuselage. Both gears depended on gravity and air loads for proper deployment.
The combination of a wheeled nose gear and aft mounted main gear skids was a very stable configuration during slideout after landing. This landing gear configuration and orientation eliminated any tendency to bounce back in the air on touchdown since the drag of the aft mounted skids forced the nose down once they contacted the ground.
The X-15 landing gear configuration was not readily steerable on the ground. There was no nose wheel steering and, with skids for main gear, it did not have conventional asymmetric braking to steer the aircraft. Applying rudder only resulted in the vehicle sliding sideways without turning. The only way to turn the vehicle was to apply roll control to load up the skid in the desired turn direction. Right roll resulted in a right turn. This was only effective, however, above 100 knots. Below 100 knots the pilot was just along for the ride. The airplane went where it wanted to. That is why we landed on the lakebed. You had a lot of room to deviate during slideout on a lakebed.
The aircraft had a rather conventional wing, although it was relatively small with only a 22-foot span and roughly 200 square feet of area. The empennage of the aircraft was unusual with two canted horizontal surfaces and two vertical surfaces. These tail surfaces resembled the feathers on an arrow and served the same purpose by keeping the aircraft stable and pointed in the right direction. The upper and lower vertical tail surfaces were quite large and thick compared to those found on conventional airplanes. They were wedge-shaped in cross section with the sharp edge in front and the thick blunt base at the rear. This shape provided additional stability to prevent the aircraft from swapping ends at very high speeds. The lower half of the lower vertical had to be jettisoned before landing because it protruded below the landing gear.
The aerodynamic control surfaces on the aircraft consisted of an upper and lower rudder and two canted horizontal stabilizers. The rudder surfaces were the outer half of the upper and lower vertical fins. These surfaces pivoted about an axis at about midspan. They were large surfaces and quite effective. The canted horizontal surfaces served as pitch and roll control surfaces. They moved symmetrically for pitch and asymmetrically for roll. We referred to the horizontal surfaces as a rolling tail. There were no ailerons or other roll control surfaces on the wings. The only movable surfaces on the wing were trailing edge landing flaps.
Speed brakes, spoilers, or other drag devices are commonly used on unpowered aircraft, as an inverse throttle, for energy control. The X-15 had four speed brake segments located on the rear portions of the upper and lower vertical fins. These surfaces opened in a V shape to produce drag. They were very effective, particularly at high dynamic pressure, and were normally used to kill off excess energy. When they were used in this manner, the approach to landing was always made with excess energy. The speed brake was then used to kill off the excess energy just prior to landing, when we knew we had enough energy to make it to our intended landing location.
Speed brakes can also be used to modulate energy by deploying them to a midposition and then opening or closing them to increase or decrease drag. Used in this manner, they work almost as a conventional throttle. Good speed brakes are essential on an unpowered aircraft to ensure that the pilot can consistently and routinely land the aircraft at the desired landing location. We could routinely land the X-15 within 1,500 feet of our desired touchdown point, mainly due to the good effective speed brakes. That is as good as some pilots can do with an engine.
The aerodynamic flight control system was a fairly conventional, hydraulically powered system with dual hydraulic systems. The control system included a high-gain, high-authority stability augmentation system to provide dynamic stability. The X-15 was one of the first aircraft to use such a system. Aerodynamic control was accomplished either through a conventional center stick and rudder pedals or a side stick and rudder pedals. The center stick was seldom used during powered flight due to the high g forces. It could be used for landing, but the pilots found that they could land the aircraft successfully using the side stick and they did not bother to revert to the center stick for landing. It also became a macho thing to fly the entire flight using the side stick. I always felt that I could fly more precisely using the center stick, but I was not about to admit that to anyone. My ego would not let me use the center stick, even in an emergency.
The reaction control system, or ballistic control system as it was occasionally referred to, was a dual hydrogen peroxide rocket system with two rockets in each control direction. Each rocket was fired by manually opening a valve using the reaction control hand controller. When the valve opened it allowed liquid peroxide to flow over a silver screen catalyst bed. As mentioned previously, the silver screen causes peroxide to decompose into water and oxygen. The heat of decomposition turned the water into steam. Thus, we basically used steam rockets to control the aircraft outside the atmosphere.
The pilot operated the manual reaction controls through a side stick controller located on the left-hand instrument panel. It was a three axis controller for pitch, roll, and yaw control. It was not a very precise control
ler and we ended up using it in a so-called bang-bang mode, where it was momentarily displaced to fire a rocket and then returned to neutral—kind of a blip-type control input. If we did not get the response we wanted, we put in another blip or two. It actually was a proportional controller, whereby the more you displaced the control stick, the more thrust you got. The pilots did not like to use it in that manner though, because we did not want to get any large aircraft motions started when we were outside the atmosphere.
Liquid nitrogen was used as the primary cooling medium in the X-15. Liquid nitrogen circulated through the ball nose and also cooled the APU bearings. Liquid nitrogen converted to gaseous nitrogen cooled the avionics and instrumentation units. The cockpit was pressurized and cooled with gaseous nitrogen and the pilot was cooled in his pressure suit by evaporated liquid nitrogen. There were provisions for ram air cooling of the cockpit, however this mode could not be used supersonic.
The X-15 was one of the first aircraft to utilize an inertial platform for attitude reference and guidance information and a computer to calculate inertial quantities. Standard barometric instruments were almost useless in the X-15. We had them on the instrument panel, but the only time we used them was in the landing pattern because they did not work at high altitudes or outside the atmosphere. For the major portion of the flight, we used inertial data for control and guidance.
On my second X-15 flight, the computer failed and I lost my inertial data. It is quite a shock to be without any speed, altitude, or rate of climb information when you are moving at more than 3,000 MPH and cannot see the ground or a horizon. Following this experience, I really took my emergency simulation practice seriously. I practiced each of my flights with all kinds of simulated failures. For example, I practiced my flights with all my flight instruments failed using only the stopwatch, attitude indicator, and the horizontal stabilizer position indicator. That obviously was an extreme case, but I could successfully fly a flight that way. I practiced my flights with many other combinations of instrument failures. This definitely paid off in my ability to handle emergencies. The one quantity that was essential was time.
All the flights were planned on the basis of total energy which was directly related to engine burn time. All the events during a flight were initiated at a precise combination of time and energy. If our clock stopped, we were in trouble. The ground controller could help us out by calling time over the radio, but it was not uncommon to have radio failures or momentary loss of communications. The cockpit timer was a critical instrument. Because of its importance, one might expect the timer to be a very sophisticated high technology electronic device. In fact it was a simple stopwatch with a 100-second sweep that started automatically with main chamber ignition. We had a backup stopwatch that we started at one minute to launch since we were too busy to start it at launch.
To fly at Mach 6, or 4,000 MPH, one must have an airplane that will survive temperatures as high as 1,200°F. The X-15 was built of steel to withstand these temperatures. The steel used was Inconel-X, a tough high-strength nickel-steel alloy that retained most of its strength up to 1,200°F. I first encountered this material while working as an engineer at Boeing Airplane Company. I was trying to find some bearing material that could be utilized in an engine thrust reverser. I ordered a number of high-temperature steels to make bearings and test them under high-temperature load conditions. One of the materials I ordered was Inconel-X. I received a 3-inch diameter cylindrical billet of the material and took it to the machine shop to have a 3-inch piece cut off to make a bearing. The shop foreman fired up a band saw and started to cut the billet. He only managed to make a small groove in the billet before the cutting action stopped. The Inconel-X had destroyed the sawteeth in just a few seconds. The foreman then tried a large power hacksaw. Those sawteeth also disappeared. I then foolishly suggested that he try cutting a chunk off with a cutting torch. He fired up his cutting torch and within a couple of minutes we had a billet of metal that looked like taffy. It would not cut; it would just soften up as it got white hot. We tried pulling off a chunk, but it was too tough to pull apart. It was indeed a piece of steel taffy when we finished. Looking back on it, the entire process was hilarious, but I really developed tremendous respect for the strength of that material. I was impressed.
The major structure of the X-15 was made of this material. Most of the skin was also made of Inconel-X. During a flight to Mach 6, some parts of the airplane would be heated to 1,300°F, but only momentarily. As soon as the engine burned out, the speed and the heating rate decreased. In addition, the heat tended to redistribute throughout the structure over a period of time and stabilize out at a lower overall temperature. As a result, the average maximum structural temperature during a flight was less than 1,000°F. Inconel-X could easily tolerate that temperature and we could therefore fly up to Mach 6 and pull 6 g without overstressing the aircraft. The X-15 was a tough old bird. It did pop and bang as it accelerated above Mach 5, but it all hung together and got us back home.
A rocket airplane sounds pretty exotic, but in many ways it is rather simple. A rocket engine has almost no moving parts other than valves. Fuel and oxidizer are forced into a combustion chamber at the right mixture ratio and are burned in a controlled explosion. Getting the fuel and oxidizer into the engine is the tricky part. The early rocket airplanes used pressure feed systems to force the fuel from the tanks into the engine. This was a simple system, but the tanks and all the propellant lines had to be designed for high pressure to equal the pressure in the combustion chamber. Later rocket airplanes, including the X-15, utilized turbopumps to build up the propellant pressure to force the fluids into the combustion chamber. This allowed the tanks and propellant lines to be designed for much lower pressures. Tank pressures in the X-15 were only 45 to 50 psi for example. The 50 psi pressure in the tank was used to force the propellant from the tank into the turbopump. Combustion chamber pressure was 600 psi. The turbopump raised the propellant pressures from 50 psi in the tanks and propellant lines to 600 psi going into the combustion chamber.
In the X-15, an igniter was required to light the propellants. The igniter in the X-15 engine was similar to a spark plug. The igniter was used in the engine to light a small stream of propellants in a small combustion chamber attached to the main combustion chamber. This small chamber acted like a blow torch. Once it was lit, it served as the igniter for the main combustion chamber. We referred to this starting ignition sequence as igniter idle. We had this small chamber firing before we launched from the B-52. We lit the main chamber after launch.
Because a rocket engine involves a controlled explosion, it is quite temperamental. The controlled explosion can sometimes blow itself out and quit producing thrust. Or, at the other extreme, it can become an uncontrolled explosion. Fortunately, most of our flights took place between these two extremes.
Throttling a rocket engine is one way to precipitate a problem. We had a throttle in the X-15, but we used it cautiously since it was not uncommon to lose the engine completely by throttling it back. The X-15 rocket engine was one of the first rocket engines to have a throttling capability. The engine initially had a throttling capability from 30 percent power to 100 percent power. The throttling worked surprisingly well, however the lower limit of 30 percent finally had to be increased because the engine did not want to run reliably at that power setting. The minimum throttle setting was increased to 40 percent power which solved most of our engine problems.
The LR-99 rocket engine in the X-15 had another unique feature. It had a restart capability. If the engine failed to light the first time, you could recycle the starting process and try again. This feature saved a number of missions since it was not uncommon early in the program to fail to get a light on the first try after launch. Most of the failures to light in the early part of the flight program were primarily due to the fact that we were starting the engine at the minimum throttle position of 30 percent power. We later changed our starting procedure to start the engin
e with the throttle at 100 percent power and then throttle back if required to reduce thrust. This cured most of the failure to light problems. A failure of the engine to light after launch or a “no light” callout over the radio could cause a few missed heartbeats both in the air and on the ground.
The engine relight capability was theoretically useful only when the tanks were mostly full. If the tanks were not full, the propellant feed line could be unported and you could not get propellants to the engine. The only relights that were made with partial fuel were those made by Scott in the LR-99 demonstration flights. I tried an engine relight once when the engine quit after forty-one seconds of burn time. I didn’t get a relight, but we later found that the engine had been damaged before I had attempted the relight. There had been a small explosion in the engine when it shutdown prematurely. That explosion prevented the engine from recycling properly. I may have gotten a relight if that had not happened.
I really wanted to try an engine relight just before landing. Quite often we shut the engine down 5 or 10 seconds before all the fuel was burned. That few seconds of burn time would be adequate to make a go-around since the aircraft weight would be down to about 16,000 pounds and the engine would produce 60,000 pounds. One could get back up to high key damn fast with that kind of acceleration. I know it would have been a spectacular maneuver if I could have pulled it off. I also know I would have been fired if I had done it, but I think it would have been worth it. It would have stunned a lot of people since half the people on the base were outside watching the X-15 land after each flight. Even the commanding general of the flight test center came out to watch each X-15 landing. General Branch, an outstanding center commander, would have loved it. Paul Bikle, our director at Dryden would also have loved it, but he would have fired me to make a point. He did not tolerate foolishness during business hours. After hours, he was a pretty regular guy. Another person who really would have been impressed with a go-around would have been the chase pilot who would have been “left in the dust.”
At the Edge of Space Page 6
