Total free flight time for the 199 flights in the program was 30 hours, 13 minutes, and 49 seconds. Total flight time including captive flight time on the B-52, was less than 400 hours for the three airplanes. The average total flight time per airplane including captive time was roughly 130 hours. We obviously did not wear them out flying them. We did, however, do a lot of ground testing. After each flight, we routinely disassembled each system, inspected it, reassembled it and then operated the system to ensure that it was ready for the next flight. Therefore, for each flight, we operated each system at least twice—once during checkout and once during flight. If the flight was an aborted flight, we still usually operated the systems during the captive portion of the flight, so we added another cycle to the total number of system operations. My personal opinion is that we wore the airplanes out testing them in preparation for flight.
The airplane and its systems did operate in a rather severe environment. Atmospheric pressures on the airplane ranged from over 2,000 pounds per square foot down to almost 0 in the vacuum of space. The g environment varied from -2.5 g to over 8.0 g. The temperatures on the various parts of the airplane varied from -245° F to 1,200° F.
Some parts of the airplane had a controlled environment. The cockpit and the equipment bay were pressurized and the temperature was regulated. In most of the airplane, however, the environment was uncontrolled and the temperatures varied widely. The hydraulic lines, for example, ran alongside the liquid oxygen tank in the side fairings. The hydraulic fluid in these lines was super-cooled such that when you started the APUs and the hydraulic pumps, the pressure went off scale over 4,000 psi. This was obviously detrimental to the integrity of the system, but the system survived this exposure. Ultimately, though, we could expect failures as a result of this environment and we did have representative failures. On the basis of our X-15 experience, the space shuttle can anticipate problems for many years to come.
PART 3
TRIUMPH AND TRAGEDY
Chapter 8
Toward Mach 7—The X-15A-2
This phase was the last distinct phase of the program. It began with the first flight of the modified number two aircraft on the 109th flight of the program. It came to a conclusion after twenty-one flights on the 188th flight of the program, when the airplane was damaged due to excessive heating during its maximum-speed flight.
An event that led to this phase occurred on November 9, 1962. Jack McKay was scheduled to make a ventral-off stability and control flight out of Mud Lake in the original number two X-15. The weather was good that morning and things went smoothly during the countdown to launch. During prelaunch checkout of the X-15 there were no symptoms that hinted of the dire events to follow. The launch occurred on time and Jack advanced the throttle to light the main chamber. During that phase of the program, the engine light was performed at minimum throttle setting, which was approximately 30 percent thrust. Jack got a main chamber light at 30 percent power, but when he attempted to increase power, he got no response. The engine was not going to cooperate this particular morning.
Theoretically, Jack could have made it back to Edwards at 30 percent thrust, but there was no way of knowing whether the engine would continue to run long enough at that power setting to get home to Edwards. The problem of energy management was seriously compromised by the low power since the decision times for an emergency landing at each of the various emergency lakebeds were calculated on the basis of 100 percent thrust. We had no way of recomputing those decision times in real time, so mission rules under these conditions dictated that the pilot shut the rocket engine down and make an emergency landing at the launch lakebed. Jack shut the engine down after 70.5 seconds of engine burn time and turned the aircraft back toward Mud Lake.
Mud Lake was circular, approximately 5 miles in diameter, and quite smooth and hard. It was a good lake for an emergency landing. Jack began jettisoning propellants immediately after shutting the engine down. Complete propellant jettison was never fully achieved on aborted missions and thus, Jack was destined to land with a considerable amount of residual fuel and liquid oxygen on board. On final approach, he selected flaps down, but the flaps failed to move. Jack was now unknowingly set up for a catastrophic landing. As he came across the edge of the lakebed, he was high on airspeed due to the excess weight. He touched down very gently, but at a higher than normal speed due to the inoperative flaps. Jack actually touched down at 296 MPH. We normally touched down at approximately 230 MPH.
In a normal landing, when the X-15 main gear skids touched down, they created a large drag load as they slid along the surface of the lake. This drag on the skids caused the nose of the aircraft to slam down. This slamdown caused a large load on the nose gear, but it also resulted in a large load on the main gear a fraction of a second later as the aircraft rebounded and loaded up the main gear.
To compound the problem, as the nose of the aircraft rotated down after main gear touchdown, the control system automatically tried to stop the nose from slamming down. It did this by deflecting the horizontal stabilizers to the full leading edge down position. This was a futile attempt on the part of the control system. It could never hold the nose up, due to the high drag on the skids. What it did do, however, was load up the main gear with a large aerodynamic down load. This was an undesirable effect since the main gear was now simultaneously subjected to a large rebound load and a large downward air load. The magnitude of the air load was a function of the touchdown airspeed. The higher the touchdown airspeed, the larger the aerodynamic down load. This sequence of events during a normal landing tended to load the main gear up near its design limits.
On this landing the combination of the two loads was large enough to cause the main landing gear to fail due to the higher landing speed. The left landing gear collapsed almost immediately, causing the aircraft to tilt over to the left. As the aircraft tilted over, the left horizontal stabilizer dug into the lakebed and tore off the aircraft, taking the left main gear strut with it. The lower ventral also struck the lakebed and was torn off the aircraft in small fragments.
The aircraft continued to roll over until the left wing tip contacted the lakebed. Shortly thereafter, the nose wheels failed and came off the nose gear strut. The aircraft began sliding on the nose strut, the left wing tip, and the right main gear skid. As it did so, the aircraft began to swerve to the left. Jack realized he was in serious trouble and elected to jettison the canopy in case the aircraft should roll over. Milliseconds later, the aircraft abruptly tilted over to the right as the aircraft swerved through 90 degrees. The right wing tip dug into the lakebed and the aircraft flipped over, slamming into the lakebed on its back. Jack’s helmet was one of the first things to hit the lakebed. One might say that the aircraft came down on Jack’s head. It was a crushing blow.
Jack was now pinned in the cockpit, hanging upside down, while propellants and peroxide were starting to leak from the aircraft. The liquid ammonia fuel was the greatest hazard because the rescue crew could not help Jack while those potent vapors enveloped the aircraft. Their emergency breathing masks were not working properly.
The rescue helicopter pilot finally realized what the problem was and set up a hover over the cockpit area to blow the ammonia vapors away from Jack and the rescue personnel. Now the problem was getting Jack out of the cockpit. The rescue crew had no way of lifting the airplane up to get Jack out. They finally decided to dig a hole below the cockpit to allow him to slide out underneath. By this time, the C-130 with the paramedics on board had landed and, after Jack was assisted out of the X-15 cockpit, they carried him aboard and headed back to Edwards.
This emergency landing was the first to benefit from all the emergency planning and procedures. It was always a major operation to get the emergency crews and equipment in position for each flight, but in this case it all paid off and justified the expense. Within seconds after the X-15 flipped over, the X-15 ground rescue crew was at the airplane after racing across the lakebed from their standb
y position on the lake. The fire truck was right behind them and the helicopter was already hovering near the X-15 when they arrived. The rescue crew vehicle and the fire truck had been flown up to Mud Lake in the C-130 before daybreak to be in position for the X-15 launch. The helicopter had also flown up from Edwards in the early dawn to be on station for the launch. The C-130 had made a trip back to Edwards to pick up another fire truck and the paramedics and had flown back up range to take up its standby station halfway between Mud Lake and Edwards. It was then in position to cover an emergency landing at any one of the intermediate lakebeds. It was in position this day to assist in the rescue effort and transport Jack back to the hospital at Edwards. We only had three landings at intermediate lakebeds during the X-15 program, but the preparations paid off. I can vouch for that, because I made one of those landings at an intermediate lake.
Jack was seriously injured when his head hit the lakebed. The force of that impact squeezed the cartilage out from between the vertebrae in Jack’s neck and upper back. He ended up 1 inch shorter, with a couple of cracked vertebrae. Jack recovered sufficiently to fly again, but ultimately his injuries forced him to retire on a disability.
The airplane survived Jack’s Mud Lake accident in amazingly good shape. That, of course, is a testimony to the steel construction. The outer 2 feet of the right wing were bent up 90 degrees, but the remainder of the wing was perfectly straight. The airplane came to rest on the upper vertical stabilizer and the upper fuselage, just behind the cockpit. The upper vertical stabilizer was slightly crushed, but easily repairable. The left-hand horizontal stabilizer had been ripped off when the left main gear failed, but the rest of the primary empennage and fuselage structure were undamaged. The aircraft was hauled back to North American’s El Segundo plant on a flatbed for repairs.
Instead of just repairing the aircraft, the program officials elected to take this opportunity to modify the aircraft for scramjet engine tests. Approximately a year after the accident, the number two X-15 emerged like a phoenix from the bowels of the North American Aviation production facility on the south side of the Los Angeles International Airport. After Jack’s Mud Lake accident, the X-15 had been extensively modified to carry an experimental scramjet engine up to speeds of over 5,000 MPH.
The modifications included the addition of a 28-inch plug in the middle of the fuselage between the LOX and ammonia tanks. A spherical tank was installed in this extra space to carry the liquid hydrogen fuel for the scramjet engine. The scramjet engine was to be carried on the lower ventral fin stub, but there was not sufficient ground clearance with the standard landing gear. The solution was to increase the length of the main landing gear struts. This was the second major modification. The nose gear also had to be modified to increase the length of the oleo travel to accommodate the extra force produced by the slamdown of the longer, heavier fuselage. (An oleo is a shock strut like the shock absorbers on a car. A longer oleo is required for a higher total force.)
The major modification was the addition of two huge external tanks for additional main engine propellants. One tank contained liquid oxygen and the other, liquid ammonia. Together these tanks carried an additional 1,800 gallons of propellants which corresponded to approximately 60 seconds of engine burn time. An additional peroxide tank was added externally at the rear of the aircraft to provide more steam to drive the engine turbopump for a longer period of time. Several additional source gas tanks were added to the aircraft to provide pressurization for the extra propellant tanks.
Figure 7. X-15A-2 with modifications.
The total propellant was almost double that carried by the standard airplane. One might assume that this would provide a big increase in maximum speed. Realistically, the actual speed increase was disappointingly small. The extra weight and tremendous increase in drag due to these tanks severely reduced the rate of rotation to achieve the desired climb angle and also reduced the rate of climb and acceleration. With full external tanks, the aircraft weighed almost 57,000 pounds compared to 33,000 pounds for the standard, fully fueled aircraft. All of the external fuel would be used up just to reach Mach 2 at 70,000 feet, where the tanks were jettisoned. The internal fuel did not provide the same total acceleration as the standard airplance, since the new airplane was heavier and had somewhat more drag, particularly with the ablative coating on the aircraft. The net result was, at most, two additional Mach numbers at the expense of a lot of additional complexity.
The tanks contained a recovery parachute system which added even more complexity to the overall system. Operationally, we had to be concerned about where the tanks might impact depending on whether or not the chute system worked. We had to plan for either case and ensure that the tanks impacted in a controlled area. Also, in case of a problem, we had to be able to jettison the tanks at any time to try and save the airplane. We thus had the problem of ensuring that we did not drop a 1,000 gallon tank of liquid oxygen on some old prospector’s shack. The California desert is very barren and uninhabited, but ironically, when you have to jettison anything in an emergency, it always finds something of value to hit. The B-52 also had to be structurally modified to carry the much heavier number two aircraft.
The basic aircraft structure had to be somehow protected from the increased aerodynamic heating predicted at the higher planned speeds. The proposed solution was to cover the entire airplane with an ablative material developed to protect missile nose cones. This material acted as an insulator. More effectively, it acted as a dissipater of heat through a burning process that produced a heat resistant char layer. Variations of this ablator material were used to protect the Mercury, Gemini, and Apollo spacecraft. This material was installed on the X-15 in both premolded pieces and as a spray-on coating.
The material was pink in color. When it was first applied to the X-15, the crew called Joe Walker out to look at it just to needle him. A pink airplane did not quite fit the macho image of a test pilot. Luckily, a protective coating was required due to the material’s sensitivity to liquid oxygen. The protective coating was white—an acceptable color.
The actual installation process proved to be a lengthy, tedious job involving many hours of hand finishing the coating. This coating was not applied until the aircraft was being prepared for the final high-speed flights. It would have been extremely hard to service, operate, and maintain the airplane routinely with that protective coating permanently installed. We were fortunate in some respects that the flight program was terminated after only two flights with the protective coating. It was a real pain to install it on the airplane. Several other modifications were required due to the decision to use the ablative coating.
The cockpit canopy windows were modified to an oval shape to withstand the higher aerodynamic heating anticipated at Mach 8. An eyelid was designed to cover one window during high-speed flight to prevent the window from being clouded by the residue from the protective ablative thermal protection material. This eyelid could be opened manually once the aircraft slowed down to subsonic speeds, providing good vision for landing. The other window was not covered and was used at the high speeds.
An extendable airspeed probe was also designed to be deployed at subsonic speeds to ensure that the pilot had a good, clean airspeed system for landing. It was suspected that ablator residue might plug the normal fixed external probe.
The initial justification for modifying the number two aircraft was accepted as a worthwhile research effort. It was generally agreed that the aircraft could be modified to successfully accomplish the desired objectives. It was not obvious how complex or painful the development process was going to be.
Once the number two X-15 was modified, we began a flight envelope expansion program on the basic airplane, without tanks, to determine the effects of the basic airplane modification on the aircraft’s stability, control, and handling qualities. The effects were not anticipated to be significant because the external configuration of the modified basic aircraft had not changed much. However, we need
ed to accurately quantify the aircraft’s flying characteristics before the external tanks and scramjet engine were installed on the aircraft.
The flight data would be used to update our simulator and allow the pilots to assess the predicted flying characteristics with the tanks and scramjet installed. It was known from wind tunnel tests that the tanks and the scramjet would definitely alter the flying characteristics and, in general, degrade them. This was verified in the simulator and was subsequently confirmed much later when the airplane was finally flown with external tanks.
The first envelope expansion flight was a flight out of Hidden Hills to a maximum speed of Mach 4.59. Everything worked fine and the airplane flew reasonably well. Bob Rushworth flew that flight and also the next one out of Delamar Lake. On the second flight, he achieved a maximum Mach of 5.23. As he was decelerating back through about 4.5 Mach, Bob heard a tremendous bang under his feet, almost like an explosion. Simultaneously, the aircraft pitched down, the nose yawed to the side and the aircraft rolled over on its side. Smoke began to fill the cockpit, compounding the problem. Bob had no indication of what had occurred. The control room could not tell him what had happened because data they were looking at showed no indication of a malfunction.
The chase aircraft were no help, as they were more than 10 miles below him. Bob could only guess at what might have happened. The possibilities included an explosion of some sort or a structural failure. Bob did note on reflection that the loud bang resembled somewhat the noise generated during nose gear deployment. Other evidence seemed to substantiate that possibility. Bob informed the control room that he thought the nose gear had come out. The control room still could neither confirm nor help since no one had ever anticipated that kind of problem and therefore there were no specific emergency procedures.
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