The next flight, number 186, was made with a complete ablative coating on the aircraft and the dummy scramjet. It was originally planned to be a flight to Mach 6.5 with external tanks, but was subsequently changed to a lower speed flight without tanks. This change was made to obtain more data prior to committing to a maximum-speed flight.
This flight was also accomplished as planned without any incidents or surprises. The ablative coating did not appear to affect the flight characteristics of the aircraft although there seemed to be a significant increase in drag. This was anticipated because of the roughness of the ablative coating and the resultant thicker airfoil sections. The maximum Mach number achieved was 4.94 at an altitude of 85,000 feet. The eyelid worked properly to prevent the ablator residue from clouding the windshield. The only significant result of this flight was the severe erosion of the ablator on the ventral leading edge. This duplicated the results of the previous flight and should have warned us, again, of the consequences of going to higher speeds. The heating on this flight was calculated to be only one-third of that predicted for the maximum Mach number flight, and yet we still saw severe erosion of the protective ablative coating. We even correctly deduced that the excessive heating was due to shockwaves off the dummy scramjet, but we did not fully appreciate the ultimate intensity of the heating. We simply replaced the ablator on the leading edge of the ventral and prepared the aircraft for the next flight.
Although not planned as such, the next flight was to be the last flight of the aircraft. It was planned to be a high-speed flight to Mach 6.5. The ablative coating applied for the previous flight was refurbished as necessary and the aircraft prepared for flight. This flight was the first flight with all of the elements needed for a high-speed flight. The configuration included the ablative coating, the canopy eyelid, the dummy scramjet, and external tanks.
Good flight test practice would have dictated that we do a build-up test program on the complete configuration. That would mean a first flight to a relatively low Mach number with the complete configuration and then subsequent flights to increasingly higher Mach numbers. We vigorously debated whether to do this, but finally decided that we had indeed already done a buildup test program. We had done it in bits and pieces, however. We had made a low and a high-speed flight with the tanks. We had made a Mach 4.75 and a Mach 4.94 flight with the dummy scramjet and eyelid, and we had made a Mach 4.94 flight with the ablative coating. The decision was to proceed with a high-speed flight with the all up configuration.
System preflight functional tests were initiated on Saturday, September 16, and were completed the following Wednesday. The drop tanks were installed on Monday, September 18. An APU ground run was made on September 20. No engine ground run was made, but the aircraft was serviced with a full load of LOX and ammonia to conduct leak checks and to check propellant transfer from external to internal tanks. The airplane was closed out for flight on September 22. Final ablative coating repairs were made on Saturday, September 23, and the aircraft was mated to the B-52 two days later. Rain in the Mud Lake area rendered Mud Lake unusable for the entire week, so the flight was rescheduled for Tuesday, October 3.
A problem with the radio headset in Pete’s pressure suit helmet caused a delay in the Tuesday takeoff. Pete was finally fitted with a spare helmet. A second problem, a persistent ammonia leak, required that the tank be pressurized to reseat the leaking jettison value. This procedure did not completely stop the leak, but it reduced it to a small dribble which was considered acceptable. Takeoff occurred at 1,330 hours, rather than the scheduled 1,200 hours. This was a rather late takeoff time for normal X-15 flights, but this aircraft required a lot of extra servicing on the day of the flight.
The flight proceeded uneventfully during captive flight up until about 4 minutes to launch. At that time, Pete had problems hearing NASA-1, but he continued the checklist activities in preparation for launch. At 2 minutes to launch, the radio communications improved and the countdown proceeded smoothly right down to launch. At launch time, Pete hit the launch switch and started to reach for the throttle when he realized that he had not launched. He hit the launch switch again and this time the airplane launched. He got a good engine light and pulled the nose up to begin the climb. The climb went according to plan and at 60 seconds Pete pushed over to 5 degrees angle of attack and prepared to jettison the tanks. The tanks came off at 67 seconds and, according to Pete, they came off hard, very hard.
At tank separation, the airplane pitched down to a -2.5 degree angle of attack before Pete could catch it and bring it back up to a +2 degree angle of attack. He maintained that angle of attack until he came level at approximately 100,000 feet and then began increasing angle of attack to maintain level flight at that altitude. Pete continued to accelerate in level flight until he reached an indicated 6,500 feet per second, or roughly Mach 6.5. At that time, 141 seconds after launch, he shut the engine down. The thrust seemed to fall off slowly and velocity increased to 6,630 feet per second, or Mach 6.7.
After shutdown, Pete began doing control pulses in the yaw and pitch axes to obtain stability and control data. As he decelerated through Mach 5.5, Pete noticed a peroxide hot light. This light was one that really got a person’s attention. Hot peroxide was like hot nitroglycerine, something one would rather not encounter. Unknown to Pete and those in the control room, the intense heat from the shock waves off the dummy scramjet was burning the ablator off and melting huge holes in the skin of the ventral.
Even if Pete had known what was happening as a result of the shock wave heating, there was nothing he could do to prevent the damage once he exceeded Mach 6. The burn through of the ventral was inevitable from that point on. He may have been able to minimize the damage by deploying speedbrakes to slow down quicker, but he could not stop the cutting torch action of the air impacting the ventral at hypersonic speeds.
The temperatures were later calculated to be over 2,700° F in this region. The flow of hot air into the ventral through the holes caused the peroxide to get hot, burned the instrumentation wiring, burned through a control gas line, and cooked off some of the explosive bolts that were used to jettison the dummy scramjet from the airplane. The burnthrough of the control gas line resulted in the loss of the tank pressurizing gas, which prevented jettison of the residual fuel.
The peroxide hot light in the cockpit distracted Pete from his planned maneuvers and his energy management. NASA-1 finally instructed him to jettison his peroxide and began vectoring him toward high key at Edwards. Pete was high on energy coming into high key due to being distracted and he came across the north edge of the lake at over 55,000 feet at Mach 2.2.
He attempted to jettison the remaining propellants during his turn to low key, but nothing came out. This lack of jettison prevented the chase from spotting him since the chase relied heavily on jettison to acquire the X-15. Jettisoned propellants created a bright vapor trail similar to a contrail which was easily seen. Pete was not going to get any help from the chase until he rolled out on final. Pete had his work cut out for him.
He was high on energy, unable to jettison propellants, and unsure of the condition of the aircraft. He finally got the airplane turned around over the south end of the lakebed but was still supersonic at 40,000 feet altitude. As he rolled out heading northbound, the dummy scramjet tore off the airplane and tumbled on down to impact on the bombing range without a recovery chute. The chute and deployment system had been damaged by the extreme heating and the chute failed to deploy. As Pete proceeded up the east shore of the lakebed for a landing on runway one-eight, he sensed that he was losing altitude at a greater than normal rate. NASA-1 noted this also and suggested that Pete consider landing on runway two-three instead of one-eight.
Pete called for an altitude check from the chase to help him decide which runway to try for, but the chase had still not acquired him. Pete decided to press on for runway one-eight. Pete made it around the turn to final on runway one-eight and, as he rolled out, the chase finally
saw him. The landing was a good one and the slideout was normal. The higher rate of descent was later confirmed and attributed to the extra drag from the charred and roughened ablative coating.
The dummy scramjet was later located and recovered as a result of some excellent investigative effort on the part of Johnny Armstrong, one of our flight planning engineers. No one was certain when the scramjet tore off the aircraft. Armstrong examined the flight records in minute detail looking for some clue. He noted a sudden increase in longitudinal acceleration at one point in the flight, and he suspected that the increase in acceleration might be due to the decrease in drag resulting from separation of the dummy scramjet. He correlated the time of the sudden increase in acceleration with the time on the radar map of the aircraft’s ground track and defined a location at which he suspected the scramjet separated from the aircraft. He then estimated the free-fall trajectory of the scramjet and established a potential ground impact point which happened to be located in the Edwards bombing range. He drove out to his predicted impact point, stopped the van that he was driving, began walking, and within several minutes found the scramjet within 300 feet of his predicted impact point.
Examination of the airplane after the flight revealed extensive damage to the ventral and the lower fuselage structure. The tough nickel-steel skin was melted through like butter, creating several large holes in the ventral. The recovered dummy scramjet showed similar heating damage. Everything inside the ventral was burned, including the hydraulic lines that connected to the speed brake actuators. These lines had not burned through. If they had, we would have lost the aircraft, and possibly Pete. The damage to the aircraft was a sobering sight.
The airplane was returned to North American Aviation for repairs. A month later, Mike Adams was killed in the number three aircraft. These two events—the near loss of the number two aircraft and the loss of the number three aircraft—precipitated a reassessment of the overall X-15 program. The number two aircraft never flew again. It was repaired and then sent to the USAF museum at Wright-Patterson AFB where it is proudly displayed at this time.
It is always more dramatic to discuss the problems, malfunctions, or near disasters when telling airplane stories. The flights highlighted in the X-15A-2 program are those types of flights wherein problems occurred or there were significant elements of risk. There were many routine flights and there were some textbook flights. Even on the last X-15A-2 flight, there were many positive aspects of the flight. Pete flew a perfect flight until he was distracted by the peroxide-hot light. Even with that distraction, he brought the airplane home and made a beautiful landing.
Most of the modifications had performed as they were designed to perform. The eyelid worked well to protect the windshield. The external tank system worked well—fuel and LOX transferred into the main tanks as intended, while maintaining an allowable center of gravity position. The tanks ejected and separated cleanly, and the parachute recovery systems functioned properly with only minor anomalies. The aircraft was completely stable and controllable with the huge external tanks and the dummy scramjet installed, and the ablative coating worked well to protect the aircraft in all areas except the lower ventral area. We burned through the ventral in several areas, but we later defined the problem and developed a fix. The aircraft was quickly repaired and modified and could have been ready to fly within a couple of months. After these last modifications, the aircraft could have successfully carried the scramjet to speeds in excess of Mach 7.
The X-15A-2 development program was a success. There were a few moments of stark terror, as the saying goes, but this is to be expected in any flight test program.
Chapter 9
Sometimes the Bull Wins
Mike Adams started his X-15 career with a bang. The bang was the noise made by the failure of the forward bulkhead of the ammonia fuel tank. As a result of that bang, Mike had to make an emergency landing on his first flight. As I have said earlier, the last thing any pilot needed on his first X-15 flight was an emergency. The mental stress level was high enough on a normal first flight. Mike handled his emergency very well.
Mike was scheduled to make his first flight out of Hidden Hills. The flight plan called for a pull-up after launch at 50 percent throttle to a climb angle of 20 degrees. At 80 seconds after launch, he was to push over to 0 g and maintain 0 g until 104 seconds, at which time he was to increase angle of attack to hold level flight at 74,000 feet altitude. He was to maintain altitude until reaching 4,000 feet per second at 129 seconds after launch, at which time he was to shut the engine down. Following engine shutdown, Mike was to perform a series of mild maneuvers to familiarize himself with the handling qualities of the airplane and then vector to high key for a landing at Edwards.
As indicated above, the flight did not go according to plan. The engine shut down 90 seconds after launch as Mike was extending the speed brakes to decrease the rate of acceleration. The engine shutdown occurred as a result of the forward bulkhead failure in the fuel tank coupled with a momentary pushover to less than 0 g. In the postflight debriefing, Mike said that, “Immediately after the engine shutdown, I saw two little lights below the igniter-ready light come on and say something bad.”
Mike immediately attempted an engine relight, but with no luck. The engine failed to light. Pete Knight, who was the controller, asked, “Did you get a shutdown, Mike?” and Mike replied, “Rog.” Then Pete said, “OK, you’re going to Cuddeback.” Mike was subsequently heard over the radio talking to the airplane saying, “Now what did you go and do that for?” NASA-1 said, “Say again, Mike.” Mike responded with the comment, “Wonder why I picked a shutdown.”
As Mike approached Cuddeback, he made the comment that, “It looks like I could make Edwards.” Mike, like all the other X-15 pilots, preferred landing at “home.” Pete would have none of that wishful thinking. He replied, “Let’s make it Cuddeback.” Mike arrived at high key at Cuddeback with ample energy and commented to Pete at NASA-1 that, “This thing is sort of fun to fly.” Pete said, “Say again,” and Mike said, “This thing is fun to fly even if I have to go to Cuddeback.” Pete was not in the mood for frivolity in the middle of an emergency and said, “Well, let’s talk about it after you get it on the ground.”
Pete’s reaction was quite typical of the reaction of most pilots who are serving as controller or flight director during a flight. They are generally under more stress than the pilot in the airplane because of a sense of responsibility for the safety of the pilot in the airplane. We verified this by monitoring the heart rate of each pilot when he acted as controller and when he flew the airplane. The pilot’s heart rate was higher when serving as a controller than when he was actually flying the airplane. Pete was so concerned about Mike that he was attempting to give Mike a GCA (ground control approach) around the pattern at Cuddeback using radar information.
This practice of vectoring the X-15 around the approach using radar was discouraged after my emergency landing at Cuddeback. In this instance, Pete forgot the ground rules and talked him all the way around the pattern, preventing the chase from getting a word in edgewise. Mike commented after the flight that “After the restart attempt, I had all the help I needed there and WE brought it into Cuddeback.”
After returning to Edwards and being debriefed, Mike had scheduled a T-38 flight in the afternoon. Even this routine flight would not go smoothly for Mike on this day. For the second time in one day, Mike got an inadvertent engine shutdown. Fortunately, the T-38 has two engines. Nevertheless, Mike now had to make another emergency landing, this time at Edwards. I never checked, but I wonder if Mike’s car made it home that evening without quitting.
Mike’s second flight was made in the number three X-15. Normally a new pilot was not scheduled to fly the number three X-15 until he had made at least two flights in the standard aircraft. The control system in the number three aircraft was more complex and unconventional. It had some good and some bad characteristics. The bad ones could sneak up on you if you
were not forewarned. The flight planners apparently felt that Mike could handle the number three aircraft after his successful emergency landing on his first flight.
This flight was planned as another pilot checkout flight from Hidden Hills. A tip pod accelerometer experiment was also carried on this flight. This plan called for a climbout after engine light at 75 percent throttle to a climb angle of 30 degrees. Engine thrust was increased incrementally on a pilot’s first three flights from 50 to 75 to 100 percent to allow the pilot to adjust to the higher acceleration levels. One hundred percent thrust was impressive, to say the least. I believe a pilot would have been overwhelmed by the g forces and the events if he had to make his first flight at 100 percent. I was thoroughly impressed by 50 percent thrust on my first flight.
Mike was to climb at 30-degrees flight path angle to 58,000 feet altitude and then push over to 0 g and hold that until reaching 77,000 feet at 80 seconds after launch. Then he was to increase angle of attack to maintain level flight until he reached 4,500 feet per second velocity, at which time he was to shut the engine down. Following shutdown, he was to perform a number of roll and yaw maneuvers to familiarize himself with the aircraft’s flying characteristics and vector to high key for landing at Edwards.
On this flight, everything proceeded as planned. The only problem Mike had was with his radio just prior to launch. He had to rely on the B-52 to relay calls from NASA-1 to verify that everything was okay for launch. After launch he did not hear NASA-1 until passing Cuddeback Lake while vectoring to high-key. During the approach, just prior to landing, Mike ballooned dangerously but managed to recover and got the airplane on the lakebed before he lost all his airspeed.
At the Edge of Space Page 32
