The XLR99 Arrives

The first ground-test XLR99 (s/n 101) arrived at Edwards on 7 June 1959, and the first hot test was accomplished without an actual X-15 at the Rocket Engine Test Facility on 26 August 1959. X-15-3 arrived at Edwards on 29 June 1959. It was essentially identical to the other two airplanes in that it was equipped with a standard Westinghouse stability augmentation system, a stable platform, and a normal cockpit instrument panel. What made it different, at this point, was that it had the XLR99 engine. X-15-3 was never equipped with the XLR11 engines. At the same time, North American removed the second X-15 from flight status after its ninth flight (2-9-18) on 26 April 1960 in anticipation of replacing the XLR11 engines with the XLR99. This left only X-15-1 on active flight status, although the XLR99-powered X-15-3 would soon be joining it.[90]

North American made the first ground run with the XLR99 in X-15-3 on 2 June 1960 at the PSTS. Subsequent inspection revealed damage to the liquid-oxygen inlet line brackets, the result of an unexpected, but easily corrected, water-hammer effect. After repairs were completed, the company conducted another ground run with satisfactory results. For all of the ground runs during the program, a pilot had to be in the cockpit since the nearby blockhouse could not operate the engine by remote control. For the early tests, the pilot was Scott Crossfield, although all of the pilots would participate in ground runs prior to their first flights. The MC-2 full – pressure suit was an order of magnitude more comfortable than earlier pressure suits, but Crossfield still had little desire to wear it more than necessary. Since there was no need for altitude protection during the engine runs on the ground, Crossfield generally wore street clothes in the cockpit. All other personnel required for the tests were in the blockhouse, with the exception of Air Force fire crews a relatively safe distance away.-191

The third ground run began on 8 June at approximately 1930 hours. The objectives were to demonstrate the restart capability and throttling characteristics of the XLR99. All pre-test operations, servicing, and APU starts were successful and all systems were operating normally.

The engine was primed, set to idle, and then ignited at 50% thrust. After the chamber pressure stabilized for 7 seconds, Crossfield advanced the throttle to 100% for 5 seconds and then moved the throttle to idle for 5 seconds before shutting down the engine. Nobody noted anything abnormal during these events. After 15 seconds, Crossfield moved the throttle to the 50% position. The turbopump started normally, first-and second-stage ignition occurred, and the main chamber start appeared normal. After the main chamber pressure stabilized, it rapidly fell off and the engine shut down automatically. At this time, a valve malfunction light came on in the cockpit, so Crossfield moved the throttle to the off position and the light went out. In order to

restart the XLR99 after a malfunction shutdown, the pilot had to push a switch that reset the automatic safety devices. As Crossfield wrote in his accident statement, "the reset button was depressed at which time the airplane blew up." It was approximately 1945 hours.-192

Crossfield later observed, "During this entire sequence except for the malfunction shut down, there was no evidence in the cockpit of difficulty." The explosion appeared to be centered forward of the engine compartment, and caused the aircraft to separate around fuselage station 483.5, just forward of the liquid-oxygen tank. Don Richter, who was in the main blockhouse, indicated that he observed the explosion originating 5 feet forward from the aft end of the airplane, with the fireball quickly expanding to about 30 feet in diameter.-192

The explosion threw the entire forward fuselage about 30 feet forward. Crossfield said, "In the explosion, which is not describable, the cockpit translated abruptly forward and to the right with an acceleration beyond the experience of this pilot." The basic X-15 airframe had been designed – largely at Crossfield’s urging-to protect the pilot in case of an emergency; it appeared to work well. Ever the competent test pilot, Crossfield turned on his Scott airpack, turned off the engine switches, and pulled all the circuit breakers. He attempted to contact personnel inside the blockhouse, but the explosion had severed communications with the ground.-194

The fire truck that had been standing by was on the scene within 30 seconds, water pouring from its overhead nozzle, and a second fire truck arrived a minute or two later to help extinguish the fires. Art Simone and a suited fireman rushed to the cockpit and Crossfield was rescued uninjured. Simone had inhaled ammonia fumes and received minor burns to his hands, but suffered no lasting effects. The fires were largely out within a few minutes of the explosion, and Crossfield was safe. It was time to figure out what had happened.-192

Representatives from the Edwards provost marshal’s office, the North American industrial security office, and the Edwards air police arrived on the scene and roped off the area pending an investigation. Around 2110 hours, North American photographer Stan Brusto arrived to photograph the wreckage; after this was complete, the Air Force removed the data recorders from the aircraft for analysis. The air police withdrew after putting into place procedures to limit access to the area, leaving one fire truck on standby just in case. Personnel spent the next 24 hours finding all the bits and pieces blown from the aircraft, tagging them, and preparing to move the remains of the aircraft back to Inglewood. Major Arthur Murray from the X-15 project office authorized the move on 10 June.-196

Engineers removed the XLR99 from the wreckage on 13 June and took it to hangar 1870 at Edwards for inspection. North American transported the remains of X-15-3 by truck from Edwards on 15 June, parking overnight at the intersection of Sepulveda and San Bernardo Road before continuing on to Inglewood on 16 June.-192 By 4 August the company had assessed the damage and determined that the airframe would have to be replaced from fuselage station 331.9 aft. The dorsal and ventral stabilizers, all four speed brakes, both horizontal stabilizers, both main landing skids, and both propellant tanks would be replaced. The company considered the wings repairable, as were the APUs and stable platform. All of the miscellaneous equipment in the rear and center fuselage, along with most of the research instrumentation in the aft fuselage, also required replacement. Reaction Motors did not consider the XLR99 repairable, although the company salvaged some parts for future use. The Rocket Engine Test Facility required major repairs, but was back on line by the end of June.-192

Force had incorporated a vapor-disposal system into the PSTS to allow the ammonia fumes to be vented from the airplane during engine testing without endangering people. Essentially the disposal system consisted of a 90-foot pipe that connected the airplane ammonia vent to a water pond where the ammonia was diluted. At the time of the explosion, the ammonia tank pressurizing gas regulator froze or stuck in the open position while the vent valve was operating erratically or modulating only partially open. North American had considered this condition a potential failure on the airplane itself, and had addressed the problem during development. However, when combined with the back pressure created by the vapor-disposal system attached to the ammonia vent, the tank pressure surged high enough to rupture the tank. In the process, debris damaged the hydrogen-peroxide tank, and the mixing of the peroxide and ammonia caused an explosion.-199

Post-accident analysis indicated that there were no serious design flaws in either the XLR99 or the X-15. The Air Force determined that the cause of the accident was a failure of the pressure regulator, exacerbated by the unique configuration required for the ground test. Nevertheless, North American devised several modifications to preclude similar failures in the future. These included redesigning the pressurizing gas regulator to reduce maximum flow through an inoperable regulator, providing the regulator with additional closing forces in the event of freezing, relocating the regulator to minimize the chances of moisture accumulation and subsequent freezing, and redesigning the relief valve and its surrounding plumbing.-11"

Rebuilding the aircraft was not as straightforward as it sounded. Besides the estimated $4.75 million cost, there would be a considerable delay in obtaining suitable replacement parts. The X – 15s were not mass-produced items, and structural spares were nonexistent. The time required to repair the airplane meant it would miss most of the envelope expansion program and was, therefore, somewhat redundant.

There had been considerable interest in testing a new Minneapolis-Honeywell MH-96 adaptive flight control system in a high-speed vehicle prior to its use on the X-20 Dyna-Soar. Given the unfortunate event, the Air Force took this opportunity to modify X-15-3 to include the system. Complicating this was the fact the X-15 program was operating under a "reduced budget"-$8.6 million for FY61 instead of the $10.5 million that had been requested. However, the X-15 program still enjoyed considerable support within the Pentagon, and in early August, Air Force Headquarters authorized the ARDC to release $1 million from existing funds (i. e., the $8.6 million) to cover the procurement of long lead items needed for the repairs. The remaining $3.75 million, along with the restoration of the $1.9 million removed from the program earlier, was to follow "at a later date." In the interim, the Pentagon directed the X-15 program to "operate on a fiscal 1961 schedule compatible with… funds of $10.5 million plus an additional $4.75 million to cover the repair of the damaged aircraft." Although money never came easily for the X-15 program, it always came.-1"

The XLR99 Arrives

Scott Crossfield was at the controls of X-15-3 when it suffered a catastrophic explosion during a ground run of the XLR99. Fortunately, Cross field was not injured. Subsequent investigation showed there was nothing wrong with either the engine or airframe design and that the explosion had been caused by the failure of a minor component and the unique configuration required for ground testing. The X-15-3 was subsequently rebuilt to include the advanced MH-96 adaptive flight control system. (AFFTC History Office)

On 10 August, the Air Materiel Command requested that North American submit an estimate for the repair of X-15-3. Twelve days later the Air Materiel Command ordered the repair using the $1 million authorized by the Pentagon, and North American proceeded with the work. The company estimated that the aircraft could be completed in August 1961 and available for flight in October. The Pentagon came through at the end of March 1961, funding the X-15 program at $15.25 million-the original $10.5 million request plus the cost of rebuilding the damaged airplane.-1102!

The new money allowed the AFFTC to increase the propellants it had ordered because of 1) the high consumption of propellants required for component testing at the PSTS, 2) the high level of development testing of the APU and ballistic control system, and 3) the increased development testing of the XLR99 at the Rocket Engine Test Facility. A quick review shows the quantities and costs involved in this usually overlooked matter;!103!


Original FY61

Revised FY61

Alcohol (gal)

48,000 @ $0.51 = $24,480

60,000 @ $0.51 = $30,600

Ammonia (gal)

140,000 @ $0.28 = $39,200

256,000 @ $0.28 = $71,680

Peroxide (lbs)

261,000 @ $0.60 = $156,600

420,000 @ $0.60 = $252,000

Helium (sfc)

2,400,000 @ $0.02 = $48,000

5,400,000 @ $0.02 = $108,000

Nitrogen (tons)

1,500 @ $15.00 = $22,500

3,500 @ $15.00 = $52,500

At Inglewood, North American was installing the XLR99 in X-15-2 and incorporating several other changes at the same time. These included a revised vent system in the fuel tanks as an additional precaution against another explosion, revised ballistic control system components, and provisions to install the ball nose instead of the flight-test boom used so far in the program. The company had been looking to conduct the first flight in early September, but discovered corrosion in the engine hydrogen-peroxide tank. While North American was taking care of the corrosion, Reaction Motors tore down one of the ground-test engines to determine the condition of the individual components after 2 hours of engine operation. The inspection revealed no outstanding deficiencies or indications of excessive wear, clearing the way for the first X-15 flight with the million-horsepower engine.-1104

The installation of the ball nose presented its own challenges since it had no capability to determine airspeed. The possibility of a failure in the ball-nose steering mechanism also made it unsuitable as a total-pressure port to derive airspeed. The X-15 was designed with an alternate airspeed probe just forward of the cockpit, although two other locations-one well forward on the bottom centerline of the aircraft, and one somewhat aft near the centerline-had also been considered. Several early flights compared the data available from each location, while relying on the data provided by the airspeed sensors on the flight-test boom protruding from the extreme nose. The primary location exhibited some velocity-indication sensitivity at speeds over 345 mph and angles of attack over 4 degrees. At 8 degrees alpha the indicated airspeeds were generally about 25 mph low. The tests indicated that the data from all three locations were about the same, so the engineers decided to retain the original location. An interesting discovery was that the error was substantially less after the ball nose was installed, which led to a theory that the extended nose boom was contributing to the errors. Fortunately, the airspeed indications were consistent at the speeds and angles of attack encountered in the landing pattern, so researchers simply adjusted the instruments to compensate. After NASA installed the ball nose, engineers compared angle-of-attack data (based on the horizontal stabilizer position) with those from previous flights using the flight-test boom. The data were generally in good agreement, clearing the way for operational use of the ball nose.-105