RETROSPECT

After the first 50 flights with the XLR99 engine, researchers at the FRC took a step back and reflected on the problems they had experienced. Excepting the single incident on the ground that gave Scott Crossfield his wild ride at the Rocket Engine Test Facility, the engine had proved to be remarkably safe during operation. Although there had been a multitude of problems, large and small, the program described itself as "engine safe."1981

One of the major factors in successful engine operation in the X-15 after launch was the amount of checkout the engine went through on the ground beforehand. This had its drawbacks, however, since "operating cycles on the hardware for ground assurance checks take a relatively large portion of the hardware life," according to C. Wayne Ottinger and James F. Maher. Illustrating this is the fact that 350 ground runs, including 100 with the XLR99 installed in the X-15, had been necessary to achieve the first 50 flights. For the first dozen flights, the FRC conducted a test of the engine installed in the X-15 before each mission. After the 12th flight, a flight attempt could follow a successful flight without a test firing-a process that saved 18 ground runs during the next 38 missions.1991

Between the conclusion of the PFRT and May 1963, 90 modifications were made to the engine configuration. In order to meet the safety criteria imposed by the Air Force, Reaction Motors used the "single-malfunction" concept, i. e., it designed the engine so that no single malfunction would result in a hazardous condition. The company used a dual-malfunction concept with regard to structural failure, meaning that if one member failed, another would carry its load. The PFRT series of tests convincingly demonstrated these capabilities, since 47 different malfunctions resulted in a safe shutdown.11001

Despite all of the effort that went into developing a restartable engine, this capability was not used during the first 50 flights, except for four flights on which it was used to start an engine that had failed on the first attempt. However, another feature proved to be a welcome addition: the ability to operate the pump and both igniter stages while the research airplane was attached

to the carrier aircraft. This allowed verification of over 90% of the moving components in the engine before the research airplane was dropped.-1401

When the engines first arrived at Edwards, several components (particularly leaking pumps and malfunctioning hydrogen-peroxide metering valves) accounted for an abnormally high percentage of the flight delays. Relaxing the operating requirements regarding certain pump leaks and limiting the duration of the pump run time did as much to reduce pump delays as did the ultimate fixes themselves. NASA also noted that "excessive time lag in obtaining approval for correction" and "excessive time required to develop the correction and complete flight hardware incorporation of fixes after approval" were significant contributors to the delays caused by the XLR99.[102]

The control box was the heart of the engine and was responsible for the control and sequencing of the engine. This was not a computer by the modern definition of the term, but rather a mechanical sequencer with some electronic components. The major problem experienced by this device during the first 50 flights was the failure of pressure switches due to ammonia corrosion of the silver contacts-echoes of the original warnings on the effects of ammonia exposure. Reaction Motors finally eliminated this problem by switching to gold contacts. In addition, there were random wiring discrepancies, servo amplifier failures, and timer failures.-103

During the latter part of 1962, several in-flight oxidizer depletion shutdowns resulted in second – stage igniter damage because reduced liquid-oxygen injector pressure allowed the reverse flow of ammonia into the oxidizer inlet. The subsequent minor explosion either bulged the igniter inlet manifold or blew the face off the second-stage igniter. Reaction Motors installed an auxiliary purge system to correct the problem. In addition, several sensing-line detonations had defied correction throughout the summer of 1963. These occurred in the second-stage chamber sense line during any thrust decrease when unburned combustible gas from the previous increasing pressure cycle entered the sense line. Interestingly, engineers initially attributed this problem to a lubricant used in the main propellant valve. They believed that the "liquid-oxygen safe" lubricant was impact-sensitive and responsible for the second-stage igniter explosions. Although further investigation later proved this theory incorrect, analysis of the lubricant revealed that some batches were out of specification on impact sensitivity.-1104!

The hydrogen-peroxide system that powered the turbopump experienced several problems, including erratic metering valve operation, catalyst-bed deterioration, seal failures, and corrosion. Engineers corrected the metering valve problem by increasing the clearance around the valve. The substitution of electrolytically produced hydrogen peroxide for organically produced product solved the catalyst-bed deterioration, although it technically violated the engine qualification since the PFRT had been run with electrolytically produced hydrogen peroxide. The development of improved gaskets and seals relieved the seal failures and solved most of the corrosion problems. The turbopump itself suffered only minor problems, mainly steam and propellant leaks. The lowering of specifications governing the allowable leakage rate provided the most progress in working with the problem.-105

The oxidizer system also created some headaches, even though it was largely a copy of the original XLR30 system. The major problems were propellant valve leakage and the need for a quick-change orifice. Improved lip and shaft seals initially helped control the leakage, and eventually Reaction Motors introduced a redesigned valve that eliminated the problem. Prior to the incorporation of the quick-change orifice, it was necessary to remove the engine from the aircraft in order to change the oxidizer-to-fuel ratio. Engineers changed the ratio based on the proposed altitude for the next flight to maximize the performance of the engine. Once Reaction Motors incorporated the quick-change modification, engineers at Edwards could insert different-sized probes into the orifice while the engine was in the aircraft. This eliminated the need to conduct a ground run after reinstalling the engine. Tailoring the oxidizer-to-fuel ratio actually allowed the engine to produce slightly over 61,000 lbf at some altitudes.-105

Although nearly everybody considered the XLR99 a good research airplane engine, the engine was far from perfect. Milt Thompson observed that "the LR99 was amazingly reliable if we got it lit, and if we did not move the throttle while it was running." Joe Vensel, the director of FRC flight operations echoed the advice: "[I]f you get the engine lit, leave it alone, don’t screw with it." This is perhaps overstating the case, but not by much. During the early part of the flight program, the XLR99 had a remarkably poor record of starting when the pilot wanted. Part of the problem was that the early flight rules said to start the engine at minimum throttle (50% for the very early engines, and 30% for the later ones). The engine simply did not like to start at those throttle settings. After the program decided to start the engine at 100% throttle, things got much better.107-

Still, even after the engine lit, it did not particularly like to throttle. As a result, Joe Vensel directed the pilots not to throttle the engine until after the X-15 had sufficient energy to make it back to Edwards. Milt Thompson talked him into changing his mind for one flight (3-29-48) in order to accommodate a research request, and Thompson ended up on Cuddeback Lake when the engine quit as he throttled back 42 seconds after launch. After that, the restriction was rigorously enforced: no throttle movement until the airplane could glide back to Edwards. Although the lower throttle limit on later engines was 30%, the program decided not to go below 40% because of the persistent vibration problem. The pilots also learned to move the throttle slowly to minimize the chances of the engine quitting. It mostly worked, and flight planner Bob Hoey does not remember any significant problems occurring later in the program.-1108!

During the flight program, eight in-flight propulsion problems resulted in emergency landings. These included one due to no ignition, one because the engine hung at 35% thrust, one shutdown when the throttle was retarded, two due to low fuel-line pressures, one turbopump-case failure, one ruptured fuel tank, and one due to a perceived lack of fuel flow from the external tanks on X – 15A-2. Overall, it was not a bad record for a state-of-the-art engine over the course of 199 flights.

Although 11 flight engines were manufactured, only eight were available to the flight program. One (s/n 105) was lost in the ground explosion that seriously damaged the X-15-3 before the XLR99 had even flown, and two other flight engines were dedicated to the ground-test program. Making 199 flights on eight engines was an outstanding achievement.

XLR99 Flight Engine Run Time Summary (Minutes per Year) Year s/n 103 s/n 104 s/n 106 s/n 107 s/n 108 s/n 109 s/n 110 s/n 111 No. of flights Pre Del 13.47 31.23 7.90 8.63 6.29 4.64 4.45 4.43 – 1960 11.42 5.88 0 0 0 0 0 0 3 1961 16.66 0 12.05 4.78 13.34 5.98 1.53 5.75 13 1962 8.72 6.13 7.02 18.32 5.77 9.45 11.75 11.87 30 1963 1.43 8.52 0 16.27 5.58 2.55 (9.10)* 11.22 6.32 21 1964 12.03 11.05 6.08 6.52 7.68 6.58 0 (6.33)* 3.24 (20.03)* 27 1965 12.03 7.86 3.26 14.22 15.10 7.73 8.40 5.93 32 1966 2.72 0 15.07 9.98 0.52 2.37 8.85 4.65 20 1967 11.45 3.98 1.23 2.63 5.50 2.72 4.72 2.30 15 1968 3.80 3.60 2.60 0.70 3.63 3.25 1.22 Lost+ 8 Total 73.73 78.25 55.21 82.05 63.41 45.77 (54.87)* 52.14 (58.49)* 44.49 (64.52) 169 *Additional time used for ground testing of second-stage igniter purge modification.

+ Lost in X-15-3.

Data courtesy of Robert G. Hoey.

As was done for most components on the X-15, all XLR99 maintenance was performed at Edwards using a local, depot-level maintenance approach. With few exceptions, the engines ran for a brief period in the PSTS before NASA installed them in one of the X-15s or stored them for future use. Since the X-15 maintenance philosophy was to provide sufficient spare engines and maintenance personnel to ensure 100% flight engine availability, it was normal to have a backlog of engines in flight-ready storage (essentially spares). The engine activity was divided into three categories: 1) installed in an X-15, 2) active maintenance, and 3) flight-ready storage. Early in the program, NASA conducted one or more ground engine runs (leak checks) after installing the engine in the airplane and before every flight. This requirement for an aircraft engine run between flights was relaxed later in the program, assuming there were no engine problems on the previous flight.1102*

Milton O. Thompson had more than his fair share of experience with the XLR99, and enjoyed sharing it during discussions with various groups after the X-15 program ended. One of his favorite stories concerned the emergency landing he had to make on Flight 3-29-48 when the XLR99 quit as he throttled back 42 seconds after launch. (NASA)

The staff of the AFFTC Rocket Engine Maintenance Shop from 1961 to 1968 in support of the XLR99 averaged about 37 people. Interestingly, in 1965 these technicians made about $4 per hour on average. This shop was responsible for all maintenance of all uninstalled XLR99s; the FRC handled minor repairs of installed engines. Every 30 operating minutes, on a test stand or in the airplane, each XLR99 had to undergo a "30-minute" inspection that took just over two weeks to complete. The Air Force overhauled the engines when needed, a process that took just over a month. Recoating the thrust chamber, done by the FRC, took a few days.-1110

Unlike many rocket engines of that era, the XLR99 was equipped with a malfunction-detection and automatic-shutdown system. For most engines, reliability is based on the number of start attempts. However, since one of the primary features of the XLR99 was its ability to restart in flight, its total reliability was defined as the number of successful engine operations per flight attempt, regardless of the number of start attempts. The resulting X-15 data and point estimates of reliability were as follows:[111

XLR99/X-15 flight attempts^112 169 Successful engine operations 165 Successful first-start attempts 159 Overall reliability 97.6%

First-start reliability 94.0%

Over the course of the X-15 program, the flight engines accrued a total of 550.53 minutes of run time, plus an undetermined amount on ground-test engines. A total of 1,016 engine starts were recorded for the flyable engines (dedicated ground-test engines incurred many more). Although there were numerous automatic shutdowns, there were no catastrophic engine failures. The safety of the XLR-99 engine (defined as the probability of non-catastrophic engine operation) may be conservatively estimated by dividing the number of successful starts (1,016) by the number of starts plus one (1,017) (assuming the next start to be catastrophic for the worst case). The resulting estimate of the probability of non-catastrophic engine operation is approximately

0.99902ДШ

In retrospect, the engine still casts a favorable impression. The XLR99 pushed the state of the art further than any engine of its era, yet there were no catastrophic engine failures in flight or on the ground. There were, however, many minor design and manufacturing deficiencies, particularly with the Rokide coating on the thrust chamber. Surprisingly, the primary source of problems on most large rocket engines-the turbopump-proved to be remarkably robust and trouble free.