The Engine

Those concerned with the success of the X-15 had to monitor the development of the aircraft itself, the XLR99 rocket engine, the auxiliary power units, an inertial system, a tracking range, a pressure suit, and an ejection seat. They had to make arrangements for support and B-36 carrier aircraft, ground equipment, the selection of pilots, and the development of simulators for pilot training. It was necessary to secure time on centrifuges, in wind tunnels, and on sled tracks. The ball-nose had to be developed, studies made of the compatibility of the X-15 and the carrier aircraft, and other studies on the possibility of extending the X-15 program beyond the goals originally contemplated. In addition to such tasks, funds to cover ever increasing costs had to be secured if the project were to have any chance of ultimate success, and at certain stages the effects of possibly harmful publicity had to be considered. With such multiplicity of tasks, it could be expected that several serious prob­lems would arise; not surprisingly, probably the most serious arose during the develop­ment of the XLR99.

Finding a suitable engine for the X-15 had been somewhat problematic from the earliest stages of the project, when the WADC Power Plant Laboratory had pointed out that the lack of an acceptable rocket engine was the major shortcoming of the NACA’s original propos­al. The laboratory did not believe that any available engine was entirely suitable for the X-15 and held that no matter what engine was accepted, a considerable amount of development work could be anticipated. Most of the possible engines were either too small or would need too long a development peri­od. In spite of these reservations, the labora­tory listed a number of engines worth consid­ering and drew up a statement of the require­ments for an engine that would be suitable for the proposed X-15 design. The laboratory also made clear its stand that the government should “… accept responsibility for develop­ment of the selected engine and… provide this engine to the airplane contractor as Government Furnished Equipment.”2′

The primary requirement for an X-15 engine, as outlined in 1954, was that it be capable of operating safely under all condi­tions. Service life would not have to be as long as for a production engine, but engi­neers hoped that the selected engine would not depart too far from production standards. The same attitude was taken toward reliabil­ity; the engine need not be as reliable as a production article, but it should approach such reliability as nearly as possible. There could be no altitude limitations for starting

or operating the engine, and the power plant would have to be entirely safe during start, operation, and shutdown, no matter what the altitude. The laboratory made it quite clear that a variable thrust engine capable of repeated restarts was essential.

The engine ultimately selected was not one of the four originally presented as possibil­ities by the Power Plant Laboratory. The ultimate selection was foreshadowed, how­ever, in discussions with Reaction Motors concerning the XLR10, during which atten­tion was drawn to what was termed "… a larger version of [the] Viking engine [XLR30].” In light of subsequent events, it was interesting to note that the laboratory thought26 the XLR30 could be developed into a suitable X-15 engine for less than $5,000,000 …” and with “ … approximate­ly two years’ work.”27

After North American had been selected as the winner of the X-15 competition, plans were instituted to procure the modified XLR30 engine that had been incorporated in the winning design. Late in October, Reaction Motors was notified that North American had won the X-15 competition and
that the winner had based his proposals upon the XLR30 engine.28

On 1 December 1955 a $1,000,000 letter con­tract was initiated with Reaction Motors for the development of a rocket engine for the X-15.-"’ Soon afterwards, a controversy devel­oped over the assignment of cognizance for the development of the engine. It began with a letter from Rear Adm. W. A. Schoech of the Bureau of Aeronautics. Adm. Schoech con­tended that since the XLR30-RM-2 rocket engine was the basis for the X-15 power plant, and the BuAer had already devoted three years to the development of that engine, it would be logical to assign the responsibili­ty for further development to the Navy. The admiral felt that retention of the program by the BuAer would expedite development, especially as the Navy could direct the devel­opment toward an X-15 engine by making specification changes rather than by negotiat­ing a new contract.30

The Navy’s bid for control of the engine development was rejected on 3 January 1956 on the grounds that the management respon­sibility should be vested in a single agency, that conflict of interest might generate delay,

The EngineThe XLR99 was an extremely compact engine, considering it was able to produce over 57,000 pounds – thrust. This was the first throttleable and restartable man-rated rocket engine. Many of the lessons-learned from this engine were incorporated into the Space Shuttle Main Engine developed 20 years later. (NASA)

and that BuAer was underestimating the time and effort necessary to make the XLR30 a satisfactory engine for piloted flight.

The Final Reaction Motors technical propos­al was received by the Power Plant Laboratory on 24 January, with the cost pro­posal following on 8 February.31 The cover letter from Reaction Motors promised deliv­ery of the First complete system “… within thirty (30) months after we are authorized to proceed.”32 Reaction Motors also estimated that the entire cost of the program would total $10,480,718.33 On 21 February the new engine was designated XLR99-RM-1.34

The 1956 industry conference heard two papers on the proposed engine and propul­sion system for the X-15. The XLR99-RM-1 would be able to vary its thrust from 19,200 to 57,200 pounds at 40,000 feet using anhy­drous ammonia and liquid oxygen (LOX)35 as propellants. Specific impulse was to vary from a minimum of 256 seconds to a maxi­mum of 276 seconds. The engine was to Fit into a space 71.7 inches long and 43.2 inch­es in diameter, have a dry weight of 618 pounds, and a wet weight of 748 pounds. A single thrust chamber was supplied by a
hydrogen-peroxide-driven turbopump, with the turbopump’s exhaust being recovered in the thrust chamber. Thrust control was by regulation of the turbopump speed.36

The use of ammonia as a propellant present­ed some potential problems; in addition to being toxic in high concentrations, ammonia is also corrosive to all copper-based metals. There were discussions early in the program between the Air Force, Reaction Motors, and the Lewis Research Center37 about the possi­bility of switching to a hydrocarbon fuel. It was finally concluded that changing fuel would add six months to the development schedule; it would be easier to learn to live with the ammonia.38 There is no documenta­tion that the ammonia ultimately presented any significant problems to the program.

The decision to control thrust by regulating the speed of the turbopump was made because the other possibilities (regulation by measurement of the pressure in the thrust chamber or of the pressure of the discharge) would cause the turbopump to speed up as pressure dropped. As the most likely cause of pressure drop would be cavitation in the pro­pellant system, an increase in turbopump



This 1956 sketch shows the controls and indicators for the XLR99. A different set of controls were used for the XLR11 flights, although they fit into the same space allo­cation. Notice the sim­ple throttle on the left console, underneath the reaction control side-stick (not shown).

The jettison controls took on particular sig­nificance on missions that had to be aborted prior to engine burn­out. (NASA)






















The Engine

speed would aggravate rather than correct the situation. Reaction Motors had also decided that varying the injection area was too complicated a method for attaining a variable thrust engine and had chosen to vary the injection pressure instead.

The regenerative cooling of the thrust cham­ber created another problem since the vari­able fuel flow of a throttleable engine meant that the system’s cooling capacity would also vary and that adequate cooling throughout the engine’s operating range would produce excessive cooling under some conditions. Engine compartment temperatures also had to be given more consideration than in previ­ous rocket engine designs because of the higher radiant heat transfer from the struc­ture of the X-15. Reaction Motors’ spokesman at the 1956 industry conference concluded that the development of the XLR99 was going to be a difficult task. Subsequent events were certainly to prove the validity of that prediction.

A second paper dealt with engine and acces­sory installation, the location of the propel­lant system components, and the engine con­trols and instruments. The main propellant tanks were to contain the LOX, ammonia, and the hydrogen peroxide. The LOX tank,
with a capacity of approximately 1,000 gal­lons, was located just ahead of the aircraft’s center of gravity; the 1,400 gallon ammonia tank was just aft of the same point. A center core tube within the LOX tank would pro­vide a location for a supply of helium under a pressure of 3,600 psi. Helium was used to pressurize both the LOX and ammonia tanks. A 75-gallon hydrogen peroxide tank behind the ammonia tank provided the monopropel­lant for the turbopump.

Provision was also made to top-off the LOX tank from a supply carried aboard the carrier aircraft; this was considered to be beneficial in two ways. The LOX supply in the carrier aircraft could be kept cooler than the oxygen already aboard the X-15, and the added LOX would permit cooling of the X-15’s own sup­ply by boil-off, without reduction of the quantity available for flight. The ammonia tank was not to be provided with a top-off arrangement, as the slight increase in fuel temperature during flight was not considered significant enough to justify the complica­tions such a system would have entailed.

On 10 July 1957, Reaction Motors advised the Air Force that an engine satisfying the contract specifications could not be devel­oped unless the government agreed to a nine-

The Engine

The XLR99 on a main­tenance stand. The engine used ammonia (NH3) as fuel and liq­uid oxygen (LOX) as the oxidizer. The XLR99 required a sep­arate propellant, hydro­gen peroxide, to drive its high-speed turbop­ump—the Space Shuttle Main Engine uses the propellant itself (LH2 or L02, as appropriate) to drive the turbopumps. (AFFTC via the Tony Landis Collection)

month schedule extension and a cost increase from $15,000,000 to $21,800,000. At the same time, Reaction Motors indicated that it could provide an engine that met the per­formance specification within the established schedule if permitted to increase the weight from 618 pounds to 836 pounds. The compa­ny estimated that this overweight engine could be provided for $17,100,000. The Air Force elected to pursue the heavier engine since it would be available sooner and have less impact on the overall X-l5 program.

Those who hoped that the overall perform­ance of the X-l5 would be maintained were encouraged by a report that the turbopump was more efficient than anticipated and would allow a 197 pound reduction in the amount of hydrogen peroxide necessary for its operation. This decrease, a lighter than expected airframe, and the increase in launch speeds and altitudes provided by a recent substitution of a B-52 as the carrier aircraft, offered some hope that the original X-l5 per­formance goals might still be achieved.39

Despite the relaxation of the weight require­ments, the engine program failed to proceed at a satisfactory pace. On 11 December 1957 Reaction Motors reported a new six-month slip. The threat to the entire X-l5 program posed by these new delays was a matter of serious concern, and on 7 January 1958, Reaction Motors was asked to furnish a detailed schedule and to propose means for solving the difficulties. The new schedule, which reached WADC in mid-January, indi­cated that the program would be delayed another five and one-half months and that costs would rise to $34,400,000—double the cost estimate of the previous July.10

In reaction, the Air Force recommended increasing the resources available to Reaction Motors and proposed the use of two 11 XLR11 rocket engines as an interim installation for the initial X-l5 flights. Additional funds to cover the increased effort were also approved, as was the estab­lishment of an advisory group.42

The threat that engine delays would serious­ly impair the value of the X-l5 program had generated a whole series of actions during the first half of 1958: personal visits by gen­eral officers to Reaction Motors, numerous conferences between the contractor and representatives of government agencies, increased support from the Propulsion Laboratory43 and the NACA, an increase in funds, and letters containing severe censure of the company’s conduct of the program. An emergency situation had been encountered, emergency remedies were used, and by mid­summer improvements began to be noted.

Engine progress continued to be reasonably satisfactory during the remainder of 1958. A destructive failure that occurred on 24 October was traced to components that had already been recognized as inadequate and were in the process of being redesigned. The failure, therefore, was not considered of major importance.41 A long-sought goal was finally reached on 18 April 1959 with completion of the Preliminary Flight Rating Test (PFRT). The flight rating program began at once.13

At the end of April, a “realistic” schedule for the remainder of the program showed that the Flight Rating Test would be completed by 1 September 1959. The first ground test engine was delivered to Edwards AFB at the end of May, and the first flight engine was delivered at the end of July.4*

A total of 10 flight engines were initially procured, along with six spare injector – chamber assemblies; one additional flight engine was subsequently procured. In January 1961, shortly after the first XLR99 test flight, only eight of these engines were available to the flight test program. There was still a number of problems with the engines that Reaction Motors was continuing to work on; the most serious being a vibra­tion at certain power levels, and a shorter than expected chamber life. There were four engines being used for continued ground tests, including two flight engines.47 Three of the engines were involved in tests to isolate

and eliminate the vibrations, while the fourth engine was being used to investigate extend­ing the life of the chamber.48

It is interesting to note that early in the pro­posal stage, North American determined that aerodynamic drag was not as important a design factor as was normally the case with jet-powered fighters. This was largely due to the amount of excess thrust available from the XLR99. Weight was considered a larger driver in the overall airplane design. Only about 10 percent of the total engine thrust was necessary to overcome drag, and anoth­er 20 percent to overcome weight. The remaining 70 percent of engine thrust was available to accelerate the X-l5.44