THE TR-139

The TR-139 engine proposed by Reaction Motors was an extensively modified version of the Navy-developed XLR30-RM-2. Reaction Motors liked to call it a "turborocket" engine because it used turbopumps to supply its propellants, a relatively new concept. The XLR30 dated back to 1946 when Reaction Motors initiated the development of a 5,000-lbf engine to prove the then – new concepts of high-pressure combustion, spaghetti-tube construction, and turbine drive using main combustion propellants. By 1950, engineers believed these principles were sufficiently well established to initiate the development of a 50,000-lbf engine. The turbopump and its associated valves completed approximately 150 tests, and Reaction Motors considered it fully developed, with the exception of additional malfunction-detection and environmental tests that were required before a flight-approval test could be undertaken. The evaluation of a "breadboard" engine had demonstrated safe and smooth thrust-chamber starting, achieved 93-94% of the theoretical specific impulse, and shown satisfactory characteristics using film cooling.-126

The engine consisted of a single thrust chamber and a turbopump to supply the liquid oxygen and liquid anhydrous ammonia propellants from low-pressure tanks on the aircraft. These propellants had boiling points of -298°F and -28°F, respectively. That meant that after the propellants were loaded into the X-15 tanks, they would immediately begin to boil off at rates that were dependent upon the nature of the tank design and ambient conditions. In an uninsulated tank, liquid oxygen has a boil-off rate of approximately 10% per hour on a standard day. Even the crudest insulation significantly lowers this, and a well-insulated tank can experience less than 0.5% per hour of boil- off. Reaction Motors pointed out that insulating a tank usually required a great deal of volume, and that the airframe manufacturer would need to conduct a trade study to find the best compromise between volume and boil-off. Since the B-36 carrier aircraft had sufficient volume to carry additional liquid oxygen to top off the X-15, this was not a major issue. Anhydrous ammonia, on the other hand, has a relatively high boiling point and very low evaporation losses. Simply sealing the tank by closing the vent valve would minimize losses to the point that the ammonia would not have to be topped off before launch.-127

Reaction Motors did have some cautions regarding the hydrogen peroxide that powered the TR – 139 turbopump and the X-15 ballistic control system. It was necessary to maintain the propellant below 165°F to prevent it from decomposing, and Reaction Motors believed that it would be necessary to insulate all the valves, lines, and tanks. North American thought that only the main storage tank required insulation, because of the relatively short exposure to high temperatures. However, not insulating the entire system allowed small quantities of propellant (such as found in the lines supplying the reaction control system) to potentially reach elevated temperatures. To counter this, Reaction Motors recommended installing a continuous-circulation system whereby the propellant was kept moving through the lines in order to minimize its exposure to high compartment temperatures, particularly in the wings. If the engineers found the circulation system to be insufficient, it was possible to install a rudimentary cooling system on the main tank.-1281

20 40 ЄО 80 100 а ALTITUDE X 1000 FT.

ENGINE THRUST ENVELOPE

The final Reaction Motors contract called for an engine capable of being throttled between 15,000 Ibf and 50,000 lbf, although this was later raised to 57,000 lbf. Some engines actually produced more than 60,000 lbf. The engine needed to operate for 90 seconds at full power or 249 seconds at 15,000 lbf. (NASA)

Engineers considered the TR-139 thrust chamber very lightweight at 180 pounds. Furthermore, it used an assembly of "spaghetti tubes" as segments of the complete chamber, and, as it turned out, the spaghetti tubes would prove to be one of the more elusive items during engine development. The thrust chamber used ammonia as a regenerative coolant, but the exhaust nozzle was uncooled and configured to optimize thrust at high altitude. Reaction Motors expected to use a slightly altered XLR30 thrust chamber. The modifications included the incorporation of a liquid propellant igniter (for restarts) and derating to operate at 600 psia instead of 835 psia. The lower chamber pressure was desired to improve local cooling conditions at low thrust levels.-129

In order to improve safety, Reaction Motors proposed the simplest igniter the engineers could think of. The igniter was located along the centerline at the top of the chamber and had two sections. The first section contained a catalyst bed that used activated silver screens to decompose hydrogen peroxide into steam and oxygen at 1,360°F. The second section consisted of a ring of orifices where fuel was injected; when the fuel and superheated oxygen mixed, they combusted. The resulting flame was used to ignite the propellants in the combustion chamber. Reaction Motors believed this simple igniter would not be subject to the kind of failures that could

occur in electrical ignition systems. Despite the apparent desirability of this arrangement, a more traditional electrical ignition system was used in the final engine.[30]

The XLR30 turbopump was a two-stage, impulse-type turbine driving fuel and oxidizer pumps. The turbine operated at a backpressure of 45 psia at full thrust. The designers matched the pump characteristics to allow varying engine thrust over a wide range of thrust simply by varying the power input to the turbine. Varying the flow of hydrogen peroxide to a gas generator controlled the speed of the turbine. The gas generator consisted of a simple catalyst bed that decomposed the hydrogen peroxide into steam. Reaction Motors expected that the engine would need only 2.5 seconds to go from ignition to maximum thrust, and only 1 second to go from minimum to maximum thrust. On the other side, it would take about 1 second to go from maximum to minimum thrust, and not much more to complete a shutdown.-131

However, using a single turbine to drive both the fuel and oxidizer pumps resulted in the XLR30 liquid-oxygen pump operating at too high a speed for the new XLR99. Haakon Pederson, who became the principal designer of the XLR99 turbopumps, modified the original XLR30 oxidizer pump section to have a single axial inlet impeller operating in conjunction with a directly driven cavitating inducer. This required a new impeller design, new casting patterns, a new inducer, and a new pump case. Essentially, this was a new liquid-oxygen pump, and it became one of the major new developments necessary for the XLR99.-132

At this point, Reaction Motors expected to take 24 months to develop the new engine, followed by six months of testing and validation. The company would deliver the first two production engines in the 30th month, and manufacture 10 additional engines at a rate of one per month.-133

All parties finally signed the Reaction Motors contract on 7 September 1956, specifying that the first flight-rated engine was to be ready for installation two years later. The Air Force called the "propulsion subsystem" Project 3116 and carried it on the books separately from the Project 1226 airframe. The final $10,160,030 contract authorized a fee of $614,000 and required that Reaction Motors deliver one engine and a mockup, as well as various reports, drawings, and tools. The 50,000-lbf engine would be throttleable between 30% to 100% of maximum output. The 588- pound engine had to operate for 90 seconds at full power or 249 seconds at 30% thrust.-134

Less than two months after the Air Force issued the letter contract, the NACA began to question the conduct of Reaction Motors. On 11 April 1956, John Sloop from Lewis visited the Reaction Motors facilities and reported a multitude of potential development problems with the ignition system, structural temperatures, and cooling. Sloop reported that approximately 12 engineers were working on the engine, and that Reaction Motors expected to assemble the first complete engine in May 1957. However, Sloop believed that the Reaction Motors effort was inadequate and questioned whether the appropriate test stands at Lake Denmark would be available in late 1956. Sloop suggested that the company needed to assign more resources to the XLR99 development effort.-133

Despite the issues raised by Sloop, the Air Force did not seem to be concerned until 1 August 1956, when the Power Plant Laboratory inquired why scheduled tests of the thrust chamber had not taken place. It was not explained why four months had elapsed before the Air Force questioned the schedule slip.-133
important for maintaining the schedule. Reaction Motors also attributed part of the delay to modifications of two available test chambers to accommodate the high-powered engine.[37]