Transtage

An important step forward occurred with the third liquid-propellant stage for the Titan III, known as Transtage, for which the air force decided on a pressure-fed engine that would use the same nitrogen tetroxide as oxidizer and Aerozine 50 for fuel as stages one and two. As planned, it would have two gimballed thrust chambers, each pro­ducing 8,000 pounds of thrust, and a capability of up to three starts over a six-hour period. Aerojet won this contract, with a Phase I agreement signed in early 1962 and a Phase II (development) award issued on January 14, 1963.51

Aerojet designed the Transtage engine (designated AJ10-138) at about the same time as a larger propulsion unit for the Apollo service module. The two engines used basically the same design, featuring the same propellants, ablatively cooled thrust chambers, and a radiatively cooled nozzle assembly. Since the Apollo service module’s engine bore the designation AJ10-137, its development ap­parently began earlier in 1962, but it also lasted longer. Although

Aerojet designed and built them both, and more information is available about the development of the spacecraft engine, it is not clear that any of the latter’s problems and solutions are relevant to the Transtage engine, which was less than half as powerful and roughly half the length and diameter of its sibling.52

Apparently, these two engines were not the only ones with ab­latively cooled combustion chambers in this period, because an important NASA publication on liquid-propellant rocket engines issued in 1967 stated that such “thrust chambers have many ad­vantages for upper-stage applications. They are designed to meet accumulated duration requirements varying from a few seconds to many minutes." Although construction could vary, in one ex­ample (unspecified), the ablative liner used a high-silica fabric im­pregnated with phenolic resin and then tape-wrapped on a mandrel. Asbestos impregnated with phenolic served as an insulator on the outer surface of the liner. A strong outer shell consisted of layers of one-directional glass cloth to provide longitudinal strength. Cir­cumferential glass filaments “bonded with epoxy resin" provided “hoop strength."53 This appears to have described the Transtage combustion chamber (as well as others?).54

On July 23, 1963, Aerojet had successfully operated a Transtage engine for 4 minutes, 44 seconds, considered “a long duration fir­ing." During that static test, the engine started and stopped three times, demonstrating the restart capability. However, a more criti­cal test of this crucial capability (which would allow it to place multiple satellites into different orbits on a single launch or to po­sition a single satellite in a final orbit, such as a geostationary or­bit, without a need for a separate kick motor) would occur in the simulated-altitude test chamber at Arnold Engineering Develop­ment Center in Tullahoma, Tennessee. In August 1963, tests at that center confirmed suspicions from the July 23 test that the combus­tion chamber would burn through before completing a full-duration firing (undefined). In addition, gimballing of the engine in a cold environment revealed a malfunction of a bipropellant valve (that fed propellants to the combustion chamber) and a weakness in the nozzle extension, made of aluminide-coated columbium and radia­tion cooled with an expansion ratio of 40:1. Information about how Aerojet solved these problems is not available in any of the sources for this book, with the official history of the Titan III merely stating that “by the close of 1963, an extensive redesign and testing pro­gram was underway to eliminate these difficulties so the contractor could make his first delivery of flight engine hardware—due in mid – December 1963." 55

One Aerojet source does not comment on these particular diffi­culties but does refer to “the error of trying to develop in a produc­tion atmosphere." The source explained in this connection that de­velopment of this small engine occurred while Titan I was starting into production, causing management and engineers/technicians to pay less attention to it. But presumably, the speed required in Transtage’s development was also a factor in these particular prob­lems. Obviously, engineers had not expected them and had to adjust designs to correct the difficulties. In any event, engine deliveries did not occur in mid-December, as initially planned, but started in April 1964.

Aerojet engineer and manager Ray Stiff recalled that after engine deliveries began, the air force started to impose new requirements. Because Transtage needed to perform a 6.5-hour coast while in orbit and then be capable of “a variety of firing, coast, and refire combi­nations," there had to be “unique insulation requirements," to pro­tect propellants from freezing in the extreme cold of orbit in space, especially when shaded from the sun. But this insulation retained the heat from combustion, which built up around the injector with presumed dire consequences for continued performance. Stiff does not reveal how Aerojet solved this problem, stating only that the engine’s injector was “baffled for assurance of stable combustion."56

Other sources reveal that the injector used an “all-aluminum flat 170 faced design" with a “concave spherical face, [and] multiple-orifice Chapter 4 impinging patterns." The baffle was fuel cooled, so perhaps an ad­justment in this feature solved the heating problem. According to an Aerojet history written by former employees and managers, “The injector design has undergone two performance upgrade programs which resulted in the very high specific impulse value of 320 lbf-sec/ lbm, and the design has been carried over into later versions of the Delta."57 (Most sources do not rate the specific impulse this high.)

In any event, the two initial Transtage engines each yielded 8,000 pounds (lbf) of thrust with a specific impulse of more than 300 lbf-sec/lbm. Pressurized by cold helium gas, each of the hyper – golic propellants was stored in tanks of a titanium alloy that the prime contractor, Martin, machined in its Baltimore Division. The titanium forgings came from the Ladish Company of Cudahy, Wis­consin. Although titanium was difficult to machine, it was gaining increasing use for liquid-propellant tanks. With a fuel tank about 4 feet in diameter by 13.5 feet in length and an oxidizer tank mea­suring about 5 by 11 feet, Transtage’s propellant containers were hardly huge but were reportedly some of the largest yet produced from titanium. Each overall engine was 6.8 feet long with its diam-

eter ranging from 25.2 to 48.2 inches. Its rated burning time was a robust 500 seconds, and its total weight was only 238 pounds.58

Transtage advanced storable-propellant technology but also rep­resented a further example of trial-and-error engineering. Other up­per stages used the technology developed for Transtage and for the Apollo service module’s engine.