Atlas Propulsion

Even though the Viking rocket used alcohol and the Vanguard first stage adopted kerosene as its fuel, the next major advance in alcohol and kerosene propulsion technology came with the Atlas missile. As with the Redstone, North American Aviation designed and built the Atlas engines, which also owed a great deal to NAA’s work for the Navaho. Unlike the Redstone, the Atlas engines burned kero­sene rather than alcohol. (Both used liquid oxygen as the oxidizer.) Kerosene that would work in rocket engines was another legacy of the protean Navaho program. In January 1953, Lt. Col. Edward Hall and others from Wright-Patterson AFB insisted to Sam Hoffman that he convert from alcohol to a hydrocarbon fuel for a 120,000-pound – 122 thrust Navaho engine. Hoffman protested because the standard Chapter 3 kerosene the air force used was JP-4, whose specifications allowed a range of densities. JP-4 clogged a rocket engine’s slim cooling lines with residues. The compounds in the fuel that caused these prob­lems did not affect jet engines but would not work easily in rocket powerplants. To resolve these problems, Hoffman initiated the Rocket Engine Advancement Program, resulting in development of the RP-1 kerosene rocket fuel, without JP-4’s contaminants and variations in density. This fuel went on to power the Atlas, Thor, and Jupiter engines. The specifications for RP-1 were available in January 1957, before the delivery date of the Atlas engines.38

On October 28, 1954, the Western Development Division and Special Aircraft Projects (procurement) Office that Air Force Ma­teriel Command had located next to it issued a letter contract to NAA to continue research and development of liquid-oxygen and

kerosene (RP-1) engines for Atlas. The cooperating air force organi­zations followed this with a contract to NAA for 12 pairs of rocket engines for the series-A flights of Atlas, which tested only two outside booster engines and not the centrally located sustainer en­gine for the Atlas. The Rocketdyne Division, formed to handle the requirements of Navaho, Atlas, and Redstone, also developed the sustainer engine, which differed from the two boosters in having a nozzle with a higher expansion ratio for optimum performance at higher altitudes once the boosters were discarded.39

Using knowledge gained from the Navaho and Redstone engines, the NAA engineers began developing the MA-1 Atlas engine system for Atlases A, B, and C in 1954. (Atlas B added the sustainer engine to the two boosters; Atlas C had the same engines but included improvements to the guidance system and thinner skin on the pro­pellant tanks. Both were test vehicles only.) The MA-1, like its suc­cessors the MA-2 and MA-3, was gimballed and used the brazed "spaghetti" tubes forming the inner and outer walls of the regen­eratively cooled combustion chamber. NAA had developed the ar­rangement used in the MA-1 in 1951, perhaps in ignorance of the originator of the concept, Edward Neu at Reaction Motors. NAA/ Rocketdyne began static "hot-fire" tests of the booster engines in 1955 and of all three MA-1 engines in 1956 at Santa Susana. The two booster engines, designated XLR43-NA-3, had a specific im­pulse of 245 lbf-sec/lbm and a total thrust of 300,000 pounds, much more than the Redstone engine. The sustainer engine, designated XLR43-NA-5, had a lower specific impulse (210 lbf-sec/lbm) and a total thrust of 54,000 pounds.40

Produced in 1957 and 1958, these engines ran into failures of systems and components in flight testing that also plagued the Thor and Jupiter engines, which were under simultaneous develop­ment and shared many component designs with the Atlas. They used high-pressure turbopumps that transmitted power from the turbines to the propellant pumps via a high-speed gear train. Both Atlas and Thor used the MK-3 turbopump, which failed at high al­titude on several flights of both missiles, causing the propulsion system to cease functioning. Investigations showed that lubrication was marginal. Rocketdyne engineers redesigned the lubrication sys­tem and a roller bearing, strengthening the gear case and related parts. Turbine blades experienced cracking, attributed to fatigue from vibration and flutter. To solve this problem, the engineers ta­pered each blade’s profile to change the natural frequency and added shroud tips to the blades. These devices extended from one blade to the next, restricting the amount of flutter. There was also an explo-

FIG. 3.4 Technical drawing of a baffled injector similar to the one used on the Atlas MA-1 engine to prevent combustion instability by containing lateral oscillations in the combustion chamber. (Taken from Dieter K. Huzel and David H. Huang, Design of Liquid Propellant Rocket Engines [Washington, D. C.: NASA SP-125, 1967], p. 122)

sion of a sustainer engine caused by rubbing in the oxygen side of the turbopump, solved by increasing clearances in the pump and installing a liner.

Another problem encountered on the MA-1 entailed a high – frequency acoustic form of combustion instability resulting in vi­bration and increased transfer of heat that could destroy the engine 124 in less than a hundredth of a second. The solution proved to be rect – Chapter 3 angular pieces of metal called baffles, attached to a circular ring near the center of the injector face and extending from the ring to the chamber walls. Fuel flowed through the baffles and ring for cooling. The baffles and ring served to contain the transverse oscillations in much the way that the 18 pots on the V-2 had done but without the cumbersome plumbing. Together with changing the injection pat­tern, this innovation made the instability manageable. These im­provements came between the flight testing of the MA-1 system and the completion of the MA-3 engine system (1958—63).41 They showed the need to modify initial designs to resolve problems that appeared in the process of testing and the number of innovations that resulted, although we do not always know who conceived them or precisely how they came about. (But see the account below of Rocketdyne’s Experimental Engines Group for some of the explanations.)

FIG. 3.5 Technical drawing showing components of an MA-5 sustainer engine, used on the Atlas space-launch vehicle, 1983. (Photo courtesy of NASA)

С 1983-781

GIMBAL BEARING

OX D ZER INLET ELBOW

OXIDIZER DOME

FUEL MANIFOLD

SUSTAINER THRUST CHAMBER ASSEMBLY

National Aeronautics and Space Administration Lewis Research Center

The MA-2 “was an uprated and simplified version of the MA-1," used on the Atlas D, which was the first operational Atlas ICBM and later became a launch vehicle under Project Mercury. Both MA-1 and MA-2 systems used a common turbopump feed system in which the turbopumps for fuel and oxidizer operated from a single gas gen­erator and provided propellants to booster and sustainer engines. For the MA-2, the boosters provided a slightly higher specific im­pulse, with that of the sustainer also increasing slightly. The overall thrust of the boosters rose to 309,000 pounds; that of the sustainer climbed to 57,000 pounds. An MA-5 engine was initially identical to the MA-2 but used on space-launch vehicles rather than missiles. In development during 1961-73, the booster engines went through several upratings, leading to an ultimate total thrust of 378,000 pounds (compared to 363,000 for the MA-2).

The overall MA-3 engine system contained separate subsystems for each of the booster and sustainer engines. Each engine had its own turbopump and gas generator, with the booster engines being identical to one another. The MA-3 exhibited a number of other changes from the MA-2, including greater simplification and bet­ter starting reliability resulting from hypergolic thrust-chamber ig­nition. A single electrical signal caused solid-propellant initiators and gas-generator igniters to begin the start sequence. Fuel flow

FIG. 3.6

Technical drawing of an injector for an MA-5 booster engine, used on the Atlas space-launch vehicle, 1983. (Photo courtesy of NASA)

National Aeronautics and Space Administration Lewis Research Center

through an igniter fuel valve burst a diaphragm holding a hypergolic cartridge and pushed it into the thrust chambers. Oxygen flow oc­curred slightly ahead of the fuel, and the cartridge with its triethyl aluminum and triethyl boron reacted with the oxygen in the thrust chamber and began combustion. Hot gases from combustion oper­ated the turbopump, a much more efficient arrangement than previ­ous turbopumps operated by hydrogen peroxide in rockets like the 126 V-2 and Redstone.

Chapter 3 The MA-3 sustainer engine had a slightly higher specific impulse of almost 215 lbf-sec/lbm but the same thrust (57,000 pounds) as the MA-2 sustainer. The total thrust of the boosters, however, went up to 330,000 pounds with a climb in specific impulse to about 250 lbf-sec/lbm. Both specific impulses were at sea level. At altitude the specific impulse of the sustainer rose to almost 310 and that of the boosters to nearly 290 lbf-sec/lbm. The higher value for the sustainer engine at altitude resulted from the nozzles that were de­signed for the lower pressure outside Earth’s atmosphere. The MA-3 appeared on the Atlas E and F missiles, with production running from 1961 to 1964.42

Most of the changes from the MA-1 to the MA-3 resulted from a decision in 1957 by Rocketdyne management to create an Experi­mental Engines Group under the leadership of Paul Castenholz, a

design and development engineer who had worked on combustion devices, injectors, and thrust chambers. He “enjoyed a reputation at Rocketdyne as a very innovative thinker, a guy who had a lot of en­ergy, a good leader." The group consisted of about 25 mostly young people, including Dick Schwarz, fresh out of college and later presi­dent of Rocketdyne. Bill Ezell, who was the development supervi­sor, had come to NAA in 1953 and was by 1957 considered an “old- timer" in the company at age 27. Castenholz was about 30. Before starting the experimental program, Ezell had just come back from Cape Canaveral, where there had been constant electrical problems on attempted Thor launches. The Atlas and Thor contracts with the air force each had a clause calling for product improvement, which was undefined, but one such improvement the group sought was to reduce the number of valves, electrical wires, and connections that all had to function in a precise sequence for the missile to operate.

The experimental engineers wanted a system with one wire to start the engine and one to stop it. Buildup of pressure from the turbopump would cause all of the valves to “open automatically by using the. . . propellant as the actuating fluid." This one-wire start arrangement became the solid-propellant mechanism for the MA-3, but the engineers under Castenholz first used it on an X-1 ex­perimental engine on which Cliff Hauenstein, Jim Bates, and Dick Schwarz took out a patent. They used the Thor engine as the start­ing point and redesigned it to become the X-1. Their approach was mostly empirical, which was different from the way rocket devel­opment had evolved by the 1980s, when the emphasis had shifted to more analysis on paper and with a computer, having simulation precede actual hardware development. In the period 1957 to the early 1960s, Castenholz’s group started with ideas, built the hard­ware, and tried it out, learning from their mistakes.

Stan Bell, another engineer in Castenholz’s group, noted a further difference from the 1980s: “We were allowed to take risks and to fail and to stumble and to recover from it and go on. Now, everything has got to be constantly successful." Jim Bates added that there were not any “mathematical models of rocket engine combustion processes" in the late 1950s and early 1960s. “There weren’t even any computers that could handle them," but, he said, “we had our experience and hindsight."

The reason the engineers in the group moved to a hypergolic ig­niter was that existing pyrotechnic devices required a delicate bal­ance. It proved difficult to get a system that had sufficient power for a good, assured ignition without going to the point of a hard start that could damage hardware. This led them to the hypergolic cartridge

(or slug) used on the MA-3. In the process of developing it, however, the group discovered that a little water in the propellant line ahead of the slug produced combustion in the line but not in the chamber; there the propellants built up and caused a detonation, “blow[ing] hell out of an engine," as Bill Ezell put it. They learned from that experience to be more careful, but Ezell said, “there’s probably no degree of analysis that could have prevented that from happening." There were simply a lot of instances in rocket-engine development where the experimenters had to “make the right guess or assump­tion"; otherwise, there was “no way to analyze it. So you’ve got to get out and get the hard experience." Ezell also opined that “with­out the Experimental Engine program going, in my opinion there never would have been a Saturn I," suggesting a line of evolution from their work to later engines.43

The experiences and comments of the members of Castenholz’s group illuminate the often dimly viewed nature of early rocket en­gineering. Without the product-improvement clauses in the Atlas and Thor contracts, a common practice of the Non-Rotating Engine Branch of the Power Plant Laboratory at Wright-Patterson AFB, the innovations made by this group probably would not have occurred. They thus would not have benefited Thor and Atlas as well as later projects like Saturn I. Even with the clauses, not every company would have put some 25 bright, young engineers to work on pure experiment or continued their efforts after the first engine explo­sion. That Rocketdyne did both probably goes a long way toward explaining why it became the preeminent rocket-engine producer in the country.

The changes in the Atlas engines to the MA-3 configuration as 128 a result of the experimental group’s work did not resolve all of the Chapter 3 problems with the Atlas E and F configurations. The Atlas lifted off with all three engines plus its two verniers (supplementary en­gines) firing. Once the missile (or later, launch vehicle) reached a predetermined velocity and altitude, it jettisoned the booster en­gines and structure, with the sustainer engine and verniers then continuing to propel the remaining part of the rocket to its destina­tion. The separation of the booster sections occurred at disconnect valves that closed to prevent the loss of propellant from the feed lines. This system worked through the Atlas D but became a major problem on the E and F models, with their independent pumps for each engine (rather than the previous common turbopump for all of them). Also, the E and F had discarded the use of water in the regen­erative cooling tubes because it reacted with the hypergolic slug. The water had ensured a gentle start with previous igniters. With

the hypergolic device, testing of the engines by General Dynamics had produced some structural damage in the rear of the missile. Design fixes included no thought of a large pressure pulse when the new models ignited.

On June 7, 1961, the first Atlas E launched from Vandenberg AFB on the California coast at an operational launch site that used a dry flame bucket rather than water to absorb the missile’s thrust. The missile lifted off and flew for about 40 seconds before a failure of the propulsion system resulted in destruction of the missile, with its parts landing on the ground and recovered. Rocketdyne specialists analyzed the hardware and data, concluding that a pressure pulse had caused the problem. The pulse had resulted in a sudden up­ward pressure from the dry flame bucket back onto fire-resistant blankets called boots that stretched from the engines’ throat to the missile’s firewall to form a protective seal around the gimballing engines. The pressure caused one boot to catch on a drain valve at the bottom of a pressurized oil tank that provided lubrication for the turbopump gearbox. The tank drained, and the gearbox ceased to operate without lubrication. To solve this problem, engineers re­sorted to a new liquid in the cooling tubes ahead of the propellants to soften ignition and preclude pressure pulses.

Repeated failures of different kinds also occurred during the flight-test program of the E and F models at Cape Canaveral. Control instrumentation showed a small and short-lived pitch upward of the vehicle during launch. Edward J. Hujsak, assistant chief engineer for mechanical and propulsion systems for the Atlas airframe and as­sembly contractor, General Dynamics, reflected about the evidence and spoke with the firm’s director of engineering. Hujsak believed that the problem lay with a change in the geometry of the propellant lines for the E and F models that allowed RP-1 and liquid oxygen (ex­pelled from the booster engines when they were discarded) to mix. Engineers “did not really know what could happen behind the mis­sile’s traveling shock front" as it ascended, but possibly the mixed propellants were contained in such a way as to produce an explosion. That could have caused the various failures that were occurring.

The solution entailed additional shutoff valves in the feed lines on the booster side of the feed system, preventing expulsion of the propellants. Engineers and technicians had to retrofit these valves in the operational missiles. However, the air force decided that since there could be no explosion if only one of the propellants were cut off, the shutoff valves would be installed only in the oxygen lines. A subsequent failure on a test flight convinced the service to approve installation in the fuel lines as well, solving the problem.44 Here

130

Chapter 3

was a further example of engineers not always fully understanding how changes in a design could affect the operation of a rocket. Only failures in flight testing and subsequent analysis pinpointed prob­lem areas and provided solutions.