McDonnell let a $32 million subcontract to North American Aviation's Rocktdyne Division, Sacramento, California, to build liquid propulsion systems for the Gemini spacecraft. Two separate systems were required: the orbit attitude and maneuvering system (OAMS) and the reaction or reentry control system (RCS). The OAMS, located in the adapter section, had four functions: (1) providing the thrust required to enable the spacecraft to rendezvous with the target vehicle; (2) controlling the attitude of the spacecraft in orbit; (3) separating the spacecraft from the second stage of the launch vehicle and inserting it in orbit; and (4) providing abort capability at altitudes between 300,000 feet and orbital insertion. The OAMS initially comprised 16 ablative thrust chambers; eight 25-pound thrusters to control the spacecraft attitude in pitch, yaw, and roll axes; and eight 100-pound thrusters to maneuvre the spacecraft axially, vertically, and laterally. Rather than providing a redundant system, only critical components were to be duplicated. The RCS was located forward of the crew compartment in an independent RCS module. It consisted of two completely independent systems, each containing eight 25-pound thrusters very similar to those used in the OAMS. Purpose of the RCS was to maintain the attitude of the spacecraft during the reentry phase of the mission.

McDonnell awarded a $6.5 million subcontract to Minneapolis-Honeywell Regulator Company, Minneapolis, Minnesota, to provide the attitude control and maneuvering electronics system for the Gemini spacecraft. This system commanded the spacecraft's propulsion systems, providing the circuitry which linked the astronaut's operation of his controls to the actual firing of thrusters in the orbit attitude and maneuvering system or the reaction control system.

At a mechanical systems coordination meeting, representatives of McDonnell and Gemini Project Office decided to develop more powerful retrograde rocket motors for the Gemini spacecraft. The new motors, similar in configuration to the old but with some three times the thrust level, would permit retrorocket aborts at altitudes as low as 72,000 to 75,000 feet. McDonnell's original subcontract with Thiokol was accordingly terminated and a new subcontract was let on July 20. Development of the new motors was expected to cost $1.255 million.

Rocketdyne completed designing and fabricating prototype hardware for both spacecraft liquid propulsion systems and initiated testing of the reaction control system. Test firing of the 25-pound-thrust chambers revealed nozzle erosion causing degradation in performance after one third the specified burn time.

Gemini Project Office identified the primary problem area of the spacecraft liquid propellant rocket systems to be the development of a 25-pound thruster able to perform within specification over a burn time of five minutes. Three-minute chambers for the reaction control system (RCS) had been successfully tested, but the longer-duration chambers required for the orbit attitude and maneuver system (OAMS) had not. Rocketdyne was three weeks behind schedule in developmental testing of RCS and OAMS components, and five weeks behind in the systems testing.

Successful achievement of the full burn-time duration specified for the orbit attitude and maneuver system (OAMS) 25-pound thruster. Gemini Project Office reported Rocketdyne's successful achievement of the full 270-second burn-time duration specified for steady-state operation of the orbit attitude and maneuver system (OAMS) 25-pound thruster. This had been the primary focus of Rocketdyne's research effort, in line with McDonnell's position that meeting steady-state life operations with the 25-pound OAMS thrust chamber assembly (TCA) was the key to resolving major problems in the development of spacecraft liquid propulsion systems. McDonnell engineers believed that a TCA design able to meet the steady-state life performance required of the 25-pound OAMS TCA would also be adequate to meet pulse-life performance requirements, and that a satisfactory 25-pound TCA would only have to be enlarged to provide a satisfactory 100-pound TCA. They were wrong on both counts. Rocketdyne subsequently shifted its primary TCA effort to obtaining life during pulse operation for 25-pound thrusters and steady-state life operation for 100-pound thrusters.

Rocketdyne reactivated the test program on the 100-pound thrust chamber assembly (TCA) for the orbit attitude and maneuver system. Through March, testing had been at a very low level as Rocketdyne concentrated on the 25-pound TCAs. Testing had ceased altogether in April because hardware was unavailable. Tests had shown, however, that a satisfactory 100-pound TCA design could not be derived from an enlarged 25-pound TCA design. The major objection of the reactivated test program was to achieve steady-state life. Two tests late in May were encouraging: one achieved 575 seconds of operation with no decay in chamber pressure and a performance efficiency of 92 percent; the other operated for 600 seconds with 10 percent decay in chamber pressure and 91.9 percent performance efficiency. Specification performance was 530 seconds with less than 3 percent chamber pressure decay and 93 percent performance efficiency.

Rocketdyne successfully tested a 25-pound thrust chamber assembly (TCA) for the reentry control system (RCS) in pulse operation. Earlier efforts had aimed primarily at achieving steady-state performance, until tests revealed that such performance was no guarantee of adequate pulse performance. Char rate on pulse-cycled, 25-pound RCS TCAs proved to be approximately 1.5 times greater than identical TCAs tested in continuous runs. Several TCAs failed when the ablative material in the combustion chamber was exhausted and the casing charred through. To correct this problem, the ratio of oxidizer to fuel was reduced from 2.05:1 to 1.3:1, significantly decreasing chamber temperature; the mission duty cycle was revised, with required firing time reduced from 142 seconds of specification performance to 101 seconds, without catastrophic failure before 136 seconds; and the thickness of the ablative chamber wall was increased, raising motor diameter from 2.54 to 3.75 inches. The development of a suitable ablative thrust chamber, however, remained a major problem. No RCS TCA design was yet complete, and no 25-pound orbit attitude and maneuver system TCAs had yet been tested on a pulse-duty cycle. Rocketdyne was already three months late in delivering TCA hardware to McDonnell, and all other components had been rescheduled for later delivery. Completion of development testing of components had also been slipped three months.

Rocketdyne completed its initial design of the 25-pound thrust chamber assembly (TCA) for both the reentry control system (RCS) and orbit attitude and maneuver system. Less than a month later, Rocketdyne recommended an entirely new design, which McDonnell approved on July 5. The redesigned TCA was planned for installation in spacecraft Nos. 5 and up. Meanwhile, however, Rocketdyne had established a thrust chamber working group to improve TCA performance. This group designed, built and successfully tested in pulse operation two 25-pound RCS thrusters much more quickly than Rocketdyne had anticipated; thus the new design configuration was incorporated in the manufacturing plan for spacecraft Nos. 2 and up. The design of all TCAs, 25-85-, and 100-pound, were now identical. In reporting these developments, Gemini Project Office attributed the success of the new design to relaxed test requirements rather than to any breakthrough in design or material. In addition to reduced oxidizer-to-fuel ratios and less required firing time, thrust performance requirements were also lowered to 22.5 pounds for the 25-pound thrusters, 77.5 for the 85-pound thrusters, and 91.2 for the 100-pound thrusters.

Rocketdyne began a series of tests to verify its new thrust chamber assembly (TCA) design for the reentry control system (RCS) and the orbit attitude and maneuver system (OAMS). The test plan called for each type TCA, 25-pound RCS, 25-, 85-, and 100-pound OAMS, to be tested to mission duty cycle, steady state life, limited environmental exposure, and performance. Rocketdyne submitted its design verification test schedule to McDonnell and Gemini Project Office on August 27, with seven of the 16 tests already completed. The remaining nine tests were to be finished by September 10. This proved an optimistic estimate; design verification testing was not completed until October.

Gemini Project Office reported that systems testing of the orbit attitude and maneuver system (OAMS) and reentry control system (RCS) was scheduled to be resumed early in October. Systems tests had begun in August 1962 but had been brought to a halt by the unavailability of thrust chambers. Three categories of systems tests were planned: (1) Research and Development Tests, comprising gas calibrations, aerospace ground equipment, evaluation, surge pressure evaluations, pulse interactions, steady-state evaluations, and vacuum soak tests; (2) Design Information Tests, comprising extreme operating condition evaluations, a group of fill-drain-decontamination-storage tests, pulse performance, skin heating, expulsion efficiency, liquid calibration, manual regulation, and propellant gauging; and (3) Design Approval Tests, comprising acceleration testing, RCS mission duty cycle tests at ambient temperature, OAMS two-day mission duty cycle tests at ambient temperature, and OAMS 14-day mission duty cycle tests at ambient temperature. Systems testing did not actually resume until May 1964.

This was to permit incorporating a drogue parachute in the system as a means of stabilizing the spacecraft during the last phase of reentry, at altitudes between 50,000 and 10,000 feet. This function had originally been intended for the reentry control system (RCS), currently suffering from serious development problems. The revised design would also permit RCS propellants to be dumped before deploying the main recovery parachute. GPO outlined a three-phase drop test program to develop the drogue chute and qualify the revised recovery system. Phase I, scheduled for January and February 1964 and using boilerplate No. 5, as a test vehicle, would develop the technique of deploying the pilot parachute by the stabilization chute. The deployment sequence was planned to begin with deployment of the stabilization chute at 50,000 feet. At 10,600 feet, the astronaut would release the stabilization chute. A lanyard connecting the stabilization and pilot chutes would then deploy the pilot chute. Two and one-half seconds later, the rendezvous and recovery (R and R) section would separate from the spacecraft, allowing the main chute to deploy. Phase II of the drop test program, scheduled for March through August 1964 and using a parachute test vehicle (an instrumented weight bomb), would complete development of the stabilization chute. From June through October 1964, Phase III tests would qualify the recovery system, using static article No. 7, a boilerplate pressure vessel and heatshield equipped with production RCS and R and R sections. Since this program was not expected to be finished before the third Gemini mission, qualification of the existing system was to be completed with three more drops in February and March 1964. Static article No. 7 would serve as the test vehicle before being diverted to Phase III testing.

Rocketdyne test-fired an orbit attitude and maneuver system (OAMS) 85-pound thruster to a new mission duty cycle requiring 550 seconds of normal operation and 750 seconds before catastrophic failure. In noting McDonnell's reevaluation of the OAMS mission duty cycles, which imposed increased life requirements on OAMS thrust chamber assemblies (TCA), Gemini Project Office pointed out that this change compounded the TCA problem: the current (and briefer) mission duty cycles had yet to be demonstrated under specification conditions on the 25-pound and 100-pound TCAs. During the next two months, Rocketdyne stopped testing and concentrated on analyzing the performance characteristics of small ablative rocket engines, while McDonnell completed revising of duty cycles. Representatives of NASA, McDonnell, and Rocketdyne met in January 1964 to clarify the new life requirements for OAMS engines, which were significantly higher: required life of the 25-pound OAMS thruster in pulse operation was raised from 232.5 seconds to 557 seconds; that of the 85- and 100-pound thrusters, from 288.5 to 757 seconds.

Gemini Project Office (GPO) reported the results of a survey of testing being done at Rocketdyne on the orbit attitude and maneuver system (OAMS). The research and development phase of testing OAMS components appeared likely to extend well into 1964, with the development of an adequate thrust chamber assembly (TCA) continuing as the major problem. Hardware availability remained uncertain, no definite method of resolving the TCA life problem had yet been selected, and McDonnell's current revision of mission duty cycles compounded the problem. Lack of hardware was also delaying system testing, which would be completed no sooner than the second quarter of 1964. Persistent delays in the research and development test program were in turn responsible for serious delays in the qualification test program. To meet the manned Gemini launch scheduled for 1964, GPO was considering the possibility of beginning qualification tests before development testing had been completed.

Persistent problems in the development of engines for the Gemini orbit attitude and maneuver system prompted a review by the management of Manned Spacecraft Center. After discussion three decisions were reached. The possibility for further reducing the oxidizer to fuel ratio (currently 1.3:1) while still maintaining stable combustion and good starting characteristics was to be investigated. Lowering this ratio would reduce operating temperatures and enhance engine life. Another investigation was to be conducted to determine the feasibility of realigning the lateral-firing thrusters more closely with the spacecraft center of gravity. Such a realignment would reduce the demand placed on the 25-pound thrusters (which had yet to demonstrate a complete mission duty cycle operation without failure) in maintaining spacecraft attitude during lateral maneuvers. The third decision was to build an engine billet with ablation material laminates oriented approximately parallel to the motor housing. A recently developed parallel laminate material in its initial tests promised to resolve the problem of obtaining the thrusters' full operational duty cycle.

Rocketdyne tested an orbit attitude and maneuver system (OAMS) 100-pound thrust chamber assembly (TCA) to the 757-second mission duty cycle without failure. The TCA incorporated a modified injector which sprayed about 25 percent of the fuel down the wall of the chamber before burning, a technique known as boundary-layer cooling. With an oxidizer to fuel ratio of 1.2:1, the ablative material in the chamber was charred to a depth of only 0.5 inch. A second TCA, tested under the same conditions, charred to 0.55 inch. The flight-weight engine contained ablative material 1.03 inches thick, indicating that this engine configuration provided an ample margin for meeting mission requirements. These test results encouraged Gemini Project Office (GPO) to believe that boundary-layer cooling answered the problem of obtaining life requirements for the OAMS 100-pound TCAs. The same technique was also tried with the 25-pound TCA, but boundary-layer cooling was much less successful in the smaller engine; a modified rounded-edge, splash-plate injector yielded better results. This configuration was tested to the 570-second mission duty cycle using a mixture ratio of 0.7:1; at the end of the test, 0.18 inch uncharred material was left. Earlier TCAs using the same mixture ratio had failed after a maximum of 380 seconds. GPO now expected both 25- and 100-pound TCAs to be ready for installation in spacecraft 5 and up.

Gemini Project Office (GPO) reported the results of a test program to determine the possible effects of cracked throats or liners on the orbit attitude and maneuver system thrusters. Because of the manufacturing process, almost all thrust chamber assemblies (TCA) had such cracks and consequently could not be delivered. The tests showed no apparent degradation of engine life caused by cracks, and Rocketdyne claimed that no TCA in any of their five space engine programs had failed because of a cracked throat. With certain restrictions, cracked throats were to be accepted. GPO expected this problem to be reduced or eliminated in the new boundary-layer cooled TCAs, the throats of which had appeared in good condition after testing.

Gemini Program Office (GPO), encouraged by several highly successful tests, reported that all orbit attitude and maneuver system thrust chamber assembly (TCA) designs had been frozen. A 25-pound TCA tested to the 578-second mission duty cycle was still performing within specification requirements after more than 2100 seconds with a maximum skin temperature of 375 degrees F. Additional Details: here....

Gemini Program Office (GPO) reported the substantial completion of all research and development testing of components. These included the thrust chamber assemblies, of the reentry control system (RCS) and orbit attitude and maneuver system (OAMS) as configured for spacecraft Nos. 2 through 5. Additional Details: here....

Gemini spacecraft No. 3 thrusters were static fired as part of a complete, end-to-end propulsion system verification test program carried out on spacecraft Nos. 2 and 3 to provide an early thorough checkout of servicing procedures and equipment before their required use at the launch complex. T Additional Details: here....

The orbit attitude and maneuver system (OAMS) 25-pound thrusters installed in spacecraft No. 4 were replaced with new long-life engines. Installation of the new engines had been planned for spacecraft No. 5, but they were ready earlier than had been anticipated. Additional Details: here....