McDonnell submitted to Manned Spacecraft Center the detail specification of the Mercury Mark II spacecraft. A number of features closely resembled those of the Mercury spacecraft. Among these were the aerodynamic shape, tractor rocket escape tower, heatshield, impact bag to attenuate landing shock, and the spacecraft-launch vehicle adapter. Salient differences from the Mercury concept included housing many of the mission-sustaining components in an adapter that would be carried into orbit rather than being jettisoned following launch, bipropellant thrusters to effect orbital maneuvers, crew ejection seats for emergency use, onboard navigation system (inertial platform, computers, radar, etc.), and fuel cells as electrical power source in addition to silver-zinc batteries. The long-duration mission was viewed as being seven days.

Manned Spacecraft Center completed an analysis of possible power sources for the Gemini spacecraft. Major competitors were fuel cells and solar cells. Although any system selected would require much design, development, and testing effort, the fuel cell designed by General Electric Company, West Lynn, Massachusetts, appeared to offer decided advantages in simplicity, weight, and compatiblity with Gemini requirements over solar cells or other fuel cells. A basic feature of the General Electric design, and the source of its advantages over its competitors, was the use of ion-exchange membranes rather than gas-diffusion electrodes. On March 20, 1962, McDonnell let a $9 million subcontract to General Electric to design and develop fuel cells for the Gemini spacecraft.

McDonnell awarded AiResearch a $5.5 million subcontract to provide the reactant supply system for the Gemini spacecraft fuel cells. The oxygen and hydrogen required by the fuel cell were stored in two double-walled, vacuum-insulated, spherical containers located in the adapter section of the spacecraft. Reactants were maintained as single-phase fluids (neither gas nor liquid) in their containers by supercritical pressures at cryogenic temperatures. Heat exchangers converted them to gaseous form and supplied them to the fuel cells at operating temperatures.

In an electrical systems coordination meeting at Manned Spacecraft Center, results of operating the first fuel cell section were reported: a fuel cell stack had failed and the resultant fire had burned a hole through the case. Another section was being assembled from stacks incorporating thicker ion-exchange membranes. One such stack, of six fuel cells, had operated for 707 hours within specification limits, and after 875 hours was five percent below specified voltage; a similar stack was well within specification after operating 435 hours.

At a meeting on spacecraft operations, McDonnell presented a 'scrub' recycle schedule as part of a continuing investigation of the capability of a delayed Gemini launch to meet successive launch windows during rendezvous missions. With no change in either existing aerospace ground equipment or the spacecraft, the recycle time was 48 hours (an earlier estimate had been 24 1/2 hours) for a trouble-free recycle. Gemini Project Office wanted to recycle time reduced to 24 hours and ultimately to something less than 19 hours to meet successive launch windows, possibly by replacing fuel cells with batteries for rendezvous missions only.

Gemini Project Office reported that the fuel cell development had slipped, although the amount of slippage had not been completely estimated. Causes of the slippage had been rejection of vendor parts, extension of vendor delivery schedules, and lack of early determination of production procedures.

McDonnell reported that spacecraft No. 2 was roughly one month behind schedule, primarily because of late deliveries of onboard systems from the vendors. Critical items were orbit attitude and maneuver system, reentry control system, fuel cells, and cryogenic storage tanks. Several systems had failed to pass vibration qualification and required modification. The Development Engineering Inspection of the spacecraft was scheduled for October 1963, but further delays postponed it until February 12-13, 1964.

Gemini Project office (GPO) requested McDonnell to do a design study of the requirements and configuration necessary for using batteries instead of fuel cells in all spacecraft scheduled for two-day rendezvous missions. Personnel from GPO had visited General Electric to review the results of experiments conducted to determine the theoretical operating life of the fuel cells to power the Gemini spacecraft. Tests results showed a life of about 600 hours, but changes in the spacecraft coolant system increased the fuel cell operating temperatures and reduced fuel cell life by about two-thirds. The theoretical life of the cells was between 150 and 250 hours; until some method of increasing the operating life of the fuel cell could be achieved, the development program would remain a problem.

Delays in the fuel cell development program prompted Gemini Project Office to direct McDonnell to modify the electrical system for spacecraft No. 3 so that either fuel cells or a silver-zinc battery power system could be installed after the spacecraft had been delivered to the Cape. A contract change incorporating this directive was issued January 20, 1964.

Gemini Project Office (GPO) reported that a silver-zinc battery power system would be flown in spacecraft 3. Gemini Project Office (GPO) reported that a silver-zinc battery power system would be flown in spacecraft No. 3 instead of a fuel cell system, which could not be qualified in time for the mission. Late in January, 1964, McDonnell reviewed for GPO the status of the fuel cell program and discussed the design of an improved fuel cell into spacecraft No. 5 and to delete fuel cells from spacecraft Nos. 3 and 4, substituting the battery power system.

Gemini Program Office reported that tests had been conducted on section I of the fuel cells planned for the long-duration Gemini-Titan 5 mission. These tests had resulted in a failure characterized by output decay. A complete investigation was in process to determine the cause of the failure.

The system was inadvertently operated for 15 minutes during a short circuit prior to the scheduled test. System performance was poor, and two of the cells would not carry loads of six amperes. The test was terminated. The product water sample obtained from the test was extremely acidic, indicating a potential membrane failure.

Fuel cells and batteries were discussed as power sources for the Gemini-Titan (GT) 5 mission (long-duration) at a meeting of the Gemini Management Panel. A study was reviewed that proposed a combination to be used in the following manner: batteries would be used during peak load requirements; the fuel cell would supply the remaining mission power source requirements. The panal accepted the proposal, and McDonnell was directed to proceed with the plan. In addition, the group decided to remove the fuel cell from GT-4 and substitute batteries, pending the concurrence of NASA Headquarters. It also decided to fly older versions of the fuel cell in GT-2 (the redesigned version would be flown in the later manned flights) to gain flight experience with the component. Additional Details: here....