Working up the flight plan, with its heavy emphasis on rendezvous and extravehicular activity, began in 1965 and lasted until Gemini IX was launched. By January 1966, three types of rendezvous had been included: third spacecraft orbit, from above the target vehicle, and a very high altitude maneuver to reach an imaginary (or phantom) target. The phantom rendezvous (which depended on the Agena's propulsion system) was soon canceled by the planners, both because they still did not completely trust the target vehicle's engines and because they did not want to expose the crew to too much radiation.26
Gemini IX soon picked up a third rendezvous, anyway, one that Gemini VIII missed doing - re-rendezvous from an equiperiod orbit. The spacecraft thrusters were used for an upward velocity change to separate it from the target. If the firing were precise and all conditions were right, the spacecraft and Agena would automatically rendezvous at the end of an orbit, because the more elliptical spacecraft orbital path would intersect the circular orbit of the target at the proper point. Theoretically, the closing maneuvers should involve only braking the spacecraft to reachieve stationkeeping (alias re-rendezvous) with the target.
Stafford was beginning to worry about doing three rendezvous; his spacecraft was the last to have the smaller tanks - 150 kilograms as opposed to (on Spacecraft 10) 208 kilograms of maneuvering fuel. But the equiperiod rendezvous was designed as a fuel-cheap way to evaluate maneuvers and lighting conditions for a dual rendezvous with a passive target scheduled for Gemini X. And Mathews decided that the lunar module abort rendezvous could remain in the flight plan for Gemini IX, but it would have a lower priority and would be contingent on fuel and time.27
So rendezvous was the first major objective on Gemini IX, and preparing for the different types produced its share of headaches. But the second most important activity, extravehicular work with the AMU, was a bigger source of trouble.28
The AMU had been ticketed for at least two flights from the start. This backpack, with its oxygen supply and radio, was powered by hydrogen peroxide, a relatively unstable chemical. Several MSC engineers were unhappy about using it. Warren North was one of them; North also worried about the high-temperature jet hitting the astronaut's space suit. Cernan's personalized jet-pack weighed 76 kilograms and its 10.2 newton (2.3-pound) thrusters operated in pairs - forward and back, up and down, but not from side to side. This caused another worry.29 But Aldrin, on a training trip to California, suddenly got an idea. He tested it on his next trip to the Ling-Temco-Vought (formerly Chance Vought) plant in Dallas. After he mounted the training  machine, a burst from the two aft thrusters sent him across the air-bearing table toward his target. A brief nudge from the small control jets at one shoulder and knee turned him to the side. He could now use his forward- or backward-firing thrusters to move sideways with respect to his path toward the goal.30 North's fears that the heat of the AMU thrusters might damage the pressure suit proved valid, and its insulation had to be changed. The Mylar insulation was replaced by 11 layers of aluminized H-film (a thin sheet of polyamide with a coating of aluminum on one side).31
The spacecraft also needed some rework to fit it for extravehicular tasks. At NASA's request, McDonnell bonded 80 Velcro hook patches to the surface of the spacecraft. Then Velcro pads, which would cling to the patches on the spacecraft, were added to Cernan's gloves to help hold him in place as he moved about. With body position so important in checking out and donning the AMU, two handholds and a footbar were installed as restraints. Velcro pile on the footbar would mate with Velcro patches on Cernan's boots. During zero-g flights, he found this was not enough. After stirrups were added, he and Aldrin had no difficulty in checking out the unit in further practice flights.32
25 "Gemini IX-A Mission Report," pp. 7-10, -11.
26 "Gemini Program/Mission Directive," Appendix A, Sec. A-9; Ted A. Guillory, Charles L. Stough, and Lt. Charles F. Davis, Jr., "Gemini IX Flight Plan," Final, 18 April 1966; Kenneth A. Young, telephone interview, 19 March 1970; memo, Mathews to dist., "Mission Planning for Agena," GV-66245, 21 Oct. 1965; Meyer notes, 11 Jan. 1966; "Trajectories and Orbits Meeting, January 20, 1965"; memo, Mathews to Asst. Dirs., Flight Ops. and Flight Crew Ops., "Gemini IX Mission Plan and Flight Plan," GV-66380, 24 March 1966.
27 Evans and Czarnik, "Summary of Rendezvous Operations," pp. 12-14; Malik and Souris, Gemini Technical Summary, pp. 28890; "Gemini IX-A Mission Report," p. 4-17; TWX, Mathews to NASA Hq., Attn: Day, GV-12393, 24 March 1966; TWX, Mathews to McDonnell, Attn: Walter F. Burke, "Contract NAS 9-170, Spacecraft Consumables Loading for Gemini IX," GV-12410, 22 April 1966; TWX, Mathews to McDonnell, Attn: Burke, "Contract NAS 9-170, Spacecraft Consumables Loading for Gemini X," GV-12453, 21 June 1966.
28 Memo, Mathews to Asst. Dirs., Flight Ops. and Flight Crew Ops., "Gemini IX Mission activities priorities," GV-66390, 11 April 1965; Mathews memo, GV-66415, 3 May 1966.
29 McKee memo, 26 Aug. 1963; "Technical Development Plan for DOD/NASA Gemini Experiments," p. 4-25; "DOD/NASA Gemini Experiments Study," Interim Report No. SSD-TDR-63-406 (McDonnell Report No. A358), 24 Jan. 1964, pp. 1.6.1 through 1.6.77; Frederick T. Burns et at., "Gemini Extravehicular Activities," in Reginald M. Machell, ed., Summary of Gemini Extravehicular Activity, NASA SP-149 (Langley, Va., 1967), p. 3-8; North interview; Harold I. Johnson, interview, Houston, 10 Feb. 1967.
30 Ronald C. Croston and James B. Griffin, "Manned Flight Simulation of the Air Force Modular Maneuvering Unit," printed in AIAA Fourth Manned Space Flight Meeting, St. Louis, 11-13 Oct. 1965 (New York, 1965), pp. 118-26; Edwin E. Aldrin, Jr., interview, Houston, 4 April 1967; Griffin, telephone interview, 10 Oct. 1969; Herbert E. Smith, telephone interview, 10 Oct. 1969; Harold I. Johnson, David C. Schultz, and William C. Huber, "Maneuvering Equipment," in Machell, ed., Summary of Gemini Extravehicular Activity, p. 6-36.
31 Memo, Mathews to dist., "Gemini extravehicular equipment integration," GS-64108, 17 Jan.1966; Larry E. Bell et al., "Life Support Systems for Extravehicular Activity," in Machell, ed., Summary of Gemini Extravehicular Activity, p. 4-8; Elton M. Tucker, telephone interview, 24 March 1970; James W. McBarron II, telephone interview, 25 July 1973.
32 David C. Schultz and Hilary A. Ray, Jr., "Body Positioning and Restraints," in Machell, ed., Summary of Gemini Extravehicular Activity, pp. 5-1, -8; Johnson, Schultz, and Huber, "Maneuvering Equipment," p. 6-33; "Additional EVA Provisions," McDonnell Engineering Change Proposal No. 597, 14 Oct. 1965; Cernan interview.