The Air Force, however, was disconcerted. Colonel Daniel McKee, head of the Air Force Systems Command Field Office at MSC, complained about being kept in the dark about the plans for White's exercise. His office should have been involved in the orderly planning that was to lead to the use of the AMU, since at about $12 million that was the single most expensive Defense experiment to be carried on Gemini. The AMU was designed to make an extravehicular astronaut independent of spacecraft systems. A boxlike backpack with sidearm controllers, it consisted mainly of three beams and supporting shelves on which such parts as tanks to store the hydrogen-peroxide propulsion system and the life-support oxygen supply were mounted. Because the spacecraft was so small, the AMU was housed back in the adapter section. The astronaut would go out the hatch tied to a tether, make his way to the rear of the spacecraft, and strap himself into the AMU. This special propulsion system weighed about 76 kilograms (168 pounds), which was no burden at all in the weightlessness of space.29
On 20 September 1965, NASA had named the crew for Gemini VIII. Neil Armstrong, a civilian test pilot with long experience in the X-15 rocket research aircraft program, was the command pilot, as he had been in the backup crew for Gemini V. His fellow crewman, David Scott, was new to the Gemini program. For the backup crew for Gemini VIII, Navy Lieutenant Commanders Charles Conrad and Richard F. Gordon, Jr., the pattern was similar. Conrad had been pilot on Gemini V, and Gordon was newly assigned to Gemini.30
Some three weeks before the crew was announced, McDonnell held a briefing on the extravehicular gear Gemini VIII's could carry. It comprised two major units: an Extravehicular Life Support System, called the ELSS by the engineers who worked on it, and an Extravehicular Support Package, known as the ESP or, more commonly, the backpack. The life support system was a chestpack designed to do just that - feed vital oxygen to the spaceborne astronaut from the  spacecraft's supply, from a primary source in the backpack, and from its own emergency supply. The backpack did more. Designed like the AMU to fit into the spacecraft adapter section, it carried, aside from its own oxygen supply, a radio and 8 kilograms (18 pounds) of propellant for a zip-gun maneuvering unit. The backpack was connected to the spacecraft systems by an 8-meter oxygen-hose tether. Once the astronaut had switched over from the spacecraft to the backpack oxygen supply, he could add a lightweight 23-meter tether to the shorter hose and, theoretically at least, maneuver as far as 30 meters from the spacecraft.31 Armstrong attended the St. Louis meeting and asked for help in the training program. The crews would need a realistic spacecraft adapter with which to practice donning the backpack. Armstrong also wanted the pilot to leave the spacecraft in the altitude chamber and test the combined backpack-chestpack.32
As soon as he was assigned to the mission, Scott began concentrating on the extravehicular exercise, eventually going through over 300 airplane zero g parabolas and more than 20 hours on an air-bearing table. The astronauts practiced EVA maneuvers, supported by an air cushion of 0.0254 millimeter (0.001 inch), on a table roughly 6 by 7 meters. They used a zip gun to move from one place to another, which gave them some idea of what it would be like to start and stop in space. This strenuous training raised some questions.33
Scott's zip gun had about 15 times more propellant than White's and used Freon instead of oxygen as fuel, further multiplying the gun's total impulse, since Freon has a density about three times greater than oxygen. How oxygen acted in vacuum was fairly well known, hut Scott worried about how Freon would behave. One problem soon showed up: at low temperatures, the Freon caused the zip gun's poppet valve to stick open when triggered, and the escaping gas threatened to tumble the astronaut in space. New seals solved the problem and two new shutoff valves added a safety factor.34
By December, Scott and Armstrong were voicing a number of doubts about the equipment, ranging from nitpicks to serious complaints. One that fell into the latter class was the threat of an oxygen ejector in the chestpack freezing and blocking the flow of oxygen from both the spacecraft and the emergency supply in the chestpack. The life support system had been icing up during tests. Although test conditions were more severe than Scott would meet in space, he could hardly be expected to ignore the warning. The designers obliged by installing 20-watt heaters near the ejector.35
Another problem was the tangle of umbilicals, tethers, and jumper cables that made donning the chestpack inside the spacecraft so difficult. During early tests, Scott found his movements restricted and his vision nearly blocked by his pressure suit while he was trying to connect everything. Late in December 1965, however, Scott satisfied himself  that he could strap on the unit, hook it up, and open the hatch in the McDonnell altitude chamber at a simulated 46,000 meters. Scott went through a full-dress rehearsal in the last few weeks before flight, in the MSC 6-meter vacuum chamber, putting on the chestpack inside the spacecraft, going outside, and then donning the backpack housed in the adapter.36
29 Reginald M. Machell, interview, Houston, 18 April 1967; Harold I. Johnson, interview, Houston, 10 Feb. 1967; Johnson, David C. Schultz, and William C. Huber, "Maneuvering Equipment," in Reginald M. Machell, ed., Summary of Gemini Extravehicular Activity, NASA SP-149 (Langley, Va., 1967), pp. 6-28, -29; letter, Col, Daniel D. McKee to George M. Low, "NASA/DOD Mission Planning," 13 June 1965.
30 Astronautics and Aeronautics, 1965: Chronology on Science, Technology, and Policy, NASA SP-4006 (Washington, 1966), p. 444; "Mission Operations Report, Gemini Flight Number Eight," M-913-66-09, 3 March 1966, pp. 31-33; David R. Scott, interview, Houston, 5 April 1967.
31 "Abstract of Meeting on Extravehicular Life Support System and Extravehicular Support Package Testing, September 2, 1965," 14 Sept. 1965, with enclosure, "Extravehicular Life Support System and Extravehicular Support Package Spacecraft 8 Test Plan," n.d.; Frederick T. Burns et al., "Gemini Extravehicular Activities," in Machell, ed., Summary of Extravehicular Activity, p. 3-6; Larry E. Bell et al., "Life Support Systems for Extravehicular Activity," ibid., pp. 4-29, -58, -59; TWX, Mathews to McDonnell, Attn: Burke, "Contract NAS 9-170, Gemini, Fit Check of Extravehicular Support Pack in Spacecraft 8 Adapter," GS-10089, 30 Dec. 1965.
32 "Meeting on Extravehicular . . . Testing, September 2, 1965."
33 "Gemini 8 Astronaut Crew Press Conference," 26 Feb.1966, pp. 2-6; Machell interview; Neil A. Armstrong, interview, Houston, 6 April 1967; Johnson, Schultz, and Huber, "Maneuvering Equipment," p. 6-7.
34 Harold I. Johnson et al., "Extravehicular Maneuvering about Space Vehicles," in Gemini Summary Conference, NASA SP-138 (Washington, 1967), pp. 92-94; "Rendezvous and Extravehicular Systems," Gemini Design Certification Report, February 1966, p. 2.1-2; MSC Quarterly Activity Report for Office of Assoc. Adm., Manned Space Flight, for period ending January 1966, p. 44; memo, Mathews to Asst. Dir., Flight Crew Ops., Attn: Chief, Flight Crew Support Div., "Qualification of the hand-held maneuvering unit," GT-65457, 5 Nov. 1965; Johnson interview.
35 Machell interview; Bell et al., "Life Support Systems for EVA," p. 4-47; Quarterly Activity Report for period ending January 1966, p. 44.
36 Machell interview; "Abstract of Meeting on Extravehicular Life Support Systems/Extravehicular Support Package, January 5 and 6, 1966," 7 Jan. 1966; "Abstract of Meeting on Extravehicular Life Support System and Extravehicular Support Package, January 28, 1966," n.d.; Bell et al., "Life Support Systems for EVA," pp. 4-41, -43, -44; "Gemini Program Mission Report, Gemini VIII," MSC-G-R-66-4, 29 April 1966, p. 12-2.