Besides its primary function as a flight-test vehicle, Apollo 6 (AS-502) served as a milestone in the site activation of LC-39. The Site Activation Board's second flow required that high bay 3, mobile launcher 2, and firing room 2 be in operation for the second Saturn V launch. Delays in the arrival of flight hardware and setbacks to the Apollo 4 schedule helped the board meet its schedule in time for Apollo 6. In April 1967, Boeing officials estimated that modifications on the swing arms, hold-down arms, and the tail service masts would require another 12,000 manhours. The mid-July date for the completion of this work was seven weeks behind schedule and threatened to delay a mid-August rollout. As events would eventually unfold, Apollo 6 did not reach pad A until February 1968, several months after the swing arm work was completed.6
The S-IC first stage arrived at KSC on 13 March 1967, and erection of the booster on mobile launcher 2 came four days later. Since the delivery of the S-II stage was another two months off, the Boeing crew substituted the S-II spacer again. The S-IVB stage and the instrument unit followed on the same day. The launch vehicle team quickly discovered that the high bay's environmental control system could not support the checkout. Portable high capacity air conditioners, used originally to protect Pegasus spacecraft on LC-37, were pressed into service. Even so, the humidity approached the maximum allowable for certain pieces of ground support equipment.7
During the month of April, a number of tests on Apollo 6 were postponed because of Apollo 4 support requirements, illuminating one of the limitations of the mobile concept in its early days. Although the facilities could physically accommodate two vehicles at the same time, their checkout could not proceed without the removal of men and equipment from one vehicle for temporary use on the other.
The S-II stage arrived on 24 May. It was mated with the interstage and moved to a low bay the next day. Further delays in the launch vehicle tests forced a postponement of several procedures including the launch vehicle overall test 1 (plugs in). Although propellant dispersion and power transfer tests were completed by the end of the month, the plugs-in test did not get under way until 13 June. The restacking of Apollo 4 in mid-June delayed the movement of the S-II to a horizontal position in the transfer aisle, and threatened the latter's erection date of 7 July. By the end of June, a new schedule for Apollo 6 was in hand, based upon the arrival of the command and service modules on 29 September.8
Apollo 6 operations in July and August continued to be marked by frequent delays. Several postponements were caused by hardware problems such as a request from Marshall that the launch team x-ray all liquid-hydrogen lines on the S-II stage. Vehicle tests were interrupted by the Apollo 4's plugs-in test on 1 August and again by ordnance installation on AS-501 during the week of the 14th. By September rescheduling had become a way of life for the checkout team.9
Another revised schedule in mid-September placed Apollo 6's countdown demonstration test in late January. Within a week the validation of swing arm 1 was four days behind schedule. Work on the service arms halted altogether on 26 September when most of the Apollo 6 crew was detailed to work on problems on mobile launcher 1. Support for Apollo 4 continued on an "as required" basis. Although the tests of the service arms for mobile launcher 2 fell three weeks behind schedule, this was not critical, because the delivery of the spacecraft was also postponed - this time by two months.10
With Apollo 4 launched and the spacecraft for Apollo 6 on hand, operations picked up. The swing arm tests were finally completed on 11 December, a day after the command and service modules joined the Saturn stack. During the remainder of the month, the launch team contended with a variety of problems: late flight control computers and flight program tapes, faulty memory in the RCA 110A interface unit, and glycol spilled on the outer surface of the spacecraft and S-IVB stage. The troubles of another plugs-in test on 21 December were typical: failure of a printed-circuit board in a digital events evaluator, a false fire alarm in the assembly building, failure of the emergency detection system test program, and a faulty battery that put an early end to the test.11
Problems with flight hardware continued to consume much time. During the plugs-out test on 28 December, the launch team had a premature cutoff of engine 5 in the S-II stage. An investigation indicated that the culprit was the engine control actuator. On 5 January 1968, North American began a three-day operation to replace the actuator. Just as this was being completed, a crack was discovered in the weld of a 2.5-centimeter LOX fill and drain purge line that paralleled a similar line inside the second stage. By the time the replacement line was cleaned and installed, the S-II crew had lost another three days. Unfortunately, the problems of the S-II stage on Apollo 6 were not limited to the checkout; they were precursors of malfunctions that would occur in flight.12
The space vehicle electrical mate, emergency detection system check, and overall test 1 were run during two days in mid-January, and the space vehicle swing arm overall test was completed on 29 January. As the launch crew reextended the swing arms after the test, the retract latch mechanism on arm 1 failed and the first stage took a blow. A gimbal joint in the support system was damaged, but the dent in the launch vehicle proved superficial.
Apollo 6 was transferred to the pad on 6 February. Under cloudy skies the crawler with its load paused briefly just outside the assembly building for the erection of the communications antenna and lightning rods on the mobile launcher. Winds and rain hit the area, and the crawler stopped when the storm disrupted communications with the launch control center. After two hours, with contact restored, the control center gave orders to proceed. The four double-track trucks moved ahead in the driving rain. A rainbow formed above the glistening height of Apollo 6 shortly before the crawler reached the foot of the pad. Two diesel engines began leveling the platform as the transporter negotiated the incline to the top of the pad. The sun had sunk behind a low bank of clouds, and the rocket inched up the pad in semidarkness. By the time the crawler reached the top of the pad shortly after 7:00 p.m., the clouds had scudded away, the winds had died down, and the stars glistened in a rain-washed sky. The mobile service structure could not be moved to the pad for two days because of high winds.13
The flight readiness test for Apollo 6 was completed early on the morning of 8 March. Three days later the flight readiness review was held at KSC. The meeting included representatives of all the major supporting elements for the mission. Apollo 6 was cleared for flight subject to the satisfactory completion of space vehicle testing and the closeout of action items identified by the review. The launch was set for 28 March. The next week the hypergolic loading team ran into some minor problems, and the stabilized platform in the instrument unit was replaced. The latter meant an extra 18 hours to reestablish the guidance system's integrity. The launch was changed twice again, first to 1 April, then 3 April.14
Preparations for the countdown demonstration test ended 23 March, and the precount began on schedule at 1:00 p.m. on the following day. The test was completed within a week. The launch countdown was picked up on 3 April at 1:00 a.m., the T-8 hour mark. There were no unscheduled holds. At the mission director's informal review held at KSC on 3 April, Apollo 6 received a "go" for launch the next day. Launch day dawned warm and humid with scattered clouds. The prelaunch countdown and liftoff, in the words of Rocco Petrone, "followed the script"; but the script included one cliff-hanger, again in the S-II stage. During the countdown demonstration test, four propellant pump discharge temperatures had been a few degrees above redline values at the engine inlets. This threatened to convert the liquid hydrogen and oxygen into gases before reaching the injector. If this happened, Petrone told a prelaunch press conference, the pumps could malfunction and upset the ratio of fuel to oxidizer. After the test, steps had been taken to improve the insulation, and the LOX redline was raised two degrees to 98 kelvins (-175 degrees C). Whether these changes would correct the condition would not be known until the countdown went into automatic sequence a little more than three minutes before liftoff. If the temperatures exceeded the new redline, the sequencer would be halted at T-22 seconds. As it developed, the launch readings were within the new tolerances.15