DESTINATION MOON: A History of the Lunar Orbiter Program
Preparing for the Fourth Mission
[269] As approved by the Ad Hoc Surveyor/Orbiter Utilization Committee on May 1, 1967, Mission IV would attempt to accomplish some of the objectives not directed towards fulfilling Apollo needs. Specifically it would "perform a broad systematic photographic survey of lunar surface features in order to increase the scientific knowledge of their nature, origin, and processes, and to serve as a basis for selecting sites for more detailed scientific study by subsequent orbital and landing missions."1
This mission, unlike the first three, required that Lunar Orbiter IV fly a nearly polar orbit. In such an orbit [270] the spacecraft would acquire contiguous photographic coverage of a minimum of 80% of the front side at 50 to 100 meters resolution. It would photograph as much of the Moon's far side as possible at the best possible resolution. The spacecraft's photographic subsystem would carry enough film for 212 frames, and ground control planned to read out all photography in the priority mode immediately after processing as a precaution against any mechanical failure in the subsystem. A final readout would be available if necessary.2
In preparation for the fourth mission the Lunar Orbiter Project and Program Offices conducted a flight readiness review on April 13, 1967. On March 13, Spacecraft 7 (the fourth flight spacecraft, or Lunar Orbiter IV) had been removed from storage at the Kennedy Space Center to begin Hangar S integration and checkout tests. Launch readiness was scheduled for May 4. and no problems were encountered during the Hangar S activities.3 The flight readiness review found Lunar Orbiter IV and the backup (Spacecraft 3) ready for launch.4
Because the fourth Orbiter would fly a high polar orbit, it would be exposed to the Sun almost the entire [271] mission, necessitating certain changes on the spacecraft. A modified charge-controller component was installed to reduce the rate of charge in the power system. Boeing engineers covered about 20% of the exterior of the equipment deck with mirrors to increase its heat rejection capability. A damaged micrometeoroid detector was removed and another unit installed. Finally the Inertial Reference Unit was removed for replacement of a failed capacitor. After reinstallation it successfully completed two attitude control system tests.5
During the weeks before the fourth launch the Program Manager showed some concern over the failure of NASA's Applications Technology Satellite (ATS II) to achieve its planned circular orbit around the Earth on April 6.6 NASA officials attributed the improper orbit to failure of the Agena rocket to reignite in orbit. Unofficially ATS program management said the cause for the reignition failure was failure of the Agena's Propellant Isolation Valve (PIV) to close after the first burn. Scherer hoped the PIV for the Lunar Orbiter IV Agena would test out successfully before April 27, the planned date for the mating of the Agena with the Atlas [272] booster.7 Lewis Research Center personnel responsible for the Agena took corrective actions and installed a reworked valve in time for the launch. The reinstallation took less than one month to complete, and it did not jeopardize the launch date.
Two areas involving previous mission and ground test problems also pertained to the successful performance of the fourth and fifth missions. The traveling-wave-tube amplifier aboard Lunar Orbiter II had experienced high helix current. Ultimately it had failed to turn on during the final readout phase, and some data were lost. The TWTA onboard Lunar Orbiter III had also experienced overheating from high helix current and power output variations from temperature changes. Worse yet, the TWTA in the ground spacecraft for the Mission D Simulation Test failed to perform successfully under mission conditions. The component was undergoing close examinations to determine the mode of failure. A delay of the fourth mission would hinge upon the seriousness of the test findings and the difficulty in resolving the problem.8
Failure in the photographic subsystem presented the other area of questionable spacecraft performance. Readout [273] problems had marred the success of Lunar Orbiter III with unwanted repetition in readout and the inability of the film transport system to move film. Program investigators had not pinpointed the causes of these failures. However, the ten-day Mission D Simulation Test, just completed on April 12, partially compensated for these failures. During the test no problems involving readout had occurred, increasing the likelihood of a successful fourth mission.