A good many factors pointed to the merit of this approach:
- A rectangular window 18 by 38 centimeters (seven by 15 inches) above the commander's head could readily be incorporated into the LEM's structure, with only minimal design changes. The weight penalty would be between 4.5 and 6.8 kilograms (10 and 15 pounds) (excluding possible effects on the vehicle's environmental control system). On the other hand, eliminating the front docking mechanism would save about 11 or 14 kilograms (25 or 30 pounds). A docking aid on the CM was essential, but the device "would pay for itself in increased reliability and decreased design load requirements and fuel requirements." Additionally, instead of two docking aids on the LEM (as currently envisioned), only the upper one would be needed.
- The top-only docking arrangement would simplify the docking operation per se. The crew would no longer have to transfer the drogue from the top to the front hatch prior to rejoining the CM. [The need for depressurizing the spacecraft to perform this task thus was obviated.] As an additional "fringe benefit," the front hatch could possibly be reconfigured to make it easier for the crewmen to get out of and back into their craft while on the moon.
- The overhead window would enable the LEM commander to see the moon during powered descent and ascent portions of the flight, and thus would afford the crew a visual attitude and attitude reference.
There existed, naturally, some offsetting factors: the pilot's limited view of his target (thought to be of "no major consequence"); and his being unable quickly to scan his instrument panel (which was not essential). Also, the maneuver called for the pilot to fly his vehicle, for a considerable period, in a rather strained physical position (i.e., with his head tossed backward). But because of the many inherent advantages, the group concluded, LEM-active docking at the upper hatch was acceptable as a backup method for docking. (CM-active docking still would be the normal procedure, because that vehicle could "perform the docking maneuver more easily and more reliably than can the LEM . . . Deletion of the front docking capability on [the] LEM will not alter this relationship, therefore the LEM should be required to dock only when the CSM or the crew member inside is incapacitated. If the CSM is incapacitated returning to it is of questionable importance.") They recommended that Grumman be directed to proceed with this concept for the LEM.