the sustainer engine ten seconds before SECO. This was to ensure that the LOX supply would be completely exhausted at SECO and prevent a LOX-rich shutdown. The PU system was set up in the Atlas C configuration through MA-6 in the interest of reliability, the standard D-series PU setup not being used until MA-7. Skin Big Joe and MA-1's boosters sported thicker gauge skin on the fuel tank but the LOX tank used the standard D-series missile skin. After the loss of the latter vehicle in flight, NASA determined that the standard LOX tank skin was insufficient and requested it be made thicker. Atlas 100D would be the first thick-skinned booster delivered while in the meantime, MA-2's booster (67D) which was still a thin-skinned model, had to be equipped with a steel reinforcement band at the interface between the capsule and the booster. Under original plans, Atlas 77D was to have been the booster used for MA-3. It received its factory rollout inspection in September 1960, but shortly afterwards, the postflight findings for MA-1 came out which led to the thin-skinned 77D being recalled and replaced by 100D. The LOX tank skin was thickened still further on MA-7 as the operational Mercury flights carried more equipment and consumables than the R&D ones and capsule weight was growing. Guidance The vernier solo phase, which would be used on ICBMs to fine-tune the missile velocity after sustainer cutoff, was eliminated from the guidance program in the interest of simplicity as well as improved performance and lift capacity. Since orbital flights required an extremely different flight path from missiles, the guidance antennas had to be completely redesigned to ensure maximum signal strength. The posigrade rocket motors on the top of the Atlas, designed to push the spent missile away from the warhead, were moved to the Mercury capsule itself. This also necessitated adding a fiberglass insulation shield to the LOX tank dome so it wouldn't be ruptured by the rocket motors. Engine alignment A common and normally harmless phenomenon on Atlas vehicles was the tendency of the booster to develop a slight roll in the first few seconds following liftoff due to the autopilot not kicking in yet. On a few flights however, the booster developed enough rolling motion to potentially trigger an abort condition if it had been a crewed launch. Although some roll was naturally imparted by the Atlas's turbine exhaust,