boosters in the field. NASA sent out a memo to GD/A requesting that subsequent Mercury-Atlas vehicles not include bulkhead insulation. LOX turbopump In early 1962, two static engine tests and one launch (Missile 11F) fell victim to LOX turbopump explosions caused by the impeller blades rubbing against the metal casing of the pump and creating a friction spark. This happened after over three years of Atlas flights without any turbopump issues and it was not clear why the rubbing occurred, but all episodes of this happened when the sustainer inlet valve was moving to the flight-ready "open" position and while running untested hardware modifications. In addition Atlas 113D, the booster used for Wally Schirra's flight, was given a PFRT (Pre-Flight Readiness Test) to verify proper functionality of the propulsion system. On MA-9, a plastic liner was added to the inside of the pumps to prevent this failure mode from recurring. Pneumatic system Mercury vehicles used a standard D-series Atlas pneumatic system, although studies were conducted over the cause of tank pressure fluctuation which was known to occur under certain payload conditions. These studies found that the helium regulator used on early D-series vehicles had a tendency to induce resonant vibration during launch, but several modifications to the pneumatic system had been made since then, including the use of a newer model regulator that did not produce this effect. The flow of helium to the LOX tank on Mercury vehicles was limited to 1 lb per second. This change was made after Atlas 81D, an IOC test from VAFB, was destroyed in-flight due to a malfunction that caused the pressurization regulator to overpressurize the tank until it ruptured. Hydraulic system The hydraulic system on Mercury vehicles was a standard D-series Atlas setup. The vernier solo accumulator was deleted as Mercury vehicles did not perform vernier solo mode. A hydraulic pressure switch on MA-7 was tripped and flagged an erroneous abort signal, so on subsequent vehicles additional insulation was added as cold temperatures from LOX lines were thought to have triggered it. Propellant utilization system In the event that the guidance system failed to issue the discreet cutoff command to the sustainer engine and it burned to propellant depletion, there was the possibility of a LOX-rich shutdown which could result in damage to engine components from high temperatures. For safety reasons, the PU system was modified to increase the LOX flow to