smooth thrust. The engines would also use a "wet start", meaning that the engine tubes would contain an inert fluid to act as a shock damper (the two failed Atlas D flight tests used dry starts, with no fluid in the engine tubes). If the booster failed the check, it would be automatically shut down. By late 1961, after a third missile (27E) had exploded on the pad from combustion instability, Convair developed a significantly upgraded propulsion system that featured baffled fuel injectors and a hypergolic igniter in place of the pyrotechnic method, but NASA were unwilling to jeopardize John Glenn's upcoming flight with these untested modifications and so declined to have them installed in Mercury-Atlas 6's booster. As such, that and Scott Carpenter's flight on MA-7 used the old-style Atlas propulsion system and the new variant was not employed until Wally Schirra's flight late in 1962. Static testing of Rocketdyne engines had produced high-frequency combustion instability, in what was known as the "racetrack" effect where burning propellant would swirl around the injector head, eventually destroying it from shock waves. On the launches of Atlas 51D and 48D, the failures were caused by low-order rough combustion that ruptured the injector head and LOX dome, causing a thrust section fire that led to eventual complete loss of the missile. The exact reason for the back-to-back combustion instability failures on 51D and 48D was not determined with certainty, although several causes were proposed. This problem was resolved by installing baffles in the injector head to break up swirling propellant, at the expense of some performance as the baffles added additional weight and reduced the number of injector holes that the propellants were sprayed through. The lessons learned with the Atlas program later proved vital to the development of the much larger Saturn F-1 engine. Electrical system Added redundancy was made to the propulsion system electrical circuitry to ensure that SECO would occur on time and when commanded. The LOX fuel feed system received added wiring redundancy to ensure that the propellant valves would open in the proper sequence during engine start. Tank bulkhead Mercury vehicles up to MA-7 had foam insulation in the intermediate bulkhead to prevent the super-chilled LOX from causing the RP-1 to freeze. During repairs to MA-6 prior to John Glenn's flight, it was decided to remove the insulation for being unnecessary and an impediment during servicing of the