was used to draw up a list of the most likely failure modes for the D-series vehicles, however simplicity reasons dictated that only a limited number of booster parameters could be monitored. An abort could be triggered by the following conditions, all of which could be indicative of a catastrophic failure: The booster flight path deviated too far from the planned trajectory Engine thrust or hydraulic pressure dropped below a certain level Propellant tank pressure dropped below a certain level The intermediate tank bulkhead showed signs of losing structural integrity The booster electrical system ceased operating The ASIS system ceased operating The ASIS system was deemed necessary because some flight failures of Atlas vehicles (for instance, Atlas 6B) occurred so fast that it would be nearly impossible for the astronaut to react in time to manually activate the LES. Other failure modes such as a deviation from the correct flight trajectory did not necessarily pose an immediately danger to the astronaut's safety, and the flight could be aborted manually. Not all of the modifications listed below were carried on every Mercury flight and numerous changes were made along the way in the interest of improvement or as a result of flight data obtained from failed Atlas launches. Quality control and checkout procedures also improved and became more detailed over the course of the program. Rate gyros The rate gyro package was placed much closer to the forward section of the LOX tank due to the Mercury/LES combination being considerably longer than a warhead and thus producing different aerodynamic characteristics (the standard Atlas D gyro package was still retained on the vehicle for the use of the ASIS). Mercury-Atlas 5 also added a new reliability feature—motion sensors to ensure proper operation of the gyroscopes prior to launch. This idea had originally been conceived when the first Atlas B launch in 1958 went out of control and destroyed itself after being launched with a non-functioning yaw gyro, but it was phased into Atlas vehicles only gradually. One other Atlas missile test in 1961 also destroyed itself during launch, in that case because the gyroscope motor speed was too low. The motion sensors would thus eliminate this failure mode. Range safety The range safety system was also modified for the Mercury program. There would be a three-second delay between engine cutoff and activation of the destruct charges so as to give the LES