(Earlier, Paine had asked whether the SPS engine had ever failed to fire during all of this developmental program.) Mueller reported that a review of the test history showed that no complete flight-configuration engine had ever failed to fire. In fact, during the entire development program (comprising some 3,200 engine starts and more than 90,000 seconds of firing time ) only four engines had failed to start. In all of these cases, the cause of the ignition failures could be traced to faulty ground support equipment or to inadequate or improper operational procedures. No engine failure could be attributed solely to the SPS engine itself. Mueller's response to Paine - with obvious overtones for the upcoming Apollo 8 circumlunar mission - bespoke a supreme confidence in the safety and reliability of the all-important main engine of the spacecraft.