Mercury-Redstone 1 - Causes of The Failure

Causes of The Failure

Investigation revealed that the Redstone's engine shutdown was caused by two of its electrical cables separating in the wrong order. These cables were a control cable, which provided various control signals, and a power cable, which provided electrical power and grounding. Both cables were plugged into the rocket at the bottom edge of one of its tail fins and would separate at liftoff. The control cable was supposed to separate first, followed by the power cable. However, for this launch, the control cable was longer than expected—it was one designed for the military Redstone missile rather than the shorter cable designed for Mercury-Redstone. This control cable had been clamped to compensate for its greater length, but when the vehicle lifted off, the clamping did not work as planned and the control cable separation was delayed, eventually occurring about 29 milliseconds after the power cable had separated.

During this brief interval, the lack of electrical grounding caused a substantial current to flow through an electrical relay which was supposed to trigger normal engine cut-off at the end of powered flight. This relay tripped, causing the Redstone to shut off its engine and send a "normal cut-off" signal to the capsule. Under normal circumstances, when the capsule received this signal during a flight, it would do two things: it would jettison its escape rocket, which was no longer of any use, and after the escape rocket had flown clear, the capsule would separate itself from the expended Redstone. In the case of MR-1, the capsule did jettison the escape rocket as it was designed to, but the capsule did not separate itself from the Redstone. The capsule was designed to suspend this separation until the vehicle's acceleration had almost ceased, so that the capsule would not be hit by a still-accelerating launch vehicle. This would happen when the capsule's acceleration sensors detected an acceleration approaching 0 g, which it would normally experience after the Redstone had shut down and was entering free fall. However, in MR-1, the Redstone was not in free fall but rather sitting supported on the ground. Thus the capsule sensors detected the effect of their own supported weight, which they read as a constant "acceleration" of 1 g. Because of this apparent acceleration, capsule separation was disabled.

The jettison of the escape rocket activated the capsule's parachute recovery system. Since the altitude was below 10,000 feet (3,000 m), this system was triggered by its atmospheric pressure sensors and followed its usual sequence, with the drogue parachute deploying first, followed by the main parachute. But because the main parachute was not supporting the capsule's weight, the parachute system did not detect any load on this chute, so it acted as if the chute had failed and deployed the reserve parachute.

Since the Redstone's automatic inflight abort sensing system was running in open-loop mode, the engine shutdown did not trigger an abort. However, the system did report an abort condition, so it did function properly.