scope:

Abstract:

On 24 April 1990, the launch of the Hubble Space Telescope, STS-31R, was held at T-31 seconds because the Main Propulsion System (MPS) liquid oxygen (LOX) outboard fill and drain valve (PV9) was not closed. The command to close the valve was blocked by Prerequisite Control Logic (PCL) and therefore never issued. A plan to override the PCL was conceived and concurrence was granted by the launch team. Once PV9 was successfully closed, the NASA Launch Director authorized resuming the countdown clock, and STS-31R was successfully launched.

The PCL blocked the PV9 closed command because a deviation to the S1003 procedure required the LOX helium transfer line purge to be turned on at T-1:20. This change was in response to a potential Launch Processing System (LPS) power failure resulting in an emergency drain to prevent vehicle damage. The procedural change was coordinated and signed by all the proper personnel but was not run against the math model because it was thought to be well understood.