Discovery’s MPS stars in STS-121 countdown

by Chris Bergin

One of the key workhorses of the Shuttle Orbiter fleet’s engineering, the Main Propulsion System (MPS), proved to be one of the unsung stars of last week’s launch of STS-121.

A hold was nearly called for the T-31 second mark, due to concerns about the liquid oxygen (LOX) inlet temperatures proving to be higher than normal. However, engineers made the call that the MPS turbo pumps could cope – and Discovery launched as planned without issue.

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The MPS is based within the aft fuselage of the orbiter, where liquid hydrogen and liquid oxygen pass through the manifolds, distribution lines and valves of the propellant management subsystem, enroute to the main engines.

During prelaunch activities, the MPS and its subsystem are used to control the loading of liquid oxygen and liquid hydrogen in the external tank, ready to be used in opposite direction to feed the main engines.

As heard by those watching NASA TV’s countdown coverage, Flight Controllers were considering holding for around a minute at the T-31 second mark, to allow the LOX temperatures to cool down, taking away some of the projected stresses on the pumps. This would have also allowed controllers to watch the inlet temperatures after LOX drainback had started.

NASA noted during the countdown, ‘based on data from the detankings, the inlet temps were a little high, which could cause an automatic hold pretty close to T-31.’ This would have been for around a minute, in the allowed four minute window the Shuttle has to launch in, before a scrub is called.

Those temperatures were coming from the sensors located in the MPS, where the 17-inch feedline – which carries the LOX from the top of the External Tank, down the side of the tank, into the MPS – meets the LOX inlet low pressure turbo pumps.

nullThe call to proceed would have been made by the highest lead for the MPS in Firing Room 4 at KSC. Named as the CPROP (Main Propulsion System/Space Shuttle Main Engine Lead Engineer) controller, he alone can make the ‘Go, No Go’ call on the MPS’ health during the countdown.

One of the calls that was made, according to sources, was to allow natural drainback of LOX back through the TSM (Tall Service Mast) lines – which is a normal procedure – which also aids cooling to SSME (Space Shuttle Main Engine) start requirements.

‘The pumps would have been stressed like they were during testing in qualifying runs,’ noted a source, highly versed in the MPS. ‘(But) the pumps are very tough.’

Also noted was the failure of an ECO (Engine Cut Off) sensor, noted as ‘The LH2 five percent sensor did not indicate ‘DRY’ during flight.’

This sensor usually shows ‘DRY’ around eight minutes into flight, but remained ‘WET’ – not indicating dry at any time, as was the case with its failure during both scrubs.

While the ECO sensors have been a concern for some time, the five percent sensor has no relevance during ascent, given the main sensors that relate to engine cut off were the ones which were replaced on ET-119 (and ET-118), following their issues during STS-114’s pre launch.

This also goes to prove to Shuttle managers that the ECO sensor faults do relate to a ‘bad batch/lot’ of sensors, given the five percent sensors were not replaced. Sources note the bad batch/lot relate to a faulty machine at the ECO sensor vendor, which has since been replaced.

While the anomalies were never confirmed to one specific area, the five percent sensor issue may have now confirmed that it was the sensors themselves that were at fault. Thus, it is likely the ECO sensor issue will not prove to be a problem on future tanks.

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