Shuttle Endeavour has MPS prevalve issue

by Chris Bergin

NASA documents acquired by this site have shown that Shuttle orbiter Endeavour has a “possible metallic particle” stuck in one of her MPS (Main Propulsion System) prevalve screens.

Inspection images show the object that is wedged into the screen is similar to that seen on Discovery, prior to her return to flight mission, STS-121.

**SPECIFIC INFORMATION AND FURTHER DOCUMENTATION** Both related presentations available to download on L2.
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Discovery launched without issue, as would Endeavour, so long as the object is shown not to be an ‘ignition source’ – with Titanium being the highest risk associated with such a contamination in the LO2 prevalve screens.

‘OV-105 (Endeavour) has a possible metallic particle stuck in the engine 1 LO2 prevalve screen,’ noted one of two NASA presentations. ‘No issue for engine orifice blockage or mechanical binding, but mechanical impact and/or frictional ignition are still a concern.

An array of images show the contamination has a ‘foil like’ appearance, jammed into one of the small holes in the screen. Ironically, the fact that the item has lodged into the screen is encouraging, given that is the screen’s job, ensuring contaminations are caught before they can enter the main mechanics of the SSMEs (Space Shuttle Main Engines).

Where the item originated from is not known, although Endeavour has not flown since 2002, and sources note the item could have been ‘bouncing around’ the MPS for some time, before finally finding its rather apt position in the prevalve screen. The object was found during the Major Modification Period evaluations, although the presentations where only presented to NASA managers this month (Aug 17 PRCB).

It may even of been the case that Endeavour has flown a number of times with this object being lodged in the screen, given the borescope inspections only take place during long stand-downs of the orbiters – usually during the Major Modification Periods.

Key to engineers is the confirmation of the object’s composition.

‘Titanium (foil in OMS pod seals) is worst-case metallic for ignition in O2,’ added the document. ‘Low ignition energy and high heat of combustion, limited data previously available to assess ignition/propagation in LO2. Engineering assessments of potential hardware pinch-points conducted for SSME and MPS LOX system hardware.’

Engineers and managers have been looking back to their test results on the similar situation that occurred with Discovery, while they issue more testing on Endeavour – with her return to flight operations not slatted in until the middle of next year with STS-118, carrying S5, Spacehab-SM, ESP3 to the International Space Station (ISS).

However, following her return to flight operations, Endeavour is required to support STS-117, as she becomes the LON (Launch On Need) vehicle for STS-117. The LON date for STS-318 currently May 8, 2007.

‘Most testing conducted for STS-121, OV-103 similar issue, but additional tests conducted to address STS-118, OV-105 concern,’ recognised the presentation. ‘Test results for OV-105, with worst case ignition source material (Ti), using actual valve and pump materials indicate the contamination can be ignited, but shows no sustained propagation to surrounding materials (Kel-f excepted).’

It is likely Endeavour will be cleared to fly ‘as-is’ – given previous history of similar events, added to the risk studies showing their is a very small chance the item could become dislodged.

‘Contamination would have to dislodge from prevalve screen and would have to be trapped and ignited at precise time of valve closure, but could lead to Crit1 failure if occurred. Likelihood considered remote (addressed in Risk-Trade analysis).’

Another reason for leaving the contamination in the screen is due to the difficulty in accessing the internals of the MPS. The process of removal could in fact cause serious damage to the system.

However, with this being the second contamination found in an Orbiter’s MPS in just a matter of months, NASA appears to be moving towards developing a specialised system that remove such an object, in a similar fashion to that of keyhole surgery.

First in line is the development of what is being called the XRF tool, which will be used to confirm what sort of contamination has been found. Removal would be required if it was deemed an ignition source – which could lead to a vacuum cleaner type device being added to the tool.

The requirement of this development being taken forward was classed as one of the recommendations of NASA Safety and Mission Assurance – part of the Space Shuttle System Integration Division.

‘Contamination identification and the possibility of titanium within the Integrated MPS will be a continuing issue for the life of the program,’ they noted.

While evaluations will continue, the same division also – and importantly – recommended to, ‘Accept Contamination As-Is.

‘The probability that the material is titanium is low. The contamination would have to dislodge from the prevalve screen to be a hazard. The probability of contamination being entrapped in one of the three susceptible components is relatively low, conservatively ~1 in 1189.’

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