NASA proposes extra sensors on the tanks

by Chris Bergin

NASA managers have requested funding – up to around $7m – to be put in place for the continuation and expansion of special monitoring equipment on the External Tanks.

Known as DFI (Developmental Flight Instrumentation), Shuttle managers and engineers will evaluate data gained from the equipment to decide on further modifications to the tanks – in order to further reduce the risks of foam liberation on ascent.
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Document information from last week’s PRCB. Yesterday’s PRCB documents (selected) now available on L2.

ET-119 – which will fly with Shuttle Discovery on STS-121 – has a number of sensors on board the tank along the cable tray, to assess how the ET performs without its PAL ramp and modified ice/frost ramps. ET-118 – which will fly with Shuttle Atlantis on STS-115 (STS-300) in August with have a larger array of instruments for the same purpose.

However, the full DFI suite will not fly until STS-118 – flight number five of the current manifest, it is proposed.

‘Assess which DFI, currently on ET-118 and ET-119 should be made a part of the baseline ET configuration,’ noted the document, titled ‘Funding Request for ET Cable Tray Instrumentation on STS-118 & -120.’

‘A standard, extensive ET DFI configuration would allow the SSP (Space Shuttle Program) to obtain a comprehensive understanding of the flight environments in the shortest number of flights.’

However, increasing to a full suite of sensors isn’t as easy as hoped, with potential interface with other Shuttle systems and a limit on available interface slots along the tank.

‘The amount of instrumentation on the ET is limited by the interface to the existing data acquisition system (EDAS on the SRB). The original (STS-1) ET/Orbiter MADS DFI interface has been removed. ET does not have a certified data acquisition system. Qualifying a new data acquisition system for ET or SRB has been estimated to require ~2 years,’ the document added.

‘The RSS interface between the ET and SRB elements has a limited number of available interface pins which can be used for DFI instrumentation. Using the MSFC-developed Excitation Power Box maximizes the number of sensors that can be installed on the ET.’

With the process likely to continue throughout the history of the space shuttle program – set for a 2010 retirement – NASA will evaluate the best options for a standard DFI suite on the tanks.

‘Increases from 14 sensors/FLT (installed on STS-114, -121, -115) to ~20 sensors/FLT (proposed for STS-118 & -120). SE&I, JSC & MSFC engineering, and ET project continue to pursue the highest priority locations that can be effectively instrumented.’

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However, the United Space Alliance is against the elaboration of the DFI on the tank, noting: ‘The first three RTF flights (STS-114, 121, and 115) will provide data for the LH2 and LOX cable tray environments. The continued instrumentation provides limited additional data without clear value added rationale.

‘Adding additional requirements for this data poses a significant threat to the ability to meet the shuttle manifest.’

Kennedy Space Center managers also noted the lack of the full DFI not being available until the fifth flight of the current manifest, with their rationale based around the question of what benefit the full DFI will have on the data received from the previous four flights.

‘While the scope of processing work for STS-118/120 EDAS/ET DFI is minimal, and equivalent to current ops, this instrumentation will not be installed until the 5th flight of this vehicle configuration.

‘Whatever flight rationale and level of risk acceptance deemed acceptable for the first 4 flights, should be applicable to future flights. This instrumentation is described as potentially providing additional insight without demonstrating knowledge of what successful results would be.’

A final decision is likely to come after STS-121’s data has been analysed.

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