T-RAD debuts during EVA-4 – ET problem could delay missions

by Chris Bergin

Delayed from several previous missions, the Tile Repair Ablator Dispenser (T-RAD) on orbit demonstration has finally taken place during STS-123’s EVA-4. Bob Behnken (EV1) and Mike Foreman (EV2) were tasked with the 6hr 24min spacewalk that also included the replacement of a RPCM (Remote Power Controller Module).

Meanwhile, an unspecified problem with the External Tanks is threatening large scale delays to STS-124 and subsequent missions. A meeting at MAF (Michoud Assembly Facility) will determine the extent of the issue.

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Its official title, DTO-848 TPS Repair Techniques (T-RAD / STA-54 Tile Repair Demo), has origins in the smaller STA-5 applicator, which was part of a scrapped CIPAA (cure in place ablator applicator) back pack system.

Originally planned for the Return To Flight mission STS-114, the system was slightly modified with a new target of making the STS-121 flight. However, following further evaluations, this moved again to STS-120 – before also being deleted from that mission.

The importance of the system is obvious, with its ability to fill damaged areas of TPS (Thermal Protection System), worse than – for example – the ‘gouge’ seen on Endeavour’s previous flight, STS-118.

However, it is not capable of repairing large breaches in the orbiter’s wing leading edge RCC panels, a task which was to be dealt with by the LAR (Large Area Repair) kit – to be developed ahead of STS-125’s mission to service Hubble, though LAR is now believed to inexplicably deleted due to costs. However, this cannot be confirmed.

T-RAD has undergone extensive testing on the ground, but lacks a test in the environment of space. EVA-4’s test will go a long way to ensuring engineers have the right formula in the dispenser’s STA-54 material.

‘Currently the T-RAD / STA-54 is part of the standard Orbiter TPS Repair kit and is certified only for flight but not for operations. Extensive T-RAD/STA-54 ground testing has been conducted but on-orbit operations have not been conducted,’ noted an extensive presentation on T-RAD.

‘A demonstration on the use of the T-RAD/STA-54 on damaged TPS tile samples will satisfy the ultimate requirement to determine if the repair material is mixed properly, to refine EVA ops procedures for tile repair, and demonstrate that this is a viable TPS tile repair method.’

The T-RAD test is expected to take three to four hours during EVA-4, before the ‘Sample Box’ – which has various examples of tile damage – is evaluated on the ground after Endeavour returns home next week. The test objectives are numerous.

‘Purpose: To dispense STA-54 material in a relevant on-orbit environment using T-RAD, to characterize material behavior during on-orbit dispense, and to evaluate on-orbit cured material post landing,’ added the presentation.

‘Objectives (Ranked): 1. Characterize material density distribution (void/bubble distribution) of on-orbit dispensed STA-54. Two open cavities in LI-900 tile will be filled. One material sample will be tamped and the other will not.

‘2. Compare thermal performance of both on-orbit and HTV dispensed STA-54 material. One 6” x 6” tile with 4” x 4” cavity will be filled on-orbit. The on-orbit filled tile will be arc-jet tested to the same test profile that previous samples prepared in Human Thermal Vacuum (HTV) were subjected to. Arc-jet test results will be compared to determine the thermal performance difference between on-orbit and HTV prepared samples.

‘3. Assess the ability to conduct a multi-depth damage repair including monitoring material expansion/bubbling and achieving required underfill. One 6” x 6” tile with a multi-depth cavity will be filled on-orbit. Thermocouples will be placed directly under each step. The on-orbit filled tile will be arc-jet tested post flight to validate the material entry ‘swelling’ model.’

‘4. Characterize adhesion properties of on-orbit cured STA-54 material. A large tile cavity exposing several TPS material (filler bar, SIP, Silica) will be filled. Verify on-orbit environment does not affect material adhesion properties. Results of the adhesion properties could be fed into a structural model.

‘5. Demonstrate T-RAD system and EVA techniques are adequate in repairing flight-like damages. One tile with actual ice impact damage is included in the tile sample. Two STS-118 damages will be machined into tile assemblies. Repaired sample can also be used to assess adhesion properties. Demonstrate minimum and maximum fill depths can be achieved when repairing flight-like damages.

“6. Qualitative assessment of the thermal performance of the T-RAD/STA-54 in expected flight environment. STA-54 requires 24 hours cure-time at -50 deg F Higher temperature will reduce material cure time. STA-54 dispense rate could be unacceptable if temperature is too low.”

Also to be taken into account is the toxic nature of STA-54. A major set of criteria was created to ensure all hazards to both the EVAers and the associated hardware in the test site would not be compromised by the material.

The box of samples that will be ‘filled in’ by the T-RAD experiment range across the variety of tile damage that an orbiter could suffer from.

“Large Cavity Arc-jet Sample: Perform hardness check. Perform photogrammetry of the returned samples and compare material cured state to what was observed on-orbit post DTO. Trim down material to 0.25” below tile OML. Perform Arc-jet testing at JSC. Dissect sample and evaluate material post arc-jet test,” listed examples of the many tests that will be fulfilled by the experiment.

“Multi-step Arc-jet Sample: Perform hardness check. Perform photogrammetry of the returned samples and compare material cured state to what was observed on orbit post DTO. Measure fill level at each step. Perform Arc-jet testing at JSC. Perform pull test (tensile strength). Dissect samples and evaluate voids within material post arc-jet test.”

Meanwhile, Endeavour is continuing to perform without an issue, with no new “funnies” – as problems are called by engineers involved with the mission – being listed during Flight Day 10 to 11.

More will follow during the EVA and Flight Day 11.

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