Discovery arrives at pad following clearance of final issues

by Chris Bergin

Discovery remains on track to make the May 31 launch date for STS-124, as she made her overnight journey to Pad 39A.

It’s been a busy flow inside the Vehicle Assembly Building (VAB) for the stack, as engineers worked through clearing a total of 21 issues ahead of rollout. Discovery arrived at the pad well ahead of schedule – at 4:25am local time, with the shuttle secured at the pad just after 6am.

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Engineers in the VAB removed the platforms that have surrounded the shuttle ahead of rollout, following a successful Shuttle Interface Test (S0008 Ops) on Thursday morning.

‘OV-103 (STS-124): In VAB, S0004 operations are complete. Monoballs connected; this operation went well. Power-up testing completed. Began configuring for roll,’ noted the Integrated Flow Status. ‘Plan is to roll at 00:01 on Saturday morning. Have 21 constraints to roll; don’t foresee any problems associated with this.’

Out of the list of IPRs (Interim Problem Reports), an issue with a camera on a Solid Rocket Booster (SRB) proved to be the biggest threat to an on time rollout, which was eventually cleared, given a pad changeout can take place if required.

‘During power-up testing, picked up a couple of IPRs. One IPR was on SRB camera which didn’t get images to recorder. Troubleshooting has been performed and believe isolated problem to video recorder,’ added the Flow Status.

‘Once get confirmation of this from SRB folks and Engineering, will be okay to roll. If it was the camera, would have to stay in VAB and solve problem.

‘(However), are pretty certain that problem has been isolated to SSVR (video recorder). Have a replacement unit in place, which can be changed out at the pad. Working with Ground Ops to coordinate this.’

The remaining issues – numerous and listed in the processing reports on L2 – were cleared while the final VAB platforms were being retracted ahead of rollout.

‘SSV (Space Shuttle Vehicle) Rollout and Mate to Pad: Preps continue and VAB platform retractions are in work. E and B platform retractions were completed yesterday. D platform retraction is today,’ added Friday processing information.

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‘Completed GN2 leak checks for OV-103/STS-124. Finished nozzle screen weld repairs. All SSME rollout preps are complete and are ready to support rollout on Saturday morning.

‘Call to Stations is scheduled for 2200 tonight. SSV first motion is scheduled for 0001L Saturday, with MLP (Mobile Launch Platform hard down at Pad A at 0700L. There are no weather concerns for rollout.’

Discovery left the VAB 15 minutes ahead of schedule, and made it to the pad in just under five hours. Hard down and securing of the shuttle was around 90 minutes later.

Discovery’s payload bay doors will be opened on Saturday, after the Rotating Service Structure (RSS) is rotated into the mate position. Monday will see the installation of the STS-124 payload.

A subject of great interest ahead of STS-121, the issue of tin whiskers – a phenomenon involving the growth of tiny, hair-like splinters from the metallic surfaces of components which can cause short circuits – has come to the forefront again, this time relating to the orbiter TACAN (Tactical Air Navigation) system.

The onboard tactical air navigation units determine slant range and magnetic bearing of the orbiter to a TACAN or VHF omnirange TACAN ground station. The orbiter is equipped with three TACAN sets that operate redundantly.

Each TACAN has two antennas: one on the orbiter’s lower forward fuselage and one on the orbiter’s upper forward fuselage. The onboard TACAN sets are used for external navigation and for the orbiter during the entry phase and return-to-launch-site abort.

‘Orbiter (USA/JSC): During OPO (Orbiter Project Office) Tagup, discussed tin whisker inspection on TACAN. Found tin whiskers on an older model. Have done things since then to retard tin whiskers,’ noted the latest Shuttle Stand-up/Integration report. ‘Acceptance is based on length, location, and quantity of tin whiskers.’

While of interest, this issue will not prove to be a problem for Discovery’s flow towards launch, as confirmed at the OPO meeting, which expanded on the background relating to their findings.

“Saw low likelihood of having any issues, so concluded this isn’t a concern for STS-124. The tin whiskers were not close to electrical components, and the rationale on tin whiskers has not changed because of this.”

“In design of box, added material components in alloy to mitigate tin whiskers. Most were found in area where a screw or bolt when through a hole, which retains the whiskers. Also, tin whiskers are in area that is part of RF assembly (Microwave Scan Beam Landing System).

“Only thing tin whiskers could do in this area is possibly short out RF side, and would not get erroneous output from TACAN. Have no concerns for STS-124. Will open boxes from later lots in about a week.”

Meanwhile, the STS-124 crew have been kept busy by checking out the new runway 22 at the alternative landing site at Edwards Air Force Base. The crew are practising at Edwards in a STA (Shuttle Training Aircraft) which mimics the handling of an orbiter during landing.

“Flight Crew (NASA/JSC): Prime crew went to Edwards and flew and STA. Nav aides need work as far as lighting intensity, being set up backwards for reds and whites,” added the Stand-up. “Will resolve this and send another STA with a pilot in several weeks.”

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