A cancelled changeout of an element related to the MSBLS (Microwave Scanning Beam Landing System) has become the main topic of engineering evaluations ahead of Friday’s launch of Shuttle Atlantis on STS-117.
A full scale Orbiter Project Office teleconference is taking place over the next few hours to discuss a failure with the system’s “range” mode, with a recommended changeout of the RF Assembly cancelled as it would have caused a slip to the launch date.
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Each orbiter has three separate MLS systems for redundancy. The MLS begins operating at about 15,000 feet and is used like a very precise airplane ILS system. It provides slant range, azimuth, and elevation data to the flight computers, allowing for very accurate landings. The full Microwave Landing System Overview Presentation is available now on L2.
The problem being worked right now is only with one of those three MLS units (apparently the receiver). The LCC say that as long as the RA (radar altimeters) are working – which give the best altitude data beginning at 5,000 feet- then it is okay to launch with only 2 of the 3 MLS units operational.
The Launch Commit Criteria (LCC) allows for 2 of 3 MSBLS to be fully operational for launch to proceed, so long the problem is confined to the MSBLS avionics boxes. Currently MLS 3 is being described as ‘suspect’ as opposed to ‘failed.’
‘(NAV) During MSBLS (Microwave Scanning Beam Landing System) Self-Test, MLS #3 failed in the ‘Range’ mode,’ noted expansive L-1 Launch Countdown information acquired by this site’s L2 STS-117 section. ‘System Engineers and Problem Resolution Team (PRT) recommended changing out the RF Assembly which is located in the AV Bay #2 Shelf 3 half moon section of the Crew Compartment.
‘It is not located on a cold plate (air cooled). The affected Launch Commit Criteria (LCC) says 1 of 2 radar altimeters required if less than 3 onboard MSBLS available. (NOTE: The preceding sentence is from the IPR write-up – the LCC CT-08 states 2 of 2 radar altimeters required if less than 3 onboard MSBLS available. VITT has asked that the IPR wording be changed to reflect the correct criteria.)’
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Engineers carried out troubleshooting, which included 10 additional self tests, which all came up nominal with range, elevation and azimiuth. However, the planned changeout was cancelled as engineers try to stop any further slips in the timeline – which is currently being caught up.
‘The Orbiter Project Office (OPO) agreed to change out the RF Assembly as long as it did not impact the Friday, June 8 launch attempt,’ added the information. ‘NTD (NASA Test Director) and KSC integration determined not to proceed with the change out as the delay in PRSD (Power Reactant Storage and Distribution) loading will not allow ops without affecting the launch date.’
PRSD loading was completed this morning, with 3947.955 lbm of O2 – allowing for 5.96 days of hold time if required, along with 472.42 lbm of H2, resulting in 4.58 days of hold time capacity. A few leaks were noted in the GSE (Ground Support Equipment), although not serious enough to cause concern.
The GSE has several leaks reported,’ added information. ‘No constraints to launch, but will require work if a scrub/turnaround is required.’
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