Atlantis targets KSC or Edwards for Sunday – Expansive ASA contingency plan

by Chris Bergin

Atlantis is still waiting on the weather in Florida, with the preferred return to KSC still on the cards for Sunday. Four landing options – including two opportunities in California – are available on Sunday, although the first KSC landing possibility has been waved off. Meanwhile, the Aerosurface Servoamplifer (ASA) channel 1 failure received a series of expansive MMT (Mission Management Team) level overviews during Atlantis’ flight, ensuring a contingency plan was ready to be activated in the event of a second “full” ASA failure.

Atlantis Latest:

With early weather reports on Saturday appearing to be more favorable for a landing at the Kennedy Space Center, Entry Flight Director Norm Knight polled his team for a go/no go for Payload Bay Door (PLBD) closure, which was completed – along with the bypassing of Atlantis’ radiators to the Flash Evaporator System (FES).

With Atlantis in OPS3 – the software transition for entry operations – the weather was to determine when and where Atlantis will land. Astronaut chief Steve Lindsey flew to the south of KSC to check for weather patterns in a T-38, before testing approaches in the STA (Shuttle Training Aircraft).

The crew donned their entry suits, with the next key decision point expected ahead of fluid loading by the crew. However, Knight has already decided to wave off the first opportunity to land on Saturday, an Orbit 180 option, due to poor weather over the Kennedy Space Center’s Shuttle Landing Facility (SLF) and approach.

Two options remained available on Orbit 181, opening with the Edwards Air Force Base in California, before the second – and final – opportunity is available to land at KSC. One final Saturday option was open to Knight and his team on Orbit 182, resulting in a landing at Edwards. However, all four options were waved off in the hope of improved weather for a KSC landing on Sunday.

“The weather at KSC has not cooperated today. There’s a chance we can land at KSC tomorrow. Waving off the day,” Mr Knight told the crew. “Edwards good for Sunday and Monday if KSC does not favor us”.

Sunday’s four opportunities relate to two at KSC and two at Edwards, with the crew currently a go for Payload Bay Door closure, ahead of donning their entry suits – which was also given a go.

“The weather is looking a little better than yesterday, so you are go for suit up,” noted Mr Knight, who later gave the go for a conservative start to fluid loading – as the KSC weather situation appears to be improving.

However, Mr Knight was not comfortable enough to give the go for returning Atlantis on the first opportunity to KSC, leaving Atlantis with one final Florida option, and two Edwards landing windows.

Update: Heading to Edwards – see here for latest article.

More will follow – refer to live update pages – linked above – for up to the second updates. New article will be published at the time a landing site decision is taken.

ASA failure investigation:

Easily the most discussed hardware item during STS-125, the ASA channel 1 failure received the bulk of the MMT’s attentions, for the event of a contingency scenario that would have forced Atlantis to land, within hours, at either Edwards Air Force Base or White Sands. Around 15 MMT presentations – all available on L2 – were created during the mission.

The failure of the single channel ASA – part of a quad channel system – held no mission impact. However, due to flight rules, a second failure – depending on the cause – forces the decision to bring the orbiter home.

a73The system is required for carrying commands from the flight computers to the aerosurfaces such as the elevons and tail – required during the latter stages of entry, along with approach and landing.

“The ASA converts MDM (Multiplexer Demultiplexer) commands into aerosurface actuator drive currents and performs servoloop closure for all but the body flap,” outlined one of the MMT presentations, outlining the system’s background.

“26VAC/400Hz excitation is used to drive its position, secondary delta pressure, and primary delta pressure transducers. Bypass control will automatically shut off elevons and rudder/speedbrake if an internal power supply failure is detected.

“26VAC Excitation wiring is distributed throughout the wing, aft body, and lower vertical tail. There is no short circuit protection inside the ASA itself – it relies on the two RPCs (controllers) that feed it to protect the wire. This kind of short has occurred on many occasions on the vehicle.”

MMT documents note the failure of the ASA channel 1 occurred prior to the vehicle departing the launch pad, at the point the Space Shuttle Main Engines (SSMEs) were powering into life.

The resulting noise from the engines managed to disrupt some of Atlantis’ avionics – which is likely why the MMT paid so much attention to the issue, in order to protect against a potentially serious issue on future launches.

“STS-125 Overview: Aerosurface ServoAmplifier (ASA) 1 26VAC/400Hz excitation was shorted to vehicle ground after engine ignition,” another document added. “400Hz noise was injected into the vehicle ground plane, disrupting MDMs in the av bay.

“1st event, short duration, started at 1801/52.598 GMT and lasted 0.02 seconds (8 cycles) from hardwire data. 2nd event started at 1801/52.860 GMT and lasted 0.85 seconds – stopping at 1801/53.71. Indicative of a hard short event. Calculated noise frequency roughly 400 Hz.”

“OA1 and FA1 single-ended measurements were affected. Noise stopped as soon as the RPCs tripped. Systems outside the ASA sustained no ill effects.

“A similar event occurred during STS-37 and had similar effects on the connected OA and FA MDMs,” referencing a 1991 occurrence, again with Atlantis, during ground processing, where “both RPCs tripped off, ASA channel 1. Pinched wire in wire clamp caused short to ground of 26 v ac (in left wing).”

A large amount of attention will be paid to the wiring in Atlantis once she returns to her Orbiter Processing Facility (OPF), although the MMT called for a startling amount of flow documentation on associated work on the orbiter’s aft avionics bay, via a Problem Reporting And Corrective Action (PRACA) search.

a92The amount of data shows how every single piece of hardware on the orbiters are heavily documented. In the case of the ASA issue, over 10 pdfs of scanned, hand written, documentation – outlining work sheets and engineer comments – were presented to the MMT for overview.

“OV-104 Baseline wire inspections have been completed in aft avionics bay 4 and the aft compartment (started in 2004 and completed in 2008 over several flows) but have not been completed in the wings or vertical tail,” noted one document outlining the work history on orbiter wiring in the avionics bay, including on Discovery and Endeavour.

“PRACA search was completed for all suspect connectors and cable segments and did not identify any work performed on this hardware during Flow 30. No connectors in the suspect wiring paths were demated during Flow 30.

“The aft compartment contains the largest percentage of suspect wire and sees significantly more traffic than any of the other areas.

“Post landing plan: No further actions required until after return to OPF. (Engineering) will lead troubleshooting effort in coordination with (other departments) after landing: Inspect, demate and re-inspect 54P92 from ASA-1 J2 receptacle. Take resistance to ground measurements to look for the existence of a short downstream towards the actuators/PDUs and upstream into the ASA.

“If short exists downstream, inspect, demate and re-inspect downstream bay 4 bus connector and take additional resistance to ground measurements. Continue on this type of path until the circuit is isolated to a single leg. Once isolated to a single cable segment options exist for visual walk-down inspection, use of an arc safe machine to determine distance to fault, etc.

“If short is not detected post flight the possibility of extensive visual inspections exists starting with prioritized ‘high risk’ areas based on wire routing.”

During the mission, the MMT set up a Tiger Team (a high level, cross center investigation body) to work the root cause and ensure no other systems were at risk from associated failures. The Tiger Team also created plans for the event of another ASA failing – which would have breached flight rules.

Due to this, had a second ASA channel failed – providing it was a full hard failure like ASA channel 1 – the MMT already had a plan of action to bring Atlantis home. However, due to required landing site margin – in the event of landing with troublesome ASAs – the large open spaces and runway options of White Sands and Edwards Air Force Base (dry lake runways) would have been prefered.

“For a single FCS (Flight Control Surface) Channel failure there are no mission duration impacts and no change to landing site selection: Priority is KSC, EDW, NOR (White Sands),” added another MMT presentation.

“For a second FCS Channel failure FR A2-207 changes priority of the landing sites to protect for the next worse failure that affects the ability to control the vehicle: Priority is NOR, EDW, KSC.

“Less than 2 FCS Channels is not certified. With a single channel across all actuators the result is likely not survivable and best case would result in slow response from the actuators possibly resulting in PIO conditions. With a single channel on a single actuator vehicle control would be adversely affected during high demand maneuvers like roll reversals, final flare, and load relief.

“Concern with a single FCS Channel is vehicle control and therefore margin at the landing site is important. NOR optimizes for channel failures where the ability to make the runway exists.”

The presentation went on to note the advantages of both White Sands (NOR) and Edwards, for aiding an orbiter with the loss of two FCS/ASA channels.

“Northrup (NOR) is highest priority for 2 FCS Channel failures. NOR provides two runways and the most lateral/longitudinal margin. Cross runways provide best ability to avoid a crosswind and target a headwind. Crosswind requires more control authority. Lateral margin for each runway is advertised as 300 feet but level and maintained to ~900 feet. Important when directional control is an issue.

“Under run advertised as 1k feet but maintained well beyond that. Length is advertised as 15,000 feet but ~29,000 feet available. Runway is termed a lakebed but hardness is comparable to concrete. Nose landing gear slap down loads are not an issue.

a83“EDW is second priority for 2 FCS Channel failures. EDW lakebed can be used as a downmode option if problems during entry that would prevent making the 22/04 concrete runway.

“Lakebed provides more margin than the EDW 22/04 concrete runway for shoulders and underrun margin. Lakebed has Nose Landing Gear (NLG) slapdown load concerns for heavy weight/forward cg vehicles due to not being able to provide 2-sigma protection against the 90k NLG loads.

“STS-125 is heavy weight with a forward cg (center of gravity). NLG loads are a function of lakebed hardness (hard, medium, or soft) and hardness measurement of the runway surface is no longer provided since EDW 15/33 is designated as ELS only. Issue still exists even with hard surface conditions.

“Flight Rules governing the EDW Lakebed are provided in backup. No longer in the Flight Rules Document since the EDW lakebed is an ELS runway.”

“KSC is last priority for 2 FCS Channel failures. No downmode capability. Shoulders are 50 feet in width. Beyond shoulders are not loadbearing. Under run is 1000 feet.”

The side benefit of the ASA channel 1 failure was the allowance to create these plans in the event of a similar failure on a future mission, with the MMT documentation transferrable to such a future scenario. It was also thanks to Atlantis’ lack of any major issues during her flight that allowed engineers to dedicate time to the ASA investigation – allowing for what is called “lessons learned.”

More information is likely to be forthcoming via the mission’s IFA (In Flight Anomaly) review, which will make up part of STS-127’s upcoming SSP (Space Shuttle Program) Flight Readiness Review (FRR).

L2 members: Documentation – from which most of the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size

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