Ensuring safe flight – engineering effort on Endeavour’s APU 1

by Chris Bergin

As the STS-123 crew enjoy a well deserved period of off duty on Flight Day 10, documentation has confirmed Endeavour’s Auxiliary Power Unit (APU 1) pressure decay is no cause for concern – as initially thought.

More telling is the large scale effort that went into the evaluations, which involved several NASA and contractor teams – exemplifying how the engineers follow extensive processes behind the scenes to ensure safe flight.

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APU 1 Evaluations:

Each orbiter has three APUs – which produces pressure for the orbiter’s hydraulic system that controls the vehicle’s wing flaps, rudder, body flap and landing gear, among other elements of the vehicle.

The evaluations were called after Endeavour’s APU 1 Fuel Tank was observed to be suffering from decaying tank pressure (tank outlet pressure transducers) at a rate of around 4 psi/day.

This was deemed ‘Out of Family’ with Endeavour’s APUs 2 and 3 and historical APU on-orbit profile – with a typical in-flight decay from thermal affects being around 1.0 psi per day.

Due to the lack of available system instrumentation that provide means to distinguish between fuel (hydrazine) or GN2 (gaseous nitrogen) leakage – engineers created documentation to prove it was not a hydrazine related leak.

That’s where the main element of evaluation was centered, due to the associated risk with a hydrazine leak – specifically a fire risk during the latter stages of landing.

‘Criticality and Consequence: If leakage is GN2, no impact to mission. Leak rate is minor and will not affect APU operations,’ opened one of the three presentation – available on L2. ‘If leakage is fuel, possible fire in aft fuselage during landing. During on-orbit operations aft environment does not support combustion.’

Upon noticing the decaying pressure readings, engineers first cleared any immediate risk on orbit – mainly through experience that a leak of fuel would freeze in the environment of space. With the leak rate unlikely to increase while Endeavour remained on orbit, along with the lack of a combustion risk, the system was – and is – deemed to be ‘stable’.

Checking their extensive flight history, engineers used the most recent example of decaying tank pressure on an APU – seen during STS-121 with Discovery, with a decay rate of around 3 psi per day – in order to find the potential cause of the issue.

‘Post STS-121 Inspections: Fuel System. No evidence of fuel leakage noted on the APU 1 fuel system. Point-to-point Drager checks with resolution to 0.1 ppm showed no evidence of external hydrazine at aft compartment lines, fuel tank, or service QD (Quick Disconnect). Conclusion was no fuel leakage occurred during STS-121 event,’ added the presentation.

‘Insulation inspections. No insulation anomalies were noted on either the fuel or GN2 system. Discredits theory that pressure decay was actually thermally induced anomaly.’

Flight history also included the pulling of post flight documentation from Endeavour’s previous flight – STS-118 – and processing work ahead of STS-123, including pre-flight pad preparations for launch.

‘No Disturbance to Fuel Tank Region Except Insulation Repair on Fuel Side of Tank. Insulation was found during surveillance inspections to not be installed per drawing (inadequate amount of batting previously installed),’ noted the findings.

‘Reinsulated the Bottom of the Tank Area Where Line Exits the Tank. Insulation work was non-intrusive to fuel system fluid lines and tank. Very low likelihood any collateral damage occurred during this rework. Fuel and GN2 QD actuated during fuel servicing. Only Fuel Components Disturbed During Flow. New/Refurbished Cap Installed.

‘No QD Leakage Noted Post Loading Operations: Post 400 psi Pressurization for Flight on 2/29/08. Both Fuel and GN2 QDs Passed Leak Check Prior to Cap Installation. Caps are inspected prior to installation. QD Cap Was Leak Checked Prior to Vehicle Install. No Pressure Decreases Noted post pressurization (11+ days). Conclusions: No Indication of Fuel or GN2 Side Leaks Prior to Flight.’

In fact, documentation over the life of the shuttle program shows engineers have a very good record for screening for any issues associated with leakage on the orbiter’s hardware, with only a handful of incidents documented in addition to STS-121.

Those are listed as STS-65’s processing flow, STS-72 post flight – though not officially a leak, even though the QD cavity was observed to be ‘wetted’, and STS-3 – which required a pre-flight waiver.

and most notably STS-9 – which is documented as an “Entry/Landing Fire” after injector stem leaks on Columbia caught fire during landing, only discovered in post flight processing.

As part of the evaluations, physics based analysis was needed to help differentiate between a Hydrazine Leak and a GN2 Leak, due to the limited ability to distinguish between the two leakage media.

This brought out the NASA big guns from JSC Thermal and Fluids Analyst department and NESC (NASA Engineering and Safety Center) Technical Fellow for Life Support and Thermal Control, who assessed whether STS-123 data indicates a hydrazine leak or a nitrogen leak. Leak physics were investigated and scoping analyses were performed to assess the leak.

The findings work on several theories associated with the gained data from STS-123’s decaying pressure readings and flight history – along with expected results on various scenarios.

“If there were a hydrazine leak in a line or at the QD, we would expect that it would be freeze over and stop flow, stabilizing the tank pressure,” noted the summary of their findings. “If there were a hydrazine leak in the tank, we would expect that it would be either be detectable in the temperature measurements or would freeze over and stop flow, stabilizing the tank pressure.

“These are not consistent with the temperature data and the persistent pressure decay seen on STS-123. STS-123 data signature is analogous to that of the nitrogen leak that occurred on STS-121. It is highly unlikely that there is a hydrazine leak on STS-123.”

In conclusion, the engineering community concluded that STS-123’s APU 1 has a GN2 leak – which holds no risk for continued on orbit operations and landing.

“Physics Based Analysis Concludes a GN2 Leak is More Likely based on Data. Confirmed by Independent Analysis Performed by Boeing, WSTF (White Stands Test Facility) and USA (United Space Alliance). General Experience from Test Facilities and the Orbiter Show Many More Problems with Gas Leaks than Liquid,” added the conclusion pages.

“Several Fuel Leakage Scenarios Would be Detectable with Available Flight Data: Leakage Near Thermostat or Temperature Sensor, Vacuum Induced Freezing causing Blockage to Stop Leak, takes a Smart Leak (coupled with large area of insulation missing) to be Fuel.

“Based on the results of this investigation, the APU community has a high degree of confidence that the decay observed in the APU 1 fuel supply system is a GN2 leak and not a fuel leak. The APU recommendation is to run the APU “Ops Nominal” for both FCS (Flight Control Surface) check-out and entry.”

A full roundup of the latest mission status will be added to the next article – to be published in-between Flight Day 10 and 11.

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