Failure Analyses of Space Shuttle APU Turbine Blades

1. Failure Analyses of Space Shuttle APU Turbine Blades A Retrospective

2. Function of APU�s Provide power for the orbiter�s three independent hydraulic systems. Each system provides hydraulic pressure to position hydraulic actuators for: Thrust vector control of the main engines by gimbaling the three SSMEs Actuation of various control valves on the SSMEs Movement of the orbiter aerosurfaces (elevons, body flap, rudder/speed brake) Retraction of the external tank/orbiter 17-inch liquid oxygen and liquid hydrogen disconnect umbilicals within the orbiter at external tank jettison Main/nose landing gear deployment (system 1)/(system 1 or 2) Main landing gear brakes and anti-skid Nose wheel steering (system 1 with backup from system 2).

3. APU Locations-Orbiter

4. APU Operation During a typical flight, the APUs are started 5 minutes before lift-off and operate through the Orbital Maneuvering System-1 (OMS-1) burn when hydraulic power is no longer required. The APUs are basically inactive on orbit. One APU is run briefly the day before deorbit to support the Flight Control Surface (FCS) checkout. The APUs are restarted for the deorbit burn and entry. They are shut down shortly after landing.

5. APU Operation-Prelaunch Circ pumps run to flow hydraulic fluid through system APU prestart at T - 6:15 minutes APU start at T - 5 minutes

6. APU Operation-Ascent The HYD system provides hydraulic pressure to Throttle and steer the orbiter main engines Actuate the orbiter aerosurfaces Retract the external tank/umbilical plates All APUs are operated from T -5 minutes through the OMS-1. If there is no OMS-1 burn then APU shutdown comes after repositioning the main engines for orbit.

7. APU Operation-Entry The HYD system provides hydraulic pressure to Actuate the orbiter aerosurfaces Deploy the landing gear Provide braking Provide nosewheel steering At D/O - 5 minutes, one APU is started to insure that an APU is operating through the entry flight phase. At El - 13, the remaining two APUs are started and are operated through postlanding.

8. APU Chronology Initial design began in the early 1970�s Reliability Efficiency Lightweight Small footprint Prototype testing in mid 1970�s. Enterprise Flight Testing-1977 Initial flight Columbia STS-1, April 12, 1981

9. Orbiter APU

10. Cross Section of APU Turbine

11. APU Turbine Cross Section

12. APU Operational Details Each APU rated at 135 horsepower Each APU weighs 88 Pounds Turbine speed of 81,000 RPM Up to Speed in 9.5 Seconds Fuel for Gas Generator is Hydrazine First stage gas temperature of 1700F Two of the three APU�s must function for orbiter to function.

13. APU Fuel Hydrazine-N2H4 Liquid hydrazine is passed over a catalyst, Iridium, in the gas generator. Decomposition of hydrazine produces ammonia, nitrogen and hydrogen. Reaction is highly exothermic

14. Challenger Accident STS-51-L Challenger explodes 73 seconds after takeoff on January 28th 1986. Space Shuttle flights halted while extensive investigation into accident and assessment of Shuttle program are conducted.

15. Post Challenger Presidential Commission Study The Commission concluded that there was a serious flaw in the decision making process leading up to the launch of flight 51-L. A well structured and managed system emphasizing safety would have flagged the rising doubts about the Solid Rocket Booster joint seal. Had these matters been clearly stated and emphasized in the flight readiness process in terms reflecting the views of most of the Thiokol engineers and at least some of the Marshall engineers, it seems likely that the launch of 51-L might not have occurred when it did.

16. Post Challenger Presidential Commission Study The Commission is troubled by what appears to be a propensity of management at Marshall to contain potentially serious problems and to attempt to resolve them internally rather than communicate them forward. This tendency is altogether at odds with the need for Marshall to function as part of a system working toward successful flight missions, interfacing and communicating with the other parts of the system that work to the same end.

17. NASA Post Challenger Systems Review On March 13, 1986, NASA initiated a complete review of all Space Shuttle program failure modes and effects analyses (FEMEA's) and associated critical item lists (CIL's). Each Space Shuttle project element and associated prime contractor is conducting separate comprehensive reviews which will culminate in a program-wide review with the Space Shuttle program have been assigned as formal members of each of these review teams. All Criticality 1 and 1R critical item waivers have been cancelled. The teams are required to reassess and resubmit waivers in categories recommended for continued program applicability. Items which cannot be revalidated will be redesigned, qualified, and certified for flight. All Criticality 2 and 3 CIL's are being reviewed for reacceptance and proper categorization. This activity will culminate in a comprehensive final review with NASA Headquarters beginning in March 1987.

18. NASA Concerns with APU Hydrazine Fuel STS-9 (SpaceLab1)-Columbia-1983 Hydrazine fuel lines cracked on 2 APU�s while in-flight Formed Hydrazine snowballs Decomposed and exploded on landing at Edwards Air Force Base. Ripped holes in aft fuselage of orbiter Cracked Turbine Blades Had been noted by engineers after first orbiter flights.

19. APU Turbine Blade Failure Analysis Investigation began in late 1986 Budgeted at $12MM Identify root cause of blade cracking Evaluate reliability per NASA standards. If reliability is unacceptable, then redesign.

20. APU Turbine Blade Failure Analysis Companies Involved NASA Johnson Space Center, Houston, TX Marshall Flight Science Center, Huntsville, AL Rockwell International, Downey, CA Rocketdyne, Los Angeles, CA Sundstrand Aviation Engineering, Rockford IL Manufacturing, Denver, CO Southwest Research Institute, San Antonio, TX Rocket Research Company, Redmond, WA

21. APU Turbine Blade Failure Analysis Quarterly meeting with Sundstrand, Rockwell and NASA Pre-meetings to support quarterly meetings Specialists meetings between quarterly meetings Personal computers in infancy No internet and no Email Photo were Polaroid Presentations by overhead and 35mm slides No videoconferencing

22. APU Turbine Blade Failure Analysis Turbine disk details Forged Rene 41 nickel-base alloy Integral forged shaft

23. APU Turbine Blade Failure Analysis Turbine blade details Blades integral with disk Blade Passages ECM�d Blades Polished by Extrudahone Process Overhung Shroud Design

24. APU Turbine Blade Failure Analysis Turbine shroud details Continuous shroud Inconel 625 Shrunk fit Electron Beam welded

25. APU Turbine Blade Failure Analysis

26. APU Turbine Blade Failure Analysis Transverse cracks noted in blades Present on both 1st and 2nd stage sides Variable length, up to 0.090� long Approximately 3/8� from base of blade Array of cracks observed Longitudinal cracks noted at blade tips outboard of shroud Cracks do Not Trend with Running Time or Start/Stop

27. APU Turbine Blade Failure Analysis

28. APU Turbine Blade Failure Analysis Fracture surfaces characterized as crystallographic Origins near edges but not always at edges No evidence of crack arrest marks No evidence of fatigue striations

29. APU Turbine Blade Failure Analysis Metallographic examination confirmed crystallographic transgranular fracture mode White layer, after etching, observed along fracture and just in front of crack tip Up to 0.0005� Deep

30. APU Turbine Blade Failure Analysis Results presented at first joint meeting Hypotheses High cycle fatigue due Forced excitation Resonance Hydrogen embrittlement from decomposition of hydrazine Brittle cracks Nitriding due to decomposition of hydrazine

31. APU Turbine Blade Failure Analysis Further investigation No white etching layer present on blade surfaces Hardness indentations indicate white etching layer is softer than unaffected blade material TEM examination revealed that white etching layer which void of gamma prime precipitates

32. APU Turbine Blade Failure Analysis SouthWest Research Institute investigation Identified 3rd airfoil bending mode as cause of cracking Frequency approximately 85 KHz

33. APU Turbine Blade Failure Analysis In Fall of 1987 Teardown of APU #3 on Atlantis revealed a partial blade separation Flown on STS-51-J and STS-61-B Transverse crack and longitudinal crack linked up APU had accumulated 4.2 hours of operation

34. APU Turbine Blade Failure Analysis Nondestructive examination of all blades of all APU turbine disks flown in all Space Shuttles Automated fluorescent Penetrant Inspection Examined under stereomicroscope at 30X Stereomicroscope modified for digital camera Cracks identified and measured using NASA developed graphics software Digital images stored on mainframe computer

35. APU Turbine Blade Failure Analysis Southwest Research Institute Component flight reliability requirement .999 Reliability, 95% Confidence Weibull-Bayesian analysis Critical crack size of 0.125� Weibull shape parameter of �=6.683 Crack growth data from field returned APU turbine disks Analysis concluded that NASA reliability requirement was satisfied with a limit of 25 hours of operation per disk.

36. Postscript Space Shuttles began flying again with STS-26, Discovery on September 29, 1988 Original design APU turbine disk used with life limit of 25 hours New robust IAPU design initiated FPI of existing fleet APU blades replaced by automated Eddy Current Inspection Increased sensitivity

37. Postscript IAPU design completed Sharp blade edges eliminated Full width shroud incorporated Resonant condition eliminated 6% reduction in fuel consumption IAPU flies on Endeavor, STS-47, September 1992

38. Postscript NASA continued to investigate alternative APU designs that did not use hydrazine Electric Hydrogen-oxygen