Structural Inspections Find Enterprise in Better Than Expected Condition

by Chris Gebhardt

As the Space Shuttle Program prepares to enter its final phase next year, with Discovery, Atlantis, and Endeavour all decommissioned and prepared for display at various museums around the United States, preparations for a ferry flight of NASA’s first orbiter, OV-101 Enterprise, from her current home at the Smithsonian’s National Air and Space Museum in Chantilly, Virginia are underway – with preliminary results from an initial round of inspections on the test bed Orbiter showing her to be in “better than anticipated” condition.

Results of Phase I Inspections:

After completing her initial series of test objectives, including mated SCA/Orbiter ferry tests, five (5) free flights for the Approach and Landing Test program, mating ops validations and launch pad fit checksvalidations at the Kennedy Space Center, she was taken on a international tour in 1984 as part of the Louisiana World Exposition, before undergoing fit checks and pad validations at the ultimately never-used Vandenberg Shuttle Launch Facility in 1985.

Enterprise was ferried to Washington D.C. on November 18, 1985 where she officially became the property of the Smithsonian Institute.

From 1985 to 1988, Enterprise remained exposed to the elements in Washington before a non-environmentally controlled hanger was built around her.

Then, 18 years to the month after her arrival in Washington, Enterprise was towed to her new home – the environmentally controlled Udvar-Hazy center – in November 2003, where she has remained on display as the centerpiece of the Smithsonian’s air and space history collection ever since.

Now, with the pending retirement of the Space Shuttle fleet next year, the Smithsonian has stated its desire to replace Enterprise with the fleet’s flagship Orbiter Discovery – the workhorse of the Shuttle fleet with a projected 39 flights to her credit at the completion of the Program.

Initial plans had called for the Smithsonian to replace Enterprise with Orbiter Columbia (the first of the shuttle Orbiters to fly in space) given Columbia’s increasingly diminished role within the Orbiter fleet due in large part to her incompatible weight with ISS construction and mission objectives.

In fact, just prior to the loss of Columbia on February 1, 2003, NASA’s Flight Assignment Working Group only listed Columbia for two future missions, the original STS-118 ISS flight (later flown by Endeavour in 2007) and the original HST SM-4 flight (which was subsequently cancelled, reinstated, and eventually flown by Atlantis in May 2009).

However, the loss of Columbia led to the decision to end to the Space Shuttle Program following completion of ISS construction – a milestone now slated to occur in February 2011 barring the addition of the potential STS-135 mission to the Shuttle manifest, which would push Shuttle operations into June/July 2011.

As such, the Smithsonian altered their plans to include the procurement of Orbiter Discovery at the end of the Shuttle Program. As part of this plan, the Smithsonian has stated that they will loan orbiter Enterprise to another museum, thus driving the requirement to ferry OV-101 to a new location in the coming years.

*CLICK HERE for the March, 2010 article, outlining the opening evalutions on Enterprise*

This shipment of Enterprise from the NASM will entail mounting Enterprise on top of one of NASA’s two SCA (Shuttle Carrier Aircraft) for the first time in over 25 years and ferrying her to a new home.

As a result, structural inspections (conducted March 1-12, 2010) of Enterprise were required to “validate the health of the airframe and subsystem config/condition for potential ferry preparations,” notes an extensive presentation on Enterprise’s condition – available on L2.

As defined by the fleet-wide Orbiter OMRSD (Operational Maintenance Requirements and Specification Document), verification of OV-101 Enterprise’s structural capability for ferry flight was performed.

This included  “physical inspections” on the following areas: “all bolted OML (Outer Mold Line) penetrations, four hoist points, three SCA attach points, elevons, bodyflap, speedbrakes, wing root bolts, vertical tail bolts, FRCS (Forward Reaction Control System), tailcone, ALTA Pods, PLBD (Payload Bay Door) hinges, bulkhead latches, and centerline fittings, and landing gear trunnion load path.”

For the OML, a combination of visual and torque sampling was used during the inspection period. Likewise, NDEs (Non Destructive Evaluations) were used on the primary structural interfaces of Enterprise (four hoist points and three SCA attach points).

Furthermore, overall structural corrosion of Enterprise’s observable areas (including the wing and fuselage struts) were performed and compared to the previous corrosion inspection of OV-101 performed in 1996.

“Integrity of unique installations and previous elevon repairs” were verified as was the condition of a Main Landing Gear weld, which – if in violation of safety limits – would be a violation of Orbiter tow regulations.

In all, Enterprise was found to be in extremely good condition. As stated by the OV-101 Inspection Report, “OV-101 condition is much better than anticipated. NDE of critical interfaces shows no anomalies.”

Only 138 “discrepancies” were noted by the inspection teams: 39 instances of structural corrosion, 25 occurrences of ground damaged structures, 60 loose or missing non-flight fasteners, and 14 “other subsystem anomalies.”

Assessments to date show that 66 of these “discrepancies” will have to be repaired or replaced before Enterprise’s upcoming ferry, 20 will require further load testing and potential repair if load tests are unacceptable, 30 are acceptable as-is by STR PRT, 12 are acceptable as-is by Stress assessment, 3 are still undergoing Stress assessment, 3 require further inspection, and 4 “other.” (The presentation did not specify what “other” meant.)

Specifically, in terms of structural corrosion, “nearly all areas with corrosion potential were evaluated save [the] nose cap [and] WLE (Wing Leading Edge) and forward fuselage [due to] limited access,” notes the presentation.

In all, every area of observed corrosion was categorized and no instances of “severe” corrosion or corrosion areas that “cannot be made acceptable by analysis or repair” were identified.

All data has, nonetheless, been submitted to the “design center” for stress analysis.

So far, analyses have determined that “galvanic reaction between TCS and STR contributed to fwd fuselage corrosion.” Since this area was not easily accessible during the first round of ferry inspections, additional inspections will be require to further clear/repair this area for ferry operations.

Moreover, the discrepancies identified in Enterprise’s landing gear were generally minor and ranged from installed non-flight hardware to missing cotter pins or safety wires – all easily fixable.

However, three fairly significant findings on the Landing Gear will need to be addressed before ferry operations.

The first two pertain to the Right Main Lading Gear (RMLG).

“RMLG Uplock Jam nut broken loose,” notes the presentation. “Hard stop contact not flush with hook in up position” making it difficult to manually cycle the gear. The landing gear have to be stowed in an “up and locked” configuration for ferry ops.

The second issue with the Right Gear is the Inboard Strut Trunnion. “Loose hardware on bearing retainer plate [due to] suspect trunnion shimming [from] suspect bolt head contact during retraction.”

The third and final issue with the landing gear is on the Nose Landing Gear (NLG) and pertains to the bungee configuration. “NLG bungee configuration differs from flight vehicles,” notes the presentation.

The bungee in question is currently in the “fired” condition making it impossible to cycle the uplock hook without removing the drive linkage.

Additionally, in terms of Enterprise’s rigging condition, the uplock/door hooks were manually cycled to the “gear up” position, with a recommendation to compare the “gear up” position measurements against those on either Atlantis, Discovery, or Endeavour – a procedure that is currently “in work.”

Also, all hard stops and “uplock” stops were verified under a “no load” configuration for the MLG center stops and NLG forward and aft door hook stops.

Finally, the MLG struts currently installed on Enterprise have “never been retracted on OV-101” since their installation in 1997. The presentation notes that this may drive a requirement to “adjust uplock roller position” before ferrying operations.

Forward Work:

As a result of the first round of inspections, a “Forward Work Phase 1B” plan has been devised to complete inspections and analysis of Enterprise’s structural condition.

Under this plan, additional inspections of the forward and aft fuselage will be performed through “external access at multiple locations,” all NDE inspections with a custom probe of the forward SCA attach point will be completed, future landing gear retraction work will be performed, evaluations of the RMLG uplock/trunnion discrepancies will be worked (including a decision on whether or not the landing gear strut will have to be removed for trunnion repair), and hydraulic line and NLG bungee functionality evaluations will be performed.

Additionally, RMLG hydraulic tubing will be reinstalled to “restore plumbing to provide hydraulics to the inlet of the gear and strut actuators.”

In all, Enterprise is in extremely good condition considering her three year exposure to the elements and 10+ years in a non-environmentally controlled hanger. With all discrepancies either repairable or cleared, one of the only things that remain for Enterprise is a decision on what museum she will next call home.

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