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STS-87 Close Call Investigation Board addressed the SPARTAN spacecraft failure to execute its pirouette maneuver and the failure to re-grapple the SPARTAN following the failure to pirouette. The failure to re-grapple investigation included both the initial tipoff of the SPARTAN by the remote manipulator system (RMS), and the failed rate matching efforts by the orbiter following the tip-off. Not included in the investigation were the separation, rendezvous, and EVA capture.
STS-87 was a 16 day flight with six crew members. The primary payloads were the United States Microgravity Payload (USMP) and the SPARTAN-201 satellite. Additionally, there was a planned Extra-Vehicular Activity (EVA). SPARTAN-201 had flown three times before (STS-56, 64, and 69) without problems except for some difficulty in berthing, due to tight clearances within the release/engage mechanism (REM). Other SPARTAN missions had been flown on the shuttle (STS-63, 72, and 77) with different scientific instruments and payload objectives. The original flight plan was to deploy SPARTAN on flight day 2 and retrieve it two days later. Due to problems with an independent cooperative satellite (SOHO), the SPARTAN deploy was rescheduled to flight day 3 (November 21, 1997).
On flight day 3, a new flight plan was uplinked and executed by the crew. The final command required to prepare SPARTAN for deployed operations was a crew input via the Payload and General Support Computer (PGSC.) This input was not received by the spacecraft. Lack of telemetry and onboard verification procedures left this condition undetected by MCC and the flight crew. The Board's conclusion based on all the evidence available is that the crew inadvertently omitted the SPARTAN Standby step. SPARTAN was grappled with the Remote Manipulator System (RMS), removed from the Release/Engage Mechanism (REM), and released per the flight plan. The missed command step resulted in the failure of the SPARTAN to execute an expected preprogrammed maneuver ("pirouette") about 2.5 minutes after deploy.
In accordance with the flight rules, and with MCC's concurrence, the crew attempted to re-grapple the SPARTAN satellite. In the process, the RMS operator executed a premature capture, followed by a release in close proximity to SPARTAN, while continuing to drive the RMS towards the grapple fixture. At some point very close to the time the release command was sent, analysis indicates the RMS End Effector snare wires contacted the grapple fixture pin and induced rotational and translational rates into the SPARTAN. A few seconds later, the RMS operator elected to back away rather than attempt capture.
The induced rotational rates (about 2 degrees per second) were judged too large for trying an immediate RMS re-grapple, and the crew attempted to match rotational rates with the SPARTAN spacecraft using the orbiter. Due to its inertial characteristics, lack of an engaged attitude control system, and the tip-off rates, SPARTAN began a 3 axis rotation which made it essentially impossible for the orbiter to match consistently. However, the CDR judged that the rates were low enough that the task was possible. The flight control team established a propellant use limit that protected the remainder of the flight objectives, including retrieval of the SPARTAN satellite. MCC called off the rate matching attempts when that limit was reached. The crew then executed two separation maneuvers, the second to correct residuals from the first. Post flight analysis has indicated there existed an unknown risk of re-contact with SPARTAN caused by substantial attitude control maneuver cross-coupling into translations.
During this rate matching attempt, additional rotational rates were imparted to the SPARTAN by inadvertent RCS jet plume impingement. These were of sufficient magnitude (> 3.8 degrees per second) to cause the SPARTAN to invoke its Minimum Reserve Shutdown mode which, over several orbits, reduced the rates to approximately zero.
A re-rendezvous was planned and executed two days later. The SPARTAN was captured by two EVA crew members and stowed in the REM with the help of the RMS. Re-deployment of SPARTAN was considered later in the mission; however, Space Shuttle Program management concluded insufficient Orbiter RCS propellant remained to guarantee retrieval of the spacecraft.
Principal recommendations of the Board include:
(1) Apply Cockpit Resource Management (CRM) techniques for critical on-orbit activities such as deploys and other RMS operations.
(2) Improve the crew interface to the PGSC/SPARTAN software.
(3) Improve payload training accountability.
(4) Improve assessment and thoroughness of crew member retention of critical training objectives.
(5) Develop and implement a final SPARTAN "Ready for Deploy" verification.
(6) Revisit safety analysis to assess expanding or removing SPARTAN 1-hour constraint.
(7) Re-examine the objectives mix in RMS training; must increase the fail-to-capture element.
(8) Improve range-to-grapple information available to the RMS operator.
(9) Establish rate matching flying techniques, procedures, and Flight Rules and then train to them.