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TheTransfer Orbit Stage (TOS) was anupper stage developed byMartin Marietta forOrbital Sciences Corporation during the late 1980s and early 1990s. The TOS was designed to be a lower-cost alternative toInertial Upper Stage (IUS) andCentaur G upper stages. The TOS was designed to be deployed by theTitan 34D,Commercial Titan III andSpace Shuttle.
The main propulsion system of the Transfer Orbit Stage was anOrbus 21solid rocket motor, similar to the one used in the first stage of IUS. Attitude control was provided by hydrazine thrusters. Theinertial guidance system used aring laser gyroscope produced byHoneywell. Its control system employed a digital optimal, time-scheduled control algorithm to provide stability whilst the Orbus 21 was burning, while aphase-plane controller was used to manage its reaction-control system.
Only two Transfer Orbit Stages were launched. The first was launched on 25 September 1992, aboard a Commercial Titan III with theMars Observer spacecraft bound forMars. Despite a successful launch, the Mars Observer spacecraft later malfunctioned. The second use of a TOS was the deployment of theAdvanced Communications Technology Satellite fromSpace Shuttle Discovery on missionSTS-51. This was also successful.[1]
The Transfer Orbit Stage was designed to separate from theSpace Shuttle via pyrotechnic devices enclosed in a frangible metal joint known as a Super*Zip. On STS-51, the primary and backup pyrotechnic devices inside the Super*Zip were erroneously fired simultaneously, resulting in minor damage to both the TOS, and the Shuttle's payload bay lining. However, the ACTS satellite deployed successfully and functioned normally on orbit. The Shuttle landed safely despite the anomaly.