Transfer Orbit Stage

The main propulsion system of the Transfer Orbit Stage was an Orbus 21 solid rocket motor.

The inertial guidance system used a ring laser gyroscope produced by Honeywell.

The second use of a TOS was the deployment of the Advanced Communications Technology Satellite from Space Shuttle Discovery on mission STS-51.

[1] The Transfer Orbit Stage was designed to separate from the Space 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.

TOS with ACTS, seen from STS-51
The planned trajectory of Mars Observer in this diagram also illustrates the purpose of the Transfer Orbit Stage. After the Commercial Titan III had transported on its upper stage the TOS and satellite into low Earth orbit, the fairing separated and the Transfer Orbit Stage accelerated the satellite in the direction of Mars .
The preparation of Mars Observer and the Transfer Orbit Stage (TOS) before their integration with the upper stage of the Commercial Titan III