Transfer Orbit Stage

The Transfer Orbit Stage (TOS) was an upper stage rocket developed by Martin Marietta on a subcontract to Orbital Sciences Corporation during the late 1980s and early 1990s. Orbital Sciences was the prime contractor for NASA for TOS. TOS was developed to be a lower-cost alternative to the very flexible but significantly expensive Inertial Upper Stage (IUS) or Titan Centaur.

TOS was composed of an Orbus 21 solid rocket motor with hydrazine-fueled thrusters for orbital maneuvering. TOS navigated via a Honeywell produced ring-laser gyro (RLG).

The TOS control system employed a digital optimal, time-scheduled control algorithm for stability during solid motor burn and a phase-plane controller for management of its reaction-control system (RCS). This provided exceptional stability during maneuvering.

Separation from the TOS was provided by Super-Zip connectors with the option for a spin-stabilized separation.

TOS participated in two missions during the program lifetime, both of which suffered mishaps:

• Delivery of the Advanced Communications Technology Satellite (ACTS) to orbit via the Space Shuttle, and
• Delivery of the Mars Observer (MO) space probe to Mars with launch provided by Commercial Titan.

During the ACTS mission, TOS suffered a misfire of its Super-Zip separation system such that both the primary and backup Super-Zip connectors were simultaneously fired. This threw shards into the lining of the Space Shuttle bay, causing safety concerns. The Space Shuttle successfully returned to Earth afterwards. For the Mars Observer mission, the boost to Mars and following Collision/Contamination Maneuver (CCAM) were as expected and the boost to Mars were either to or superior to the plans. However, the Mars Observer failed to contact the mission controllers after the transmitter was turned off for Mars Orbital Insertion (MOI).

After these two mishaps, no further TOS missions were booked and the program was eventually cancelled.