The Transfer Orbit Stage (TOS) was an upper stage developed by Martin Marietta

Martin Marietta

Martin Marietta Corporation was an American company founded in 1961 through the merger of The Martin Company and American-Marietta Corporation. The combined company became a leader in chemicals, aerospace, and electronics. In 1995, it merged with Lockheed Corporation to form Lockheed Martin. The...

 for Orbital Sciences Corporation

Orbital Sciences Corporation

Orbital Sciences Corporation is an American company which specializes in the manufacturing and launch of satellites. Its Launch Systems Group is heavily involved with missile defense launch systems...

 during the late 1980s and early 1990s. The TOS was designed to be a lower-cost alternative to Inertial Upper Stage

Inertial Upper Stage

The Inertial Upper Stage , originally known as the Interim Upper Stage, is a two-stage solid-fueled booster rocket developed by the U.S...

 and Centaur

Centaur (rocket stage)

Centaur is a rocket stage designed for use as the upper stage of space launch vehicles. Centaur boosts its satellite payload to geosynchronous orbit or, in the case of an interplanetary space probe, to or near to escape velocity...

 upper stages. The TOS was designed to be deployed by the Titan 34D

Titan 34D

The Titan 34D was an American rocket, used to launch a number of satellites for mostly military applications. After its retirement from military service, a small number were converted to the Commercial Titan III configuration, which included a stretched second stage, and a larger fairing...

, Commercial Titan III

Commercial Titan III

The Commercial Titan III, also known as CT-3 or CT-III was an American expendable launch system, developed by Martin Marietta during the late 1980s and flown four times during the early 1990s. It was derived from the Titan 34D, and was originally proposed as a medium-lift expendable launch system...

 and Space Shuttle

Space Shuttle

The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

.

The main propulsion system of the Transfer Orbit Stage was an Orbus 21 solid rocket motor. Attitude control was provided by hydrazine thrusters. The inertial guidance system used a ring laser gyroscope

Ring laser gyroscope

A ring laser gyroscope consists of a ring laser having two counter-propagating modes over the same path in order to detect rotation. It operates on the principle of the Sagnac effect which shifts the nulls of the internal standing wave pattern in response to angular rotation...

 produced by Honeywell

Honeywell

Honeywell International, Inc. is a major conglomerate company that produces a variety of consumer products, engineering services, and aerospace systems for a wide variety of customers, from private consumers to major corporations and governments....

. Its control system employed a digital optimal, time-scheduled control algorithm to provide stability whilst the Orbus 21 was burning, while a phase-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 the Mars Observer

Mars Observer

The Mars Observer spacecraft, also known as the Mars Geoscience/Climatology Orbiter, was a 1,018-kilogram robotic space probe launched by NASA on September 25, 1992 to study the Martian surface, atmosphere, climate and magnetic field...

 spacecraft bound for Mars

Mars

Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

. Despite a successful launch, the Mars Observer spacecraft later malfunctioned. The second use of a TOS was the deployment of the Advanced Communications Technology Satellite from on mission STS-51

STS-51

STS-51 was a Space Shuttle Discovery mission that launched the Advanced Communications Technology Satellite ACTS in September 1993. The flight also featured the deployment and retrieval of the SPAS-ORFEUS satellite and its IMAX camera, which captured spectacular footage of Discovery in space...

. This was also successful.

When deployed from the Space Shuttle

Space Shuttle

The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

 Super-Zip connectors were used to secure the Transfer Orbit Stage. It could be spun up before deployment if necessary. On STS-51, the primary and backup separation systems were erroneously fired simultaneously, resulting in minor damage to both the TOS, and the Shuttle's payload bay lining.