Reaction Control System - Location of Thrusters On Space Capsules

Location of Thrusters On Space Capsules

Two Apollo spacecraft (the Service Module and the Lunar Module) had translation thrusters grouped into external blocks of four, which served to translate and orient the spacecraft. Other designs used separate sets of thrusters for these two tasks. The Apollo thrusters were configured to allow "coupled" RCS firings (where thrusters on opposite sides of the spacecraft fired together), which allowed adjusting the vehicle attitude without affecting the critical accuracy of their orbital, trans-lunar and trans-earth trajectories.

The Mercury and Gemini spacecraft each had groupings of two nozzles inserted into their forward compartments, with slots cut out from which the exhaust could escape. These thrusters were used for orientation, not translation. (Indeed, the Mercury spacecraft had no separate capacity for translation at all.) Similarly, the command modules of both the Apollo and Soyuz spacecraft have their re-entry RCS thrusters ungrouped.

Gemini, due to its relatively low mass, was able to change its orbit using its thrusters, and did not require an engine (unlike its heavier descendants).

A pair of translation thrusters are located at the rear of both the Gemini and Soyuz spacecraft; the counter-acting thrusters are similarly paired in the middle of each spacecraft (near the center of mass) pointing outwards and forward. These act in pairs to prevent the spacecraft from rotating. The thrusters for the lateral directions are mounted close to the center of mass of each of these spacecraft as well, but Gemini has only one engine for each of the directions while Soyuz again uses a pair.

None of these engines is intended for orientation. For that purpose, both Gemini and Soyuz have engines at the extreme rear of the spacecraft. Here Soyuz uses engines only one-tenth the power of the others.

The placement of the translation thrusters (which are used to alter the spacecraft's velocity) has one important requirement that the placement of the orientation thrusters (which are used to rotate and orient the spacecraft) does not: if the direction of thrust of the translation thrusters does not pass through the center of mass of the spacecraft (when tracked backward from the nozzle) the spacecraft will rotate — an unwanted side effect. Current and past spacecraft are not operated by automatically firing the orientation thrusters to counteract this rotation because such a system might fail, so manual re-orientation is required afterward. Because of these constraints, translation thrusters can generally be placed in fewer locations than orientation thrusters.

Finally, Soyuz has a thruster at the rear of the spacecraft that points parallel to each solar panel. This thruster is used for orientation, but has the unique application of keeping the spacecraft's solar panels pointing towards the sun. Without this thruster, a computer system would have to keep the panels properly aligned, wasting electricity. The spin is dampened by a counterpart thruster on the other side.