Command module RCS

Having heated the command module's thrusters, the rest of its reaction control system could be primed, ready for use.

Throughout the mission, all small-scale manoeuvres were carried out using the RCS thrusters on the service module. The command module's RCS was a simpler system primarily because there was no need to perform translation manoeuvres. It controlled the command module as re-entry began, presenting it to the atmosphere in its naturally stable, aft-first attitude until aerodynamic forces took over. Then once the CM was in the atmosphere, it performed the roll manoeuvres that would steer the spacecraft to an accurate landing while simultaneously damping any excessive motions in the pitch and yaw axes. It comprised two entirely independent systems or 'rings' to provide complete redundancy, each of which had a fuel tank and an oxidiser tank mounted in the space around the periphery of the CM. These supplied propellants to six engines recessed into the outer hull of the spacecraft - a total of 12 engines. For the majority of the mission, these tanks remained unpressurised and the system maintained in an inert state.

Now as they neared re-entry, pyrotechnic squibs were detonated, opening up valves to feed pressurising gas from small high-pressure helium tanks to the fuel and oxidiser tanks. Like the RCS systems on the service and lunar modules, the tanks were not pressurised by simply pumping high-pressure gas into them; the helium did not come into contact with the propellant within. In a weightless environment, blobs of liquid in a simple tank are liable to float about with voids in between that could allow gas to enter the propellant lines to the engines. Instead, the propellant was isolated within a Teflon bladder inside the tank, while helium gas occupied the space around it, squeezing the bladder and forcing propellant through the feed lines to the thrusters. This technique prevented helium bubbles from entering the engines even when the propellant was weightless.

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