Undocking

To begin the process of splitting the two spacecraft, the electrical umbilical that connected them was disconnected within the tunnel, and the probe and drogue docking mechanism put back in place. Two other umbilicals were reconnected to the docking equipment to pass telemetry and commands to and from the probe and to supply power to fire its retraction mechanism. The LM hatch was installed by the LM crew as the CMP put on his helmet and gloves, a safety measure for the next task of preloading the probe.

Up to that point, the two spacecraft had been held together by the 12 docking latches that gripped across the two docking rings and their seals. These latches, however, had to be manually released prior to undocking, thereby removing the primary means by which the two spacecraft were joined. Therefore, to prevent the spacecraft from being pushed apart by the cabin air pressure, the CMP extended the probe to engage the three capture latches, each the size of a thumbnail, with the rim of the hole at the centre of the LM's drogue. The probe was then tensioned to firmly engage the latches against the air pressure that would try to push apart the two vehicles, with a total mass of nearly 34 tonnes, when the main latches were released -hence the need for the CMP to be wearing his space suit. Before any of this, however, he had to disable some thrusters.

The strength of the probe was more than adequate to hold the spacecraft, except in one direction - roll. If the thrusters of the CSM were to impart a rolling motion to the stack, the force would be primarily transmitted to the LM through the probe arms and the little capture latches, subjecting them to dangerous shear. At this point, therefore, the CSM was inhibited from firing its roll thrusters. Once the probe was tensioned, it was safe to release all 12 docking latches - an operation that also re-cocked them, ready to engage again when the LM returned to dock after its journey to the surface. The CMP then installed the hatch at the apex of the command module and checked its integrity. Only when the CM cabin was secure, could he remove his helmet and gloves.

As with many operations on board Apollo, the procedures surrounding undocking and separation were carefully choreographed. Undocking was always carried out at a specific attitude and at a specific time. Planners wanted the stack to be oriented with the CSM towards the Moon and the LM away from it. An attitude was given in the flight plan for the event and the stack was manoeuvred to this attitude some minutes prior to the undocking. Being in an inertial attitude, the stack would reach the correct orientation with respect to the Moon at a specific time, and this would be the moment of undocking.

Coordinating the undocking with the event timer helped the crew to run through a time-dependent sequence accurately, as so often was the case for major mission events. With 30 seconds to go, the CMP set the EMS to monitor changes in velocity and started the movie camera. At zero, a switch that controlled the extension and retraction of the probe was momentarily pushed up to execute the undocking. Undocking was only ever carried out once during a normal mission. The second time the LM departed, it was actually cut free, along with the tunnel and all the docking equipment - a very final event that disposed of the ascent stage at the end of its mission. The two procedures available to achieve the undocking were based on how the switch that extended the probe was operated. The momentary action of this switch had two effects: it commanded the probe to fully extend, regardless of how long it was held for; it also caused the probe to pull in the capture latches for the duration of the switch action, thus disengaging them from the drogue. Achieving a simple undock therefore merely required the switch to be held closed for the length of time it took the articulated probe to extend, so that when it reached its full 25-centimetre extension, the latches would still be disengaged, allowing the LM to sail away.

The preferred method, however, was the 'soft undock' for which the extend switch was only held for a short period. Although this fully extended the probe, it allowed the capture latches to re-engage with the drogue, causing the LM to be held at the end of the fully extended probe. This method minimised unintended LM velocity with respect to the CSM. Once the motions between the two vehicles had stabilised, the latches were released by cycling the extend switch once more. The separation could then be completed by controlled firings of the RCS thrusters.

If the electrical command to release the capture latches were to fail, the probe included arrangements to allow a suited crewman to manually release them from either side of the tunnel: either the CMP could pull a handle from the CM side or a LM crewmember could access a button in the centre of the probe tip, poking through the hole in the centre of the drogue. In either case, the respective cabin would have

Antares, the Apollo 14 LM recedes from Stu Roosa in the CSM Kitty Hawk.

had to have been depressurised and the corresponding hatch removed to allow access.

Undocking generally occurred over the eastern limb of the Moon just prior to the spacecraft coming back into view of the Earth. When Apollo 15 reappeared after its planned undocking and separation, Ed Mitchell in mission control enquired how it had gone. Dave Scott didn't have good news.

"Okay, Houston; this is the Falcon. We didn't get a Sep, and Al's been checking the umbilicals down on the probe.'' When Al Worden had pushed the extend switch, neither the latches nor the probe extension had operated. The suspicion that the probe umbilicals were not connecting properly was confirmed by Mitchell's next message.

"Falcon, Houston. We have no probe temp[erature data], which indicates the umbilical is probably not well connected.''

''Okay. Well, that's just what he's checking,'' Scott informed. Worden had removed the forward hatch to give him access to the plugs and sockets of the probe umbilicals within the tunnel. Scott realised the danger in the situation and checked that Worden was aware of it also. ''Hey, Al, I hope you made sure the extend/ release switch was off when you went in there.'' Scott's fear was that if the switch to extend the probe had been placed in the 'on' position, and with the docking latches released, then when Worden reconnected the umbilical, the probe would immediately extend, separating the craft and, with the CM forward hatch removed, evacuate the cabin.

As soon as Worden had reseated the plugs in their sockets, mission control saw their telemetry change. ''Apollo 15, Houston. We're seeing the telemetry on the probe now. I presume that may have been our problem.'' A new separation attitude was sent to the crew, rescheduling the event for 26 minutes later.

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