UNDOCKING

To begin the process of splitting the two spacecraft, the electrical umbilical between them had to be disconnected within the tunnel and the docking mechanism put back in place. Two other umbilicals were reeonneeted to the docking equipment to pass telemetry and commands to and from the probe and to supply pow? er to operate its retraction mechanism. Then while the LM crew closed the hatch at the Lop of their spacecraft, 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 Lo undocking, thereby removing the primary means by w’hich the tw’o spacecraft w’ere 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 at its tip, 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 these latches. When the main latches were released, the capture latches would have to hold against the air pressure that would try to push 34 tonnes of spacecraft apart hence the need for the CMP to be wearing his spacesuit. 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 axis – roll. If the thrusters of the CSM were to impart a rolling motion to the stack, the force would be transmitted to the LM primarily 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 re­cocked them, ready to engage again when the LM returned to dock after its journey to the surface. The CMP then reinstalled the hatch at the apex of the command module. Only when he had checked that the air pressure in his 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, with the stack’s long axis towards the Moon’s centre. 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.

Undocking w’as 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. Coordinating the undocking with the event timer helped the crew to accurately run through a time-dependent sequence, 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 /его, a switch that controlled the extension and retraction of the probe was momentarily pushed up to finally execute the undocking.

There were two procedures available to undock the spacecraft and it depended on the precise operation of the switch that extended the probe to decide which one was used. The switch had a momentary action which had two effects: it sent a command to the probe to fully extend, w hich it did regardless of how long the switch was held for; it also caused the probe to pull in the capture latches thus disengaging them from the drogue, but only for the duration of the swatch action. Therefore, to achieve a simple undock merely required that 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 and the LM would sail away.

The preferred method, however, was the ‘soft undock’ for w hich the extend switch was held for only a short period. Although this fully extended the probe, it allowed the capture hitches to re-engage with the drogue so that the LM would 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 CSM would then complete the separation by controlled firings of its 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 which poked through the hole in the centre of the drogue. In either case, the respective cabin would have had to have been depressurised and the corresponding hatch opened to allow access.

Undocking generally occurred shortly after the spacecraft came 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. David 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 Л1 Worden had pushed the extend switch, neither the latches nor the probe extension had operated. The suspicion that the probe umbilicals were not properly connected was confirmed by Mitchell’s next message.

”Falcon, Houston. We have no probe temperature 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 in order to gain 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, separate the craft and 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. 1 presume that may have been our problem.” A new separation attitude was sent to the crew to reschedule the event for 26 minutes later.