Controlling the burn

The next two items in the PAD - 01696, plus 00584 - gave the crew information about the orbit that was expected to result from the burn. They indicated, in tenths of a nautical mile, the expected altitudes of the orbit's apolune and perilune using the 5-digit format of the computer's display. Therefore, they showed that the initial elliptical orbit should measure 169.6 by 58.4 nautical miles, or 314.1 by 108.2 kilometres. The plus sign was a heart-warming confirmation that the perilune was above ground. A negative value here would have been a cause for some worry, as it would mean that the spacecraft was headed for a point below the surface and was therefore doomed!

If the three delta-v components given earlier in the PAD were added together using vector addition, the result would be the total velocity change given in the 5-digit number - 30001. This vector sum totalled 3,000.1 feet per second (914.4 metres per second) and was labelled 'delta-vt'. It represented the total velocity change that the spacecraft should experience along its longitudinal axis. When it was time to execute this burn, the crew were able to watch as the computer's display showed this number descend to zero as the engine worked on the delta-v it had to achieve in all three components.

Next in this PAD was the expected duration of the burn - 641 - in this case, 6 minutes 41 seconds. The eventual duration depended on the actual weight of the spacecraft and whatever thrust was actually achieved by the engine. Within limits, the engine was not shut down until the required delta-v had been achieved.

Following on was another 5-digit number - 29939 - that was of a remarkably similar magnitude to the total delta-v, but slightly lower. Known to the engineers as 'delta-vc', this was related to one of two automatic mechanisms for shutting down the engine in a normal burn. The primary means was the computer in association with the accelerometers mounted on the guidance platform. As soon as the desired delta-v had been achieved, it sent a command for engine shutdown. The secondary means was the EMS and its delta-v function.2 Prior to the burn, the crew entered this delta-vc value into the EMS display. As with the primary system, this represented the delta-v the EMS should experience as the burn progressed. During the burn, the output from a dedicated accelerometer in the EMS measured the resultant change in velocity and caused the displayed delta-v to count down to zero. When it reached zero, the EMS generated a signal to shut down the SPS engine.

At first glance, it might appear that the value for the velocity change that was entered into both the primary and secondary systems should have been equal, but in fact delta-vc was always slightly smaller where the SPS was involved. This reflected the difference in the sophistication of the two control systems. When a rocket engine is commanded to shut down, the thrust never falls to zero instantly; there is always an appreciable extra thrust that tails off over a short span of time. The Apollo SPS was no different. The primary guidance and control system was sophisticated enough to take this tail-off thrust into account, its magnitude having been measured during ground tests and perhaps confirmed by earlier short firings of the engine en route to the Moon. The secondary system within the EMS was a much simpler affair and ignored the tail-off thrust, leaving it to the flight controllers to adjust down the value that they issued to the crew. If the EMS did have to shut the engine down, it did so early enough to allow the extra thrust to complete the desired manoeuvre.

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