Attitude checks

Continuing to the bottom of the LOI PAD form, Capcom Karl Henize read, "25, 2671, 228; the rest is NA." This refers to six lines on the form that were concerned with two methods of double checking to ensure that the spacecraft was in the proper attitude for the burn. On this PAD for Apollo 15, only the first was brought into play; it exploited the fact that the spacecraft's sextant could be aimed precisely at the stars. If the spacecraft had been placed in the correct attitude for the burn, then flight

2 The EMS or entry monitor system was discussed in Chapter 5 where we saw how its ability to measure velocity change could be used for manoeuvres to retrieve the LM from the S-IVB. It will be discussed further in Chapter 14, where we shall see how it is used for its prime purpose, monitoring re-entry into Earth's atmosphere.

controllers had calculated that a particular star should be visible through the sextant when its shaft and trunnion angles were set to specific values. In this case, the crew were to expect Star 25, which is commonly known as Acrux, in the constellation Crux, to be visible in the sextant when the shaft and trunnion angles were preset to 267.1 and 22.8 degrees respectively.

Henize indicated that nothing else need be entered on the form by pronouncing it as 'NA' for 'not applicable'. What was skipped was the boresight star method. This used the crew optical alignment sight (COAS) - a unit with an illuminated graticule similar to a gunsight that could be mounted in a window and whose aim could be calibrated. It was not required for LOI because all the windows would be facing the Moon.

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