The Entry Pad A Worked Example

As was done before all major manoeuvres encountered during the flight, and with over 4 hours remaining to the landing, Capcom read up a PAD to the crew - a large list of numbers and notes that, in this case, defined the parameters of re-entry for the crew and the computer. This list included checks to be made of their attitude; and the times, angles and velocities to be expected at various points along their trajectory. Much of the information was to be fed into the computer and the EMS so that this equipment could be properly initialised prior to entry.

In April 1972, rookie astronaut and Capcom Henry Hartsfield made a call to Casper, the returning CSM of Apollo 16, commanded by John Young, to pass up a preliminary version of their re-entry details. They were only 4.5 hours away from splashdown yet they were still 63,000 kilometres out. "Apollo 16, Houston. Have an entry PAD for you."

"Okay. Go ahead with the PAD," replied Young.

Hartsfield then launched into the long, monotonous, yet precise string of digits and comments that would bring the crew safely to Earth. "Okay, MidPac; 000 153 000 2900632 267; minus 0071, minus 15618..." As with previous PADs, there was no punctuation. Confusion was only avoided by the pro forma sheet onto which the information was copied, along with the crew's experienced expectation of what each number was likely to be.

Hartsfield continued: "069 36196 650 10458 36276 2902332 0027 Noun

69 is N/A; 400 0202 0016 0333 0743; sextant star 25 1515 262; boresight N/A; lift vector UP. Use nonexit EMS; RET for 90K, 0606; RET mains, 0829; RET landing, 1321; constant-g entry, roll right; moonset, 2902026; EMS entry, reverse bank angle at 20,000 feet per second.''

Young then read it all back to Hartsfield to check he had made no mistakes. This is what the PAD meant:

The planned landing area was in the mid-Pacific Ocean - MidPac - as distinct from the Atlantic or Indian Ocean. With the Earth making a complete revolution once every 24 hours, it was straightforward to target the spacecraft to land in any of these bodies of water, just by altering the timing by a few hours. NASA was very particular in ensuring that everyone knew what the planned landing site was.

By the time of the re-entry, the guidance platform would already be aligned with an appropriate orientation, so the next set of digits, three groups of three - 000 degrees, 153 degrees, 000 degrees - defined the CM's attitude at the 0.05-g event with respect to that orientation. The careful choice of the platform's orientation is shown

lukar entry

Use non-exit EMS pattern.

M

I

D

p

A

C

area

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r 0.05 g f> o.os £ r o.os £

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1

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*

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RET 9 OK [2

9

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6

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get h0r f «

RET mains is 08:29.

RET landing is 13:21.

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-

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long

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kax g

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rtg0 ems vie

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rrt

Cons tant-g entry: roll right

Moonset: 290:20:26.

EMS entry.

Reverse bank

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ret 0.05 g

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0l 11« vl max vl win

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do ret «circ

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retbb0 seteb0 ret0r0

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5xts sft trn

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angle at 20,000 feet.

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sra 5xp

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U

P

lift vector

The PAD for Apollo 16's re-entry.

The PAD for Apollo 16's re-entry.

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