Entry REFSMMAT

A brief recap of the REFSMMAT might now be appropriate. The computer’s idea of the direction in which the spacecraft was pointed was always given with respect to the orientation of the stabilised platform inside the IMU. However, to make any sense, the platform itself had to be aligned to some known reference – one that was related to the universe around the spacecraft. The REFSMMAT numbers defined such an orientation in space to which the guidance platform could be aligned. Flight controllers could choose this orientation arbitrarily to suit the mission’s operational needs.

For re-entry, an orientation was chosen that would help the crew to make sense of their 8-ball displays, essentially turning them into artificial horizons that would show attitude relative to the ground below. It was based on the point of entry interface, whereby the v axis was aligned along the azimuth of their flight path but parallel to the horizontal plane. The г axis was parallel to a vertical line at the point of entry interface. In other words, at entry interface, the г axis would be pointing towards the Earth’s centre. By default, the у axis was aimed to the right of the flight path. The upshot of this arrangement was that at entry interface, if the heatshield was presented exactly forward and the crew heads-down, the FDAI display would read 0 degrees for roll, 180 degrees for pitch and 0 degrees for yaw. In this way, the display was much easier to interpret. Of course, for re-entry, the heatshield did not face exactly forward but was tilted towards Earth so that the FDAI would display 153*.

Returning to the entry PAD, the next two items were related to the first of many checks of their attitude and trajectory. This check would be made at 290 hours 06 minutes 32 seconds into the mission, 17 minutes prior to hitting the atmosphere. It did not require any fancy instru­ments. All that was required was for the crewman in the left couch, in this case CMP Ken Mattingly, to look out of the rendezvous window in front of him and see if Earth’s horizon aligned with a set of angle marks inscribed on the window. The spacecraft’s pitch angle at this time should be 267 degrees. A leeway of 5 degrees was allowed.

The next two items referred to the latitude and longitude of the planned point of splashdown.

Apollo 16 was targeted to land 00.71 degrees south of the equator
and 156.18 degrees west, a point about 2,200 kilometres south of Hawaii, near Christmas Island. In the event, it landed about 4.5 kilometres west of this point.

Even though the approach into the atmosphere was quite shallow, the crews had to sustain quite high deceleration forces as the CM rammed into the air. Most returning crews endured a peak force of about six g. The Apollo 16 crew were warned to expect a peak force of 06.9 g. Records show’ that their deceleration peaked at 7.19 g, the highest for any Apollo crew.

When the spacecraft reached entry interface, the arbitrary altitude where re-entry was said to begin, it was expected to be at the extraordinary velocity of 36,196 feet per second or slightly over 11 kilometres per second (11.033 kilometres per second to be exact). At this point. Retro believed the flight path would form an angle of 6.50 degrees to the horizontal which was considered ideal. Subsequent analysis showed that the flight path angle was actually 6.55 degrees, well within the 1-degree tolerance allowed for re-entry.

The spacecraft was expected to travel 1,045.8 nautical miles from the time the guidance system sensed 0.05-g switch until landing on the ocean, a distance of nearly

2.0 kilometres.

Another velocity value of 36,276 represented, in feet per second, how fast they believed the spacecraft w’ould be travelling when the 0.05-g point was reached. As well as going into the computer, this number and the previous item went into the EMS, which allowed them to choose to monitor either velocity to be lost or distance still to travel. Note that the value given here was slightly higher than that quoted for entry interface. This demonstrated that mission control expected the spacecraft to gather yet more speed between the arbitrary point of entry interface and the estimated 0.05-g event.

Mission control expected the spacecraft to reach entry interface at a time of 290:23:32 since launch. It was then expected that 27 seconds would elapse between entry interface and the 0.05-g event being sensed. The actual time would depend on the local atmospheric conditions.

The next item concerned a computer entry, Noun 69, that was not applicable, or. Y A to this PAD. As entry commenced, the computer ran Program 64 and its display was set to show’ the contents of Noun 74. This had three numbers derived from the primary guidance system which told the crew: (a) how much drag they were experiencing (i. e. the g-force to which re-entry was subjecting them), (b) their current inertial velocity, not taking into account the rotation of the Earth, and (c) the angle their flight path made with the horizon. If the entry profile required the spacecraft to skip out of the atmosphere for a time, the computer was programmed to move to Program 65, whereupon the display w’ould show Noun 69. which again displayed drag and velocity, this Lime relating to the skip-out flight path. The PAD form had spaces in which mission control could advise the crew of the expected maximum and minimum values for drag and velocity for this phase of the entry, but on Apollo 16 these w’ere not needed. Of all the Apollos that returned from the Moon, only Apollo 11 had to deal with this skip-out condition and that was to extend their flight path and avoid a storm.

During part of the entry, the spacecraft was flown in a trajectory that

maintained a constant, g-force. In all cases, this was set at 4.00 g. As the spacecraft continued to slow, there came a point when its velocity was equal to that of an orbiting object. In other words, on reaching this speed, the spacecraft no longer had enough momentum to return to space on a long-duration elliptical orbit that would have threatened the crew. Also, since the spacecraft would be in the atmosphere already at this point, drag would continue to reduce its velocity, and a landing somewhere on Earth was assured. The next item in the PAD informed the crew that they were expected to reach this safe milestone 2 minutes 2 seconds after entry interface.

The extreme heat generated by the shock of compression ionised the air and created a sheath of plasma that effectively blocked radio signals to and from the spacecraft. This period, known as blackout, was expected to begin at 00:16 or 16 seconds after entry interface and continue until 3 minutes 33 seconds after entry interface.

Once the spacecraft had shed most of its speed, the Earth landing system (ELS) would convert it from a 6-tonne lump falling through the atmosphere to a gently descending vehicle returning explorers from the Moon. The first part of this system was a pair of drogue chutes which the Apollo 16 crew could expect to be deployed 7 minutes 43 seconds after entry interface.

The Apollo 16 entry PAD continued with six further items that were all concerned with two methods of ensuring that the command module’s attitude was correct just prior to re-entry. The first of these would use the sextant to aim at a particular star. The second, not used on Apollo 16, was a similar thing using the COAS mounted in a window. Apollo 16’s sextant star check said that two minutes prior to entry interface, the crew should expect to see sextant star 2.5, Acrux – which is the major star in the southern constellation Crux – through the eyepiece when their sextant was aimed to a shaft angle of 151.5 degrees and a trunnion angle of 26.2 degrees.

The final entry on the form used for the PAD told the crew that the lift force generated by the shape of the hurtling spacecraft would be ‘up’ at least at entry interface. This was another way of saying that they should be in a ‘fcci-up, hcads – down’ attitude as they approach the atmosphere. This information was passed on to the computer by entering a ‘1′ in the right place in its memory.

A number of additional comments were read to the crew at the end of the PAD. The first concerned the scroll pattern to be used to monitor the entry on the EMS. The appropriate pattern was one that was calibrated for the type of non-exiting re­entry they planned to make. In other words, since there was no intention to exit the atmosphere on a long, skipping re-entry, they should line up the scroll pattern at the start of the non-exit EMS pattern.

Next were three Limes, stated with respect to entry interface, for important milestones in the latter part of the flight. The first {RET for 90K) was for the command module’s descent to an altitude of 90,000 feet, or 27.4 kilometres, which was expected 6 minutes 6 seconds after entry interface. The spacecraft’s tltree main parachutes (RET mains) were expected to be deployed 8 minutes 29 seconds after entry interface while the entire re-entry, from entry interface to splashdown {RET landing) on the Pacific Ocean, was expected to take 13 minutes 21 seconds.

During the constant-g entry phase of the flight, the spacecraft had to roll to aim the lift force it generated up or down to maintain a constant deceleration. Mission control included a note to say that if the crew needed to do this manually, perhaps through equipment failure, they should roll right.

As the crew returned from their lunar exploration, the view out of their windows afforded them a view’ of the Moon setting behind Earth’s horizon. The precise time it did so w-as calculated to the second and mission control informed the crew they could expect Moonset at 290:20:26 – that is. at 290 hours 20 minutes 26 seconds into the mission. This item was more of a confidence booster. If Moonset didn’t occur on time, there was nothing they could do about it. The CM was on its own and had no propulsion to correct any error in their trajectory prior to entry.

The final item concerned a detail of the re-entry in ease the crew had to fly the profile manually. Manual re-entry w’as flown by following the cues provided by the EMS (EMS entry) with which the CMP controlled the g-forces as the spacecraft’s velocity fell. Mission control’s note was to reverse their angle of bank as their velocity passed 20.000 feet per second.

After their flights, crews were often asked to talk about the non-scicntifie aspects of flying to the Moon; the meaning, symbolism and the raw human emotion behind it. Typical replies have said that they were too busy for such things. Some have suggested that for such answers. NASA should have sent artists and poets. A few crewmen have pointed out that had artists and poets comprised the crews, they would never have returned alive. The sheer degree of detail involved in flying the Apollo spacecraft explains why NASA preferred highly proficient pilots as astronauts.