Apollo 13

Apollo 13's long return home is almost over. With four and a half hours to go before their expected splashdown, the time to return to Command Module Odyssey is at hand. They have just released the Service Module and witnessed the evidence of the catastrophic destruction within its innards caused by the oxygen tank explosion.

Noun 46 sets up their DAP routine as 31021 - docked configuration, 2 jets on RCS system B, fine scale (4 degrees/sec) large deadband (5 degrees) and .5 degrees rate. Noun 47 gives the weights (or masses) of the Command Module and the Lunar Module in pounds. With the Service Module gone, the computer needs to readjust its firing commands based on the new weight distribution and center of mass in the docked stack.

Comm break.

As NASA and government officials file in to wait for the splashdown, the ship is speeding 63,616 km distant from Earth, and moving at 3,233 m/s.

Jack has now powered the Command Module Computer, as per their reactivation checklist.

Remarkably, the computer's warning lights are not powered, while the computer is already running. This spares a little bit of power by not needing the AC inverter on yet.

It sounds like Jack has tried the procedure on both at his station at the controls and at the Lower Equipment Bay, where the second DSKY is located by the optics.

Comm break.

Jack is talking about Verb 50 Noun 25, option 00062, which should put the computer into the standby mode.

Comm break.

Distance to go, 61,799 km; velocity, 3,279 m/s.

Long comm break.

Long comm break.

It appears that they are drawing just a little more power than would be to their liking, and want Jack to see if the Command Module is configured as per the checklist.

Main A is already powered from the Command Module's own batteries, while Main B still receives power via the umbilicals from the Lunar Module. They obviously want to save all of the CM battery power now that they still can do so.

The Command Module's RCS jets are the prime suspect for the moment as the source of their power drain. Turning them off didn't improve the situation, however.

Two moments are of particular importance when understanding an Apollo Entry PAD. The first is Entry Interface, an arbitrary altitude of 400,000 feet or 121.92 km, a figure that is used when planning the trajectory. The second is the moment when the accelerator within the EMS (Entry Monitor System) detects a deceleration of 0.05 g (one-twentieth of a g). This event is used as a trigger fo the computer's entry software to change for an initialisation program to one that begins to control the spacecraft's passage through the upper atmosphere. The event also starts the EMS's monitoring function, giving the crew displays that let them know how the automatic entry is proceeding, and if it fails, allowing them to fly the entry manually. The entry PAD is interpreted as follows:
Purpose: Reentry from trans-Earth trajectory
Landing target: The landing target is the Mid-Pacific Landing Area, which is their primary recovery area.
IMU gimbal angles required for trim at 0.05g: Roll, 000°; pitch, 153°; yaw, 000°.
Time of the Moon check: 142 hours, 38 minutes, 17 seconds GET.
Spacecraft pitch at Moon check: 178°.
Estimated Splashdown point: minus 21.66° latitude, minus 165.37° longitude.
Maximum number of g's during entry: 6.7.
Velocity at Entry Interface (400,000 feet altitude): 36,211 feet/second ( meters/second).
Entry flight path angle at Entry Interface: 6.51°.
Range to go to splashdown point from 0.05g event: 1,168.9 nautical miles. To set up their EMS before re-entry, the crew need to know the expected distance the CM would travel from the 0.05 g event to landing. This figure will be decremented by the EMS based on signals from its own accelerometer.
Predicted inertial velocity at 0.05g event: 36,292 feet/second. This is another entry for the EMS. It is entered into the unit's Delta-V counter and will be decremented based on signals from its own accelerometer.
Time of Entry Interface: 142 hours, 40 minutes, 40 seconds GET.
Time from Entry Interface to 0.05g event: 00:28 (seconds).
The next four items are not applicable as they are only used then a skip-out re-entry is being flown.
Planned drag level (deceleration) during the constant g phase: 4.00g.
Time from Entry Interface until their velocity slows sufficiently to allow a circular orbit around the Earth: 2 minutes, 4 seconds.
The practical implication of this is that this is the "capture point" where the CM will no longer have sufficient kinetic energy to make an orbit of Earth, should it exit the atmosphere. Since the spacecraft will already be within the Earth's sensible atmosphere at this point, drag will continue to slow the spacecraft and the return to Earth is assured.
Time from Entry Interface that the communications blackout begins: 00:17.
Time from Entry Interface that the communications blackout ends: 03:22.
Time from Entry Interface that the drogue parachutes will deploy: 08:14.
Sextant star: 33 (Antares, Alpha Scorpii)
Sextant shaft angle at Entry Interface minus 2 minutes: 353.1°.
Sextant trunnion angle at Entry Interface minus 2 minutes: 29.9°.
The next three items refer to an attitude check made using the COAS sighted on a star two minutes before Entry Interface.
Boresight star: Sigma Libra.
Boresight Star pitch angle on COAS: Up 08.8°.
Boresight Star X position on COAS: Left 0.4°.
Lift vector at Entry Interface: Up. Since the direction of the lift vector is towards the crew's feet, they will re-enter in a heads-down attitude.
GDC Align stars: The stars to be used for GDC align purposes are 31 Arcturus and 33 Antares. The align angles are roll, 41°; pitch, 245°; yaw, 24°.
Comments in addition to the PAD:EMS non-exit pattern will be used. Maintain mooncheck attitude until moonset. Constant g entry is performed by maintaining roll to right.

A most interesting note that Kerwin makes in his report is the mention of the 'constant g backup reentry area'. This is a theoretical target point for a landing made with a very rudimentary backup reentry procedure, where the crew will use only the accelerometer data to perform a manual landing. By maintaining constant g - approximately 4 g's, they are sure to decelerate the Command Module enough for it to slow down and then plunge towards the sea and coming to a landing there. This should be performed only if the computer guidance and the backup landing maneuver using the Entry Monitoring System (EMS) should both fail.

Long comm break.

The Summer Triangle refers to a triangular pattern of stars where the points are formed by Vega, Deneb and Altair, all Apollo navigation stars and hence a good target for their sightings.

Long comm break.

Their voltage is nominal. They're doing good after removing some potential ghost loads.

Distance: 55,978 km, velocity: 3,441 m/s.

Very long comm break.

Their earlier advice on how to spot stars with the Command Module optics will not apply if they remain in the new attitude as suggested by Jim.

Comm break.

Mission Control has the ability to visualize what stars are visible in the optics at any given time and spacecraft orientation, allowing them to provide this information with almost no delay.

One of the LM batteries has finally been drained almost completely, and they are switching it off after approximately 76 hours of service. Plenty of power still remains, especially in the Ascent Stage batteries they've been saving up.

Cone Crater is a landmark at their Fra Mauro landing area. It is a 370-metre crater that was a major science objective of the mission. Its rim would eventually be visited by the Apollo 14 surface crew.

Long comm break.

Distance to go, 51,297 km, velocity is up to 3,590 m/s.

Comm break.

Jim thinks they aren't able to see any stars for the moment, hence requiring to use gimbal angles computed at Mission Countrol to start aligning the IMU.

PICAPAR is a computer routine that will select suitable stars within the optics' field of vision for the purpose of navigational sightings. Having them within the field of vision saves up RCS fuel, for not having to rotate the spacecraft around to find more stars to look at.

Should their star sightings fail, there is a plan to use the still-attached LM to turn the Command Module's optics to face the Moon and the Sun and use them for navigational reference.

Very long comm break.

Long comm break.

Noun 20 (Present ICDU Angles R,P,Y) will display the gimbal angles on the Lunar Module DSKY. Mission Control will use this information to calculate the corresponding angles in the Command Module's reference system.

Comm break.

It seems that the rapidly changing procedures and the tiredness are catching up with Jim - besides the poor radio signal - and he has to ask for repetition.

Very long comm break.

If everything else fails, they will adjust the IMU to a pre-set orientation designated for the entry, with the guidance X-axis aligned to the local horizontal plane.

Distance, 46,720 km; velocity, 3,751 m/s.

Very long comm break.

These values will be used to set the initial state of the IMU platform to correspond with their present attitude.

Distance to go, 44,212 km.

Distance to go, 42,959 km; velocity up to 12,798. Their velocity at Entry Interface, an altitude of 400,000 feet or 121.92 km, will be 11,037 m/s.

Very long comm break.

The LM power transfer has been terminated, and from now on the Command Module relies on its batteries only.

Long comm break.

The Command Module's telecommunications have come back online, and they are using the four alphabetically named Omni antennas once more.

Distance, 39,062 km.

Long comm break.

Distance to go, 37,516 km; velocity, 4,152 m/s.

Comm break.

Very long comm break.

They are now uploading crucial data into the computer's erasable memory that they will need to set up their guidance system after several days of cold.

Comm break.

Distance to go, 34,490 km; velocity is up to 4,311 m/s.

Long comm break.

Comm break.

Jack's DSKY is showing him Alarm 00220, 'IMU orientation unknown.' Although he has started setting up the IMU platform, the computer doesn't know this yet.

Long comm break.

The 'flags' are yes/no data points in the computer, which will tell the navigational programming the status of a particular value. The drift flag refers to the amount of drift from optimal accuracy the gyroscopes have degraded since launch - something that can be compensated for via the computer's program. The REFSMMAT flag tells the computer whether the IMU has a known alignment or not.

Distance, 32,338 km; velocity up to 4,435 m/s.

Distance, 31,900 km; velocity, 4,459 m/s.

Comm break.

Swigert, from 1970 Technical debrief: "The two stars were Altair or Vega. At that time, Jim was saying to hurry up because the Earth was getting bigger. He was chomping to get out of the LM."

Jack is now pushing on with the P52 IMU alignment, trying to sight stars he can use for navigational measurements. This would provide them with the most accurate guidance platform, although they are ready to go with the more crude Sun-Moon alignment, if he can't see the stars. Having just separated from the Service Module, however, debris from that is apparently visible in the optics and hampering his visibility.

Long comm break.