Apollo 17

The crew of Apollo 17 have entered lunar orbit and have made a single pass across the nearside of the Moon. They have just disappeared around the far side for a second time. When they do so for a third time, they will make a 22-second burn of the SPS engine in order to lower their nearside altitude down to only 14.9 nautical-miles [27.6 km]. This will be the descent orbit from where Cernan and Schmitt will descend to their landing site at Taurus-Littrow

While they have access to the computer, and in particular to its 2K words of random access memory, Mission Control upload the following data: The target load represents the goals of the engine burn that will refine their orbit. The state vector is the most recent determination of their velocity and position based on measurements from Earth. It will be loaded into the section of memory reserved for the CSM's state vector. With the SPS engine having made a major burn in space, the engineers now have a better idea of how much thrust it continues to impart after shutdown. This is the tail-off thrust. The computer can take this into account when deciding the correct moment to stop the engine. Overmyer added that they are not uploading numbers for the PIPA bias. A PIPA is a Pulse Integrating Pendulous Accelerometer and there are three of them in the IMU to measure acceleration along three orthogonal axes.

The PAD is interpreted as follows:
Purpose: This PAD provides the details of a burn of the SPS engine that will be fired while the spacecraft is around the far side of the Moon. The burn will reduce their velocity in order to lower their near side altitude from its current 170-nautical-mile or 315-km value down to only 14.5 nautical miles or 26.9 km.
Systems: The burn would be made using the large SPS engine at the rear of the Service Module, under the control of the Guidance and Navigation system.
CSM mass (Noun 47): 40,035 pounds (18,160 kg).
Pitch and yaw trim (Noun 48): +1.90° and -0.64°. These angles represent an initial direction for the Gimbal-mounted engine to aim the thrust through the stack's centre of mass. As the burn progresses, slow shifts in the stack's centre of mass will be compensated for by the nozzle's aim being altered by the TVC (Thrust Vector Control) system.
Time of ignition (Noun 33): 93 hours, 11 minutes, 36.60 seconds.
Change in velocity (Noun 81), fps (m/s): x, -191.6 (-58.4); y, 0 (0); z, +47.8 (+14.6). The change in velocity is resolved into three components which are quoted relative to the local vertical frame of reference. The large negative component indicates the largely retrograde nature of the burn, slowing them down.
Spacecraft attitude: Roll, 0°; Pitch, 228°; Yaw, 0°. The desired spacecraft attitude is measured relative to the alignment of the guidance platform which itself is still aligned per the LOI REFSMMAT.
H_A, expected apogee of resulting orbit (Noun 44): 58.9 nautical miles (109.1 km).
H_P, expected perigee of resulting orbit (Noun 44): +14.5 nautical miles (26.9 km). The burn will shape their orbit in preparation for the Lunar Module's eventual manoeuvres for the descent. They are keeping the perilune a little higher than previous missions to take account of the way it will be altered by the Moon's uneven gravity. Like Apollo 15 before it, Apollo 17 is orbiting over two of the Moon's strongest gravitational anomalies, the mascons (short for mass concentrations). As the 15 crew slept, their perilune descended much more than had been expected. The flight dynamics people are prempting this drop with this burn. Note the emphatic plus sign on the figure. This indicates that the altitude is a positive value; i.e. above the surface and not below it!
Delta-V_T: 197.4 fps (60.2 m/s). This is the total change in velocity the spacecraft would experience and is a vector sum of the three components given above.
Burn duration or burn time: 22 seconds.
Delta-V_C: 192.1 fps. This figure is entered into the EMS Delta-V counter. The EMS's independent accelerometer will shut the engine down in case the G&N system fails to do so. This figure, Delta-V_C, is slightly lower than Delta-V_T because the EMS does not take account of the engine's tail-off thrust. Instead, the engineers have factored it into the number that will be entered into the counter.
Sextant star: Star 45 (Fomalhaut, Alpha Piscis Austrini) visible in sextant when shaft and trunnion angles are 187.5° and 19.1° respectively. This is part of an attitude check.
GDC Align or Set stars: Should the IMU fail to provide a trustworthy attitude reference, the crew can use the spacecraft's other gyros (the BMAGs) and their associated Gyro Display Couplers as a reference. To align these, the spacecraft is rotated so as to place Sirius and Rigel against the telescope's graticule (or reticle) in a predetermined fashion. If they were to do so, the spacecraft's attitude with respect to the desired REFSMMAT would be 133°, 200°, and 30° in roll, pitch and yaw respectively.
Additional notes are that an ullage burn would be required to settle the contents of the propellant tanks to the rear. This would be achieved using four rear-facing thrusters firing for 15 seconds. An additional note concerns the real dangers of the engine overburning, slowing the spacecraft more than intended. If only 22 seconds of burn can drop their nearside altitude by 155 nautical miles to 14.5 nm, it is clear that an overburn of only 2 seconds would be enough to put them in real danger of hitting the surface. The Flight Plan includes details of what to do if the computer indicates an overburn. Further, if radio tracking after AOS indicates possible impact, a bailout burn is scheduled to place them into a safe orbit.

The later AOS time that results from burn having occurred shows how the spacecraft will have been slowed down.

After they come around in the descent orbit, Ron will carry out an exercise where he will track a landmark at the landing site using a combination of the spacecraft's rotation and the movement of the sextant. Combined with a knowledge of the spacecraft's orbit based on tracking from Earth, this will help flight controllers pin down the position of the landing site.

Prior to Apollo, most photographic mapping of the Moon from orbit was carried out using imagery provided by the Lunar Orbiter probes that flew in 1965 and 66. These provided high resolution coverage of much of the nearside near the equator in support of the first landings. However, terrain to the north and south was only photographed using the wide-field camera on the probes, and often from some distance away. Thus, mission planning for Apollos 15 and 17 had to deal with an inherent lack of precision in knowing exactly where the landing site is. This tracking exercise will help refine that. The three values represent the landmark's latitude (20.284° north), its longitude divided by two (+15.151 which yields 30.302° east.) and its altitude below the lunar datum of 1.96 nautical miles. The reason they divide the longitude by two is to increase the precision of the numbers. In its early simplicity, the computer was limited to storing values of five digits with the position of the decimal point being fixed. Since latitudes only go up to ±90.000°, they can be represented to three decimal places. Longitude has a range of ±180.00° which would limit the precision to two decimal places. Dividing my to bring the range of values to ±90.000°, restoring three decimal place accuracy, though only five times more precise instead of ten times.

The PAD is interpreted as follows:
Purpose: This PAD is another in a series of contingency PADs that are read up to the crew for burns that are intended to get them home in case of an emergency, the idea being to ensure that while the radio os working, they always have a valid get-you-home PAD. This one, if used, would be fired after they have completed five orbits around the Moon.
Systems: The burn would be made using the large SPS engine at the rear of the Service Module, under the control of the Guidance and Navigation system.
CSM mass (Noun 47): 38,570 pounds (17,495 kg).
Pitch and yaw trim (Noun 48): +0.49° and +0.92°. These angles represent an initial direction for the gimbal-mounted engine. As the burn progresses, the nozzle will be slowly and automatically steered to track shifts in the stack's centre of mass.
Time of ignition (Noun 33): 98 hours, 39 Minutes, 43.24 seconds.
Change in velocity (Noun 81), fps (m/s): x, +2,329.8 (+710.1); y, -2,403.1 (-732.5); z, -1,152.8 (-351.4). The change in velocity is resolved into three components which are quoted relative to the local vertical frame of reference.
Spacecraft attitude: Roll, 193°; Pitch, 99°; Yaw, 318°. The desired spacecraft attitude is measured relative to the alignment of the guidance platform.
Subsequent items on the PAD are not applicable. These are H_A, H_P, Delta-V_T, burn duration, Delta-V_C, sextant star, boresight star and parameters that relate to Earth landing.
GDC Align (or set) stars: The stars to be used for GDC Align purposes are Sirius and Rigel. When these two stars are viewed through the telescope in the correct manner, the spacecraft's attitude will be: roll, 133°; pitch, 200°; yaw, 30°.
Additional notes in the PAD state that an ullage burn of 12 seconds duration using the four rearward-firing RCS thrusters would be required to settle the contents of the SPS tanks. The burn's details assume that the LM has been undocked and that they have carried out the Descent Orbit Insertion (DOI) burn.

Long comm break.

The gravity gradient effect comes about when the strength of gravity from a large nearby body (like Earth or the Moon) changes across the width of an irregular body nearby which causes a torque. In the case of Apollo, the Moon's gravity is very slightly stronger on the side of the spacecraft nearest the lunar surface than it is on the opposite side of the spacecraft. This tends to make the spacecraft turn very slowly in order to make its long axis point moonward, the most stable arrangement. Another term for the gravity gradient effect is the tidal effect. It explains why the Moon always has one hemisphere facing Earth. The Moon feels the gravity gradient due to Earth's gravity weakening across the lunar diameter. Billions of years ago, the Moon gradually aligned its axis that had the minimum moment of inertia to point to Earth and so we have a near side and a far side.

Long comm break.

Ron has completed the seventeenth platform realignment of the mission using P52 in the computer. For this, he used stars 15 (Sirius, Alpha Canis Majoris) and 22 (Regulus, Alpha Leonis). As a result of these sightings, the computer determined that the gimbals that support the guidance platform have to be rotated, or torqued by 0.046° in X, 0.014° in Y and -0.069° in Z. As alluded to in the conversation, Ron achieved a star angle difference of 0.00°, this being a comparison of the measured angle between the two stars and the angle that the computer knows is between them from its internal tables. Earth's proximity to Regulus caused a lot of spilled light in the optics so Ron had to trust that when the computer had slewed the sextant to view the star, it really was pointing at Regulus.

P40 is the SPS thrusting program and it is being set up with the parameters for the upcoming LOI burn.

Comm break.

Long comm break.

Jack reported seeing a flash as they passed over the night-time, Earthlit part of the Moon at 090:18:11.

Long comm break.

Long comm break.

Long comm break.

Long comm break.

Very long comm break.

Very often on Apollo, the crews would take photos of Earthrise over the Moon's sunlit horizon. On this occasion, one of the Apollo 17 crew has had the wit to take a unique series of shots showing Earth setting behind the Moon's night-time horizon.

VHBW refers to very high-speed black & white film, type 2485, whose light sensitivity is rated at 6,000 ASA. For the Hasselblad 70-mm cameras, this is loaded into magazines QQ and RR (photo serials 154 and 155 respectively).

The critical nature of the burn's timing is illustrated by the fact that if the burn were to last only 2 seconds longer than required, it would drop their nearside altitude to a negative value; i.e. it would put them in real danger of hitting the surface.