Apollo 16

The Apollo 16 spacecraft, together with the S-IVB third-stage of the Saturn V launch vehicle, has almost completed its first orbit around the Earth. After a long transit across the Pacific, during which they were out of communication with Mission Control, John Young, Charlie Duke and Ken Mattingly are about to start the preparation for Trans Lunar Insertion - the firing of the S-IVB booster to send Apollo 16 out of Earth orbit and towards the Moon. To help readers find the more significant parts, the following is a short index of the main events.

Break in CM tape until 001:48:38.

The PAO means that the pressure has not released as planned. However, he is suggesting that the valve is releasing some pressure, if not as much as it is intended to.

Throughout the mission, large lists of numbers, called PADs, will be read up to the crew which give them the information necessary to carry out a particular manoeuvre. PAD stands for Pre-Advisory Data. Some of these "block data" are for planned manoeuvres such as the TLI (Translunar Injection) or LOI (Lunar Orbit Insertion) burns. Other PADs, such as the "TLI plus 90" and "Lift-off plus 8" mentioned here are the first of 27 abort options which will be read up to the crew at scheduled times throughout the early and middle portions of the mission. Note that the TLI+90 PAD has nothing to do with TLI itself but would occur 90 minutes after a successful TLI burn in the event of an abort. However, mission planners have decided that at no time beyond Earth orbit will the crew be without a get-you-home PAD. If at any time the crew subsequently lose communication with Earth, they will have the information to hand to get themselves back manually.

To simplify the voice transmission of these huge lists of numbers and reduce the likelihood of errors, each type of PAD was precisely formatted both in Mission Control's and the crew's paperwork - all the crew needed to do was fill in the blanks.

The abort PADs are the responsibility of RETRO, one of the flight controllers in the front row of the MOCR (Mission Operations Control Room). According to Chuck Deiterich, who was one of those who occupied the RETRO console throughout Apollo, "There is quite a bit of protocol in the PAD process. Empty PADs were in tablets of no carbon required (NCR) paper. We would make about six copies and use a red ballpoint on the top (original) so the CapCom would be sure what was part of the printed form and what was data." (2003 correspondence).

As Fullerton reads the PAD, a complete P30 type, Charlie Duke copies the values to the appropriate form in the checklist.

Interpretation of this particular PAD is as follows:

• Purpose: In the event of a serious problem soon after a good Translunar Injection, this PAD has the details of a burn which would return them quickly to the Earth. It has an ignition time of 90 minutes after TLI. In the event of such an emergency, the SPS (Service Propulsion System) engine would be used to slow the spacecraft to below Earth escape velocity, resulting in a high altitude ballistic arc that would take them directly to splashdown without any further orbits. Full procedures for the TLI plus 90 abort are given on Page L4-13 of the CSM Launch Checklist
• System: The burn would be under the control of the Guidance and Navigation System. Burns can also be controlled by the SCS (Stabilization Control System).
• CSM weight (Noun 47): Calculated to be 66,973 pounds (30,378 kilograms). Note that here, and in all the NASA documentation of the time, the term "weight" is freely used in the context where, strictly speaking, "mass" ought to be used. Weight is defined as the force applied by a mass in a gravity field. In free fall, mass remains the same while weight becomes essentially zero.
• Pitch and yaw trim (Noun 48): -0.54° and +1.89°. The SPS engine bell is gimbal mounted to allow its thrust vector to be aligned with the spacecraft's centre of mass. Two thumb wheels on the left of the Main Display Console allow adjustment of these pitch and yaw trim angles. Once a burn begins, the bell's aim is slowly altered automatically to compensate for shifts in the centre of mass.
• Time of ignition, T_IG (Noun 33): 4 hours, 3 minutes and 18.55 seconds Ground Elapsed Time.
• Change in velocity (Noun 81), fps (m/s): X, -356.2 (-108.6); Y, +0.1 (+0.03); Z, +3,600.1 (+1,097.3). This is the change in velocity as defined along three orthogonal axes with respect to the local vertical.
The three velocity components in a PAD are always expressed with respect to the local vertical. Imagine a coordinate system based on a line from the spacecraft to the centre of Earth or the Moon. (Which you use depends on the sphere of influence you are in, i.e. which gravitational field is stronger.) This line is the Z-axis and is vertical at the point where it intersects the surface. The X-axis is perpendicular to this in whatever direction the spacecraft's orbit is taking it. It is therefore parallel to the local horizontal. The Y-axis is perpendicular to the orbital plane. This arrangement holds even for a very extended elliptical orbit like the one Apollo 15 will take to the Moon, where the spacecraft is clearly not travelling parallel to the local horizontal. For this burn, we can see that the largest component of velocity change is in the plus-Z direction which is towards Earth, countering the spacecraft's velocity away from the planet.
• Spacecraft attitude at T_IG: Roll, 181°; Pitch, 232°; Yaw, 2°. Although the velocity change is expressed with respect to the local vertical, spacecraft attitude is expressed with respect to the current alignment of the guidance platform. That alignment is currently set to the launch pad REFSMMAT.
• H_A (Noun 44): This is the height of the resulting orbit's apogee. Since the computer cannot display an altitude higher than 9999.9 nautical miles, and the maximum height of the CSM during a TLI+90 abort will be much higher than that, H_A is not applicable.
• H_P (Noun 44): 18.9 nautical miles (35.0 km). This is the height of the resulting orbit's perigee. Targeting a perigee so low will cause the spacecraft to intercept Earth's atmosphere and cause it to re-enter.
• Delta-V_T: 3,618.3 fps (1,103.8 m/s). This is the total velocity change that would be experienced by the spacecraft. It is a vector sum of the three components given earlier.
• Burn duration or burn time: 5 minutes, 04 seconds.
• Delta-V_C: 3,604.0 fps. This figure would be entered into the EMS (Entry Monitor System) Delta-V display.

Delta-V_C requires a little more depth of explanation. SPS engine burns are normally controlled by the G&N system. If it is a long burn, that is, greater than six seconds, the control is closed loop. The system monitors the achieved Delta-V and shuts down the engine at the appropriate time to reach the required Delta-V. In doing so, it takes account of the engine's tail-off impulse, which is the amount of thrust that the engine continues to impart after the shutdown command. It knows the thrust that is expected from this tail-off and can calculate the resulting tail-off Delta-V based on this and the spacecraft mass.

If the G&N system were to fail during a burn, the EMS provides a backup means of shutting down the engine at the right time. This equipment carries a separate accelerometer which measures Delta-V along the longitudinal axis of the spacecraft. Prior to the burn, the crew enter the expected Delta-V into a display on the EMS. As the burn progresses, the figure showing the remaining Delta-V drops towards zero, at which time the EMS sends a shutdown command to the SPS in case the G&N system has not already done so. However, the EMS has no knowledge of the tail-off thrust. The flight controllers take this into account and give the crew a low Delta-V figure for entering into the EMS so that if it is called upon to shut down the engine, it will do so early enough for the tail-off thrust to effect the correct total Delta-V.

As CapCom Gordon Fullerton was reading up the PAD, the communication route was transferred to a ship in the Atlantic, USNS Vanguard. The changeover has caused Charlie to miss some of the information.

The communications link has transferred to the Vanguard...

Carrying on with the PAD.

Continuing with the explanation of the PAD:

• Sextant star: Star number 26 (Spica, Alpha Virginis) should be visible through the sextant when its shaft and trunnion are set to the values 73.4° and 15.1° respectively.
• Boresight star: Star number 37 (Nunki, Sigma Sagittarii) which should be visible through the COAS when its angles are set to the values 25.7° up, and 2.9° right.

The next five parameters all relate to re-entry, during which an important milestone is "Entry Interface," defined as being 400,000 feet (121.92 km) altitude. However, in this context, a more important milestone is when atmospheric drag on the spacecraft imparts a deceleration of 0.05g.

• Expected splashdown point (Noun 61): 21.31° south, 165.00° west; in the mid-Pacific.
• Range to go at the 0.05 g event: 1,093.2 nautical miles. To set up their EMS (Entry Monitor System) before re-entry, the crew need to know the expected distance the CM would travel from the 0.05g event to landing. This figure will be decremented by the EMS based on signals from its own accelerometer.
• Expected velocity at the 0.05g event: 34,867 fps. 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.
• GET of 0.05g event: Expected at 23 hours, 08 minutes and 28 seconds GET. This is when it is expected that the EMS and P64 will be triggered.
• GDC Align stars: Stars Sirius (in Canis Major) and Rigel (in Orion) are to be used to align the gyro assemblies if it is not possible to use the guidance platform for this purpose.
• GDC Align angles: Roll, 317°; Pitch, 108°; Yaw, 005°.

GDC Align is another term that needs a little more explanation. The spacecraft has two independent systems for determining attitude and changes in attitude. The primary system is the IMU and its stable platform, held orientated to the stars by gyroscopes. A secondary system, usually tied to the SCS, comprises a set of gyros that are attached to the spacecraft structure. In other words, being mounted directly to the body of the spacecraft, their mounts are not stable. Unlike the IMU, which measures absolute attitude, these gyro assemblies only measure the rate of attitude change. However, absolute attitude can be derived from these measurements by integrating the change, a job carried out by the GDCs (Gyro Display Couplers). This technique is imprecise so at regular times, the crew presses the GDC Align button to make the GDCs knowledge of attitude match the IMU's. The question then arises; what happens if the IMU is not working? The crew then have a backup method of aligning the GDCs by sighting two stars through the scanning telescope in a particular way. They know what the spacecraft's attitude should be when this is achieved and can dial this into the GDCs, properly aligning them.

The final note in the PAD concerns the ullage burn. Since the SPS propellant tanks are full, there is no need to perform a small RCS burn, known as the ullage burn, to settle their contents. To confirm the figures, Charlie Duke reads back the PAD.

The Lift-off+8 PAD carries data for P37, a program in the computer that will calculate the details of a burn that will return the crew to Earth. An important condition for P37 is that the spacecraft must still be in Earth's sphere of influence, thus simplifying the calculations. The program takes the four values from the PAD; the specified time for ignition of the engine, a specified maximum change in velocity (or Delta-V), the longitude of the splashdown and the GET for the start of re-entry. These act as a set of constraints with which it calculates the desired trajectory and the details of the burn to achieve it.

And now the PAD for the TLI Burn itself.

The data read up by Fullerton is structurally different to other PADs as the manoeuvre is controlled by the IU on the launch vehicle, and not the computer in the CM. A form for filling in the numbers is available on L2-21.

The timings for events relating to the launch vehicle are defined relative to a number of time bases, each of which start with a particular event. This allows controllers to move complete sequences of events relative to the overall mission time. The restart sequence for the S-IVB's single J-2 engine is tied to Time Base 6 (TB-6). When TB-6 begins, all subsequent events to restart the engine such as tank repressurisation, engine chilldown, ullage, etc., follow on, leading to the engine start command 9 minutes, 30 seconds later, and ignition 8 seconds after that.

The crew also have tasks to perform in the minutes leading up to the TLI burn and they use their event timer to help them. Around 002:23:57, the 'S-II Sep' lamp is illuminated for ten seconds, this being the start of TB-6. At 9 minutes to ignition, John will start the event timer counting up, having previously set it to 51:00. This will give a visual count-up to and beyond ignition to aid the crew in sequencing their final tasks before and during TLI. Items in the checklist are therefore shown with times from 51:00, through (1:)00:00 and upwards.

The PAD is interpreted as follows:

• TB-6 predict light: 002:23:57. This is predicted start of time base 6. It implies that T_IG (time of ignition) will be 9:38 later, at 002:33:35.
• Attitude for TLI: 179°, 113°, 000° in roll, pitch and yaw respectively. This attitude is stated with respect to the orientation that the guidance platform has held since launch.
• Duration of burn: 5 minutes, 43 seconds.
• Delta-V_C': 10,373.0 fps. This figure will be entered into the EMS to allow the crew to monitor the remaining velocity that is still to be added. This should be at zero when the burn comes to an end.
• V_I: 35,589 fps (10,847.5 m/s). Inertial velocity at engine cut-off.
• Separation attitude: The correct attitude for separation of the CSM from the launch vehicle is 359°, 146°, 319° in roll, pitch and yaw respectively, again with respect to the orientation of the guidance platform. Among the criteria for adopting this attitude is solar illumination of the LM to assist the docking procedure.
• Extraction attitude: The correct attitude for extraction of the LM from the S-IVB is 301°, 326°, 41° in roll, pitch and yaw respectively.
• R2 align: 112.7.
• R2 ignition: 107.2.
• ORDEAL Start: Relative to the count-up on the event timer, the ORDEAL should be started at 57.20. The ORDEAL (Orbital Rate Display - Earth And Lunar) drives the FDAI (Flight Director Attitude Indicator, or "8-ball") to make it display the spacecraft's attitude relative to the ground below.
• Yaw: 001

During Apollo 16's TLI, the crew are going to use the ORDEAL to drive the FDAI at a rate which matches the pitch rate of the S-IVB during its powered flight. This way, the crew can monitor the progress of the TLI as far as the vehicle's attitude is concerned, and they can take over manual attitude control if required during the burn. They must start the ORDEAL working at a precise time if the FDAI is to show zero attitude errors, otherwise the spacecraft's further motion around the Earth will cause it to be offset one way or the other.

Ellington Air Force Base (now Ellington Field) is located approximately 24 km (15 miles) south of downtown Houston, near the major highway heading from the City to NASA and Galveston Island. It is where NASA bases its small fleet of aircraft in support of the Manned Spacecraft Centre.

At this point, the Command Module's computer, the CMC, must be set to receive the updated state vector. This involves setting the Up Telemetry (Tlm) switch to the right of the DSKY to Accept. The CMC must also be in an idle status (running P00, essentially a 'do-nothing' program). P00 is commonly pronounced 'pooh' by all the Apollo crews as in the character from A. A. Milne's book Winnie the Pooh. As recounted in Murray & Cox's book, Apollo: The Race to the Moon, this program name even entered the daily lingo of the flight controllers. To 'go to poo' meant to go to sleep. While the CMC is receiving data link information by up-telemetry, the top left DSKY status light will illuminate.

Mission Control has some final updates to the procedures for TLI, starting on Page L2-28 of the Checklist.

And on Page L2-29.

With the state vector updated, the CMC Up Tlm switch is set to block further data from the ground

See explanation of the SECS Pyros at 002:13:46

Comm break.

Break in Tech transcript until reacquisition by Canary at 001:50:06. Onboard tape restarts one minute after Loss Of Signal.

Towards the end of the TLI burn, the S-IVB's thrust will actually produce an acceleration of nearly 1.4g, so all the items in the CM have to be properly secured.

If the attitude of the spacecraft changes, there could be a small amount of lateral acceleration. Placing heavy objects on the spacecraft's shelf under the side windows would not be a good location.

The technical transcript and the CM tape transcripts do not agree for the period 001:50:39 to 001:50:52. The technical transcript gives continued conversation between Charlie Duke and Gordon Fullerton, lasting up until 001:54:23, about 30 seconds before LOS. The CM transcript details a conversation between all three crew members, but this does not fit in with the other conversation. It appears that the CM transcript timings may be in error. The following details both conversations in turn.

Comm break.

Comm break.

The suspect CM tape follows. The two tapes agree again from 001:54:21.

No more CM tape data recorded until 001:54:21.

The Apollo Range Instrumentation Aircraft or ARIA are US Air Force Boeing EC-135E aircraft modified from C-135B transports similar to the Boeing 707 airliner. With a huge tracking antennae in the bulbous nose, the ARIA provides a mobile "ground" station where the Manned Spaceflight Network cannot provide communications. Four of these aircraft were converted at the time of Apollo.

Very long comm break.

The crew are now starting to prepare the spacecraft for Trans Lunar Injection, starting at the top of Page 2-27 of the Launch Checklist.

Setting the Trans Lunar Inject (XLunar) switch on the MDC to tells the Instrument Unit that the crew are ready for TLI. At 42 seconds into TB-6 the IU will make the first of two checks to see the status of this switch.

The Launch Vehicle Emergency Detection System is powered up so that any failures in the S-IVB during the burn can be indicated to the crew. At this stage in the flight, there is no automatic abort option, so the crew would have to manually initiate separation of the CSM from the S-IVB.

The Entry Monitoring System provides a secondary function of measuring velocity changes during the TLI burn and later SPS and RCS burns. During Entry, the output of the EMS's accelerometer is integrated twice to calculate the distance travelled by the CM after the entry interface. During engine burns, the accelerometer output is integrated once to calculate the velocity change (Delta-V) along the CM's X-axis. The electro-luminescent indicator can read up to 99,999.9 feet per second, with a precision of 0.1 foot per second. In each case, the crew set the desired Delta-V on the counter, which then counts down to zero.

The use of the EMS is a good example of system redundancy in the Apollo design. Changes in spacecraft velocity could be measured from the main CM Guidance and Nav inertial system, but this is more complex and liable to errors due to drift. The fixed EMS accelerometer provides a simple, reliable means of making this critical measurement. As another example of multiple use of the same system, the EMS display also provides an indication of the range between the CM and the LM during rendezvous operations, using the separate VHF radio ranging system.

The EMS Delta-V Test and Null Bias Check is carried out in accordance with Page G2-5 of the G&C Checklist.

This value is set as part of the test; it is not related to the earlier PAD values.

While John Young carries on with the EMS test, Charlie Duke and Ken Mattingly ready the CMC for TLI using P15. This provides a back-up for initiation of the S-IVB ignition sequence, monitors the TLI burn if the IU is in control and provides automatic shutdown of the S-IVB if the CMC is in control.

Charlie Duke is reading the TLI PAD details to Ken Mattingly, who is then entering them in the DSKY. The first set of figures is the GET of Time Base 6, 2:23:57.

The target is between -0.1 and -41.5 feet per second, so the actual value is right in the middle of the range.

Verb 06 Noun 14 is the command to load the velocity change.

Not clear what John Young is referring to.

From the TLI PAD, the Indicated Velocity at S-IVB cut-off will be 35,589 feet per second (10,848 m/s).

From the TLI PAD, the change in velocity during the burn will be 10,373.0 feet per second (3,161.7 m/s).

John Young and Charlie Duke continue with the P15.

The crew are halfway down Page 2-27 of the Launch Checklist. However, they have become slightly out of order, and have carried out P15 without having carried out Verb 83. If they now call up the Verb 83 routine, they will lose the information they have stored in the CMC ready for the TLI burn. Without Verb 83, they will not have an exact pitch angle when they reset the ORDEAL.

Applying power to the two Translational Controllers mounted on John Young's and Ken Mattingly's couch arms.

Applying power to the two Rotational Controllers mounted on John Young's and Ken Mattingly's couch arms. There are two supplies; the Normal supply powers the breakout switches which control the RCS through the SCS and the CMC. The Direct supply powers the direct switches which give the crew direct control of the RCS thrusters in event of a malfunction of the SCS. A more detailed description of the RCS and its controls will be given in the chapter on TD&E.

Enabling SCS control through Translation and Rotation Controls.

Setting the dual indicator to monitor S-IVB oxidiser pressure. See 000:06:02 for more details.

The term "ullage" comes from the brewing industry, where it means the space above the beer in a brewer's vat., but in this context it means the volume in the spacecraft propellant tanks that is filled with helium gas. In a spacecraft, fuel will float freely in the tanks, unless it is constrained. However, it is essential that there are no bubbles of helium in the propellant before it is fed into the engines. In smaller tanks, such as those of the RCS, Teflon bladders are used to control the fuel and to separate it from the helium used to pressurise the tanks. But in larger tanks, such as the four main ones of the SPS, bladders cannot be used and so a short period of acceleration is required to force the fuel to the bottom of the tank (and the helium to the top) before the SPS fires. This acceleration is provided by the RCS firing for a few seconds. Normally, the SPS will command this automatically, but in event of a failure of the automatic system, the Direct Ullage switch on Panel 1 of the MDC provides a back-up capability for initiating the RCS ullage burn. The Direct Ullage Switch is backed up by two circuit breakers on Panel 8, which must be energised to enable the switch to work. A complex system, but necessary to provide the necessary safety.

It is not clear what the next conversation is about. John Young and Ken Mattingly seem to be discussing a deviation from the procedure. Possibly linked to the Verb 83 issue?

The crew have now completed Page L2-28 of the Launch Checklist. Charlie Duke is highlighting the change in FDAI slew setting on Page L2-29 that was read up from Mission Control at 001:42:12.

This may be a transcription error. The time for R2 ignition in the R2 ignition in the TLI PAD is 107.2, and John Young may be referring to this as "ten, seven, two".

John Young is reviewing the procedures on Page L2-28. During the S-IVB re-start sequence, the booster propellant tanks are pressurised. Because the tanks share a common bulkhead, any large difference in pressure could cause a catastrophic failure of the bulkhead, followed by an explosion of the booster. If the LOX pressure exceeds the LH₂ pressure by more than 36 psi, the LH₂ pressure exceeds the LOX pressure by more than 26 psi or the LOX tank pressure exceeds 50 psi, then the crew must carry out an emergency separation of the CSM from the S-IVB, as detailed on Page Emer 1-1.

And back to more housekeeping, preparing the cabin for acceleration.

A number of the major systems on Apollo are operated by explosives. This provides a very reliable and low-weight means of achieving events such as separating the LM from the S-IVB, or of separating the docking ring from the CM to jettison the LM after docking in lunar orbit. As a down-side, they are one-shot devices that cannot be re-set if operated in error. With the SECS Pyro Arm circuit breakers live, all the relevant switches are also live. John Young is making sure that all the crew are aware of this, since even though the switches are guarded, it would be all too easy to press the wrong one and, for example, jettison the docking ring while leaving the LM attached to the S-IVB - thus losing any chance of a lunar landing. The two controls, although not adjacent, are on the same row of six switches. The even more important switches that jettison the SM from the CM are powered by different circuit breakers and are not live at this point.

A similar problem, but with even more serious potential consequences, faced the crew of Apollo 13 as they approached the Earth at the end of their calamitous mission. They had to jettison the SM from the CM, while keeping the LM attached until the last minute. The CMP, Jack Swigert, had to resort to sticking large labels on the two switches to make sure that he did not press the wrong one.

Ken Mattingly is referring to his life jacket.

Not clear what the discussion is about. Pro (Proceed) is normally the end of a CMC Program, so the crew may have not completed P15.

In zero gravity, the body has a natural tendancy to move its's fluids to the head, since the normal counterbalancing effect of gravity is removed. The sinuses also tend to fill, both due to increased leakage of fluid into them and because they do not drain normally. The result is a full-headed feeling and a swelling of the face.

Although the Launch Checklist shows the periods that the spacecraft will be in contact with the ground stations, it does not include the same for the ARIA.

In other words, the CM will separate from the S-IVB.

Five minutes to Time Base 6 start.

The crew are re-reading the checklist. Having bypassed some of the steps in the TLI preparation on Page L2-27, they are now checking that nothing has been omitted in their earlier actions from 001:54:54.

The SECS Pyro switches are being selected, as this step was bypassed at 002:02:55.

See comment at 001:55:17 on the significance of the Translunar Inject (XLunar) switch.

John Young is setting the ORDEAL to 300 and Lunar, in accordance with the Launch Checklist Page L2-28.

The crew are waiting for the Uplink Activity Light to go On, signalling the start of Time-Base 6, nine minutes and 38 seconds before ignition (9:30 before Engine Start Command).

The S-II SEP light is now being used as an indicator.

Long comm break.

The FDAI needs to be slewed to the correct position.

Setting the ORDEAL. See earlier for the ORDEAL controls.

Break in CM Tape, perhaps due to vox.

Comm break.

Comm break.

Comm break.

One minute, 15 seconds to S-IVB ignition.

On Launch Checklist Page L2-29, coming up for 58:15, 105 seconds to S-IVB ignition.

The DSKY display blanks for five seconds at 105 seconds before S-IVB ignition, while in P15.

To ensure that there is a record of spacecraft systems performance, even if the data link with Mission Control is lost during the TLI burn, Charlie Duke sets the Data Recorder Reproducer (DRR) - the CM's tape recorder - to High pulse-code modulation (or data bit rate), to Record and to Forward, to achieve the best recording conditions. He also resets the Up-Data Link equipment, and then sets it to Normal, in case Mission Control need to up-link any commands to the spacecraft. However, the crew still have the final control over up-link activity as the Up-Tlm switch is set to Block (see 001:41:45).

John Young sets the EMS to display the velocity change during the burn, as discussed at 001:55:26

The S-II Sep light illuminates at Time Base 6, 58:36, two seconds before S-IVB APS starts thrusting in the ullage manoeuvre.

The spacecraft is over the Pacific, facing into the sunrise. Ken Mattingly is suggesting that John Young increase the brightness of the displays to help him read them.

The engine start command is sent at 002:50:29.51 GET. The restart sequence is essentially identical to the start of the S-II stage's J-2s and of the first start of the S-IVB 's J2. The main difference is that fuel is allowed to run through the engine walls for eight seconds before the final ignition.

See comments at 002:09:57.

As the J-2 engine burns, the ratio of oxidiser to fuel is changed from approximately 4.5:1 to 5 to 1 at the 56-second point. This increases the thrust from around 183,000 lbf (814 kN) to 207,000 lbf (921kN).

Comm break.

Mission Control cannot directly observe the vibration levels that the crew are sensing. During the 1972 Technical Debrief, the crew discussed the vibration.

Mattingly, from the 1972 Technical debrief: "On TLI, it seemed to be increasing in amplitude; although I thought that the frequency was still the same."

Young, from the 1972 Technical debrief: "Yes. That's what I thought all the way to ECO (Engine Cut-Off) . But, there was a buzz on the S-IVB all the way to engine cut-off. And, it was a high frequency buzz. ... It was too high a frequency to be characterized as pogo. The amplitude was so low you couldn't characterize it as pogo either. It didn't seem that anything was in danger of coming apart. I was more worried about it quitting."

From the AS-511 Flight Evaluation Report: "In a post-mission debriefing the Apollo 16 crew reported that the vehicle had experienced some low amplitude vibration or 'buzz' during portions of the S-II stage burn, and throughout the S-IVB first and second burns. The crew also noted that the vibrations did not appear to be oriented in any particular axis. Analysis of flight data indicates the presence of low amplitude, approximately 65 hertz, vibration during the S-II stage burn and both S-IVB stage burns. The data show lateral amplitudes of ±0.10 g at the IU during S-IVB first burn and ±0.20 during second burn. The vibrations can also be seen on selected propulsion pressure measurements. A review of AS-510 [Apollo 15] data shows similar vibration at approximately 72 hertz. Because of the data characteristics, the vibration is suspected to be related to normal stage propulsion system operation and probably characteristic of the J-2 turbomachinery. These vibrations pose no pogo or any other structural concerns, and are of such low amplitude as to be virtually obscured in the measurement background noise."

A rather bland engineering report on a phenomenon that gave the crew considerable concern!

From the AS-511 Flight Evaluation Report: "The S-IVB second burn time was 341.9 seconds, 2.4 seconds less than predicted. This difference is primarily due to the slightly higher S-IVB performance and lighter vehicle mass during the second burn."

The lighter vehicle mass was due, in part, to a slightly longer (0.4 seconds) than anticipated first S-IVB burn, due in turn to lower than anticipated S-IC and S-II performance.

The crew are now supposed to read the following parameters, but were slightly late in freezing the display:

• Time From Cutoff (TFC) giving the burn time in minutes and seconds.
• Velocity to be gained (V_G) in feet per second.
• Indicated Velocity at cutoff(V_I in feet per second.
• The remaining velocity to be added (Delta V_C)

The crew are now at the top of Page L2-31, as they reconfigure the spacecraft after the successful TLI burn.

The CMC is now in P00, to update the State Vector.

Verb 66 sets the updated CMC State Vector into the LM Computer.

Long comm break.

Apollo 16 is now well on its way to the Moon. Travelling at over 10 km per second (22,000 miles per hour), John Young, Charlie Duke and Ken Mattingly are on course for the Moon. Next, they have to conduct the Transposition and Docking manoeuvre, to link up with the LM.