Reports/Apollo 17/Saturn V flight evaluation/8 Structures

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[edit] 8.1 Summary

The structural loads experienced during the S-IC boost phase were well below design values. The maximum bending moment was 96 x 106 lbf-in at the S-IC LOX tank (less than 36 percent of the design value). Thrust cutoff transients experienced by AS-512 were similar to those of previous flights. The maximum longitudinal dynamic responses at the Instrument Unit (IU) were +0.20 g and +0.27 g at S-IC Center Engine Cutoff and Outboard Engine Cutoff (OECO), respectively. The magnitudes of the thrust cutoff responses are considered normal.

During S-IC stage boost, four to five hertz oscillations were detected beginning at approximately 100 seconds. The maximum amplitude measured at the IU was +0.06 g. Oscillations in the four to five hertz range have been observed on previous flights and are considered to be normal vehicle response to flight environment. POGO did not occur during S-IC boost.

The S-II stage center engine LOX feedline accumulator successfully inhibited the 16 hertz POGO oscillations. A peak response of +0.4 g in the 14 to 20 hertz frequency range was measured on engine 5 gimbal pad during steady-state engine operation As on previous flights, low amplitude 11 hertz oscillations were experienced near the end of S-II burn. Peak engine No. 1 gimbal pad response was +0.06 g. POGO did not occur during S-II boost. The POGO limiting backup cutoff system performed satisfactorily during the prelaunch and flight operations. The system did not produce any discrete outputs and should not have since there was no POGO.

The structural loads experienced during the S-IVB stage turns were well below design values. During first burn the S-IVB experienced low amplitude, +0.14 g, 16 to 20 hertz oscillations. The amplitudes measured on the gimbal block were comparable to previous flights and within the expected range of values. Similarly, S-IVB second burn produced intermittent lot amplitude oscillations of +0.10 g in the 11 to 16 hertz frequency range which peaked near second burn cutoff.

[edit] 8.2 Total Vehicle Structures Evaluation

[edit] 8.2.1 Longitudinal Loads

The structural loads experienced during boost were well below design values. The AS-512 vehicle liftoff steady-state acceleration of 1.21 g was slightly higher than predicted (1.19 g), resulting in slightly higher longitudinal loads but no associated problems. Maximum longitudinal dynamic response measured during thrust buildup and release was +0.21 g in the IU and +0.40 g at the Command Module (CM), Figure 8-1. Comparable values have been seen on previous flights.

The F-1 engine thrust buildup rates were normal. The ignition sequence was 2-1-1-1 with engines 3 and 4 igniting early relative to the center engine. While the desired 1-2-2 start sequence was not achieved, the time deltas between pairs of diametrically opposed engines were within the 3a dispersion used in preflight loads analyses (229 ms). The desired start sequence apparently cannot be expected with high confidence, but the structural loads on the SA-513 vehicle have been analyzed using start sequence stagger times both less and significantly larger than experienced on AS-512 with no problems arising. Thus the AS-512 ignition sequence has been established as not detrimental to SA-513.

The longitudinal loads experienced at the time of maximum bending moment (79 seconds) were as expected and are shown in Figure 8-2. The steady-state longitudinal acceleration was 2.02 g.

Figure 8-2 also shows that the maximum longitudinal loads imposed on the S-IC stage thrust structure, fuel tank, and intertank area occurred at S-IC CECO (139.3 seconds) at a longitudinal acceleration of 3.79 g. The maximum longitudinal loads imposed on all vehicle structure above the S-IC intertank area occurred at S-IC OECO (161.2 seconds) at an acceleration of 3.87 g.

Combined compression and tension loads were computed for the maximum bending moment, CECO and OECO conditions, using the loads shown in Figures 8-2 and 8-3 and measured ullage pressures. Those loads which produced minimum safety margins are plotted versus vehicle station along with the associated capabilities in Figure 8-4. The minimum ratio of capability to load is at Station 1541 for the OECO condition.

[edit] 8.2.2 Bending Moments

The peak vehicle bending moment occurred during the maximum dynamic pressure phase of boost at 79 seconds, Figure 8-3. The maximum bending moment of 96 x 106 lbf-in at vehicle station 1156 was less than 36 percent of design value.

[edit] 8.2.3 Vehicle Dynamic Characteristics

[edit] 8.2.3.1 Longitudinal Dynamic Characteristics

During S-IC stage boost, the significant vehicle response was the expected four to five hertz first longitudinal mode response. The low amplitude oscillations began at approximately 100 seconds and continued until S-IC CECO. The peak amplitude measured in the IU was +0.06 g, the same as seen on AS-510 and AS-511. The AS-512 IU response during the oscillatory period is compared with previous flight data in Figure 8-5. Spectral analysis of engine chamber pressure measurements shows no detectable buildup of structural/propulsion coupled oscillations. POGO did not occur during S-IC boost.

Engine 2 outboard fuel suction duct 1 pressure data (D146-115) showed a high amplitude (8 psi peak) 11 Hz oscillation throughout most of the S-IC stage burn. The 11 Hz frequency content was also found in the related fuel suction inlet pressure measurement D4-102 where it appears as an aliased 1 Hz frequency of similar amplitude.

This 11 Hz oscillation has been observed on previous flights for various time periods and comparable amplitudes. In particular, the fuel inlets on Engine 5 on AS-501 (D146-115 and D149-115) exhibited a 12.5 Hz, 8 psi peak amplitude oscillation throughout flight.

This observed oscillation is a combined pump-propellant feed line pressure oscillation that occurs under certain Net Positive Suction Pressure (NPSP) conditions which were met for Engine 2 for most of the AS-512 S-IC burn time. This is not a POGO phenomenon. No significant vehicle response occurred at this frequency.

The AS-512 S-IC CECO and OECO transient responses were equal to or less than those of previous flights. The maximum longitudinal dynamics resulting from CECO were +0.20 g at the IU and +0.50 g at the CM, Figure 8-6. For OECO the maximum dynamics at the IU were +0.27 g and +0.80 g at the CM, Figure 8-7. The minimum CM acceleration level of -0.60 g occurred at approximately the same time and is somewhat lower than on previous flights but considered normal.

The S-II stage center engine accumulator effectively suppressed the 16 hertz POGO phenomenon. The flight data show that the 16 hertz oscillations were inhibited with amplitudes comparable to those seen on AS-511, Figure 8-8. The peak 14 to 20 hertz center engine gimbal response was approximately +0.4 g, as compared to +-0.5 g on AS-511. POGO did not occur.

The usual transient response in the center engine LOX pump inlet pressure was experienced shortly after accumulator fill was initiated. The peak response was approximately 34 psi peak-to-peak with a frequency of approximately 70 hertz, Figure 8-9. The LOX pump inlet pressure on AS-511 had a higher frequency content, a longer duration, and lower amplitude (13 psi peak-to-peak) but AS-512 is similar to AS-510 (45 psi peak-to-peak at 68 hertz). Such variation: are not unique and the causes are attributed to the individual pump characteristics. There are no parallel increases in responses among the other engine pressures and the structural accelerations which again indicates the lack of strong coupling between the transient pressure response and the structural accelerations.

As on prior flights, very low 11 hertz oscillations were noted near the end of S-II burn. The AS-512 peak engine No. 1 gimbal pad response was +0.06 g as compared to +0.07 g on AS-511.

During S-II burn, between 184 and 207 seconds range time, the vibration level on the S-IVB gimbal block was discernible above the noise floor, Figure 8-10. The maximum acceleration of the gimbal block in this interval was about +0.06 g. The signature of this signal appears to be wide band random. No signature similar to the S-IVB gimbal block oscillation was apparent on the various S-II dynamic parameters, i.e., the structural vibrations, the LOX pump inlet pressure fluctuations and the combustion chamber pressure fluctuation. Figure 8-11 compares the spectrum of the S-IVB gimbal block signal with the spectrum of the S-II center engine thrust pad. The spectrum associated with the center engine indicates a very low level response concentrated in the 20 hertz region. The S-IVB gimbal block has the character of a random response across the frequency spectrum. This demonstrates that the S-IVB phenomena is not the result of a forced response due to an excitation emanating from the S-II. The S-IVB gimbal block vibration spectrum shows an order of magnitude increase when the noise occurs whereas the S-IVB LOX pump inlet pressure shows little change, Figure 8-12. The higher levels at frequencies from 5 to 20 hertz on the gimbal block do not occur in the LOX pump inlet pressure. Therefore it is concluded that the disturbance is not valid vibration data. Also, the amplitude during this disturbance, if valid, would produce insignificant dynamic loads on the stage.

During AS-512 S-IVB first burn, low frequency (16 to 20 hertz) longitudinal oscillations very similar to those observed on AS-511 were evident. The AS-512 amplitudes (+0.14 g at gimbal block) were well below the maximum measured on AS-305 (+0.30 g) and within the expected range of values.

AS-512 S-IVB second burn produced intermittent 11 to 16 hertz oscillations similar to those experienced on previous flights. The oscillations began approximately 135 seconds prior to cutoff and had a maximum value of +0.10 g measured on the gimbal block. This compared to +0.05 g on AS-51D and +0.08 g on AS-511.

[edit] 8.2.4 Vibration

There were no significant vibration environments identified on AS-512.

A comparison of AS-512 data with data from previous flights show similar trends and magnitudes.

The "buzz" reported by the astronauts on AS-511 flight is again apparent on AS-512 at approximately 63 hertz in the pump inlet pressure measurement as it has been on previous flights. The vibrations can also be seen on selected propulsion pressure measurements (Figure 8-13). The AS-512 data show amplitudes similar to AS-511 (less than 1.0 psi rms). A review of AS-510 data showed similar vibration at approximately 72 hertz. The vibration is 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 very low amplitude.

[edit] 8.3 S-II POGO Limiting Backup Cutoff System

The backup cutoff system provides for automatic S-II CECO if vibration response levels exceed predetermined levels within the preselected frequency band. The system consists of three sensors, a two-out-of-three voting logic, an engine cutoff arming function, and an automatic disable function which is effective until the arming operation has occurred.

The system did not produce discrete outputs at any time. The accelerometer analog outputs were well below the levels which would produce a discrete output even during the engine start period when the system was not armed. After arming, the analog output did not exceed one g.