Reports/Apollo 17/Saturn V flight evaluation/5 S-IC Propulsion

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[edit] 5.1 Summary

All S-IC propulsion systems performed satisfactorily. In all cases, the propulsion performance was very close to the predicted nominal. Overall stage site thrust was 0.30 percent higher than predicted. Total propellant consumption rate was 0.16 percent higher than predicted and the total consumed mixture ratio was 0.002 percent higher than predicted. Specific impulse was 0.14 percent higher than predicted. Total propellant consumption from Holddown Arm (HDA) release to Outboard Engines Cutoff (OECO) was low by 0.14 percent.

Center Engine Cutoff (CECO) was initiated by the Instrument Unit (IU) at 139.30-seconds, 0.02 seconds earlier than planned. OECO was initiated by the fuel depletion sensors at 161.20 seconds, 0.47 seconds earlier than predicted. This is well within the +5.99, -4.22 second 3-sigma limits. At OECO, the LOX residual was 36,479 lbm compared to the predicted 37,235 lbm and the fuel residual was 26,305 lbm compared to the predicted 29,956 lbm.

The S-IC hydraulic system performed satisfactorily.

[edit] 5.2 S-IC Ignition Transient Performance

The fuel pump inlet prestart pressure of 45.3 psia was within the F-1 engine acceptable starting region of 43.3 to 110 psia. The LOX pump inlet prestart pressure and temperature were 81.3 psia and -287.3°F and were within F-1 engine acceptable starting region, as shown by Figure 5-1.

The planned 1-2-2 F-1 Engine start sequence (Engines 5, 3-1, 4-2) was not achieved. Two engines are considered to start together if both thrust chamber pressures reach 100 psig within 100 milliseconds. By this definition, the starting order was 2-1-1-1 (Engines 5-3, 1, 4, 2). The buildup times of all five engines as measured from engine control valve open signal to 100 psig chamber pressure, Table 5-1, were faster than predicted, although within specifications. The 2-1-1-1 start sequence had no adverse affect on either propulsion system performance or on the structure.

• Figure 5-1. 5-IC LOX Start Box Requirements
• Table E-1. F-1 Engine Systems Buildup Times

BUILDUP TINE, SECONDS ENGINE 1 ENGINE 2 ENGINE 3 ENGINE 4 ENGINE 5 Predicted* 4.057 3 ;65 3.925 3.990 3.933 Actual* 3.862 "..861 3.605 3.669 3.819 Difference 0.195 0.104 0.320 0.321 0.114 Direction 1 Fast j Fast iFast Fast Fast

• Time from 4-way control valve open signal to 100 psis combustion chamber pressure All times corrected to nominal prestart conditions

The desired 1-2-2 start sequence was also not achieved on flights AS-507, AS-508, and AS-510. The timing of the start signals to each engine is adjusted to achieve the desired start sequence and is based on data from individual engine firings and the single data sample in the stage environment obtained from static firing. Typically, a wide dispersion of start times is observed at the stage static firing. This dispersion is attributed primarily to the differences between the stage conditions and single engine test stand conditions. Adjustments made between stage static firing and launch have been effective in reducing the dispersions substantially. However, it is apparent from review of data from all the Saturn V launches, that the system cannot be fine tuned accurately enough to consistently assure the desired start sequence within the 100 ms criterion. This fact is probably attributable to a combination of the limited data sample in the stage environment and typical engine start time dispersions even under controlled conditions.

The structural implications of a non-standard engine start sequence for the Skylab mission have been examined considering significantly larger dispersions than experienced on AS-512 and other Saturn V flights, and there is no concern. Accordingly, no modification of the present engine start sequence implementation is planned.

The reconstructed propellant consumption during holddown (from ignition command to holddown arm release) was 75,090 lbm LOX (67,031 Ibm predicted) and 22015 lbm fuel- (18,764 lbm predicted). The greater than predicted propellant consumption during holddown was due to the faster engine start and longer burn before holddown release. The reconstructed propellant load at holddown arm release was 3,239,298 lbm LOX (3,243,932 lbm predicted) and 1,409,906 lbm fuel (1,415,766 lbm predicted).

Thrust buildup rates were as expected, as shown in Figure 5-2.

• Figure 5-2. S-IC Engines Thrust Buildup

The engine. Main Oxidizer Valve (MOV), Main Fuel Valve (MFV), and Gas Generator (GG) ball valve opening times were nominal.

[edit] 5.3 S-IC Mainstage Performance

S-IC stage propulsion performance was satisfactory. Stage thrust, specific impulse, mixture ratio, and propellant flowrate were near nominal predictions as shown in Figure 5-3. The stage site thrust (averaged from time zero to OECO) was 0.30 percent higher than predicted. Total propellant consumption rate was 0.16 percent higher than predicted and the total consumed mixture ratio was 0.002 percent higher than predicted. The specific impulse was 0.14 percent higher than predicted. Total propellant consumption from HDA release to OECO was low by C.14 percent.

For comparison of F-1 engine flight performance with predicted performance the flight performance has been analytically reduced to standard conditions and compared to the predicted performance which is based on ground firings and also reduced to standard conditions. These comparisons are shown for the 35 to 38-second time slice. The largest thrust deviation from the predicted value was -7 klbf for engine 2. Engines 1 and 5 had lower thrusts than predicted by 6 and 1 klbf, respectively. Engines 3 and 4 had higher thrust than predicted by 1 and 2 klbf, respectively. Total stage thrust was 11 Klbf lower than predicted for an average of -2.2 klbf/engine. These performance values are derived from a reconstruction math model that uses a chamber pressure and pump speed match.

An 11 Hz, 8 psi peak amplitude, oscillation was observed in the S-IC Engine No. 2 fuel suction line inlet pressure. This oscillation was also observed during S-IC-12 static test and disposed of at that time as no problem. This phenomenon is a self-induced oscillation characteristic of the F-1 fuel pump and has been observed on previous flights. The oscillation is Net Positive Suction Pressure (NPSP) dependent and its sensitivity varies from engine to engine. The stage accelerometer data are nominal at 11 Hz and comparable to that of previous flights, indicating the vehicle structural gain at this frequency is small.

The ambient gas temperature under Engine No. 1 cocoon increased shortly after liftoff and exceeded previous flight data from approximately 30 to 65 seconds by a maximum of about 13°C. After 100 seconds the temperature returned to a normal level and remained similar to the cocoon ambient temperature level for the other engines. The increase in the ambient gas temperature did not affect engine performance during flight. The two most probable causes of the temperature increase are: 1) a minor hot gas leakage from the Gas Generator drain port plug which subsequently sealed, 2) a temporary loss of cocoon insulation integrity (possible loose combustion drain access cover) which later corrected itself. Both of these possible causes for the cocoon ambient temperature rise are discussed in detail in Section 13.2 Vehicle Thermal Environment.

• Figure 5-3, S-1C Stage Propulsion Performance
• Table 5-2. S-IC Individual Standard Sea level Engine Performance

[edit] 5.4 S-IC Engine Shutdown Transient Performance

The F-1 engine thrust decay transient was nominal. The cutoff impulse. measured -from cutoff signal to zero thrust, was 669.632 lbf-s for the center engine (0.1 percent less than predicted) and 2,593,423 lbf-s for all outboard engines (3.0 percent greater than predicted). The total stage cutoff impulse of 3,263,055 lbf-s was 2.3 percent greater than predicted.

Center engine cutoff was initiated by the IU at 139.30 seconds, 0.02 second earlier than planned. Cutoff signal to the outboard engines was initiated by fuel depletion and occurred 0.47 second earlier than the nominal predicted tire of 161.67 seconds. The fuel depletion cutoff was caused by the higher than predicted fuel density due to chilldown of the fuel during the 2 hour 40 minute hold and the slightly higher than nominal batch fuel density for this flight. The early cutoff was due mainly to slightly higher than predicted stage site thrust (0.03 percent higher) and the accompanying higher propellant flowrates.

[edit] 5.5 S-IC Stage Propellant Management

The S-IC stage does not have an active propellant utilization system. minimum residuals are obtained by attempting to load the mixture ratio expected to be consumed by the engines plus the predicted unusable residuals. An analysis of the residuals experienced during a flight is a good measure of the performance of the passive propellant utilization system.

The residual LOX at OECO was 36,479 lbm compared to the predicted value of 37,235 lbm. The fuel residual at OECO was 25,305 lbm compared to the predicted value of 29,956 lbm. A summary of the propellants remaining at major event times is presented in Table 5-3.

[edit] 5.6 S-IC Pressurization Systems

[edit] 5.6.1 S-IC Fuel Pressurization System

The fuel tank pressurization system performed satisfactorily, keeping ullage pressure within acceptable limits during flight. Helium Flow Control Valves (NFU) 1 through 4 opened as planned and NFU No. 5 was not required.

The low flow prepressurization system was commanded on at -97.0 seconds. The low flow system was cycled on a second time at -3.1 seconds. High flow pressurization, accomplished by the onboard pressurization system, performed as expected. HFCV No. 1 was commanded on at -2.7 seconds and was supplemented by the ground high flow prepressurization system until umbilical disconnect.

• Table 5-3. .S-IC Propellant Mass History

Fuel UM ullage pressure was within the predicted limits throughout flight as shown by Figure S-4. HFCV So.'s 2, 3 and 4 were commanded open during flight by the switch selector within acceptable limits. Helium bottle pressure was 3000 psis at -2.8 seconds and decayed to 475 psia at OECO. Total helium flowrate was as expected.

Fuel pump inlet pressure was maintained above the required minimum Net Positive Suction Pressure (PSP) during flight.

[edit] 5.6 S-IC LOX Pressurization System

The LOX pressurization system performed satisfactorily and all performance requirements were net. The ground prepressurization system maintained ullage pressure within acceptable limits until launch omit.

The onboard pressurization system performed satisfactorily during flight.

The prepressurization system was initiated at -72.0 seconds. Ullage pressure increased to the prepressurization switch band and flow was terminated at -58.3 seconds. The low flow system was cycled on three additional times at -42.9. -20.8, and -5.4 seconds. At -4.7 seconds. the high flow system was commanded on and maintained 'ullage pressure within acceptable limits until launch commit.

• Figure 5-4. S-IC Fuel Tank Ullage Pressure

Ullage pressure was within the predicted limits throughout flight as shown in figure 5-5. GOx flowrate to the tank was as expected. The maximun GOX flowrate after the initial transient was 48.8 lbm/s at CECO.

The LOX pump inlet pressure met the minimum NPSP requirement throughout flight.

[edit] 5.7 S-IC Pneumatic Control Pressure System

The control pressure system functioned satisfactorily throughout the S-IC flight.

Sphere pressure was 2970 psia at liftoff and remained steady until CECO when it decreased to 2850 psia. The decrease was due to center engine prevalve actuation. There was a further decrease to 2475 psia after OECO. Pressure regulator performance was within limits. The engine prevalves were closed after CECO and OECO as required.

[edit] 5.8 S-IC Purge Systems

Perfornance of the curve systems was satisfactory during flight.

The turbodump LOX seal storage sphere pressure of 2955 psia at liftoff was within the prestart limits of 2700 to 3300 psia. Pressure was within the predicted envelope throughout flight and was 2805 psia at OECO.

The pressure regulator performance throughout the flight was within. the 85 ±10 psig limits.

[edit] 5.9 S-IC POGO Suppression System

The POGO suppression system performed satisfactorily during S-IC flight.

Outboard LOX prevalve temperature measurements indicated that the pre-valve cavities were filled with gas prior to liftoff as planned. The four resistance thermometers behaved during the AS-512 flight similarly to the flight of A5-511. The temperature measurements in the outboard LOX prevalve cavities remained warm (off scale high) throughout flight, indicating helium remained in the prevalves as planned. The two thermometers in the center engine prevalve were cold, indicating LOX in this valve as planned. The pressure and flowrate in the system were nominal.

[edit] 5.10 S-IC Hydraulic System

The performance of the S-IC hydraulic system was satisfactory. All servo-actuator supply pressures were within required limits. Engine control system return pressures were within predicted limits and the engine hydraulic control system valves operated as planned.