Hydraulic Controls for Gimbaling Saturn V Engines

1. First-stage booster of Saturn V space vehicle has five engines. Hydraulic system gimbals four outboard engines in flight.

Manifold Supply—Figure 1 shows the fluid power system. The ring manifold supplies fluid to the engine gimbal system and the engine start control valves.

The filter manifold has two inlet ports so fluid can flow into it from either the ground supply or the engine fuel turbopump. The two check valves prevent backflow from one power source to the other,

Checkout Procedure—The engine gimbal system is checked out at 2000 psi. After checkout, and several minutes before launch, ground pressure is lowered to 1500 psi to match the pressure of the F-1 engine-start control system timing.

The engines must be held on center to start them because the thrust chamber jacket chambers are filled with an inert fluid.

Gimbaling the engines just before ignition would spill some of the fluid overboard, causing air pockets, or allowing fuel to precede the first fluid to the injectors. Either could result in rough combustion. The GSE requirements for checkout of the gimbal system at 2000 psi are shown in Figure 2. The 140-gpm flow represents the leakage of eight servoactuators and five engine start control systems. The peak flows shown Figure 2 are for checking out the gimbal system.

2. Flow requirements for actuator checkout gimbal system.

Flow requirements at 1500 psi during engine start are shown in Figure 3. Immediately before engine start, requirements are due to servoactuator leakage and engine start control valves. Peak flows after engine startup represent the opening of the main LOX valve and the main fuel valve.

3. Flow requirements for engine start during static firing and launch.

Ducting—Two of the system’s requirements are zero leakage all joints and the use of flanged connections. To do this, the fluid-power system uses universal gimbal joints, which provide the required line flexibility in the high-pressure supply lines and flexible metal hose in the return lines.

Ground Supply Fluid—Because the flash point of RP-1 fuel, which supplies the system in flight, is 110 to 139° F, it is classified as a Class Ill flammable liquid, not suitable for ground operations. A study was made to find substitute fluids with properties similar to RP-1 that could be used in the laboratory and in ground operations.

Such a fluid was found in RJ-1, a ramjet fuel. It has properties very similar to RP-1, but a flashpoint ranging from 190 to 208° F. If purchased with a specified flashpoint exceeding 200° F, it is not classed as a flammable liquid by the National Code and would not have the same stringent handling requirements of RP-l fuels. Therefore, the RJ-l fuel can be used in the laboratories or in GSE with the same precautions as taken with MIL-H-5606.

Contamination—One of the greatest problems in using fuel as a hydraulic fluid was the effect of contamination. To determine the particulate contamination level of RP-l fuel used in the Saturn S-l booster, a fuel-sampling program was conducted at the Test Division of the Marshall Space Flight Center and at the Atlantic Missile Range. This program was to determine:

• what contamination would be encountered in the direct fuel system and
• how well present fuel loading facilities could meet filtration requirements for this system.

Figure 4 shows results of contamination counts. The samples show a high percentage of samples within specified limits. These levels were obtained in facilities not intended for use with servo control systems. With a careful redesign (and this is being done), the new facilities should be able to meet requirements consistently.

4. Particulate contamination count of S-1 fuel samples. Curves compare SAE-4 contamination level with Marshall Space Flight Center specifications.

Filtration—It is not enough to know that the required cleanliness levels can be met. Because of what would happen if dirty fuel entered the booster, a study is now being made of techniques for continuous monitoring of the contamination level of entering fuel. Remote control, automatic contamination particle counters may be used to do the job.

During the studies, the question was raised that even if the fuel was clean when it entered the fuel tank, that the wetted area of the tank would contaminate the fuel and thus make flight filtration still almost impossible. An analytical study was undertaken to determine how much contamination the tank added and what techniques, if necessary, could he used to filter the fuel in flight.

Filter Maintenance—To determine the expected total life of the power source filter, previous data and experience on the Saturn I vehicle were used. This included functional checkout, manufacturing checkout, static firing, launch operation checkout, and flight, which showed a total life from 10 to 14 hours. This is not exceptionally high, but at the time of this study it was not certain what contamination levels could be expected from each operation, especially from the static firings and flights. At that time these were the alternatives:

• Monitor filter pressure drop and change elements as required.
• Use a large, conventional filter with a very high dirt-holding capacity.
• Use a self-cleaning filter to achieve very long filter life.

To meet manufacturing and delivery dates, the first alternative was adopted.

Study of the third alternative uncovered a filtration technique used in jet engine fuel control systems. This technique is based on a wash filter concept developed to meet the stringent requirements of MIL-E-5007B. Figure 5 shows a schematic of the filter. It appears to have virtually infinite life and is extremely simple. It requires a constant flow from which to tap. This filtration concept is being pursued and could be used as an alternative if there are unforeseen contamination problems.

5. Wash filter design is used in filtering jet engine fuels. It could also be used in filtering fluids for engine gimbal controls.

More Contamination Studies: Servovalves—A study conducted by Franklin Institute of the effects of fluid contamination on H-1 engine hydraulic systems and components shows that servovalves could operate satisfactorily at much higher contamination levels than presently permitted. These tests were run at SAE contamination levels 2, 3, and 4 (see table).

MSFC’s contamination level, as specified in MSFCSPEC-166, corresponds closely to SAE-2. The test showed that the servovalves could operate up to 55 hours at SAE-4 contamination level and still meet specification performance. This level is 4 to 5 times greater than the present level.

John Hadel is Research Engineer, Saturn Booster Branch, The Boeing Co., Huntsville, Ala. Royce Church was a Research Engineer at the Boeing Co. when this article was submitted for publication. He is now Aerospace Engineer with NASA at Marshall Space Flight Center, Huntsville, Ala.