This article was originally published in Hydraulics & Pneumatics in December 1963.
The S-IC stage of the Saturn V launch vehicle will be propelled by five F-1 engines generating a total thrust of about 7.5 million pounds. This compares with the eight H-1 engines of the S-1 stage of the Saturn I vehicle, generating a total thrust of 1.5 million pounds.
Booster for the Saturn V is this S-IC stage. It has five F-1 engines, each producing about 1.5 million pounds of thrust. The four outer engines are gimballed. The center one is fixed.
Preliminary design calculations indicated that considerable beefing-up of gimbal systems of the type used for the Jupiter and Saturn I vehicles would be necessary to meet the greatly increased requirements of the S-IC. Thus, it became appropriate to consider whether it would be more advantageous to design an altogether new system employing the latest concepts in hydraulic actuation.
The final choice was between a high-pressure, closed-loop, conventional hydraulic system utilizing MIL-H-5606 and a fueldraulic system in which RP-1 (kerosene) fuel, taken from the high-pressure side of the turbopump, is used as the fluid medium in a single-pass system. Flow from the actuator returns to the fuel system, rather than to a hydraulic reservoir. The figure permits a comparison of the essential features of the two systems. Because each became a matter of considerable controversy, with different groups favoring whichever system most nearly fulfilled their specific requirements.
Conventional System vs. Fueldraulic
A major factor favoring selection of a beefed-up conventional hydraulic system was the extensive experience gained from previous vehicles. However, increasing the operating pressure and flow rate for such a system from 3,000 to 4,000 psi and from approximately 15 to 100 gpm, respectively, to meet S-IC requirements would necessitate redesign and qualification of most major components. Therefore, it was argued that these changes would exceed the state-of-the-art to such an extent as to partly negate the value of previous experience.
Gimbal systems proposed for the F-1, oxygen-kerosene engine. The fueldraulic system (left) is simpler, with fewer components than the high-pressure hydraulic system. To simplify the illustration, flexible fittings and hose, and lines to other gimballing actuators and to other fluid power systems on the S-IC stage are not shown.
Estimated weights for flight weight hardware were approximately equal for the two systems. Also, the dynamic response of each system appeared adequate. Cost and delivery schedules appeared to favor the fueldraulic system, principally because of the smaller number of components. This smaller number of components also suggested that the fueldraulic system would have fewer potential leak points and would constitute a less complex and, therefore, more reliable system.
A conventional, high-pressure hydraulic system has more components and is more complicated than the fueldraulic system shown in the previous illustration.
On the other hand, certain system aspects, such as development of redundancy and mechanical position feedback, appeared to be easier to achieve with the high pressure hydraulic system. Also, areas of uncertainty associated with contamination control, flammability and chemical compatibility of materials with RP-1 tended to favor the high-pressure hydraulic system.
After consideration of the many aspects of the problem, the decision was made to adopt the fueldraulic system; to give immediate attention to potential problem areas associated with this system; and to continue development of the high pressure hydraulic system for possible future applications.
Low Flash Point—The most pressing problem arose from the fact that during quality checkout, the temperature of the RP-1 exceeds its flash point. Any leakage would, therefore, constitute a serious fire hazard, particularly since the checkout operations are performed in closed hangers. Use of a less hazardous fluid for system checkout, therefore, was considered mandatory. After reviewing numerous fluid specifications, RJ-1, a commercially available jet fuel, was found to be very similar to RP-1 with respect to chemical composition, viscosity, and density. However, the flash point was sufficiently high that by means of special purchasing requirements (including batch-wise testing) it was possible to insure that it did not fall below 200°F, the lower limit recommended for this application based on safety considerations.
Exposed to Three Fluids—While use of RJ-1 for checkout alleviated one problem, it contributed to another. Because MIL-H-56J6 has much better lubricity properties than RP-1, component checkout will be done with MIL-H-5606 to insure maximum life for checkout equipment. System checkout will be with RJ-1 as indicated above and static firing will be with RP-1. Thus, the system will be exposed successively to each of several different fluids. Although the properties of these fluids are sufficiently similar that compatibility problems are not expected (see the table), the practice of sequencing different fluids so many times certainly is not desirable and consequences are being evaluated.
Contamination in a Quarter Million Gallons—Of all the problems encountered with the fueldraulic system, the area of greatest uncertainty probably was that associated with the effects of particulate contamination. Thus, the design and location of filters and the allowable levels of contamination for conventional hydraulic systems of at most a few gallons capacity would be expected to differ markedly from those for a one-pass system for which the fluid capacity is almost a quarter million gallons.
Studies of particulate contamination in RP-l as supplied to existing vehicles have been implemented to obtain background information for the S-IC vehicle. Because of the lower viscosity and density, it was expected that foreign particles would settle more quickly and contamination levels, therefore, would be lower in RP-1 than in MIL-H-5606. Samples of RP-1 collected from ground support equipment tended to confirm this expectation, except for a much larger concentration of fibers, which, apparently, were released by the water separator. A limited evaluation of supply line filters, therefore, has been initiated, emphasis being placed on removal of fibers.
Automatic Particle Counters
Because filter evaluation studies and contamination surveys require analyses of hundreds of fluid samples, a comprehensive investigation of automatic particle counters was initiated to facilitate selection of counters for contractor and in-house use. One of the major problems in this respect is that of withdrawing representative samples of approximately 100 ml volume from supply lines of 80 to 200 gpm capacity. When ordinary bleed valves are used for sampling, clean samples frequently are obtained from dirty systems due to non-uniform flow conditions.
Studies are continuing on these and other problems such as compatibility, gum formation, bacteria growth, lubricity, and susceptibility to corrosion. Many questions remain to be answered. However, in many respects it appears that the technology of the fueldraulic system, particularly with respect to contamination control problems, is further advanced, relative to development of the overall vehicle, than has been the case with previous vehicles at this stage of development.