Publication Year

1970

Keywords

liquid-feed system, servovalve, fuel, engineering, design, combustion

Disciplines

Aerospace Engineering | Biomedical Engineering and Bioengineering | Engineering | Heat Transfer, Combustion | Physical Sciences and Mathematics

Abstract

With the advent of space exploration the demand for variable-thrust rocket engines grew appreciably. A fairly straightforward solution to this need is a hybrid system consisting of a solid fuel and liquid oxidizer. A servovalve can then be employed to regulate the oxidizer flow to the combustion chamber, thereby controlling combustion pressure and ultimately engine thrust. The increased demand for controlled-thrust rocket engines has therefore called for the use of servovalves; unfortunately, standard servovalves currently available in industry are designed upon the premise of a constant pressure drop across the valve as is provided in operation between a constant supply pressure and the atmosphere. Propellant flow to a combustion chamber is not a similar arrangement, since the system's pressure will vary greatly as the flow is throttled, resulting in pressure-drop variations across the servovalve of nearly an order of magnitude.

A large variation in pressure-drop across the servovalve destroys the desired linearity of flow gain factor with respect to valve stem position. As an example, this means a typical servovalve must travel eighty-four percent of its total stroke to cut the flow in half. If throttling ratios on the order of five to one are desired, the remaining flow must be throttled over only sixteen percent of the stroke. Obviously, the system becomes more sensitive to valve stem position as the flow is reduced. True, an adaptive control system can be sued to compensate for this variable gain, but such electronic systems are complex and costly. A common solution employs a compromise constant-gain electronic system, with the gain set to give acceptable performance at some point of the stroke. Such a compromise cannot provide consistent performance over wide ranges, thereby restricting throttling to some range about the compromise point.

This paper details a suggested servovalve design which could give the liquid-feed system designed a third alternative. The new design attempts to determine a variable flow gain factor which yields a liquid-feed system with constant-gain characteristics notwithstanding wide pressure-drop variations across the servovalve. The description of the design is not the major goal of the paper, but is included as a starting point in order to answer two major questions concerning the design:

(1) Does the design's dependence upon system parameters make it impractical for industry?

(2) Does the design accomplish its goal despite the use of linearization techniques?

The first question involves a parametric study to determine the extent and nature of the dependence upon system parameters. A digital simulation of a complete system is the major tool in answering the second question. Both questions were answered favorably by the studies performed. The parametric study indicated that a single constant-gain servovalve is applicable to a family of systems, placing only a reasonable restriction on the system designer. In the simulation, the constant-gain servovalve showed the desired characteristic of stability with a simple control system, whereas the standard servovalve caused system instability at reduced flows because of the inherent system gain change. These favorable answers signal an important step in simplifying liquid-feed systems driven by constant supply pressures and requiring constant overall system gain.

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