Abstract: The critical stable cross-sectional area of an air cushion surge chamber (ACSC) determined by the open surge chamber stability theory is related to the height of the gas column in the chamber, and both are the factors of the closed gas chamber volume. To understand the relationship of the stability of ACSC water level fluctuations versus the cross-sectional area and the indoor gas volume and verify the factors that determine the water level stability, this paper presents an analysis on the stable conditions of small fluctuations in an ACSC system, and reveals the stability is related to the cross-sectional area and indoor gas volume. We confirm that under the volume minimization conditions that meet the requirements for the structure, layout, construction, etc., the small fluctuation stability of an ACSC system mainly depends on its indoor gas volume. The concept of critical stable cross-sectional area is inaccurate, and an ACSC need not be limited to the equal area type. Then, we come up with new ideas for an ACSC critical stable gas volume and the corresponding design optimization, and develop a calculation formula for the critical stable gas volume with its parameters determined according to strict derivation and determination rules. And a variety of formulas for practical calculations are also derived.

Key words: air cushion surge chamber, stable gas volume, stable cross section area, critical stability, simplified formula, safety factor, shape optimization