A two-stage free-piston driver

The overall cost of free-piston driven facilities can be substantially reduced if the contraction between the compression and shock tubes is replaced with a constant area section. However, with such an implementation, a new driver concept is required in order to achieve a realistic facility length. This paper describes a new free-piston driver type for expansion tubes which satisfies the above criteria. The technique is known as the two-stage free-piston driver where the driver gas is compressed in two distinct stages with a unique compound piston design. A new facility has been constructed (X-2) which is described in some detail. A quasi-one-dimensional numerical model of the compression process is also developed which agrees well with driver tube experimental results. This new driver is coupled to an expansion tube arrangement where super-orbital test flows are generated. The results show that a two-stage free-piston driver is capable of driving hypervelocity expansion tubes and therefore new facilities of increased size but reduced cost are now possible. Received 18 May 1998 / Accepted 18 October 1998