Design of lightweight pistons for the X2 and X3 expansion tube free-piston drivers

Initial experimentation using X2 to simulate a Mach 13 scramjet flight condition indicated the requirement to develop a tuned free-piston driver. This involves running the piston at much higher velocities around the time of diaphragm rupture in order to sustain high driver pressures for significantly increased duration. Central to the tuned driver is a lightweight piston, which enables the piston to be accelerated to high speeds, and subsequently decelerated, over the same fixed length of compression tube. This paper discusses the background to the tuned driver requirement for X2, and describes the design, stress analysis, and final configuration of a new lightweight aluminium piston. Experimental results for X2 using the new tuned driver are presented, indicating the successful implementation of the concept.