Centre for Hypersonics

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X3 is a large super-orbital expansion tube.  Over 300 shots have been fired. It is currently being upgraded with an improved driver.

  • X3 is a free-piston driven expansion tube with a test exit diameter of 180 mm and is capable of producing stagnation enthalpies above 100 MJ/kg.

Recent interest in interplanetary atmospheric entry calls for manoeuvers at superorbital velocities (above 8 km/s). In order to understand the gas dynamics which occur around such hypervelocity craft, unique ground test facilities are required.  The University of Queensland has pioneered the development of the free-piston expansion tube as a device capable of studying superorbital gas dynamic flows (Morgan and Stalker 1992; Neely and Morgan 1994; Neely et al. 1991; Sutcliffe and Morgan 1997; Palmer and Morgan, 1997).

Free-piston drivers generate a high temperature driver gas used for the generation of strong shock waves by rapidly compressing a light gas (such as helium) in a tube using a heavy piston driven by a large air reservoir. Stalker (1966; 1967) developed the free-piston driver for use with shock tubes and reflected shock tunnels. Conventional free-piston driver designs make use of a large area change situated at the primary diaphragm station (Stalker 1964) which allows a shock of higher strength to be driven compared with the equivalent driver conditions in a constant area driver. Also, by maintaining piston speed after diaphragm rupture, a reasonably constant pressure exists in the driver region which increases the test time by delaying the arrival of unsteady expansion waves at the test section.

In an ideal, free-piston expansion tube is the piston compresses the driver gas in an approximately isentropic process. After the piston stroke, the driver contains hot, compressed gas (usually helium) used for the production of a shock wave in an air filled shock tube through the bursting of a steel primary diaphragm. The air test gas is heated and accelerated in the shock tube before the primary shock arrives at a light secondary diaphragm, which immediately ruptures and a fast secondary shock travels along the acceleration tube. The test gas is processed by an unsteady expansion wave to the test conditions which are a function of the initial fill pressures in each tube. The unsteady expansion wave has the desirable properties of increasing the stagnation pressure and enthalpy of the test gas. This phenomena is called enthalpy multiplication and a full description of the theoretical operation of the expansion tube can be found in Trimpi (1962).


Morgan RG, Stalker RJ (1992) Double diaphragm driven free piston expansion tube. In: Proc 18th Int Symp on Shock Tubes and Waves, Springer-Verlag

Palmer RA, Morgan RG (1997) Stagnation point heat transfer measurements in superorbital expansion tubes. AIAA Paper 97-0280

Neely AJ, Morgan RG (1994) The superorbital expansion tube concept, experiment and analysis. Aeronautical J March:97

Neely AJ, Stalker RJ, Paull A (1991) High enthalpy, hypervelocity flows of air and argon in an expasion tube. Aeronautical J June/July:175

Stalker RJ (1964) Area change with a free-piston shock tube. AIAA J, 2:396

Stalker RJ (1966) The free-piston shock tube. Aeronautical Quart, 17:351

Sutcliffe M, Morgan RG (1997) Experimental flat plate investigation of hypersonic carbon dioxide flows. AIAA Paper 97-0475

Stalker RJ (1967) A study of the free-piston shock tunnel. AIAA J, 5:2160

Trimpi RL (1962) A preliminary theoretical study of the expansion tube, a new device for producing high-enthalpy short-duration hypersonic gas flows. Tech Rep R-133, NASA