The University of Queensland has completed extensive research into the flow of fluid around objects to simulate the re-entry of spacecraft into atmospheres and flow through scramjets (Jacobs et al., Unknown). This has been made possible through the use of expansion tubes which can reach gas flow velocities in excess of 10km/s (Jacobs et al., Unknown).The expansion tubes operate on gas expanding from an extremely high pressure through to a very low pressure. Objects are placed in the path of expansion to take measurements (Davey, 2006).Due to an X3 expansion tube upgrade at UQ the negative pressure volume (dump tank) is insufficient in size to keep sub-atmospheric pressures at a maximum operational envelope. This thesis looks at the design of a new dump tank.The new dump tank has been designed to have an optimised volume in order to minimise negative pressure pumping time yet still hold a sub-atmospheric pressure after a maximumenvelope shock has been created (R Morgan 2007, pers. Comm., March).Comparisons between measured pressure and predicted pressure were used on the X2 and T4 expansion tubes to find an accurate technique to calculate the required new dump tank volume. The most accurate analytical technique investigated was found to assume isentropic expansion of the gas with a specific heat ration of 1.4. One-dimensional modelling was run with unrealistic computation times and axisymmetric modelling was found to over estimate the spent gas pressure.