By Prof. Tony Musker (DELTACAT Ltd)
The isochoric (constant-volume process) hybrid rocket that is being developed in HYPROGEO requires a constant supply of high temperature gaseous oxygen. This oxygen will be generated using a catalytic decomposition chamber (a chamber where propellants decompose when in contact with a catalyst), which is fed with liquid hydrogen peroxide, a strong oxidant. The higher the concentration of this peroxide, the higher the temperature of the oxygen being fed to the hybrid rocket’s combustion chamber. HYPROGEO seeks to utilise the highest concentration possible in order to achieve high performance, as measured by specific impulse, which is often the most used indicator of performance and efficiency of a rocket engine. The catalyst being used for HYPROGEO is a material called PX1. PX1 has a ceramic substrate, which is susceptible to thermal shock. The material has been used extensively by DELTACAT and AIRBUS Stevenage in conjunction with 87.5% mass concentration of hydrogen peroxide. What is not known is whether PX1 will withstand the initial thermal shock when using peroxide concentrations that are significantly higher than 87.5%. To investigate this, DELTACAT has designed and manufactured a so-called characterisation chamber, which contains a PX1 catalyst bed (referred to within HYPROGEO as the characterisation bed). The complete device is shown in the first picture on the left.
Peroxide enters from the left and passes through an injector plate and nickel diffuser. After negotiating an internal ring baffle, or fence, the fluid is directed through a stainless steel gauze (a weave structure in which the weft yarns are arranged in pairs and are crossed before and after each warp yarn keeping the weft firmly in place) and a perforated retainer plate. The nozzle provides a sonic choke, i.e. accelerates the fluid to the speed of sound, to allow the chamber pressure to reach an appropriate value. By the time the gas reaches the retainer plate, the oxygen (and superheated steam) will have reached a stagnation temperature of between 600-950 degrees Celsius (At a stagnation point the speed of the fluid is zero and all of the kinetic energy has been converted to internal energy), depending on the peroxide concentration used. Temperatures and pressures will be measured using transducers attached to the tops of the standpipes shown in the above CAD model. The device is manufactured from Inconel 625, which provides the required strength at high temperature. The completed system is shown in the second picture below.