After much design work, planning and a lot of machining, a small collection of solid propellant test motor nozzles are finally ready for action. The nozzles are primarily designed to cover the throat diameter range of 6mm to 16mm in suitable steps.
In combination with core burning solid propellant grains, different nozzle throat sizes can be selected to characterize a solid propellant formulation at different pressures and burning rates.
Since a specific solid propellant formulation has a burning rate and pressure that primarily depends on the ratio of burning area to nozzle throat area, it is advantageous to cover a certain throat area interval by nozzles having a fixed relative throat area step size. In this way the smallest number of nozzles are needed to be able to select an appropriate nozzle with a throat area as close as possible to the desired throat area in a relative sense (ie. the smallest percentage difference).
In our case we use approximately 25% throat area increase between successive nozzle sizes to acheive a total throat area range of 711% using a total of 10 different nozzles.
A secondary test objective is to determine the performance of a solid propellant formulation. To facilitate this the nozzles are equipped with a diverging section with an exit area ratio that should achieve a close to optimum expansion when a suitable nozzle is used.