Moon mission

Researchers at Purdue University are conducting NASA-funded research to develop rockets faster and less expensively for missions to Mars and the moon. The work will focus on liquid-fuelled rockets and in particular understanding how fuel and a component called the oxidiser interact inside the rocket engine’s fuel injectors to cause unstable combustion.

This instability is a complex phenomenon which has continually plagued rocket development as it causes extreme bursts of heat and pressure fluctuations that could lead to accidents and hardware damage. Therefore more knowledge is needed before future engines can be developed and used for space flight.

Heat from combustion naturally fluctuates inside the combustion chamber. At the same time, the combustion chamber generates resonant sound waves that cause ‘acoustic pressure’ which also fluctuates. When heat and pressure fluctuations coincide, the combined result can be devastating, causing accidents and damage to rocket engines.

A paper published by the university demonstrated that an experiment can be designed to study instabilities occurring simultaneously, something which has been virtually impossible in the past. Engineers used a carefully designed injector and varied the length of the combustion chamber to see how changing acoustics affected the heat-driven pressure fluctuations.

Data are collected using pressure and heat sensors inside the chamber, and the researchers also take high-speed video of the combustion process to analyse instability.

At the American Institute of Aeronautics and Astronautics’ joint propulsion conference in California in July the researchers presented new findings. Nicholas Nugent, at Purdue’s School of Aeronautics and Astronautics explained that the work was carried to generate combustion and instability data so that other researchers can develop better computational models for designing rocket engines.

Without effective simulations, engineers have to rely on trial and error, which is costly, time consuming and potentially dangerous. Nugent added that more computer modelling at the beginning of the research reduces the risk of damaging expensive hardware as well as reducing the amount of testing required.

Future work will use optical sensors to measure more precisely the dynamic interactions between combustion fluctuations and fluctuations in acoustic pressure.

The team are hopeful that the work may also benefit the US Air Force since the injector used in the experiment is similar to those employed in advanced, high-performance rockets that use kerosene fuel. These rockets require less time to prepare for launch than conventional rockets, meaning they could be quickly sent on military missions. Unlike space shuttle engines that require foam-insulated tank for the cryogenically cooled liquid hydrogen propellant, the ‘oxidiser-rich-stage-combustion cycle’ engines on which the experiment is based use a kerosene fuel that can be stored at room temperature. Using kerosene would enable engineers to create sleeker, more compact and lighter rockets that possess the same power as liquid hydrogen rockets.