University of Canterbury PhD students Malcolm Snowdon and Avinash Rao are developing flight control systems for rockets. They use electronic sensors to measure any changes in motion so that moveable fins automatically respond.

Controlling pitch, yaw and roll

Pitch is a measure of how high or low the nose cone is pointing.

Yaw is a measure of how far to the left or the right the nose cone is pointing.

Roll is a measure of how much the rocket has rotated on its longest axis.

Pitch, yaw and roll are called the ‘three degrees of freedom’ that refer to the rotational movement in any direction. The easiest way to think of these is to imagine you are a rocket. If you lean forwards or backwards, that’s pitch. If you lean from side to side, that’s yaw. If you spin around, that’s roll

The first problem in controlling which direction a rocket will fly is to get it pointing in the right direction.

Malcolm and Avinash began their project at the University of Canterbury to automatically control the roll of a rocket. An electronic sensor was used to measure how much the rocket rolls as it moves through the air. Moveable fins at the tail of the rocket were programmed to automatically correct the roll to keep the rocket flying without roll

They built vertical wind tunnels to test their designs and systems before launching their rocket for a full launch test. Their rockets are designed to fly between 600 and 900 metres high. Altitude was not the major focus. Their focus was to build a rocket to respond to changes in roll and to correct itself during flight. All systems performed as expected.

After successfully developing roll control, Malcolm and Avinash are now developing methods to control yaw and pitch as well. This has required more advanced engineering, electronics and programming.

Translational movement

There are three more degrees of freedom that refer to translational movement – movement of the whole rocket in the x, y and z axes. Measuring and controlling the position and orientation of a rocket as it is moving through the air with all of these possible directions is a complex task.

Six degrees of freedom

The electronic sensor measures any movement in all six degrees of freedom. The rocket is programmed to move small aerofoils near the front end of the rocket to control the rocket fully. This is a more sensitive and accurate method of control than just moving flaps on the large fins at the back of the rocket. Malcolm and Avinash are working towards developing their control systems to fully control the flight of a rocket in all six degrees of freedom.

Subsonic versus supersonic flight control

Research began with speeds of up to half the speed of sound. Once the problems associated with this have been mastered, they intend to develop control for rockets travelling faster than the speed of sound.

Near the sound barrier, shock waves from the small fins no longer act in the same way. Shock waves form off all leading surfaces as they punch their way through the air. This changes the way the air flows over the small fins near the base of the nose cone. Powerful and rigid actuators are required to maintain control of the fins as they are battered by these shock waves, and they no longer control the rocket as much. Controlling the rocket becomes much more complex.

Malcolm and Avinash are based at the University of Canterbury. Their current research is supported by scholarships and collaboration with Rocket Lab in Auckland.

    Published 30 November 2011