Engineers at University College Dublin have recently been awarded a contract by the European Space Agency to find new ways to control vibrations in the rocket designs of tomorrow, writes Dr William O’Connor
Mech

 

Author: Dr William J. O’Connor, BE PhD FIEI CEng, senior lecturer, School of Mechanical & Materials Engineering, University College Dublin

Engineers at University College Dublin (UCD) have recently been awarded a contract by the European Space Agency (ESA) to find new ways to control vibrations in future rocket designs.

The Dynamics and Control Group at UCD has been researching the general problem of controlling the motion of flexible mechanical systems. The problem arises in many fields of engineering, from computer disk drives, through long-arm manipulators, up to large cranes. The challenge of controlling flexibility is particularly acute in large structures with flexible appendages, which are frequently used in space. This is partly because putting mass into space is expensive, and making a structure light inevitably increases its flexibility.

The control challenge then becomes how to control the motion of a flexible structure while simultaneously absorbing vibrations – that is, to get it to move in such a way that it responds almost as if rigid. The UCD group has developed a new technique for this problem, which has many advantages over existing control strategies. It interprets the motion of the flexible system as mechanical waves propagating through the system.

By understanding and controlling these waves, the technique combines position control and active vibration damping in a rapid, robust and seamless way. The approach is simple and easy to implement, with many advantages over other techniques.

One strong feature is that the control algorithm does not depend on having a good model of the system dynamics, and so continues to work very well even when the system changes, perhaps in an unknown way. It continues actively to dampen flexing and other vibrations while continuing to execute the desired manoeuvre and gross motion of the structure.

European Space Agency and Ariane 6


The European Space Agency's Ariane 6

The European Space Agency’s Ariane 6 (image: ESA)

In a previous contract with the ESA, the UCD group successfully applied their techniques to the control of a satellite structure for a large space telescope, as well as a robot arm for a planetary explorer. In the current contract, the ESA has asked them to develop a control toolbox that facilitates implementing and testing their control strategies, with a special focus on the control of launchers, such as the planned new Ariane 6. The work aims to design more stable launchers giving improved performance and a smoother ride than is currently possible, despite the structural flexibility and the sloshing of on-board liquid propellant.

When standing on the launch pad, about 90% of the weight of the rocket is fuel. In other words, at lift-off, some 90% of the thrust coming from burning the fuel is used just to lift the fuel itself. All the other weight, including the rocket structure, rocket motors, control systems, communication systems and of course the precious payload, must fit into the remaining 10%.

The weight calculations are precise and every gram counts. If the ratio of thrust to weight from the chemicals in the fuel was a few percent lower, we would be stuck on earth.

With design therefore tending towards lighter and lighter structures, the launcher structure is prone to bending vibrations. The natural frequency of such vibrations can be as low as two Hertz, implying a larger mass and/or lower stiffness than those found in most mechanical systems. As structures go, it is quite wobbly. Furthermore, the structure is so delicate that if it was inclined at about 30 degrees, it would break up.

Too much vibration can damage the rocket structure and certainly reduces payload comfort. The payload could be, for example, supplies for the international space station, a large astronomical telescope, or a special satellite for communication, for earth monitoring, or for global positioning. The less vibration such delicate devices experience, the better.

Reducing flexural vibrations also helps to maintain the finely calculated trajectory of a rocket from launch into orbit. Any departure from this course must be corrected, usually by steering the rocket motors, but this reduces the main thrust forward and so consumes more fuel, independently of the consequences of the flexing and vibrations.

Major control challenge


The rocket-engine thrust acts at one end of a long slender structure. This presents a major control challenge. Think of the chaotic flight of a party balloon when air is escaping out the hole, or think of balancing a brush on your finger while simultaneously giving it a large acceleration. The control challenge is significantly increased by the sloshing of tonnes of liquid fuel which can easily cause standard control algorithms to go unstable, particularly if something suddenly perturbs the trajectory. Space missions have been lost due to the over-reaction of rocket control systems to sloshing of fluid propellant, which, instead of tending to reduce the sloshing, made it progressively worse, with disastrous results.

To lift off, overcome the Earth’s gravity and arrive in orbit, rockets must quickly reach a speed of the order of 28,000km/h from standstill. This requires an immense amount of fuel. They also need enough additional propellant to complete their mission. At these upper stages, the fuel is entirely in fluid form and the fluid dynamics are complex, especially under changing gravity effects and rocket accelerations.

A further complication is that the launcher dynamics change considerably at different stages of the flight, from take-off to orbit, in ways which are not fully predictable. So the control system must be robust to modelling errors, to unmodelled dynamics such as the fluid sloshing, and it must be able to cope with disturbances. The UCD strategy is very well suited to such challenges.

Ultimately, it is all about safely delivering the payload into space with the most precision and minimum cost, including minimising fuel consumption.

The work will form part of the ESA’s Future Launchers Preparatory Program (FLPP), which oversees studies and research activities and technologies capable of delivering high performance and reliability for the next generation of European Launchers.

Enterprise Ireland co-ordinates the participation of Irish companies and research teams in ESA programmes. The space sector is of growing importance for Irish industry, with an increasing number of Irish companies are playing active roles in a range of space activities. This project on the control of future launchers an important new addition to this work, with the potential for commercial spin-out of the research.

http://www.engineersjournal.ie/wp-content/uploads/2014/11/1406_EngMech_ESA_030a-1024x819.jpghttp://www.engineersjournal.ie/wp-content/uploads/2014/11/1406_EngMech_ESA_030a-300x300.jpgDavid O'RiordanMechaerospace,European Space Agency,UCD
  Author: Dr William J. O'Connor, BE PhD FIEI CEng, senior lecturer, School of Mechanical & Materials Engineering, University College Dublin Engineers at University College Dublin (UCD) have recently been awarded a contract by the European Space Agency (ESA) to find new ways to control vibrations in future rocket designs. The Dynamics...