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Förslaget inkom 2007-10-22

Deployment simulation of lightweight inflatable space structures

Very long tubes made of thin film are considered for use in space applications, for example, in solar sails and antennas. Prior to the launch of the spacecraft, the tubes are folded in various ways in order to fit into the compartment of the launch vehicle. Once in orbit, the tubes are deployed from the spacecraft by inflation of gas. Because the tube is very light, inflation experiments cannot be performed on Earth due to the effects of gravity and atmospheric pressure. Instead, one must rely on numerical simulations.

A previous MSc project studied the inflation of a framework of tubes by the finite element software LS-DYNA. The approach used in that work was the uniform pressure method, where it is assumed that the pressure is constant within the tube. To simulate a pressure gradient in the framework, it was divided into several so-called control volumes. The pressure is constant in each control volume, but the control volumes can ventilate into each other through permeable walls. In this way a gas flow through the structure can be simulated.

A disadvantage with the uniform pressure approach is that it may not be accurate if the deployment is fast or if the structure comes into contact with itself during the deployment as is the case in out-of-position car airbag deployment simulations. A more accurate method is the Arbitrary Lagrangian-Eulerian (ALE) method, but it is computationally more expensive and quite problematic to tune for convergence. To overcome the problems with the ALE method, a new method called the Corpuscular or Particle method has very recently been incorporated in LS-DYNA. This method is very simple and based kinetic gas theory. In this method, the gas molecules are modelled as spheres bouncing around inside the airbag or tube impacting each other and the membrane of the structure. The tube deploys when the particles hit the walls of the structure. Early studies of the particle method have shown that it is very robust when analysing complex geometries.

The aim of the present MSc project is to analyse the deployment of long and slender tubular space structures using the new corpuscular or particle method in LS-DYNA and compare the results with those of the uniform pressure approach. Another important aspect that should be investigated is the use of implicit time integration in the latter phases of the deployment through the explicit-implicit switch in LS-DYNA. The implicit scheme allows for larger time steps, which may be of importance when simulating deployments of several seconds (compared to car airbags which are deployed in tens of milliseconds).

Good knowledge in mechanics, solid mechanics, dynamics and finite elements. Experience in MATLAB programming and ANSYS is preferred.

The structural mechanics group at KTH Mechanics has extensive experience in the simulation of flexible and lightweight space structures. Recently, a study of the deployment of a large cable net in space was performed for the European Space Agency. For the simulations of the space structures, the software LS-DYNA, ANSYS and ABAQUS are used.


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