6DOF Model Rocket Stability with Changing Mass

LWhitson2 · March 18, 2016, 10:43

I am currently using CFX to study the effects of rocket design on stability. My basic simulation and mesh are set up using a cylinder that represents the bulk flow around the item and a sphere that encompasses the flow near the rocket. The sphere is attached to the item and decoupled from the the surrounding mesh to allow the rocket to freely rotate in the flow field. The sphere is attached to the item at the Center of Mass location and there is a second coordinate system that is also attached to the rocket. This is similar to the method used in the Buoy Tutorial problem in CFX.

My method works great for pre-ignition, but the design of my rocket uses a large amount of propellant that significantly affects the overall mass, CoM location, and inertial components as it burns. I have been trying to come up with a way to deal with this in CFX as it is my understanding that the rigid body coordinate system has to be collocated with the center of mass. It would also mean that the center of the sphere would have to move with the center of mass as well, I think.

So, does anyone have an idea of how I might accomplish this?

ghorrocks · March 18, 2016, 19:38

The easiest way would be to do a series of models, each one moving the centre of mass a bit to account for the fuel burn; with the rest of the meshing redone to suit. Then use the output of the previous simulation as the input to the next one.

March 18, 2016, 10:43	6DOF Model Rocket Stability with Changing Mass	#1
LWhitson2 New Member L.Bryce Whitson Jr. Join Date: Jun 2011 Posts: 18 Rep Power: 14	I am currently using CFX to study the effects of rocket design on stability. My basic simulation and mesh are set up using a cylinder that represents the bulk flow around the item and a sphere that encompasses the flow near the rocket. The sphere is attached to the item and decoupled from the the surrounding mesh to allow the rocket to freely rotate in the flow field. The sphere is attached to the item at the Center of Mass location and there is a second coordinate system that is also attached to the rocket. This is similar to the method used in the Buoy Tutorial problem in CFX. My method works great for pre-ignition, but the design of my rocket uses a large amount of propellant that significantly affects the overall mass, CoM location, and inertial components as it burns. I have been trying to come up with a way to deal with this in CFX as it is my understanding that the rigid body coordinate system has to be collocated with the center of mass. It would also mean that the center of the sphere would have to move with the center of mass as well, I think. So, does anyone have an idea of how I might accomplish this?

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March 18, 2016, 19:38		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	The easiest way would be to do a series of models, each one moving the centre of mass a bit to account for the fuel burn; with the rest of the meshing redone to suit. Then use the output of the previous simulation as the input to the next one.