Boundary conditions for the modal analysis of a rocket

- July 6, 2022 at 9:09 pm
  
  shantashreejena97
  Subscriber
  
  Hello,
  I want to do the modal analysis of a rocket to know the mode shapes and relative stress and relative deformation at the large deformation areas. The analysis has to done for just after the take off condition. I couldn't able to figure out what boundary conditions do i need to give for performing my modal analysis? I have only given payload as a point load to the payload attachment frame region. Could you please help me to solve this?
- July 7, 2022 at 12:04 pm
  
  peteroznewman
  Subscriber
  
  No boundary conditions are required for a Modal analysis. This is sometimes called a Free-Free modal analysis and the first six modes are the six rigid body motions and should have modal frequencies that are practically zero. If they are not zero, that indicates there is a mistake in the model and something is connected to ground. That mistake must be fixed.
  Request 12 or more modes. Mode 7 is the first bending model of the rocket and it should have a non-zero modal frequency. If mode 7 has a practically zero frequency, that shows there is an unconnected part of the model and that mistake must be fixed.