General Lumped Parameter Modeling Tips

Before generating the lumped parameter model of the projectile or the gun tube, read the following general hints and tips on generating lumped models using Balans. The basic steps are shown in the example.

Let BALANS attempt to help with modeling using the Generate Analysis Option, and then hitting the Generate Projectile Model button. It will make a first pass at lumping the model and establishing beam elements.
After BALANS has determined a preliminary suggested model, make sure the nodes are placed where you need them.
- Nodes should be placed at structural transition points (places where the structure makes significant changes in axial or bending stiffness).
- Keep in mind that nodes should be ideally placed to create beams that do not have dramatically different stiffness (this makes the solution more efficient and accurate).
- Most projectiles can be represented with sufficient accuracy using fewer than 30 nodes, and many can be modeled with under 20 nodes.
Edit the Node data table by deleting nodes where they are not necessary and adding nodes (coordinates only) to refine the model. Node coordinates can also be changed if that is appropriate. The node data is edited in the Node/Springs tab.
Regenerate the model with the new node locations (if you fail to properly select the re-generate analysis option, the model may reset, causing you to re-edit your node changes). Regenerate recalculates the elements based on the nodes in the node table. If you edit the nodes and do not regenerate then the nodes and elements will be out of sync.
Elements may then be addressed. Elements are "composite" sections composed of cylindrical and/or hollow cylindrical sections. By composite, I mean that there is shear transfer between the beam sections within an element; thus, they act together in bending and compression/tension. If some of the beam sections within an element do not act this way in your model, you can do one of the following:
- Create a material (in the table provided) that has the same density as the subject beam section, but has a low bending modulus relative to the main structural section in that element. Two orders of magnitude less works well.
- Delete the beam section so that the beam element no longer considers the mass or structural contribution of that section. This is typically my approach with plastic components that are a very low percentage of the projectile total mass.
- Delete the beam section and replace it with equivalent mass at nodes in the "Added Weight" column of the node table.
Once your element data is adjusted (this may take a few iterations), you can Redisplay the model. Once again, if you fail to select the proper analysis option; your hard work at fine-tuning the model will be overwritten by the regeneration of the model.
These same steps need to be repeated for the tube model.