Munition on Aircraft

[Omer-Gino]

Is there an example for munition test (only cpp jsbsim)?
I want to create aircraft carrying munition, and release.
A reference will be most helpful.

[seanmcleod70]

What are your requirements after the release? Simply that the mass and drag of the aircraft change, or do you want to spawn another FDM to model the trajectory of the munition?

[omergino]

You are right I didn't specify, I want to spawn another fdm.

[seanmcleod70]

I took a brief look at FlightGear in terms of how they handle munitions, for example see -https://wiki.flightgear.org/Scripted_AI_Objects#Missiles. So they don't spawn another JSBSim FDM, rather it looks like they run Nasal based FDMs for the missiles.

There are some references to ChildFDM in the current JSBSim source code:

https://github.com/search?q=repo%3AJSBSim-Team%2Fjsbsim+ChildFDM&type=code

But also this comment saying it's not yet supported.

jsbsim/src/FGFDMExec.cpp

Lines 1039 to 1042 in f794e04

	// Lastly, process the child element. This element is OPTIONAL - and NOT YET SUPPORTED.
	element = document->FindElement("child");
	if (element) {
	result = ReadChild(element);

So for now, if you want to go the route of spawning another FDM for the munition etc. and you mentioned

only cpp jsbsim

I'd suggest something along these lines:

• Add your munition as a point mass to the aircraft's fdm config
• Add some config options in aircraft's fdm config to modify drag etc. based on whether the munition is on the aircraft or not
• Create a munition specific fdm config file
• Run a regular update loop and when the user releases the munition then:
  • Remove/zero point mass in aircraft's fdm config, will update mass and moments of inertia and also reduce drag based on config
  • Create a new JSBSim FDMExec instance loaded with munition's fdm config
• During further update loop iterations perform time step for both FDMs (aircraft and munition)

[Omer-Gino]

Thank you!