Kerbal Space Program

Some time later, with the game paused, the game's memory usage has creeped up by just under 17MB to 28.9%! It's just been sitting here in the pause menu for the last half hour.

I watched my memory usage on the Windows 1.3 (64-bit) version and agree that switching around frequently between different physics enabled scenes causes memory usage to rise. Playing nine missions at once and hopping around between vessels, I was easily loading over 5GB on KSP alone with 16GB in my computer (eventually crashing as my total memory got near full). I've since upgraded to 32GB and after a day of playing have noticed the memory for KSP get as high as 9GB without a single reboot.

The heap isn't getting its garbage cleaned up often enough. Returning to the KSC or the main menu should cause all unused objects during physics play to be cleaned up. I know in Java and C# you can request the garbage collector to do a collection, but it doesn't guarantee it will happen right away. Any time the pause menu is up is a good time to request it too as it won't affect physics calculations.

A feature I could suggest to help with this is to set a time stamp on a vessel such that when you leave it to jump to another one, the vessel retains all active references to its parts in memory so you can quickly access those same parts when you switch back and after thirty minutes pass of real time, then dump the references and allow the garbage collector to clean it up. Having some sort of indicator that the user can use to flag a ship as needing to stay in memory could be helpful too as this will certainly reduce the number of duplicate copies of the ship end up in memory while waiting for older copies to be collected. My Minmus refueling ship is used frequently and would certainly qualify. keeping the KSC physics loaded and ready for each new launch will help too. Too many crashable buildings and structures to be duplicating this space every time. I understand though that reverting a flight to the design means a new and completely different ship might be coming out next time, so the old ship needs to be deleted from memory, but reverting from launch doesn't requiring dumping and rebuilding the ship if you can just reset its resource values and location/direction/speed variables.