legRoom

100% usage on at least one core is common for 3D games; if you're seeing lower usage it is likely that KSP is GPU-limited on your system. Modern CPUs and GPUs cannot be damaged merely by running them at full load; they will automatically slow down as needed to protect themselves if they actually start to overheat. Moreover, a relatively beefy desktop like yours should have adequate cooling to run at full load indefinitely without overheating, unless it's really dusty or something.

If your computer is overheating enough to cause crashes, there is something very wrong with your computer: Modern CPUs and GPUs (the main heat sources) will automatically reduce their clock speed and operating voltage as needed to prevent errors or damage. When doing this "thermal throttling", the computer will continue to work correctly - just slower than normal. For a game, this does not cause crashes or hangs - just a lower framerate. On a desktop or full-size laptop, user-space software (such as KSP) can't cause significant overheating in the first place, even if it's really buggy. The cooling system should be sized to allow running the CPU at 100% load indefinitely. The GPU might throttle a bit under a really heavy load like Furmark - but not very much, and throttling doesn't cause crashes anyway. If your computer is actually overheating, the most likely cause is either lots of dust blocking the cooling airflow - or that you're using a dinky netbook or tablet type device which isn't really powerful enough to run heavyweight games like KSP properly to begin with.