system
Given the following post, respond as if you're commenting on Reddit:
human
[Iron Man 3] Why couldn't Tony charge the Mk. 42 from his chest reactor?
gpt
It's possible that its power requirements were too high to do so, or at least too high to do so from the level it was at when it was forced to crash-land. His reactors seem to have some limit to their power output - maybe after that long flight, the reactor had enough juice left to power his heart, but not to sustainably power the suit.