I've carried a regular Space Pen for over 30 years (well had to replace one I lost on an Arizona mountain top while recovering a buck/filling out my tag)...
I replaced the standard medium cartridge with a fine tipped blue one and NEVER had any problems!!
It's ALWAYS with me! Comes in handy when trying to write in the rain, below freezing, or at angles for sure!!
You can get just the refills and they typically come with a little plastic cap that allows you to use them in Parker pens.