"Geez, Solomon. You'd have to be nuts to do a solo space flight testing that experimental engine you developed." 
 "Hold my beer." 
 This is a free short story set in the The Expanse series that tells us about how a Martian engineer named Solomon Epstein developed the drive system used by all the space ships. That sounds like it'd just be nerd bait for the kind of hardcore fans who look for schematics of fictional starships on the interwebs, but this actually has a couple of really solid hooks that make it something more than that. 
 One of the critical underlying elements of The Expanse series is the Epstein Drive. Not only is it the concept that makes constant travel around the solar system feasible, the way it functions is an integral part of the stories. The force of acceleration and what it does to the human occupants of the ships always has to be accounted for, and it's been used to great dramatic effect repeatedly in the series. 
 What this story does is explain how that drive came to be, and it also acts as quick primer on how this was a key moment in The Expanse timeline that sets up all the conflicts between Earth, Mars, and the Belt that were already established in the first book. 
 So it's a solid prequel set-up that sets up the structure of the series. It's also connects emotionally by telling us about Solomon and what happened to him after he fired that engine up the first time. 
 And it's free!