Who is Tom Mueller? He is SpaceX's first employee and a top expert in rocket engine technology.

The engine is the most difficult component to manufacture, the most prone to problems, and a key component that determines the progress of a rocket.
Musk personally approached Tom and invited him to join SpaceX. It's fair to say that without Tom, SpaceX would have had a very difficult early period.

Now, Tom has left SpaceX and founded Impulse Space, a space logistics company, which recently completed a $500 million Series D funding round.