On this page of the guide to Mr Prepper, you will find information on where to find the hydrogen necessary to build a rocket.
To build a space rocket - one of the main objectives in Mr Prepper - you will need hydrogen. You will find it as a large, blue bottle, hidden somewhere in the mine (this area is generated randomly when you start a new game) at level -5. To get to this mine level, you need to get an access card on level -4. In order to get the card, you must complete 3 story quests for Joe from the Junkyard.