The Planet Strappers by Raymond Z. Gallun

The Planet Strappers started out as The Bunch, a group of student-astronauts in the back room of a store in Jarviston, Minnesota. They wanted off Earth, and they begged, borrowed and built what they needed to make it. They got what they wanted--a start on the road to the stars--but no one brought up on Earth could have imagined what was waiting for them Out There!