Stephen Marlowe 
Jungle in the Sky [EPUB ebook] 

Famous Science Fiction Novel
Excerpt from Book:
It was Kevin Mc Gann who showed him around the Gordak after Brennschluss. Newton’s second law of motion carried the ship forward through the near-vacuum of space now, and it would continue that way, plowing ahead at seven miles per second until it was caught and slowed by the space-station’s gravity. There the bunkers would be reloaded with slow-fission plutonium for the long dash sunward to Mercury.