Weight is very important in Space Agency. If a rocket weighs too much, it won't be able get into orbit. Every time something is added to a rocket, its weight increases.
There are several ways to conserve fuel so that weight matters less, unless the progress to orbit made is under 57%.
Tricks to conserve fuel
- Turn off the first stage when it reaches maximum velocity and the boosters will keep going the rocket going (unless using Delta-IV Heavy Boosters)
- Quickly keep tapping the engines during ascent will maintain velocity and save fuel.
- In space, using gravity assists when passing by a planet or star at faster than orbital speed. It points the spacecraft in a different direction without you using any fuel for turning.
- Also in space, don't speed up too much, because it will require more fuel just to slow back down. (A similar tactic can be used in Mission 15 with the Voyager-X.)