The solid boosters are small-sized boosters that can be attached to the side to any first stage that has room for boosters. These are the most basic boosters in the game. Solid boosters come in two variations: solid boosters and long march solid boosters. They both cost 1m and have almost identical stats. Long march solid boosters have 2/6 lifting power and basic ones have 1/6 lifting power. They have the same amount of fuel so there is no reason you would choose the basic ones.
The solid boosters,unlike the Liquid boosters,once ignited,can't be throttles or turned.They burn at their full capacity until their fuel is over and they are automatically ejected.Their ejection need not be done manually,and they usually run out of fuel shortly before the first stage does in most cases