Solid Rocket Boosters are cheap and dirty.
As a procedural part, they are sizeable like a fuel tank, but with a nozzle on the bottom which can be reshaped to alter the vacuum/atmospheric performance of the booster--longer nozzles are better in space. The thrust is proportional to the length of the booster, the burn time proportional to the width, and presumably with some tweakable sliders in the editor to fine-tune those--and maybe even a thrust curve editor?
SRBs can not be throttled or stopped. But they can produce some absolute UNITS of thrust--the highest-thrust rocket in the world is the SRB on the Space Shuttle, if I'm not mistaken.
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