A workhorse rocket in most space agencies; the humble SRB is little more than a tube of steel encasing thick rubbery soild propellant. This propellant, usually a mix of ammonium perchlorate (an oxidizer), polybutadiene acrylonitrile (or PBAN, the combustible fuel) and a little bit of aluminium powder provides a massive amount of thrust ideal for surface level boosting roles getting launch vehicles off the ground. The Space Shuttle SRB's provided about 85% of the power to get the stack flying, and lifted the orbiter on a trajectory where it could circularize and complete its orbit under its own power.
The propellant is shaped within these canisters to dictate their ideal flight performance - "grain geometry" extrusions through the center of the booster control thrust and burn time through surface area exposure. A star shape for example gives high initial thrust at the start of the flight but tapers off at the end as the fins (and surface area) of the geometry ablate and burn away as they're consumed.
And this is very important - there is no other way to control a SRB. By design they have no turbopumps, no thrust chamber (aside from their hollow grain geometry), nothing but a small charge at the top of the stack that ignites them, and hopefully a decoupler to remove them from the stack once they start to burn out. A few larger designs include actuators to power clever interlocking plate mechanisms embedded within the propellant allowing them to gimbal - but that's it. Once they're on, they're on - you can't throttle solids.
But cats love making noise and breaking stuff so here we are! Hunter's commissioned a set of large Meowdyne SRB's to round out KSA's existing small sepatrons. Following the design conventions of real life; these are assembled out of repeating canisters that can be stacked on top of a base thrust assembly piece, which feature thrust vectoring actuators on larger models. Smaller models have simple fixed nozzles, giving the early game a little bit of calamity. Budgets only go so far after all!
Each of these canisters allude to grain geometry on their endcap plates; not unlike the covers actual SRB segments have before they're loaded and stacked. Ideally we'll give the player a way to customize grain geo down the track; probably with a tweakable UI window.
All up we have a 2.0m stack (actuating), a 1.0m stack (actuating), 0.5m stack, and a 0.25m stack. Sepatrons still sit at 0.125m, perfectly kitten sized. The nozzles have also been designed with separate geometry to enable us to do cool things in the future™. Stay tuned!
On top of the modularity of the solid propellant canisters - each SRB comes with two new parts that help cap them off. The Shuttle SRB's were famously reusable, each having forward skirts and nosecones that housed recovery hardware (parachutes, tracking devices, gyros, etc).
There are no parachutes yet, but why not pivot what would cover them into segmented petal adaptors? These both terminate at common sizes so you could create tidy stacked sounding rockets that transition cleanly into different diameters. Or just use them as nosecones.
Hopefully we can get them all into the game soon - in the meantime I'm on to batteries, solar panels, structural trusses and docking ports. Thanks for reading :)