The THK "Pollux" Solid Fuel Booster (also known as an SRB) is a solid rocket booster used to provide a short burst of additional thrust to a rocket. It comes as a single unit. The engine and fuel tank are one single part which means that it isn't possible to increase the burn time for it by adding more fuel tanks.
|| The Pollux is an engineers best friend. Is your rocket a little too low thrust to get off the pad efficiently? Put one on. Still not enough? Put on a pair. Try six of them if that's still not enough. Or if you're feeling really craft, you can launch your entire rocket with just these tried and true solid rocket motors.