In a dual-thrust solid fuel rocket motor, the propellant mass is composed of two different types or densities of fuel. In the case of a tandem dual-thrust motor, the fuel closest to the rocket nozzle burns fast and the fuel further into the motor's body burns slower. This has the effect of giving the rocket a lot of thrust initially, accelerating it up to high speed rapidly, and then once all the fast-burning propellant has burnt, the slow-burning propellant begins delivering a much lower level of thrust. The first phase of acceleration is called "boost" and the second phase "sustain". Not all dual-thrust motors are in a tandem arrangement but non-tandem motors function much the same; they just have a different physical layout of fuel, for example they might burn from the inside to the out