In a dual-thrust

solid propellant A propellant (or propellent) is a mass that is expelled or expanded in such a way as to create a thrust or another motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid payload. In vehicles, th ...

rocket engine A rocket engine is a reaction engine, producing thrust in accordance with Newton's third law by ejecting reaction mass rearward, usually a high-speed Jet (fluid), jet of high-temperature gas produced by the combustion of rocket propellants stor ...

, the propellant mass is composed of two different types (densities) of fuel. In the case of a

tandem Tandem, or in tandem, is an arrangement in which two or more animals, machines, or people are lined up one behind another, all facing in the same direction. ''Tandem'' can also be used more generally to refer to any group of persons or objects w ...

dual-thrust motor, the fuel nearest to the rocket nozzle burns fast, and the fuel further into the motor's body burns slower. This gives the rocket higher thrust initially, accelerating it rapidly to high speed. When all the fast-burning propellant has burnt, the slow-burning propellant delivers a 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 outside (core burning), rather than from the end in (end burning). The advantage of dual-thrust motors is that if the fuel were entirely the fast-burning type, the rocket would accelerate to a higher speed initially but because

air resistance In fluid dynamics, drag, sometimes referred to as fluid resistance, is a force acting opposite to the direction of motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers, two solid surfaces, or b ...

increases quadratically with speed, the rocket would slow very rapidly. This would give a higher peak speed but a lower average speed. Instead, the boost phase accelerates the rocket to a high enough speed (high enough to propel the rocket to its destination fast, but not high enough to cause excessive air resistance), and then the sustain stage allows the rocket to maintain this high speed until it burns out. Then it is able to coast, slowly losing speed. Dual-thrust motors are most prevalent in rockets which are atmosphere-bound since they have to deal with air resistance over most of their flight. It is similar in concept to

multistage rocket A multistage rocket or step rocket is a launch vehicle that uses two or more rocket ''stages'', each of which contains its own engines and propellant. A ''tandem'' or ''serial'' stage is mounted on top of another stage; a ''parallel'' stage is ...

s, but much simpler to design and build since there is no requirement to detach stages, have separate components, etc.

References

* {{DEFAULTSORT:Dual-Thrust Rocketry