A rocket engine that maintains the velocity of a rocket vehicle once it has achieved its programmed velocity by use of a booster or other engine.
http://encyclopedia2.thefreedictionary.com/sustainer rocket ...

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PL stałe heterogeniczne paliwo rakietowe dwuzakresowy silnik rakietowy materiał wysokoenergetyczny
EN composite solid propellant dual thrust rocket motor high energetic material

http://yadda.icm.edu.pl/baztech/element/bwmeta1.element.bazt...

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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 outside (core burning), rather than from the end in (end burning).

http://en.wikipedia.org/wiki/Dual-thrust