and is surrounded by an annular throat, which exhausts gases from the same (dual-throat) or a separate (dual-expander) thrust chamber. Both throats would, in either case, discharge into a bell nozzle. At higher altitudes, where the ambient pressure is lower, the central nozzle would be shut off, reducing the throat area and thereby increasing the nozzle area ratio. These designs require additional complexity, but an advantage of having two thrust chambers is that they can be configured to burn different propellants or different fuel mixture ratios. Similarly, Aerojet has also designed a nozzle called the "Thrust Augmented Nozzle", which injects propellant and oxidiser directly into the nozzle section for combustion, allowing larger area ratio nozzles to be used deeper in an atmosphere than they would without augmentation due to effects of flow separation. They would again allow multiple propellants to be used (such as RP-1), further increasing thrust. Liquid injection thrust vectoring nozzles are another advanced design that allow pitch and yaw control from un-gimbaled nozzles. India's PSLV calls its design "Secondary Injection Thrust Vector Control System"; strontium perchlorate is injected through various fluid paths in the nozzle to achieve the desired control. Some ICBMs and boosters, such as the Titan IIIC and Minuteman II, use similar designs. See also Choked flow – when a gas velocity reaches the speed of sound in the gas as it flows through a restriction De Laval nozzle – a convergent-divergent nozzle designed to produce supersonic speeds Dual-thrust rocket motors Giovanni Battista Venturi Jet engine – engines propelled by jets (including rockets) Multistage rocket NK-33 – Russian rocket engine Pulse jet engine Pulsed rocket motor Reaction Engines Skylon – a single-stage-to-orbit spaceplane powered by hybrid air-breathing/internal-oxygen engine (Reaction Engines SABRE) Rocket – rocket vehicles Rocket engines – used to propel rocket vehicles SERN, Single-expansion ramp nozzle – a non-axisymmetric aerospike Shock diamonds – the visible bands formed in the exhaust of rocket engines Solid-fuel rocket Spacecraft propulsion Specific impulse – a measure of exhaust speed Staged combustion cycle (rocket) – a type of rocket engine Venturi effect References External links Exhaust gas velocity calculator NASA Space Vehicle Design Criteria, Liquid Rocket Engine Nozzles NASA's "Beginners' Guide to Rockets" The Aerospike Engine Richard Nakka's Experimental Rocketry Web Site "Rocket Propulsion" on Robert Braeuning's Web Site Free Design Tool for Liquid Rocket Engine Thermodynamic Analysis Nozzles Rocket propulsion Pyrotechnics