Expanding nozzle
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The expanding nozzle is a type of rocket nozzle that, unlike traditional designs, maintains its efficiency at a wide range of altitudes. It is a member of the class of altitude-compensating nozzles, a class that also includes the plug nozzle and aerospike. While the expanding nozzle is the least technically advanced and simplest to understand from a modelling point of view, it also appears to be the most difficult design to build.
The expanding nozzle consists of two engine skirts (bells, etc.), one inside the other. One, attached directly to the engine nozzle, is designed for use at lower altitudes and is short and squat. The other, sitting outside the first, fits over the lower altitude bell to extend it into a longer and narrower (measured in terms of length) bell used for higher altitudes. At liftoff the outer bell is pulled up from the inner bell, out of the way of the exhaust. At some point as the spacecraft climbs, the outer bell is pushed back down over the inner bell to increase the efficiency.
Generally simple in concept, the expanding nozzle is considerably more complex to build than it might seem. Engine bells must be cooled to avoid damage from the hot rocket exhaust. The cooling is done by running either the oxidizer or fuel (in the case of LH2 fueled engines) though tubing in the bell under high pressure. With one part of the bell moving, some part of the plumbing has to be flexible, and this becomes so complex that the advantages of the design are often considered too costly.
The Space Shuttle Main Engine was originally designed to use an expanding nozzle. Since these engines are fired from takeoff right into space, any sort of altitude compensation could dramatically improve their overall performance. The expanding nozzle was later abandoned in a cost-cutting phase, and today the SSME suffers a 1/4 drop in performance at low altitudes as a result.
Glushko has used an expanding nozzle on one design, the RD-701 tripropellant rocket. Funding ran out with the fall of the Soviet state, but the designers are convinced the engine has potential and have approached several parties for additional funding.
