Plug nozzle
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The plug 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, but unlike the aerospike, the plug design is considerably more "traditional".
The plug nozzle looks from the outside like a normal engine bell, as used on most rocket engines. Inside, however, there is a "plug" (referred to as the centerbody) that can seal off the exit from the combustion chamber above it. It is similar in form to a sink turned upside down, with the exhaust from the combustion chamber coming out the drain, past the plug (stopper), and into the bell (bowl).
The key to the plug system is to use a bell designed for high altitude use. In a normal nozzle this would result in the ambient air pressure forcing the exhaust in from the walls of the bell, leading to a loss of thrust. However in this situation the plug is "closed" by moving it closer to the combustion chamber, and the exhaust is forced out from the plug to the sides of the bell again.
Two similar designs, the reverse-flow and horizontal-flow, squeeze the flow between two complete bells which are moved in relation to each other. The existence of three such designs, along with the similar aerospike, has led to all such designs becoming known as radial out-flow nozzles.
The plug nozzle or Aerospike nozzle has an annular doughnut-shaped chamber with an annular nozzle slot. An alternate version has a number of individual small chambers each with low area ratio short nozzles, a round throat, and a rectangular exit arranged in a circle around a common plug or spike. The outside aerodynamic boundary of the gas flow in the divergent section of the nozzle is the interface between the hot gas and the ambient air; there is no outer wall as in a conical or bell-shaped nozzle. As the external or ambient pressure is reduced during the ascending flight, this gas boundary expands outward, causes a change in pressure distribution on the central spike, and allows an automatic and continuous altitude compensation.
[edit] Advantages
Maximum thrust can be achieved throughout the flight envelope if the nozzle have an ability to freely adapt to external conditions. This provides a higher specific impulse, larger payload and thus reduces the cost to orbit. Moreover, plug nozzles allow for a very effective propulsion system integration and the use of multiple combustion chambers brings additional maneuverability in both pitch and yaw making this nozzle gimbals free, which permits weight saving over Bell-shaped nozzles. The use of multi-combustion chamber makes plug nozzles inherently safer than the single combustion chamber Bell-shape nozzles in the case of engine failure. A plug nozzle design, or truncated aerospike, can be seen as a Bell-shaped nozzle turned inside out.
However, the performance of plug nozzles is much more difficult to evaluate than standard contoured nozzles because the flow field is highly complex and turbulence is largely involved.
