Annular Nozzles II

Rocket Nozzles
Nozzle Shapes
Radial Out-Flow
Radial In-Flow
Altitude Compensation
Performance Losses
Advantages & Disadvantages
Having introduced the three principal families of nozzle shapes and discussed the radial out-flow nozzle, we will now look more closely at the second class of annular nozzles.

Radial In-Flow Nozzles:

The second major variety of annular nozzles is the radial in-flow type, exemplified by the spike shown below.

Size comparison of optimal nozzles
Size comparison of optimal cone, bell, and radial nozzles for a given set of conditions [from Huzel and Huang, 1967]

This type of nozzle, named for the prominent spike centerbody, is often described as a bell turned inside out. However, the nozzle shown above is only one of many possible spike configurations. Variations of this design, shown below, include

    (a) a traditional curved spike with completely external supersonic expansion
    (b) a similar shape in which part of the expansion occurs internally
    (c) a design similar to the expansion-deflection nozzle in which all expansion occurs internally.
Comparison of spike nozzle shapes
Comparison of spike nozzles with (a) external expansion, (b) internal-external expansion, and (c) internal expansion [from Berman and Crimp, 1961]

Note that each of the above spike nozzles features a curved, pointed spike, the most ideal shape. This spike shape allows the exhaust gases to expand through an isentropic, or constant entropy, process. In so doing, the nozzle efficiency is maximized and no energy is lost because of turbulent mixing in the exhaust flow. While the isentropic spike may be most efficient, it also tends to be prohibitively long and heavy. However, theoretical studies have shown that replacing the curved shape by a much shorter and easier to construct cone results in very little performance loss. The following graph illustrates that the thrust decreases by less than 1% for cone half-angles up to 30. Furthermore, the graph gives an indication of how much the spike length can be reduced by employing a cone-shaped spike.

Changes in nozzle performance and length due to replacement of the lower centerbody
Changes in nozzle performance and length due to replacement of the lower centerbody [from Berman and Crimp, 1961]

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