Aerospike Nozzles

Rocket Nozzles
Nozzle Shapes
Radial Out-Flow
Radial In-Flow
Altitude Compensation
Performance Losses
Advantages & Disadvantages
A further subclass of the radial in-flow family of spike nozzles is known as the aerospike.

Aerospike Nozzles:

Previously, we discussed methods of reducing the length of a spike nozzle centerbody by replacing the ideal spike with a conical spike. While this method does indeed result in a much shorter nozzle length, we can go even further by removing the pointed spike altogether and replacing it with a flat base. This configuration is known as a truncated spike, an example of which is shown below.

Example of a truncated, conical spike
Example of a truncated, conical spike [from Berman and Crimp, 1961]

As any fluid dynamicist recognizes, the significant disadvantage of the "flat" plug is that a turbulent wake forms aft of the base at high altitudes resulting in high base drag and reduced efficiency. However, this problem can be greatly alleviated in an improved version of the truncated spike that introduces a "base bleed," or secondary subsonic flow, into the region aft of the base.

Example of an aerospike nozzle
Example of an aerospike nozzle with a subsonic, recirculating flow [from Hill and Peterson, 1992]

The circulation of this secondary flow and its interaction with the engine exhaust creates an "aerodynamic spike" that behaves much like the ideal, isentropic spike. In addition, the secondary flow re-circulates upward pushing on the base to produce additional thrust. It is this artificial aerodynamic spike for which the aerospike nozzle is named.

Linear Aerospike:

All of the nozzles we have studied thus far have been annular, or circular when viewed from below. Still another variation of the aerospike nozzle is not an annular nozzle at all. A second approach, pioneered by the Rocketdyne company (now a division of Boeing) in the 1970s, places the combustion chambers in a line along two sides of the nozzle:

Rocketdyne RS-2200 linear aerospike engine
Rocketdyne RS-2200 linear aerospike engine [from Flinn, 1996]

This approach results in a more versatile design allowing the use of lower-cost modular combustors. These modules can be combined in varying configurations depending on the application.

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