Performance Losses

Introduction
Rocket Nozzles
Nozzle Shapes
Radial Out-Flow
Radial In-Flow
Aerospike
Aerodynamics
Altitude Compensation
Performance Losses
Advantages & Disadvantages
Development
Summary
References
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In the previous section, we introduced the all-important concept of altitude compensation, theoretically the greatest advantage of the aerospike nozzle over more traditional nozzle shapes. Unfortunately, reality does not always match theory, and it this section we will explore sources of performance losses in aerospike and linear aerospike nozzles as well as some potential solutions.

Performance Losses:

When we first introduced altitude compensation, we compared a theoretical aerospike nozzle with a bell nozzle and an ideal nozzle. According to theory, the aerospike should meet or exceed the performance of the bell at all altitudes, thanks to its inherent altitude compensation characteristics. However, experimental data shows that these predictions are not necessarily true, for many potential sources of losses exist in the design of a practical engine, as illustrated below.

Comparison of performance results for theoretical and actual spike, aerospike, and bell nozzles
Comparison of nozzle thrust coefficient efficiencies vs. nozzle pressure ratios (NPR) for theoretical and actual spike, aerospike, and bell nozzles [from Tomita et al, 1999]

The most important cause of these losses is the elimination of the full-length isentropic spike previously discussed. The following graph indicates the loss in performance resulting from the replacement of a "full" (80%) spike by a truncated (20%) spike.

Comparison of performance results for full and truncated spike nozzles
Comparison of nozzle thrust coefficient efficiencies vs. nozzle pressure ratios (NPR) for full (80%) and truncated (20%) spike centerbodies [from Tomita et al, 1998]

Even though the aerodynamic spike does behave similarly to an actual spike, performance will still be lost due to oblique shock formation and the transition from an open to a closed wake. The effect of this transition can be seen in the above figures in the sudden decrease in thrust efficiency at pressure ratios around 20-30. Another important loss results from the replacement of an annular nozzle by a linear one. In a linear engine, exhaust may flow to the side of the engine rather than along its axis, as illustrated in the following figure. Other typical sources of performance loss are usually functions of individual designs, such as the type of thrusters employed and their arrangement, but these are beyond the scope of this discussion.

Cross-flow in an aerospike nozzle
Cross-flow in an aerospike nozzle [from Tomita et al, 1999]

Many researchers working on the X-33 and similar projects in other countries are currently conducting a great deal of research into methods of alleviating these performance losses. The most important of these, already mentioned several times, is the introduction of a secondary flow aft of the nozzle base, or a base bleed. The pivotal question surrounding the secondary flow is how great it should be. Research has shown that too small a bleed has no impact on performance while too large a bleed can have a detrimental effect on overall performance.

Aerospike nozzle performance with base bleed
Aerospike nozzle performance with base bleed [from Tomita et al, 1999]

Some experimental results are provided above comparing the full spike, aerospike without base bleed, and aerospike with minimum base bleed (0.3% of the primary flow) and maximum base bleed (2.4%). Beyond a bleed of about 3%, performance begins to suffer because the base flow actually re-circulates too quickly. As indicated in the above figure, the maximum bleed results in far better performance, matching the full spike over a very large range of operating pressures.

The same study also looked at the use of endplates, or sidewalls, along the aerospike, as illustrated below. Two types of sidewalls were evaluated, one extending well into the base flow region and another extending only to the end of the centerbody.

Comparison of aerospike nozzle side walls
Comparison of aerospike nozzle side walls [from Tomita et al, 1999]

The experimental results shown below indicate that the use of these walls does increase the base pressure, and thus the thrust coefficient, but the shorter walls do not increase performance nearly as much as those extending into the secondary flow region.

Aerospike nozzle performance with side walls
Aerospike nozzle performance with side walls [from Tomita et al, 1999]

From these results, and similar studies performed by others (see Fick and Schmucker, 1997; Ruf and McConnaughey, 1997; Tomita et al, 1998 and 1999; and Weegar, 1996), it is clear that space agencies, like NASA, seriously considering aerospike engines for future vehicles must carefully analyze characteristics of the secondary flow of these engines. Although the concept of altitude compensation appears simple at first glance, tools like base bleed and sidewalls may be key to actually achieving the required levels of performance.





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