Experimental Investigation of the Evacuation Effect in Expansion Deflection Nozzles

Paper number

IAC-08.C4.3.6

Author

Dr. Neil Taylor, University of Bristol, United Kingdom

Coauthor

Mr. Mark Hempsell, Reaction Engines Ltd., United Kingdom

Coauthor

Mr. Alan Bond, Reaction Engines Ltd., United Kingdom

Coauthor

Mr. Richard Varvill, Reaction Engines Ltd., United Kingdom

Coauthor

Mr. James Macfarlane, Airborne Engineering, United Kingdom

Coauthor

Mr. Richard Osborne, Airborne Engineering, United Kingdom

Year

2008

Abstract

Expansion Deflection (ED) nozzles have long been considered for use in trans-atmospheric vehicles due to the potential increments in performance offered through altitude compensation. However, there are several difficulties associated with the type, primarily a more complex combustion chamber and throat design, and the importance non-isentropic flow phenomena such as viscosity and shock waves on flow field composition. This means that the flow solution techniques required for the design process are more complex than conventional nozzle analysis, and were until relatively recently (the last decade or two) beyond the capabilities of standard aerodynamic tools in widespread use. This greater complexity results also in a broader design space, increasing the length of the design process as a greater number of issues must be investigated. Most significantly, however, interest in the ED nozzle has been limited by to a general consensus that the type produces relatively poor altitude compensation, as evacuation effects within the central flow region reduce the pressure at the free pressure boundary between the supersonic flow and the atmosphere. As it is through this interface that altitude compensation is effected, this would inevitably reduce the performance of the system as it would in effect compensate to a higher altitude than that at which the vehicle was flying. Coupled to this, a similar class of nozzle (the Aerospike), appeared to offer the same benefits without the drawbacks, and hence was much more extensively studied.

However, the Aerospike nozzle has a number of flaws which have delayed its introduction as a common replacement for bell type nozzles. The ED concept has been re-evaluated by the current authors over a number of years, and it has been concluded that provided that care is taken in specifying nozzle geometry, the pressure adjustment is more effective than generally assumed. For instance, cold gas tests of an ED nozzle recently reported elsewhere at off-design conditions show little or no such evacuation effect. 

The STERN project (Static Test Expansion deflection Rocket Nozzle) was therefore set up with the intent of producing the first published results from a hot firing of an ED design, with the intention of resolving the issue. This engine operates on gaseous hydrogen and air, as a secondary aim was to demonstrate combustion technology for the air-breathing phase of combined cycle rocket engines. The static thrust of the motor is approximately 5 kN, developed from a mass flow rate 2.0 kg at a chamber pressure of approximately 100 bar. The test programme ran from February to April in 2008, and the results of this programme are reported here for the first time. 

Abstract document

IAC-08.C4.3.6.pdf

Manuscript document

IAC-08.C4.3.6.pdf (🔒 authorized access only).

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