Computer Science – Performance
Scientific paper
Jan 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002iaf..confe.781t&link_type=abstract
IAF abstracts, 34th COSPAR Scientific Assembly, The Second World Space Congress, held 10-19 October, 2002 in Houston, TX, USA.,
Computer Science
Performance
Scientific paper
ED nozzles have long been considered for launch vehicle applications, due to their postulated twin benefits of reduced length and altitude compensating capability. However, the difficulties involved in modelling the inviscid/viscous jet boundaries and associated flow phenomena during wake closure, have prevented the creation of a reliable method for the prediction of the performance characteristics of the type through atmospheric flight. However, if the operating regime of the nozzle is restricted to vacuum conditions (e.g. upper stages, and OTVs, etc), the wake region of the ED nozzle is permanently closed. Under these circumstances, the prediction of the pressure distribution along the viscous/inviscid flow boundary, and the complex interaction of the fluid flows during wake closure, becomes unnecessary. Therefore thrust calculation may be accomplished by conventional techniques, provided reliable methods are available for the prediction of the flow in the throat region (which may be arbitrarily displaced and inclined to the axis of revolution), and estimation of the pressure acting on the base of the central pintle. Prediction of ED nozzle throat flows has been accomplished by the use of CFD techniques, described in a previous paper. The analysis in this paper has been extended to complete nozzles by including a conventional method of characteristics based optimisation routine for the outer shroud contour, and a semi-empirical method for prediction of pintle base pressures. A brief parametric study is presented, outlining the effects of throat configuration (including throat wall radii, and radial displacement and inclination) on the performance of axisymmetric and planar ED nozzles under vacuum conditions. Whilst the method used for base pressure prediction requires several simplifying assumptions which affect the accuracy, results from an ongoing experimental program are reducing this uncertainty. Further as nozzles designed for vacuum operation are likely to have extremely high area ratios to maximise thrust coefficient, the relative magnitude of the pintle base pressure is small compared to the forces generated on the shroud, reducing the sensitivity of overall thrust calculations to errors in base pressure prediction. A comparison of thrust performance of bell and ED nozzles reveals that considerable reductions in length are possible, in the region of 30%. By implication this should result in a significant lowering of system mass. This conclusion is further supported by consideration of the unrealistic worst case scenario, which is the assumption of zero thrust contribution from the pintle. This analysis still produces length savings of over 20% when compared to conventional optimised bell nozzles.
Hempsell C. M.
Taylor N. V.
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