Analysis of flowfields over four-engine DC-X rockets

Abstract

The objective of this study is to validate a computational methodology for the aerodynamic performance of an advanced conical launch vehicle configuration. The computational methodology is based on a three-dimensional, viscous flow, pressure-based computational fluid dynamics formulation. Both wind-tunnel and ascent flight-test data are used for validation. Emphasis is placed on multiple-engine power-on effects. Computational characterization of the base drag in the critical subsonic regime is the focus of the validalion effort; until recently, almost no multiple-engine data existed for a conical launch vehicle configuration. Parametric studies using high-order difference schemes are performed for the cold-flow tests, whereas grid studies are conducted for the flight tests. The computed vehicle axial force coefficients, forebody, aftbody, and base surface pressures compare favorably with those of tests. The results demonstrate that with adequate grid density and proper distribution, a high-order difference scheme, finite rate afterburning kinetics to model the plume chemistry, and a suitable turbulence model to describe separated flows, plume/air mixing, and boundary layers, computational fluid dynamics is a tool that can be used to predict the low-speed aerodynamic performance for rocket design and operations. /IJ/S::__:_ 620For launch vehicles using clustered engines, it is well known that the base environment significantly affects the overall drag _.2 and integrity 3 of these vehicles. Hence, it becomes very important to be able to predict the base drag during the vehicle design phase. Al- in an earlier effort, the McDonnell Douglas Navier-Stokes threedimensional (MDNS3D) CFD code 7 was calibrated for a plugnozzle DC-X configuration through comparisons with cold-flow data. Also, a separate effort benchmarked the finite difference Navier-Stokes (FDNS) CFD methodology s.9 with a cold-flow fourengine clustered nozzle base-flow experiment without the influence of the external flow over a vehicle body. In the current study, the FDNS CFD formulation is further benchmarked with the windtunnel data for an exact replica of the four-nozzle DC-X rocket.Here, the base-flow physics is complicated by the external flow past the forebody and aftbody. The DC-X ascent flight-test data, where the full-vehicle combined base environment with the hot engine exhaust and afterburning of the excess hydrogen with entrained air, are used to complete the validation process. Previous benchmarks s,9 have covered a range of P,/P,_ from 5 to 510 and equivalent altitudes from 7000 to 37,500 m, whereas the current effort completes the critical lower spectrum of P+/P_ from 1.2 to 1.7, equivalent altitudes from 1500 to 3000 m, and Mach number from 0.1 to 0.3 during ascent at zero angle of attack. Computations were performed to evaluate the forebody, aftbody, and base pressures and the total drag. The effect of afterbuming plumes on the base-flow physics is studied, and the scaling practice using cold-flow tests to infer flight vehicle conditio...