AeroCFD
MODELS THAT INCLUDE LAUNCH LUGS
HART Missile Example

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HART Missile and Launch Lug AeroCFD Analysis, Mach = 0.5

AeroCFD is classified as an axisymmetric 3-D and planar 2-D Computational Fluid Dynamics (CFD) computer program. However, AeroCFD solutions are not limited to bodies of revolution even though AeroCFD's bodies of revolution can be very complex. Results of an AeroCFD analysis is presented that determines the forces and pressure distribution acting on a typical launch lug and superimposes these effects on the main body of the HART Missile. The analysis involves first solving the flow field acting on a rocket and its fins in 3-D axisymmetric form. Then, in a separate analysis performing a 2-D analysis of the launch lug using the Import Shape geometry defined in figure-1. The Import Shape of the launch lug is 2-D, however AeroCFD has the capability to define launch lug thickness in the Z direction where for this example the thickness is defined to be 0.188. This approximation allows the analyst to define a launch lug as a 3-D body for the purpose of determining the axial force (FX) and normal force (FY) acting on a 3-D body. Flow field interference between airframe and launch lug is a consideration. However, a good "engineering" approximation for the forces acting on a launch lug and its contribution to total force acting on a rocket can be made using the principal of superposition. This useful "engineering" approximation is especially good in regions where flow field interference of the airframe, fins and launch lug is minimal. For example, launch lugs, or most objects located on the airframe, are located ahead and midway between the fins of a typical model or high power rocket. This approximation is especially good for Mach 0.5 flight as illustrated in Figure-3 because the background flow field of the inserted launch lug closely matches the pressure contours of the HART Missile by itself. This condition indicates minimal airframe-launch-lug interference and says that superposition is a good estimate for Mach 0.5 flight of the HART Missile. For the Mach 1.2 flight condition, airframe-launch-lug interference is increased but the aerodynamic forces acting on the launch lug can still be successfully modeled using this technique. However, this "engineering" approximation is less accurate as Mach number is increased well above Mach 1 due to interference with the nose cone induced shock layer as illustrated in Figure-5. In conclusion, this analysis demonstrates that AeroCFD solutions are not limited to bodies of revolution and with a little imagination AeroCFD can be applied to attached objects like launch lugs if flow interference is not severe but is very cost effective. The true cost to perform the same analysis in full 3-D using a commercially available CFD program involves taking weeks to generate a 3-D CAD model, performing complex meshing around fins and launch lug and then spending hours of expensive computation time on a supercomputer. For example, this analysis took only two hours from initial set-up to final solution using AeroCFD.


Figure-1, X, R (Y) coordinates used to define Import  Shape geometry of a 2-D launch lug with 0.188 width (Z).

 


Figure-2, AeroCFD 2-D launch lug results displaying drag force (FX) and other aerodynamic coefficients for Mach 0.5.
AeroCFD contour plot image reduced by 27%.

 


Figure-3, AeroCFD plot showing launch lug pressure contours superimposed on Hart Missile pressure contours for Mach 0.5.
Launch lug contour plot inserted into the main rocket contour plot and then scaled and located using Photoshop or similar
processing software. AeroCFD contour plot image reduced by 27%.

 


Figure-4, AeroCFD 2-D launch lug results displaying drag force (FX) and other aerodynamic coefficients for Mach 1.2.
AeroCFD contour plot image reduced by 27%.

 

Figure-5, AeroCFD results showing launch lug pressure contours superimposed on Hart Missile pressure contours for Mach 1.2.
Launch lug contour plot inserted into the main rocket contour plot and then scaled and located using Photoshop or similar
processing software. AeroCFD contour plot image reduced by 27%.

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