Tuesday, February 24, 2015

Electron Cont'd (5)

Aerodynamic coefficients are specified in the aerodynamics section in the aircraft definition file.
Coefficients must be defined per axis; for Electron, I specified DRAG (axial) and LIFT (pitch/normal) axes.
Normally, I would also need to define the SIDE (yaw) axis, but due to symmetry about the longitudinal axis, SIDE is simply equal to -LIFT.

DRAG force is a factor of the following:
  • Dynamic pressure (JSBSim property: aero/qbar-psf): pressure of air exerted on moving rocket; increases with speed
  • Cross-sectional area (JSBSim property: metrics/Sw-sqft): for a cylindrical rocket, equal to the circular cross sectional area
  • Angle of attack alpha (JSBSim property: aero/alpha-rad): angle between wind and rocket, related to pitch angle
  • Mach number (JSBSim property: velocities/mach)
RASAero was able to plot the drag coefficient CD versus alpha at Mach 0.1, 0.5, 1.1, 2.0, 5.0.
It was also able to plot CD versus Mach number from 0 to 25.
I defined tableData elements in JSBSim for these two plots. The first plot has two independent variables alpha and Mach number so the table is two-dimensional with alpha increasing from top to bottom, and Mach number increasing from left to right.
The second table is just CD with Mach number increasing from top to bottom. I took the "power-on" sim values (exhaust plume effects included) as the rocket will likely be thrusting through most of its flight path.

LIFT force, like DRAG, is also a factor of dynamic pressure, cross-sectional area, angle of attack, and Mach number.
I defined another two-dimensional table with independent variables alpha and Mach number from a RASAero plot of lift coefficient CL.

Next I will define the GNC parameters for stabilizing the rocket during flight.

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