Tuesday, February 24, 2015

Electron Cont'd (3)

JSBSim still requires that I specify aerodynamic data before I can realistically simulate atmospheric flight. These could be the following:

  • Drag coefficient for different Mach numbers (CD)
  • Side force coefficient (CY, or alternatively, normal force coefficient CN since a rocket is symmetric about its center)
  • Lift force coefficient (CA)

Normally, one would need to go about building a scale model and run a CFD sim or perform measurements in a hypersonic wind tunnel. However, I found a free program (unfortunately Windows-only, with no source code) called RASAero that can calculate various aerodynamic parameters through a wide range of Mach numbers in the specific case of a rocket with a nose cone and fins.

The following is the main screen of the program where I input the dimensions of the Electron rocket. As there are no fins on Electron, I set 0.0001 for the fin dimensions (since setting them to 0.0 would cause an exception). Also the program can display a sketch of the rocket:

This is an example of the plot output of drag coefficient versus airspeed (Mach number). You will notice the drag increasing dramatically just before Mach 1 (speed of sound). This illustrates the "sound barrier" nicely. There are two graphs: power-off is a simulation for gliding (unpowered) flight and power-on includes effects from the engine such as the rocket exhaust plume.

The next step is to copy and paste numbers from RASAero into JSBSim.

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