Tuesday, February 24, 2015

Electron Cont'd (2)

To simulate the atmospheric phase of an Electron launch, I tried using OpenRocket, a Java GUI originally designed for simulating model rocket launches.
However the limitations of using a model rocket simulator to simulate a 10.5-ton orbital launch vehicle quickly became apparent:

In the above screen, I roughly sketched out the two stages of the Electron vehicle using the measurements in my Blender model.
As Electron will use composite materials for its hull, I specified carbon fibre for my OpenRocket model as well.
There is no publicly available data on the thrust curve of the Rutherford engines powering the Electron, so I took the highest thrust motor O8000 available on ThrustCurve.org and scaled up the maximum thrust to match the peak thrust figure quoted for Rutherford on the Rocket Lab web site (146 kN for the 9-engine 1st stage cluster, or 16.2 kN/motor, and 18 kN for the 2nd stage vacuum motor). I also set the burn times for the engines to 114 seconds, which is about how long ESA's Vega burns for its 1st stage, and increased the propellant masses until the wet mass of the rocket was in the ballpark of the quoted figure of 10.5 metric tons.

This gets Electron up to about 1.6 km altitude and nearly Mach 0.4, but without active GNC (guidance, navigation and control), the LV starts to "dance" around and tumble out of control and hits the ground after only 70 seconds in the air. Here is a GNU Octave (Matlab) plot of the simulated flight profile from OpenRocket:

The problem is that all model rocket simulator programs like OpenRocket assume that you are using appendages such as fins to passively stabilize the rocket and cannot simulate active stabilization using a GNC system. Still, it's quite interesting to see how long such a large rocket can stay aloft even without stabilization.

To be able to simulate active control, a different program is required. Apparently JSBSim, which is also used by the open source flight sim FlightGear, allows you to model a rocket GNC system. This is not surprising, as the designer of JSBSim is Jon S. Berndt, who did rocket dynamics engineering work previously for NASA. My plan is to try this next.

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