Rocket Flight Simulators

Simulators are great for giving you a rough idea of how high your rocket will fly, help in selecting an appropriate motor and delay, determining if the launch speed is adequate and some help you determine the stability of the rocket (the CG/CP relationship). My two favorite simulators are wRasp (www.wRasp.com) and Rocksim (www.apogeerockets.com).

But they really only give you a rough idea. Let’s face it – there are a lot of variables that affect rocket flight – wind at launch, wind at various elevations, launch angle, rod whip, fin misalignment, motor performance variability, temperature, humidity, the surface finish of the rocket – you get the idea. While some simulation programs allow you to change these variables, the difficulty in measuring them may be giving you the illusion of greater accuracy than is warranted.

A key parameter in rocket simulations is Cd, drag coefficient. Rocksim calculates Cd using one of three different methods, which you select. Cd is dynamic, changing during flight, yet wRasp uses a single Cd. Obviously both simulator programs have limitations with regard to Cd. Change the Cd and watch how altitude changes. Now some rocketeers will tell you that by using an altimeter the Cd for a particular rocket can be determined after the fact. But in reality, that Cd is not accurate, it’s just a plug to make things work. All other errors or flight conditions get lumped into that plug number.

Although the results are just approximations, simulations are very useful. I wouldn’t fly a midpower rocket without first running a simulation. wRasp is free, quick and simple to use. All you need to know are the diameter and weight of the rocket, estimate the Cd or use the default, select a motor and launch. However, it will not tell you the stability of the rocket. Rocksim will, but it’s not free and is more complex; its strength is designing custom rockets.

Roadrunner has provided downloadable files of its motors to use with both wRasp and Rocksim.

Sim first, fly second.