Gimbaled Model Rocket Simulator


Informatique - 08-05-2018

Project Overview

Thrust vectoring is the ability of an aircraft, rocket, or other vehicle to manipulate the direction of the thrust from its motor in order to control the attitude (angle) of the vehicle. It is used in rocketry it is the primary means of attitude control, as aerodynamic control surfaces are ineffective outside the atmosphere.

Rocket Thrust Vectoring animation (Credit: NASA)

In a gimbaled thrust system, the exhaust nozzle of the rocket can be swiveled from side to side. As the nozzle is moved, the direction of the thrust is changed relative to the center of gravity of the rocket.

Rocket Thrust Vectoring torque generation (Credit: NASA)

I started designing and building a model rocket with a gimbaled thrust system. Its controller should allow it to be stable and follow a pre-determined trajectory without the help of fins, reducing its aerodynamic drag.

Because of the complexity of this project, it will be divided into two articles:

  • Part 1: Impulse - the rocket physics simulator
  • Part 2: design, build & tests of the rocket [Work in progress]

The software

A rocket is a complex system and the impact of every variable must be known prior to the design process. The usage of a simulation software that computes the kinematics of the system solves this issue by allowing us to visualize these impacts, and tweak the variables to optimize the flight performances.

User interface of Impulse

Impulse is a open-source and cross-platform simulator for gimbaled thrust rockets:

  • It accurately simulates the physics and outputs real-time data;
  • The simulated rocket is fully configurable;
  • To visualize the kinetics of the rockets, there are several graphs (for linear and angular acceleration, speed and position) and a 3D view;
  • A rocket controller board can be connected through a serial link.

The last point is a very useful feature of Impulse: it means that it is capable of testing the control algorithm on the rocket hardware.

Download and usage

To launch Impulse, execute the jar file. As an alternative, in can be executed from the command line with java -jar ImpulseRocketSimulator_v0.1.jar. Java Runtime Environement 8 must be installed.

Download release v0.1

The help tab in the software has all the required instructions in order to get started.


Technical Details

In this section, I will be doing a short technical overview of Impulse.

Physics engine

Impulse's engine is based on a physics model of the rocket I designed. It takes into account as many variables as possible to estimate the position and attitude of the rocket at any time: mass, repartition of mass, gimbal angle, motor thrust, etc. I am currently writing a small report to justify the equations used in this model.

This basic flow diagram represents the way the discrete computation can be implemented in computer code.

Flow diagram of the physics engine

As the model advances to get more accurate, the physics engine is intended to evolve.

Code

Impulse is developped in Java, making it both efficient and cross-platform. All of its code is released under MIT license.

Download Eclipse project

The source code is very modular, with extensive usage of the object-oriented design principles. Therefore, it is very expendable: more motor types (hybrid, liquid fuel), multi-staging, etc. Every class is fully commented using standard Javadoc format.

Special thanks to Alexey Sokolov (JSSC), Ralf Sternberg (minimal-json) and Konstantin Bulenkov (DarculaLaf) for their great open-source libraries!

Limitations

There are still limitations to this simulator:

  • It does not simulate aerodynamic drag. I advise using the great OpenRocket software for accurate, open-source simulation of those forces.
  • Currently, one degree of freedom is completely absent: the rotation around the vertical axis of the rocket. Although it is not required in my physics model, it might be added in the future.
  • It makes several reasonable assumptions:
    • The rocket's mass repartition is radially symmetrical;
    • The rocket's mass doesn't change throughout the flight (yet).

In the future, I plan to:

  • Priority: implement simulation of aerodynamic forces. This is quite a bit harder than in most other simulators because of the assumptions I can't make, due to the absence of fins!
  • Import a complete motor database to provide the most often used thrust curves.
  • Create an Arduino library to make the usage of Impulse with Arduino much easier.

Auteur : Charles Grassin


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