GUIDANCE / ACTIVE STABILISATION, COLD GAS JETS AND MONOPROPELLANT THRUSTERS
Initially, this started out as a project to build a cold gas jet system following a bet with the builder of the GYROC gyro stabilised, gimballed rocket, James Macfarlane. The system is heavily, heavily experimental, and I would hardly recommend it for general use. Subsequently, the issues encountered have meant that the project has become more wide ranging, and now encompasses a number of other control systems, as well as the vertical active stabilisation system, more often referred incorrectly to as a guidance system.
For all those budding active stabilisation acolytes out there, let me say this now at the start...
This topic is not for the faint hearted!
Guidance is not normally guidance in a rocketry context, but active stabilisation, and more specifically vertical active stabilisation.
Whilst not beyond the capabilities of anyone who is an experienced rocketeer, is comfortable with developing their own electronics systems, and has a decent grasp of mathematics, it is very involved, eats up lots of time, and can take a long time to make progress sometimes.
First of all, an explanation...... Guidance generally refers to guiding a vehicle to a target, normally at low altitude, or on the ground. Sub orbital, experimental and amateur rockets are designed to go upwards, so guidance systems are not really much use.
Vertical active stabilisation however, in which the rocket can maintain a vertical trajectory, is a lot, lot more useful, since maintaining the trajectory as straight and vertical as possible, enables the rocket to go higher.
So when rocketeers talk about guidance systems, it is not in the weapons sense, but in the keeping a rocket going vertically upwards sense, i.e. vertical active stabilisation. Big difference.
With this project, it seemed that rather than going straight to a vertical active stabilisation system using the cold gas jet control system, it would be better to break the development down into smaller, more manageable (and cheaper) stages. Consequently, the project now consists of the following stages:
- - Aerodynamic control surfaces using a moveable forward fin system.
- - Sun sensing vertical active stabilisation system.
- - Horizontal Auto Levelling (HAL) vertical active stabilisation system.
- - Gyro platform stabilised active stabilisation system.
- - Cold gas jet control system.
Aerodynamic control surfaces using a moveable forward fin system.
Front mounted control fins are less effective than rear mounted control fins, but when used in conjunction with fixed rear fins, their less effective nature actually becomes an asset, since any control system movement results in less responsive and less sensitive pitch changes. The front mounted fins effectively act as a active stabilisation trim control system (assuming the combination of fixed rear fins and moveable forward fins still results in a stable rocket). To ensure the combination of fixed rear fins and moveable forward fins does result in a stable rocket, the rocket's stability needs to be simulated in rocket trajectory software such as Rocksim.
From an ease of construction point of view, choosing controllable forward fins as opposed to controllable rear fins is a no brainer. Fitting the moveable fins and control system at the front of the rocket results in a much more compact active stabilisation, or guidance and control system, since there is no motor to fill up the airframe. Control systems do not need to be routed past a hot motor, and placing the active stabilisation or guidance and control system all at the front improves the vehicle''s static stability.
The Cold Gas Jet Control System.
Using a small, steel gas cylinder (A CO2 cylinder, in fact), a solenoid valve releases gas through a simple 4-way circulator, thus altering the trajectory of the rocket. The circulator port is determined by an onboard active stabilisation system coupled to the cold gas jet control system. On a small rocket it is quite heavy, especially when you take into account the active stabilisation system and power system as well, but on larger rockets it is ideal since it forms only a small fraction of the total mass.
Before plunging in to vertical active stabilisation development, before emailing others with questions about vertical active stabilisation or guidance, the best way to learn about vertical active stabilisation or guidance is to read the information on the links listed below. There is some very useful information contained on these webpages, and to anyone interested in active stabilisation or guidance, it is a very useful place to start. Read, assimilate the information, and run your own figures through various calculations.
There is a lot of work involved in developing these systems, years of development. A good grasp of physics and applied mathematics is highly advised!
- HPR Guidance Article
- Gyro Guidance
- Model Rocketry Guidance System
- The RMR Guidance FAQ
- PSAS Amateur Inertial Navigation Systems
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