Rocket Avionics - Sutton Program Article 6Follow article
This article will accompany the sixth video in our series made possible by RS Components Grass Roots aiming to introduce key rocketry concepts and principles. This article will introduce each of the components required for a small-scale rocket flight computer and explain the fundamental principles behind their operation. Written by James Balharrie, Kieran Webb and Sean Clark
Avionics is the term used to cover all electronics on a rocket, these systems need to work together to allow everything to function correctly. It covers functions such as monitoring the state of the rocket, controlling certain flight parameters, air-to-ground communication and recovery event management. All of these functions are made possible by correctly (and simultaneously) functioning avionics.
The principal piece of rocketry avionics is a flight computer and this generally consists of a microcontroller and sensor suite combined on a printed circuit board (PCB).
Summary of a PCB
PCBs are boards that consist of conductive and insulating layers. These layers are designed in such a way whereby conductive traces are formed, providing a reliable electrical connection between components just like wires. Components are securely affixed to the boards by means of solder, a fusible metal alloy, provides a reliable connection between the PCB and the components.
In order to design a PCB, specialised CAD software is used and the design process behind each piece of software is largely the same. First, a schematic is created. Below is a basic circuit diagram, much like you would draw by hand, and provides the structure of the design – showing components, values of components and the way in which they are connected.
The second stage is placing the components on a board in the orientation you desire and from here, the traces can be routed on the board from component to component (matching the previously made schematic). Once the board is printed, the components can be placed onto the board using solder as mentioned before.
The importance of the Microcontroller
While commercial flight computers do exist, such as the Stratologger, the majority of this article will be focused upon our custom flight computer, made in-house at GU Rocketry. At the centre of the computer is the microcontroller, this is a small form factor computer coming equipped with a processor, memory and input/output capabilities.
Common commercial microcontrollers that you might recognise include the Raspberry Pi or Arduino boards. These contain extra functions such as display outputs (HDMI) or network connections (usually Ethernet or WiFi). Both of these systems as well as GU Rocketry’s own, Teensy 4.0, all provide the necessary hardware on a single PCB.
While the microcontroller is the effective ‘brain’ of the flight computer, without additional peripherals such as sensors, it would be unable to interact with the real world. All the computers are useless if they have no data upon which to act, this is why sensors are so crucial to ensure predictable and safe flight.
Sensors and their uses
The first sensor is GPS or Global Positioning System. We have all heard of GPS – it locates the position of the rocket through triangulation of satellites but unfortunately, it is unable to calculate the altitude of the rocket, only its position in 2D space.
This problem can be overcome with the addition of a barometer. A barometer calculates ambient air pressure and using the rule whereby air pressure decreases with increasing altitude, a simple calculation can be done onboard to determine the altitude. The last of the main 3 sensors is the IMU, or Inertial Measurement Unit.
This uses accelerometers to measure the rocket’s acceleration, gyroscopes to measure the angular velocity of the rocket, and in this case, a magnetometer to measure the magnetic field relative to earth. Individually these sensors are not without their downfalls but when combined they provide all the data needed to accurately calculate the altitude and kinematics of a rocket.
There is an SD card contained in the rocket as well which records flight data in order to be analysed later and also pyrochannels. These control our pyro events such as parachute deployment. Finally, there is a status LED, which extremely useful for telemetry debugging (or, correcting for flight path inconsistencies).
A popular method in combining values representing the same variable is called a Kalman Filter. A Kalman Filter takes readings of a state (for example, velocity) from multiple sources and over time estimates a more accurate reading than could be achieved through one source alone. This is particularly useful when relying on accurate data for calculations when using sensors with notable errors in their readings.
The Internal Communications
Next are the communications protocols, split into two primary types- SPI (Serial Peripheral Interface) and I2C (Inter-Integrated Circuit).
SPI uses a master/slave communication hub. In the case of our flight computer, the master device is the microcontroller and the slave devices are the GPS sensor and SD card reader. Each of the devices have different types of connections in order to discern which device is returning information to the microcontroller. There are 2 data connections taking place within the flight computer:
- Master In, Slave Out (MISO)
- Master Out, Slave In (MOSI)
These two connections are regulated by a serial clock and a chip select. The serial clock allows for the alignment of the clock cycles, essentially making sure the exchange of data between the master and slave devices occurs at the correct time, and the chip select allows for the interchange between MISO and MOSI communication types to allow the microcontroller to understand which device it is currently communicating with.
I2C uses two connections, a serial data and a serial clock. These are all connected on a single bus with each slave device having a unique address. The main benefit of I2C is that it allows multiple slave devices to be connected together using one singular bus.
Communicating with the Ground
During powered flight, all of the data gathered by the sensors is constantly transmitted back to the ground, allowing for in-flight tracking and analysis. There is a wide range of communication types used to transmit this data, and they each have their own benefits and shortcomings. Choosing the communication type depends on a multitude of factors:
- The range over which the data must be transmitted
- The frequency at which the data must be transmitted
- The power consumption of the rocket’s transmitter
Our Saltire-02 flight computer uses two RFM9 LoRa transmitter modules, which use SPI to communicate with the microcontroller. These modules transmit data at a frequency of 433MHz- this allows for our flight computer to transmit within the ‘license-free’ frequency band in the United Kingdom.
This transmission of data is crucial for the rocket in flight, as it helps the launch team monitor the rocket’s flight path as well as providing data to allow for an easier recovery. Article 7 in the Sutton Program will cover a range of recovery techniques and strategies, both on small and large-scale rockets.
As with any electrical system, a robust and reliable power system is absolutely vital for that system to operate successfully. When we are designing our power supply (in this case, a range of batteries) there are multiple important factors to consider. These include:
- the output voltages and currents
- the capacity of the batteries
- the size & weight of the batteries
- the reusability of the batteries
For this example, we will use the same batteries as we have for Saltire-02 i.e., two batteries which are each rated at 3.7V.
When combining batteries, an important design aspect to consider is the circuit configuration- whether the circuit is in series or parallel. The configuration of the circuit will affect its current, capacity and voltage as shown below.
- When in series, the voltage output will double
- When in parallel, the current and capacity will both double
When placing the batteries in series, the voltage will increase by 3.7V every time we add another battery. In this case, the voltage would be 7.4V, which exceeds the 3-5V load the microcontroller can safely handle. In order to rectify this, a voltage regulator can be used.
Time for fireworks!
Pyrochannels are outputs which connect to electronic matches (otherwise known as e-matches) which ignite pyroevents. The term ‘pyroevents’ include a range of rocketry-related events, such as the explosion to launch a parachute or the ignition of a new stage. The most common pyrochannel design makes use of an electronic switch to ignite the match when a signal is applied- this switch is called a MOSFET. There are two types of MOSFET, ‘n-type’ and ‘p-type’.
All MOSFETs contain 3 connections, a gate, a drain and a source. For an n-type MOSFET, current flows through it when a voltage which exceeds the MOSFET’s gate threshold is applied.
The opposite is true for a p-type. When the gate threshold is exceeded, current cannot flow through.
In the context of our flight computer, we use an n-type MOSFET, meaning that when the microcontroller wishes to activate the pyroevent, it sends a high signal to the gate. This then exceeds the gate threshold, allows the current through and ignites the e-match.
Using the microcontroller, and the sensor suite outlined before, we can calculate the stage of the rocket, this will then be used to determine when to active the pyroevents.
The stages of rocket flight
The stage of flight is always monitored and stored into memory. While in the current stage of flight, the flight computer will detect a pre-determined change of specific variables that go on to indicate the stage threshold has been past. Some key stages to implement in a flight computer’s software include pre-ignition, accent, apogee, drogue descent, main descent, and landing. The following table summarises some of the basic kinematic changes you would expect to see for a respective stage transition.
Velocity zeroAcceleration zero
Acceleration increase (towards positive)
Velocity decreases to zero
Acceleration changes direction (positive to negative)
Velocity decreases notably as drogue deploys
Acceleration changes direction (negative to positive)
Drogue Descent/Main Descent
Velocity decreases notably as main parachute deploys Acceleration increases (towards positive)
These stages can now be used to sequence events during flight. For example, to remove the prospect of ballistic descent (free-fall descent) the drogue parachute must be deployed as close to apogee as possible, therefore the signal to the pyrochannel to deploy the parachute can be sent as soon as the stage transition is detected. Events can also happen timed from a stage transition, using a similar example, a back-up drogue charge can be activated at a set time from the apogee detection.
The avionics which make up a rockets systems are some of the most vital components within a rocket- without them, there is no way to collect data or communicate to the ground team. As well as this, they can be rather complex and difficult to build and fully understand, and as such most educational bodies do not cover rocket avionics in depth until the student has a good understanding of general electrical systems. In any case, we hope this article provides a practical look at the thought process behind flight system development. Once again, thanks for reading, and happy learning!
Parts in this series:
- An Introduction to GU Rocketry
- The History of Rocketry - Sutton Program Article 1
- Rocket Dynamics - Sutton Program Article 2
- The Rocket Equation - Sutton Program Article 3
- Rocket Propulsion - Sutton Program Article 4
- Rocket Aerodynamics - Sutton Program Article 5
- Rocket Avionics - Sutton Program Article 6
- Rocket Recovery - Sutton Program Article 7