A student society of undergraduates and PhD students dedicated to pushing the limits of high power rocketry and high altitude ballooning. We are based out of the Cambridge University Engineering Department and hold regular meetings.
Below are tests we carried out (prior to the Coronavirus lockdown) of the subsystems that will be needed for our next rocket rocket project, the Martlet 4. Powered by our Pulsar hybrid rocket motor, Martlet 4 will be our largest rocket yet while its hybrid propsulsion provides some unique challenges.
Fill line disconnect system: ‘The Tower’
The oxidizer for our motor is nitrous oxide, which must be loaded onto the rocket’s tanks entirely by remote control for safety reasons. We thus need a mechanism to separate the high-pressure hose used to fill the tank prior to launch. Our disconnect system, nicknamed ‘The Tower’ involves an electric winch, a tower-mounted pulley and a quick disconnect. Well done to the team that put the tower together!
Drogue deployment test
Below is a test of the deployment of the drogue parachute. The drogue is the first parachute to deploy, which later pulls out the main parachute for landing. It is ejected by pressurizing the parachute bay with carbon dioxide. This test was carried out on the ‘Martlet 3C’ airframe, a prototype airframe of the same diameter of the eventual Martlet 4 rocket.
Other milestones included the test of the rocket’s main feed valve. This lightweight aluminium valve is driven by an electric motor and opens to a wide orifice to admit the oxidizer into the combustion chamber. It has so far been tested with a back-pressure of 70 bar.
Martlet 2 is powered by an O-8000 Pro150 Cesaroni motor. This is the largest certified motor available and, over the course of its 5.12s burn time, produces an average thrust of 8000N. We purchased the motor last weekend from Animal Motor Works and will be meeting it for the first time at BALLS.
In other exciting news all the aluminium stock is now out of the basement lab and being machined. WPPE are machining the various couplers, fins and the aft cone whilst the CUED workshop are kindly machining the fin can ( at a length of 480mm and with a diameter of 186mm it is Martlet 2’s biggest metal component).
Current radios on High Altitude Ballons (HABs) are laughably slow, on the order of 50 bits/s. While this can suffice for transmitting simple data strings such as GPS co-ordinates, it’s useless for anything else. There have been several attempts at in-flight image transmission for amateur HABs with existing radios, though these took around 6 minutes to transmit a single small grainy image.
Lynx is an experimental digital radio transmitter for HABs, capable of data rates an order of magnitude above existing amateur systems. The end goal of the project is to have a live video stream coming from a balloon in flight. It features a powerful ARM chip to deal with the error correction codes and signal processing.
This development board will allow a number of the key radio systems to be thoroughly tested before implementation in the first flight model. Currently, we are waiting for some RF components to arrive from the US before the board is assembled.
The Joey-M flight computer was soldered up this week, all going fairly well apart from some minor rework required on some of the finer pitched components.
Joey-M uses the uBlox NEO-6Q GPS with a Sarantel SL1202 passive antenna, along with a small I2C EEPROM such that configuration can be retained when Joey is powered off. USB is broken out to a Micro USB connector such that configuration and debugging can be achieved via the excellent u-Center software from uBlox, and the configuration saved to EEPROM.
Joey-M uses the Micrel MICRF112 434MHz ISM band 10mW FSK transmitter with a twin varactor diode crystal pulling arrangement. Its 13.56MHz crystal has an 18pF capacitor on one side, and the varactor arrangement designed such that under no bias voltage, it also rests at 18pF.
One varactor has a large range of capacitance that allows the center radio frequency to be moved around by about 30kHz. The other varactor has a much smaller range, such that it can be used for very small shifts for FSK. Voltage is applied to the varactors by a twin 16 bit DAC from Linear Technology.
The radio stage of Joey-M is shown below:
To ensure a clean spectrum and thereby increasing the ability to decode, the modulator input waveform is digitally filtered in a Gaussian shape to provide GFSK. The step response of the order-50 Gaussian filter is stored in EEPROM on the AVR, and is written out to the DAC on each change of modulator input voltage. A trace of the modulator input at 300 baud is shown below:
A working firmware is currently running on Joey-M in preparation for a first flight this week. Later, temperature compensation for crystal frequency drift and more complex radio modes (MFSK) will be explored using the Joey-M platform.
Finally, thanks again to Cambridge Circuit Company for manufacturing these boards for us.
We got the PCBs for the Joey-M flight computer! These are experimental flight computers using the Micrel crystal pulling radio (see an earlier blog post), along with an Atmel AVR and a uBlox NEO-6Q GPS.
Thanks very much to Cambridge Circuit Company for manufacturing these PCBs for us!