Due to a late minute design change we could not get the parachutes to fit inside the booster section of Martlet 3. Perhaps inspired by his PhD on Dyson vacuum cleaners Jack suggested that vacuum bags might help, no one thought it would work. A short trip to Asda and some experimentation later and the chutes were in the rocket. What’s more they could be pulled out with negligible force! The technique involves fitting the parachute bay and parachute (in it’s deployment bag) inside a large vacuum bag. The vacuum then assists the packing of the chute into the bay.
The standard in model rocketry is to use a small (about 2 grams) black powder (BP) charge to deploy the parachutes. The black powder can be ignited with an e-match which can be fired from a flight computer. The rapid combustion produces hot gases which pressurise the sealed parachute bay causing the rocket to separate at a specially designed section.
Smokeless powder (SP) has been designed as a replacement for BP in firearms
applications. It does not require an explosives license where as BP does, which makes its use attractive. However the chemistry of SP is quite different to BP, in particular its reaction rate is far more sensitive to pressure. When used as an ejection charge the powder is far less contained than in the barrel of a gun.
After learning this I made some aluminium charge holders. The first one had an internal diameter of 7mm, this proved too small as the holder was fired through the 5mm plywood bulkhead during the deployment test. The second version had the below dimensions. I made up a charge with the tip of the e-match positioned at the bottom, about 2 grams of SP, masking tape acting as wadding and a section of old inner tube cable tied over the top as a burst diaphragm. The ground test worked perfectly. As it was a launch day I jumped in the car and headed to the launch site where I made up 3 identical charges (one for the drogue, two for the main). But the main failed to deploy. Both charges had fired but had not produced enough pressure to break the shear pins. On closer inspection of the failed charges the inner tube (burst diaphragm) had slipped past the cable tie rather than rupturing. This must have reduce the containment just enough to curtail the combustion.
In conclusion SP can be used for rocket deployment charges but it is very sensitive to how it is confined. Black powder is much more forgiving and is a much more suitable explosive, a suitable license is not hard to get.
So quite a while back some PCBs arrived courtesy of Cambridge Circuits for the first revision of Dora, a data logger for use in our custom static fire test rigs. The device itself consists primarily of an STM32 processor, a micro SD card, and analogue front ends for a thermocouple, strain gauge and pressure sensor. We also included a USB port for debugging and some GPIO headers in case we wanted to experiment with extra functionality such as an LCD screen.
I’ve managed to get my final year project to be based on rockets, what’s more a guided rocket! The project has a few key areas: Airframe construction, IMU and simulation/control. I chose to use canards for a few reasons:
- They can be used over the entire ascent of the rocket not just the motor burn, this should give about 15s to play with.
- The rocket can be designed to be passively stable and as such can be tested without an active control loop.
- The control can then be introduced gradually, starting with roll.
The past few weeks I’ve been working intensively on the airframe design and construction. The rocket is made from 4″ tube, is almost 2m tall and weighs in at about 4.5kg.
Canard and upper flight computer assembly. The canards are 3D printed out of nylon using an SLS printer. I was hopping there would be sufficient resolution to print the servo splines. However there wasn’t. I think it is possible using the correct type of 3D printer though.
Adapted from a page by Adam Greig 14/08/2015.
This post provides links to more information about the radio transmissions from Martlet 2, such that they may be publicly documented formats.
For details on the physical radio modulation and data link layer, please see:
For details on the packet structure, please see:
For any other queries please get in touch using the email address provided on this website.
The flight computer PCBs have been collected from Cambridge Circuit! As ever they’ve done a fantastic job. We had a quick tour of their facility when we collected these PCBs which was great to see. Thanks again Cambridge Circuit! The renderings of the PCB above were generated by GerbLook, click the link to check out the other layers.
All the orders have now been placed for the various electronic components for these PCBs (the bill of materials for one flight computer alone is around £150!). We also have stencils and stencilling jigs in-hand so soldering should take place next week, after which all that’s left is to write the firmware that will operate the avionics. This firmware is responsible for deciding when the moment is right to fire the various pyrotechnics and let the ground crew know what’s going on.
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).
Today Cambridge Circuit Company finished manufacture of the PCBs for M2R, the radio board on Martlet 2. We drove over to pick them up this afternoon and they’re looking good! Hopefully we’ll have the designs for the flight computer finished soon and then we’ll get both boards assembled and start on the firmware.
A big thank you to Cambridge Circuit Company for their continued support of CUSF; they’ve now been sponsoring us since 2012 and have always given us fantastic PCBs very quickly.
The radio modules we’ll be using on the M2R radio boards arrived from Radiometrix today. They’re generously sponsoring Martlet 2 with these radios, thanks! We’ve been using their radios for many years now, starting with the venerable NTX2 for some early high altitude balloon payloads, and they’ve always performed perfectly. We’re excited to try out these very new MTX2 modules — they are much smaller than the NTX2 and should have excellent frequency stability thanks to a temperature compensated crystal, plus they are in-flight frequency reprogrammable in case we find frequency clashes in the field.