To strengthen the fins, a fillet was put between the fins and the airframe, and then two layers of carbon fibre added over the top of the fillets. The fillets are made of two different parts: (1) epoxy clay at the front and back of the fillets (black), and (2) epoxy lightened with fillite in the centre of the fillet (gray). The epoxy clay is extremely strong and easy to shape, but is also very dense. It was only used at the front and back of the fillet, where it will not be covered by the carbon fibre.
After the fillets have been laboriously sanded smooth, carbon fibre was added over the fillets. When rockets fly at high speed the fins are prone to ‘flutter’, where the fins bend and twist at a high frequency. This can result in broken fins,. so some strengthening is required. We used two layers of 200gsm uni-directional carbon fibre, and Sicomin epoxy. The fabric is orientated so the fibres run down one fin, across the body tube and up the other fin, and this prevents the fins from fluttering.
The fabric is wetted out with epoxy and covered in peel ply (red) to help release excess epoxy and give a smooth finish. The peel ply is a plastic film that is designed to not stick to epoxy, and it is perforated with a series of small holes to allow excess epoxy to escape.
After a fair amount of hand finishing with a sander, and a thin gloss coat of the epoxy the result is very smooth and shiny.
The stages of the Martlet 1 rocket fit together with some finely machined aluminium coupling rings. The coupling rings are then restrained together by some 6mm plastic bolts. In flight, the primary method of separating the stages is to ignite the rocket motor in the next stage. As the pressure in the rocket motor ramps up, the high pressure causes the plastic bolts to shear, and the stages separate. The stage that has separated then deploys its parachutes from the open end, by using explosives protractors to release the spring drogue parachute.
If the rocket motor fails to ignite, however, it is important to have a secondary separation mechanism, so that the parachutes can be deployed. In this case, the flight computers detect that the motor didn’t ignite, as there is no acceleration, and separate the stages. The explosive protractors that deploy the spring drogue parachutes are strong enough to shear the interstage plastic bolts, and the stages separate, with the spring parachute close behind.
This video was taken at 300 frames per second and shows the front of one stage (bottom tube) and rear of the next stage (top tube) separating. As the protractors are activated the cap that restrains the spring parachute pushes the stages apart with 4kN force applied within 20ms of activating. The plastic bolts, whose heads are the white dots, are sheared and the orange and black spring parachute jumps out of the parchute section. In this video the spring parachute does not jump fully clear, as (a) the spring was not fully compressed, and (b) the bottom tube hits the floor before it can get out.
Over the weekend we assembled Martlet 1 in its 2-stage configuration (Stage1+3 of the 3-stage configuration). It is likely that the Martlet 1 rocket will be flown as a 2-stage rocket for its first outing at the Big Range launch, in little over a weeks time. The 2-stager is still pretty big – it should reach Mach 2 and ~35000ft, and is almost two stories high as shown in the picture below.
The Martlet 1 rocket has three stages, and each stage has two parachutes: (1) a small ‘drogue’ parachute that is deployed at apogee, so that the rocket comes down from high altitude in a controlled manner, but without drifting too far. (2) a large main parachute that is deployed near the ground for a soft landing.
Both the drogue and main parachutes were bought from Fruity Chutes and modified for better deployment. The drogue parachutes are 12″ in diameter, and we have added two features: (1) Nylon netting around the parachute lines, which means that it is now impossible for the drogue parachute lines to tangle. (2) A 50mm diameter spring has been sown into the centre of the drogue, and sown into a netting sock to keep it straight. The spring has two purposes, to keep the drogue stretched and inflated, and to jump out into the airflow when it is deployed. The drogue is packed into the rocket behind an aluminium cap, and the cap is restrained by two plastic bolts. To deploy the parachute, the plastic bolts are sheared by an explosive protractor, and the spring makes the drogue jump out of the rocket into the airflow.
The second feature that we have added to the recovery system is a proper parachute deployment bag. A deployment bag keeps the parachute packed in a neatly folded state, and makes sure that the parachute lines and the parachute are unfurled correctly into the airflow. The main parachute is deployed at 40m/s for this rocket, so it is important that the parachute deploys properly. The standard HPR practice of rolling the parachute, then wrapping it in its own lines, is much more likely to end up in a tangled parachute and a hole in the ground.
The deployment bags are made of red Cordura nylon, and white nylon tape. The inside of the bag is coated with polyurethane to reduce the friction when the parachute pulls out. On one side of the bag there are two tunnels for the parachute lines, on the other side there are five tunnels for the larger nylon tape that runs from the base of the parachute to the rocket. The parachute lines are folded into a U-shape, and the bend of the U is pulled through the tunnel (as shown on the left of the bottom photo). When the bag is deployed in the airflow, the lines start to go taut and then pull out of each tunnel in turn, then pull the parachute out of the bag. In this way the parachute and lines are fully stretched out when entering the airflow and will deploy without tangling.
Today we tried a method for vacuum bagging the tip-to-tip carbon fibre onto the fins of the rocket. The tip-to-tip carbon fibre has uni-directional fibres that run down one fin of the rocket, over the body tube and up the next fin. At supersonic speeds, the fins tend to flex and ‘flutter’, and can snap off the rocket. The tip-to-tip carbon fibre gives the fins a lot more flexural strength and should prevent this.
We created a vacuum bag that uses putty tape to seal on the aluminium leading edge of the fins. Because we only wanted to test the method, we covered the surface of the rocket in teflon sheet before adding the epoxy.
Although the vacuum bag gave a reasonable finish to the carbon fibre, it was quite a pain to get a good vacuum seal, and was quite an involved process. For the proper tip-to-tip carbon fibre we have decided to go for a normal wet lay-up and to manually smooth down the carbon fibre using rubber rollers. The epoxy that we are using for the lay-up is Sicomin, from Matrix Composites, which has good temperature stability without a post-cure. It is quite ‘fragrant’, however, hence some fairly hefty respirators….
Each stage of the Martlet 1 rocket has two parachutes: (1) a small sprung drogue parachute to bring the rocket down fast from high altitude (2) a much large maion parachute that is released near the ground to give a soft landing. The small parachute is deployed at apogee, and then near the ground the riser cable from the small parachute is released by a pyrotechnic mechanism. As the small parachute is released it pulls the main parachute out of the rocket body.
The release mechanism is shown below mounted on the bulkhead that sits between the motor and parachute sections of each stage. The pyrotechnic device is a protractor (gold in the photo below), which has an explosive charge and a piston inside it. The release mechanism has two parts, a housing that is bolted to the bulkhead,. and a slider with two 4mm pins. The kevlar riser cable (here just string) is looped around one of the pins until the mechanism is fired.
When the pyrotechnic device is fired, a piston is ejected with 2kN force, pushing out the slider and releasing the cable. The slider is then stopped from flying off by a catching plate (not shown).
Since Martlet 1 is a multi-stage rocket, we decided it would be great to have video of the stages separating, so we’ve installed an HD video camera in the fillet of the stage 3 fins. The camera is a #11 HD808 keyfob ‘spycam’, which with a LiPo battery upgrade can give 2 hours worth of HD video – long enough for us to arm the rocket and faff around before launch.
The camera had its casing stripped off, battery replaced, LEDs added for ‘power on’ and ‘recording’ (using some ideas from this post), and the USB connector and switches broken out, so that they can be accessed from outside the rocket. Caution: There is some bodgalicious soldering in the photos that may cause offence to some people.
To charge the battery, or download video, you just have to plug a USB cable into the back of the fin fillet (which is not yet added in the photos below).
There are two switches, one for power and one for the shutter. The two SMD LEDs are just visible to the right of the switches.The final product should give great video as the separation happens, a screenshot from the installed camera is below. We’ve now adjusted the position so that there is less aluminium at the top of the picture.
Lots of progress on the Martlet 1 rocket. The last few days we have bonded a lot of components, using a variety of alignment jigs machined on our CNC router.
The machined aluminium leading edges have been bonded into the C-F honeycomb that forms the fin structure, for stages 1 and 3. Together they form a lightweight fin with a sharp, high temperature leading edge, suitable for supersonic speeds. The fins have locating pins that locate into centering rings that are bonded into the rocket airframe.
The stage 3 fins are shown here gluing with a protective layer around the HD camera (white blob) – see following post.
The nosecone has also been bonded into its aluminium mating ring, using an alignment jig knocked together with a machined mating flange, a length of aluminium profile and an aluminium plate we had in the lab. After testing it on a rotating table, it’s pretty well aligned and looks great in the rocket. Just needs a lick of paint now.
The stage 1 and stage 3 fins have been tacked into the airframe, using the locating pins and a foam jig. Thie jig has fin slots milled at exactly 120 degrees to align the fins.
We’ve now bonded the coupling rings into the airframe sections of the Martlet 1 rocket, shown standing together in the photo below (from left Stage3 Stage2 Stage1). We have also cut down the nosecone to fit our rocket diameter. The rocket nosecone was sent in record time from MadCowRocketry in the states. It’s filament wound glass fibre with a metal nosetip, and seriously smooth finish. We’d definitely recommend buying from them.
Now we just need some fins…….
We’ve now bonded in the coupling rings for the stage 3 parachute section. The parachute section is coupled to the motor section by an aluminium bulkhead. The parachute section contains two parachutes: (1) a small sprung parachute, that is deployed at high altitude to bring the rocket down quickly but safely, (2) a large main parachute that is deployed 1200m above the ground, to give a soft landing.
Using 12 high grade, countersunk M3 bolts, the bulkhead attaches to coupling rings that are bonded into the motor and parachute sections.
At the end of the parachute section is an aluminium ring that couples to the back of the nosecone (or back of the next stage, in the case of stage 1+2). The parachute section has 2 wiring tunnels for the explosive protractors that control parachute deployment. The protractors are golden coloured in the image below, and shown in the as-fired state with the pistons extended. The third tunnel guides the kevlar riser from the small primary parachute to a release mechanism on the bulkhead. When the riser is released at 1200m altitude, the primary parachute pulls the main parachute into the airflow.