By Mark Uitendaal and Leon Krancher
Disclaimer: all liability waved! The contents of this page is presented for informational purposes only. Do not try to recreate any experiments presented in this page. The NAVRO and the author of this article cannot assume responsibility for any use readers make of this information. In The Netherlands it is forbidden by law to own this type of propellant if you do not have an exemption of the "Wet Explosieven Civiel Gebruik" (WECG).
After the loss of our airframe the Vortex XL, which was lost in the trees at the launch-site, it was time for a bigger project. A project with (more) ambition. After some inspiration from a photo shoot of HPR magazine about Ray Dunakin III and some cool onboard movies from people like the Gates brothers, we agreed that we also wanted to fly with a photo camera. The whole thing is not new; a guy named Philippe Reignier has done it in France. We also agreed that the rocket would have two stage parachute recovery. Also, the rocket will have to fly on our own PVC motors and on commercial HPR motors. This is necessary because we want to fly on NLD18 at "'t Harde" and at ALRSIV in Switzerland.
The Spectre is a rocket of about 1,8m long. It has an elliptical nose cone. This is done because we don't want to pull the Cp to the front. The trailing edge of the fins are somewhat tapered to the front. This is done that when the vehicle is landing safely on its parachute, no energy of the impact is going through the fins, even when it hits the ground at an angle.
The maximum diameter of the PVC motor is 55mm. This motor will have a somewhat similar performance as an AeroTech J350-W (we hope). At ALRS we want to fly an J350-W. This is a 38mm motor, so we will need some sort of motor-adaptor.
The camera is a fixed focus camera from Canon. We soldered some wires to the PCB of the camera so that we can attach a relay in the future. We are going to make a NE555 circuit for the timing of the photos, probably with an interval of just over one second. The camera is going to look sideways to the direction of flight, so we will need to build it very accurate to prevent high speed rolling during flight.
The electronics will be an AED R-DAS with a TRAX ART (Dekker) timer as backup. The reason we use commercial products like these is the reliability of the systems. Systems like recovery electronics are somewhat "mission critical" for the successful outcome of the flight.
The Motor concept
The motor we intend to fly with is a PVC motor. The general design is the same as its predecessors. It uses a concrete nozzle which is pressed into a PVC tube. This design also includes some PVC couplers.
The propellant is potassium nitrate and sorbitol in an oxidizer/fuel ratio of (65/35). In previous designs the propellant was potassium nitrate and sucrose (the Vortex motor), but to get a higher burn-rate we have chosen sorbitol as fuel.
The nozzle will be incorporating two washer-rings. This is done to minimize erosion. In our previous motors, erosion was not that bad. The erosion kept the KN (klemmung) low. In this motor we also want a high thrust coefficient, so the shape of the nozzle must be somewhat the same through out the burn.
|The general dimensions of the motor will be:|
|Nominal Diameter:||50mm (standard PVC-tubing in The Netherlands)|
|Maximal diameter:||55mm (due to couplers and end-caps)|
|Isp (aim):||105 sec|
|Total Impuls (aim):||700+ Ns|
|Burn-time (aim):||2,0 sec|
|Kn max:||150 (which probably gives a pressure of about 20 Bar)|
The design pressure is very low, but this is done for reliability. We also got a static test stand which we will use to verify the motor. A problem raised in previous tests, with another rocketry club called "DARE", is the slow start of big sucrose and sorbitol motors. We are going to see what this motor will do. If it is a real slow starting motor, we are going to boost up the burn with a pyro-mix of cellulose and black powder.
The Spectre update
At first we made a concept design of the outside of the rocket. Of course this is just a concept, so during the project the design can always be altered. Any alterations will depend on the real position of the centre of gravity (Cg) in the rocket. This enables us to "customize" the centre of pressure (Cp) by altering the fins. The general approach of recovery will be a separation in the middle of the rocket, when deploying a drogue chute at apogee. The main chute will be deployed from the nose cone at 200 metre altitude.
At this time we only made the fuselage and some parts of the electronic canister. The fuselage is made of standard PVC tubing. Normally this tubing is used for drainpipe. The outer diameter of the tube is 80mm, with a thickness of 1.5mm. The nose cone has been made of birch-wood which was made on the lathe we have on loan from our friend Bert Koerts.
The electronic canister:
The electronic canister will be the brains of the vehicle. It will contain the flight computer and backup device, the camera unit with the interval circuit, and a small localizer transmitter. In this design the stresses due to deployment of the parachutes will be transferred through the canister, so the design must be very strong. Also due to the size of the camera, we can't hook up the canister through a ring. This means that the lowest bulkhead on the canister must contain an O-ring to prevent hot gasses from the ejection charge to enter the payload area.