The Bethania Project
This page details the construction, and eventually the flights, of my first non-Estes model rocket. It is designed to be flown on my experimental hybrid rocket motor, although if I ever get around to sorting out my explosives licenses, it could fly on Aerotech motors as well.
In case you were wondering, the rocket is named "Bethania" after Morgan the Moon's rocket in a song by Max Boyce called "The Ballad of Morgan the Moon". If you are Welsh you'll know who Max Boyce is. If not, he's a Welsh folk singer, who writes songs and poetry, mostly based on rugby and coal-mining. "The Ballad of Morgan the Moon" is about a Welsh rocket scientist who was actually the first man to walk on the moon (not Neil Armstrong as many believe). Unfortunately Morgan never returned, although a postcard was received from him which said simply "It is cheese (Caerphilly)".
Bethania is constructed from a mixture of off-the-shelf components and homemade parts.
The airframe is 4' of 3" PML phenolic tubing, made from a 12" piece and a 36" piece of phenolic tubing (as opposed to a single 48" piece).
The airframe is reinforced with West System epoxy resin and a single layer of 7 oz/yd2 (235 g/m2) woven kevlar cloth from Shadow Composites for added strength and impact-resistance. Shadow Composites heat-shrink tape was used in to ensure that a nice compact laminate was obtained. The rocket was left close to radiator for the first couple of hours of curing, to help the tape to shrink and squeeze out the excess resin.
Imperfections on the surface of the tube (such as holes where the epoxy hasn't completely filled the weave of the kevlar, or ridges from the edges of the heat-shrink tape) were removed by sanding and filling repeatedly (a very tedious job!). The filler used was Wilkinson's own-brand Wilko Fine Surface Filler. I know, I should use a proper epoxy-based filler. However this stuff was (a) cheap (£1.25 a tub), and (b) easily available. It also seems to work fairly well - it sticks to the cured epoxy just fine (as long as you give it several hours to dry), and doesn't flake off when it is sanded (which I thought it might).
The nose cone is a standard PML 3" nose cone.
More details on this coming soon!
The fin can is of my own design, and homemade. It slides over the 38mm motor mount tube, and the fins protude through slots cut into the 3" airframe. They are held in place by a PML tailcone which slides onto the motor mount tube behind them.When I get around to it I'll add some photos to show exactly what I mean. The fins themselves are made from a balsa wood core, laminated with epoxy and kevlar (the same stuff used on the airframe). The tubular part is a kevlar epoxy composite again, constructed with the aid of Shadow Composites heat-shrink tape - excellent stuff. The fins are then epoxied to the tube using Araldite Rapid epoxy, and then filleted with epoxy with added kevlar pulp (again from Shadow Composites) for extra strength.
I'm not sure how the strength of fins constructed in this way compares to proper through-the-wall construction, but my way has the advantage that if the fins break I can simply remove the fin can and pop a new one in with out any hassle. Also, I can add different sized fins if necessary, i.e. if I decide my rocket needs a larger or smaller margin of stability.
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The rocket is designed around my 37mm hybrid rocket motor, but as it has a 38mm motor mount, it should be suitable to fly on Aerotech 38mm solid-fueled motors, when/if I ever get around to sorting out my explosives permits.
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The main flight computer is my home-made altimeter. This will record air pressure, acceleration and air temperature, and control deployment of the drogue and main chutes. There is also a back-up timer, in case of failure of the main flight computer. In addition, to aid in recovery of the rocket, a radio transmitter will be on board, which with a suitable YAGI antenna on the ground should enable tracking of the rocket. A small attack alarm will be situated in the drogue compartment also, to aid in recovery.
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The recovery system is a two stage design, to (a) increase my chances of getting the rocket back, and (b) minimise the walk to get it. Deployment of both the drogue and main chutes will be controlled by my homemade altimeter, probably with a PIC-based timer for backup.
The drogue chute is home-made, 18" diameter, similar to the design on Richard Nakka's page. It is deployed at apogee from a hatch just above the motor mount section, below the electronics bay.
The main chute is a 66" chute from Bob Fortune at Aerocon Systems. It will be deployed at a predetermined altitude above ground level (yet to be decided).
Shock cord is 3mm static rope from Outside (a climbing and camping store). It is attached to the anchor points using a figure-eight knot.
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