M-Python
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I had such a good time with my Dynacom Scorpion project that I wanted to do something similar but in a higher performance minimum diameter project. I worked up an initial design based on Dynacom components and sketched it all out in Rocksim. The design is a 3" minimum diameter single stage with three fins, and dual deployment, designed around the Aerotech M1315W motor. I wanted to vacuum bag the fin can, so I designed a separate fin can that was small enough to bag and attach to the main airframe using a relatively short section of motor mount tube. The M-Python is actually a pseudo-minimum diameter; it does have a motor mount tube, but the motor mount tube OD is equal to the ID of the main airframe. This design allowed me to use a motor retainer that was flush with the main airframe.
I initiated this project in November of 2001, and had a great
time going back and forth with Eric Haberman, Dynacom's proprietor, finalizing the
design. Starting with my sketch, Eric produced an AutoCAD drawing that was the
basis for all the subsequent tweaks. Eric solved a lot of the problems
with this project and the final design is the result of a true collaboration.
Eric was going to be taking some time off over the winter, so I knew it would be
a long wait. I used this cold weather (and epoxy unfriendly) time period
to tool up my rocket fab, commissioning the universal fin alignment fixture from
the machine shop and a John Coker tip-to-tip jig from a cabinet maker. I
also bought the Shadow Composites composite curing oven plans and am making that
myself.
I received the components in early April 2002, and they are shown in their initial state in the photograph on the left. The motor mount tube is snugly inserted into the fincan airframe on the lower left. The motor mount tube has grooves for the fins machined into it that perfectly match the slots in the fincan airframe. Additionally, the forward end of the motor mount tube has circumferential grooves machined for better adhesion to the main airframe section, which is in the middle of the photo. The motor retainer slides on to the rear of the motor mount tube, with the result being that the OD of the retainer is flush with the OD of the main airframe. The aft-most railbutton is threaded into the aluminum retainer.
The timer compartment in this photo is inside the payload airframe section in the upper right portion of the photo. It also serves as a coupler between the main airframe section and the payload airframe. In this photo it is hidden behind the Elvis CD which is shown for scale. The timer compartment bulkheads and associated hardware are shown in the upper left.
The fins are a custom design clipped delta, and although the glare hides it, are very nicely beveled by Eric.
The nose cone is shown assembled, and has an allthread
centerpiece running from the tip to the base, to accommodate weights for
adjusting the center of gravity. I am quite happy with the quality of the
machining and the fiberglass, so from here on out it is up to me.
The first step is to epoxy the bare aluminum motor retainer to the aft end of the motor mount tube. West Systems makes a two-part aluminum etch kit which is supposed to increase adhesion of aluminum to fiberglass and epoxy. Although probably not necessary, I decided to use it just for fun and perhaps education. Part A cleans the surface and removes the aluminum oxide surface layer, part B temporarily prevents re-oxidation for two hours.
The etch treatment seemed to work well, the aluminum (roughed up
at the bonding section with a rose-patterned 80-grit sanding job) was brightened
significantly and the cleaner/etchant produced a satisfyingly dark by-product.
Eric's retainer system is very innovative; a steel wire is captured between a
slot in the fiberglass motor mount and a slot in the aluminum retainer. Although
it is a little bit tricky to install, the result
is a mechanically
keyed attachment that is quite reassuring. If you look closely at the photo of
the attached retainer on the right, you can see a dark circumferential line in
the shadowed fiberglass area that is the steel wire resting in its slot. I had
intended to use the opaque (black) shadow composites high temperature epoxy, but
the installation process was dependent on visual feedback, so I went with West
Systems 105/206 epoxy so that I could get some visual feedback. I don't think
that will be a problem since one ends up with a mechanical lock taking most of
the load rather than the strength of the epoxy bond. This is a very clever
design, my hat is off to Eric once again.
The Fins...
After bonding the motor mount tube/retainer section to the airframe, the next step is to mount the fins. I finally received my Universal Fin Alignment Fixture from the machinist in early August. The photo at left shows it in action. The design and procedure for using it can be found by following the hyperlink above.
The results were great, and I couldn't see or measure any deviations from the desired alignment at all.
I decided to reinforce the fins from tip to tip using three types of cloth, 5 oz. bidirectional carbon fiber, 2.5 oz. carbon fiber, and 2 oz. fiberglass, in that order from inside out. I have studied John Coker's tip-to-tip reinforcement technique from his website, and decided to copy his method. Easier said than done...To do this right, I had a cabinet maker build yet another jig; an almost exact copy of the jig, following the advice on John's site. This technique aims to simulate vacuum bagging, but since the geometry of fins bonded to a cylinder doesn't really accommodate vacuum bagging, weight is applied directly on the lay-up, and the structure is supported from beneath with a wooden jig. More information about this jig can be found in the tools section, which includes a link to the original source on John's website.
You can see the tip-to-tip jig in action in the photos below:
I used Shadow Composites' High Temperature epoxy thickened with
kevlar pulp for the fin fillets. This stuff is quite viscous prior to adding the
hardener, and I had good luck dispersing the kevlar pulp by mixing it into the
resin first, and adding the hardener second. The high amount of shear involved
in stirring just the resin overcomes the kevlar's tendency to clump. Once the
hardener is added, the viscosity drops dramatically and I've had problems with
clumpy kevlar in the past when adding it after the hardener. Also, since the
epoxy
is opaque, it is difficult to find and break up any clumps there may be.
After the fillets had partially cured (four hours) I did the lay-up using West Systems epoxy (don't snicker at the lay-up shown above...I wasn't finished!) Following the method of Mr. Coker, I used mortar mix inside a smooth plastic bag atop the peel ply and breather layers in order to squeeze out the excess epoxy. I was only able to get about 0.5 psi using this method, as opposed to 14 psi when vacuum bagging. However, the results looked good to my eye with a healthy amount of resin transferred. Having to use the mortar mix inside a plastic bag is definitely the weak point with this method, however, and I will keep my thinking cap on for improvements.
Above left is a "before and after" shot for the filling, priming, and sanding process. Above right is the entire rocket assembled for the first time.
Electronics
For this project, I wanted to use the RDAS "kompakt" flight computer from AED Electronics. The RDAS is relatively small, and can fit in the same space as the Blacksky Altacc that I normally use except that the battery and switch are externally mounted. However, the RDAS is infinitely expandable, with outboard igniter boards, GPS, telemetry, magnetic apogee detect, etc. available. Also, it is well supported by a relatively large community of nerds (I mean that in the good sense of the word) that provide advice and new expansion capability. The primary gathering place of these folks is the "topica" discussion board for RDAS, found here.
My first task in laying out this project's 8" x 3" rectangular electronics board was to select a switch and 9V battery holder. The consensus opinion I got for the switch (aside from various home-brews) was the missle-works switch. I bought two and upon examination they seemed like they would do the trick.
The next decision was the 9V battery holder. Primarily, this is deciding between a fixed, encapsulating style or a floating design that holds the battery with tie-wraps and only attaches at the battery terminals. After doing some legwork, I came up with four candidates, shown below:
At left, is the Missle-Works version, second from left is an aluminum version from Keystone Electronics (definitely the coolest looking), second from right is the supplied holder from AED electronics, and furthest right is another Keystone version. After evaluating all these, I decided to go with the holder on the extreme right, from Keystone. The problem with the encapsulating holders is that they couldn't compensate for variations in battery dimensions, and the slop necessary to allow change-out of batteries reduced their ability to hold the battery in place during rough duty. Between the two free floating versions at right, I preferred the Keystone (extreme right) because it was rigid, but covered the battery terminals preventing any accidental shorts. Additionally, you can order the Keystone model with any length wire leads you like.
After buying a Weller soldering station and teaching myself to solder, the board was finished and is shown below. The board slides into the timer compartment and is sandwiched between the bulkheads, preventing up and down movement. The board fits so precisely that it does not rotate within the tube. Note that there is only a single altimeter (no backup!). I will use the "smart deployment" mode: inertial detection of apogee for the drogue and barometric main deployment. I will use the timer circuit as a back up for drogue deployment.
