SR&ED Eligible Zombie Apocalypse Prevention Series – Circular Sawblade Launcher

Source: Shooting Circular Sawblades With The Slingshot via Gizmodo

It’s a popular myth that only work done in lab coats is SR&ED eligible; however, the majority of the everyday awesomeness that can be found on the internet could be SRED-able if they meet the corporate requirements. First in our series on preventing the zombie apocalypse using R&D tax credits. Read on to find out how one of our expert technical writers might slant an SR&ED report on the circular sawblade launcher.

SR&ED Advancements

The ultimate goal of this project was to develop a slingshot design that would be able to use a circular saw blade as the operational projectile.  Traditional slingshots work quite well for throwing solid objects with a symmetric moment of inertia, but the slingshots failed when attempting to throw a disc.  In order to consider our project a success, we had to achieve the following advancements:

Launch Mechanism

The design of the payload holder in traditional slingshots is quite simple, in that a basic sling pouch is used.  The length of the strings holding the pouch help determine both windup and release characteristics. This direct approach isn’t feasible when launching saw blades; consequently, we required a new method of imparting the initial energy to the saw. We would need to transfer 1000 N of elastic force directly into the saw blade to achieve an equivalent rate of change of linear momentum of the projectile upon its release. Another requirement was that a method of constraining the disc to travel in a straight line needed to be created. By doing so, we would be able to achieve a minimum launch distance of 50m. We also wanted all modifications to be easily constructed from materials found in the post-apocalyptic ruins of a small city. By the end of the project, we had successfully completed these objectives. A symmetrical handle was mounted on both sides of an 8” saw blade and two large elastics were used to provide the power. A smooth channel was installed along the barrel of the slingshot to guide the shot.  The trigger had to be redesigned to handle the extra weight and power required; a mechanical lock was included to hold the elastics in place while the saw blade was loaded. We measured our initial velocity of the blade to be 20 m/s, and achieved a maximum launch distance (parallel to the ground) of 56m with an effective target range of 40 m. The blade was found to be lethal against all fruit and vegetable test subjects, and we anticipate that it will work equally well against the zombie hordes.

Angular Accelerator

In order to get the disc to successfully fly any distance at all, we needed to get it to spin. To obtain both maximum flight distance and penetrating power, the disc would have to rotate at a minimum angular speed of 600 RPM when it left the barrel of the slingshot. We also required that any additional equipment would not interfere with the actual launching process, and that all equipment modifications would need to be handheld (<20 lbs total). We successfully integrated a spin inducer into the final design. A thick rubber strip was imbedded within the launch guide so that the teeth of the saw blade dug into the strip as it was projected along.  We measured our rotational speed to be ~500 RPM, and the spinning disc increased our projectile distance by 1120% (5m to 56 m) as compared to a non-spinning launch.

SR&ED Uncertainties

Weight Concerns

Basic slingshots are all handheld devices, but the saw blade is such a different projectile, that we realized it was likely we would have to redesign the appearance and function of the slingshot. Since the primary purpose of this device was for the use of soldiers charged with zombie cleanup duty, the weight and unwieldiness had to be minimized. With all the additional equipment we would need (launch stabilizer, spin inducer, heavy-duty trigger), it was uncertain if we could achieve our performance goals while still allowing the average foot soldier to wield these for an entire tour.

Attachment Points & Spinning

The primary difficulty we encountered with this sawblade launcher was trying to figure out a way to handle the discs and cause them to spin.  We wanted the loading process to be simple and minimize the user’s contact with the razor sharp edges for safety reasons; this drastically limited the number of different ways we could think about launching the disc. The moment of inertia profile of the disc meant that any attachment point would have to be axially aligned to prevent eccentric from appearing during the flight, which also limited how much freedom we had to deal with attachment points.  The fact that the spin inducer needed to be reusable was another concern to us.  Many methods involved direct contact with the sawblade teeth, which could damage the launcher itself and cause it to fail at a critical life-or-death moment.

SR&ED Work Performed

We first began this project by investigating to see if any other survivor groups were also investigating this technology to see if we could pool our resources and knowledge base. We found that other groups were either a) focusing on extending the traditional slingshot design to larger and larger payloads, or b) using powered launchers which required electricity and carefully machined parts to send the saw blade out. None of other groups were focusing on a low technology solution for the hundreds of people still living around abandoned wood mills, so we began our own project from scratch.

We first tried applying the traditional slingshot design to our problem. We mounted the saw blade on a ballista platform, and proceeded to test its performance. We mounted the blade in a variety of different orientations, with the blades parallel to each of the 3 major axes.  In all cases, it failed regardless of how carefully we initially balanced the blade against the sling.  The blade would reach a maximum distance of 5m, but the random rotations induced by slight tension imbalances in the elastics meant that the penetration power was very low, and the plane of the blade which would impact the target vegetable was completely random.  Sometimes the blade teeth would hit and bite in a few millimetres, sometimes the face of the saw would simply smack the target head-on.  This was unacceptable, as any damage would be minimal and more likely to enrage the target then incapacitate.

We realized that our first goal was to get a better grip on the sawblade.  Our initial attempts with a traditional sling proved useless, as the blade would often cut through the material, or we would get an uneven force projection, again due to unevenness in the tension of the elastics.  We examined the sawblade and realized that an attachment point located directly through the centre hole of the disc could work.  Any force applied along this axis would coincide with the inertial centre of the disc and eliminate any torque.  We experimented with different materials (aluminum, melted down steel girders etc.) but settled on pine wood.  The other materials required advanced melting and machining tools which may not be available to the average patrol.  Dowels of pine can be scavenged from many houses and businesses which are already burned out, and cut to length as needed.  We encountered an initial problem with keeping the two sides of the dowel connected, as we weren’t sure how durable we needed this connection to be in case we needed to reuse the disc after scavenging later.  Nails were tested, but we realized that simple wood glue would be enough to hold up for several shots.  Our initial tests with this handle showed that the elastics would slip off; grooves were cut into the sides which solved this issue.

Attaching the elastics to the new handle doubled our previous distance record, and we noticed more reliable teeth first impacts, but we realized a guide would be required.  We constructed a simple channel which ran along the top of the slingshot and forced the sawblade to follow it during launch.  This improved the accuracy instantly.  We found that the disc would travel at least 5m in a straight line at full power before wobbling for the next 10m.  We experimented with different barrel lengths, trading off additional control for extra weight.  We settled on a 3’ long shaft, which was still short enough to be handled by one person.  Additional lengths could be constructed depending on the desired role of the weapon (sniper, skirmisher etc.).

We finally realized that the only way to get the disc to travel an adequate distance would be to get it rotating in space.  We performed theoretical calculations to see the stabilizing effect that the spin would have, and decided that a 600 rpm rotation would be sufficient.  We tested several methods to get the disc to rotate.  We tried wrapping the elastic bands mounted to the handle multiple times so that the disc would begin to spin as the elastic unravelled.  This increased the strain on the bands, reducing their lifetime, increased the strain of the mechanical trigger we had designed, and also introduced reliability issues, as occasionally the disc would not launch at all.  We also tried introducing a slight curve to the barrel to see if that would induce spinning, but it did not have a noticeable effect.

Examining the blade again, we realized we could use the teeth as a source of friction.  A softer material imbedded in one side of the track only would induce the angular momentum we needed.  We tested several materials, including solid machined metal which fitted the grooves of the disc exactly.  This increased the production cost of the launcher, and also produced a good deal of noise when fired, so we abandoned this.  We eventually found a rubber manufacturer, and after testing several rubbers of varying toughness and strength, we chose one.

Tests were conducted with this new design and the results were very promising.  A firing range consisting of various fruits and vegetables at various distances was constructed.  Varying the elastics used allowed us to measure how much force was required to get a certain distance, and at what point the impacts stopped being lethal.  We fine tuned the design after this, until we were able to get a reliable ‘kill’ impact on a watermelon (approximate zombie flesh consistency) at 40 m.

Keep checking back with the “Fun” section of SREDucation.ca for future instalments of the SR&ED Eligible Zombie Apocalypse Prevention Series.

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