Design Case Study : Pedal Powered Hot Dog Launcher

So you're given a bunch of hot dogs. Then you're asked to come up with a team, and build a home made contraption measuring 6x6x6 ft so as to launch the dogs to the greatest possible distance in just 3 tries. As if that's not enough cause your head to itch, you're told you can't use chemical reactions, fire, sparks, batteries or compressed air to achieve this task. Hmm?

A colleague of mine at work and a friend of his decided to team up for this serious challenge. Steve Peifer and Jeffrey Ohliger are both mechanical engineering alumni of Alfred University at Alfred, New York. Earlier this year, they found themselves wanting to compete in the school's 2009 Hot Launching Competition. The event would be held on April 26 in the University premises.

Left to Right : Steve Peifer & Jeffrey Ohliger

Now launching a hot dog is not as easy as it sounds. The item is delicate and must be prevented from breaking apart after launch. You have air resistance to tackle. No, the hot dogs cannot be encapsulated with fairings, neither can they be frozen. In fact, rules stated that all the hot dogs would be supplied on the day of the competition and would be of the same size and brand.

The rules also stated that the hot dog would be thrown in the direction of a straight line from the center of the launch site. Every inch it deviated from this line in either direction would be penalized from the distance you threw in the direction of the straight line. If the hot dog did split after launch, only the distance of the piece falling to the shortest distance in the direction of the straight line would be taken into account, no matter how far the other pieces went. Teams could be as big as 5 but only 2 people could be present at launch. The team with the greatest distance achieved would win a gift certificate of 75 dollars.

Interestingly, the pair came up with the idea of using pedal power to launch their hot dog. After lots of coin spent buying parts, welding and assembling, they had a working model. Off they went to the competition. At the end of the day, from 15 or 20 teams, their design achieved the best distance and they were proud winners. This, in spite of an unpredicted headwind straight from hell on that day.

Some days back, I caught up with Steve to help me unravel the mechanism behind the launcher. Enjoy the read, pictures and videos! Drop a comment if you have any questions.


Q. Steve, where did you pick this idea for your design? Did you consider any alternatives?

SP : The idea to build a centrifugal trebuchet came from high school when we built one for a Science Olympiad competition. The advantage of a centrifugal trebuchet is that it allows the object, in this case a hot dog, to be brought up to the desired launch speed slowly. This avoids extreme accelerations on the object which would have caused the hot dog to split and break. Our goal was to throw a hot dog completely unmodified (no freezing or encapsulating) over 350 feet, so there really weren't any other types of launchers (allowed in the rules) that would have thrown the dog at 150 mph without breaking it.

The design

Q. That makes a lot of sense! Explain to readers how this mechanism operates.

SP : Sure. Its pedal powered, as you can see. We used a chain to couple the rear sprocket with the shaft of the rotating throwing arm. The person on the bicycle pedals until the desired launch speed is reached (there is a speedometer measuring the throwing arm tip speed). Then the launch is initiated by pulling a lever that releases a spring loaded rod that snaps into the path of the throwing arm. The rod strikes a pin attached to the hot dog holder. When they collide, the pin on the holder gets knocked off and allows the holder to open. The angular location of the launch mechanism (the spring loaded rod) sets the launch angle which can be adjusted in between launches.

Hot dog holder

Driven sprocket and shaft

Bearings

Q. So was that your bike?

SP : No. It was my uncle's. Its an old clunker.

Jeff, like a master, lays the torch to steel

Q. I'm sure the rules must have been a little tight. What, according to you, were some of the biggest challenges to overcome?

SP : Oh, there were many.

1) Air resistance, both for the hot dog and the throwing arm : In order to decrease the drag on the throwing arm, which was made out of square aluminum tube, we used foam to teardrop the throwing arm. This allowed us to increase the speed from around 90 mph to 160 mph.

2) Keeping the hot dog in one piece : Once we tweaked the release mechanism, this wasn't much of an issue with our design.

3) Making sure the hot dog is released at the optimum angle and not breaking anything during the launch : Initiating the launch at the desired angle wasn't hard, but making sure the hot dog didn't extrude out of the holder at high speed was a huge challenge. The more we secured the hot dog in place, the more likely the holder was to break during the launch. It took a lot of tweaking before we found a happy medium.


Q. I'm sure! So if you don't mind me asking, how did you go about calculating the effective range of the hot dog when launched?

SP : I created an Excel spreadsheet to calculate the required velocity of the hot dog so that it would reach our goal of 350 feet. The spreadsheet also calculated the energy required to power the launcher at that speed based on the physical properties of the throwing arm.

Above are screenshots from Steve's Excel spreadsheet

Q. Interesting. I recall from Physics 101 that 45 degrees of launch angle gave you maximum launch range, but that didn’t take air resistance into account. You know, R = [(v)^2 x sin2(angle)]/g.

SP : Right, pretty much everything we learned in physics was for an ideal world with no air resistance. You are correct that 45 degrees gives max range with no air resistance. In the real world with our situation, we calculated that a launch angle of 35 degrees would do the trick.


Q. I'm sure you did a bunch of test runs before the big day. Just curious - how many hot dogs did that involve?

SP : You're right. We tested the launcher for a few weeks leading up to the competition in our backyard. Testing started off well right from the first launch. Our initial problems were with the hot dog breaking, but we eventually determined it was from a rubber band slicing through the dog as it left the holder. Once we got that issue fixed, we started throwing faster and faster. Each time we increased the speed, we had issues with the hot dog exiting the holder prematurely. We had to tweak the holder so that it required more force to open yet still open freely enough for a clean release without breaking anything. We probably threw about 40 hot dogs during testing.

Steve pedals the launcher in his backyard

This video, shot at a high 420 fps, shows the launch sequence

Q. So the big day arrives. You guys do your thing. You win. Did your design actually work as planned?

SP : The design worked perfectly during the competition, just as it had during the testing that week. Unfortunately, we had no control over the wind which was blowing straight in our face at over 30 mph. We still won the competition, but we only threw a little over 290 feet compared to 400 feet during testing the day before (with no wind). To give an idea of how much the hot dog was blown off course, the head wind was coming at a slight angle, maybe 15 to 20 degrees from the right. To compensate, we had to launch about 100 feet to the right of our target, and the dog still blew back a couple feet left of center line!

This slow motion video captures the launch on the day of the competition.

Congratulations! Excellent work from you two!

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