Rocket Pictures From an Old Fighter Pilot

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Colonel
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Rocket Pictures From an Old Fighter Pilot

Post by Colonel » Fri Apr 20, 2012 9:45 am

Image

Image


Here are two photos of the winning rocket



BHT412
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Re: Rocket Pictures From an Old Fighter Pilot

Post by BHT412 » Fri Apr 20, 2012 11:15 am

Thanks for the detailed information it really helps us first timers.

I had a couple of questions if you have a minute. Weight, I found that without enough weight the rocket would spin around the fishing line several times. The torque was to great and the rocket would expend all it's energy spinning around the line and basically go no where. Can you tell me how much your completed rocket weighs?

This is the fist time for our pack to have a rocket derby so they decided to limit the prop winds to 100. As mentioned above I had to keep the weight up for the rocket not to spin but I'm only getting about 25' on my test track. Did you happen to test at 100 winds, if so do you remember how far down the track your rocket went?

Thanks



Colonel
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Re: Rocket Pictures From an Old Fighter Pilot

Post by Colonel » Mon Apr 23, 2012 2:17 pm

Your questions illustrate why we test fly. We have to validate that our design preforms the way we intended. Right up front...I do not think that adding weight is a good solution to your torque roll problem. Extra weight will stop the roll, but only by making sure the powerplant does not have enough power to move the rocket over the string. Extra weight will also make sure your rocket does not go very fast for the same reason that it will not roll over the string. We want as high of a thrust-to-weight ratio (high power-low weight) as possible to make the fastest rocket.



Our rocket (body, rubber bands, hangar, plastic tail piece, prop assembly, fins and 3 rubber bands) weighed in at 6.8 oz. I think that can be cut down to 5 oz without sacrificing too much strength.



Tighten the line track: We tested at 50-75-100-125 and 150 turns (once). The rocket did not make the 40 feet during the first tests. We kept tightening the track (50 lb test mono) until we got scared and then tightened it a bit more. We also hung a 35 lb weight plate on one end of the line to keep it tight. Once the track was super tight, the rocket made the entire 40' trip consistently. We did not prep the track (lube) or the plastic carrier. Our rocket had no problems making the 40' at the actual race. Maybe they did something to make the track as efficient as possible. Check with your Scoutmaster to see what they use for the official race.



I gathered a group of giant brain guys during lunch to tackle your torque roll problem. We actually built a rubber-band powered "rocket" out of a straight stick and a prop (maybe 3 oz total weight) and got it to fly down a line without rolling...so weight is not the problem/solution.

We believe that there must be some yaw (nose to left or right of the rocket longitudinal centerline) and some positive pitch (nose above longitudinal centerline) built into your rocket to cause the torque roll.

Explanation: Newton's law (an action causes an equal and opposite reaction) explains why the ship wants to roll. So, if the prop turns clockwise (as viewed from the "cockpit") the aircraft body wants to rotate counter-clockwise on its longitudinal axis (assuming the prop is not misaligned with the longitudinal axis of the body). But this can't happen to the rocket because the hangar attached to the track holder and line keeps the rocket from rotating around its own longitudinal axis. A rocket with the powerplant alligned to the track line with 0 degrees or a little nose down pitch should not roll.
However, If you've accidentally built in a bit of nose-left yaw and nose up pitch, the prop torque will drive the rocket to climb up and left (reverse that if your prop is turning counter-clockwise and the yaw is nose right). Once this vector is established, the high-power of the rubber band engine at the start of the run will continue to climb the rocket over the string because the track line is now the longitudinal axis of the circle the rocket is trying to describe. As the power decreases (or the weight increases), the rocket will not have enough power to get all the way over the string.

The Fix: I doubt the space derby kits are made to exacting standards at the factory so the prop and rubber band holes may not be centered, etc. Check to make sure the powerplant (rubber bands, prop, prop shaft, plastic tailpiece notch that holds the rubber bands) of your rocket and the hangar align with the track longitudinally and align parallel or a little nose down to the track line (0 degrees or a slight nose-down) in pitch. Don't bend the prop shaft to fix a misalignment! Shim the plastic prop holder to counteract yaw and pitch in a misaligned powerplant. Make sure the hangar is not inducing yaw by being misaligned longitudinally. Change the hangar position alignment to establish the proper nose pitch.
Good luck! Let me know if this doesn't work and we'll try to develop another solution.



Colonel
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Re: Rocket Pictures From an Old Fighter Pilot Weight Correct

Post by Colonel » Mon Apr 23, 2012 2:42 pm

Our rocket weighed 0.68 oz not 6.8 oz! The stick rocket weighed 0.3 oz. Sorry for the confusion.



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