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BsSmith

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For my tech class, our chapter "project" is to design and build a balsa wood drag racing car powered by a CO2 canister. Being the resident rocket nerd of the class, I wanna win!

Are there any tips or suggestions from the smarter rocket nerds out there?
 

Indiana

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We did this when I was in junior high shop class.

As I see it, these things are the most important:
  • Keep mass as low as possible
  • Eliminate as much friction as possible from the wheel/axel interface.
  • Make it as aerodynamic as possible.

We had to keep our wheel diameter, but we were allowed to put them in the lathe and make the width as narrow as we could.

If I remember right, mine did 60 feet in 900 milliseconds.
 

Meat

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I failed with mine i went for a cool looking non aerodynamic design . Tip DO NOT PLACE A ROCKET ENGINE IN THE CAR . I did this when I went home of course it was one with an ejection charge . I am pretty sure the neighbors dodg never grew back his fur and I can still not hear a certain tone .
 

georgegassaway

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As I see it, these things are the most important:
  • Keep mass as low as possible
  • Eliminate as much friction as possible from the wheel/axel interface.
  • Make it as aerodynamic as possible.
That is a very good list. Keeping it light was the first thing that came to mind before I saw your list.

I would also suggest some things to consider, in no particular order:

Keep it more low-slung than tall, so that it not top-heavy.
Use a long enough wheelbase (distance from front axles to back axles) that the model will not tend to veer off to one side easily.
Try to make sure the wheels are as perfectly aligned as possible, so the car will roll as dead-straight as possible. Be prepared to be able to “tweak” to adjust that after you do some roll tests and may find it goes to the left or right.
Make sure the rocket engine mount is perfectly centered to the body (in yaw), so that the thrust will not tend to make it veer left or right.

The lightest and aerodynamically slickest car will not win if it veers off to one side, out of bounds (or however the contest is set up). So keep in mind it has to be able to run straight too.

It also has to be strong enough to hold together, and not break too easily. This is where trying to make it light can sacrifice strength, so you have to be careful in the design approach. I feel comfortable giving some generic advice but not to actually suggest a specific design.

BTW - some of the above about alignment presumes the car is free-running and not using a tether line. If a tether line is going to be used, then some of those I mention will not be nearly as critical as with a free-running car. But top-heavy is an issue regardless (you do not want it flipping over), and the more it tends to veer to one side, the more friction there will be on the tether line guide lugs.

- George Gassaway
 

KDRaven

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keep your Co2 cartridge warm. I remember when I done my back in JR high, I did not warm up my cartridge for the first run and the car just puttered and spit down the wire slowly.
 

Gillard

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i did this experiment with a year 7 (11 year olds) class about 10 years ago, the winning team's car did not use wheels and was simply a sledge type car. the runners very thin and very smooth.
the surface they used was very smoth, so that helped.

if using wheels, use three wheels and not 4 - wheels cause drag.
also use a fin to help keeping the car straight - if you are not using a guide wire.
 

rocketace

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Oooo, this was fun, I was top in my class :)
I used a shell design where the wheels wee inclosed. I used some powdered graphite on the axles (you can rub pencil led on the metal axles to make them a little slicker) I spent hours with my dremel just slowly eating away at the inside...there are parts where when you hold it up to a light you can see through (make sure it wont break, the deceleration is violent) Then sand the outside as smooth as possible and I put one coat of lacquer sanded and one coat of paint and sanded down to 600 grit. My school now does not allow for shell cars because i beat everyone so bad ;)

Here is a pic:
http://web.me.com/tennisace1/Site/Fleet.html#28

And the pic after that is one I made in my free time for a 18mm motor :) Yes it was on a long guide wire and it was awesome

So like they said above, LIGHT, frictionless, and aerodynamic
~Jason
 

pyrovette20

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I had done this in jr high also. My car beat everyone in class and was to race the winner from the other 2 classes. Someone about a week later stole my car from the classroom. So we never found out who would of won between the 3 classes. That same class was where I found out about rockets 30+ years ago.
I trick I did (dont know if it worked) was to compare the weights of the co2 canisters and try to pick the heaviest one thinking there might be more compressed air inside. Mine was about as thin as a pencil in the middle and I removed as much mass as possible. Also painted it with a super high gloss and waxed it and the axles. The graphite sounds like a better idea. We all had standard wheels,wheel base,height,etc.
 

CharlaineC

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I was forced to do this in jr high right after we had built and flew model rockets. and i admit I had a blast. I went for the indy car look. the old school almost bullet/dragster look. so i turned it on a lath so she was narrow in the front and streamlined in the back the front wheels where cut to 1/8" or 1/4" i forget they where solid though. the back wheels where 1/2" hollow. graphit was used on the axels and teflon washers where the wheels touched the body. ours where theathered i dont remember if i won or lost my race but i remember the teacher ptting my car in the display case. un like my rocket because i snapped at him lol.
 

gpoehlein

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Gee... isn't it funny that the same design consideration apply here as to competition rockets? ;)

Seriously, I keep wondering about adapting this whole concept to rocketry. Not quite the old Vashon cold power concept, but might be fun.
 

rsynoski

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I did these races a couple time in Jr. High. If you are constrained by silly things like rules and need to keep the wheels, then key is mass, the lower the better. A close second is rolling resistance. The axles and wheels need to be true, and all bearing surfaces need lubrication. If possible try and reduce the mass of the wheels, because in the end they will represent a significant part of the overall mass of the vehicle. Since the absolute speed of the vehicle is pretty low, aerodynamic does not play a big part in the overall performance.

The fastest vehicle I made was built for two wood dowels and some coat hanger wire for skids. It was probably twice as fast as the wheeled vehicles.
 

BsSmith

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We have to use wheels and are given a balsa wood wedge with a hole already cut in the back for the Co2. We are also allowed to use the CAM milling machine to cut out the basic outline, so I don't think the axles will be a huge problem.

I did my rough sketch today for it, the main body will be a very thin airfoiled shape, and then it will have a rased "pod" where the canister hole is. I also am going to "cheat" at home and use some laminating Aeropoxy to strengthen it up and smooth it out with filler. Chrome paint too! Should be fun!
 

matthew

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Anyone have a plan for making the CO2 canister piercing starter thingy?
 
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