Rocket Scientists???

[C00LR]

Need some input. My girlfriends daughter is working on a school project. A balloon car that goes by the force of air coming out of the baloon to make it go. SO far the team has a balsa wood frame about 24 inches long and 8 inches wide. They are trying to make wheels and axles. The hardest part of me helping is to keep my hands in my pockets and NOT take over and build it for her. I'm suggesting ideas here. Anyway there can be NO pre made wheel parts. NO bearings. I know I was heart broken when she told me that. I was ready to go to the local RC car shop and get wheel bearings and wheels. So instead we are going to make an axle of sorts running inside of a tube. I'm planning on getting her a piece of all-thread and putting a nut and washer on each side of a baby food jar lid for wheels on the ends of the all thread axle. I am going to spray some dry teflon sparay lubricant in the axle tube to help out with any resistance. After she finds out if thats acceptable of course.
They are going to use 2 baloons with a piece of PVC pipe on a T and two elbows to combine the force of TWO balloons to have extra thrust.
Here is my question or idea and I dont know if I am on the right track with this.....
Where the air will be exiting I want to add a reducer to the tube so it will make the opening smaller. Shouldnt this increase the power or thrust coming out? Sort of like the way a power nozzle works on the garden hose am I right??
To keep this on topic on a bus website, I even suggested a Lexan school bus body on top of the whole thing but was shot down on that.

 

[C00LR]

And for the record. I have withheld the idea I had of telling her to fill the balloons with the natural gas for the bunsen burners in Science class and lighting it as it goes down the hallway. Real rocket power!!!

 

[C00LR]

whoah! hold on there! A weather balloon would be too big!! Its going to run down the hallway in the school. And they have to use the regular party ballloon size as far as I know.
This thing only has to go 12 meters minimum. They have to be able to inflate the balloons by the normal way by blowing them up. I should have added that in the first post.

 

[Sojakai]

Look up Model RailRoad bearings. You have a set of wheels fixed to an axle that is sharpened to a point on both ends and fits into a cone shaped impression. Very little friction to overcome. Just try and use something a little sturdier than balsa for these bearings... But maybe you could use the heat generated by them to light the "After Burner"... Anyone else have a mental of a burning 1950s windup car shooting across a table?

 

[bbbrt76]

a smaller nozzle would increase the max speed possible but would reduce the acceleration, since you are not going to have enough volume of compressed air to get anywhere near max velocity, you want lbs of thrust, set it up so you can easily change sizes and experiment (that's how she'll gain an understanding of the cause and effects involved) instead of an axle turning in a tube and creating friction over the whole length of the tube, I'd mount a smooth solid axle that didn't turn and let the area of friction be only the width of the wheels, also taller wheels will have less resistance, thinner, and harder are also better, and I think I'd go with a 3 wheel tadpole (2 in front 1 in rear) design for further drag and weight reductions, weight is probably your most important factor and the pvc plumbing is going to add up fast use as thin walled as you can and if you can make the pvc do double duty as the frame that would be even better

 

[Kirbystud]

I went through that when my son was in school. We could only use foam cups,straws.tape and ballons. So things have changed.....

 

[C00LR]

I've looked at some of these on Youtube and from what I've seen, we have this thing way overbuilt. LOL, OVERBUILT is the way my Dad always built stuff so thats kind of how I have learned to do things. BUT, At least we know it will be strong enough!!
As far as I know, it doesnt matter how fast or slow it goes just as long as it goes. We have 90% of it built so far and I think it will go pretty good.
Something else we thought about is getting a couple more exit nozzles and possibly making the air come out close to the floor so thoeretically it will try to push away from the floor. which should keep it going...I THINK. But I will get her some extra pieces and then let her experiment with that part.

 

[C00LR]

Final update on the balloon car.
The baby food jar lids were swapped with CDs The PVC pipe was scrapped because it made it too heavy and it didnt even go. She used a frame made from balsa wood and used masking tape stretched across to make a deck for the baloons to sit on. She used a bopper balloon and two long skinny balloons for propulsion. The long skinny ones ran out quick but give the extra boost to get the thing moving and the big bopper balloon kept it going.

She wound up with 1st place for distance and 2nd in speed.

Thanks you guys for the advice and ideas!!

I cant wait till they do the mousetrap car!!