Balloon powered car

[Paul69camaro]

Believe it or not, I have to design and build a balloon powered car for my final exam in one of my engineering classes. Kinda childish but sounds like it could be fun. The rules are basic, you get one ballon, one plastic straw, any type of glue and any type of paper product to build it with. Also wire and a rubber band can be used but only to secure the balloon. The grading will be done by design and performance, (how fast you cover 15 ft from a stop), I'd like to hear how you guys would build it. I'm thinking about using the ballon as a rubberband and rubbing it around the axel. I also toyed around with the idea of using a "paddle" attached to the axel and blow the air from the ballon on it. My main problem is deciding how to build axels to minimize friction. Anyways give me some ideas.

Paul

[TonyL]

why not use the balloon as a jet to power the car? too easy?

[Matt@RFR]

How about this for axle bearings. Divert a tiny bit of the balloon's air to bearing blocks that have tiny holes (.005"?) drilled through them and into the axle cavity. Air bearings. You could make the bearing blocks out of Lexan or UHMW or similiar.

The wheels should be very small in diameter since you have to accelerate this contraption, and should have very very little contact area with the 'road', but probably not be sharp so they don't have a chance to dig in. Maybe a wedge shape with a .020 radius?

Edit: Also, I'm sure that you're aware that if you use the balloon as a jet, a tapered nozzle in the end of the straw would work wonders.

[Jim Nilsen]

A turbine is the right way to think even if it is a paddle wheel. A pump for moving water is a good design too. Remember that air is basically the same as liquids, think of aerodynamics and how they treat air as a liquid. Blowing a jet of air on a paddle wheel in the right proportion will give you excellent results. Just gear it(diameter of the paddle wheel) like you would a 1/4 mile car and make sure you just run out of air at 15ft.

Jim Nilsen

[myclone]

Thinking out loud..

Balloon used like a rubber band would work but youd have to weigh in the possibility of the balloon shredding or breaking since they make pretty lousy rubber bands. If that happens and the car moved very little or not at all youd prolly end up with an "F".

I like the turbine/paddle wheel mounted on the drive axle type set up except for the fact you'll need some type of grippy tires for the drive wheels since youd have an enormous amount of thrust at the very start which would spin the drive wheels if they were slippery. Blow the tires off from lack of traction (even though it'd look cool IMO) and use up the limited stored energy in the balloon then youd prolly get an "F".

Using the jet propulsion method of putting the straw in the outlet of the balloon is about the safest bet as far as reliability but it prolly wouldnt be very exciting to build/watch.

If I read what youre allowed to use in construction correctly you only have paper as a material to work with other than securing the balloon. Yes? If so then minimizing friction is gonna be tough using paper products but something you might consider is a material called "Strathmore" (<sp?). I have used it when building model train projects and it comes in sheets as well as rods...or at least it used to. Four strathmore discs for tires/wheels and strathmore rods as axles would be incredibly light weight but pretty strong structurally. Last place I bought any of that material was a well stocked hobby shop but you can prolly find it on line too but either way the stuff is pretty cheap to buy.

Can you use any lube on the axles? Graphite powder maybe? Some type of lube on the axles/wheels would go a long way to cutting the friction down if you can use it.

Slightly off the subject but didnt Captain Kangaroo do a similar project back in the day for one of his shows. It seems so as he was always showing how to build stuff out of milk cartons and rubber bands (I swear he could build a battle ship out of a milk carton, a rubber band, and some clothes pins LOL).

[TonyL]

the classic



Here's how rocket scientists build theirs some good ideas there.

[Jagarang]

Maybe you could do to something to decrease wind resistence on that big inflated ballon.

[formula]

I had to do a similar project, not quite as limited though. we used two balloons and pretty much built a jet propelled p-38j lightning with wheels on it, if you can picture that in your mind. The two straws were supported by a triangulated frame made of thin styrofoam and split in the rear to allow for a center wheel. the front two wheels were mounted outboard of the two straws. both tips were cut, tapered, and taped solid again. the balloons were on bendy straws, allowing us to bend them up off of the ground and keep them from causing any problems with the frame. the tips were aimed slightly out in hopes that they would improve stability a bit.

The thrust levels were incredible. our car, which always had some interesting problems with keeping the thrust from each balloon the same (we had to build it in 15 minutes, so it was all hack-and-slash with absolutely no measurement), ran the entire length of the track, turned around, ran all the way back, turned around again, and ran probably halfway back. it was hilarious.

after some more work and tuning, we got it to run the entire distance of our lab (probably a good 50 feet).

dunno if this'll give you any ideas or not, but at least now you know that jet power is extremely viable.

[Derek69SS]

I did this for physics class my freshman year in H.S., and of course, I won in my class Ours was done for max distance rather than accelleration, and mine travelled ~120' before hitting the rear wall of the gym.

I used the "jet" method, and did not use the straw. I used a cap for a sharpie marker, and drilled out the end... this gave me adjustability, I was able to test several sizes of holes to get the most distance. Yours is short distance, so a big hole will get you fast accelleration. You'll want your balloon empty by about 10' and coast the rest of the way.

Mine was a 3-wheel design for min. drag, weight, and friction. 2-wheels in front, and one in the rear. It was built entirely out of plastic straws, and had wooden axles sanded smooth and covered with graphite. My wheels were pickle-jar lids... the kind for canning at home, about 1/32" thick, and perfectly round and straight.

[Paul69camaro]

Hey guys, thanks for the ideas.

-Myclone, you read it right, I can only use paper products so I'm gonna have to look into that Strathmore stuff your talking about.

-Matt, as far a air bearings go, they are a great idea, but a little more complex than i wanna try to get into for this balloon car, plus it has to be paper products.

As far as just jet propulsion, not much design there so i bet i would'nt get as good as a grade. Plus their has got to be a more efficient way.

Great ideas though, keep them coming.

[Derek69SS]

As far as just jet propulsion, not much design there so i bet i would'nt get as good as a grade. Plus their has got to be a more efficient way.

Speaking as a CAD guy who works with engineers on a daily basis, I have this piece of advice... take this with you and apply it to every project you ever work on.

Keep It Simple Stupid

Complexity is a horrible thing if the same results can be achieved with simplicity.

[Rubes]

kind of a "jet" idea, but addresses the question earlier on controlling the wind resistance of the big inflated balloon. Put the balloon inside a paper towel tube. This should give you a higher pressure jet and give awsome acceleration (think of a welding tank having its valve knocked off with a hammer )

Rubes

[Fuelie Fan]

After going through a number of engineering competitions, I've learned this: They're not looking for the most complicated design, they're looking for the most effective and JUSTIFIED design. If your professor asks you why you used air bearings, for example, and you say "they sounded bitchin", then you better win the competition by a landslide or you will not get a lot of credit for it. He'll tell you to stop wasting customer and company money on undue complexities that only offer another failure mode with no proven benefit. If air-jet propulsion with cheesy straw axles works like a champ, that's your answer.

However, If you can PROVE the benefit of the air bearings (again just for example) either through calcs or testing, or both, then you get an A.

As a starting point, you may want to calculate the potential energy stored in the balloon either through pressurization or through spring storage, this should help give an indication of which way to go with your design, and also gives a lot more meat for your professor to review. If you show that you measured the spring rate of the balloon to determine how much energy could be stored as a rubber band, and then also measured how much volume and pressure could be stored when used as a balloon, that's easy points with minimal work. Don't forget to place an efficiency on each method before making your decision. I'm guessing that a rubber band should be more efficient storage since you don't have the materials to build a good nozzle for the jet.

[TonyL]

using glue the straw could be cut up and glued back together to make a nice fat, cone shaped jet. just a thought. I seriously doubt you will find better propultion from any other idea other than a straight jet. use the paper mache to make a nice teardrop shape around the balloon, nothing is more aerodynamic than a teardrop.

[CarlC]

I totally agree with the KISS principle. Just like cars, the less stuff that directly relates to performance, the better the performance will be.

Jet power is likely going to be the choice. Trying to structurally design a framework using wires to only secure the ballon is going to be very difficult. Transmitting torsional work, either wrapped around the axle or twisted like a balsa wood airplane, is going to be difficult to manufacture and keep reliable. I don't know if a paddle efficient enough to be equal a jet is even close to practical using paper of that size. It sure as heck would be tough.

Controlling the thrust output is key. You want to keep the thrust just high enough to maintain a reasonable speed, but not overspeed. Air friction goes up exponentially with speed. Using a straw that you can trim to a nozzle, or apply a paper nozzle to, will allow you to change the output volume, straighten the air, and give you an easier way to make changes in nozzle direction. A few minutes of tuning will help a bunch. Attaching the ballon to the straw/car framework is likely where your rubber band will be used.

For the wheels, one way to take advantage of the rules is to laminate paper using glues. Using laminations, and a coating of glue on the outer rim, a very thin stiff wheel with a hard surface can be obtained. Carefully sanded, you can have a hard surface with low rolling and air friction. It will also allow you to precision drill a hole to better fit a plastic straw axle. The plastic straw likely has the lowest Cf of your available axle materials.

Other structrual members can also be made this way. Just don't get too elaborate on stuff that does not matter.

Use the least number of axles/wheels as possible (3).

[TonyL]

This is my favorite one. and it could be easily made with only paper, straws, glue and rubberbands.

are there limits to the materials? only one straw? only one rubber band?

the wheels could be made of paper sandwiched together like plywood with glue like Carl suggested. The plumbing is the problem. You'd have to fabricate elbows and fittings from the straws. hard, but not impossible. I'd make a 45 degree elbow so the balloon was cocked back at an angle to reduce drag from the balloons initial size.

Cut lengthwise down the straw, coat it in epoxy, then roll it to decrease the diameter of it,(like a poster) thus reducing drag. a paper one could be made but it would surely have more drag. unless coated with an epoxy and then polished to a shine.

[CarlC]

If you decide to use the inflated ballon, you might want to have a discussion with your professor about drag on a teardrop shaped object and which orientation is the most benefical for your application ;-)

Are there any limitations on how much air, or other substance, can be put into the ballon?

[Paul69camaro]

There is a limitation on how much air can be put into the balloon, but the prof. never let us know exactly. He said he was gonna have a box the balloon had to fit in the day of the competition.

I know simpler is better, but looking at the way he graded some of the past cars, the jet-powered ones did'nt get the highest marks. But then again they just had a straw attached to the balloon, nothing that really controlled the air.

Tony, Yes I am limited to only one straw and one rubberband.

Here's what i'm thinking: Make the chassis out of cardbord, use only three wheels, Use those small, stiff paper plates, the ones that are kinda thick, and cut and drill those into wheels. I'm about to go to hobby lobby to try and get some more ideas on axles and bearings (and look for that Strathmore stuff)

[Jim Nilsen]

I got to thinking about the 15' issue and started to think about the size of the wheels allowed. If you have 6" wheels you have 18" of travel per revolution approx. and if you stretched the balloon with ten wraps around the axle you will go 15'. How big the balloon is will make a big diff but if you have a rubber band too you could link the balloon and the rubberband together and get the balloon to unwrap from the axle and make the size of the spool for the balloon to wrap up on bigger for just the right amount of torque to get some speed up. Once it gets to the end of the balloon it will fully disengage andcoast the rest if it has to.
This is all pending on whether you have to have air in the balloon but if you did you could use the air to squeeze the unraveling of the balloon off of the spool part of the axle to help with more speed.

I could draw a sketch of what I am talking about but I think I decsribed it enough for you to get the idea and go from there.

Jim Nilsen

[Paul69camaro]

Jim, I think I'm going to do that.

I just finished the first proto-type and it sucks. It moves but it's not exactly fast. It's a paddle design, I even got the friction pretty low by using a coffee straw as an axel housing and the sticks off of q-tips,coated with Silicon Lube. I also made a housing for the padde wheel, kinda like a turbo housing. This is the funny part, I could'nt get enough flow from the straw so I got a tampon out of the women's restroom at work, and used the tubes off of it. Worked great as the center of the paddle wheel, and as a discharge tube for the balloon. (kinda wierd when your blowing through it to refill the balloon ) It's gonna suck when I have to present it to the prof.

Well Tomorrow I'm going to put the motor into my real car, And I might try to start The secong Balloon car, I'm gonna try Jim's idea. Simple and easy.