Q&A: How to make a “Soda-Bottle launcher” project?

Question by Darkroxas: How to make a “Soda-Bottle launcher” project?
We were asked to build “http://users.soe.ucsc.edu/~karplus/abe/soda-bottle-rocket.pdf”

But thing is, the design sucks. It doesn’t have anything to have a grip on it tightly, and etc. And i can’t just change the design and choose a length of choice bla bla, i could however add something to make it better.

I have to get it to reach higher than 10meters. Mine is going up around 5m. i attached a hook using foam and cloth hanger, but 1) it isn’t THAT secure. 2) it tilts and shoots sideways (like not directly up). However doing this gave me time to add more pressure. Usually in 5-6 maybe 7 pumps it shoots up. but this time i had to pump atleast 10+ times till it finally shot up.

Is there a way i could get it to .. stay there, till i finish pumping? or a way that i’d be able to pump more?

Best answer:

Answer by Damon
Yes the launcher design leaves a bit to be disired. Ones I make can be charged to the capacity of the plastic bottles themselves, which is about 200 psi.

The higher the pressure achieved in the bottle, the fewer launches one can make before the plastic breaks due to fatigue. At 100 psi, the number of launches available usually exceeds the attention span of the audience, and the height is spectacular so I usually aim for near that pressure.

If you cannot do anything about the launcher, then what is left is what you can do to the bottle. I would suggest experimenting with adding a wad of model clay to the top of the rocket bottle.

When the bottle runs out of ‘fuel’ of pressurized water and air, the energy of velocity is stored in the mass of the empty bottle (Kinetic energy KE =½mv²) , which by design the mass of a pop bottle is quite small. The friction in the air quickly brings the bottle to a stop on its vertical rise ( U = Fd ) That is, the work done by friction equals the friction force multiplied by the distance it acts.

As energy is neither created nor destroyed, then the kinetic energy of the empty bottle equals the loss due to friction plus the gain in potential energy.( PE = mgh )
KE = PE + U

You will find that the clay will become the actual projectile and the empty bottle following it will act somewhat like feathers on an arrow keeping it lined up better with the direction of flight thus lowering air resistance

If you put on too much clay, the rocket energy of the water being driven by the air will not accelerate the bottle to a high enough speed,

Not enough clay and the bottle quickly looses velocity due to the friction force from air.

The correct amount of clay and you will find your bottle rising 3 to 4 times higher than it does without any added mass.

You can also tape on some tail fins to the sides of the bottle near the neck. The fins help keep the bottle oriented in an upright position which reduces the drag from air. A slight twist to the fin allignment will cause the rocket to rotate like a well thrown football, thus helping keep the bottle oriented along a vertical axis and reducing air resistance.

have fun!

If you can modify your launcher, I’d suggest this.
Most soda bottles have a very rigid collar just below the cap.
Find a plastic pipe fitting which has an inside diameter slightly larger than your bottle collar.
fit the new pipe fitting to your taped up male pipe adaptor and drill two holes through the new fitting so that a construction nail can slide through the holes and keep the bottle secured to the male pipe adaptor until you are ready to launch.
Launch your rocket by pulling both nails out quickly at the same time using a length of stout string tied below the head of each nail.

I like to have a small pipe fitted into the neck of the bottle and ending above the water line. This keeps my “fuel” from leaking out into the launcher tubing and being wasted. I also use an “O”ring seal in the neck of the bottle rather than pipe threads and tape.

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