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My first bottle rocket!

Written by Sherwin, published on May 20, 2013

A simple plane reminds me of the joy of physics and making a bottle rocket

I never had a bottle rocket growing up. At least, not that I remember. And a couple of weeks ago, a few friends and I put Mentos in a bottle of diet Coke and that was really exciting and it got me to thinking about bottle rockets.

And on the weekend, we launched one. We had several tries and only a couple worked. Here are our various recipes with notes on the results.

Things we used:

  • one empty 2 litre pop bottle (used in every trial as the rocket)
  • 6 litres of diet pop
  • two packages of Mentos
  • 2 litres of vinegar
  • small string (dental floss)
  • baking soda
  • a cork
  • a home made bottle rocket launch pad: we made a simplified version of this launch pad, without the air pump assembly (chemical/cork launch only): wood, metal braces, nails and strong string
  • table spoon
  • measuring cup

Test #1

We used our three litre pop bottle as our rocket in this test and all subsequent trials. In it, we put:

  •  750 ml water
  • 500 ml diet coke
  • 2 Mentos

We poured the liquids in first and then wrapped the Mentos in paper towel and tied a string to them and dangled them inside the bottle, not touching the diet Coke. We put the cork on so that the string was held in place and the Mentos and the Coke didn’t mix. Then we turned the bottle upside down and put the bottle in the launching position on the launch pad. Locking it into place we all raced back out of the way in preparation to pull the launch cords.

Well, almost. This didn’t work at all. There was no pressure build up in the bottle and I pushed the cork too far inside and the whole thing was a fail. We needed more pressure. This trial was a fail. More fuel!

Test #2

  • 500 ml of diet coke
  • 3 Mentos

This was also a fail. We guessed that the process of pouring diet coke into a measuring cup and then into another bottle resulted in too much loss of gas.

We needed more pressure.

Test #3

  • 750 ml diet coke
  • 3 Mentos

Fail.

Test #4

  • 2000 ml diet coke
  • 3 Mentos

This time, we were more hopeful about the amount of pressure in the bottle. We didn’t have to pour the coke from one bottle to another so the pop didn’t go flat. More pressure!

But when we pulled the launch pins, the bottle only sort of skipped about a half metre in the air and then sort of skittered about two metres along the ground. It was definitely more exciting and it was the first time we saw any propulsion at all.

One reason we guessed that it didn’t have more pressure, and more propulsion, was that the Mentos ended up kind of jammed too close to the cork in the neck of the bottle and we weren’t sure if they were properly reacting with the diet coke.

Test #5

  • 2000 ml of diet coke
  • 4 Mentos attempted/ 3 Mentos used successfully
  • no floss!

We actually tried to jam 4 Mentos into the bottle, but we were too slow, or the reaction was too quick, and the diet Coke started spraying all over us so I quickly jammed the cork on preventing the last Mentos from getting in. But then there wasn’t really any pressure build up in the bottle. Probably there was too much fuel all over me and my collaborator. So we just manually pulled the cork.

Test #6

  • 250 ml dihydrogen monoxide
  • 500 ml vinegar
  • 2 tbsp of baking soda

We poured the baking soda into a bit of paper towel and rolled it up and tied it with the ends closed and dangled it from the cork. When we turned it upside down the reaction began and it was kind of exciting. But there was not enough pressure build up. We pulled the cork and there was definitely a pop. Looking back I would say we didn’t wait long enough. Also, at the the time we noticed that the baking soda was really rolled tight and with too much paper towel and was kind of compacted. None of this is very good for getting the chemical reaction happening freely.

We were getting kind of skeptical.

Test #7

  • 250 ml water
  • 750 ml vinegar
  • 3 tbsp baking soda

Holy heck. The pressure built up. We pulled the launch pins. There was this amazing ppppppsssssssshshhshshsht sound and the suddenly our rocket was in the air. It was a little dark and we were startled so we’re not really sure how high it went. Did I mention that we were doing this by moonlight? I should report that the height was giant (not more than 30 metres) and there several screams.

One big factor in the success of this launch, we think, was that we waited. This gave the vinegar and soda a longer chance of reacting more completely. We still found some baking soda on the launch pad and we wondered if we used too much or if we didn’t wait long enough.

Test #8

  • 1000 ml vinegar
  • 4 tbsp baking soda
  • no water!

This final trial was also awesome. Someone screamed in my ear. And we think the rocket went even higher. How high? Not sure. Fun high.

Final notes

Our rocket did not have any aerodynamic additions so it totally tumbled in the air. That would create a lot of unnecessary drag and shorten our trajectory. Also, it’s possible that by making the bottle opening smaller, we could slow down the “burn” time. Word on the street is that this could result in going higher: having too much acceleration to soon can result in too much initial air resistance. Slowing down or extending the thrust period can result in less initial air resistance. It’s a matter of calculus. So for us, it’ll be a matter of trial and error.

I would love to have a parachute. Also, I would love to put a camera on it. And a nose and some fins. And some decorations!

Also, I should mention that one of our intrepid team totally had a gaping hole torn out of the ass of their pants during the night. It was an embarrassing and hilarious act of heroism, from scaling a fence into the park. I’m proud to say that team members went to their aid quickly, even before taking photos. In the end, duct tape saved the day.

No animals were injured.

 

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