Coke Bottle Rocket

Here is an impressive demonstration that can be carried out in complete safety in schools or indoor events.

The demonstration uses a 500ml plastic Coca-Cola bottle as a miniature rocket. Unlike some similar water based propulsion systems, this technique uses an alcohol / air vapour as a fuel.

Safety and risk assessment:
The rockets can be launched from a smooth surface inclined between 20 and 45 degrees. A heatproof mat should be placed just behind the 'launch platform' to avoid damage from hot gas being expelled from the rockets exhaust.
A track to guide the rocket can be made by sticking two meter rules in a narrow V shape to the launch platform. These hold the rocket in place during launch and guide it as it accelerates.

A safety screen (I use a sheet of Perspex about 3 feet by 2 feet) should be placed between the side of the launch platform and the audience
No one should stand in an arc of +/- 45 degrees in front of the rocket.
The rocket should be fired using a 6 inch wooden splint placed close to the lid. Matches should not be used as your hand will be too close to the exhaust.
The person lighting the rocket and anyone in close proximity (20 feet) should wear safety glasses.
No one should stand behind the rocket.
 

Method:
Take a 500ml Coke bottle and carefully drill a 4 mm hole in the center of the plastic lid. The bottle selected should be in 'pristine' condition and should not have any signs of scratches or other abrasions anywhere on its surface. For the demonstration to work, the bottle should be washed out so no Coca-Cola remains. It's then dried. The drying process should be natural, do not be tempted to speed up the process by heating in an oven as this will damage the structure of the bottle. Incidentally do not use any form of still water bottles as this experiment requires a bottle that has been designed to contain pressurised 'fizzy' drinks.

Next, the fuel.
Measure 3ml of any suitable alcohol.  I have used Methanol, Ethanol, Propan 2-ol also known as Iso propyl alcohol. But by far the most commonly available fuel is Methalated spirit.

Put the alcohol into the bottle, replace the lid and cover the hole with your finger. Shake vigorously for 30 seconds to form the alcohol/air  vapour.
Next, open the lid slightly and empty any liquid alcohol into a sink with running water or into a bucket of water. This step is very important as excess alcohol in the bottle is a fire risk. By the time the rocket lands we want all the fuel to be burned. Any excess alcohol removed will be diluted in water and will no longer be flammable. Finally, replace the lid keeping your finger over the hole.

The Fun Bit:
The rocket is now placed between the meter rules on the launch platform. Standing to one side and not behind the rocket bring the lighted splint close to the hole in the lid. After a second or two the vapour ignites and the rocket accelerates away from the platform with a loud 'whoosh'.
A typical distance covered is about 20 to 25 feet.

Real Science:
As well as some fun for 11 to 14 year olds in my local school I have also used the coke bottle rocket for some real school science.
Using a Force sensor by Pasco, we have conducted a 'static firing test' with the bottle held down and with the a sensor placed ahead of the bottle to measure thrust.
The results are useful for teaching some basic Force = mass x acceleration physics and can be developed into more complex projects

Note that the duration of the powered section of the flight is less than 200mS and the mass of an empty coke bottle is 31.14 grams.
 

Thrust profile for the Coke bottle rocket.

If you have access to the Pasco range of sensors with DataStudio software, then the graph above was produced with the following parameters.

Samples per second.       500
Auto Start                >0.1N
Auto stop                 T = 0.5 seconds
Scale                      -0.4 to +5N

Lab testing the Coke bottle rocket.

 

Besides firing the rockets across the lab, it's also possible to calculate other parameters of a typical rocket.  Let's examine what we know and what we can discover about the rockets characteristics.

Mass of bottle when fuelled with alcohol / air vapour.  (full) 31.39 grams
Mass of bottle after launch. (empty)  31.14 grams

Now for a little Rocket Physics.   You can calculate several parameters from the graph and the full and empty mass data.

Total impulse.  This is the product of the thrust and time and has the units  Newton seconds.  You can use the graph to estimate / calculate the value. 
e.g. If a rocket can provide 5 Newtons for 10 seconds then its total impulse is 50 Newton seconds.

Average thrust.  This is the average thrust in Newtons provided over the total burn period.  For our rocket the peak thrust is 4.3 Newtons. You're welcome to calculate the average thrust from the graph above......but my estimate is around 2 Newtons.  If you have a more accurate answer please let me know.

Mass flow rate.  This is the rate of flow of fuel and is used in the efficiency calculation below. The units are kg/sec  or kilograms of fuel used per second.
Our rocket uses 31.39-31.14 grams of fuel in about 160ms.  

Total Impulse or ISP.  This is a very important measure of a rockets efficiency.  The unit is the 'second'

ISP  =  Average thrust / mass flow rate / gravitational constant.    Where the gravitational constant is 9.81m/s^2

Rocket thrusters have an ISP from apx. 60 seconds for a cold gas vented under pressure into space to the cutting edge Ion engines that have an ISP above 3000.  Professional rockets using liquid oxygen and kerosene have an ISP of around 280 seconds.

Can you estimate or calculate the ISP for the Coke bottle rocket. ???     Have fun with the numbers, or build one for yourself, but don't forget the safety precautions because 4.3 Newtons of thrust applied to a 31 gram bottle gives a lot of acceleration !!    checkout the formula and rearrange:

 Force = mass x acceleration.    

(force in Newtons.  mass in kg.  acceleration in metres per sec squared)

Home