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