Butane Bottle Rocket Physics Experiment

This entry was posted on by (updated on )
Butane Bottle Rocket
All you need for a butane bottle rocket is butane and a bottle of water or soda.

The butane bottle rocket is a simple and exciting physics experiment where you add a thin layer of butane to a partially full soda or water bottle, invert the bottle, and watch it take off like a foam-fueled rocket.

Butane Bottle Rocket Materials

This project only requires a few basic materials:

  • Plastic bottle containing your beverage of choice
  • Butane

Find butane as lighter fluid or for butane torches. Use any drink you like that comes in a plastic bottle. Bottled water works. Cola produces a highly visible display. The bubbles released by a carbonated beverage like a soda give the rocket a bit more power. Keep in mind, sugary beverages ultimately result in a sticky mess (for better or worse).

Bottle Rocket Procedure

Before you start, select an open outdoor area. Put on eye protection (in case your aim is off) and gloves (to prevent frostbite from the butane).

  1. Open the bottle and pour out or drink some liquid until the bottle is around two-thirds to three-quarters full.
  2. Dispense butane into the open bottle. Make sure the butane container opening is facing downward or you’ll just get vapor and not liquid. You only need a thin butane layer on top of the bottle contents.
  3. Tip the bottle to the side until it is upside and watch it take off! Don’t point the bottle toward people, animals, or property. Don’t point it away from you unless you want showered with liquid.

How the Butane Bottle Rocket Works

Butane (C4H10) comes as a liquefied gas that has an extremely high vapor pressure (~170 kPa at 283 K) and boiling point right around 0 °C or 32 °F. It immediately boils at room temperature and transitions from a liquid into a gas.

When you dispense butane, the vapor and liquid are very cold because the contents of the can are pressurized. After dropping it onto the water or soda, it bubbles on top of the liquid. With a density around 2.5 g/cm3, liquid butane actually is slightly more dense than water (~1 g/cm3), yet it floats on the surface rather than sinking. Why? Notice the bubbly interface between the water or soda and the butane. The liquid butane rests on a layer of butane gas that separates it from the bottle contents due to the Leidenfrost effect. The same buffering layer of vapor forms when water droplets skitter across a hot pan or liquid nitrogen splatters onto a surface. The bottle is open, so some of the vapor released by the boiling butane escapes into the air.

But, when you turn the bottle upside down, the insulating vapor layer around the liquid butane makes it lighter overall than the water or soda and it rises to the base of the bottle. The gas pressure of the boiling butane quickly builds and forces the bottle contents out the opening, forming a rocket. Carbonated beverages enhance the effect as carbon dioxide bubbles join the butane bubbles. There isn’t a chemical reaction here; it’s just the effect of pressurized gas escaping a confined space, much like in the mentos and soda volcano.

Finally, the shape of the bottle opening is essentially a Venturi tube. Fluids flowing through this shape experience expansion and compression. The pressure change accelerates the fluid through the opening. So, you get a rocket.

Safety Information

Be sure you perform the butane bottle rocket project safely:

  • Butane is highly flammable. Only perform this project in an open area where there are no flames or sparks.
  • Just in case, have a fire extinguisher or garden hose handy.
  • With all rocket projects, wearing safety goggles is advised.
  • Wear gloves when dealing with compressed gases, as they are very cold and potentially cause frostbite.
  • Use care when turning over the bottle rocket so that it does not launch toward people, animals, or breakable objects.

See the Butane Bottle Rocket in Slow Motion Action

The action starts around 2:47 into the video:

A Safer Bottle Rocket Using Canned Air

The butane bottle rocket is pretty safe, providing you are not careless. Even so, there is some concern about the flammability of the butane. If you search YouTube, there is an example of what happens if you intentionally ignite the rocket (very ill-advised). One way of avoiding the flammability issue entirely is by replacing butane with canned air.

Despite its name, compressed or canned air doesn’t actually contain air. Instead, it is a compressed hydrocarbon or hydrofluorocarbon. If it’s a hydrocarbon (like butane, isobutane, or propane) then it’s not a substitute. However, if the can says it contains 1,1-difluoroethane, 1,1,1-trifluoroethane, or 1,1,1,2-tetrafluoroethane, then you have liquid with a very low melting point and also low flammability.

The project works basically the same way as with butane and it poses a comparable frostbite risk. However, the resulting rocket is not quite as powerful. One way of increasing the effect is by using warm water or a warm soda, which speeds the rate of liquid boiling. You can experiment with different temperatures and bottle shapes.

References

  • Balabin, Roman M. (2009). “Enthalpy Difference between Conformations of Normal Alkanes: Raman Spectroscopy Study of n-Pentane and n-Butane”. J. Phys. Chem. A. 113 (6): 1012–9. doi:10.1021/jp809639s
  • Bernardin, John D.; Mudawar, Issam (2002). “A Cavity Activation and Bubble Growth Model of the Leidenfrost Point”. Journal of Heat Transfer. 124 (5): 864–74. doi:10.1115/1.1470487
  • Incropera; DeWitt; Bergman; Lavine (2006). Fundamentals of Heat and Mass Transfer (6th ed.). ISBN 0-471-45728-0.
  • Zivenko, Oleksiy (2019). “LPG Accounting Specificity During ITS Storage and Transportation”. Measuring Equipment and Metrology. 80 (3): 21–27. doi:10.23939/istcmtm2019.03.021