Physics Photo of the Week

JANUARY 27, 2012

Liquid Nitrogen Rocket
The Physics class last December made a "rocket" using a pint-sized plastic bottle "fueled" by extremely cold liquid nitrogen (about -200 deg C).  The bottle was partially filled with the liquid nitrogen and corked.  (Note: this is extremely dangerous, the cork could be ejected in the hands of the experimenter and blast the hands and face of the experimenter with cryogenic fluid causing causing severe "freezer burns").  The corked bottle, partially filled with the liquid nitrogen, was placed in the bottom of the 3-inch PVC pipe with the cork pointing down. 

When the corked liquid nitrogen is sitting in the capped bottle, the liquid rapidly heats up to ambient temperature, vaporizing the liquid nitrogen.  The pressure rapidly builds up to the point where the internal pressure blows out the cork and the remaining liquid.  Because the massive liquid is forced out of the bottle, Newton's third law dictates that an equal and opposite force is produced on the bottle by the liquid, ejecting the bottle from the pipe.  The fog tail is caused by the liquid nitrogen streaming through the air condensing water vapor from the surrounding environment.  In the animation one can see the drops of liquid nitrogen spewing radially outward from the path of the rocket and falling to the ground.  The outward trajectories of the liquid droplets - perpendicular to the path of the "rocket) - is presumably caused by the boiling of the liquid in mid air once the pressure is released.  Notice also that there is considerable discharge out of the open back end of the pipe.  The rocket is propelled even if there is no earth to push against.

This experiment is rather anti-climatic - there is hardly any noise compared with a combustion-propelled rocket.  The ejection of the liquid nitrogen from pressing out the cork is quite silent.  The Chemists at WWC like to put a screw cap firmly on the plastic bottle, place the bottle in a well-constructed metal waste can, and wait.  The rupturing of the plastic bottle creates a satisfying bang!

Students performed the filming of this event while the professor handled the corked LN2 bottle.


Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

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