Paper Rocket Engines

Why?

I’ve been interested in rocket engines for some time now, especially the design of combustion chambers. I’m no physicist, and I’ve never claimed to be, nor do I have a budget. On the other hand, there’s no way to test out one’s understanding of combustion but to try it - so I decided to build a small rocket engine.

A First Attempt

The easiest way to build an engine is simply to roll up a tube of paper and fill it with butane, and light it. This will produce a little tiny bit of thrust out both ends. Capping one end directs this thrust out the other end, and boom. A tiny, pointless rocket engine.

I built a number of these, with increasing sophistication. I found that, with the addition of two spools from Scotch tape reels to reinforce the ends, I can create a double-walled chamber that won’t burn my hands. Here’s how it works.

Take four index cards and align them. Then roll them up and slot them into the slot that runs around the circumference of the tape roll. Place the other tape roll on top and use Scotch or duct tape to secure them, with a pressure seal! You can blow into either end with one hand over the other to see if the seal is good.

Now cap one end with some layered tape. Duct tape works, but Scotch lets you see the combustion inside. Then take a fifth index card and wrap it around the outside, securing it with tape. The air between the inner and outer wall will absorb radiated heat and protect your hand.

Hold the engine with the open side up and place a lighter upside-down with the head inside the engine. Depress the release valve without igniting and allow butane gas to enter the engine. It’s heavier than air, so it will fall to the bottom.

Now flip the engine so the open end is just a few degrees down from horizontal and hold the lighter two to three centimeters from the opening, and ignite it. Whoosh! Thrust.

I added a conical nozzle to the end, but I made little effort to optimize its geometry; this is a project for the future.

[caption id=”attachment_318” align=”alignright” width=”225”]A photo of the bottle connected to the engine. This simple rocket actually kicks pretty hard when the bottle is filled with butane.[/caption]

[caption id=”attachment_320” align=”alignright” width=”225”]Just the engine, with the bottle detached. Just the engine. This can be sealed over and fired by itself for a very tiny impulse.[/caption]

[caption id=”attachment_321” align=”alignright” width=”225”]A schematic diagram of the engine, the engine with oxygen and fuel feeds, and the engine with nozzle and bottle. These are some schematics I sketched while working on this tiny engine. The two-tube-feed idea is a no-go, for reasons that I hope are obvious when one considers the problems with gaseous oxygen being fed through a flammable tube.[/caption]

A Simple Fuel Tank

In order to produce some real impulse, we need much, much more butane/air mix to feed through our engine. I made a first stab at this problem by taking a simple Pepsi bottle and drilling out the end of the cap. I then removed the seal at the end of the engine and glued the cap into place such that it sealed with the top Scotch tape roll. I could now simply fill the “fuel tank” about half full, screw the engine onto the top, and tip it so that the butane begins to flow. Lighting the engine then resulted in an appreciable impulse - and a very hot Pepsi bottle, because the bulk of the combustion occurred in the “fuel tank”! Not at all what we want.

I’m currently trying to find a good gas pump, or some way to atomize some kind of liquid fuel, which, along with an 80mm fan to feed atmospheric oxygen, will create what is essentially a jet engine - I’ll post about that if I ever get it working.

*****
Written by Leo Tindall on 18 February 2017