This page documents the construction of a hybrid rocket motor constructed (almost) entirely from PVC pipe. The motor is designed to be very cheap and very easy to construct, with low labor and tooling costs, and a very low per-flight cost (<<>$5). It is modelled after Albert Gassol's work, but I believe it is simpler, cheaper, and generally improved. This page is not yet a complete howto, though I intend it to be eventually. It should answer some questions about the general construction, though. Those wanting to make similar motors should be warned that making PVC pipe withstand nitrous pressures is non-trivial; I will have more details on how to do this later. The major improvement over Alberto's version is that the tank is completely reusable, and the chamber swaps on and off very easily, so prep times are very short.

The general construction technique is a mix of standard PVC fittings, plus making use of the fact that pipe sizes from 1/2" to 1-1/2" almost nest (1/2" will nest in 3/4", and 3/4" in 1", with the use of a sledge hammer; 1" is slightly loose in 1-1/4"; and 1-1/4" will nest in 1-1/2" with significant sanding or other removal of material). See Notes on Pipe for PVC dimensions and other data.

The tank is made from a length of 1-1/4" PVC pipe (optionally 1-1/2"). The forward closure is a standard end cap. The aft closure and injector is made by nesting pipe down to 1/2", and attaching an NPT brass compression fitting to for the UC valve injector assembly. The tank is connected to the combustion chamber via a 1-1/4" female threaded adapter.

The combustion chamber is made from a layer of 1-1/4" pipe plus a layer of 1" pipe, which together act as both fuel and chamber wall. The combustion chamber mates to the tank via a 1-1/4" male threaded adapter. The nozzle is formed by nesting pipe down to 1/2".

This page is still very much under construction, but since I successfully flew one of these on Nov. 20, 2005, I've decided to put up some construction pictures for those interested. The motor I flew used 3/16" OD nylon tube for the injector. The rocket was my Blue Moon, a 54mm minimum diameter rocket. It flew on the 1-1/2" tank version of this motor. The rocket weighs ~4 pounds dry, and the loaded motor is ~ 2.5 pounds. The rocket seemed underpowered off the pad, but this may be due in part to the fill tube cutting well below the top of the chamber, thus likely reducing flow rate and degrading combustion. The same geometry should work on larger injector diameters, like 1/4" and likely 3/8" without trouble, but I have not tested those. Unfortunately, I do not have altitude data for the rocket, as the altimeter switch turned off on landing. The flight was otherwise perfectly successful, and the rocket was recovered easily with no damage.

All the parts:

External taper on a pipe to be nested (formed by sanding):

Forming an internal taper:

Internal taper:

Nested pipe nozzle assembly:

Injector assembly insert:

And finally, the static test:

Flight video evan_hybrid.wmv (97 Kb)

For more information:
The Art of the All-PVC Amateur Hybrid Rocket Engine

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