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After two years of working with PVC in the construction of solid propellant rocket engines, it occurred to me the possibility of making an all PVC hybrid rocket engine. What motivated such an endeavor was:

• The versatility and availability of PVC.
• That the pressure stress involved in hybrid systems seemed within the boundaries of PVC.
• Of course, the challenge of trying something unusual and new.

An unavoidable question pups up: Why make a hybrid engine that is not reusable? Two answers come to mind:

1. Why not.
2. Same reason one makes a non reusable solid engine.

In reality PVC is cheap, easy to work and a good medium to learn.

If you need to know about hybrid rocket principles:
Hybrid Rocket Science
Univdersity of Illinois Project Prometheus - A Hybrid Rocket Motor

Perhaps some history:
Amateur Hybrid Genesis by Bob Fortune and Bill Colburn.

Based on previews work done on PVC engines, I decided that the prototype all PVC engine would be done on 1.5" schedule 40 tubing. After asking around for basic hybrid specs, I found out that a typical hybrid engine works within the following parameters:

1. Chamber pressure between 150 and 400 psi (10.34 and 27.57 bar)
2. Oxidizer tank pressure between 460 and 1000 psi (31.71 and 68.94 bar) dependent on temperature (see table 3).

Next I proceeded to calculate the structural properties of the 1.5" schedule 40 PVC I was intending to use. For this I used two methods:

1. A program written by Richard Nakka (casing.xls)
2. A procedure outlined in John H. Wickman's book "How to make amateur rockets" Motor Structural Analysis - chapter 7

Here are the results of both procedures.

From Harvel Plastics Inc. ( http://www.harvel.com/pipepvc-sch40-80-dim.asp ) I got this table:

After plugging the numbers into the casing.xls spreadsheet The results were (Illustration 1):

Illustration1

On the other hand, according to John Wickman's book :"How to make amateur rockets"

Min. Wall Thickness = [(MEOP) * (Chamber Diameter)] / [ 2 * (Ultimate Tensile Strength)]

Where MEOP = Maximum Expected Operating Pressure
The Ultimate Tensile Strength for PVC = 5,000 psi (344.73 bar).

Moving the equation around.

MEOP =[ Min. Wall Thickness * 2(5,000) ] / Chamber Diameter

MEOP = [ 0.145 * 10,000 ] / 1.592 = 910.80 psi = 62.79 bar

All these numbers are indicative that the PVC engine chamber and oxidizer tank are viable.

The first part of the project then was to get a feel for the oxidizer and see how it behaved with different fuels, at the same time to learn how to prevent it from burning the PVC (which is frequently used as fuel in other systems).

Next, to pressure up a PVC oxidizer tank to see if it could hold.

Finally, to put everything together, perform a static test, evaluate the results and ....