Mark-IIId PVC Hybrid Motor

Stephen Daniel

Last update, July 2007


Project Summary

For some time Evan and I have been working on building low cost hybrid motors. This site documents our latest attempt, the Mark-IIId PVC hybrid motor.


  • Build a low-cost medium power hybrid motor
  • Must be buildable without a machine shop
  • Cost to build should be moderate
  • Cost to fly should be as low as possible


The Mark-IIId is our third major PVC hybrid design, and the fourth version of that design.

The Mark-I was our first attempt. The design did not automatically align the chamber with the thank, a crucial failing. We never got as far as hot-fire testing a Mark-I.

The Mark-II is a UC-valved hybrid. It works well on small injectors and light rockets. It is documented here.

The Mark-III is designed to fly a 4' rocket and should achieve J- or K- class impulse and a lift-off thrust of perhaps 1000 newtons. During the development we've learned a lot about how to make these motors safer and more reliable. Along the way we've destroyed at least 3, two during hot-fire tests, one during fill.

The Mark-IIIc has been very successful as a test vehicle, with 8 successful hot-fire tests and 1 hot-fire failure. However, the Mark-IIIc is a test-only motor. The plumbing stack does not support mounting the motor in a rocket.

The Mark-IIId is designed to be a flyable version of the Mark-IIIc.

Design Overview:

We believe the goals of the project have easiest to achieve if we build the motor out of PVC, augmented with bits of brass plumbing, copper tubing, and an occasional valve.

Early on in the design of the Mark-III we abandoned the UC valve fill/ignition system. The Mark-III uses a pyro-valve to open the injector and ignite the rocket. Nitrous is filled from the top of the motor.

In order to improve reliability, saftey and total impulse, we decided to regulate the pressure of our nitrous flight tank to 400 PSI. This chills the nitrous to about 23 degrees F, which is workable. By regulating the flight tank pressure we ensure consistent performance. Both the pressure and density of nitrous vary widely over the temperatures typically found at an amateur launch. On hot days the motor produces more thrust but less total impulse.

The aft end of the motor is a detachable chamber. It is made of 2" schedule-80 PVC pipe. The fuel grain is usually a piece of 1-1/2" schedule-40 PVC pipe that slides inside the 2" schedule-80. It is perfectly possible to burn other plastics, such as ABS, or acrylic. The nozzle is made of steel washers and PVC pipe fittings. The forward end of the chamber contains the pyro-valve. It screws onto the aft end of the flight tank. The injector is built into the aft end of the flight tank.

The injector is a brass pipe-cap that has 6 precisely drilled holes in it.

The flight tank is made of PVC pipe. It can be made out of 1-1/2" or 1-1/4" schedule 40 pipe. The length is somewhat arbitrary. The dimensions of the tank and the chamber must be balanced with the injector design to ensure the motor will work well. The current Mark-IIId design represents one set of trade-offs. Using the hsim simulator one can design larger, smaller, taller, or fatter versions of the Mark-IIId.

The forward end of the flight tank contains the plumbing stack. This includes a quick-disconnect fitting for the fill tube, a check-valve for the fill line. a solenoid dump valve, and a mechanical pressure relief valve to regulate the flight tank pressure. The design and implementation of this plumbing stack is new for the Mark-IIId. All other aspects of the Mark-IIId are identical to the Mark-IIIc.

The current design doesn't worry too much about the cost of the plumbing stack. This portion is viewed as indefinately resuable. The PVC tank / injector complex is expected to last about 10 flights. (The injector itself is brass and resuable indefinitely.) The chamber and nozzle are single-use, and are designed to be as inexpensive as possible.

Site Map

The rest of this site contains information on the design, construction, and testing of the Mark-IIIc and -IIId motors.