P1: Nitrous at Balls in '09

Motor Project


Last Update, 6/16/09 by Stephen Daniel


This page documents version 4 of the pyrovalve, both design and fab instructions.

The pyrovalve serves as the main nitrous valve. Pyro material blocks the flow of nitrous into the combustion chamber. When this material burns away the nitrous flow begins and the motor fires. Additionally the non-combustable portions of the valve structure are an ablative insulator for the injector plate.

Unlike previous versions of the valve design this valve is not respondible for main engine ignition. An external ignition source (discussed below) is required.

Successful test runs with this valve discussed here.

Revision History

This page documents the fourth version of the pyrovalve.


The pyrovalve is a 0.7" thick plastic disk. It fits inside the motor tube just aft of the injector plate. When the nitrous tank is pressurized, the injector plate rests on the valve, which in turn rests on the fuel grain.

The valve is made of a ring of cast fiberglass with a 1" diameter hole in it. This hole is filled with the pyro material. The fiberglass mates against the injector's aft-face o-ring seal. The fiberglass insulates the injector plate during engine run. The center of the injector plate is not insulated but presumed to be cooled by nitrous flow.

The 1" center hole is filled with pyro-material to a depth of 0.5". Both faces are basically flat. The forward face is notched with a 1" diameter by 1/8" deep notch. The goal of this design is a valve that fails abruptly and completely exposes all 8 injectors holes simultaneously.

Ignition is done by resistors and pyrogoop. There are two resistors on the valve itself.


Fiberglass disk

  • The mold is a ring of 4" aluminum tube, cut from the same stock used to make the motor. The ring is greased with lithum grease as a mold release, and lined with a piece of acetate to slightly reduce its diameter. This is set on a piece of greased plexiglass. The center hole is formed with a greased piece 1" teflon rod acting as a mandrel.
  • The mandrel must be exactly centered. We use a printed drawing of disk set under the plexiglass as an alignment template.
  • The fiberglass is a mix Mr. Fiberglass medium cure epoxy and 1/32" milled glass fibers, 3:1 by weight. The mix is:
    • 81g Mr. Fiberglass thin resin.
    • 27g Mr. Fiberglass medium-cure hardner.
    • 36g 1/32" milled glass fiber.
  • Mix the epoxy first, then mix in the glass fibers.
  • Vacuum de-gas the mix. We use a hand-held pump (a brake-line bleeding pump purchased from Harbor Freight). We pump down to about 24" of mercury worth of vacuum and hold for 2 minutes.
  • After we pour the mix we vacuum de-gas again.
  • After the disk has cured, remove it from the mold. The top surface of the disk will be not very flat. This becomes the aft surface of the valve. It should be trimmed and sanded on a belt sander until more-or-less flat.


We also made a blank valve for use in various tank tests. This valve has no center hole and no pyro material. The pour ratios for this were

  • 87g Mr. Fiberglass thin resin
  • 29g Mr. Fiberglass medium-cure hardner
  • 39g 1/32" milled glass fiber.
  • We prepare the center hole for the pyrovalve by cutting some somewhat random notches inside with a dremmel and a cut-off wheel. We make several notches about 1/32 to 1/16 inch deep. The inside of the hole is then roughed with 100-grit sand paper.