P1: Nitrous at Balls in '09

Motor Project

PBAN Fuel Grains

Last Update, 5/24/09 by Stephen Daniel

Introduction

This page documents the PBAN fuel grains we plan to use for medium scale testing. See test plan here.

Links

Information on our original PVC fuel grains is here.

Obsolete PBAN page is here.

Design

The PBAN grains are cast into a section of 3" PVC pipe. This piece of pipe serves as the casting tube and as the chamber liner. PVC is cheap and durable enough for this purpose.

When we get to full scale runs we will probably shift to phenolic liners and tubes to get complete use of the full diameter of the chamber.

The initial design calls for using a 1-1/2" PVC pipe as the mandrel. This gives a web thickness of 0.57". (Web OD will be a nominal 3.042", ID nominal 1.900".) Note, however that we wrap the mandrel in plastic film, increasing the ID by an estimated 0.010".

We cast into the PVC pipe by plugging the bottom end with a cast epoxy plug that performs 3 functions. It seals the end, it centers the mandrel, and it ensures that the PVC pipe is about 0.75" longer than the PBAN grain at each end. The mandrel is center at the top using a 3" couple and 3" to 1-1/2" bushing. This top cap is trimmed with a Dremel sander so that the bushing slides all the way over the mandrel.

The mandrel is wrapped in about 2 layers of heavy self-adheasive plastic film. We use film purchased from U-Haul, sold as a moving wrap to keep awkward objects fastened down.

The epoxy rings were cast using 24g resin, 8g medium-cure hardner, 2g phenolic micro-balloons, and 0.1g fused silica. The additives were put in to reduce the viscosity of the uncured mix in order to make it easier to cast.

The fuel grains will be 7.5" long, centered inside the 9" long tube. The 3/4" gap at each end ensures good mixing of the fuel and oxidizer for a more complete burn.

Advantages of this design:

  • Inexpensive.
  • Rugged.

Disadvantages:

  • Wastes a fair amount of space. The PVC pipe is both too thick and its O.D. is not quite big enough.

Formula

Based on reading Experimental Composite Propellant by Terry W. McCreary, Ph.D., we are planning on using the following formula for our PBAN fuel grains:

  • 81.2% PBAN
  • 17.8% DER-331 (an Epoxy resin)
  • 1.0% carbon black

We will also add about 1 drop of silicone oil (PDMS) per kg of mix as an anti-foaming agent.

In concrete terms, we will use

  • 1,380g PBAN
  •  303g DER-331
  •   17g Carbon black
  • 10 drops of silicon oil

We expect a small (135g) amount of leftovers.

Construction

Notes

  • Mixed and then degassed to 29.25 inches. Volume of the mix still expands to the top of the bowl (extimated 2x) at 28.5 inches.

Results

  • This formula produces a sticky black rubber that is not very strong.
  • A small mount leaks at the bottom during curing. We should work to add more grease to the bottom of the 3-1/2" PVC to make this extra easier to remove.
  • Using plastic film makes it easy to get the mandrel out. Getting the plastic film out is a little more challenging. Should grease the outside of the film to aid in this process.

This page documents the grains used in tests 12 and 13, on May 25.