Overview

By casting a liner in place the amount of propellant in a given case size can be increased while furthermore decreasing the cost of the motor by not requiring a commercially wound liner. This technology will open up a multitude of non-standard case diameters.

Material Trade

The first set of choices that need made is a material selection. This will drive infrastructure design, thermal design, and chemical acquisition. Commonly used in industry and the hobby, EPDM rubber provides a pliable, strain relieving, insulating layer. The other alternative is a HTPB based cocktail of various additives.

EPDM has a large body of prior art. A suggested EPDM rubber formulation is 93% EPDM rubber and 7% kevlar fiber.

HTPB has also been used before. Literature suggests that it is more suited for high-G flights where case adhesion is paramount. HTPB used as a liner should be around 50% solids for strength, with carbon being the primary additive, with TiO2, fiber, zinc borate, and other additives potentially useful. A plasticizer is also strongly suggested for HTPB based liners to improve casting performance. Chemical etching can significantly improve liner adhesion to the aluminum case, as can other binding agents. A suggested etchant is Chemloc. Use IPDI as a curative to allow for a slower reaction and provide time for the propellant to cure into the liner matrix. The propellant is slightly mobile in the liner.

Discussion with Zach B. lead to the discovery of the industrial use of a 'primer' to improve chemical bonding between the 'insulator' (traditionally called the liner by the team) and the 'liner' (traditionally called 'casting tube' on the team). Note that the conflict between amateur and industry nomenclature can lead to confusion when reading literature.

Primer: https://www.sigmaaldrich.com/catalog/search?term=16219-75-3&interface=CAS%20No.&N=0&mode=partialmaxfocus=product&lang=en&region=US&focus=product&gclid=CjwKCAjwt7PcBRBbEiwAfwfVGBi03pzu-bslpr0ERIClr61SfIyLXUha6eelaiQuUae-aPtuvS7c6BoCuBkQAvD_BwE