In broad terms, there are two kinds of paper-phenolic tubing that we are familiar with in HPR.

One is convolute wound paper-phenolic, which comes in many grades normally distinguished by their NEMA designation: XXX-34, X, XX, XX23AC, bla bla bla. These are convolute wound, with a high resin content, and somewhat brittle making it lousy airframe material. They are wound oversize, cured at elevated temperature, then centerless ground to the specified outside diameter. Single use motor cases, and insulating liners for both SU and reloadable motors is where this tubing will show up.

Insulating applications are a commercial forte for this type of material, and maybe the most common application we’d see is in large fuse casings. HRC (high rupturing capacity) fuses often go a step further with glass/phenolic, glass/epoxy or other materials with higher mechanical strength. They cost a lot more too.

A common variant is linen-phenolic, still convolute wound and looking a little like glass cloth-epoxy G10 tubing, but made with a fabric material and phenolic resin. Type CE is a common industrial grade. It’s pretty tough though still somewhat brittle, easy to machine and of reasonable cost. BlackSky uses this in it’s durable Optimal series of rockets, and it plus it’s rod and sheet counter parts show up in all kinds of electrical applications as structural insulating material.

The other material used commonly in airframes is spiral wound paper-phenolic. I know of it being made two ways - one is using pre-impregnated with resin, which is wound exactly as is “normal” paper tubing a la Estes or LOC, then post cured. The other is similar, but impregnation is done after winding using a vacuum process. The resin % is generally higher with the latter method. This material is lower cost than the convolute varieties generally, and the only place I’ve seen it is in less demanding insulating applications in electrical equipment. I presume there are other applications. It is a poor choice for use as a case insulator or grain inhibitor for various reasons. It’s not really that great for airframes as it does shatter or crack easily, but for equivalent dimensions it is stiffer than a kraft paper equivalent.

These paper-phenolic and linen phenolic materials bridge the gap mechanically and cost-wise between low cost paper tubes, and “higher-end” GRP structural materials like filament wound glass-epoxy (G12), convolute would glass-epoxy (G10) or those based on Kevlar and carbon. Thus they represent a typical trade-off between mechanical properties and cost.

Submitted by: Michael Dennett