Mark-IIId PVC Hybrid Motor

Lessons Learned

Stephen Daniel

Last update, October 2007


This section contains a few of the more valuable lessons we've learned while trying to build PVC hybrid motors.

High Pressure PVC Tanks

You can run PVC at well over its rated pressure, if you do not abuse it. The Gizmologist gives PVC pressure ratings. We've taken 1-1/2" PVC pipe (rated to 330 PSI) up to 650 PSI without problems. This does not mean you can abuse it, but if the joints are carefully cleaned, built using heavy-duty PVC cement, and allowed to cure for a full 24 hours, you can do it.

Warning: Pressured PVC pipe is dangerous. Never work near PVC pipe that contains pressurized gas. Test all systems using pressurized water or oil, never pressurized air or nitrous.

PVC into Brass, Not Brass into PVC

Our early work on PVC hybrids was simplifed by drilling holes in PVC pipe fittings, tapping them, and screwing in brass fittings. This often works, but if it doesn't you cannot fix it. Tightening leaking fittings makes them worse. We've also had failures where the PVC cracked and broke, apparently at the hole where the brass was tapped in.

The Mark-IIIc and -IIId designs mate brass to PVC by using a male-threaded PVC pipe and screwing a brass fitting over it. We never use female threaded PVC pipe, and we never drill and tap PVC.

Use Sane Nitrous Supply Pressures

We lost a test engine when we filled it using a nitrous supply that sitting in the sun on a hot summer day. We believe that our relief valve simply could not vent nitrous fast enough to keep the flight-tank pressure down. The tank failed dramatically during fill.

Our current designs are hydrotested to 750 PSI, and we require that the supply tank read 625 PSI or lower before opening the fill valve.

Take Advantage of PVC Pipe's Good Features

PVC pipe is built to very high tolerances. If you build something out of unmodified PVC, and work carefully, you will get a reproducible and well aligned product. If you start making glue joints on surfaces you've altered or in holes you cut then the odds of a good product are low. For some of our designs we nest PVC pipe and glue the nested pieces, but we avoid this trick and we never use it where alignment is critical.