In a previous post, I had groused about NASA removing plans for a common compressed-air launcher from their educational web site for “safety reasons” without explaining what the hazards were. (There are several possible hazards, including shooting the rockets at people, firing more massive projectiles, and exploding PVC pressure containers.) I asked for a copy of the engineering report, and (10 weeks later) I’ve finally gotten a reply.
They have a FAQ page which they have sent out as PDF files to people who inquired, but they don’t seem to have put it on their website anywhere (a strange oversight—I would have put it up on the web site first and sent people links to it, rather than sending out PDFs). They did not send me the engineering report I requested, insisting that it was only available through a Freedom of Information Act request. Again, a very strange, anti-education, anti-safety approach—I would have put the engineering report on the web, since there was clearly public interest and a need for the information to be disseminated. I get the impression that NASA is being run by lawyers and politicians, whose first instinct is to make everything require expensive intervention by lawyers and whose second instinct is to prevent the spread of information if at all possible. There may have been a time when NASA was run by scientists and engineers, but they certainly aren’t now.
In any event the FAQ is quite clear on what hazard they were talking about:
NASA does not recommend use of PVC pipe with compressed gasses, including air. Under pressure, PVC can shatter or explode under pressure or from an external force.
At this time NASA has no plans to redesign an activity using PVC pipe to construct a launch system that utilizes compressed air. NASA will assess other materials and designs and may release a new high-powered paper rocket launcher at a future date.
NASA completed an inquiry into this activity and determined that the launcher, or design equivalents, should not be used. NASA does not recommend use of PVC pipe with compressed gasses, including air. PVC can shatter or explode under pressure or from an external force. NASA recommends that individuals and organizations should immediately discontinue use of these launchers.
Q8: Can I obtain a copy of the engineering report referenced in the discontinuation notice?
You may request a copy of the report under the Freedom of Information Act. (http://www.hq.nasa.gov/office/pao/FOIA/agency)
Exploding PVC is one of the possible hazards I had considered, and perhaps the most dangerous one, since not all people who build with PVC are aware of its brittleness and that it produces very sharp shards when it shatters. The brittleness and sharp are why toy swords are not made from PVC pipes. (The Society for Creative Anachronism experimented with PVC for their swords in the 1980s, when rattan was getting expensive, and decided that PVC was far too dangerous.)
There is a standard workaround that reduces (though does not completely eliminate) the hazard: wrapping all pressurized PVC components with a few layers of strapping tape. The strapping tape does not prevent the PVC from shattering, but may contain the shrapnel or slow it down enough to reduce the danger zone. The strapping tape only has to hold the pieces together for long enough for the air to escape—the energy of the pressurized air is dissipated in tearing the tape rather than in propelling the shrapnel.
Even more common is keeping the pressure fairly low. This is not as big a win as one might think, since even fairly modest pressures (like 40psi) can still store significant energy in a large pressure vessel, and PVC can shatter at low pressures if it is struck or if it has gotten UV damage from being left in the sun. Keeping the vessel small and the pressure low is probably safe enough, so I have no concerns about the soda-bottle rocket launcher plans that I’ve published in the past—the friction-fit launching keeps the pressure down to around 20–40 psi and the volume is small. I have no idea what plastic the commercial soda-bottle rocket launcher uses (maybe ABS, which is not as brittle as PVC), and they have a pressure release around 60psi (4 bar), so again is probably safe enough.
The one project we’ve done that potentially hazardous is the foam-dart launcher, which is very similar to the NASA design for the paper-rocket launcher (indeed, we have used it as a paper-rocket launcher). We were pressurizing to 120psi (8 bar), and we had not wrapped the pressure vessels with strapping tape. That project is on hold currently, not for safety reasons, but because the students in the robotics club got too busy to continue the robotics. If we pick it up again, we’ll wrap the pressurized PVC pipes with a few layers of strapping tape, then add a layer of colored duct tape (for UV protection of the strapping tape and to make it look nice). I think that would reduce the hazard to an acceptable level for experimenters, though not for a commercial product.