Gas station without pumps

2012 October 14

NASA releases reasons for removing paper rocket activity

Filed under: Robotics — gasstationwithoutpumps @ 10:37
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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.

2012 July 12

Nerf gun barrels

Filed under: Robotics — gasstationwithoutpumps @ 19:08
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In Nerf gun progress, I discussed possible approaches for handling the problem of Nerf-brand darts not fitting in the barrel:

  • Find a source of (probably non-Nerf) foam darts that are 1.45cm (9/16″) diameter with heads that are no wider than the body. I think that they came with an NXT generation crossbow, so replacement foam darts for that may be what we need.
    I’ve ordered a couple dozen NXT generation darts.
  • Buy Nerf  (or other) darts with the right size bodies but oversize heads, remove the heads, and make new ones (out of what?). This would be cheap, but tedious, and the darts would probably fly poorly, unless we made the new heads have a decent weight.
    I’m not willing to try this yet.
  • Use clip-system darts for compatibility with the popular Nerf guns, but find a smaller diameter tube than the ½” PVC pipe (where? and how would it be connected to the solenoid valve?) It looks like Schedule 40 3/8″ steel pipe has a inside diameter of 0.49″, which is just right, but steel pipe is rather heavy.
    I’m still looking, but I’ve not found any lightweight tubing that I think will work.
  • Use clip-system darts, but convert to the Nerf-standard tube-inside-the-dart launching system.  This limits the effective barrel length to the inside length of the dart (about 4.5cm) and the barrel diameter to the inside diameter of ¼”, which will limit the top speed of the darts (OK for safety, but probably not as much fun).
    I tried this out today, as the outside diameter of ¼” copper pipe seems to be just the right size.  We already had a piece of copper pipe stuck into a 3/4″ threaded end cap (it was part of the vacuum bottle for the ROV), so I could test this easily.

Here are the two barrels. The long barrel of 1/2″ PVC has an NXT Generation dart poking out the end. It can be loaded all the way into the barrel with a ramrod. An old-fashioned NERF-style dart with the same body size, but a head too big to fit into the barrel is shown below the barrel. The NERF Clip-system dart is shown mounted as far as it goes (only 4.3 cm) onto the 1/4″ copper tube.

The Nerf method of firing darts (with the dart surrounding the launching tube) is similar to our firing of paper “rockets” from the outside of the 1/2″ PVC barrel.

Loading the darts onto the tube is a bit finicky, as the foam is a tight fit over the copper. It might help to smooth the end of the tube, and perhaps use a dry lubricant (soap?). Rapid loading might be a problem.  I can also see why so many Nerf enthusiasts modify clip-system darts—only 4.3 cm fit onto the tube, but one would expect closer to 6cm to fit.  The depth the tube goes into the foam corresponds to the length of the barrel in the bullet-and-barrel system, and the longer the barrel the higher the muzzle velocity (to first approximation).

The darts fire fine though and seem to go fast enough.

Firing the darts this way makes a much higher pitched “pop” rather than the deep “thump” of the long barrel, because the shorter tube has a much higher resonance frequency.  Actually, I don’t know whether this arrangement is better modeled as a closed tube as I did for the long barrel in Nerf gun on the oscilloscope or as a Helmholtz resonator, which would have a resonant frequency of \frac{v}{2\pi} \sqrt{\frac{A}{V_0L}}, where v is the speed of sound (about 34320 cm/sec), A is the area of the neck (about 0.217 cm^2), V0 is the volume of the pipe between the valve and the neck (about 22 cm^3), and L is the length of the neck (about 9cm).  Hmm, the Helmholtz resonator is at about 180 Hz, and the pitch is definitely much higher than that, so perhaps an open-pipe model [Wikipedia’s article on acoustic resonance] is called for: \frac{v}{2(L+0.3d)}, which gives 1750 Hz.

2012 July 10

Nerf gun progress

Filed under: Pressure gauge,Robotics — gasstationwithoutpumps @ 15:30
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The Nerf gun prototype is coming along nicely.  The students have tested the launcher up to 120 psi, built a prototype tilt and pan mechanism in Lego, and today hooked up a small reservoir behind the solenoid valve on the barrel to a bigger reservoir using an air hose.

They hope to be able to keep the bigger reservoir up to pressure by occasionally turning on a 12v compressor.  The compressor takes 10A and can’t be run for very long at a time (it brings the system up to 120psi  from 0psi in about 15 seconds), so they’ll have to run it off a relay.  I couldn’t find cheap relays that looked easy to use with 5v control and 12v 10A contacts, but automotive relays are cheap (I found 5 relays and 5 sockets for $5 on Amazon AGT (5 Pack) 30/40 AMP Relay Harness Spdt 12V Bosch Style (40AMP-HRNS)—even with shipping that is only $2.25 each for relay plus socket).  The relay can be controlled by half an H-bridge of the Hexmotor board.  The other half of the H-bridge controlling the solenoid should be fine, as we never need to run the compressor and fire at the same time—we can stop the compressor for a fraction of a second while firing.

They want to have the running of the compressor be automatic, which would require a pressure sensor.  The Freescale MPX5999D would work and is one of the few sensors I’ve seen with a large enough range, but I’m not sure how to mount it.  Standard tire-pressure monitoring sensors and transmitters are cool, but I don’t know if they go up to high enough pressures and I don’t know how to interface to their transmitters—that is almost certainly a more expensive solution. Honeywell has a differential sensor with ports that will go to ±150psi, which may be easier to connect up, but it costs about twice as much and is uncompensated and unamplified: I suspect it would be a lot fussier to work with than the Freescale part.  I’ve ordered a sample of the Freescale part, and read their AN936 application note on mounting (epoxy is your friend).

It turns out that the relays may be useful for other functions, like a linear actuator for the tilt mechanism. Two relays can be controlled from one H-bridge to get forward-backward-stop action on motors up to 30 amps (but no PWM!). Unfortunately, 12v linear actuators seem to run $100 and up, which is more that I want to spend on a single part.  I may ask the students to redesign—either building their own lead-screw mechanism or coming up with a different tilt mechanism.  I don’t think a simple servo motor will do—the beefiest one I have claims only 69 oz-in (0.49 Nm) of torque, which I don’t think will be enough to tilt the gun, even if they can get the hinge very close to the center of gravity.

Another problem has come up: getting more darts.  We have 5 darts that fit the barrel perfectly (1.45cm diameter).  There are plenty of darts sold like that, but they almost all now have larger heads on the end, and the heads don’t slide down the barrel.  The new Nerf clip-system darts are all mini-darts, that have a 0.5″ (1.25cm) diameter instead.  These do not fire well from the ½” PVC, which I measured as having an ID of 1.485cm (0.585″). A chart of PVC sizes I found on line says that 1/2″ ID Schedule 40 PVC is supposed to have an inside diameter of 0.622″, which is almost 5/8″, but that ID can vary by 10%, even along a single piece of pipe—only OD is held to tight specs.  Thicker-walled Schedule 80 is supposed to have 0.546″ ID, which would still be too loose for clip-system darts.

I see four possible solutions:

  • Find a source of (probably non-Nerf) foam darts that are 1.45cm (9/16″) diameter with heads that are no wider than the body. I think that they came with an NXT generation crossbow, so replacement foam darts for that may be what we need. They’re nowhere near as cheap as clip-system darts, but this is still probably the cheapest solution.
  • Buy Nerf  (or other) darts with the right size bodies but oversize heads, remove the heads, and make new ones (out of what?). This would be cheap, but tedious, and the darts would probably fly poorly, unless we made the new heads have a decent weight.
  • Use clip-system darts for compatibility with the popular Nerf guns, but find a smaller diameter tube than the ½” PVC pipe (where? and how would it be connected to the solenoid valve?) It looks like Schedule 40 3/8″ steel pipe has a inside diameter of 0.49″, which is just right, but steel pipe is rather heavy.
  • Use clip-system darts, but convert to the Nerf-standard tube-inside-the-dart launching system.  This limits the effective barrel length to the inside length of the dart (about 4.5cm) and the barrel diameter to the inside diameter of ¼”, which will limit the top speed of the darts (OK for safety, but probably not as much fun).

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