Holy cow, Bigelow Aerospace has launched an inflatable habitat! They’re saying that it’s testing out fine and everything is working well so far. This is getting almost no play in the media, unfortunately. It’s a huge step toward the building of a cheap on-orbit habitat (which Bigelow wants to turn into the first space hotel). Now if we just get the whole commercial passenger launch thing in place we’ll be really cooking. Whee!
Sean and I have performed another feat of Mad Science. Yesterday we successfully completed a pneumatic potato cannon, and let me tell you, the reality is a lot more terrifying than it sounds. We started the day by determining that we would build two different cannons. One would be a normal “in-line” potato gun using 2″ diameter PVC pipe, while the other experimental model would use the much more voluminous 3″ PVC for the pressure chamber (a significant difference in air volume and thus, hopefully, final altitude). The other difference with the experimental model was that we decided to shape it like a military mortar launcher:
First stop, a trip to the hardware store to pick up supplies. We discovered that the PVC pipe makes an excellent musical instrument if you beat on the end. Sean and I had a great Blue Man Group groove going in the plumbing supplies:
After our trip to Lowes we returned to Sean’s place and started cutting and gluing the PVC together until we finally had our two models. Unfortunately, we used some older PVC cement that we weren’t sure about, and either this, or some mis-fit in our assembly, created a problem that would come back to haunt us later.
Sean’s whole family joined in the assembly fun:
While we waited for the glue to dry, Erin (who had bravely accompanied me for Mad Science Day) and I joined Sean and his family down at the incredible new San Marcos dam recreation area. We had fun hanging our feet in the water while we watched Sean and his son Liam shoot the rapids. We also partook of some most excellent fajitas courtesy of a friend of Sean’s. He was the kind of guy who would throw a mean tailgate party.
Racing the daylight, we went back to Sean’s house and walked the weapons… er, I mean, science projects, officer… down to the schoolyard about 100 yards away. There’s a really large field down there, but it turns out it was almost not big enough.
Here are the two finished launchers ready to go:
Charlotte, the Mortar Launcher, which Sean ended up keeping:
And my personal gun. It’s my very favorite gun in the world. I call her Vera:
I was surprised at how simple a pneumatic potato gun is to construct. It’s also surprisingly inexpensive (although using the larger diameter PVC pipe upped the price a bit). The most expensive part is the sprinkler solenoid that is used to fire the gun. The solenoid cost $13. Next to that, the pipes and fittings were fairly cheap, and even using the much more expensive large diameter high pressure PVC (an absolute must), each gun was still pretty reasonable. If you stepped down to smaller diameter PVC you could probably build it for less than $20. Just make sure to use the high pressure stuff, please… you don’t want a grenade going off! In the picture above you can see that I taped another piece of pipe to my gun to minimize flexing. The gun was so long that it would tend to bow and flex as you carried it. I was concerned that this might weaken the narrow joints around the solenoid so I reinforced it with the extra tube. It seemed to do the trick. Ideally I think it would be good to get a 2″ diameter solenoid to avoid all the adapters, but Lowes only carries the 1.5″ ones. Even if it was more expensive to get a 2″ solenoid, the difference would probably be saved in not having to buy all the step-down adapters.
You can clearly see the green solenoid on both guns in the pictures above. There is a tiny black lever that activates the solenoid and releases the pressure to fire the gun. The pressure vessel is filled via a standard schrader-type valve that is pushed through a hole cut into the tube. It’s fairly easy to jam it in place from the inside, and the pressure holds it against the tube much like an airplane door is held in place by cabin pressure. The only downside to the gun is that you have to actually hold it or stand near it to fire. Next month I’m going to spend the $10 to buy the 24 volt adapter that enables you to fire the solenoid remotely. Then I’ll hide behind a big wall and push the thing up to really high pressures.
So how does it perform? Much better than I ever thought. For our first test of Vera we loaded up a potato and pressurized it up to 40psi. I angled the gun at about 45 degrees and flipped the switch. FOOOOMP. The potato easily traveled the length of the soccer field and had a good 5-6 seconds of airtime. It was really impressive. Then we loaded up an egg and sent it flying at 30psi. Surprisingly, the egg stayed intact until the end of its flight! It traveled about half the length of the soccer field and splattered spectacularly.
Once we were comfortable that Vera was structurally sound, didn’t leak, and performed well (!), we started gradually creeping up the pressure. The PVC is rated at 280psi, and I’m sure that this number contains a very conservative safety margin (x2 isn’t unreasonable). Furthermore, we had no intention of even approaching this limit since Sean’s small car-powered compressor couldn’t handle it, and we didn’t have nearly enough downrange room to shoot something at that pressure. We opted to go up to 60psi and angle the gun at a higher azimuth and thus go more for altitude and hangtime rather than absolute distance. And what magnificent hangtime we got. At 90psi (the highest pressure we dared), we were able to get almost nine seconds of flight time. That may not sound impressive, but stop reading this and count off nine seconds on a clock. That’s a long time for a potato to fly. Our back of the envelope calculations showed over 300 feet in altitude, but I still think we exceeded that. Anyone know a link to a hang time/altitude calculator?
Unfortunately, we had less luck with Charlotte. We used the much bigger tube for our pressure vessels, and also decided on a double air chamber design. Unfortunately, one of our main pvc joints had a small leak that prevented us from ever pressurizing the gun. We put it on hold for a while and will cut out the offending part and try again at a later date. I’ll be sure to update you on that, though I think we’ll need to find a much larger field to test that one.
So what did I learn? First, a potato cannon is simple and cheap to build. Second, just because this is true doesn’t mean that it’s not dangerous. These things are extremely powerful, with a muzzle velocity easily over 100mph (Sean, I think our calculations were wrong and it was higher than we thought). Even loaded with a squishy potato, at close range I’m certain that they could kill. They’re definitely not toys. We shot a potato through a chain link fence and ended up with Julienne fries on the other side, and the potato that got launched at the metal pole was mashed in a fraction of a second. Most of it just exploded into mist. It was a very impressive performance and I gained a new respect for the power of pressurized air… especially as I was holding it next to my body!
Sean took video of the day and I’ll let you know when he gets an edit done and posted to google video or YouTube.
We’ve now completed three Mad Scientist projects: the trebuchet, the water rocket, and our potato cannons. We’re busy planning our next Mad Scientist Day. Ideas welcome!
Saturday night we hosted our Supper Club over at our house for a wine tasting. We had about 15 people and eight bottles of wine, about half of which are left in our fridge today (thanks, guys!). I think everybody got to try something they loved (an excellent Riesling!), something they hated (I didn’t know I’d dislike Pinot Noir so much), and something they had never tasted before (Ice Wine!). I even found a merlot I could tolerate, and was turned off by a Cabernet Sauvignon, which is unusual.
We were initially worried because we thought that we wouldn’t have enough food and that everyone would just bring the vino, but people really came through and our table was stuffed with finger foods, bread, cheeses, crackers, bruschetta, Jim’s excellent fruit trays and Karen’s yummy turkey pesto rolls. We topped it all off with a tiramisu that I made yesterday. Very challenging, but fun (and a little too sweet, I think). We absolutely destroyed the kitchen with dirty dishes and plates, which, of course, is the sign of a successful shindig. The party started at 6 and we didn’t empty out until after 11 so I think it was a huge success. Thanks, guys!
Barry and Catherine stayed with us last night, so of course we had a great time sitting up far too late drinking coffee (and Barry’s incredible ice wine) and talking about fun stuff. A great night with great friends.
Our Supper Club has a core group of about 13 people (though we’ve added the very interesting Bob and Serita lately) and different people host every month. Erin and I want to make our hosting time somewhat different than the normal dinner fare, hence the wine tasting. We’re thinking of a Supper Club breakfast or something for next time, but it’s going to take awhile to recover from this one.
Here’s the spread:
