Ask Jörg Sprave About Building Dangerous Projectiles

Jörg Sprave's day job is as a manager in the world of consumer electronics. But he has been for many years making manifest the sort of things that once filled my school notebook margins with doodles: slingshots and other devices for launching bolts, steel balls, and other stuff at high speed at targets or just into the air. (Some of his "slingshots" are hard to recognize as such; he eschews the classic American wrist-rocket braced design as well as the old Tom Sawyer forked branch in favor of things a bit more elaborate.) Thanks to the Internet, hobbies that were once obscure are now easy to follow, and Sprave's homemade slingshots are no exception; you can follow his exploits through an ongoing series of YouTube videos and a forum site that builds on these videos. He's doing it in Germany, too, where firearms may be harder to come by than in the U.S., but giant honkin' firecrackers are available (at least for part of the year), and acts accordingly. Amazingly, he has yet to lose an eye; his goggles are a wise precaution. Sprave has agreed to answer your questions about his own take on physics as a hobby. As usual for Slashdot interviews, you're invited to ask as many questions as you'd like, but please divide them, one question per post.

How Much Testing Do You Do?

[eldavojohn]

From watching a few of your videos, I see a lot of end results which are fun to watch. Take, for example, the firecracker slingshot mentioned in the summary [slashdot.org]. But one thing I can't seem to find is any testing videos or explanation of testing (maybe you have it in German and I can't find it). So how much testing do you do on these? When I saw your firecracker launcher design I immediately thought that the worst case scenario is that the firecracker detonates before leaving the chamber or possibly barrel. Now, a firecracker that goes off in the palm of your open hand versus one that goes off with your fist wrapped around it produces two very different results. A short or dry fuse in your device could be problematic. Did you detonate firecrackers inside the chamber from a distance to see if there was enough room in the chamber to allow the explosion to vent without splintering your device? How much testing did you do on it before you were confident enough to pick it up and use the butane lighter on it? Is this discussed somewhere?

Gun debate

[schneidafunk]

What are your thoughts on the current gun debate in the U.S.A.

space launch

[j00r0m4nc3r]

We still don't have terribly reliable/safe/cheap ways to deliver payloads into space. How about giant slingshots as a space launch platform?

Alternatives to Thera Band Gold & Springs?

[eldavojohn]

I see the majority of your projects depend on Thera Band Gold and then the throwers (like your circular saw blade launcher) use a spring. Have you investigated other methods of using elastic energy (which I guess is technically potential energy?) to propel a an object forward by transferring to kinetic energy? Are you engaged in any areas of materials science study in order to find more efficient and reliable materials? Or do you just use Thera Band Gold as your gold standard and leave it at that?

Injuries?

[BoRegardless]

I did similar things in high school, though not as elaborate. Made it through with all things, including eyes and fingers, intact.

How many injuries have you and your friends suffered?

Legality

[operagost]

What legal issues have you encountered? If your authorities are anything like they are in the more "progressive" states of the USA, they consider anything that looks like a firearm or throws any solid object (or not so solid [cnn.com]), via any means of propulsion, a dangerous weapon.

Performance

[PPH]

Have you tried to measure the projectile velocity (and calculate the energy) that your devices produce?