Observations from an Extra
These are my notes as a bystander ("consultant") during the video production in Arizona.
I was pretty excited to be able to play an extra in several bits. I may show up in the background lugging jerry cans with Neil, or pulling some camo netting off the coilgun to reveal it to Basil, or just standing in the background. This was all new and fresh to me, but I can see how the waiting around could eventually become tedious. Ultimately, my appearance is but one second of bending over in a truck bed lifting jerry cans when Basil is about to hop out of a shiny black SUV to meet Tod.
Danger: High Voltage
I thought the whole project was a smashing success, and the coilgun was totally reliable. Sure, some of the extra bits didn’t perform exactly as they wanted, but I still think the coilgun clearly demonstrated its immense power and that one could believably scale it to stupendous size to launch things into space.
That would have been okay, but when it switched to safe mode, one bank of capacitors still showed a full charge. So Basil emoted under camera about how dangerous this was – we couldn’t fire it, we couldn’t discharge it, and we couldn’t safely approach it to repair it. I worked out the RC time constant to be 5 hours, meaning it would only reach safe voltage levels overnight (3RC). Of course the crew is on a schedule and can’t wait that long.
Fortunately, I brought 10KV linesman gloves which allowed Tod to convince people it was safe for him to work on the coilgun. With tension in the air and cameras rolling, Tod approached this big dangerous coilgun while wearing giant comically-sized black rubber gloves up to his elbows, tapped the gauge smartly with his knuckle and it fell to zero. Problem solved.
The g-force meter was temperamental. Although it reliably responded to the flick of a stiff wire past the sensors, it gave readings perhaps half of the time while firing the coilgun. Tod tightened up procedures to clean the chamber before each shot, including vacuuming the dust out. We stayed late the first night to improve the power supply wiring to remove possible noise from the system.
Working at night in the desert was an interesting experience too, with flashlights among the cactus with coyotes howling in the not-so-distant dark. However the g-meter never did provide readings that seemed really reliable. Finally the aluminum disk got somewhat caved in by the forces and stress involved, and Tod switched to a thicker disk for which the g-meter was not set up.
I double-checked the design engineer's math and it all looks correct. The readings do seem quite high, but they may well be plausible. However, two “identical” back-to-back shots had g-meter readings of 5093 and 7785 which is a substantial variation. Also one low-power shot had a g-meter reading of 9999 which seemed unlikely. For corroboration, Dr Angel took the bent disk back to his lab where he hopes to determine how much force was necessary to bend it that amount.
The projectile was fired horizontally several times into a one-foot cubic block of gelatin. The slow-motion video is fantastic. You can see a round shockwave ring move outward from the impact, after the projectile is almost fully into the gel. This will make great TV.
On the first vertical shot with a projectile, it came apart on the way up, with its black plastic pieces fluttering down all around. Even the small brass tube for vacuum evacuation came out. Tod had only one of them, so the entire crew searched the dusty desert sand floor for something that looked like a spiky cactus needle, of which there were millions. Thankfully Basil or someone thought to look inside the firing chamber and found it there.
Subsequent vertical shots held together better. However, the fragile mirror always broke. This could have been due to impacting the hard desert floor. Or perhaps, more likely, as the ones who knew how it worked believed, the mounting scheme allowed a bending stress across the silicon, causing it to crack.
I never did hook up my oscilloscope. It became apparent that any extra equipment could only slow down production of a schedule that was very time-constrained. The whole crew worked from sunup to sundown, using every bit of daylight time they could in this extremely scenic location. The special effects crew worked even longer. You simply must see the pictures taken by Tod and Neil and others.
My only regret is that very little science could be done on the coilgun. We did not get accurate readings of things like voltage, current, waveforms, mass and velocity. I collected what details I could but it is difficult to put quantitative numbers on its performance. Also we never did get a chance to crank it up to maximum power for one last shot. Oh well, I guess that means I need to build my own, right? I came away with some good ideas about things that worked well on a coilgun. Not the least of which is the copper arrowhead slider for commutation.
Overall the coilgun was very successful, very impressive, and will make a great episode. I know it will get a lot of attention from the coilgunning community when it finally airs. I can’t wait to see the finished product.
Last update 2012-06-05
©1998-2013 By Barry Hansen