One of the interesting things about launching a free balloon is that providing you stay within a few restrictions, you don't need a waiver from the FAA to fly them. In a nutshell:
That's it.
There's one big problem with flying a balloon to high altitudes, generally speaking you want to get the payload back. As anyone who's flown a rocket to any altitude can tell you, it isn't always the easiest thing in the world to find your rocket after it's landed. Even with dual deployment a rocket flying to 10,000' can drift a mile from the launch site. Now consider trying to find something that travels maybe 50 or 100 miles from the launch site, then comes down from 80,000 to 100,000'!
My proposed solution to that problem is to fly a GPS unit with a transmitter. I started a separate page on the GPS/transmitter here so I won't get into the details. As a back up, I'm going to include my small tracking transmitter as well. I'll likely lose the GPS radio signal as the payload reaches the ground, but I will have the last set of GPS coordinates to use as a starting point, then I can use the small tracking transmitter to home in on the exact location. But, I really think the MaxStream transmitter will have better range, and I doubt if I'll even need the small transmitter, but better safe than sorry.
In theory, as the balloon increases in altitude and ambient pressure decreases, the latex balloon expands to a point where it finally breaks. And that should work just fine. But, I'm using a larger balloon than most people do, and I'm at least a little worried it might not break when it should, and sort of just hang there for hours and hours. So I've decided to include a cut down device, just to be sure my payload doesn't wind up over the Atlantic ocean.
So I decided to see if my little 555 timer could be pushed to a couple of hours, rather than seconds as used in my rockets. While there is no theoretical limit to increasing the capacitor and resistor values used to adjust the time out cycle, there is a practical limit. The capacitor will always have some current leak, and if it's enough it could get to the point the timer never timed out.
The formula for calculating the time duration: Time in Seconds=1.1 x (resistance in K ohms) x (capacitance in uF). The larger the capacitor the greater the current leak, so I'm trying to compromise with a 470 uF capacitor and use large resistors. After doing the math, my initial calculation of a 470 uF cap and an 18 meg resistor would give me a time out at 2 hours 35 minutes. After testing it, at 2 hours 47 minutes it still hadn't timed out. I really didn't want to go over 2 1/2 hours, so I replaced the resistor with a 14.7 meg, it timed out at almost exactly 2 hours on the first test, and 1 hour and 58 minutes on the second test. I wanted a good 2 hours, so I added another 1 meg resistor for 15.7 meg total, this gave me a time out of 2 hours 7 minutes +-38 seconds on three tests.
The timer while running is drawing 6 milli amps, or .006 amps on a 9 volt battery.
I've been running a time test on my transmitter using a battery pack of 8 x 1.5 volt rechargeable 2000 mA batteries. At the moment it's been transmitting non stop for over 10 hours. I'm not sure if having the transmitter farther from the receiver will consume more power, but I don't think so. I plan to use lithium batteries for the real flight, which have much better power density than the rechargeables I'm testing now.
One last power requirement is my laptop and the radio connected to it. Of course the laptop has its own battery, but it won't last long enough. So I'm going to use my PC UPS as a power supply in the tracking vehicle. Using one of my 12 volt AGM batteries, I should be able to power the laptop on AC power for about 3.5 to 4 hours, saving the internal battery for remote use if needed. I tested it for 2.5 hours, and the battery voltage was down to about 12.2 volts. I think what I'll do is run a cable from the lighter plug in the vehicle to the battery, that should keep the battery charged should I need the extra time. Since I can get some 10 hours or more from a battery pack on the radio, I'll just power the receiving radio with a AA battery pack.
April 8, 2007:
Despite my lack of updating the web page, I have been working on a lot of little things. I ordered another Aiptek mini camera, while I intended to use it to record video during the flight, I think now I'll use it to record still images instead. Using a 512 MB memory card, I was only getting about 1 hour and 15 minutes worth of video. The batteries held up fine, but the max capacity is the 512 MB and I really wanted to get a full 2 hours of video. The camera takes still pictures as well, so the plan now is to use the Aiptek for stills and my old VHSC video camera for in flight video. The VHSC camera is about 1.8 pounds, but it looks like I'll still be under my 6 pound weight limit by about 1/2 pound.
The Aiptek camera will take about 960 pictures at 1200 x 1600 resolution before the memory card fills, alkaline batteries are supposed to take about 600 pictures. I'll use lithium batteries instead, so I believe I'll get the card full before the batteries drain.
The VHSC camera is giving me about 1 hour 35 minutes on a full battery. Still short of what I wanted, so I'll have to look into a larger battery pack for this camera as well. Maybe more lithium batteries...
In this picture you can see the new Aiptek camera, a solenoid, a new timer and a lithium battery.
To trigger the camera for still pictures, I was going to open the camera up and hard wire the switch to a relay from the camera timer board. But I decided to make a generic device that would mechanically trigger any camera shutter button. So I ordered a rotary solenoid from American Science and Surplus. I added a thin aluminum plate for mounting and you can see a small sleeve I machined to go over the solenoid shaft, more on that design later... I went ahead and did a printed circuit board for the camera timer circuit. It uses a 555 timer in the astable mode to switch on and off at an interval determined by a resistor and capacitor. I've used this design before, in my first ill fated camera rocket. I'll set the timer to take a picture about every 10 seconds, or some 720 pictures over 2 hours. The battery at the bottom is a surplus lithium battery, it's a real horse at 3.6 volts and 8.5 amp hours. One battery weighs about the same as 2) AA alkaline batteries, but has some 6 times the watt hour capacity. I'll use these lithium batteries on the radio transmitter, and perhaps the camera timer as well.
Here is a screen capture of online software that predicts the balloon flight path using National Weather Service data. This software is courtesy of Near Space Ventures and is available here.
As you can see from the map above, I'm planning the balloon launch from just East of the Iowa Lakes area and just below the Minnesota border, my old stomping grounds as a matter of fact. I need to stay out of the MOA restricted air space, and this seemed like my best option to keep the balloon out of the MOA. The flight track shown here is of course using the current upper wind data, and will change on the actual launch day. Upper level winds tend to be West to East of course, so the general flight path should be similar. Another nice aspect of this type of flight is that you don't need a waiver from the FAA to fly, so your launch day can be flexible. I'll notify the FAA regardless, but it isn't even required.
April 11, 2007:
I discovered the reason my VHSC camcorder wouldn't record over 1.5 hours is because that's all the longer the tape is! Duoohh! So I ordered some longer tapes to give me my 2 hours. The batteries seem to have plenty of life, providing they stay warm enough... The still camera shutter release system is working, the video recording system should be ready to go, the GPS/transmitter system is working, so it looks like I'm about ready to go. So I ordered my helium today, a 300 cubic foot "T" cylinder. It should be here in the next week or so. Right now I'm looking at a launch date of either the last weekend of April, or the first weekend of May.
This is the cut down timer package. I'm using 3 of the 3.6 volt lithium batteries (wrapped in black tape) through a relay to heat a strand of steel wire to melt the suspension line. A standard 9 volt battery will power the timer, allowing full lithium battery current to the cutter. I'm using a screw down terminal to connect the steel wire strand to the power wires.
Here is the payload. Top right is the digital still camera, center is the VHSC camcorder, top right is the radio modem. I have 2) 4 cell lithium battery packs, one powers the solenoid on the still camera, the other powers the radio modem.
I haven't decided where I'll locate the GPS yet, but I'm leaning towards attaching it to the inside of the lid, to move it as far as possible from the other electronics. The payload container is simply a 12"x14"x12" Styrofoam cooler, very light weight and should help keep the electronics warm. I used a soldering iron to melt holes in the Styrofoam for the camera ports. The individual electronic items are held in place with blocks of Styrofoam epoxied in place.