Bob- Sounds like you've got it solved! Hopefully your post will get read by a lot of people. Thanks for your help getting birds in the air. -David, N9KT
On Sun, Jan 31, 2021 at 12:20 PM Robert Bruninga [email protected] wrote:
IMO one of the largest difficulties for Cubesat creators is that passive spring
deployment of antennas... ,,, a contrivance of nichrome wire and
monofilament
fishing line is mainly used ...to ...releases whatever spring there
is.holding the
antennas in place...
The legacy of the "nichrome wire" never dies... But all of my satellites used a better, reproducible version of this. Instead of using very low impedance nichrome wire requiring a complex high current driver, we simply use a 2 cent film resistor. Not only is it the ultimate in reliability, it can match any direct battery voltage from 3 to 28 volts using a single transistor driver and can be reused many times to prove the actual flight configuration will work.
THe key is that the physical size of a 1/4 Watt thin film resistor is standard and has a nice neck in the middle. Perfect place for a nylon line to rest. And the resistance can be *any* value. Just match the resistance so that you are providing about 3.5 Watts into the 1/4 W resistor and it will burn the string identically no matter whether you are using a 3 volt bus or a 28 volt bus. Only the value of the resistor changes. You can vary the blow-wattage to get a burn time of anything between 1 to 5 seconds. We use the 3.5 W nominal value for about a 3 second burn time (and to allow for reusability).
We have found you can reuse the same resistor over 50 times and it still works. as long as power is cut immediately after each short burn time. You can also adjust the delay from 1 second to maybe 5 seconds depending on how much you choose to overpower the resistor. And replacement resistors (the flight critical component) are highly available. And we surface solder them on the outside of the spacecraft so post-assembly replacement is trivial if needed.
See sketch at top of ParkinsonSat (PSAT) Page (aprs.org) http://aprs.org/psat.html
In fact, we provide a burn-trace on every external solar panel so that the final burn location of the wire whip antennas can be made very late in the design process and can change as things evolve. The PSAT design above used a single resistor to release 4 whip antennas at once!
Nichrome? Nah, forgedaboutit.
Bob, WB4APR
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