LOHAN seeks failsafe for explosive climax
As true patriots last weekend rolled out the bunting and unfurled a celebratory pint in honour of her Maj Queen Liz II's 60 years atop the throne of Merry Old Blighty™, we here at El Reg's Special Projects Bureau took a few moments from shouting "Gawd bless yer ma'am" to ponder matters of perhaps greater import, viz: just how to …
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Re: Electronics are going to be much more reliable
Correct me if I'm wrong but afaik:
- Instrumentation Op-Amps are finicky little things who refuse to work at the slightest hint of mistreatment (ie, low temperature, buffeting, etc)
- Straingauges and their amplification electronics suck power like mad
Re: Electronics are going to be much more reliable
Sorry, you're wrong :-)
OK, partially right, but depends on the circumstance and device you choose
if your source is relatively low impedance (e.g. wheatstone bridge device like a strain gauge), then they're pretty stable and will work down to -40 C (industrial grade devices). They will typically consume less than 50mW (an AA battery has around 3Wh of power, so that would last 60 hours.. plenty of time to launch)
older devices are less stable and use more juice, but modern devices are pretty good these days
Re: Electronics are going to be much more reliable
Ok, consider me schooled then :)
My only experience with straingauges was a beast of a setup in my university days. To be honest the electronics might qualify at antiques on that setup though :-P
Re: Electronics are going to be much more reliable
Ahhh, University electronics.. those where the days
you problem was probably the prototyping breadboard.. lack of groundplane is normally what instrument amps don't like too much.
I've been using them without problems for years
Re: Electronics are going to be much more reliable
Was using a pre-setup system. But with some very old instrumentation op-amps looking at a a TINY signal. (Barely registerable) So we had its sensitivity cranked up all the way. The system didn't like that though :-P Maybe it was just a bad design, but those measuring instruments were rather finicky in the most sensitive range.
Re: Electronics are going to be much more reliable
try measuring the signal from a fuel cell (1mV full scale deflection), when you've got TETRA radio signals bouncing around (80V/m.. normal EMC test is 3V/m or 10V/m at the most) :-)
modern devices, properly laid out and implemented have incredible common mode noise rejection
Still.. ho hum, that's life
Re: Electronics are going to be much more reliable
Okay, now your going outside my knowledge band :-P. (I'm a mechanical engineer, and just a hobbyist electro-person)
Re: Electronics are going to be much more reliable
Sorry :-p well guess I should know this stuff seeing though I'm an Electronics Engineer... only mechanics I know is from A-level maths (I did Pure and Mechanics). I'm sure there's stuff you could teach me about fluidics, thermal flow, stress etc etc etc
Just goes to show, doesn't matter how much you know, there's always someone that knows more :-p
Re: Electronics are going to be much more reliable
More correctly, there's always something who knows more about something else. And isn't that great! Imagine how dull life would be if you knew everything. :-P
Alternative mechanical release
Having given it a bit of thought I still haven't really come up with a good "balloon burst detection" method.
What I did think up is an alternative and much smaller mechanical release (You have to admit the PARIS design was rather bulky). See http://i231.photobucket.com/albums/ee238/Kloppertjes/LOHANrelease.png (Is there a way to directly post images?)
Basic idea, a sealed chamber with a small diaphragm. Affixed on this diaphragm is a pushrod that in normal conditions keeps a microswitch pressed. Now suck the bejeezus out of the chamber and bring it down to the release altitude pressure. (With a good vacuum gauge this should be easy to calibrate) Once the device reaches altitude the diaphragm pops into the relaxed position and pushes the switch.
Alternative, fix an internal string to the back of the diaphragm pulling it back, and seal the chamber at sealevel pressure. Once the internal pressure becomes sufficient the diaphragm again moves forward and pushes the switch.
Re: Alternative mechanical release
Bad plan, because it will be impossible to calibrate remotely accurately. By the time you get to 1/3 of our expected altitude, 3/4 of the pressure drop will already have occurred. Much better to use a simple timer.
OR, FFS, A ZERO G SENSOR, ARE YOU LISTENING LESTER?
Re: Alternative mechanical release
It will be just as easy to calibrate as the original PARIS release. And in case you can't read, that is what I'm proposing it as an alternative for!
The Rocket Should Be The Bursting Mechanism.
Already been done in the Patriot Missile System.
Encase the Rocket in an insulated box with a bit of acceleration space with a window that will break by the passing of the rocket headed to the balloon.
The rocket will burst the balloon on contact and continue space ward. This Mechanism also allows for recovery of the rocket in case of failure for another attempt.
Two separate ignition systems.
Mechanical timer with power and timing device attached to the rocket box. This system operates on a best estimate of optimal launch time.
Radio command and control launch system contained in balloon basket/launch subunit with a power source separate from the mechanical timer unit. Fired by radio from the ground.
Both should be insulated and painted flat black to absorb as much solar energy as possible.
Mechanical Apogee Detector
There is a Youtuber, Solidskateboards who built a mechanical apogee detector for his rockets that may work or help at least with launching at apogee, and it will not prematurely ignite due to wind and such...
http://www.youtube.com/watch?v=ldaHifNAxsg
pressure switch
or you could just use a commercial mechanical pressure switch like these :-)
http://www.impress-sensors.co.uk/low-range-pressure-switch.htm
Saturn
It's easy to see now why NASA didn't bother with helium balloons for the Apollo mission but just went with big MF Saturn rockets. Too much hassle otherwise. Maybe a chap with binoculars and a remote controlled detonator a la something out of a spy movie would be a more workable plan.
(Icon == this way up)
Use a spring scale
Attach the launch truss to the balloon via a simple spring scale.
Chose spring scale with a range of 1/2 to 1/4 (guesstimate) of the total weight of the truss (incl Vulture 2) so that once the balloon's inflated and the truss is suspended the spring scale will be fully extended and at the limit of its travel, so as to avoid any bouncing off the upper stop during the ascent.
Botch a new lower limit stop, at about 1/8 (another guesstimate) of the full range, so that the spring is still under some tension when there's no load upon it and it tries to fully retract. Then epoxy an insulated contact onto the scale, beside the new lower stop, and another to the moving spring scale indicator: when the balloon bursts, the load drops to zero, the spring scale retracts and makes a firing circuit when the scale indicator hits the lower stop.
Simple microphone
So I just had a brainwave. Why not have a simple electret microphone, and analyse the signal for noises above a threshold? Because the balloon mostly moves with the wind there shouldn't be much wind-noise on the way up. Once the rig starts dropping, there should be a noticeable amount of rushing air going on. Simply detect this and trigger LOHAN. The bang of the balloon popping MIGHT be noticeable, but I can't remember clearly hearing that in any high altitude footage(And a simple mic can run for days off a single 9v block.)
And beer, because its friday.
Works at altitude?
Can that parachute open forcefully enough to pull the chord in the thin air at ~100,000ft?
Could I suggest an alternative which detects lift from the baloon: http://iamtheb.org/me/weightdetector.GIF
Aerodynamics have no part to play. No unreliable pin to pull. Spring still ensures electrical contact.
When the weight of the payload is zero, it is in freefall. Keep the tube long to account for turbulence.
Post Burst Chaos (PBC)
Those of us who have flown video cameras on high-altitude latex balloons are quite familiar with a phenomenon that we've tagged "Post Bust Chaos"; in fact, it's so familiar to us that "PBC" is a broady-recognized acronym. In a nutshell, the behavior of the payload string becomes unpredictable within one second of the burst. This is due in part to the sudden loss of lift and partly due to the shock wave produced by the sudden expansion of now-unrestrained lift gas.
Just google for "balloon burst video" if you're not convinced. YouTube has almost as many of those as cute kitten vids.
So my recommendation, if you're taking the latex route, is to launch the rocket while the balloon is still intact, e.g., by using a GPS-based altitude measurement (far more precise than any reasonable absolute barometric pressure measurement at 1% of sea level pressure), and setting the firing altitude setpoint about 3000 m below the predicted burst altitude.
Alternately, you could use a pricey and fragile zero pressure floater or an even more pricey super-pressure floater. Or, you could pop open a vent in the neck of the latex to slow its ascent. But all of these alternatives violate the KISS principle. One could also try a minimal fill in the hopes that the rising tension in the latex envelope will halt fill gas expansion before the latex reaches its yield point; good luck computing THAT!
Use a spring
The chute may become frozen in a closed position due to the cold, and the tether may become frozen to the rig, meaning that the whole thing would be in free fall for some time after the balloon bursts.
How about suspending the rig from the ballon by a spring which, when the balloon bursts contracts causing two electrical contacts to be brought together igniting the rocket motor? There would still be the issues of stopping premature ignition due to turbulence on the ascent and preventing the contacts and spring icing up of course, but these apply to all solutions.
KISS the balloon
Surely the simplest solution would be to have two electrical contacts glued to the balloon such that they close when the balloon is deflated but open as the rubber skin stretches apart during inflation. when the balloon pops the rubber will contract again, closing the contacts and firing the ignition circuit. Simple, light and rugged. You may want to cover the contacts to prevent ice forming over them but otherwise failsafe and fast.
The advantage of this system is that it is not affected by turbulence or altitude variations as it triggers only and exactly when the balloon bursts.
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