22 posts • joined Friday 27th June 2008 04:13 GMT
"HI" is Morse Code laugh
Contrary to your post's suggestion that "HI" is a greeting, it's actually a ham CW (Morse Code) abbreviation for a laugh, or the IM equivalent of "LOL". And it's supposed to be sent at an agonizingly slow speed of 1/30 Baud.
Not sure how to translate that to words per minute but suspect it's more like words per hour.
73 (CW for "best regards") de ("from") W5VSI (my ham callsign)
"HI" is Morse Code laugh
Contrary to your post, the "HI" that hams are gonna send is the CW equivalent to an IM "LOL". And it's supposed to be sent at an agonizingly slow rate - equivalent to 1/30 Baud, where one "dit" symbol lasts 30 seconds!
73 (that's CW for "best regards) de ("from") Mike W5VSI (my ham station call sign).
However, it actually works. In one demo, it was able to get off 5 rounds before it jammed, and it never blew up in the gunman's hands.
Out of 180 high altitude balloon flights, the only times when we've encountered icing problems is when flying through low-altitude clouds, where the dew point was just a tad above freezing. And except for the one where we launched in pouring rain (a rarity in Colorado!), the only icing we observed during ascent was frost on the camera viewport glazing. This is because the latent heat in the materials kept the temp high enough to get us thru those risky regions, In the rainy case, our mechanical burst-sensing release mechanism did indeed freeze up and brought the shards back home. So rather than deal with the risks of excessive viscosity of lubricants, I'd recommend simply not ascending through clouds. Which I presume excludes launching from the plains in Spain :=P.
Re: Can be grumpy when cold and wet
The phenomenon that you describe is known in the ARHAB community as PBC (Post-Burst Chaos), and it's quite common, but tamable. The most effective solution is to use a much larger parachute than the typical 2m diameter ones in common use. PBC comes about when the drag of the descending payload string approaches that of the inflated parachute. A conventional spherical parachute is meta-stable in that it likes to dump its high-pressure bubble of air out under its skirt, and that usually happens asymmetrically, thus creating a lateral blast of air that pushes the canopy farther off upright and so on until the 'chute is flying nearly sideways. In that condition, the drag of the payload string can easily exceed the 'chute's; it's not unknown to recover the payload string buried INSIDE the canopy after it fell into it!
But one need only review the videos of the Martian entry of the Curiosity rover to see that it can be done right. The key is to provide a huge canopy that cannot be so easily tipped by a draggy payload; but this also carries a price in terms of an initial investment plus tare weight that eats into the payload weight budget. The other method is to construct the payload enclosures in the form of low-drag bombs with tail fins to keep them falling fast and upright, thus serving as "plumb bobs" to keep the 'chute erect, despite its meta-stability.
Sadly, the LOHAN truss looks as though it would act more like a kite than a low-drag bomb. But then again, if PBC occurs after the aircraft is launched, there's not much downside to PBC relative to the mission requirements.
73 de Mike W5VSI, EOSS
Re: Launching into the wind.
We've tried the weathervane approach in an effort to mitigate spin rate and consequent mal de mer when viewing live video. It doesn't work very well, and here's why:
The dominant airflow vector is vertical, since the balloon envelope has far greater drag area than any reasonable vane. However, even minor wind shear will cause the payload line to "swing and sway with Sammy Kaye", and the tail fin (weathervane) will respond nicely to that with a series of snappy 180-degree turns.
What DOES work quite well, however, is increasing the rotational inertia around the payload's vertical axis. On our GoPro DVR, we installed a pair of 1m long 1mm x 3mm carbon fibre flats rigidly cantilevered horizontally from the payload structure in opposing directions with 10 g of Pb weight added to each extreme end. This resulted in a rather profesional-looking slow and smooth pan rate looking at the horizon.
Since there's negligible horizontal airflow velocity, there doesn't seem to be any point in your endeavor to launch LOHAN into the wind, but keeping the spin rate down might make for a more manageable launch trajectory.
CR-123s will do the job
EOSS has used a 3S stack of CR-123s for its termination cord burner (melter, actually) with excellent results for many years. The "burner" is a 1.5 ohm coil of #28 AWG NiCr wire wrapped around the lift line atop the parachute, and that stack is strapped across the coil via a 5A electromechanical relay and < 1m of #16 AWG - 2 cord. There is only < 1 cm of foam core separating the CR123s from the slip stream, and they consistently melt a 3 mm nylon cord within 4 sec. The command receiver delivers 10-sec bursts of over 5A to the coil, and one set of cells is good for about 10 shots. We test the stack before each flight using a Simpson 260 VOM on the 10A scale in place of the coil - if the needle pins, we're good to go.
Electrical contact to the cells are soldered joints. We use a 250W solder gun and rosin flux on the cells after
they have been cleaned with 180 grit crocus cloth. Just get the gun tip real hot so the job is done swiftly to prevent overheating the cells.
Just be SURE to avoid CR123s with internal fuses!
Pyrotechnics on balloons
I realize that the weather across the pond from us in the colonies has been diametrically opposed to ours; witness
the numerous wildland fires, the worst of which was about 80 km S of my QTH. Due to the possibility of an ignitor control failure, Edge of Space Sciences has laid a severe kabosh on carrying ANY pyrotechnics aboard student payloads. We're at great enough risk at starting a grass fire simply from the heat of a tracking vehicle's catalytic converter! To the point: any balloon flights carrying a rocket motor would be well advised to have predicted trajectories well clear of tinder-dry areas. This option is essentially denied us in eastern Colorado, but not to you.
73 de Mike W5VSI, CTO EOSS
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!
Re: Why use an altimeter when you have a GPS tracker?
The "anti-scud" rules require that civilian GPS's cease reporting when the altitude is over about 65,000 ft AND the speed is over Mach 1. Some suppliers read that as an OR, but many don't, and those are the ones that high altitude balloonists use for flights up beyond 130.000 ft (40 km) MSL. This rule would very likely kick in if the GPS were mounted on the LOHAN rocket, however.
Re: Why use an altimeter when you have a GPS tracker?
Yeah, that +/- 100m error could totally ruin one's day during an instrument landing, but it's FAR more accurate at altitudes where the baro pressure is just 1% of that at sea level, e..g at 30 km (100K').
Re: Pressure Gauge
C'mon, blokes. You already have a GPS aboard which has been qualified to report MSL elevation (altitude) above 65,000 ft. That data will be FAR more accurate than the reading from an absolute pressure instrument operating at just one percent of full scale. Except for an instrument-grade transducer that will be too heavy and costly to fly, most have cumulative errors on the order of +/- 1%, and that's the absolute pressure you're trying read! We learned this lesson early on when we first started flying GPS beacons along with telemetered baro pressure, which we had previously relied on for determining altitude. We were crestfallen to learn that what we had believed to be apogees in excess of 120K' turned out to be more like 90 - 100K' per the GPS. And given that the rocket ignition altitude is mission critical, wouldn't you want the most precise measurement available up there?
Re: Performance loss at altitude?
Hey guys, your new vane pump is working like gangbusters! The problem you cite lies more in how you're reading your vacuum gauge. It reads the pressure at its measurement port >relative< to the lab ambient pressure, which at 1100 m is in fact about 27" Hg. What you should do is replace that relative, or "gage", pressure instrument with one which reads >absolute< pressure, such as an anaeroid guage which read against a built-in hard vacuum reference. This is how barometers work. I use a Honeywell 142PC15A and a panel digital voltmeter in my altitude chamber and have selectable gain to read out in either mbar or Torr (mmHg). Works great against a 0 - 50 Torr Stokes gauge.
Mike Manes, CTO EOSS
Natural gas, or "street gas" is 99% pure Methane, CH4, which has a molecular weight of 16, compared to that of air, which is about 28, so it's also a "lifting gas" and has been used as such in a pinch. But it's still a lot heavier than H2 (MW=2) and He (MW=4). The "lifting power" of a balloon gas is determined by the >difference< in MW's of the gas and air. So even though
H2 has half the MW of He, in air, it's only got about 8% more lift for a given volume.
It's also worthy to note that a H2-air flame is nearly invisible - a very pale blue which, unlike carbon based flames, emits negligible infrared radiation. The flames seen in the HIndenburg newsreel were thus obviously not H2 flames. It turns out that the fabric that formed the skin of the airship contained highly flammable additives,
which burned with bright yellow flames. Of course, once the airship's structure failed, the H2 in the cells was released, mixed with O2 in the air and added more fuel to the fire.
In the US, the National Weather Service launched some 120 weather balloons daily from sites across the continent, and for several years now, all of them use H2 vice He due to the latter's
soaring cost. The primary added precautions are avoidance of H2 leaks and accumulation in buildings and avoidance of ignition sources such as static discharges and other electrical arcs, and the like. H2-air mixtures less than 4% and over 75% H2 concentration, as would be
the case in a balloon, are not flammable.
Nonetheless, it's easy to gripped by "Hindenburg Paranoia", as apparently have the LOHAN guys.
Mike Manes W5VSI, CTO EOSS
Re: Fire at point of bursting
Balloon burst is a very chaotic event. Not only is there a mess of latex flying every which way,
there's an abrupt change in line tension, and an abrupt change in G's on the payload which
would likely alter your carefully tweaked launch lug orientation. So it's best to launch the rocket while the balloon is still ascending. EOSS has flown 175 flights up to the 10 mbar region, and
one nasty event which we have yet to reliably overcome is PBC (Post Burst Chaos), which
rapidly evolves in just a few seconds.
We have flown a number of solid state barometric sensors, and they're OK below about 50K',
but I'd recommend simply carrying a GPS engine and feeding the NMEA serial output to your payload uP; use the $GPGGA sentence to read MSL elevation in meters.. Our experience is that GPS is good to +/- 500 ft or better over an entire flight. But you MUST get a GPS engine that is designed to report above 63K' - many don't. We use old Rockwell Jupiters (newest firmware) and there's a list of others that are proven to operate well above 100K' on www.arhab.org.
Mike W5VSI, CTO EOSS
Get a better vacuum pump
So this surplus fridge compressor only gets down to 0.15 bar or 150 mbar? That's about 15 times greater absolute pressure than one will see at 100,000 ft (33 km) MSL, and assuming that this won't affect rocket ignition is, may I say, whistling past the graveyard. I'd advise using a more suitable pump, such as what are used by auto shops to evacuate air conditioning systems, or simply buy one sold by Harbor Freight for under US$100 (60 quid) - the latter has been used by several amateur high altitude balloon groups down to less than 5 mbar (110K'), Or just beg or borrow one from an A/C service shop for the duration of this test.
73 de Mike W5VSI, CTO EOSS (www.eoss.org)
The buried wire is an elegant method, but it does incur some sweat equity, and after all, isn't that what we're evading here? The idea of using a mower-mounted camera an QR signposts is a great alternative that allows one to program alternate mowing patterns to avoid embedded streaks.
But it also requires some sophisticated optics, and could get foiled by obstructions such as trees.
I'd propose another alternative navigation system which retains the programming flexibility of the QR code / camera scheme, and that would involve planting ultrasonic transponders around the yard, and triangulating based on the time delay from the mower's interrogation "beep" to the receipt time of the transponders' response beeps. The slower acoustic propagation velocity will allow for high-precision location determination without requiring nanosec timing measurements.
Beacons could be identified by unique ultrasonic frequencies, and use of parallel -channel DSP could be applied to time-of-arrival determination. And the physical layer hardware would be trivial to implement. Beacons would require little power, and could run indefinitely given small solar panels.
CNSP record-busting balloon flight
I've been doing this amateur radio balloonacy for over 20 years out here in Colorado, and I must admit that of the 160+ flights that I've personally participated in, this one by CNSP this one is the most memorable. What boggles the mind the most is this Chinese-made latex balloon tolerated over 2 full days of intense solar UV flux at over 30 km altitude, defying conventional wisdom that it would last less than 12 hours up there. In fact, that assumption is what drove the original landing forecast. That robustness coupled with a "sweet spot" Helium fill before launch get the credit for this awesome adventure.
Kudos to Ron K6RPT and the CNSP team for blowing away the prior records for unmanned amateur balloon flight duration and distance and setting some really challengingly high bars for the rest of us in this avocation.
Mike Manes, CTO, Edge of Space Sciences www.eoss.org
LOHAN altitude chamber vacuum pump
Edge of Space Sciences (www.eoss.org) snagged a decent used Welch 1/2 HP vane vacuum pump from an industrial salvage outlet for US$75. It was rated new to pull down to 0.1 Torr (mmHg), but gets a 1/2 m^3 chamber to 2 Torr in about a minute. Or, you could just buy a new vacuum pump from Harbor Freight Tools for about US$75 and save yourselves plenty of sub-minimum-wage pro bono hours trying to kluge one.
Mike W5VSI, CTO EOSS
H2 vs He fill gas
Pete 2's right, you know. The best reason for opting for H2 is economic. Even out here in Colorado, near the He wellheads, compressed H2 costs but 25-30% of the same volume of He. Filling a 3000 gram bag for 14 kg neck lift with He costs some US$250.
EU Medical Device Directive EMC susceptibility conformance?
DId the affected devices conform to the 3V/m EMC susceptibility requirements
of IEC 60601-1-2? Electromagnetic compatibility is a two-way street.
US TAG IEC SC62D/MT17
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