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back to article Readers fret over LOHAN's chilly bits

As the ongoing saga of our Low Orbit Helium Assisted Navigator (LOHAN) rocket motor igniter continues, readers have been expressing concerns that the igniter batteries are going to suffer from the extreme cold at altitude. Click here for a bigger version of the LOHAN graphic As we explained earlier this week, our current …

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Good luck.

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Heaters gonna heat

8 batteries weigh quite a bit ... which is lifting capacity you'd rather use to get your balloon to a higher launch altitude.

Is the payload weight going to have any impact on launch altitude?

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Anonymous Coward

Re: Heaters gonna heat

Within reason I don't think so, just ascent/descent speed?

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Re: Heaters gonna heat

That's what I was thinking and launch is going to be at a pre-determined height, isn't it? Presumably as near to the balloon burst as it is prudent to leave it.

So as long as the thing gets aloft, differences in weight didn't ought to have any effect.

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Go

I get irrationally excited whenever I see a LOHAN item in my RSS feed.

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Happy

I apologise for butting in

as I've only been following LOHAN sproadically (and that pic of the two old geezers really isn't selling it to me) but is there no scope for an ignitionless system ? ISTR the return motors on the Lunar Lander were designed to avoid the need for anything that could go wrong.

Anyway - inspiring stuff. I speak as someone who turned down a job with the ESA in 1988 - one of my regrets.

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Re: I apologise for butting in

> return motors on the Lunar Lander were designed to avoid the need for anything that could go wrong.

IIRC the big downside of those motors was they once started they were damaged, so they couldn't be tested pre-flight and then cleaned/refueled. If they didn't fire first time, RIP Armstrong & Aldrin. Scary.

Also, if Lester & co. don't want to use hydrogen in the ballon I doubt they'll be keen on using hydrazine in the rocket engines...

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Boffin

Re: I apologise for butting in

The term you are looking for is "Hypergolic propellant".

But they are all really nasty corrosive liquids. Not really something that will work for LOHAN.

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Re: I apologise for butting in

Hydrazine AND nitrogen tet. Don't mix with beer in any proportion.

Anyway, this idea means liquid bi-propellant engine, which in turn means, at the very least, 2 high pressure tanks and plumbing, highly engineered combusion chamber and nozzle, propellant injection system, high pressure valves and automatic valve control system. In Apollo thrusters the chamber/nozzle was made of "welded coated columbium (niobium)", which as any fule kno is one step away from pure unobtainium, and that alone weighed almost 4 kg ;-)

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Boffin

Battery warmer idea.

Disposable hand warmer - the kind you get from outdoor shops. http://www.handwarmers.com/id71.html

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FAIL

Re: Battery warmer idea.

It's almost like you didn't bother reading the article at all...

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Happy

Re: Battery warmer idea.

Aha! You are right. Something about brackets in text.

On the plus side you got to take me down a peg and got up voted for it. Yay you! :)

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Mushroom

Need heat??

Disposable hand-warmers....

They are light flexible and last for hours.... it dosnt have to be elegant it just has to provide heat for the length of the mission.

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Re: Need heat??

It's almost like you didn't bother reading the article or comments at all...

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Hand warmers

I do not know, and its a question,

hand warmers I think work on a reaction between iron and oxygen.

at altitude, I'd imagine there is not sufficient oxygen to actual make them heat,

are they 'just' working by thermal mass ?

If so, would thicker insulation,

and a hotter lump at the start be more efficient ?

sort of thinking heat a LUMP inside the unit up to a HIGH temprture on the ground,

? mains power ?

then unplug and launch. no need for access flap to chuck in hand warmers,

so could make better insulation ?

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Re: Hand warmers

Not all handwarmers use an oxidation process to produce heat. The gell type handwarms have some sort of meterial that gives of heat as it crystalizes.

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Boffin

replace batteries

with an ultracapacitor or fuel cell (assuming the budget stretches that far)

If the budget does not stretch, but the batteries in a thermos.

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Re: replace batteries

A fuel cell seems possible, but it weighs about a kilo for a 100W PEM fuel cell before you start adding the hydrogen tanks. Given the cell also requires oxygen, you'd also need that taking up as well. It's also about £800 for the cell, before you start going into specially-designed low pressure storage tanks for the boom juice.

Definitely not cheap, probably about a kilo and a half to two kilos all in, but you get a steady 100 watts at a flow rate of 1.3 litres of hydrogen per minute according to this source.

Yeah, a stack of D cells would last long enough and probably weigh less. Including, perhaps, the vacuum flask.

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i've seen them there soler pan'ls on spacecraft

would they work?

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Launch Rail

I'm slightly concerned about the launch rail. From the sketches, it looks like there is a good distance between the axis of the rocket motor and the rail. Any friction between the hangers and the rail, or any kinks in the rail itself, will result in a moment. I doubt that the connection between the rail and the aluminium support will be stiff enough to resist it, and the control surfaces on Vulture 2 will not work as the speed will be negligible (as well as the air density being very low). The net result will be that the rail deflects upwards into the truss, potentially resulting in the hangers fouling on the truss before Vulture 2 separates. Which would be a shame.

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Use AAAs

Energizer Ultimate Lithiums are also available in size AAA.

http://data.energizer.com/PDFs/l92.pdf

1200mAh but only half the weight.

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Stack o' button cells.

Nice idea, but does a series stack of CR2032s give enough current to set a fuse off?

If not, three cheap-ish lipo cells (100-150mAh or so) probably will work. Might even be able to make the whole affair light enough to have two or three firing circuits just to make sure.

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coin cells

You'd be surprised how much current you can get out of CR2032s. Try measuring it sometime.

As for joining 'em up - simples! - use the variety which have solder lugs and do what comes naturally to any geek with a hot stick in his hands.

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Coin Cells vs AAAs

Duracell CR2032, shorted through the multimeter. Maximum current reading, about 25mA. This may have been a faulty battery, as a much smaller L1131 button cell gave about 450mA, rapidly dropping to 400mA and lower.

Duracell Plus AAA, shorted through the multimeter. Maximum current reading, about 1.5 amps.

Just out of curiosity, an Energizer Ultra+ D cell, maximum current draw of around 2 amps.

Make of that what you will. I'd go for solder-lugged AAAs, myself.

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Pint

Small batteries and capacitors

Back in the late eighties, I said 'I think I could do better' to a friend of mine and found myself designing control equipment for an outdoor model electric railway. Total power wasn't a problem - 2 x 24V x 5A outputs - but this had to do everything. I designed and built a considerable number of points controllers which used a capacitor charging through a current limiting resistor, 12mA if I remember, and a transistor switch to discharge the capacitor through the points relay to throw it.

12mA was used because (a) I had plenty of that value resistor, and (b) it gave a reasonably quick recycle time. A much lower current could be used in this application because of the relatively long time to altitude, and a simple timer circuit could be used to trigger the discharge through the igniters. I am happy to say that these units proved extremely reliable.

If of any interest, I could dig out the designs.

Chris Cosgrove

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Boffin

LOHAN gets tighter as well as colder?

The nozzle diameter is 3mm at room temperature, in a Spanish summer, but what is the coefficient of thermal expansion of the nozzle material? I expect that a metallic nozzle would shrink with a 90K temperature drop. I can't do the maths to know if the shrinkage would be significant.

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Boffin

The nozzles are usually made of a graphite material. Which has a thermal expansion coefficient of 7,9*10^-6 m/m K. If the nozzle has an opening of 3 mm at 20 C, and we cool it to -60 degrees (80 K delta T). That means the circumference of the hole shrinks 632*10^-6 m/m. Original circumference is 9,425 mm. The new circumference is 9,425-0,006=9,419. Meaning a diameter of 2,998 mm. In other words, not really a problem.

If the nozzle was made of austenitic stainless (304 stainless) the new diameter would be 2,996 mm. Another 10 or 20 K is not going to matter all that much. Especially when talking about feeding a somewhat flexible material through the hole.

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Unhappy

weird

That should have been a reply tot he post above it...

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GPS Tracking

The UHF tracker that was on the plane is low power and light weight. You can however use it with out a licence with a move in frequency. I would suggest changing the LiPo cell on it. it was designed to track rockets so is robust.

The High Power VHF tracker that was on the balloon I still have, and will happily loan for the project, it has the advantage that the Spanish APRS infrastructure will feed it all to the internet for you, you will however need someone with an amateur licence, in the UK 25 multiple choice questions. Am pretty sure you could find a Spanish Amateur interested. You can get chinese vhf/uhf radios for about £25, feed the audio into a laptop with a couple of bits of software and good to go.

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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!

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Possibly interesting for the LOHAN D-CUPS team

Just came across some interesting blog-posts from a high-altitude cross-atlantic balloon team. (Thanks to hackaday I've got to add):

The first is some extra code for an arduino allowing more flexible task scheduling. (In case you decide there will be an arduino based system

http://www.meatandnetworking.com/code/introducing-arduino-simple-task-scheduler/

The other is a small (40 grams) radio enabled flight computer:

http://www.meatandnetworking.com/projects/a-tiny-balloon-computer/

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@mike

It's bloody dangerous to solder lithium cells, even with a high power gun. I've seen the mess when someone gets cocky. The standard way of attaching tags/wires is to spot weld them because it puts less heat into the canister

Farnell do a pretty good range of tagged CR123-ish cells (although they're always cheaper elsewhere as a rule). You could start at http://uk.farnell.com/jsp/level5/module.jsp?moduleId=en/230466.xml (or search for "CR123 solder tag"). Proper tags are more than adequate for 30A.

(You can start a car with a stack of CR123s, or a couple of Tamiya Racing packs)

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