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back to article LOHAN cranks up old-school clockwork failsafe

We're obliged to Low Orbit Helium Assisted Navigator (LOHAN) fans for answering our call for suggestions as to just how a back-up ignition system for our Vulture 2 spaceplane's rocket motor might work. Click here for a bigger version of the LOHAN graphic Our primary system is the Special Project Electronic Altitude Release …

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WTF?

Failsafe SPOC

Is it wise to have a failsafe system using the same battery pack as the primary ignition system?

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Re: Failsafe SPOC

Who cares?

It has a switch that says "Arm"

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(Written by Reg staff) Silver badge

Re: Failsafe SPOC

It's a weight issue.

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Re: Failsafe SPOC

battery size/weight - do you need voltage or capacity?

If only the former, then how about a stack of the tiniest button cells (hearing aid batteries?) to give the 9 volts

It would fit inside the casing.

You could also save some grammage but soldering the wires directly to the terminals instead of using the Lucar (spade) connectors.

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Regarding accuracy, it's pretty good. Here are the results:

etc.

Just one thing, did you do at least one of those tests in a freezer?

It wouldn't surprise me if the clockwork ran at a different rate when at low temperatures.

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Re: Regarding accuracy, it's pretty good. Here are the results:

To counter the cold, you could put a tube of dessicant (say Magnesium percholorate) in the casing and leave it deliberately open to the exterior at one end but initially capped

the inner end would remove water vapour from the interior so that if it did get cold there would be no icing up of the works. Remove the outer cap before launch and the exothermic reaction of the MgClO-whatever with the moisture in the air would produce heat.

In my experience about 20ml of granular MgPerc can make a glass tube uncomfortably hot to the touch.

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Stop

Re: Regarding accuracy, it's pretty good. Here are the results:

I'd be more worried about the cold causing shrinkage, er, I mean parts of the clockwork shrinking at different rates which might affect the timing of the clockwork.

Of course, this particular model is designed to go in an oven, so might have some features to avoid temperature differentials, but testing it in a freezer would be a good first step.

tl/dr cold causes shrinkage, shrinkage bad!

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Joke

Seeing the parachute laid out like that...

...they've finally found SPEARS's knickers.

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Joke

Re: Seeing the parachute laid out like that...

would that be Billie-Jo or Britney?

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Re: Seeing the parachute laid out like that...

Which was photographed by the paparazzi with them conspicuously absent?

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Re: Seeing the parachute laid out like that...

Which was photographed by the paparazzi with them fortuitously absent?

ftfy

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Parachute?

Rather than the tricky parachute wouldn't it be better to use the tension on the support line (or sudden lack thereof) to trigger the mechanism. The balloon tether pulls upwards (along as the balloon has lift) this holds a firing mechanism in the 'off' position. A weaker spring is rigged to pull against it . When the balloon bursts lift fails, the spring 'wins' and pulls the switch into the fire position.

This then doesn't rely on the untested low-air-density parachute so (in theory) will definitely fire very soon after burst. The downside is the chance of turbulence setting it off early but this is also an argument against the parachute. Some sort of timer would be required to make sure the upper atmosphere has been reached before it is fully armed.

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Go

don't forget to grease the locking pin

or the failsafe line might prevent the parachute from opening!

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(Written by Reg staff) Silver badge

Re: don't forget to grease the locking pin

Yes, that could be an issue too. The plan's not ideal, and only a last-ditch back-up.

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Pint

Re: don't forget to grease the locking pin

A foolproof switch is easily made with two drawing pins pressed into either jaw of a sprung wooden clothes peg. Strip of credit card for insulator / firing pin.

Probably less than 10g total weight. 9v battery for back up power, say another 40g. Sorted.

Mine's a pint. Cheers.

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Pint

Top class shed boffinry there guys!

Have a beer on me.

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Childcatcher

Oh look...

'The Anarchists' Cookbook' has all growed up...how cute...

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Alert

Make sure the timer is not based on a fan fly...

Doesn't look like it, but it's worth confirming that the timer does not use fan fly to regulate it's speed. A fan fly mechanism will practically guarantee premature ignition.

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Just a suggestion

You might want to shop around any local industrial suppliers for a Phenyl methyl silicone based lubricant for the clock work. water repellant, and good for extreme cold.

Something to keep the gears turning and ice free.

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Joke

Suspicious Object

"Bomb disposal and anti-terrorism teams were dispatched to the countryside today. There an armed drone, containing what authorities are describing as an explosive device wired to a kitchen timer was recovered. A sophisticated electronics and imaging system was also contained in the drone. Local authorities are now working with INTERPOL to arrange extradition of the suspects."

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Alert

Timer accuracy?

How does the temperature (and air pressure, as Alex 71 mentioned) affect the accuracy of the timer?

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Anonymous Coward

careful with the parachute opening

https://en.wikipedia.org/wiki/Parachute#Parachute_malfunctions

With respect, I'd worry that the 'signal line' running under the fringe of the canopy, may actually act to keep the canopy closed, especially in the thin air at altitude, or lead to an 'inversion' or 'line over' kind of malfunction. IMVHO, the best thing to do with the canopy edge is leave it alone ! I realise I don't immediately have a solution for the original problem, but I don't think this is it ....

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Really not sure you have enough fidelity with any timer for this to be worthwhile - you don't want this to go off before you hit max-altitude, so if the balloon bursts a minute or two earlier than you expect, how far will the load fall before the timer goes off?

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I have doubts

I've voiced my doubt before, especially about the line getting entangled around the chute. With a bit of experimenting and fiddeling to choose just the right amount of slack it could work, but it'll still be pretty error prone IMHO. If you do go with the hollow bearing idea, keep in mind you will also have to add a swivel to the pull line, spin of the chute would otherwise still wind up the string, causing it to bunch up and pull in.

As for the oven timer, I agree it would be a good idea to redo the tests after a good trip through the freezer. There is a good chance the cold would alter the properties of the clockspring enough to seriously throw off the timing.

I know mechanical FEELS more reliable over electronics (With things like flat batteries and such) but "the industry" has long since moved just about everything to electrical systems as far as I see. I simple 5v electronic timer with a single solid state relay, fed by a volt scavenger circuit (ala minty boost or similar) from a pair of penlites has a lifetime measured in years, while being pretty darn reliable.

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Addendum

I'd also like to add the following: Well before launch, stick all parts and payloads in a low vacuum chamber to dehydrate. If at all possible, keep the chamber nice and warm. Keep it there for as long as you can. An atmospheric pressure box with a good amount of dessicants would also work. Transfer it all to a thermo/cooler box with a load of dessicant bags (Silicagel would probably work best) and keep it all at a nice warm temperature. Don't take anything out until it is needed.

This way all parts will be bone dry. Thus much less chance of lenses developing condensation and such.

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It will still twist!

Even if you pass the line through the centre of a bearing (great idea by the way) the line itself may still twist, which means you also need a small swivel in the line itself. Fishing supplies shops sell very cheap small ones which will be ideal, they look like this:

https://www.google.com.au/search?q=fishing+line+barrel+swivel&safe=off&source=lnms&tbm=isch&sa=X&ei=y9HQUdLcHOPSiwKb5IHADg&ved=0CAkQ_AUoAQ&biw=1083&bih=904

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