Original URL: http://www.theregister.co.uk/2010/08/03/paris_release_mechanism/

PARIS pumps up a Mk 2 release mechanism

Second shot at unleashing Vulture 1

By Lester Haines

Posted in SPB, 3rd August 2010 11:27 GMT

Followers of our Paper Aircraft Released Into Space (PARIS) project will be aware that our first attempt to put together a pressure-operated Vulture 1 release mechanism didn't exactly go according to plan.

For those of you who're not up to speed on this vital mission component, we should explain that it's the device which will, at the required altitude, drop the Vulture 1 to begin its historic flight.

Our initial cunning plan was to use a big glass syringe filled with a small amount of air, which would expand as it rose into the heavens. The syringe's plunger would gradually extend, allowing us to connect a mechanical release mechanism to it.

That was the idea, but tests at QinetiQ's hypobaric chamber facility showed it just didn't work at the planned ascent rate - 1,000 feet per minute - probably due to air escaping between the ground glass plunger and the inside surface of the syringe.

There's more on that here, but suffice it to say our QinetiQ jaunt wasn't entirely a write-off. Thanks to some top improvised boffinry by Tim D'Oyly and Chas Taylor, we were able to show that an expanding rubber oxygen tube would do the job, if we could create a suitable piece of kit to house it.

Here's a graphic of the concept:

A graphic of our Mk 2 release mechanism

It's simple enough: just house the oxygen tube (maximum diameter 36mm) in a 40mm PVC pipe from the local builders' merchants. Seal both ends of the oxygen tube with rubber bungs, one of which features a fill valve, while the other acts as a mount for a steel rod designed to actuate the release mechanism.

Stick end caps on the PVC pipe, drilled out to accept the fill valve and steel rod, and you've got a simple way of mechanically exploiting the expansion of air inside the oxygen tube.

The liquid, we should explain, is needed because the amount of air required inside the tube at ground level is just 15cc, which expands to around 150cc at 20,000 metres. The tube when fully compressed has a volume of 70cc, and without liquid "ballast", would expand pretty quickly to the point of bursting.

The advantage of the liquid is that we can tweak the exact amount put into the tube (through the fill valve, natch) to deliver the exact level of expansion, or better put, the exact distance of travel of the steel rod as it exits the PVC tube housing.

And yes, we have thought about the liquid freezing. Accordingly, antifreeze would appear to be in order. The tube will also be heavily insulated and all the moving parts lubricated with this low-temperature grease (good to -73°C, the manufacturer assures).

Enough theory, now down the nuts and bolts...

First up, here are a few components as we initially laid them out on the bench:

The Mk 2 release mechanism components

The threaded bush into which the steel rod screws is actually from the plunger mechanism of a broken cafetiere, which proved an effective way of attaching a standard threaded rod to one of the rubber bungs.

The rubber bung, steel rod and threaded bush

We just glued the bush into the bung with epoxy:

The steel rod fitted through the bung into the bush

For the other end of the oxygen tube, we inserted a bog-standard bicycle valve into the bung, and fixed it with rubber glue:

Bicycle valve glued into rubber bung

Here's how we attached the bung to the oxygen tube - with steel wire. Note that we rejected our first idea of using worm drive hose clips, because they wouldn't fit inside the PVC tube.

The bicycle valve bung attacked to the oxygen tube

Here's the same passing through the PVC end cap. This differs slightly from our concept graphic, which shows the entire bung protruding from the PVC tube:

The bicycle valve passing through the PVC tube end cap

At the other end, we similarly drilled out the end cap to allow the steel rod to pass. Here we're using an off-the-shelf length of threaded rod, as getting a sufficient length of smooth rod delivered and threaded at one end was going to take weeks. We just inserted the rod into a rubber tube, to prevent it snagging as it passes through the PVC end cap.

In case you're wondering, the 4mm steel rod seen in the component snap above is just 15cm long - not enough for our purposes.

The metal rod passing through the PVC tube end cap

So, with the oxygen tube sealed at both ends, we could now conduct a high-tech pressure test...

The sealed oxygen tube attached to a bike pump

...which demonstrated just how much the tube can expand:

The oxygen tube pumped up

Of course, we tested the assembly for leaks, in the time-honoured fashion:

Testing the tube for leaks in a bowl of water

Finally, we put the whole thing together and mounted it on a test base:

The release mechanism fully assembled

It just remains to see how this performs at altitude, as we'll be bringing you the result in due course. For the record, the downside of this simple yet elegant set-up is that the PVC tube alone is 60cm long, and of course the steel rod is seen here in the "fully compressed" position.

This extravagant dimension will impact on the size of the main payload box, but we think we can use this to our advantage, as you'll see in due course. ®

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