PARIS pumps up a Mk 2 release mechanism
Second shot at unleashing Vulture 1
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.
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:
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 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.
We just glued the bush into the bung with epoxy:
For the other end of the oxygen tube, we inserted a bog-standard bicycle valve into the bung, and fixed it with rubber glue:
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.
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:
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.
So, with the oxygen tube sealed at both ends, we could now conduct a high-tech pressure test...
...which demonstrated just how much the tube can expand:
Of course, we tested the assembly for leaks, in the time-honoured fashion:
Finally, we put the whole thing together and mounted it on a test base:
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. ®
Additional PARIS resources
- Our dedicated PARIS section, with all previous updates, is right here.
- New to PARIS? We have a basic mission summary here (pdf).
- Our fledgling Flickr page, with all previous photos.
- Check out our YouTube channel - currently featuring a few camera tests.