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LOHAN seeks failsafe for explosive climax

Mulls back-up system for Vulture 2 motor ignition

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As true patriots last weekend rolled out the bunting and unfurled a celebratory pint in honour of her Maj Queen Liz II's 60 years atop the throne of Merry Old Blighty™, we here at El Reg's Special Projects Bureau took a few moments from shouting "Gawd bless yer ma'am" to ponder matters of perhaps greater import, viz: just how to make sure rocket motors go pop at altitude.

Click here for a bigger version of the LOHAN graphicSo, it was off to the pub in the quiet village of Much Mulling, to join contemplative locals in some serious chin stroking while staring into pints in search of inspiration.

To recap, we're currently investigating a barometric altimeter method to fire the solid rocket motor of our Vulture 2 spaceplane. That's Plan A, but what happens if the electronics fail?

Obviously, we need a back-up failsafe system, which should fulfil the following requirements:

  • A purely mechanical system, with no electronics
  • Triggered at, or after, balloon burst
  • Guaranteed to work, no matter what
  • Easy to test

Some of our readers have already suggested we use the balloon burst itself as the trigger. This falls down mainly because it's not possible to physically attach anything to the balloon: it's just too fragile.

Well, after a few ales and beermat sketches, we started to wonder if the recovery parachute couldn't be used in some way. Cue a couple more jars and voilà: one mechanical failsafe system...

Graphic of our proposed mechanical failsafe mechanism

To clarify, as the LOHAN rig ascends, the recovery parachute is much like a folded umbrella, kept closed by the weight of the flying truss and Vulture 2 hanging under it.

The cunning plan is to tie a cord to one of the parachute cords, where it joins the canopy, then pass it though a ring tied to the parachute cord on the opposite side of the canopy.

The free end of the failsafe cord can then be attached to a locking pin in the mechanical trigger device shown above. It's nothing more than a spring-loaded plunger which will create the electrical contact necessary to fire the rocket motor when the pin is retracted. This apparatus is housed inside the flying truss insulated electronics box, so won't freeze at altitude.

When the balloon bursts, and the parachute deploys, the failsafe cord is pulled taut across the underside of the canopy, and this action retracts the pin. It's just a matter of calculating how much slack to have in the failsafe cord to ensure it'll pull taut with force when the chute opens.

Simple as that, and this system offers the advantage of operating only when the the parachute deploys, when presumably the flying truss should be more or less at its intended attitude.

However, we have no doubt you'll already have spotted a few possible pitfalls. Firstly, since the flying truss swivels freely under the balloon - the standard set-up for High Altitude Ballooning (HAB) payloads - it's possible that the failsafe cord will wrap itself around the flying truss tethers. This will take up the slack in the line, and prompt premature pin retraction.

Secondly, the parachute may not deploy in the thin atmosphere at altitude, and only open when there's enough air to fill its canopy, presumably after a pretty lively tumbling descent for the flying truss.

We also have another concern regarding post-balloon-burst launches. The remains of LOHAN's mighty orb and the tether attaching it to the top of the parachute could fall into the path of the Vulture 2, with catastrophic results as the spaceplane thunders from its launch rod.

As you can see, nothing on LOHAN has an easy fix, so we'll throw the failsafe concept over to you lot for your comments/suggestions. If you think you have better idea, let us know. We're always happy to listen... ®

Further LOHAN resources:

  • New to LOHAN? Try this mission summary for enlightenment.
  • You can find full LOHAN coverage right here.
  • Join the expert LOHAN debate down at Reg forums.
  • All the LOHAN and Paper Aircraft Released Into Space (PARIS) vids live on YouTube.
  • For our SPB photo archive, proceed directly to Flickr.
  • We sometimes indulge in light consensual tweeting, as you can see here.

LOHAN - A Special Projects Bureau production in association with...

  • 3T RPD logo
  • University of Southampton logo
  • Applied Vacuum Engineering logo

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