Parachute cord and springs?
I initially thought of using the parachute as the trigger, but at high altitude it may not open, so I came up with this idea that uses gravity as a trigger instead:
1) Take a length of parachute cord and make a double figure of eight shape, making sure the cord doesn't pass through itself and that the continuing lengths of cord 'depart' from roughly the middle of the 8.
2) Fit a small rubber band over each loop of the eight to secure them in place. These bands should not be tight, so use very small bands at low tension.
3) Now create a metal plunger wired to the launch battery + and fit it to a powerful spring. The face of the plunger should be smooth and flat, but should also be larger in diameter than our figure 8 cord is long.
4) Fit the plunger and spring into a casing that will see the plunger in contact with a copper stop plate at rest (but still under considerable tension from the spring). Connect the copper stop plate to the + launcher circuit. This is our trigger mechanism. The casing should be open and unencumbered at the bottom, so either the stop plate is mounted separately on the truss or it is fixed to the spring casing at the top only.
5) Fit the trigger mechanism to the Truss on the centre line. On the outer left strut, directly in line with where the plunger and stop plate meet should be a strong cable anchor.
6) Tie the uppermost free end of the figure 8 cord to this Anchor.
7) Tie the other free end of the figure 8 cord to the bottom of a small weight. This weight is held in a cage mounted on the right strut in line with where the plunger and stop plate meet. This cage is open at the bottom so the weight falls out easily, but has a small hole in the top. A cone shape would be ideal in a closely mated cone-shaped cage...
8) Tie the top of the weight to another length of parachute cord which is attached to the balloon tether directly - that is above the parachute mounting. This cord should be shielded from atmospheric buffeting, so running it through a drinking straw attached to the balloon tether would protect it.
9) Place the figure 8 of cord between the plunger and the stop plate so that it prevents the two plates touching. It should stay in place through the force of the plunger trapping it against the stop plate.
Note: Before launch, cut the two elastic bands to make doubly sure the figure 8 cord will 'zip' out of position.
The method of operation is pretty simple.
a) The Balloon bursts
b) Tension is released from the balloon tether and the cable holding the weight in its cage
c) Weight falls free from cage and pulls figure 8 loop from between plunger and stop plate
d) Plunger and Stop plate meet and launch circuit connection is made
e) LOHAN launches
f) Weight fall stopped by anchor.
The only issue I can see is the 'yanking' factor that may be applied by the weight being arrested in its fall by the anchor cable pulling LOHAN off course. This could be alleviated by use of pulleys or a really long anchor cable ensuring that the weight remains in free fall until LOHAN has launched successfully. However don't be tempted to add all that extra anchor cable between the plunger and stop plate because the more cable between the two, the longer it will take to close the circuit. Perhaps coiling the anchor cable up inside a small parcel of tissue paper attached to the truss would be the way to go....