moon IV and PVC I, nozzles taped up to keep the rains out
the penultimate launch [assuming 5 launches] took place on Gav's farm again,
on the 22nd of january, '08. the grass had be recently and conveniently turned into hay, so a
devastating bushfire was unlikely.
the last launch finished off the first 2cm diameter nozzle, by melting the aluminium throat out of it:
so i made a new one, much the same but a bit longer in the exhaust side, and with a polished STEEL throat.
i tested the new nozzle with a half-catalysed and solid, half-normal and hollow propellant grain, in a short cardboard casing:
it worked well except for the bit where i didnt hold it onto the plank properly. even so, the thrust was not satisfactory.
the ejection charge fired, but only because the delay block leaked gasses, due to me being over-economical with epoxy.
i didn't bother to fix the fins as they werent very wobbly in a relative sense. i had a new idea to try anyway, that of a "moon" grain, instead of the standard hollow one. its core runs up the side of the grain, against the casing, and it burns across its diameter. hopefully this would burn for longer and produce a bit less thrust.
top and bottom of james yawn's 'moon' grain, bottom has a inward-streched core for the igniter
this was all made alongside the PVC rocket and following it's success,
alan turned up to watch this one. it was a bit windy out in the stubble and i was a bit worried
about what the untested design might do, namely explode or fly erratically and take someones eye
out or flatten grandad's new chicken shed. so we got gav's longest extension cord to add to mine
and stood about 50m away, hence the small-ness of the rocket.
spectacularly wrong! the nozzle came out again. thats the loud pop, then the rocket coasts up the rod, and
the propellant starts to burn again just as it enters free flight. the winding motion is the moon grain producing
off-center thrust, and the thin trail as it falls is the delay burning.
somewhere in between, the catalysed solid sustainer also burnt, but the ejection charge didnt fire, proving itself
totally unrealiable and requiring a complete re-think...
it really stuck in the ground like that! wouldn't want it sticking in me.
damage report: some charring to the aft, scuffed the nosecone a bit, the nozzle is all grassy and the grass has a hole in it. all easily repaired. the nozzle-coming out was probably due to a bad bond to start with, however with the amount of thrust the propellant created out of the 2cm open motor tube, perhaps the current nozzle is simply too narrow...
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