trials by fires
here is the first test motor.
the casing is a 25mm OD, 22.5mm ID aluminium tube, 310mm long.
the nozzle is installed against a dowel to locate it vertically and center it, then
PVA glue and water putty fills the cavity to a depth of 30mm.
3 meccano screws further anchor the putty to the casing, while at the other end the top closure
is an 8mm thick layer of 2-part epoxy covering and sealing a sizeable plug of solid propellant,
which was melted and packed in place.
the ignitor has a stainless steel ribbon filament and a gunpowder / aluminium /celulose
pyrogen, and its moulded into the middle of the propellant plug and protrudes into the top of the
first propellant grain core.
there are 3 rather misshapen grains of about 40mm long, 22mm in diameter and wrapped with masking
tape as a basic inhibitor, with a 6mm core. this is about half as much propellant as designed
so it should be nice and safe...
this is old propellant left over from batches, so each grain is different. i rolled them to
size but it was pretty horrible to work with, so next time i'll mould them into a tube and
divide it up in 40mm lengths.
this is what happens when you dont put putty, bolts, more putty and even more bolts in a motor closure. and you're me:
-it comes out
-you're gonna have a Bad Time
what happened here is both clear, and unclear. im not surprised the top end gave way,
but i am surprised how fast it happened: as soon as the propellant caught.
its possible the top plug fractured and contributed to a pressure spike, but that
nozzle throat was 8mm
, not small at all for the motor diameter.
that aluminium plate was flat and clean before, and the ignitor sire is cleaved in twain.
maybe the nozzle got blocked somehow. it happened so fast that my camera really didnt
pick up much:
it really moved fast, and the propellant, the middle grain of which was a tight fit,
ejected itself over the other side of the bearer. the motor ditched on the driveway a
couple of meters away, and cracked the nozzle all the way down its bell, on a seam D=<
-motor test inconclusive
-broke first nozzle
+ignitor worked well in a large core
+propellant burned fine
+nozzle didnt come out
+casing is fine
overall, a big motor wont just scale up. it needs bolts and bulkheads.
its also going to be important to hold it down really, really positively
at both ends when testing - i dont want another impact breakage.
finally, the propellant needs work and i should address that first, making
SURE i have good, strong grains that are inhibited properly and wont shatter.
Tuesday 2nd December: Trial 2
after the first test i didnt want the motor getting loose again.
so, i rounded up some big strong u-clamps for steel piping that were
hanging in the shed, and attached them to a bit of wood so they could
hold the motor without obstructing either end or crimping the casing.
I also used some innertube rubber to pad the clamps, and wedged some soft
lead wheel weights under the front one because it didnt quite do up enough.
the second thing i didnt want to happen was the forward closure to come out
quite so easilly, so i coated it with epoxy, sealed the ignitor similarly
and pressed it in warm, and then used screws the same as the nozzle, but
with a slightly smaller margin of casing, to retain it.
-> all the propellant was from a new batch with a 3rd less honey, and a bit less dry.
it was almost workable this time, was pressed into an inhibitor tube, and the cores
were drilled to 6mm, with a DRILL! there were 4 Bates grains and a 20mm header.
-> the nozzle was held in exactly the same, except the putty only came up to the screws.
i clamped it down with 3 big g-clamps, ran out the extension cord and hooked up the ignition box...
extreme overpressure blew apart the nozzle and front end simultaneously.
the pressure wave made my camera jump on its tripod. my ears kinda hurt.
the front closure whistled about 50m through the trees and was lost somewhere near the boundary fence.
theres nothing left of the nozzle except half the inlet and throat still in the motor.
the black lump is a burnt inhibitor. as usual im really glad i was 30m away behind a big bush.
why did this happen? well, i think i left the airtight plug blu-taked into the nozzle.
i dont usually make mistakes of that magnitude but as you can see in the before pictures,
it kinda hides in there =(.
in summary, the casing itself is very strong, and the ends take some force to remove.
the test cradle is a success too, and i think the propellant held together. and, my neighbours
didnt call the police on me. yet. there is only one good nozzle left now...
but these are the remains of the first nozzle - chipped, charred, but hopefully usable as a sonic
one for evaluation of another 4-grain, UNPLUGGED motor. i'll repair the casing and the next test
will be somewhere else, and done vertically.
test number 3
above are pictures of the 3rd test motor. following advice i doubled the number of retainer screws
for the header, and cast the propellant into proper rolled and glued cardboard inhibitor tubes.
nozzle is the stub of the first one, installed in the same way as always. the propellant is a new
batch that is much easier to work with and cools to be very strong. the ignitor is epoxied into
the middle of the header as before, with the wire holes sealed with epoxy.
the test stand is attached head-down to a steel post which is attached head-down to the firebreak
out the back. i ran the extension cord down hill, through the fence and behind a tree...
it worked! the pressure got out the header end a bit early, but for a moment or two there it
was all systems go from the 3 propellant grains. im guessing the header burnt back a little bit and
then the gas found a way past the ignitor wires, so that'll need to be revised for next time...
teardown pictures. nozzle is crazed but intact. it took some getting out though...
test 4 is similar to test 3, except with DOUBLE THE OOMPH
OOOOOMPH OOOOOMPH! 150 grams!!
the head end is a new idea - putty sealing a large nut, retained by the same
6 screws on its corners. the igniter goes down the thread with more putty,
and below the nut is a cardboard disc to keep the putty out of the propellant.
the nozzle has twice as many screws this time [twice as much pressure! its logical!]
finally both ends of the casing were greased well, in the hope that the putty would
thrust is left to the imagination, but it lasted from 0.7 to 0.85 of a second, i think.
the thrust flexed the steel post and pushed the mount against the ground, and the hiss
was much louder this time, reverberating around the firebreak.
this was the second attempt, as the first the day before didnt light due to a low battery.
a bit of inhibitor cardboard got loose and was honourably discharged, the rest were pretty
much intact except for the ends which got variously charred. the masking tape came off easilly
which means the casing didnt get very hot.
without measuring actual thrust, the motor burned a bit longer than expected and pressure
tailed off a bit more gradually, probably due to the variation in core diameters, that
weren't always exactly centered either. fuel residue was pretty minimal too.
on taking to it with a screwdriver, the top end came out pretty easilly - unscrew, hit it inward
then poke it out. there was a bit of leakage at this end which had bubbled what can best be
described as gross black gunk out around the screws, but the igniter holes held tight.
the nozzle end didnt leak at all, but was MUCH harder to get out. exhaust had cut a small, shallow
notch in the edge of the casing beyond the screw holes, i guess due to the higher pressure and
thus greater expansion of the flow. apart from that everything was completely ok. like facebook.
totally fine, nothing to worry about.
oh except the nozzle literally fell apart when i started to pick the putty off it :(.
so, the answer
to the ceramic nozzle question
is "excellent once, might work twice, but not
tough enough to be a daily driver"
they are much easier to duplicate than a steel one though. lighter too :).
next step: LAUNCH 1!
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