The future is in Imperial

I wanted to make a proper sort of rocket motor, since i was now making a proper sort of rocket.
by 'proper motor', i mean it wont burn through and damage everything, it will ignite cleanly and deliver decent performance from the propellant, and it will go according to plan.
a plan which will be drawn up in advance.

the "proper' rocket is one that goes forth and comes back so you can display it.
having laid out the mission objectives and extensively failed to accomplish them with pyrotechnically delayed parachute deployment, i built as small an electrical switch as possible and literally stuffed it into my existing development and a breakthrough was made: pyro sux.

problems remained, mainly that the rocket was much too small for the electrics, and much too tired to keep pace with SCIENCE, which usually means increased power.

that gets us to now, very late 2008 where i am building a much larger and more pointy rocket to fill with doo-dads. and one of them will be this motor, and so it begins.

my first blueprint


the general idea is to make a decent sized motor to test a few ideas. the problem with smaller ones has been their fiddlyness - getting propellant inside the casing in the right shape and order, and then lighting it up. when they go, they go well, but every single one is a frustration to make.
at least they dont use much fuel.

thats about to change. i went to the hardware shop and bought about 1.2m of aluminium tubing with an O.D of 25mm. [pic] it should be a while lot more aerospacey than cardboard tube =D

then i thougth about nozzles for this tubing. aluminium is too soft, steel is impossible to machine, water putty is crap at best. i wanted an accurately shaped nozzle that wasnt going to erode and wasnt going to weigh 60 tonnes. i decided to investigate ceramics with a call to the local shoppe.
he was little help but did advise me that lab crucibles are made of porcelain.
i talked to some class mates at the artschool and all of them recommended i take a walk on the wild side.
Ian is the ceramics tech of the sculpture department, on top of which all other departments are built [its in the basement] and it is a place i'd never been, a cool, airy labyrinth of weird and fascinating constructions, half of which were intentional.
upon finding him [thanks alice!] it was found by me that he's not only the master of an intimidating array of kilns, but also a really nice dude. he advised porcelain too, but recommended slip-casting it. I went away to make forms...

the outer form

form from formlessness

I made the forms for the moulds in the same way as i made the nosecone for the PVC rocket so long ago
they're 3 separate pieces, the outer nozzle shape [above], the intake side inner cone, and exhaust cone. I sanded them smooth and sealed them with epoxy, and then found i had to make plaster duplicates of the internal cone forms anyway, because one cannot slipcast against a non-porous surface.

heres how it should go together:

Making drop-moulds for the cones from the originals was touch-and-go, and the resulting plaster surface wasnt as smooth as it could have been, but i thought it'd do.

the second phase, in which pooh casts plaster into plaster, was a disaster. it stuck pretty badly, but didnt quite fuse. i hammered the stainless pins into the mould, trying to catch the end of the cones and pop them free.

result: total destruction. the little intake cone was sprung when the pin entered its mould bore, the exhaust one had to be excavated and then have a sizeable gouge repaired in its surface = C..

but all is indeed well that ends well, and these cones run true on thier axel and should be servicible after all.

here's a bunch of pictures of the making of the 3-piece external mould, which went almost smoothly because i WAXED the form this time.

Just a bit of undercut on the front end that needed to be dug out, and it came apart beautifully X-D

NOW to make the nozzle itself! the mould was carefully cleaned and assembled, stood on end with plasticine, and the axel end steadied with an improvised steadier! time to pour 8-D

the first attempt, the cast was very careflly filled with slightly thinned porcelain clay slip, pouring down the axel to direct the flow. I left as much resevior as possible to allow for absorbsion shrinkage, to make sure the mould was filled completely.

results werent promising :( but i wasnt expecting much. the mould filled but the slip didnt dry enough to hold together on removal of the miriad mould parts. i tried much thinner slip, but this didnt help, so after 4 castings i had to leave my moulds, turn my back, and get on with my final submissions.

recently, i returned to my moulds, having had some time to think and research. i knew the vital mechanism here was absorbsion of the water from the slip, without which there could be no escape from the mould. i also realised that my inner cones didnt have a lot of total volume to absorb this water. clearly, the less H20 the better.

fortunately in my absence, the mould had dried to a state of almost absolute dryness, dryer even than parliament question time =O. all that remained of the slip i had been using was an empty, upended bucket, and the alternative was the end of an old bucket of stoneware slip. i poured it in, and it was slow going, but go it did. with my newfound time, i checked on it regularly and when i saw it would pull away from the mould slightly, i tried my luck...


a beautiful, fragile object! it was quite soft about the thickest point [outer thrust bevel] and got a bit squashed during removal, but i managed to get it back into acceptable form!

with later ones, i learned to let them dry for about an hour, then carefully trim them back and free the edges. the cones are always hard to get out and need to go back in a few times to correct the profiles. the moulds also need to be really dry and clean, so no water washing. now, i build a resevior out of plasticine, leaving one 3rd of the top cone as a vent, and this gravity-fed pressure fills the mould quite rapidly. its important to top up the vent port though, once it has overflowed, to avoid cavities.

trials by fires

on friday the three nozzles i managed to extract from the cast were bisque fired [1040 degrees?]

Ian fired them all by themselves and they passed act one intact. they were hard but dusty like terracotta, and the inside surfaces were radiosity-pink under natural light.
the camera muted this somewhat, but here they are all sanded, checked to tollerances and ready for act two:

the next step involved particular absurdity: the nozzles are tiny but require a very high temperature - 1200 degrees C at least.

this requires a serious kiln, more so than the sensible-sized one used previously. fortunately there are two of these kilns capable of getting hot enough. unfortunately the smaller of the two is acting suspect with its solenoids rattling and arcing. so Ian built an altar inside the other kiln, which is rated at 43 kilowatts and almost big enough to stand in, and atop this firebrick-and-shelf structure the 3 nozzles were placed.

door closed, it took some time to program the controller, which of course had too many modes and funtions and not enough buttons to select them with. but finally it was done:

ramp-up to 1200 deg.C @ 100 deg. / Hour, cool off for 24 hours, and 10-ishAM wednesday the 26th of November, 2008, the temperature was 52 deg. and the door was opened...

thats a "guard cone" - a number 3 pyrometric cone rated at 1186 deg, stuck at a slight angle in a glob of firebrick... and its resting.

and these are NOZZLES =D. and they are standing upright. in my microwave, but whatever. they are incredibly hard too. the surface is a raw granular one, like fine, worn sandpaper. they ring if you tap them together and at about 60mm, down from about 70mm, the shrinkage of 14% is about spot-on :)

a compound extreme UV radiation / vitamin B / crushing pressure test: PASSED!

just prior to being clawed and gnawed. few survive the Wrath of the Beast.
but these are by definition, few.

now its time to build a rocket motor, stick one of these in the end and see what happens to it..

[= oh my god what is going to happen next?! ::----> =]