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
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...