5 replies to this topic

[Lil_Guppy]

Going through some of my odds-n-ends boxes, I found a batch of 5mm bottle rocket tubes which had had the clay endplug rammed in (but no propellant/header/etc), and I thought I might as well complete and use them so they are not taking up space (that's my excuse anyway ). I noticed, however, that the endplugs were quite easy to push out, as if either the tube had expanded slightly (unlikely), or the bentonite had shrunk slightly over the months of storage. I've never had this problem before, as everything I have made has been used within the week. However, it is a problem I would like to fix in case I decide I want to make a large batch of rocket motors one weekend, for storage and use over the months.

The bentonite 'clay' I am using is from a local ceramics supplier which lists it as Bentonite with the following description:

--
BENTONITE
Used in glazes as a suspending agent.

Al2O34SiO22H2O.

An extremely plastic colloidal volcanic clay that is invaluable as an additive to bodies to increase plasticity (up to 5%) and to glazes as a suspending agent(up to 3%),but it must be added to the dry ingredients.It has a high shrinkage rate at stoneware temperatures.
--

The reason I cut-n-pasted that was in case there are varaitions of bentonite which would have a different description/use.

Anyway, back to the issue at hand. I have read that treating bentonite with wax dissolved in a solvent (such as the tea-light candle wax and petroleum spirit described on Alan's site) reduces crumbling and sticking of the resulting plug after ramming. Could this also provide a "waterproofing" characteristic to the bentonite to help prevent/retard the plug absorbing moisture and causing it to shrink?

If anyone else has any other suggestions, that would be great.

Cheers,
G

[Nova]

Did you wet the bentonite before ramming it into your casing? I've heard of lot's of people who experience problems with the shrinking or swelling of bentonite endplugs and nozzles after they used wet bentonite.

[Yugen-biki]

My suggestions:

Dry the paper tubes for a few weeks until you are quite sure they are very dry. This way you can rule out the tubes as the problem.

Dry the bentonite caly on in room temperature (similar conditions were you ceep the finished rockets). Or dry the bentonite on the stove or similar.

I have never needed to mix vax in the bentonite, but why not. It sounds as if it would be a good id?.

Mix the bentonite with some finely ground sand. This will inhibit the bentonite clay to shrink if it is moist.

[Lil_Guppy]

I am ramming the bentonite dry, from a container. The bottle rocket tubes had been dried for a few weeks before loading with the plug. It has been a little humid here the past month or so, so maybe that caused the shrinking. Because it does tend to shrink when drying, I thought giving it a coating of wax (say 5%) might help stop the bentonite taking up the water from the air, and shrinking upon drying. I will also try putting in some fine sand to the mix to see how that goes.

The other day I also found an old spent helecopter device that went MIA when it was lit. The whole thing had been exposed to the elements for about two weeks, and its clay plug was shrunken quite a lot. The 1/2" diameter plug had shrunk by about 1/8", and looked like a blob of plastic on the surface. Thats why I was wondering if there were different types (chemically) of bentonite in case the type that I have is one of the ones not ideal for this purpose.

[Yugen-biki]

I mill some of my cat?s "citty litter" sand.
This is mainly bentonite with gravel. I ball mill it for some time and separate the finely round clay with some sand from the gravel.

I have used this with good results in different rockets, stored for some time.

[Lil_Guppy]

I used to use ground cat litter to with very little problem (apart from having to grind it up ), but I thought the proper stuff would be a bit easier. If I can't figure out a way to get it to stop shrinking soon, then Im going back to cat litter.

I did have a thought last night that maybe the bentonite powder has absorbed some moisture already, and when rammed into a plug, it then shrinks as it dries. I don't know.

Anyway, like I said, I'll see how my tests go (if i can be that patient that is), and I might end up changing back to cat litter for my source of bentonite.