Clay Ain't Always Clay


We use clay to make rocket nozzles.  Two popular clays to use are Hawthorne Fire Clay and Kitty Litter.  Bentonite and other clays can also be used.

I'm a Kitty Litter fan but...

So... I was driving by a ceramic shop in the middle of town and decided to dart in and see if they had any of the mystical pixie dust (aka Hawthorne fire clay). They did - $16 or something like that. I brought home 50 lbs of dust.

I treated it with 70% paraffin wax and 30 percent toilet bowl wax. The waxes were melted and then dissolved in an equal volume of Naphtha. The mix was stirred into the Hawthorne to make a 5% mix by weight.

For testing, I used the Hawthorne treated with wax, and some Tidy Cats treated with oil.

First thing that was immediately noticeable was the Hawthorne was MUCH denser. I managed to snarf my end burner tooling because I didn't put enough in the first time.

I made a new set of tooling, made a new motor with the Hawthorne and then made a motor using the Kitty Litter. I tried to make both motors identical.

I fired them on the test stand.

Motor size: 1/2"
Configuration: End burner
Fuel: Fuse Powder (a bit hotter burning than meal)
Grain length: 2.0"

Hawthorne Clay: Weight of nozzle: 4 grams
Avg thrust about 120 grams steady after the initial spurt
Time of burn: 4.43 seconds
Nozzle diameter before burn: .109
Nozzle diameter after burn: .113

Kitty Litter: Weight of nozzle 2.5 grams
Avg thrust about 120 grams for first two seconds tapering quickly to about 75 grams during last two
Time of burn: 4.83 seconds
Nozzle diameter before burn: .109
Nozzle diameter after burn: .136

We can probably yak about the results quite a bit but I think the decreased burn time of the Hawthorne motor was because the nozzle stayed together fairly well and the pressure remained constant and high. Once the pressure died down on the Kitty Litter engine, the burn rate decreased.

The increased weight of the Hawthorne is actually more of an issue than it seems - a difference of 1.5 grams is the weight of a header in those small motors.

The nozzle diameter in the Hawthorne was not affected too much although the measurements I used didn't allow for a very slight oblong erosion to the round nozzle port.

In all, I think the Hawthorne 'won' the battle of the clay nozzles.