Compression tests of recrystallized KN/sucrose rocket propellant

Click here to jump to new tests of 8/20/01

A recent submission to the Arocket list by Mr. Russel McMahon (8/15) invited us to imagine placing a 25kg weight on a 1kg sugar propellant grain, and assess the prospects for failure under compression.  I did so, and imagined three possibilities:

  1. The propellant would fail its compression test
  2. It would not fail
  3. Something else would happen.

The last of these prospects intrigued me enough to try some crude (as usual) static compression tests with my propellant.

A slug of propellant* was heated to 200 degrees, softening it enough to mold, shaped by hand into a cylinder and allowed to cool.  "Cool" being a relative term here in North Florida where heat and humidity seem to be in a contest, currently tied at 98.  The cylinder is about 1.5 inches diameter, and 1.92 inches long.  Formed by hand, it is not a perfect cylinder, thus the diameter is approximate.

* Propellant used in this test was made according to the process described here.

My plan is to stack weights on this cylinder until it breaks.  Unfortunately, all my cinder blocks are engaged, holding important posts around the house.  Most are holding up old cars.  But I have some extra foundation blocks just waiting for the next house sag.  They weigh somewhere in the range of 50 to 70 pounds.  I plan to buy a bathroom scale tomorrow so I can weigh them.  (Note of 8/20:  I bought a scale today and weighed them.  The lighter one, shown here on the sample, weighs 66 pounds, the larger one, leaning on the table weighs 77.)

The cylinder of propellant is wrapped in Handi-Wrap and enclosed in a ziploc bag to keep it clean and provide some hope of retrieving the pieces.  I left the bag unzipped so that it would not contribute substantially to the support of the block.  A piece of soft wood is placed on the propellant to pad it a bit from the unkind concrete.

The cylinder held this weight with no complaints.  I tried it several times just to make sure.  I laid the block on flat, hoping to stack the second block on top, but the first block kept slipping off, usually aiming for my toes.  So I gave up playing with the blocks.

Well that wasn't very dramatic.  A failure to achieve failure.  Now I need to find something heavier that I can manage to balance on top of the propellant slug.  After long thought, it occurred to me that I have access to a substantial weight that is quite mobile, and can be balanced on a small spot with a good degree of control.  It is my body.  I will perform a comedy act atop the slug!  That should break it - from embarrassment if not pressure.

I am a lightweight in so many ways, but was still surprised that the propellant did not fracture under my 165 pounds.  Guess I need to drink more beer (click here for a slightly offensive joke on this matter.)  I stood on it five or six times, trying to balance without holding the pull-up bar.  That didn't last long.  Gravitational anomalies abound.

Having been under the 60-pound-or-so block for a total of about a minute and under me for maybe 30 seconds, the cylinder had deformed a bit.  Its length is now 1.77 inches, about 8 percent shorter.  It is somewhat flatter on the ends, and angled slightly.  I measured from midpoint to midpoint of the cylinder faces to get an average length.

I will not be defeated.  The propellant will crack.  Now what else might I place on it - I need a heavy but steady load which can be gradually increased...then I heard the music from the Red Green Show and a light came on!

Still no failure.  No, it did not support the weight of the car, but kept slipping out as the pressure was increased.  I have no idea how much pressure this applied, but the cylinder did not deform any further, so it is probably not as much as in the previous tests.

I welcome any suggestions as to how I might test this to failure, short of buying a calibrated press.  (I would love to have one, so I can try Al Bradley's method, but there are a few other things I must buy first.)

So what does this mean.
For the moment, assume that I am creating a rocket engine using several Bates grains of similar proportion to my test cylinder.  They are not case-bonded nor otherwise supported by the airframe.  How much fuel could be loaded on top of a grain like this and still have a good chance for safe flight?

In the message referred-to above, Mr. McMahon outlined the challenge, suggesting a rocket that might reach an acceleration of 25 g's, subjecting the fuel to great pressure, perhaps too much pressure.  For the moment, I will assume that this propellant slug would have failed at any weight greater than mine.  So if we make an engine, say, 2 feet long with these cylinders formed into Bates grains, the perforation would make each perhaps 2 inches long.  So we stack 12 of them end-to-end and the total weight is 1320 grams.  That multiplied by 25 g's is 33000 grams, so the lowest grain would be subjected to a pressure of just under 73 pounds.  That leaves a considerable margin of safety relative to compression failure, assuming that other factors are negligible.

But other factors may not be negligible.  For instance, when fired, the grains will be burning, thus getting hot and getting smaller.  Will their loss of strength be compensated by their reduced mass?  Don't know.  Is the hollow cylinder of a Bates grain weaker than the solid cylinder I used in these tests?  Conventional wisdom suggests no, that the hollow cylinder should be stronger, if anything, but I have not tested this.

And the way I put weight on, slow and gradual, is not exactly rocket-like.  A sudden shock can break an otherwise-strong component.  I think of glass, which can be incredibly strong when gradual weight is applied, but which shatters when dropped a few feet.

Thanks for enduring my drivel so far.  I look forward to hearing any suggestions, comments, criticisms - even praise is welcomed.

Jimmy Yawn