[sugpro] Re: KNSB sintering/casting in an oven

From: Lincoln Ross <lr21643@xxxxxxxxx>
To: sugpro@xxxxxxxxxxxxx
Date: Thu, 7 Mar 2024 13:47:41 -0500

I looked into hydraulic jacks. They're actually pretty inexpensive, though
I don't know if ones with gauges on them are. Still wondering about the
coefficient of thermal expansion. I wonder if it's similar to that of
regular rocket candy? Of course, I don't know that either. Maybe I'll have
to make a pellet and measure it instead of doing a test burn. I don't have
wonderful test equipment, though. Maybe I can stack a bunch of pellets and
use a 6 inch caliper. That might be a little better than a 1 inch chunk and
a 1 inch micrometer. Hmmm....

On Sun, Feb 25, 2024 at 4:20 PM Lincoln Ross <lr21643@xxxxxxxxx> wrote:

I appreciate your information, thanks again for your answers.

If the deformation is all in the steel, that makes things easier, not
harder, because the propellant won't spring back much.

You were working with 100 or more times as much propellant. I'm only
working with a couple of grams. I think, with a known amount of energy,
reasonable engineering solutions are possible. Just have to find a way to
absorb 700 foot lbs, or whatever amount it is. Remember that, unless I
misunderstand c* it's comparable to the energy in five .22 bullets. One of
those can be stopped with a few cubic inches of sand. If I misunderstand
c*, I hope someone will correct me.

Your experience leads me to think that a split mold may be the right
approach. Just so there are enough high grade bolts to take the load. If
you propped your mold over a hole, were you able to push out the propellant
with the hydraulic press? Did that just shred it?

As far as the vertical force goes, you may have noticed that there are
several holes in the bottom of the mold I designed. That's so it can be
bolted to a plate, to hold it down.

Notice that I'm planning as if the propellant WAS going to explode, not
just hoping that it won't. BTW, if the threads are slathered with
anti-seize, can a  tiny speck of propellant still go off? I should think it
would have a problem getting hot enough.

Thanks again.

On Sun, Feb 25, 2024 at 2:23 AM Paul Kelly <tech@xxxxxxxxx> wrote:

The deformation iss all in the steel. That's why that die for the 38mm
grains is so massive,

There's no real way to make the dies explosion safe given that you'd
still be looking at a release of hot gas at >> 25000 psi that being the
normal operating pressure of the setup.

I did hone a VERY slight taper into some of the dies. The problem this
causes is that it generates a vertical force on parts of the tooling that
needs to be reacted by something. Even if you had just a 1% taper you would
be dealing with more than 200lb of vertical force. In practice a 1% taper
locks just about as well as no taper. Surface finish is the key.

I earnestly encourage you to not use threads to generate the force for
this application. Not the least because it implies that you will be close
to the apparatus.  Hydraulics are pretty good. Done right, nothing happens
quickly and they are implicitly low friction.
Just one spec of propellant ( and the stuff gets everywhere ) on a thread
has a high likleyhood of ignition. I had a school buddy who lost his thumb
screwing a pipe bomb shut.

PK

------ Original Message ------
From "Lincoln Ross" <lr21643@xxxxxxxxx>
To sugpro@xxxxxxxxxxxxx
Date 25/02/2024 10:05:25 AM
Subject [sugpro] Re: KNSB sintering/casting in an oven

As far as the problem with removing fuel, do we know the elastic modulus
and coefficient of thermal expansion of the pressed rocket candy? I
couldn't nail down a number for the CTE of potassium nitrate, let alone
that of rocket candy, but I have the impression that the CTE of the former
is much higher than that for steel. I found a paper with different values
for CTE, depending on what orientation of crystallized KNO3 you were
talking about. But one direction, apparently, had a far higher CTE than the
other directions. I don't know how this translates into bulk CTE or what
the effect of sugar would be. I'm guessing that the elastic modulus of
pressed rocket candy is lower than that of steel, so that it's more
important to this problem than the CTE of steel. Anyway, if we knew these
numbers, we could figure out the strain we had to relieve by changing the
temperature.

Another approach might be to line the mold with something that melts or
softens at a low temperature. I suppose mercury is a bad idea, and Woods
metal has cadmium, but there are alternatives which aren't
especially toxic. Maybe even West epoxy, unless there's some other epoxy
which has a worse (i.e. lower)Tg. Hmmm... maybe at higher temperatures less
pressure would be required..

Did you guys play around with tapered molds? Seems like that might ease
the required forces quite a bit.
-------------------
I wonder if it wouldn't be possible to make a mold that could safely
withstand an explosion. Is there a way to derive the pressure that
propellant that was trying to explode produced?  Is characteristic velocity
what you'd get if all of the thermal energy of combustion was turned into
kinetic energy? If that's true, I think we're talking about maybe 700 foot
lbs of energy from a couple of grams of propellant. For reference, I think
that's comparable to the muzzle energy of 5 .22LR bullets, which,
collectively, might have a gram and a half of powder in them. So we're not
necessarily talking about a global disaster here.

If we want to absorb that energy, a crushable piston might only have to
yield an inch or an inch and a half, but I don't know what the ideal,
crushable piston material would be. Alternatively, house the whole mess
inside a bigger pipe, and turn the cap to tighten with some kind of wrench
from outside that bigger pipe.

On Sat, Feb 24, 2024 at 6:15 PM Bruce Beck <bbeck7@xxxxxxxxx> wrote:

The pharmaceutical technology is rather advanced in producing pressed
pellet pills and has a lot of machinery for doing it. The presses they use
have been empoyed to produce ignition pellets for solid propellant
ignition.ALCLO and KNO3/boron pellets come to mind.

On Sat, Feb 24, 2024 at 2:20 PM Paul Kelly <tech@xxxxxxxxx> wrote:

No heat just pressure.
At first glance, the tooling may seem a little complex, but, as you
have found, you need to be able to press the grain off the tool and out of
the die....

A guy by the name of Jack that I flew with many years ago used pressed
candy to light U/C valve hybrids. Which is impressive if you've ever seen
how damp they get inside as they chill.

PK

------ Original Message ------
From "Shawn McHatten" <shawn.mchatten@xxxxxxxxxxx>
To sugpro@xxxxxxxxxxxxx
Date 25/02/2024 5:06:47 AM
Subject [sugpro] Re: KNSB sintering/casting in an oven

Very interesting work. I looked at this at a very small scale a few
years ago by trying to make small pellets. Like peppermints. But had little
success because I could not get them out of the mold. However I thought
sintering implied pressing AND heating. Are you heating any of this in an
oven or just the heat from the pressure.
thanks
Shawn

On Sat, Feb 24, 2024 at 4:14 PM Paul Kelly <tech@xxxxxxxxx> wrote:

Thanks for your response. It's not something I would just jump in and
do. I'd investigate to determine what would happen if it did go off. Maybe
even setting off test charges several feet underwater or buried in the
ground. Having the propellant be only about half an inch in diameter ought
to make things a bit more manageable, too. We're only talkng about two to
four grams of propellant here. I might think of a way to do this from a
distance.  As for keeping the halves together, I've already calculated
that
I'd need either very high grade bolts or absurdly large ones. Judging by
your picture, I imagine you broke a bunch.

I started by looking at what the pyrotechnic guys do. They press a lot
of comps. But this is different.

My advice would be to start with small diameters. At around 38mm (31mm
grain diam) elastic deformation of common steels becomes a problem because
the die expands under pressure then shrinks down onto the solid grain
making it hard to press out without cracking. Surface finishes on the
tools
are important for this reason.
the tooling for the 24mm motors was a little less OTT.

In any case, I find your discovery fascinating.

It's pretty cool,  but I can't claim credit. Bill Colburn once
mentioned that he had had a grain f pressed candy sitting on a shelf for
years and it was still bone dry. I just filed that away for future
reference..

PK

On Sat, Feb 24, 2024, 12:52 PM Paul Kelly <tech@xxxxxxxxx> wrote:

What you are describing will probably kill you.

One tiny amount of propellant in thosethreads and the friction of

doing it up will ignite everything.

When pressing I had blast shields in place and a remote control that

let me stand on the other side of a concrete wall.

I tried split molds
And this pic illustrates the problem
There's almost no way to keep the halves together at the pressure you
need. I tried large steel sleeves with driven wedges and many other
things.

For the 38mm grains, I ended up pressing at around 12t and this was
my die
That hole is about 31mm diameter and the die is made from 4140 steel.
When you get it right the propellant feels like porcelain.

------ Original Message ------
From "Lincoln Ross" <lr21643@xxxxxxxxx>
To sugpro@xxxxxxxxxxxxx
Date 24/02/2024 9:33:06 PM
Subject [sugpro] Re: KNSB sintering/casting in an oven

I continue to be interested in this pressing thing. I drew up a half
block, such that two could be used to mold a Jetex style and size
pellet. Each had 4 blind threaded holes (which I might be able to get
down to 2), two through threaded holes, two plain through holes, and
a
semi-cylindrical trough. More than $850 for the pair, in mild steel,
shipped, from Protolabs. (I sometimes bounce my ideas off their
automated quote system to reality test the amount of money). It's
never
been worth it for me privately, but they did some cool stuff for me
at
work in the past. The neat thing here is that one wouldn't have to
press
the pellets out of the mold. I suppose a very slightly tapered mold
might also make extraction easier.

mold block thingie

After that price shock, I had another idea. I looked around the net
and
it turns out that if you tighten down screw threads, you can generate
big forces. So may be a 1/2 inch schedule 80 steel pipe nipple with
two
caps, used as a mold, with a rod to compress the propellant when the
caps are screwed down. When the rod got too short, one could insert
washers to give more travel. I don't know if schedule 80 is enough
for
25 kpsi. But there are bigger, more expensive pipe sizes that would
be,
I think. Then one still has to press the completed grain out, which
is
apparently a problem.

BTW, I'm guessing you can't just pound sugar fuel that hard without
setting it off? Because that's probably the easy way.

On Thu, Feb 22, 2024 at 8:19 PM Lincoln Ross <lr21643@xxxxxxxxx>
wrote:

Mr. Kelly's method sounds pretty interesting. I suppose one could
use a split collar tube joiner to make it easier to extract the fuel.
This
one's just over 38 mm, i.e. 1.5":
https://www.ebay.com/itm/114322448156
I wonder if they have any like that but at 1/2 inch diameter. I also
wonder how hard you'd have to hold it down so that the fuel didn't
squish out underneath, and how close the fit would need to be. I think
the
collar itself might be strong enough, but the bolts had better be really
high grade.

If the components "turn liquid", does that mean they don't have to
be finely milled?

On Tue, Feb 20, 2024 at 2:32 PM Bdale Garbee <bdale@xxxxxxx> wrote:

William Schmiedlin <wschmiedlin@xxxxxxxxx> writes:

Another experiment would be to press the powder (much like BP
rocket motors).  Not sure how much pressure you would need, or if

you could

make it stick together well enough to survive a firing.

Paul Kelly in western Australia was playing with this in 2019.  In a
multi-topic email in which he included a photo of his hydraulic shop
press, he said:

  One of the problems with candy propellant is that its
hygroscopic. It
  turns out that when you press it instead of melt it, and by
"press" I
  mean north of 25000 PSI both the oxidiser and the fuel turn
liquid and
  you get really, really fast propellant that doesn't soak up water.
  That's a 40000kg press and, at 15000kg it will make 38mm grains.
The
  stuff comes out looking and feeling like a ceramic...

In later messages he mentioned that a big problem had been how to
get
the grains out of the casting structure once pressed.  That led me
to
think a lot about a suitable casting structure.  Then the pandemic
hit.

Note Paul's "really fast" comment.  It's easy to believe this is a
fundamentally different propellant than a thermoset mix, ergo why
folks who play with it without understanding that might get CATOs.
A
repeatable pressing process and full characterization would be
required
to make using this approach practical.

Anyway, another one of those "this might be fun to try one day"
things
stored away in the back of my mind.

Bdale

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