Well, the hobby grade liners typically have the spirals on the outside
exposed. I typically sand into those, but yes, there are some voids. It
is just how it is when you are paying the hobby prices for liners.
I think the grease just fills voids and provides an in-compressible fluid
to help support the liner.
I've spin cast both EPDM and HTPB, HTPB needs degasing and a high spin rate
(as fast as my wood lathe goes). EPDM I've had better results when it is
slowly rolled (5ish RPM) so it evenly coats everything. Otherwise I've had
it come out wavy - it agglomerates on one side.
Edward
On Tue, Apr 24, 2018 at 11:27 AM, William Claybaugh <wclaybaugh2@xxxxxxxxx>
wrote:
Ed:
Interesting.
How do you know that there are no voids on the liner surface? I’m
thinking that surface grinding the OD of the liner (which the manufacturer
does anyway) should get a void free surface. The issue then becomes—as
Korey observed—the out of roundness of the tube.
In theory, a limited amount of flexibility should allow the phenolic to
conform to the tube. For these 9” OD tubes (worked to + / - 0.010 on the
ID) it appears that the 8.480” nominal OD phenolic is a very tight fit (I
just started fitting the phenolic this morning).
I’ve seen folks use grease but always thought it was for getting the
phenolic in and out; it seems unlikely it would provide enough support to
prevent cracking of the liner.
I like the parchment paper idea; I will give it a try if there proves to
be any room.
Bill
On Tue, Apr 24, 2018 at 11:07 AM Edward Wranosky <edwardcw@xxxxxxxxx>
wrote:
In HPR rocket motors some people slather an excess of grease on the
outside of the liner to fill the gap (if present) between the liner and the
casing. I prefer having to sand down the liner and using some elbow grease
to get it into the casing over having a gap. I got tired of small diameter
liners cracking (1.5"-3" diameter motors) so I typically build the liner up
with epoxy, sand down and then apply single wrap of parchement paper so it
doesn't stick to the casing.
Edward
On Tue, Apr 24, 2018 at 10:58 AM, William Claybaugh <
wclaybaugh2@xxxxxxxxx> wrote:
Troy:
I agree that getting (cool) gas to the back of a phenolic liner is
crucial. I burnt through three very thin (0.050”) walled 2.5” diameter
motors until I worked out a labyrinth gas path that cooled the gas against
the bulkhead mass before letting it pressurize behind the phenolic.
On the whole, I’m not a big fan of phenolic for insulation exactly
because it requires gas on the motor tube wall. But for very small
diameters, it is easier than laying in epdm sheet....
Bill
On Mon, Apr 23, 2018 at 10:35 PM Troy Prideaux <
troy@xxxxxxxxxxxxxxxxxxxxx> wrote:
To me, your methodology – if enough scrutiny and care is taken – is
sound enough. There should be enough stretch (I know phenolic isn’t exactly
the most flexible of materials) to accommodate that level of clearance over
a large diameter although I’ve never used a ¼”(ish) thick liner before so…
In smaller motors, this issue is problematic for designs that
completely seal the liner with o-rings or whatever sealing so that no
chamber gas is ever exposed to the outside of the liner. Designs that allow
the gasses to pass to the outside of the liner (where the sealing is done
against the motor casing) tend to not be so susceptible to liner cracking.
Not sure how common or sound that approach would be for larger motors.
Troy
*From:* arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@
freelists.org] *On Behalf Of *William Claybaugh
*Sent:* Tuesday, 24 April 2018 1:52 PM
*To:* arocket@xxxxxxxxxxxxx
*Subject:* [AR] Solid rocket insulation, yet again
I had a nice visit w/ Kory Kline over the weekend. Whilst looking over
my two-stage hardware he noted that because extruded aluminum tubes are
always out of round, he and found that phenolic liners failed by cracking
into the inevitable gap upon ignition.
His solution to this issue was to spin cast a liner of htpb mixed w/
carbon black and then cast a monolithic grain into that lined tube. This
seems to me clever; he suggested that it does work.
My own approach has been to straighten the tube until the bulkhead will
pass through it, end to end. This leaves a maximum of 0.010” of clearance
on the phenolic liner (which has the same OD as the bulkhead); that has not
proven to be a problem in a total of eight firings at 9” diameter.
Are there other solutions to the extruded tube out of roundness issue?
Bill