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@xxxxxxxxxxxxx]
*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