Stop me if this has already been tried and found wanting... But I
gather an EPDM liner is generally superior but a pain to install,
especially at smaller diameters?
I see that EPDM is available as tubing. Perhaps start with a piece of
EPDM tubing of slightly smaller diameter and slightly greater length
than the metal tube to be lined. Seal both EPDM tube ends, with a
shrader valve (or similar) installed in one. Coat the EPDM tube
exterior, the metal tube interior, or both with a suitable adhesive -
it'd need to be relatively slippery long enough for the next step.
(Possibly add solvent, which might also help the EPDM expand to the
metal tube diameter without residual tension?) Then pull the EPDM
tubing through the metal tube. Pressurize the EPDM tubing to expand it
firmly against the metal tubing. Maintain that pressure till adhesive
sets, trim the ends, done.
Assuming there aren't already known show-stoppers, I expect it would
still take a certain amount of cut-and-try on someone's part to get this
working reproducibly. But given the current procedures I've seen
described here, it still might end up as an overall simplification.
Henry
On 4/24/2018 10:27 AM, William Claybaugh 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 <mailto: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 <mailto: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 <mailto: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>
[mailto:arocket-bounce@xxxxxxxxxxxxx
<mailto:arocket-bounce@xxxxxxxxxxxxx>] *On Behalf Of
*William Claybaugh
*Sent:* Tuesday, 24 April 2018 1:52 PM
*To:* arocket@xxxxxxxxxxxxx <mailto: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____
__ __
__ __