Lars:
Larger (than around 2”) drawn tube generally requires a mill run. If
somebody happens to order one there will be extra and that will end up in
inventory at places like TW and Yarde. But—despite ASTM—that is very rare;
I once obtained some 0.050” wall 6” OD Titanium that was surplus to a run
but that was—literally—a once in a lifetime opportunity...I wish I had
bought more....
Bill
On Mon, Apr 23, 2018 at 11:37 PM Lars Osborne <lars.osborne@xxxxxxxxx>
wrote:
Bill,
This might be a stupid answer, but have you considered using drawn tubing?
It will be much more accurate (and straighter). 9 inches diameter will be
more difficult to source, but should be possible to get. TW metals claims
to draw 6061 up to 9", but doesn't appear to have any in stock at this
moment.
If your drawn tube is from ASTM B210, the tolerance is controlled by
tables 12.20 to 12.32 of the linked document. (Note there is also a table
12.0, which is different!) For that size in a heat treated tube the maximum
out of round of the outer diameter is 0.040 in.
https://www.mkmetal.net/images/uploads/Alum%20Extrusion%20Standard%20Tolerance.pdf
Thanks,
Lars Osborne
On Mon, Apr 23, 2018 at 9:35 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