Bill,
That makes more sense. I'll refrain from giving much for "actual
recommendations" as I've never run a motor in your configuration
before. Though I'd imagine sealing the gap with EPDM rubber, an
ablative epoxy mix (epoxy and phenolic microspheres) or ablative of
your choice would be sufficient for something like that. Might work
better to seal the joint prior to casting your grain in the liner, as
you could put a bit of an ablative cap on the inside to shore it up a
bit better; and give you a little more safety margin for gasses trying
to get by.
- Wyatt
On Thu, Aug 23, 2018 at 4:03 PM William Claybaugh <wclaybaugh2@xxxxxxxxx> wrote:
Wyatt:
There are no grains to glue to the liner, the propellant is poured and cast
directly against the liner. Thus the concern about a backface ignition at the
joint.
The liner fit to the tube wall is w/i 0.005”; I will use grease (mostly for
ease of insertion and removal).
Bill
On Thu, Aug 23, 2018 at 5:56 PM Wyatt Rehder <wyatt.rehder@xxxxxxxxx> wrote:
As far as I know that is standard practice on longer motors, just
having the joint be in the center of a grain.
You can also glue the grains into the liner for some additional
sealing between the grain and the liner. You hopefully shouldn't see
much gas between your liner and the tube, so long as you have a decent
fit between the two. The gas in the core should expand your liner out
to fit against your case, as the case is the load bearing element. If
you have your seal between your grain and your liner good enough there
shouldn't be a path for gases to travel. Some grease on the inside of
the case might also discourage your hot gas flows a bit.
-Wyatt
On Thu, Aug 23, 2018 at 2:39 PM William Claybaugh <wclaybaugh2@xxxxxxxxx>
wrote:
My 9” motor uses two pieces of phenolic tubing 34.75” in (nominal) length
as liner for 67.5” of case bonded propellant. This means that the joint
between the two pieces of liner is exactly at the center of the propellant
grain.
Given that some hot gas will—intentionally—fill any voids between the
liner OD and the tube ID, it follows that such gas could ignite the back
of the propellant grain at the joint unless that joint is sealed.
Is there an experience in sealing such a joint against hot (or, at that
location, probably just warm) gas?
Thanks,
Bill