If the burn time is under 5 sec steel might even work with a graphite
throat insert for low cost ots materials.
K
On Thu, Apr 22, 2021 at 11:26 AM <tom@xxxxxxxxxxxxxxx> wrote:
As I am reading all these answers, my feeling is: go for a graphite nozzle
(although a heck of a dirty job turning one one the lathe), scrub the
thoughts of saving 150g of weight when firing an O motor und reuse the
graphite nozzle 2-3 times depending on the grade of abrasion depending on
fuel used and burn time.
Am 2021-04-22 19:52, schrieb Anthony Cesaroni:
Wrong carbon probably as well. PAN and pitch-based carbon are conductive
and will tunnel the matrix rapidly. Rayon based carbon such as Enka and C2
is what is used in nozzles and ablative structures. It's not thermally
conductive. Epoxy/PAN-based carbon will not work for this application at
all. Epoxy will not pyrolyze like phenolic or retain char at the
appropriate level.
There is only one manufacturer remaining in the US that produces
rayon-based carbon, National Electrical Carbon Products that I'm aware of.
The market was so small that Enka bailed a few years back leaving many
including NASA in a panic. Park Electrochemical et al outsource it and
compound it as prepreg. In the case of C2, under exclusive agreement with
Arian. Expect to pay over $100/lb. for plain weave, rayon carbon fabric and
$250/lb. compounded with SC-1008 phenolic. Silica or quartz phenolic
prepreg is only slightly less expensive to purchase. If you want to try
making your own product with silica and SC-1008, you can save money but
keep in mind that single part resole phenolics have to be molded and
cross-linked under at least 50 psi. and preferably 100 psi or more. It is
possible to vacuum bag cure the laminate, but you will have higher porosity
and lower density than optimum. An autoclave, compression mold or silicone
bladder, pressure molding is typically employed.
Two part catalyzed phenolic is available and can be used for hand lay-up
but its performance doesn't come close to SC-1008 under nozzle conditions.
Anthony J. Cesaroni
President/CEO
Cesaroni Technology/Cesaroni Aerospace
http://www.cesaronitech.com/
(941) 360-3100 x1004 Sarasota
(905) 887-2370 x222 Toronto
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*From:* arocket-bounce@xxxxxxxxxxxxx <arocket-bounce@xxxxxxxxxxxxx> *On
Behalf Of *JAMES ROSSON
*Sent:* Thursday, April 22, 2021 12:38 PM
*To:* arocket@xxxxxxxxxxxxx
*Subject:* [AR] Re: nozzle problem - problem nozzle...
+1 Wrong resin
Need to be using a oven cured phenolic novolac or resol based resin
system. The novalac are solid at room temp and molded at high
temp/pressure. The resol based are available as liquid system.
IMHO - There are several aspects of the nozzle that are improperly
designed beyond resin:
Nozzle design requires careful combination of ablative materials,
insulation materials, and structural materials. Most common old school,
long burn time nozzle fairing is produced via filament winding various
layers of materials. The often contain 2+ layers of different materials
wound on same mandrel. There is an ablative layer in contact with flame
front. There is middle insulation layer. And last a structural layer to
support the pressures, and mounting. Look up the designs on AMARM or HARP
motors for examples. Even the venerable shuttle motor is excellent
teaching aid in the challenges of rocket motor nozzles, and documentation
is easy to find.
Use of graphite fabric is not good. Graphite fabric conducts heat almost
as well as metal. So as soon as the surfaces sees 2000°, so does the
resin, and the adhesive becomes Jello. Graphite fabric is last thing you
want near flame front. For rocket nozzle fibers, the lowest temp fibers
you can get away with are Kevlar (polyamide) materials. But even these
polyamide fibers melt above 700-800°. What you really want is silicate
based ceramic fibers.
We amateurs tend to use solid graphite as; thick section acts like an
insulator, and has low erosion rate. The nozzle exit pressure is supported
by casing. But these can be heavy when mass fraction is important.
Amateurs worry way to much about mass fraction, for all wrong reasons. Did
you run an FEA simulation on how many bolts you needed on end closures? My
guess is used to much hardware, that could be eliminated and the mass used
for nozzle. LOL
Achieving highest mass fraction tends to be very expensive. Very few folks
are willing to spend hundreds of hours engineering the best nozzle, or the
extra 1000's of $$ it takes to achieve lowest mass fraction.
Have flown several "O"/"P" class motors using graphite throat, and
machined fiberglass exit cone. The exit cone had a flange, and acted like
the nozzle insulation washer too. For small 1-2 second burn motors, have
used a stainless steel exit cone. :)
One of the newer technologies used in medium burn time nozzles is
stainless steel or titanium cone, spray coated with thick ceramic coating.
Last but not least: Need to pay careful attention to propellant
chemistry, and more temperatures/pressures. A motor with large amounts of
metal has higher exit temperatures, which needs more insulation and suffers
more erosion. Nitrate based propellants tend to generate complex ceramic
oxides that can be more abrasive than aluminum oxide found in AP/Al motors.
Another design element is chamber and exit pressure. Running higher
pressure and flow rates tends to reduce the exposure time of erosive
particles on exit area. If you really want to understand, try using FEA to
model the heat/pressure exposure of the mass flow.
PS - The suggestion for JB weld is not going to help. JB weld is filled
with aluminum. It conducts heat, and you want an epoxy with filler that
insulates, like silica glass or phenolic micro-balloons.
Best Luck.
Jim
On April 22, 2021 at 12:16 AM Hagen Hübner <hagen.huebner@xxxxxx> wrote:
Hello everyone,
a few days ago we (Forschungsgemeinschaft Alternative Raumfahrt - FAR
e.V.) tested an O-motor with a nitrate-based fuel. The part of the nozzle
near the combustion chamber was made of graphite and withstood the high
temperatures and the abrasive exhaust without any problems. The nozzle
extension - for weight reasons made of carbon fiber reinforced epoxy resin
(tempered, but otherwise standard resin), material thickness 3 - 7 mm -
however, failed after about one second of burning time (see attached
photos).
Question(s): What can be improved to prevent the extension from burning
through in the future? Is there any chance at all chance to make a
weight-saving nozzle extension made of composite, designed for a burn time
of 5 - 10 seconds? Which more temperature-resistant resin should I use?
Which reinforcing fibers are (more) useful? Do basalt fibers have a
significant advantage over carbon fibers?
Regards,
Hagen