+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
