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/ [1]
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(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