Any reason you use CA glue to tack rather than the epoxy, is it just faster? With a 14”x0.2” fin root I don’t know if I would want to give up that epoxy surface area.
Tony
O3400 Min Diameter L3
- Thread starter airforcetp1991
- Start date
Help Support The Rocketry Forum:
Any reason you use CA glue to tack rather than the epoxy, is it just faster? With a 14”x0.2” fin root I don’t know if I would want to give up that epoxy surface area.
Tony
Rocket dreamer
Member
- Joined
- Mar 14, 2021
- Messages
- 16
- Reaction score
- 5
I’ve not flown to Mach yet but am about to start a project that will go Mach 2+. My question is, with the loctite epoxy having a temperature range of 300 degrees, is that temperature that it can withstand or curing temperatures. I ask because I’ve read a lot about fin and nose cone temperatures far exceeding that temperature at Mach 2-3+ flights. I will add that I don’t think there’s anything wrong with pushing boundaries as long as you have an experienced mentor to guide you thru the process. I like it.
You would have to look at the manufacturer website but the epoxy will start to lose strength as the temperature increases. My understanding and experience has shown that while there is heating at high speed it is concentrated most on the nose cone tip and the fin leading edges. Additionally, the time spent at these high speeds is generally not more than a few seconds and for the fin leading edges they will be behind 2 or so oblique shocks and so will experience a lower Mach number than the free stream. If you use a metal nosecone tip you shouldn't have to worry about the heating on the nose in most cases. This was my fin attachment for my 54mm MD build. It is using Rocketpoxy for the attachment to include fillets and the fins are 0.1875 G10. The rocket hit Mach 2 in 2 secs so a relatively low altitude (higher stress) for that speed and the only difference in how the fins looked after flight was some small paint scratches from the gentle landing. The paint is 4 coats of Rustoleum 2000 deg grill paint.
I was part of a discussion on another thread that talked about heat soaking the aft portion of the airframe due to motor case/nozzle heating. That is certainly a valid concern and so I would not want an epoxy with only a 100 degree temp rating but I would think 300 deg would be sufficient and if you aren't doing MD then that wouldn't be a concern at all.
-Tony
Attachments
Nytrunner
Pop lugs, not drugs
I think "4 fins is more stable" for extreme flight profiles is a misnomer. 4 fins make a symmetric side profile of the rocket, reduces roll induced by sideslip, and can decrease the possibilities of fecal turbine interactions in the middle of a flight where there are other factors attempting to rekit your rocket at the same time.
Additionally you don't flood the whole root with CA, just a couple minute dabs will hold it (held still, not held tight. Don't flick it for no reason)
I agree, there are certainly advantages, all of the missiles we employ (AMRAAM, AIM-9) are 4 finned and while a lot of that most likely is due to guidance requirements stability is also a huge concern. The roll aspect is also something I have thought a lot about. My biggest concern would be a rapid roll rate combined with come asymmetry in the mass distribution along the longitudinal axis causing procession (I think that is the right term) and the rocket developing a large angle of attack as a result. My current thought on mitigating this is to be as precise as possible in attaching the fins straight.
For the CA glue "tacking" do you apply epoxy along most of the root around the tacking dots of CA glue so you get the best of both worlds? What I have done in the past is used a layer or 2 of tape on the fins to make a snug fit with the fin alignment guides (If needed) and then use epoxy on the full length of the root with a very small "fillet" on the sides just to make sure the whole root is covered. I will then go back later and add the full fillets.
-Tony
Attachments
Nytrunner
Pop lugs, not drugs
This is ultimately a preference and process thing. Folks prefer superglue so they can get rid of the fin guides quicker. If your timeline allows you to leave the guides on for tacking epoxy to set, there's no reason why not.
Hey, it's rocketry. When stating you'll do something one way, you'll immediately be provided 3 other often contradictory ways to accomplish the same thing which must be discussed at length before the thread returns to baseline
Great project, I hope it works well! I've got my own plans for the O34k that are a little different, but this should be real good
Faster, use zip kicker. You can always epoxy it on and just use the CA on the ends to hold while epoxy dries.
This is ultimately a preference and process thing. Folks prefer superglue so they can get rid of the fin guides quicker. If your timeline allows you to leave the guides on for tacking epoxy to set, there's no reason why not.
Hey, it's rocketry. When stating you'll do something one way, you'll immediately be provided 3 other often contradictory ways to accomplish the same thing which must be discussed at length before the thread returns to baseline
Great project, I hope it works well! I've got my own plans for the O34k that are a little different, but this should be real good
That makes sense, that is always one of the most nerve racking parts of the build for me. If that fin guide shifts in any way you are screwed so I don't hate at all the idea of getting the guides off sooner and have the fin glued in place. I check the build every 20 mins it seems like and look done the long axis as if I could detect a tiny shift..
For this build I plan to use Wildman G10 guides and anchor them to a board so that I have a more rigid fin guide single unit rather than 4 separate guides. -Tony
Before I found out about zip kicker, I just sat and watched them dry
I would have to go back and look, but are you using Wildman's tubing? I thought his guides were sized for his tubing???
I would have to go back and look, but are you using Wildman's tubing? I thought his guides were sized for his tubing???
Before I found out about zip kicker, I just sat and watched them dry
I would have to go back and look, but are you using Wildman's tubing? I thought his guides were sized for his tubing???
Probably everyone's worst nightmare is to walk out an hour later and see the last fin all messed up...
Yes I was using Wildman tubing for the 54mm MD build and I plan to use it for the L3 as well. For the 54mm build though the Wildman fin guides were sized for 0.125" fins (If I remember right) not the 0.1875 that I was using so I used some Apogee fin guides. The 98mm guides from Wildman are sized for their tubes and 0.1875 so they should be good. That said the 0.2" Dragon Plate may take some slight sanding of the guides to make it work but I would prefer that over a loose fit obviously.
-Tony
blackjack2564
Crazy Jim's Gone Banana's
If u want the best bond, just tack 1 fin on with CA, only using a drop or2.
Using the one fin to hold alignment guide in place
After set, then glue remaining fins on with epoxy, when they are cured, remove that 1st fin, clean off the Ca and re-attach with proper epoxy
A better way for a better bond.
Edit PS.. U can build L-3 rocket in 24 hrs ready to fly....IF U have the skill set.
I built 6....2 -6in Wildman....2-3in Wildman ...1-4in Wildman and a 6 in Gizmo all in 5 days PAINTED and ready to fly.
By the way I personally would never drill holes or rough up my bond areas to the extreme on an Xtreme rocket. Properly using aero-space glues is all that's needed... CTI flew one with fins just glued on...no T-T with Hysol that went over M-5. BUT u must really know what U are doing to achieve this.
Using the one fin to hold alignment guide in place
After set, then glue remaining fins on with epoxy, when they are cured, remove that 1st fin, clean off the Ca and re-attach with proper epoxy
A better way for a better bond.
Edit PS.. U can build L-3 rocket in 24 hrs ready to fly....IF U have the skill set.
I built 6....2 -6in Wildman....2-3in Wildman ...1-4in Wildman and a 6 in Gizmo all in 5 days PAINTED and ready to fly.
By the way I personally would never drill holes or rough up my bond areas to the extreme on an Xtreme rocket. Properly using aero-space glues is all that's needed... CTI flew one with fins just glued on...no T-T with Hysol that went over M-5. BUT u must really know what U are doing to achieve this.
Last edited:
After some great discussion and Fin simulation help from @ether it looks like the span I choose was not going to make it based on flutter even with CF fins. Rerunning my sims a 4 fin design with 0.2" CF looks like it is more realistic based on trying to avoid adding excess length to the build and/or ballast to get the SM I need. Net drag seems to be about the same as well.
Question for the group, is the CTI boattail compatible with the O3400 reload? My understanding from their literature is that the O3400 uses an XL nozzle and the boattail is compatible with the XL nozzle. Thanks again.
-Tony
It's compatible, as long as you use a Gen1 casing (integral thrust ring). The newer Gen2 casings have a finer thread pitch and I haven't seen a tail cone yet for those.
Reinhard
Thank you. I was told the Gen2 hardware would be back in stock around next month so hopefully the Gen2 boat tail will soon follow.
Tony
From what I've been told (by CTI), the current boat tail that is offered is compatible with the gen 2 hardware but you need the adapter ring to make it work (part P98-AR)
Thank you!
plugger
Well-Known Member
- Joined
- Apr 30, 2009
- Messages
- 759
- Reaction score
- 454
I'd have to do some digging but there's a NASA paper out there regarding optimal fin numbers from a stability/drag perspective. IIRC the sweet spot was 6 fins but I could be wrong.
Anecdotally if 3 fin configurations were better for high speed at low altitude unguided rockets when compared to 4 fins I expect NASA's Sounding Rocket Program would be flying 3 fin rockets. Yet from what I can see the entire NASA SRP fleet are 4 finned vehicles.
Edit: added 'at'
Attachments
Last edited:
plugger
Well-Known Member
- Joined
- Apr 30, 2009
- Messages
- 759
- Reaction score
- 454
From my understanding heat soak from the motor case is a complete non-issue as to be certified the exterior of a casing has to stay below 200 deg? I feel like Mark Clark had some relevant comment on this in the past but I can't find it. Regardless, from my understanding case heating is a complete non-issue.
plugger
Well-Known Member
- Joined
- Apr 30, 2009
- Messages
- 759
- Reaction score
- 454
I certainly won’t argue with NASA… From a hobby practical perspective though I think it would be more difficult to properly attach and align 6 fins. I wonder too what Mach range the trade off occurs to where you simply would need the span to be smaller and so more than 4 fins would be needed.
4 fins seems to be the solid answer for this project though I still like the 3 fin look for my smaller builds but that’s totally due to aesthetics.
-Tony
That would makes sense. From what I remember the question from the other thread was specifically about the nozzle heating the far aft portion of the airframe as the nozzle will certainly be above 200 deg. I have no experience with the motor cert process so I don’t know how that is handled.
-Tony
The more I think about this I realize it doesn’t make sense to drill or serrate the fins. I didn’t really understand the process as much as I thought. First because it is still going to be the tube peel strength that is the limiting factor, I can’t imagine a fin being pulled out of a properly bonded fillet. Second, the more I have learned about the fin flutter considerations I would not want to create any place for a stress concentration to form which I fear could happen around holes drilled in a fin.
For those with FinSim experience I’d like a sanity check of my methods. First I am looking at using Dragonplate Ultra-High modulus 3/16” for the fins. For the sims my method has been to ensure the flutter speed is below the max free stream Mach expected. Do any of you plan on the Mach experienced by the fin being less than the free stream due to forward oblique shocks or do you use the conservative assumption of the free stream Mach? Also, do you run Sims with a reduced span of .25 or .5 inches to approximate the effects of the fillet?
None of this is meant to talk about a fincan type design which is very much a different solution.
-Tony
Have you thought about slotting the airframe and inserting the fins into the slots? That's how I do mine. You could choose to alternate between in the slot and on the surface if you like, something like I did on the Nike Apache. It helped set the fin depth in the airframe in my case.
https://forum.ausrocketry.com/viewtopic.php?f=6&t=5019&start=45
That would get you actual tensile and compressive strength of the epoxy working in your favour, rather than just peel.
Consider that my fincans are in tension, due to taking the motor thrust to the frame at the forward end, so any weakening of the hoop strength at the aft end is of no concern.
https://forum.ausrocketry.com/viewtopic.php?f=6&t=5019&start=45
That would get you actual tensile and compressive strength of the epoxy working in your favour, rather than just peel.
Consider that my fincans are in tension, due to taking the motor thrust to the frame at the forward end, so any weakening of the hoop strength at the aft end is of no concern.
- Joined
- Jan 18, 2009
- Messages
- 3,185
- Reaction score
- 1,399
Slotting a FW tube is questionable.
One of my 99% N's ripped the side walls between fins right off my buddies' 4.5" near-MD rocket.
Found the fins with chunks of the tube between fins still attached.
One of my 99% N's ripped the side walls between fins right off my buddies' 4.5" near-MD rocket.
Found the fins with chunks of the tube between fins still attached.
Very impressive build!
I have thought about this but like the post above I would be concerned about weakening the tube significantly. Especially with a 14" fin root, even if I broke up the slot into multiple cuts like your image, I don't think that the 0.06" CF wall in-between fins would have the stiffness to handle such a high speed flight and/or 800# of thrust. Do you always keep your fincans in tension, if so what do you see as the benefits as opposed to having the motor case seat against the aft end of the airfram?
My other thought is that even with the slots the only additional surface are is the wall thickness of the tube. Given a 0.06" wall thickness is that really a consequential amount of bonding area? My biggest concern on a high speed flight like this would be the fin peeling from the tube in the forward portion of its root and I don't know that the bonding area of the slots would reduce that chance. There could also be flutter concerns if the stiffness of the airframe is compromised by cutting the slots. Just my thoughts.
-Tony
Here is a comparison of the flutter analysis with and without a fillet (approximate). I am new to using this tool so please feel free to throw BS flags so that I can hopefully learn the easy way and not at Max Q...
-Tony
DAllen
Well-Known Member
I like the way 4 fins look better than three. Therefore I avoid unneeded aerodynamic analysis to determine if 3 or 4 are better since I already know the answer.
“If it looks right then it flies right” can’t argue with that!
That is how I always build. The length of the airframe in compression is shorter. According to Mr Euler a shorter column is much more difficult to break, and the law is a square law (1/L^2). So having only half the length of the rocket in compression means that section can support roughly 4x the loads before buckling.
https://en.wikipedia.org/wiki/Euler's_critical_load
This also keeps the back end of the rocket in tension, which is where composites have better properties
.
So the increase in buckling strength for the airframe forward of the motor mount makes sense to me, have you seen many rockets fail due to CF buckling?That is how I always build. The length of the airframe in compression is shorter. According to Mr Euler a shorter column is much more difficult to break, and the law is a square law (1/L^2). So having only half the length of the rocket in compression means that section can support roughly 4x the loads before buckling.
https://en.wikipedia.org/wiki/Euler's_critical_load
This also keeps the back end of the rocket in tension, which is where composites have better properties
How fast have you flown this type of “fincan”? I’m interested to know how the stiffness of the airframe in the fin section is effected and if it would be a concern at higher Mach for flutter.
What kind of motor mount do you use and how do you attach it? I have used the Aeropack MD retainer several times and I like it but I don’t know that I would want to put 800# of thrust through it for over 4 secs.
-Tony
Similar threads
