Nytrunner
Pop lugs, not drugs
I don’t know that I would want to put 800# of thrust through it for over 4 secs.
More like 1000lb for 4s
What's the basis for your concern? Retainer structural failure? Adhesive bond failure?
I don’t know that I would want to put 800# of thrust through it for over 4 secs.
None personally, but I am sure there have been some fail. Most airframes are thicker than they need to be IMHO. Not necessarily a bad thing in most cases.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 particular rocket flew to M2.14 successfully. I have had a MD 54mm F/G go to M1.8 and it had no tip-tip, and the fin slots were the entire length of the fins.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.
The Aeropack-style retainers are glued into the tube. I design with enough surface area to take the maximum peak thrust of any motor I can fit. If you have enough surface area, calculable using the epoxy data sheet, it works reliably. I use a safety factor of around 1.5 in that part of the design.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.
My concern would be adhesive failure. My method up until now with the Aeropack MD retainer has been to position it such that the motor case “pushes” through the aft and of the airframe and the retainer is there to keep the motor in place during drogue and more so main deployment. My thinking is that even during a very aggressive opening shock the load will be transmitted through the shock cord to the eyelet in the retainer then through the aluminum into the motor case. I make sure the retainer is epoxied well but my thinking is that this bond should not ever experience that much force. Does that check?More like 1000lb for 4s
What's the basis for your concern? Retainer structural failure? Adhesive bond failure?
If you are concerned you can place a piece of coupler inside the airframe but directly above the MD coupler. I have been known to do that on bigger rockets just to provide the MD retainer a bit more support.
What epoxy do you normally use? Do you thin it usually? With a 98mm tube there is certainly no shortage of surface area for epoxy. When I say I am more concerned with adhesive failure I meant more so that I wouldn’t be able to sufficiently get the epoxy between the tube wall and retainer.You will be surprised how much force epoxy can withstand in shear. Run the calculations based on the data sheet to indicate how much surface area is actually required. It is likely less than you think.
Any time you thin epoxy it gets weaker.What epoxy do you normally use? Do you thin it usually? With a 98mm tube there is certainly no shortage of surface area for epoxy. When I say I am more concerned with adhesive failure I meant more so that I wouldn’t be able to sufficiently get the epoxy between the tube wall and retainer.
You will be surprised how much force epoxy can withstand in shear. Run the calculations based on the data sheet to indicate how much surface area is actually required. It is likely less than you think.
I use West Systems 105/206. No thinning.What epoxy do you normally use? Do you thin it usually? With a 98mm tube there is certainly no shortage of surface area for epoxy. When I say I am more concerned with adhesive failure I meant more so that I wouldn’t be able to sufficiently get the epoxy between the tube wall and retainer.
This project is 'way outside my experience so I'm probably misunderstanding, but -- don't you want to keep flutter speed *above* max free stream Mach expected?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?
This project is 'way outside my experience so I'm probably misunderstanding, but -- don't you want to keep flutter speed *above* max free stream Mach expected?
Would changing your fin shape help you accomplish that? A clipped Delta might be slightly more flutter resistant.
The "Safe Launch Practices" was just updated to reflect 100,000 ft. so maybe an update is in the works in the High Power Code which the other document is based off of?Interesting. The safety code on the Tripoli site still shows 50k flights requiring committee approval.
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
View attachment 458631
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
This is the material that analysis was attempting to simulate. (110 msi option on the drop down). Have you heard of/seen this used before?I don't think you're going to get a modulus of 20 msi with a plate product. The individual plies might have a modulus of 20 if they're made from a unidirection tape, but the effective modulus of the laminate is going to be closer to 9 than 20.
Agreed, although my plan is to have the motor “push” from the aft end of the airframe and use the motor retainer as only a motor retainer so it should not have to experience the thrust load.If you’re concerned about the epoxy bond for the retainer, try drilling and tapping into the retainer after it’s epoxied in. Then use mechanical fasteners to keep it in place in addition to the epoxy.
Yeah that makes sense. Use the thrust ring for its intended purpose.Agreed, although my plan is to have the motor “push” from the aft end of the airframe and use the motor retainer as only a motor retainer so it should not have to experience the thrust load.
-Tony
It certainly isn't the only solution but I find it to be the easiest for me.Yeah that makes sense. Use the thrust ring for its intended purpose.
. . . my philosophy behind doing this as a L3 project is that if I intend to fly a rocket like this as a L3 why should it not have the higher level of review and approval required of a cert flight.
That's a great point - mentors are great to have!The Technical Advisory Panel Charter has as the first item listed under Scope:
1. Members of the Technical Advisory Panel shall be appointed by the TAP Chairperson to provide technical assistance and guidance to Tripoli Members desiring to design, construct and fly rockets having a high or total impulse in the M, N and O ranges and Certifying to Level 3.
People don't need to make a certification attempt to get that level of input on a project. I think it shortchanges both TAPs and flyers to see the TAP's involvement mainly as sign-off approval pre- and post- cert attempt.
Yup. For sure.Finding a mentor on high velocity, multi-stage, high altitude, or high impulse flights is incredibly difficult in this hobby for many reasons.
I looked over your file and I agree 100%. I simmed several other materials and I think you'll be just fine with the regular stuff. Carry on!After some great discussion with JB at Dragonplate it seems as though their Ultra-high modulus productive doesn’t make the most sense. The fiber orientation combined with the very high cost would basically double the fin price all to allow for a 3/16” vs 1/4” fin thickness. To me .06”, while certainly meaningful at these speeds, isn’t worth that much money and is likely starting to push the margin for error in the RASAero sims. The 0.25” quasi-isotopic allows for a flutter Mach of 3.77 even without any allowance for fillets, this plate is also rated to 250 degrees.
-Tony
Thank you again for the help understanding the composite strength considerations!I looked over your file and I agree 100%. I simmed several other materials and I think you'll be just fine with the regular stuff. Carry on!
Tony, great work. Impressive use of the new FinSim.View attachment 458631
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
Tony, very impressive.View attachment 458055
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
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