O3400 Min Diameter L3

The Rocketry Forum

Help Support The Rocketry Forum:

OverTheTop

Well-Known Member
TRF Supporter
Joined
Jul 10, 2007
Messages
5,149
Reaction score
2,436
Location
Melbourne Australia
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?
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.

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.
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.

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.
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.
 

airforcetp1991

Cave Putorium
TRF Supporter
Joined
Nov 22, 2020
Messages
172
Reaction score
73
Location
New Mexico
More like 1000lb for 4s :cool:

What's the basis for your concern? Retainer structural failure? Adhesive bond failure?
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?

Using the MD retainer as a thrust plate (I think I am using that term correctly) seems risky to me with a very large motor. Even with the grooves on the outside of the Aeropack MD I am unsure of how much epoxy contact area there is. You could enlarge the grooves and/or use fillets above and below the retainer, and I do that, but I still weary of it. Thinning epoxy also reduces its final strength from my understanding. That could certainly be my inexperience or just my intuition of the forces and bond strength being off but I don’t see a net benefit for my project though I certainly won’t categorically say it is bad.

I love hearing different methods though, so much learning to be had.

-Tony
 

OverTheTop

Well-Known Member
TRF Supporter
Joined
Jul 10, 2007
Messages
5,149
Reaction score
2,436
Location
Melbourne Australia
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.
 

airforcetp1991

Cave Putorium
TRF Supporter
Joined
Nov 22, 2020
Messages
172
Reaction score
73
Location
New Mexico
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.
Very good point. I plan to do this for another reason as well, to give the AvBay something to positively seat against in addition to the switch band. I suppose that could also help mitigate some of the possible buckling concerns as it would effectively double the wall thickness in that section.

-Tony
 

OverTheTop

Well-Known Member
TRF Supporter
Joined
Jul 10, 2007
Messages
5,149
Reaction score
2,436
Location
Melbourne Australia
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.
 

airforcetp1991

Cave Putorium
TRF Supporter
Joined
Nov 22, 2020
Messages
172
Reaction score
73
Location
New Mexico
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.
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.

-Tony
 

dhbarr

Amateur Professional
Joined
Jan 30, 2016
Messages
7,236
Reaction score
1,652
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.
Any time you thin epoxy it gets weaker.
 

Landru

Additive Aerospace
TRF Sponsor
Joined
Nov 21, 2011
Messages
430
Reaction score
136
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.
Epoxy has an extremely conservative shear of 1000psi, with most being 3000psi or more.

Even if you only attain 1/2" of epoxy around the retainer, that is 6 square inches of bond area for a strength of 6000lb.

Even with extremely conservative estimates you have a 6x factor of safety.

Consider a 98mm composite motor bulkhead has a force of nearly 4000lbs retained by epoxy...
 

OverTheTop

Well-Known Member
TRF Supporter
Joined
Jul 10, 2007
Messages
5,149
Reaction score
2,436
Location
Melbourne Australia
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.
I use West Systems 105/206. No thinning.
 

JoePfeiffer

Well-Known Member
Joined
Jun 15, 2013
Messages
207
Reaction score
161
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?
 

airforcetp1991

Cave Putorium
TRF Supporter
Joined
Nov 22, 2020
Messages
172
Reaction score
73
Location
New Mexico
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?
That would be the most conservative in my understanding as well. The reason I ask is because it is difficult to get flutter speed about about M 3.5 with the stock modulus Dragonplate in 3/16". I would like to use stock modulus to avoid the high cost of their ultra-high modulus product or having to increase fin thickness.

-Tony
 

airforcetp1991

Cave Putorium
TRF Supporter
Joined
Nov 22, 2020
Messages
172
Reaction score
73
Location
New Mexico
Would changing your fin shape help you accomplish that? A clipped Delta might be slightly more flutter resistant.
Yes, that is certainly possible and I am aware that previous builds like this have used that fin shape. Those builds, at least from my research/understanding have used some very advanced tip-2-tip layup techniques for the fin can. I do not have the equipment and more importantly the skills to do that and so I am weary of having a relatively short root length for the fins. My concern is having enough bonding area for the fillets and initial glue attachment. While the fins may be more flutter resistant if the bond isn’t strong/stiff enough it may result in a failure or a lower true flutter velocity.

Anyone with experience on building a non tip-2-tip delta type fin setup like this?

-Tony
 

T34zac

Well-Known Member
Joined
Jul 23, 2014
Messages
944
Reaction score
31
Location
Massachusetts
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
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.
 

AeroAggie

Well-Known Member
TRF Supporter
Joined
Jan 21, 2017
Messages
355
Reaction score
158
Location
DFW
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

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.
 

airforcetp1991

Cave Putorium
TRF Supporter
Joined
Nov 22, 2020
Messages
172
Reaction score
73
Location
New Mexico
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.
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?


-Tony
 

airforcetp1991

Cave Putorium
TRF Supporter
Joined
Nov 22, 2020
Messages
172
Reaction score
73
Location
New Mexico
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.
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
 

T34zac

Well-Known Member
Joined
Jul 23, 2014
Messages
944
Reaction score
31
Location
Massachusetts
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
Yeah that makes sense. Use the thrust ring for its intended purpose.
 

rokit

Well-Known Member
Joined
Aug 4, 2010
Messages
80
Reaction score
17
. . . 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.
Not looking to discourage anyone from attempting something like this, but I'd like to make a point that has larger implications than just for this project.

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.

Good luck on your project. I'm going to keep an eye on this as there's lots of good info here.
 

GrouchoDuke

Well-Known Member
Joined
Oct 18, 2016
Messages
1,113
Reaction score
547
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.
That's a great point - mentors are great to have!
 

TimothyG

Well-Known Member
Joined
Nov 1, 2018
Messages
471
Reaction score
200
Location
Northern Utah
Finding a mentor on high velocity, multi-stage, high altitude, or high impulse flights is incredibly difficult in this hobby for many reasons. The biggest of which, from my perspective, is the number of people who want to do a big project, talk and plan it out, but never actually do anything, "Tire kickers". Anybody qualified to mentor or advise big projects must then wade through a sea of grandiose plans before finding someone putting a motor in an airframe. Even the people who are willing to complete a big project are incredibly limited compared to the people who actually start a big project. I had no mentors until after the launch button was pressed on my first big project. Fly big projects and if you fail on the first attempt, be prepared to have a lot more quality input on the second attempt.
 

airforcetp1991

Cave Putorium
TRF Supporter
Joined
Nov 22, 2020
Messages
172
Reaction score
73
Location
New Mexico
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.

I plan to mock up the AvBay over the next week or so and am planning on using the Eagle CO2 system from Tinder.

-Tony
 

AeroAggie

Well-Known Member
TRF Supporter
Joined
Jan 21, 2017
Messages
355
Reaction score
158
Location
DFW
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
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!
 

airforcetp1991

Cave Putorium
TRF Supporter
Joined
Nov 22, 2020
Messages
172
Reaction score
73
Location
New Mexico
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!
Thank you again for the help understanding the composite strength considerations!

-Tony
 

aerocfd

Active Member
TRF Supporter
Joined
Dec 13, 2015
Messages
35
Reaction score
38
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, great work. Impressive use of the new FinSim.
 
Last edited:

aerocfd

Active Member
TRF Supporter
Joined
Dec 13, 2015
Messages
35
Reaction score
38
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
Tony, very impressive.
 
Top