Mach 2 with min diameter

[Performance nut]

I see folks like to tip to tip their surface mounted fins when the feet the minimum diameter rockets faster than mach 1.

My project is a Go Devil 38 which will be using a fly away rail guide. I'm debating on using the nose cone E-Bay so I can shorten the length to one body tube.

Now the question is how to keep the fins on. I have West Systems epoxy and Hysol E-120HP. I will be following the instructions on the thread above on how to properly bond using epoxy. Question is if it is enough.

PS: how do you apply constant pressure to surface mounted fins? Only thing I can think of is press it towards the BT and clamp it to my fin alignment tool as I'm pressing it. Not sure if it will maintain the pressure but it will keep it from moving. Not sure if that is sufficient.

 

[GalantVR41062]

They have simulation programs for fin stability. I have 2 fiberglass MD rockets I am working on also. A MC 2.2" tomach and a WM Blackhawk 38.

I have rocketpoxy, west systems epoxy, micro balloons, carbon fiber chop, thin carbon cloth, and the epoxy from WM, it's supposed to be Proline 4500. I plan to do some more research and minimize the mass I add to the tip of the fin but make sure the epoxy can handle the heat, and surface prep.

I would use a set of ridged fin guides and use a elastic band or tape to put constant pressure on the fins for the root attachment.

~John

 

[dnp]

Personally, I'd go with the E-120HP for a number of reasons. Primarily, I like to have accurate performance data on any epoxy I use, and as far as I know, there's not much available on West Systems in structural applications. Also, since the West Systems comes in two components, and calls for additives, the E-120HP has more consistency in physical characteristics between batches. The E-120HP also comes in cartridges, so it's just much easier to apply and work with.

With regards to T2T, unless you're flying an unusually aggressive motor, it's most likely unnecessary. As long as your fins are on straight, they will experience fairly minimal lateral forces, and shouldn't require any reinforcement. The fins that come with the BH38 are also plenty stiff on their own, and you're not going to have to worry about flutter with any motor you can fit in that rocket.

David

 

[manixFan]

I have flown 38mm and 54mm MD rockets past Mach 2 with nothing more than good fillets, no TTT fiberglass at all. That’s been with all carbon fiber rockets and the pro-line epoxy. Fastest was Mach 2.4. Hard on the paint but other than that all have survived just fine. On smaller MD rockets with well proven designs you don't need to over build it.


Tony

 

[CoyoteNumber2]

I have flown 38mm and 54mm MD rockets past Mach 2 with nothing more than good fillets, no TTT fiberglass at all. That’s been with all carbon fiber rockets and the pro-line epoxy. Fastest was Mach 2.4. Hard on the paint but other than that all have survived just fine. On smaller MD rockets with well proven designs you don't need to over build it.

How big did you make your fillets?

 

[Performance nut]

Thanks all, this is the feedback I'm looking for.

With regards to T2T, unless you're flying an unusually aggressive motor, it's most likely unnecessary. As long as your fins are on straight, they will experience fairly minimal lateral forces, and shouldn't require any reinforcement. The fins that come with the BH38 are also plenty stiff on their own, and you're not going to have to worry about flutter with any motor you can fit in that rocket.

David

My rocket is the Go Devil 38. Not sure how different it is from the BH38. Out of curiousity, what do you consider "unusually aggressive motor"? I'm planning on going with a Cessaroni J530 or J600 eventually to get her to Mach 2.

 

[Performance nut]

They have simulation programs for fin stability. I have 2 fiberglass MD rockets I am working on also. A MC 2.2" tomach and a WM Blackhawk 38.

I have rocketpoxy, west systems epoxy, micro balloons, carbon fiber chop, thin carbon cloth, and the epoxy from WM, it's supposed to be Proline 4500. I plan to do some more research and minimize the mass I add to the tip of the fin but make sure the epoxy can handle the heat, and surface prep.

I would use a set of ridged fin guides and use a elastic band or tape to put constant pressure on the fins for the root attachment.

~John

I don't have any fin stability sim software, just RockSim. Has anyone done fin stability on the Go Devil 38 fins?

I'm not sure how to get an elastic band to work with my fin jig. I have an Ultimate Fin Guide which I really like but unsure how to utilize it for something like this. I'll have to give it some thought as I really like this idea.

 

[manixFan]

How big did you make your fillets?

The fillets on my 54mm are about 3/8", on the 38mm about 1/4". One 54mm was a CF Mongoose, the one in the photo is a second one I built from scratch using a 54mm tube made by SinfulDarkLord (he really needs a new user name!) The 38mm is also a CF Mongoose. All were built with Pro-line epoxy. Fastest flight on the 54mm was on a CTI L935 to about 23,500' and Mach 2.4.The 38mm coned a bit when I flew it on a J150 (CTI 6XL long burn 38mm motor, went to about 14,200', right at Mach 1). I also flew it on either the CTI J453 or J530 but I don't have that file on hand at the moment.

It seems like smaller MD rockets with well designed fin profiles and made from stiff material don't need as much reenforcement as larger rockets. I'm working on a 98mm MD CF Mongoose and that will have TTT overlays. The weight of the bigger rocket is so much greater even when empty that it seems prudent to help keep the fins on in case of a hard landing or being dragged across the playa. I also think that alignment is critical - well aligned fins should be less likely to run into trouble than if one is at a bit of an angle.


Tony

38mm fillets:

54mm fillets:

 

[dnp]

My rocket is the Go Devil 38. Not sure how different it is from the BH38. Out of curiousity, what do you consider "unusually aggressive motor"? I'm planning on going with a Cessaroni J530 or J600 eventually to get her to Mach 2.

Oops, got my kits mixed up! Given the Go Devil's thinner fins, I'd go ahead and apply reinforcement. Finsim gives a divergence velocity of around 1280 FPS, which is far below where you'd be with a 6GXL J motor.

David

 

[manixFan]

Sorry, I just now realized you are dealing with fiberglass fins not CF. For that I would use extra reinforcement if you are planning on pushing it really hard. I spent a lot of time beveling mine so they have a very sharp leading edge. That definitely helps performance. I set up my fin alignment jig so that all three fins are in place while I am gluing them on and then use rubber bands around the fins to apply positive pressure against the body tube. You have to make sure there is no play in the fins though when you are mounting them so they stay true.

I think the biggest issue with MD fins is shape and size. As long as they are designed for speed and are fairly small, you should be good. The down side is you may need nose weight for stability if you keep the body tube short. I looked into fin placement and from now on I don't plan on having my fins so far forward. The rule of thumb seems to have them one body diameter from the base to help reduce base drag. But in sims I found it did not really seem to make much difference. Keeping the fins father back of course keeps the center of pressure back so less weight may be needed for stability.


Tony