Minimum Diameter - 2 Questions - Fin Can vs. One Piece Body/Fins, and Flyaway Rail Guides

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Active Member
Feb 5, 2023
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Tempe, AZ
Hello all!
I have two questions regarding a MD 38mm build that I am in the process of making.

The first question - how much acceleration is too much for flyaway guides? Is there any reasonable acceleration that runs the risk of smashing fins on the guides as they fly off, and if so, is this even a problem in real life? The rocket will be accelerating at ~50 Gs, but I don't think I have the time to build a tower and I'd prefer not to use rail guides.

The second question - I saw in BPS.Space's L3 builds that he used a detachable fin can that fit over the motor casing, as well as another body tube, for the full lower tube/fins section. Is this something that other people have experience with doing on this smaller 38mm scale, and if so, what are some pros and cons, versus just making it all out of one tube? Will either approach work just fine, and I'm just overthinking it?

Thank you!
I’ve seen a lot of fly away rail guides (FARGs) broken by striking fins. I’ve never seen fins broken by FARGs.

My experience with fin-cans only includes ones that attach to the outside of a body tube, so I cannot help with your second question.
I can answer 1st question. You should be ok as long as they are installed right against the leading edge of fins which I believe is stated in instructions. This way zero impact. Probably more likely to damage rail guides if they land on hard surface.
I've built a fair number of MD 38 mm rockets. I've personally had poor luck with the FARGs, but that was using some of the earliest designs. They were either easily broken or hard to find, depending on the field conditions. Nowadays, I would consider using them for a typical flight, but not for a high-G ascent. One issue I've seen is if there is any 'slip' in the fit between the FARG and the body tube. On a high-G launch, the rocket hit the FARG hard enough that the leading edges of the fins basically slid under the FARG and caused it to expand, which then put pressure on the rail guides. The whole thing then got bound up on the rail. It did fly, but it clearly lost altitude due to the excessive force require to slide up the rail. So you want to make sure there is a very good fit between the FARG and the body tube.

I could not find the L3 build you mentioned, so I can't comment on that. But if you are going well past Mach, you want to minimize any breaks in the airframe to get the smoothest airflow. Just something to consider.

I used additive aerospace’s fly away rail guides on my L2 certification. They worked great and had no damage after the flight! They are built really well though so I cant speak for any other brand or self printed ones. I launched a minimum diameter madcow Go Devil on a J570W. I cant remember my peak acceleration off the top of my head but Im certain it was well above 50Gs. My max speed was about mach 1.6 and 14k ft apogee. The apogee was actually really close to my openrocket sim so unless you are trying to break altitude records I would recommend saving money with fly away guides instead of building a launch tower. Aluminum is super expensive right now.
Also regarding fin cans and flying case design: Not a fan.
The step created by the differing diameters of a fin can and motor case creates a lot of drag compared to just using a thin wall airframe and nosecone that will have a flush fit. Simple fins and a thick fillet of good epoxy like hysol on a well prepped surface should be more than enough for most amateur size rockets (especially 38mm) too if you were thinking about tip to tip.

Edit: if you were thinking of skipping the fin can altogether and gluing fins directly to the motor case with a custom nosecone for an altitude record, I can get behind that!
I've used FARG with success on many MD flights, but use no fin cans.

The extra lip on the fin can is unneeded drag, just like the rail buttons, and minimizing drag is the secret to gain high altitudes on an MD rocket.

The prevailing advice on MD flights is to use a low thrust motor (moonburner), which limits the velocity, or otherwise reaches highest velocities at higher altitudes, where the air is thinner, and thus minimizes the drag.

Because of lower thrust off the rail, there is less chance of damaging a FARG when it hits the fins. I've had good luck using FARG's from Additive Aerospace, as he prints them using PET filaments. The early versions were PLA, which is more brittle.

Another area I've been experimenting with are airfoil cross-sections on my fins. I can accurately 3D-print fins with a symmetrical van Karman profile, and glass them over for strength. Because most of my flights cross the Mach barrier, van Karman profile is good. I've also used some NACA airfoil profiles for sub-Mach flights, and they make a difference. 3D printed/glassed fins are fine up to about 38mm rockets, anything above that may need a stronger core material. I'm considering using a flatbed milling machine like the Shapeoko 4 to make plywood fins in airfoil profiles, glue two halves together around a layer of glass or CF, and glass the exterior, strong enough for a 98mm MD flight. Need the milling machine first ;)