Q: Slotting the airframe for MD altitude/speed attempt?

[prfesser]

I didn't see anything specific on this topic, so...what about slots in a minimum-diameter airframe, just wide enough for the fin? (Airframe is not the motor case.) Good idea, bad idea?

Slot airframe, sand slots and root edge and sides thoroughly (wet-sanding with epoxy+slow curative would not be amiss). Wrap wax paper or other release material around motor case. Slide motor case in place, epoxy fins in place. Cure, remove case and release material, then fillet or tip-to-tip as desired.

I don't know how much additional support for the fins could be expected by having them recessed the tube-wall-thickness. Though I've seen/bought kits, not MD, that had TTW fins with tabs only as long as the tube thickness. Sounds like it's an improvement over surface bonding.

There's the issue of weakening the airframe but that ought not to be terribly serious if the motor itself isn't the airframe. Then again, as an old and emphatically not beloved teacher used to say, I may be full of old bootlaces and sardine juice...

Anyone out there done it this way? Results? Discussion? Questions? Comments? Physical threats?

 

[dhbarr]

Check out the crenellations on the LaserLOC

 

[mikec]

I've done it both ways and I'm sure strong opinions will be expressed, but I think it's fair to say that good quality surface bonding and fillets win over slots. With slots you don't have as good control over the bondline volume or coverage. But I'll be curious to see what others say.

I've also had friends who did tip-to-tip for MD fins in slots who had the BT distort when vac-bagging. I guess you need to put your motor case in there to defeat that and somehow not glue it in by mistake.

 

[sharkbait]

I've done it both ways and I'm sure strong opinions will be expressed, but I think it's fair to say that good quality surface bonding and fillets win over slots. With slots you don't have as good control over the bondline volume or coverage. But I'll be curious to see what others say.

I've also had friends who did tip-to-tip for MD fins in slots who had the BT distort when vac-bagging. I guess you need to put your motor case in there to defeat that and somehow not glue it in by mistake.

 

[sharkbait]

Agree a good surface mount, prep and good epoxy with strong filllets.

I actually have gotten lazy lately and just use Fisher fin cans

 

[robopup]

I think @mikec nails it - good quality surface bonding is the name of the game.

I've never really understood slotting or otherwise cutting the tubing on a minimum diameter rocket - you compromise the tubing in return for a (very) little extra surface area and I doubt that extra surface area is doing much to help react the bending loads. Similarly, unless tip-to-tip is done properly, I think it only hurts you - not a ton to gain, but plenty of room for a bad laminate with voids and/or poor bonding at the leading edges of fins.

 

[dhbarr]

Seems pretty straightforward to me that the twisting portion of flutter would be greatly resisted via binding the root... guess someone needs to fly the fins off a bunch of LaserLOCs for science.

 

[mikec]

Seems pretty straightforward to me that the twisting portion of flutter would be greatly resisted via binding the root... guess someone needs to fly the fins off a bunch of LaserLOCs for science.

I guess the same principles apply for cardboard/plywood, but what I think of as altitude/speed MD is using fiberglass or CF.

I've flown many MD rockets 38mm and 54mm trying to break 2000 mph. The only time I've had the fins come off was when they were too thin, but otherwise slotting or not slotting hasn't made a difference. Best I've done to date was surface-mounted fiberglass fins with tip-to-tip CF.

I agree that slotting seems like it would offer advantages, but they may be offset by the difficulty of bondline control, at least for epoxy bonding.

 

[robopup]

Seems pretty straightforward to me that the twisting portion of flutter would be greatly resisted via binding the root... guess someone needs to fly the fins off a bunch of LaserLOCs for science.

My guess is the component of the twisting I think you're talking about (see my bad paint art below at my guess) is not what usually causes shreds though - there's a lot more arm along the fin root to react that twist component than to react any bending load the fin sees. This is assuming the shred is caused by flutter in the first place and not just a bad quality structural bond or tip-to-tip layup.

 

[dhbarr]

I guess the same principles apply for cardboard/plywood, but what I think of as altitude/speed MD is using fiberglass or CF.

I've flown many MD rockets 38mm and 54mm trying to break 2000 mph. The only time I've had the fins come off was when they were too thin, but otherwise slotting or not slotting hasn't made a difference. Best I've done to date was surface-mounted fiberglass fins with tip-to-tip CF.

I agree that slotting seems like it would offer advantages, but they may be offset by the difficulty of bondline control, at least for epoxy bonding.

Lot cheaper to test in cardboard :-D

 

[OverTheTop]

I didn't see anything specific on this topic, so...what about slots in a minimum-diameter airframe, just wide enough for the fin? (Airframe is not the motor case.) Good idea, bad idea?

Good idea.

Slot airframe, sand slots and root edge and sides thoroughly (wet-sanding with epoxy+slow curative would not be amiss). Wrap wax paper or other release material around motor case. Slide motor case in place, epoxy fins in place. Cure, remove case and release material, then fillet or tip-to-tip as desired.

Sounds like exactly what I do. I mill the slots in a milling machine. Fillets only, no tip-to-tip. All of my rockets are built this way, unless they are TTW anyway.

I don't know how much additional support for the fins could be expected by having them recessed the tube-wall-thickness. Though I've seen/bought kits, not MD, that had TTW fins with tabs only as long as the tube thickness. Sounds like it's an improvement over surface bonding.

They are held in by compression/tension, rather than just peel if you stick them on the surface. Improvement in attachment IMHO.

There's the issue of weakening the airframe but that ought not to be terribly serious if the motor itself isn't the airframe. Then again, as an old and emphatically not beloved teacher used to say, I may be full of old bootlaces and sardine juice...

I always use a motor retainer in the Aeropack minimum diameter style. Take the motor thrust to the forward mount. This keeps the aft section of airframe in tension, where the composites work better than in compression. I keep the rear thrust ring clear of the rear of the airframe.

Here is one I prepared earlier:
https://forum.ausrocketry.com/viewtopic.php?f=6&t=4758

 

[tsmith1315]

Take the motor thrust to the forward mount. This keeps the aft section of airframe in tension, where the composites work better than in compression. I keep the rear thrust ring clear of the rear of the airframe

I've wondered why this isn’t more common in HPR.

 

[OverTheTop]

Me too.

 

[teepot]

Ok, I am not getting it. How do you keep the rear thrust ring clear of the airframe?

 

[Rocketjunkie]

Surface mounted fins with fiberglass in the fillets. Survived an O3000 motor in 1990. Fins were still 0n after it augered in from 20k feet at Black Rock. The SU motor was drilled and tapped for 20 to hold it in.

 

[OverTheTop]

Ok, I am not getting it. How do you keep the rear thrust ring clear of the airframe?

The MD retainer is set on such a position that when the motor is screwed onto it the thrust ring is just off the airframe. If using shorter motors then spacing rods are used.

 

[mikec]

I have never seen a fiberglass tube fail in flight in compression, so I'm not getting the advantage of that. Or cardboard, for that matter. All failures I've seen have been bending failures.

 

[tfish]

I cut fin slots in my current 98mm min dia..and will never do that again!

Even with my normal tip to tip..i need to redo a fin about every 3rd flight or so.
Out of all of my other min dia rockets..I've never had to do any fin attachment repairs..

Tony

 

[ksaves2]

Lot cheaper to test in cardboard :-D

I slotted in a cardboard rocket years ago but I don't know if it made any difference. I applied fillets as if the fins were surface mounted per my usual routine. I made certain I applied one layer of wax paper to a motor casing and the seam wasn't under a fin slot. Motor and wax paper came out of airframe fine. More work if you ask me. If cardboard, balsa or plywood finned rocket, if one puts a large enough motor in it's gonna shred.

Now those that laminate a cardboard rocket with CF or fiberglass cloth, that's a "whole" other issue and I believe that does increase strength (and weight). Did that a couple of times too. I think it's a PITA to do but if one has a lot of time on their hands, nice diversion slotting and mounting fins on MD cardboard. Like dipping igniters in wintertime that I think is akin to knitting socks and hats by hand.

If one is looking for speed, why not just use 'glass components and use bigger mother motors to fly'em? Will need extra weight in the form of a tracker of some kind if one wants to see 'em again though. O.k., ok. Bigger motors "cost" more.

If sticking with cardboard, I think using 3/4 oz. cloth on the fincan adds to the durability of the rocket so it will be flyable over a longer period of time if one doesn't lose it. Paint will still get ratty anyways after numerous flights. Especially if the rocket gets dragged by the wind on the ground.

Also, glassing balsa fins and the fincan would likely make the fins less susceptible to fin flutter if going super fast.

As evident, I've done permutations of the above over the years and you know what? I didn't want to lose my projects I spent a long time building using some of those techniques and generally used motors to keep them in sight to minimize the out of sight time. Winds aloft can jigger a rocket into the opposite direction of the ground wind speed and all eyes could be looking in the wrong place to get a visual.

I've seen that with GPS tracked rockets I was using a mapping program to put the APRS packets on a map program on a laptop. Had several flights (I tracked others rockets too if they used an APRS trackers) that showed the winds aloft were flipping the rocket under drogue all over the place. I'd look up and sometimes see the crowd looking in the wrong place to get a visual based on ground wind direction. I would scream spastically and point to where the rocket "was" so they could look in the right direction to get a possible visual.

Soooooooo.......... My take on small rockets, reinforce if you want but only expect it to increase durability for longer life. Otherwise, use a large enough motor and one is going to lose it or shred it. (If there is no tracker onboard.)

 

[ksaves2]

Good idea.


Sounds like exactly what I do. I mill the slots in a milling machine. Fillets only, no tip-to-tip. All of my rockets are built this way, unless they are TTW anyway.


They are held in by compression/tension, rather than just peel if you stick them on the surface. Improvement in attachment IMHO.


I always use a motor retainer in the Aeropack minimum diameter style. Take the motor thrust to the forward mount. This keeps the aft section of airframe in tension, where the composites work better than in compression. I keep the rear thrust ring clear of the rear of the airframe.

Here is one I prepared earlier:
https://forum.ausrocketry.com/viewtopic.php?f=6&t=4758

One has to be careful with the Aeropack MD retainers and plan ahead what's the longest motor casing one wants to use. Will use a threaded forward forward motor closure to use a threaded rod to screw into the AP retainer. Think hard and have the longest motor one wants to use on hand one might use. On shorter motor casings, just use a longer threaded rod. I believe it's a neat setup but one has to "think" when installing as once the AP retainer is set in place with epoxy, that's it.
Oh, don't screw the motor in tight. Just a little bit firmly. I had the experience in the past that the motor screwed out without the threaded rod and the threaded rod was retained. I had some long necked medical grade clamps (from a throw away set) I padded the jaws and able to extract the threaded rod. Kurt

 

[lakeroadster]

I have yet to try this, but @AeroTech posted it back in 2020 on the thread linked to below.

It's a great way to integrate the body tube into the fin.

Note that every other slotted section remains intact, so the integrity of the tube is maintained, thus it will stay round. It really is a genius construction technique.

"This particular rocket flew to over 13,000 feet on an H13 motor and was successfully recovered with no damage. "

https://www.rocketryforum.com/threa...-minimum-diameter-rocket.161479/#post-2038035

 

[SolarYellow]

I can't figure out why someone would pay $30 for something that could very easily and quickly be fabricated from plywood, a tee nut, and either a screw or some threaded rod.

 

[cwbullet]

I can see how slotting might strengthen the fin attachment.

 

[ghostfather]

I slot the fins on MD rockets, always leaving at least an inch of the fiberglass at the bottom intact. Otherwise the tube tends to deform.
Slotting the tube requires careful sanding on the inside of the tube to let the motor casing slide in smoothly, but I believe it's structurally stronger than a simple surface mount. If I could only slot half the thickness of the tube, that would work as well, but I'm not that precise.

When making 3D-printed fins with an airfoil outer shape, I leave a hollow part in the fin to accommodate a rectangular piece of fiberglass, which I mount in a fin slot. This stiffens the fin, and provides a strong bond to the airframe. Works with MD airframes as well as TTW