Perhaps my Crowning Achievement?

Zeroignite said:

At what altitude/speed are you expecting the crowns to cease being part of the rocket?

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Well, I am not expecting to reuse them, but I don't want them too fall off either (they're heavy enough to injure someone if they just fell). So, I will try to attach them in such a way that they don't fall off during flight (up to Mach 1.2), but can be removed after the flight without causing any damage to the fins. Method TBD.

Jim

Just spit balling here....what about some sort of slip cover/sleeve in the form of a slightly larger fin can? The cover would look like a fin can but be able to slide over your existing fins from the forward end of the rocket. The drag would (in theory) hold the crown on the body/fins but other anchoring may be necessary. This means the crown would be open at the aft end of your fin but you can put a plug to cover that part of the fin. Probably the worst description ever but I can see it in my head. I will try to draw/model what I am picturing.

Matt

Edit: Added the model of what I was thinking. Its a little rough but it is the general idea of what I was thinking. Would have taken me a lot longer to make if I made it look like actual fins. I will make a better one when I am home from work.

redsox15 said:

Just spit balling here....what about some sort of slip cover/sleeve in the form of a slightly larger fin can? The cover would look like a fin can but be able to slide over your existing fins from the forward end of the rocket. The drag would (in theory) hold the crown on the body/fins but other anchoring may be necessary. This means the crown would be open at the aft end of your fin but you can put a plug to cover that part of the fin. Probably the worst description ever but I can see it in my head. I will try to draw/model what I am picturing.

Matt

Edit: Added the model of what I was thinking. Its a little rough but it is the general idea of what I was thinking. Would have taken me a lot longer to make if I made it look like actual fins. I will make a better one when I am home from work.

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Hey, that idea has some merit. I could use a ring around the tube at the front and back - directly under the frames at each end - to hold the crowns to the airframe. Need to let that gel a bit, but I hadn't thought of that. Thanks.

I had considered glueing those points to the airframe, but the alternative of putting a ring there and then glueing to the ring instead might be more secure.

Jim

I made a sanding jig out of leftover parts and got the leading edges beveled. Aftere a little finish sanding, I coated the wood parts with epoxy to harden them up. So, the crowns are mechanically complete. Just need to fill and paint them, and then attach them.

One thing I didn't mention before is that I cut all of the G10 using a diamond cutoff wheel. That thing is in the running for tool of the year! It does through the G10 like butter.

Jim

Looks very promising.. great work...

Nice - which diamond cutoff wheel do you use?

Zebedee said:

Nice - which diamond cutoff wheel do you use?

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I'm curious too. I have had mixed results with diamond cutoff wheels. The ones from Harbor Freight just seem to "burn up." I use the reinforced fiberglass cutoff disks. They work pretty well. I've even gone to buying them in bulk (100 pc packages off eBay).

dixontj93060 said:

I'm curious too. I have had mixed results with diamond cutoff wheels. The ones from Harbor Freight just seem to "burn up." I use the reinforced fiberglass cutoff disks. They work pretty well. I've even gone to buying them in bulk (100 pc packages off eBay).

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I can post the brand tonight. I just got it at Lowes and I would guess it's dremmel.

Jim

This guy.

Or this guy.

I have the EZ-lock one and it works great.

patelldp said:

This guy.

Or this guy.

I have the EZ-lock one and it works great.

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Yep, the "or this guy" EZ Lock one. I bought several carbide hacksaw blades thinking I was going to need to cut out all of those G10 crown covers. Was not looking forward to it. Then, I remembered that I had this cutoff wheel. Hmmm. On the second batch of six skins, with the 0.047 G10, the total cutting time was not more than 10 minutes, and most of that was moving and clamping the G10 after each cut. Then, after I glued them up with the frames, I used the cut off blade to trim the G10 overhang. Saved literally hours of sanding.

As good as this was, I suspect that by the end of the year, the other candidate for tool of the year - double sided tape - will probably be back in the lead.

Jim

A quick update ...

I got the crowns filled and primed. The filler I used was the Elmers wood filler. I've used that general product for years and it worked great, but it appears that they changed the formula. I recall hearing that a while back, and it appears to be the case. There is now a "pro" material that has wood fibers in it (this stuff is useless) and an "interior" wood filler that at least appears similar to the original. However, this new stuff can't be sanded. It is much harder than the original, and if you generate any heat while sanding it, it gets even harder - to the point where it can't be sanded at all. Plus, it forms hard "chips" on the sandpaper, which then tear up the filler that you're sanding. What a pain. It's safe to say that it no longer does what I need it to do. Going to need to find something else.

Anyway, I declared victory and got the crowns primed before the weather turns this weekend.

Jim

JimJarvis50 said:

It's safe to say that it no longer does what I need it to do. Going to need to find something else.

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Jim, take a look at SuperFil (https://www.polyfiber.com/epoxy/). A bit more expensive. But you'd be surprised; a little goes a long way. Also being epoxy based (great adhesion) and very light it aligns more with our rocketry needs.

dixontj93060 said:

Jim, take a look at SuperFil (https://www.polyfiber.com/epoxy/). A bit more expensive. But you'd be surprised; a little goes a long way. Also being epoxy based (great adhesion) and very light it aligns more with our rocketry needs.

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Tim, I think we've had this discussion before, but let's revisit. I've used a lot of superfil, but my impression is that it's a bit on the dry side and doesn't get into the fine scratches I was trying to fill here. That's what made the woodfiller approach work (that is, after you water down the woodfiller just a bit). Am I wrong on this point? This certainly would have been a good opportunity to try this (with the flat surface making the application easier). I don't have any superfil, but I do have aeropoxy light. I should have tried it.

The other product I was thinking about was Kilz (some flavor of it). I tried it once before but I didn't like it. But it would have been better than what I did.

Jim

JimJarvis50 said:

Tim, I think we've had this discussion before, but let's revisit. I've used a lot of superfil, but my impression is that it's a bit on the dry side and doesn't get into the fine scratches I was trying to fill here. That's what made the woodfiller approach work (that is, after you water down the woodfiller just a bit). Am I wrong on this point? This certainly would have been a good opportunity to try this (with the flat surface making the application easier). I don't have any superfil, but I do have aeropoxy light. I should have tried it.

The other product I was thinking about was Kilz (some flavor of it). I tried it once before but I didn't like it. But it would have been better than what I did.

Jim

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Yes, AeroPoxy Lite is probably equivalent. Regarding SuperFil you do have to work with it and press it in, but I find it fills most any size gap or undulation. Saying that, I do often do a round of Bondo putty fill after my first coat of Rustoleum Filler-Primer for the smaller stuff.

Have you tried Durham's Water Putty?

CarVac said:

Have you tried Durham's Water Putty?

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Not on rockets. When applied in thin layers and impacted it cracks easily.


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Overall I like super fill. It can be a little dry as Jim mentioned. I have noticed that it seems to get dryer with age, but I could be mistaken.

Any thoughts on a material/liquid that could be added to thin it slight?


Mark Koelsch
Sent from my iPhone using Rocketry Forum

I got the crowns painted and finished. Once again, one of my tried-n-true techniques is headed for the crapper. I've always had good luck painting using Krylon, then wet sanding, and then polishing with 3M Finesse-it II. Gives a great shine. Well, looks like they've changed Krylon (for the second time perhaps?). The new stuff is something called ColorMaster. You're supposed to put on multiple coats every minute. The painting and wet sanding went OK, but the polishing didn't. Seems the polish dissolves the paint a bit more than it should. The result is about a C-, but it will work. At least I wasn't painting anything that I really wanted to look good.

Now, I just need to determine how I'm going to mount these. My current candidate approach is to put a little epoxy at perhaps 6 spots around the perimeter of the fin and then slide on the crowns. I should be able to pry them off later on. Still open to alternatives here.

This project has been a bit frustrating and has taken longer than I though it would. But, it was necessary to do something, as I don't think the stability of the unmodified design was adequate.

Jim

...that schematic is terrifying.

There's more empty space in the middle stage than I would have expected.

Zeroignite said:

...that schematic is terrifying.

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But it is possible, and we'll have fun!

One thing that does worry me a bit is that the top speed of the second booster/sustainer is around Mach 2. I'm a bit worried about separating the stages at that velocity, even though the booster has a transition. These pieces will be shear-pinned together, and I'm wondering if I should keep them together until they slow down a bit. It doesn't seem to affect the altitude simulation very much.

Jim

CarVac said:

There's more empty space in the middle stage than I would have expected.

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That section has a relatively long electronics bay and also a piston. Modifying the bay to remove about 2" isn't going to happen, but it might be possible to reduce the tube length a bit.

The sustainer tube is long too, but I've added a camera section to it below the cone, and that takes up the excess.

Jim

Looks awesome - when are you flying it?

Zebedee said:

Looks awesome - when are you flying it?

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Most likely Balls.

Jim

tfish convinced me to go with a hardware attachment. I used #6 machine screws and I don't think they'll be coming off. One nice thing is that I can travel with them off and then attach them at the launch. On to the next thing!

Jim

A quick question: this is nearly minimum diameter, right? What does the screw in the rail guide anchor into?

CarVac said:

A quick question: this is nearly minimum diameter, right? What does the screw in the rail guide anchor into?

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Didn't mean to stifle questions or comments. This particular tube was rolled on an airframe mandrel. So, there are some fiberglass rings in that area that serve to center the motor. The rail guide anchors into one of those. However, with a five wrap tube and three layers of tip to tip, there is plenty of thickness to anchor this particular style of rail guide (since it has two screws and also gets glued down). Having these guides (the PML ones) is very handy, except that they have to be opened up a bit to fit on a 1515 rail.

Jim

JimJarvis50 said:

Didn't mean to stifle questions or comments. This particular tube was rolled on an airframe mandrel. So, there are some fiberglass rings in that area that serve to center the motor. The rail guide anchors into one of those. However, with a five wrap tube and three layers of tip to tip, there is plenty of thickness to anchor this particular style of rail guide (since it has two screws and also gets glued down). Having these guides (the PML ones) is very handy, except that they have to be opened up a bit to fit on a 1515 rail.

Jim

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Ah, that's the one meant for the 1010 rail instead of the massive 1515 rail ones?

CarVac said:

Ah, that's the one meant for the 1010 rail instead of the massive 1515 rail ones?

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No, these are 1515. To clarify, the guides are a tight fit on the so-called "solid" T-slot frame. They fit just fine on the "hollow" T-slot. All 1515 is not the same. My rail happens to be the solid version, so I have to sand the guides a little to fit.

Jim

Two ideas for attachment, basically expanding on what's already been said:

1. Simple method: drill through the crown on one side and and through the fin. Tap the fin and add helicoils, then use vibratight or similar on launch day. Do some from each side. Not sure how many you'd need, but this is the route I would probably go. It lets you tap the fins instead of the far side if the crown, which is thinner. The bolts can be countersunk flush.

2. If you don't want to drill your fins: find or make a short section of tubing about the length of your new fins to fit over your body tube. Cut sections to fit between the fins, then make some sort of L section that gets either epoxied to the crown and bolted to the new can section, or vise versa. You could even just slot the tube instead of making 3 pieces and attach the crowns the same way. This is basically what the one other post said, without having to leave the back of the crown open so you can slide the whole thing on at once. You could also tap the crowns fore and aft where there is more thickness instead of the thin skin section.

Or you could fillet the crowns to a temporary can for real, then dremel them off after the flight. But then you trash the crowns... Then again if this really is a one show only affair, this might be your best bet.