source for JT-80C?

I'm looking for a source of the JT-80C coupler/ring fin to allow me to properly clone the Hyperion and Manta Bomber. As its diameter and finish aren't the same as the standard BT-80 coupler, I'm reluctant to simply cut down a coupler to make it (though I will if I have to). It does not appear that the part is available from Semroc/eRockets, which I find somewhat surprising. And the 2.55/2.50 diameters don't match anything that BMS sells. Does anyone know of a vendor that makes this part, or am I stuck being "close enough" with standard coupler?

I just checked the JT-80C from Estes against the BMS C80. The JT-80C is ID 2.47, OD 2.55 and the BMS C80 is ID 2.474, OD 2.554 . That's only 4 thousandths of an inch different.

I hope that helped. Otherwise you'll just have to see if Estes sells the JT-80C as part of the large coupler pack.

You could always take a 3 or 4" section of BT-80, cut out a wedge and make your own coupler.

https://modelrocketbuilding.blogspot.com/2011/07/semroc-iris-build-part-7-diy-making.html
https://modelrocketbuilding.blogspot.com/2011/07/semroc-iris-build-part-8-diy-making.html

Kruegon said:

I just checked the JT-80C from Estes against the BMS C80. The JT-80C is ID 2.47, OD 2.55 and the BMS C80 is ID 2.474, OD 2.554 . That's only 4 thousandths of an inch different.

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Same tubes, BMS just listed the dimension to the third decimal point.

According to the Estes Tube Reference by John Brohm, the appendix that deals with the JT-80C states that it has a 2.55 OD and a 2.50 ID. Even if the OD is only .004" off or just carried out to the third decimal place, the ID is off by .026" - again, not huge, but I know that some folks here are very, um, specific about the accuracy of their clones. I'd be surprised if that sort of individual would fuss over having a molded vs balsa nose cone, yet not care about having the correct tube. Anyway, I was asking about it with regards to its use as a ring tail, in which case, its glassine finish is also helpful for finishing. A typical coupler isn't going to paint up as nice without a bit more work. That those numbers make it a bit thinner walled than the typical coupler - even if you wouldn't spot the difference in diameter, you would see the difference in wall thickness.

I gathered my info from the Estes catalog. It's an original scan. I can email it to you if you like. Just give me an email to send it to.

EXPjawa said:

According to the Estes Tube Reference by John Brohm, the appendix that deals with the JT-80C states that it has a 2.55 OD and a 2.50 ID. ...

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The remark you refer to in the Appendix entry for JT-80C was provided in the context of how the part was listed in the typo-prone 1974 Parts Catalog (the best example of egregious typos are the BT-52 dimensions); the remark wasn't meant as the actual specification of the part.

We know that the original JT-80C was indeed a slip fit inside BT-80, as any good coupler should be. We also know that it was a thin walled tube (0.021", I believe). Knowing the OD for BT-80 (2.6"), then its ID has to be 2.558", manufacturing tolerances set aside. The original JT-80C having a wall thickness of 0.021" means that its ID has to be 2.516", manufacturing tolerances set aside.

JT-80C was also only 1" long; one could argue that length wasn't the best for it to be used as a coupler, although it made sense in the Saturn V application. To your point, it was its glassine finish that made it perfect for those tail ring applications. Why Estes used it for a tail ring as opposed to say, a 1" length of BT-80, is anyone's guess, but I imagine it was because they had a bunch beyond the K-36 kit run, and so engineered it into follow on kits to use it up. Just a speculation on my part.

To my knowledge, any JT-80 made today is much longer and thicker walled, and generally better suited for use as a coupler. Those original JT-80Cs are hard to come by unless you pirate an old kit, which in my mind makes kits like the older Super Big Bertha susceptible, because frankly a Super Big Bertha would better joined by a heavy duty JT-80 coupler than with the old JT-80C. So look for an old Super Big Bertha, scavenge the JT-80C for your ring tail clone, and you're still left with a great kit (the Super Big Bertha) to build, no one the wiser about the internal coupler.

John Brohm said:

To my knowledge, any JT-80 made today is much longer and thicker walled, and generally better suited for use as a coupler. Those original JT-80Cs are hard to come by unless you pirate an old kit, which in my mind makes kits like the older Super Big Bertha susceptible, because frankly a Super Big Bertha would better joined by a heavy duty JT-80 coupler than with the old JT-80C. So look for an old Super Big Bertha, scavenge the JT-80C for your ring tail clone, and you're still left with a great kit (the Super Big Bertha) to build, no one the wiser about the internal coupler.

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IIRC, the Maxi Alpha 3 used the JT-80C as well. Does the current Maxi Alpha 3 still use an old-style JT-80C? If so, it'd be easier to find than a Super Big Bertha, although it'd still be an expensive proposition to get a bunch of JT-80Cs that way.

It was in the Maxi Alpha (#1291) and in the Maxi Alpha III (#1321), but I've not bought the new MAIII so I couldn't comment. As for an excuse to buy another kit, I think this is as good as any!

John Brohm said:

It was in the Maxi Alpha (#1291) and in the Maxi Alpha III (#1321), but I've not bought the new MAIII so I couldn't comment. As for an excuse to buy another kit, I think this is as good as any!

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Clone the MAIII the new one is crap with the plastic fin units, but it does use a short section of coupler listed on the parts list as.....JT-80C.