Airframe - where to put the 'splice'?

[David_Stack]

Working on a design (4" diameter, Loc tubing based, L2 motor capable) and the booster section is 44" long, 10" longer than than the Loc tubing. Payload section will be approximately 22" long. Avionics bay will be Loc, with the Stiffy coupler.

It seems that from a strength perspective the best place to splice the additional length of airframe is at the aft end, where any bending moment would be minimal. That of course puts weight (coupler, epoxy) aft of the CG, which would likely be less than ideal.

Am I worrying needlessly about resistance to bending, and it would be no big deal to use a coupler to join the two tubes either at the 22" point (middle of the booster) or even at the top of the 34" tube to add the needed 10"?

Thanks as always

 

[user 30299]

Working on a design (4" diameter, Loc tubing based, L2 motor capable) and the booster section is 44" long, 10" longer than than the Loc tubing. Payload section will be approximately 22" long. Avionics bay will be Loc, with the Stiffy coupler.

It seems that from a strength perspective the best place to splice the additional length of airframe is at the aft end, where any bending moment would be minimal. That of course puts weight (coupler, epoxy) aft of the CG, which would likely be less than ideal.

Am I worrying needlessly about resistance to bending, and it would be no big deal to use a coupler to join the two tubes either at the 22" point (middle of the booster) or even at the top of the 34" tube to add the needed 10"?

Thanks as always

If this is a typical dual-deployment setup, with the avionics bay between the 22" section and the 34" section,
you need to ask yourself how much packing space is lost in the 22" section due to the nose cone's shoulder
and the avionics bay. If your main chute, harness, etc. are in that 22" section - you run out of room quickly.

The lower section, 34", is only seeing a harness and maybe a drogue chute. What we don't know is your
motor tube's length and what's the longest motor case you plan to use.

 

[David_Stack]

If this is a typical dual-deployment setup, with the avionics bay between the 22" section and the 34" section,
you need to ask yourself how much packing space is lost in the 22" section due to the nose cone's shoulder
and the avionics bay. If your main chute, harness, etc. are in that 22" section - you run out of room quickly.

The lower section, 34", is only seeing a harness and maybe a drogue chute. What we don't know is your
motor tube's length and what's the longest motor case you plan to use.

Wasn't clear in describing my design.

Yes the model currently is envisioned to have a 22" long payload section, but my issue of the moment is that the length of the booster section below the avionics bay is 44 " long, 10" longer than a stock length of Loc tubing, so will need to be spliced with a coupler.

Where along the length of the booster to make that joint is my question; at the lower end, at the upper end, or split the difference and make the booster from two 22" lengths of tubing...

Longest motor case would likely be on the order of 20" long (Loki 54/2000 or equivalent).

 

[user 30299]

Wasn't clear in describing my design.

Yes the model currently is envisioned to have a 22" long payload section, but my issue of the moment is that the length of the booster section below the avionics bay is 44 " long, 10" longer than a stock length of Loc tubing, so will need to be spliced with a coupler.

Where along the length of the booster to make that joint is my question; at the lower end, at the upper end, or split the difference and make the booster from two 22" lengths of tubing...

Longest motor case would likely be on the order of 20" long (Loki 54/2000 or equivalent).

I'm still confused about your splicing requirement.

You currently have a 22" long body tube, and a 44" long body tube.

What's the issue with the 44" length and leaving it as is ?

 

[David_Stack]

I'm still confused about your splicing requirement.

You currently have a 22" long body tube, and a 44" long body tube.

What's the issue with the 44" length and leaving it as is ?

Loc tubing is only 34" long. To make up a 44" long booster I need to splice two lengths together. Where to place that joint from a structural perspective is the question...

 

[Titan II]

Am I worrying needlessly about resistance to bending....

Yes you are. Just put it at the top of the 34 with a coupler at least 8" long.

 

[user 30299]

Yes you are. Just put it at the top of the 34 with a coupler at least 8" long.

Thank you Titan II

I agree.

 

[Rschub]

Why not a 34" and a 34" section?

 

[David Schwantz]

Call Jason at LOC and ask him to cut you a 44" tube.

 

[David_Stack]

Thank you all for the input.

I was not aware that Loc would do a longer length (though the cost to ship may be more than offset by Titan II's feedback that I'm worrying unduly about the splice location being a weak point in the airframe).

As for why not a 34" booster and a 34" payload, I prefer to have the Av-bay 'biased' more toward the top of the airframe, for weight if nothing else.

 

[user 30299]

Call Jason at LOC and ask him to cut you a 44" tube.

Just be sure to get a shipping rate first! Been there, done that.

 

[Steve Shannon]

Another alternative is to build your rocket using Stu Barrett’s zipperless construction. In that case the coupler is permanently mounted at the front of the bottom section with a bulkplate closing it off. An eyebolt is attached to the bulkplate. The extra ten inch piece of body tube is attached to your e-bay using removable rivets or screws after you load your ejection charges. The drogue chute comes out the aft end of that piece.

 

[bjphoenix]

I'm thinking there are pros and cons to either location. I think overall the rocket could be stronger with the splice at the bottom end. I gather that the motor tube would extend beyond the splice and help strengthen the combination. The if there are zippers or problems with deploy you could cut off the front part of the 34" section and splice on a new piece easy enough. Putting the splice in the front part of the booster would be easiest but might interfere with space for recovery system and it would be harder to do a future repair. That's probably what I would do though.