Extending the bottom tube on a DD

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SoCalChris

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I've got a kit that I've built, based off of the 4" fiberglass Super Striker from Mad Cow. I'd like to extend the bottom tube so that I can put a longer motor in it than what I currently can.

Is there any reason that I can't put a 12" extension between the bottom of the AV bay and the existing bottom tube, and connect it by epoxying the two tubes together with a coupler, then having the AV bay connect to the new body tube extension? The extra length doesn't cause any issues with stability according to the sim.

If I go this route, is there anything that I need to watch out for?

Thanks
 

rharshberger

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Just make sure the drogue or its harness won't snag in the new airframe coupler.
 

SoCalChris

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Yeah, I was planning on sanding the coupler down so there's a smooth transition there. Thanks for the reply
 

K'Tesh

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While lengthening the rocket will likely add stability, don't forget to check the numbers with the new larger motor. A sim program like RocSim or OpenRocket would probably help quite a bit.

[EDIT] didn't read the whole OP, before replying. D'OH!
 

sharkbait

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Not sure how you were planning to make your connections but, if you epoxy the coupler into the extension tube and then attach the extension tube to your booster with Pem-Nuts, you would be able to revert back to your original configuration anytime you wanted to also. Like previously mentioned, watch out for snagging of your drogue, cover the Pem-Nuts with some strips of electrical tape, or build up some tapered transitions around them with JB Weld.
 

bobkrech

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Don't be offended but it doesn't sound like you have built many high power rockets.

There is no need to extend the motor tube. It does not have to be longer than the motor casing. High power rockets do not use a motor block inside the motor tube, so the motor can extend beyond the motor tube. In a high power rocket, the motor has a lip on the aft nozzle flange that prevent the motor from sliding forward thru the motor tube. You should be using positive motor retention so the motor will not kick out when the ejection charge fires. The stock fin can is 31" long and the e-bay coupler is 9" long. Depending on how you configure the e-bay, you should be able to use a motor casing that is ~22" long. That would be an AT RMS 54/2560 (6G) or CTI Pro54-6G case.
 

sharkbait

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I didn't read the post as extending the MMT, the OP was asking about extending the bottom or booster airframe tube, indicating that a longer motor would hit the bottom of his E-bay and push it up out of the booster section or just not leave enough room to pack his apogee recovery gear By extending the booster airframe 12" or making a removable additional 12" section for the booster, he will be able to load and fly longer motors into the rocket.
 

dixontj93060

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Don't be offended but it doesn't sound like you have built many high power rockets.

There is no need to extend the motor tube. It does not have to be longer than the motor casing. High power rockets do not use a motor block inside the motor tube, so the motor can extend beyond the motor tube. In a high power rocket, the motor has a lip on the aft nozzle flange that prevent the motor from sliding forward thru the motor tube. You should be using positive motor retention so the motor will not kick out when the ejection charge fires. The stock fin can is 31" long and the e-bay coupler is 9" long. Depending on how you configure the e-bay, you should be able to use a motor casing that is ~22" long. That would be an AT RMS 54/2560 (6G) or CTI Pro54-6G case.
He may have the 75mm Super Striker.
 

SoCalChris

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Not sure how you were planning to make your connections but, if you epoxy the coupler into the extension tube and then attach the extension tube to your booster with Pem-Nuts, you would be able to revert back to your original configuration anytime you wanted to also. Like previously mentioned, watch out for snagging of your drogue, cover the Pem-Nuts with some strips of electrical tape, or build up some tapered transitions around them with JB Weld.
I had considered doing this, but will likely just epoxy the extension in. The weight from it is minimal, and I like the sleek slender look that the extra length gives it. Thanks for the suggestion though.

I was searching for a thread like that, thanks. There's some good ideas and great innuendo in there.

Don't be offended but it doesn't sound like you have built many high power rockets.

There is no need to extend the motor tube. It does not have to be longer than the motor casing. High power rockets do not use a motor block inside the motor tube, so the motor can extend beyond the motor tube. In a high power rocket, the motor has a lip on the aft nozzle flange that prevent the motor from sliding forward thru the motor tube. You should be using positive motor retention so the motor will not kick out when the ejection charge fires. The stock fin can is 31" long and the e-bay coupler is 9" long. Depending on how you configure the e-bay, you should be able to use a motor casing that is ~22" long. That would be an AT RMS 54/2560 (6G) or CTI Pro54-6G case.
This is my first L2 level rocket. I've built a few fiberglass 29mm before, but this is by far the largest most complicated build that I've undertaken, and the first with electronics. As was mentioned below, I'm not extending the motor mount, but the body tube so that I can fit a larger motor in the airframe. For motor retention, I'm using and Aeropack retainer ring.

I didn't read the post as extending the MMT, the OP was asking about extending the bottom or booster airframe tube, indicating that a longer motor would hit the bottom of his E-bay and push it up out of the booster section or just not leave enough room to pack his apogee recovery gear By extending the booster airframe 12" or making a removable additional 12" section for the booster, he will be able to load and fly longer motors into the rocket.
This is exactly the case. As the kit was built, the largest motor I can fit and have room for the drouge and harness is a 4 grain CTI case. I extended it so that I can fit a 6 grain and have room for the recovery equipment.

He may have the 75mm Super Striker.
Nope, this is 54mm. My next will be a 75mm in pursuit of my L3 :)

Thanks everyone for the advice and suggestions. One of the great things about where I live is that Madcow is a half hour drive away. I called them this morning, and they were in the shop and had no problem with me coming down to pick up the pieces. They cut exactly what I needed for me, gave my kids some stickers, and had some great conversation. I can't say enough good things about Madcow, I've never had anything but a great experience dealing with them.

IMG_20160813_173606.jpg

Here is the rocket with the extension already in place, just not epoxied in yet.
 
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sharkbait

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NICE!!!

Make sure you post an inflight pic with that big 6g "K" in there next.

Good luck
 

K'Tesh

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Here is the rocket with the extension already in place, just not epoxied in yet.

NICE! How tall is it now? Of course, now that I've looked at the image full size, I realize that it's not a full grown adult holding it up, so it's shorter than my first perception.
 

SoCalChris

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Final height is 8'2", so it's decent size.

I wound up ordering a CTI 6 grain XL case, with an L640. I don't plan on flying this one too often, so hopefully CTI will be back up and producing by the time I'm ready to fly it again. I really want to try the L265 with the 10 second burn, but those are currently out of stock.

Electronics wise, it has an RRC3 as the main altimeter, with an Egg Timer Quark as backup. I also have an Eggfinder in it.
 
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markkoelsch

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Final height is 8'2", so it's decent size.

I wound up ordering a CTI 6 grain XL case, with an L640. I don't plan on flying this one too often, so hopefully CTI will be back up and producing by the time I'm ready to fly it again. I really want to try the L265 with the 10 second burn, but those are currently out of stock.

Electronics wise, it has an RRC3 as the main altimeter, with an Egg Timer Quark as backup. I also have an Eggfinder in it.
What does this rocket weigh prepped for flight sans motor?
 

SoCalChris

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Including the extension tube, electronics, batteries and recovery gear, it's just under 12.5 lbs.
 

Bat-mite

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One of the great things about where I live is that Madcow is a half hour drive away. I called them this morning, and they were in the shop and had no problem with me coming down to pick up the pieces. They cut exactly what I needed for me, gave my kids some stickers, and had some great conversation. I can't say enough good things about Madcow, I've never had anything but a great experience dealing with them.
I can't get to YouTube from my work computer, but if I could, I would post Eric Idle from Monty Python's Life of Brian saying, "You lucky ... lucky ... bastard!" :lol:
 
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