First timer with a few questions

[Tamir Friedman]

Hello,

Recently we established a new rocketry club in my University and we are currently working on a high powered rocket using the J425R Aerotech motor. As part of our design we are planning to have the rocket built of two carbon fiber tubes connected via a coupler at the middle. The top part of our rocket is going to have some sensitive electronics so we decided to have our rocket separate midway via the ejecting charge of our motor. I am currently working on designing the volume in the rocket dedicated for the recovery system and I have few questions as a first timer:

1. Would closing the volume via bulkheads glued using epoxy to the body tube would be sufficient to sustain the ejection charge and separate the rocket? Or will the whole thing will break into pieces if we attempted it? we are currently planning on using plywood bulkheads, but we also have fiberglass sheet that we can use to make the whole thing tougher.

2. Currently our design is very much "dumb design". It includes the coupler being epoxied to the top tube of the rocket and slided into the bottom tube, with no reinforcements (it will basically hold together via drag only). Are we underthinking it and the rocket will randomly separate mid flight by accident?

3. How do I calculate the amount of PSI required to separate the rocket?

Those are the dimensions of the recovery/ejection volume in the rocket:
- diameter - 10cm
- length - 28 cm
- coupler length - 20cm (10 cm into each tube)
- coupler and body tubes made of carbonfiber.


Thank you so much in advance!

 

[user 30299]

Hello,

Recently we established a new rocketry club in my University and we are currently working on a high powered rocket using the J425R Aerotech motor. As part of our design we are planning to have the rocket built of two carbon fiber tubes connected via a coupler at the middle. The top part of our rocket is going to have some sensitive electronics so we decided to have our rocket separate midway via the ejecting charge of our motor. I am currently working on designing the volume in the rocket dedicated for the recovery system and I have few questions as a first timer:

1. Would closing the volume via bulkheads glued using epoxy to the body tube would be sufficient to sustain the ejection charge and separate the rocket? Or will the whole thing will break into pieces if we attempted it? we are currently planning on using plywood bulkheads, but we also have fiberglass sheet that we can use to make the whole thing tougher.

2. Currently our design is very much "dumb design". It includes the coupler being epoxied to the top tube of the rocket and slided into the bottom tube, with no reinforcements (it will basically hold together via drag only). Are we underthinking it and the rocket will randomly separate mid flight by accident?

3. How do I calculate the amount of PSI required to separate the rocket?

Those are the dimensions of the recovery/ejection volume in the rocket:
- diameter - 10cm
- length - 28 cm
- coupler length - 20cm (10 cm into each tube)
- coupler and body tubes made of carbonfiber.
View attachment 465938

Thank you so much in advance!

Are you able to post your whole rocket design? And if you're using Rocksim or Open Rocket, how about the design file.

 

[Zeus-cat]

1. Most of use plywood bulkheads and they work just fine. Just make sure they are thick enough and good quality plywood. I use 1/4 inch Baltic birch plywood. I'm not sure what the metric equivalent is, probably 6mm. Do not use 4mm if that is an option, too thin and can break apart.

2. You need a tight fight or you could have drag separation (research the term). Most rockets are fine if you pick them up by the nose and shake them gently and they stay together. If the fit is loose, use masking tape on the coupler to make it tighter.

3. What are you using as a separation charge? There are charge calculators that will tell you the required black powder needed based on the volume you have. I don't have a link, but someone will post one. ALWAYS do a test on the ground to make sure the charges are sufficient to separate the rocket!

 

[Handeman]

You are using a 100mm diameter body tube, which is not a standard kit size. That leads me to believe you are making your own body tubes and coupler.

I see two choices, glue the upper and lower tubes to the coupler and eject the nose cone, or put bulkheads on the ends of the coupler and eject the upper half from the lower half of the rocket.

I would recommend the latter. Build the couple like an av-bay. Even if you never use electronics, this will give you the option.

This calculator should tell you if the standard ejection charge on the J425R is sufficient for your design. https://www.insanerocketry.com/blackpowder.html

Good luck

 

[Tamir Friedman]

You are using a 100mm diameter body tube, which is not a standard kit size. That leads me to believe you are making your own body tubes and coupler.

I see two choices, glue the upper and lower tubes to the coupler and eject the nose cone, or put bulkheads on the ends of the coupler and eject the upper half from the lower half of the rocket.

I would recommend the latter. Build the couple like an av-bay. Even if you never use electronics, this will give you the option.

This calculator should tell you if the standard ejection charge on the J425R is sufficient for your design. https://www.insanerocketry.com/blackpowder.html

Good luck

Thank you for the reply. We were planning on gluing a bulkhead on top of the coupler as you suggested. From what I have found in my research it seems the only that matters is volume of the bay that the separation charge activate in. All the calculators I have found online considered a scenario where the separation happens at the nosecone and I wondered if it makes any difference if I was attempting to separate two body tubes or a nosecone from a tube

 

[Tamir Friedman]

1. Most of use plywood bulkheads and they work just fine. Just make sure they are thick enough and good quality plywood. I use 1/4 inch Baltic birch plywood. I'm not sure what the metric equivalent is, probably 6mm. Do not use 4mm if that is an option, too thin and can break apart.

2. You need a tight fight or you could have drag separation (research the term). Most rockets are fine if you pick them up by the nose and shake them gently and they stay together. If the fit is loose, use masking tape on the coupler to make it tighter.

3. What are you using as a separation charge? There are charge calculators that will tell you the required black powder needed based on the volume you have. I don't have a link, but someone will post one. ALWAYS do a test on the ground to make sure the charges are sufficient to separate the rocket!

1. We got a bunch of 1/4 plywood bulkheads (good quality) from the apogee store and wondered we would require to cover it with a layer of fiberglass to make sure it won't break.

2. Will try the shake test as you suggested. By "using masking tape to make it tighter" you mean circling the inner part of the body tube with masking tape so the space between the free tube and the coupler would shrink, or do you mean actually gluing the coupler to the free tube?

3. We plan on using black powder that arrived in the motor kit for the J425R that ejects after the motor runs out of fuel. We plan on having a secondary charge stored in the separation tube to be ignited via a computer if for some reason the motor's separation charge doesn't explode (a failsafe). I have found a calculator for the amount of black powder needed, but it seems to talk about a scenario where there is an attempt to eject the parachute through a nosecone, and not about separating the rocket at the midpoint. I wonder if more black powder is required in this scenario since the top tube of our rocket will be filled with some heavy electronics and might apply force in the opposite direction. It might not really matter, because by logic it would just mean that the tube that will get pushed away will be the bottom tube, but I would rather have the opinion of an expert considering this is our first attempt in a project like this.

Thank you so much for the help!

 

[tsmith1315]

No difference in where separation occurs, because there is no standard for tightness of nose cone & coupler fit. It is up to the builder. Beyond that, you are just pressurizing a volume. That's why Zeus-cat recommended ground testing.

 

[Zeus-cat]

1. 1/4 inch plywood is solid stuff. I have shattered bulkheads that were 3/16ths of an inch, but never had a problem with the 1/4 ones.

2. Wrap the tape around the outside of the coupler, about halfway down the coupler. Then insert the coupler into the upper section and push them together. It doesn't take much tape to go from loose to a good fit. If you have to really pull the two pieces apart you have used to much tape.

3. Don't overthink this. Do the calculations and do several tests on the ground to make sure it works. You want the two sections of the rocket to come apart with confidence that it will come apart when you fly it. If the upper section comes off and falls down 10cm from the rear section you need more black powder. If the upper section flies 100m away you are using too much black powder. Do your test as realistically as possible; so put wadding and your parachute in the rocket. Put something behind the rocket so it doesn't fly backwards and hit something when the charge goes off. Keep everyone away from the rocket during the test; especially the front section that will get pushed off. I run the wires for the test charge up through the motor mount and set it off with a battery. Wear safety glasses when you wire the charges and during the test; you are dealing with explosives.

 

[Tamir Friedman]

1. 1/4 inch plywood is solid stuff. I have shattered bulkheads that were 3/16ths of an inch, but never had a problem with the 1/4 ones.

2. Wrap the tape around the outside of the coupler, about halfway down the coupler. Then insert the coupler into the upper section and push them together. It doesn't take much tape to go from loose to a good fit. If you have to really pull the two pieces apart you have used to much tape.

3. Don't overthink this. Do the calculations and do several tests on the ground to make sure it works. You want the two sections of the rocket to come apart with confidence that it will come apart when you fly it. If the upper section comes off and falls down 10cm from the rear section you need more black powder. If the upper section flies 100m away you are using too much black powder. Do your test as realistically as possible; so put wadding and your parachute in the rocket. Put something behind the rocket so it doesn't fly backwards and hit something when the charge goes off. Keep everyone away from the rocket during the test; especially the front section that will get pushed off. I run the wires for the test charge up through the motor mount and set it off with a battery. Wear safety glasses when you wire the charges and during the test; you are dealing with explosives.

Thank you so much for the answer, We will do it for sure

 

[David Schwantz]

Good luck with your flight. Myself, I would build it as dual deploy. I would install 2 removable bulkheads in the coupler, use shear pins and steel screws to insure the airframe does not come apart in flight. One 2-56 nylon screw will hold it together and one steel screw to keep coupler in place is all that should be needed for this size rocket. If you glue in the coupler you can not remove it to service anything. Your electronics could be housed in the coupler ( av bay ), this way they are protected. Can still use it just as single deploy with motor eject.

 

[Tractionengines]

Tamir,

A couple of quick questions. If you have said elsewhere sorry I missed it.
Where is your university?
Does your club have a L2 certified mentor, or a L2 certified member who will be responsible for the flight? If in the USA, or most areas, you can't just buy "J" motors. Someone would need to get L1 certified ("H & I" motors). Then pass a L2 test. Then they can buy 1 ("J, K, or L") motor for certification flight. After certified L2 that person can buy more motors...
If multiple numbers of the club want to get certified....each MUST build their own rockets, fly certification flights, and pass L2 test.
Good luck with your build, and keep asking questions...
Mike

 

[Zeus-cat]

I don't think he is in the U.S.

 

[Tractionengines]

Agree likely not USA, but he said buying parts from Apogee.. so maybe... just thought I would check.

 

[Tamir Friedman]

Tamir,

A couple of quick questions. If you have said elsewhere sorry I missed it.
Where is your university?
Does your club have a L2 certified mentor, or a L2 certified member who will be responsible for the flight? If in the USA, or most areas, you can't just buy "J" motors. Someone would need to get L1 certified ("H & I" motors). Then pass a L2 test. Then they can buy 1 ("J, K, or L") motor for certification flight. After certified L2 that person can buy more motors...
If multiple numbers of the club want to get certified....each MUST build their own rockets, fly certification flights, and pass L2 test.
Good luck with your build, and keep asking questions...
Mike

We are not based in the US, no xD
Our club is in the Hebrew University of Jerusalem (Israel), and our club has a certified mentor