New to the rocketry model hobby and also new to the forum. I have successfully assembled and launched the KLIMA rocket starter kit with parachute deployment and B4-4 engines. I have also 3d-printed some ready designs of rockets and launched them successfully but I also wanted to design my own rocket with the help of Openrocket for stability. I made this rocket and had some troubles maybe because the engine was not a tight fit with the body so the rocket did not go up in a straight line. Now I am improving the design by making the body hole for engine tighter, 18mm instead of 18,2mm) and also making the fin bigger for better stability and everybody’s advice will be helpful. Material I use is PLA which has proved its temperature resistance. My regards to all the members.
Hello from Athens Greece
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Welcome!
A few quick comments on your design:
1) It is more typical for the motor to extend a bit out the back of the rocket (say, 5mm) to (a) make it easier to grab the motor to remove it, and (b) to avoid cooking the inside of the body tube. This will, however, move your CG back a bit, so you'll have to take that into account. You'll also need some sort of retention for the motor, which could be a hook, friction fit, external screw-retainer, or something else.
2) Yes, you will want the motor to be a close fit into the mount (but it shouldn't be hard to insert the motor).
3) It is not clear from the ORK file exactly how you have fashioned the motor mount, but you need to be sure that, when the motor is installed, that the front end of the motor is sealed off from the rear, so the ejection charge can work.
4) When the rocket is built, you'll want to measure the mass and CG of the whole thing *without the motor* and then override the "sustainer". That'll be more accurate than measuring and overriding each of the parts, and it'll let you make a final determination of stability of the rocket (*with* motor).
A few quick comments on your design:
1) It is more typical for the motor to extend a bit out the back of the rocket (say, 5mm) to (a) make it easier to grab the motor to remove it, and (b) to avoid cooking the inside of the body tube. This will, however, move your CG back a bit, so you'll have to take that into account. You'll also need some sort of retention for the motor, which could be a hook, friction fit, external screw-retainer, or something else.
2) Yes, you will want the motor to be a close fit into the mount (but it shouldn't be hard to insert the motor).
3) It is not clear from the ORK file exactly how you have fashioned the motor mount, but you need to be sure that, when the motor is installed, that the front end of the motor is sealed off from the rear, so the ejection charge can work.
4) When the rocket is built, you'll want to measure the mass and CG of the whole thing *without the motor* and then override the "sustainer". That'll be more accurate than measuring and overriding each of the parts, and it'll let you make a final determination of stability of the rocket (*with* motor).
Welcome! I'm surprised PLA holds up that well to the heat. I've printed in PLA+, but have not flown those yet (they're heavy). The PLA+ is supposed to handle heat much better and is not that much more money.
You’ve got a good start. Welcome aboard!
H Neil! I am more than happy to share with the community my design for further comments, advice and suggestions. See attached the zip file containing the stl's. This is the model of the ork file I shared in my previous post.
Regarding your comments, you are absolutely right because in the ork file the details are not shown, so you couldn't figure out how it works. With the stl files I am sharing this is clarified, so for this design I don't actually need a motor retentions because the lowest part gets from 18mm hole to 14mm in order to retain the motor.
The design of screwing parts in the middle allows me to put the motor a bit in front in order to gain some millimeters of CG upfront without the need to extend the motor out of the back for grabbing it.
So, I am now trying to figure out why the rocket didn't go very well while the ork file says it's stable... Maybe it happened? I forgot to mention 2 things:
1) that I used a 30 degree angle to do this launch, maybe this was the problem?
2) apart from not fully tight fit of the engine due to hole with diameter of 18.2mm, I noticed that there was also approximately 0,5mm (yes, 0,5mm) tolenarce vertically (engine could go up/down 0,5mm). I know it's too short distance, but maybe it should be very tight fit, I don't know.
Update: Some hours ago I 3d-printed a more tight-fit body (18mm instead of 18.2mm) and I then used a used 18mm engine to check if it fits, but I couldn't fit the engine all the way. The rear side of the engine (the side that we put the fuse in) cannot fit while the other side can! This makes me conclude that after the burnout of the engine the fuse-side becomes a bit thicker that's why this side cannot fit into a 18mm hole while the other can??? Or this side is thicker even before the launch? Unfortunately, I ran out of brand new engines so I can't test with a new one at the moment.
And last but not least, can you please clarify number 4? Why should I find the CG without motor? The CG with motor is not what really matters?
Thanks again for your comments!
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Hi Huxter, I think that PLA+ is just another version of PLA with some more ingredients in it which increase its melting temperature by 10-20 degrees Celsius. On that perspective, for optimal results for the PLA I use, I have set my 3d printer's nozzle temperature at 208 degrees Celsius which is approximately 18 degrees more than the normal PLA temperatures (190 degrees Celsius), so this may be called PLA+. Furthermore, in my design (see above the stl files I have uploaded) I use 9mm material thickness below the engine in order to tolerate the temperatures. While in other designs I printed the PLA did not melt, in my design I don't know because when I launched the rocket it flew away and I couldn't retrieve it to check it. Maybe in the next launch I will be more lucky!
Regarding your printed PLA rockets, why are they heavy? What is their diameter and what is the wall thickness? I have noticed that above 1,5mm they’re getting really heavy and they don’t fly high unless a C or bigger motor is used.
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Interesting. I would still be inclined to keep the motor at least flush with the tail end to avoid cooking the end of the BT and also to avoid Krushnic effect (although based on this ORK file you are not in Krushnic territory, but it's something to watch out for if you're recessing the motor).
That is normally steeper than recommended. Safety code in the US limits rod angle to 20 degrees. It may or may not have anything to do with your unstable flight, but you should keep the rod straighter than that.
If your 3D print varies even a bit from 18mm you're not going to fit. You might have to make it very slightly larger to ensure good fit. Used motors might be very slightly enlarged, but remember you will need to be able to remove the used motor from your rocket, so you don't want it to be super-tight beforehand.
The way it works is: you measure mass and CG of the finished rocket and enter it into the override fields of the Sustainer in OR. Then, when you configure it with a motor, OpenRocket will add the mass of the motor and calculate the adjusted CG and stability margin. And *that* is the number that you use to judge the stability of the rocket with that motor.
Thanks Neil, pretty good advice. Will do that with OR. I found the CG. I am pretty sure it’s what OR calculated in the first place though.
Actually after the discussion here I don’t see any big problem with my rocket design. Maybe the small fins made it unstable or maybe it just happened randomly. Maybe I should print this version again and give it one more try.
Neil, if you notice in my design the hole from 18mm becomes 14mm in order to act as motor retainer. Do you think that 14mm affects the smooth gas exhaustion? Should I make the hole 16mm instead?
14mm hole should be fine for an 18mm motor.
Hi Neil, your comment number 4, on how to properly use Openrocket, really helped me a lot. I printed a very stable rocket which went on a straight line on the sky. Thanks a lot.
