Charles_McG
Ciderwright
In the Pro24,29, and 38 lines, which have offset cores? And is there a way to tell from the CTI website (as opposed to buying one of everything and looking)?
CTI Moonburners?
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look to the far right of each motor. it says long burn. (looking at the thrust curves looks like moon burners to me? I'm not sure what the grain geometry is with Mellow Yellow)
https://www.pro38.com/products/pro24/motor.php
https://www.pro38.com/products/pro29/motor.php
https://www.pro38.com/products/pro38/motor.php
Tony
https://www.pro38.com/products/pro24/motor.php
https://www.pro38.com/products/pro29/motor.php
https://www.pro38.com/products/pro38/motor.php
Tony
stealth6
insert witty tagline here
First of all, I definitely think that "buying one of everything and looking" is a great plan (or at least a usable justification!).
Second, I'm guessing that you are worried about the whole "offset core means the possibility of a wobbly flight, especially in something like a minimum diameter / small fins rocket" thing. This seems to be a common concern, or perhaps an assumption. I myself am not so sure it's as big an issue as folks seem to assume it is. And this seems like a good place to have a discussion about it.
So, what are folks finding in the real world as regards this issue? Are people really seeing odd/strange/wobbly flights with these motors? Is it actually a problem, or is it something we THINK is going to be a problem?
I myself never had any bad flights with these motors (or rather with this grain geometry). I haven't flown a ton of them, but the ones I did all had nice straight boosts, and that includes some tiny/light/optimized for altitude rockets. In my (admittedly limited) experience, if your rocket is built well (ie: straight fins, proper CG/CP, proper mass/thrust relationship, etc.) it will fly straight and true with these motors.
Is this true for others, or have I just got lucky?
s6
Second, I'm guessing that you are worried about the whole "offset core means the possibility of a wobbly flight, especially in something like a minimum diameter / small fins rocket" thing. This seems to be a common concern, or perhaps an assumption. I myself am not so sure it's as big an issue as folks seem to assume it is. And this seems like a good place to have a discussion about it.
So, what are folks finding in the real world as regards this issue? Are people really seeing odd/strange/wobbly flights with these motors? Is it actually a problem, or is it something we THINK is going to be a problem?
I myself never had any bad flights with these motors (or rather with this grain geometry). I haven't flown a ton of them, but the ones I did all had nice straight boosts, and that includes some tiny/light/optimized for altitude rockets. In my (admittedly limited) experience, if your rocket is built well (ie: straight fins, proper CG/CP, proper mass/thrust relationship, etc.) it will fly straight and true with these motors.
Is this true for others, or have I just got lucky?
s6
Charles_McG
Ciderwright
I didn’t believe it either, until l watched it.
BT55 body tube apparently failed about 1.4sec into 2.2sec G65 burn.
Look at the twists in the smoke trail.
BT55 body tube apparently failed about 1.4sec into 2.2sec G65 burn.
Look at the twists in the smoke trail.
Charles_McG
Ciderwright
I didn’t believe it either, until l watched it.
BT55 body tube apparently failed about 1.4sec into 2.2sec G65 burn.
View attachment 360002
Look at the twists in the smoke trail.
View attachment 360003
BT55 body tube apparently failed about 1.4sec into 2.2sec G65 burn.
View attachment 360002
Look at the twists in the smoke trail.
View attachment 360003
Charles_McG
Ciderwright
This is about the 5-6th flight for my Nike Apache. The second successful staging. Others were boosted dart mode, or failed ignition. But they flew straight.
Rocketjunkie
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Mellow motors are bates grain. VERY slow propellant. The rest are moon burners or in 54mm, C slot.
mikec
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Yes. I've had some terrible wobbly flights on moonburners with rockets that were fine on BATES motors. The Pro54 J140 has been a particular offender for me.
boatgeek
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The 29mm G54 Red is also a moonburner grain but isn't labeled that way on the website. Looking at burn time, I think that's the only offset core that isn't labeled as a longburn. If you're worried about the wobble, the Mellow motors are probably just the ticket.
Andrew_ASC
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The motor burn itself, I believe is offset from a cylinderical core. The rocket is stable CP/CG wise in a sim but unstable in actual flight due to a moment about an reference axis down the center of rocket as the thrust isn’t centerline compared to a center core variant propellant grain. It’s got a torque. Some lightweight rockets or large rockets have problems. Somebody with more experience might want to speak up. The propellant core geometry and how it burns is the issue and the manufacturer does not indicate core geometry before you buy.
There wasn’t a yellow offered in the Pro24-6G line. The G-65 was the longest burn option and its core grain option sucked. The G-65 is the only MPR load for that Pro24-6G casing. The rest are HPR.
I have second hand knowledge of the CTI Pro29-5G H-53 Mellow load working fine for multistage sustainers. It has slow burn no offset.
What I can do for the OP is crack open a new L-1 G-145 reload for the Pro24-6G and we all can see the grain geometry of the Pink load if anybody cares to start a list with motors lying around. Might be cheaper than buying one of all.
Here is the Pro24-6G G-145 Pink core geometry, a cylinderical hole. So this one has centerline thrust.
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That is not a moon burner.
Andrew_ASC
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Which is why I posted it so next time I go to buy one I’ll remember.
Charles_McG
Ciderwright
Other threads here on TRF discuss the effect as coming from an off axis center of mass, rather than angled thrust.
Charles_McG
Ciderwright
Andrew_ASC
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Did a moonburner cause this?
Keisling
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I'd bet more on coning...the thrust from a moon burner has to come out a hole in the center of the nozzle. Likely this rocket had coning happen.
Charles_McG
Ciderwright
I think the question is: do moonburners make a rocket prone to coning cone more?
A lot of other threads suggest that it’s not the offset thrust - as noted, the gas comes out a nozzle that’s centered- that should recenter it. I’m mostly, but not completely convinced. Someday I’d like to build a test stand to specifically look for a small off axis force. Maybe in the next 20 years or so
. Also, it wouldn’t surprise me if the off-center core resulted in an exhaust plume with a subtle torque- not off angle, but rotating. That’s not math, just my intuition - which could suck.
The off axis mass makes a great deal of sense. If coning is roll-pitch coupling, an off axis weight would cause both a small pitch, and a roll to put it toward the ground. They could reinforce each other. It would be worse in a min diameter (or near MD) because the motor would be be a much bigger portion of the total radial moment - no airframe at a bigger radius diluting the effect.
My personal ‘feel’ is that it’s not either/or, but both.
A lot of other threads suggest that it’s not the offset thrust - as noted, the gas comes out a nozzle that’s centered- that should recenter it. I’m mostly, but not completely convinced. Someday I’d like to build a test stand to specifically look for a small off axis force. Maybe in the next 20 years or so
. Also, it wouldn’t surprise me if the off-center core resulted in an exhaust plume with a subtle torque- not off angle, but rotating. That’s not math, just my intuition - which could suck. The off axis mass makes a great deal of sense. If coning is roll-pitch coupling, an off axis weight would cause both a small pitch, and a roll to put it toward the ground. They could reinforce each other. It would be worse in a min diameter (or near MD) because the motor would be be a much bigger portion of the total radial moment - no airframe at a bigger radius diluting the effect.
My personal ‘feel’ is that it’s not either/or, but both.
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5x7
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It doesn't matter where the hole in the vessel is relative to what's burning inside it. There could only be vectoring if there was some uneven pressure across the tiny nozzle. The gasses flow through the nozzle just under mach 1, there is no vectoring. They would all veer off the rail if there were and they would all do it and you could see it in photos.
Loki Research
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You should try contacting the Florida Institute of Technology. They have a test stand that does this. I don't know if they have ever tested offset cores, but I would think it's likely. Maybe they can shed some light on this for everyone.
I'd like to see the off axis data. I'm actually more interested in the off axis data of MR and LMR motors. As you should know, the expected (mean and S.D.) and worst case thrust misalignment is the key input to determining the optimal static margin and fin cant (or spin program) to minimize flight dispersion. I've been waiting for such data for decades. It is such a massive problem that is best left to S&T which routinely tests lots of motors anyway. I have had a batch of AT F80 motors with visibly off center nozzles!
rharshberger
Well-Known Member
That Pink motor is a Bates grain type configuration.
Charles_McG
Ciderwright
Been thinking of it.
