Moonburner offset core effect on stability

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mpitfield

mpitfield

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It was suggested to me by someone at NYPOWER, as well as confirmed by Scott from Loki today, that the off-set core of a moonburner motor can have an effect on the center of gravity along the vertical center axis of the rocket. In both cases they felt that a 54mm minimum diameter would "likely be okay" or don't worry about it", however they both suggested that I post a question on TRF.

My 54mm minimum diameter rocket is 77", approximately 5.5lbs or 2500g dry, and has 3 fins. The motor I am interested in using is a Loki K-350 moonburner (Thrust curve profile here https://www.thrustcurve.org/motorsearch.jsp?id=311). On the pad the rocket is about 11.5lbs or 5200g which puts it roughly @ 2.5 over-stable.

Thank you for your opinions.
 
I feel you'll be OK too. You have a very long and heavy rocket and have good starting stability. Most of the ones that I can remember having issues are those that are short 50ish"and light 8 poundish 1 cal stability on the pad. The off set core has more affect on these types of rockets.

Tony
 
tfish said:
I feel you'll be OK too. You have a very long and heavy rocket and have good starting stability. Most of the ones that I can remember having issues are those that are short 50ish"and light 8 poundish 1 cal stability on the pad. The off set core has more affect on these types of rockets.

Tony
Click to expand...

Thank you for your feedback Tony, much appreciated.

You're not the first one to say that my rocket s heavy. Admittedly I normally overbuild, however this rocket is carbon fiber and there isn't much on the rocket that is not needed. To lighten it up I removed with 3 quick-links by looping the harness through the loops of the eye-bolts and at the end of the bridle. On the nosecone I did away with the eye-bolt all together and simply looped a short bit of Kevlar though some holes, tied and applied some CA on the knots.

It is amazing how light the rocket is when empty then how heavy it is when all full.
 
CA on kevlar will make the kevlar brittle. Kevlar stitching or heat shrink over your knots will allow the kevlar to remain flexible.
 
KBlack said:
CA on kevlar will make the kevlar brittle. Kevlar stitching or heat shrink over your knots will allow the kevlar to remain flexible.
Click to expand...

Thanks KBlack, I will try the heat shrink next time.

I did notice it makes it brittle, however the bulkhead and nosecone assembly is very modular in that it can be completely disassembled and serviced. In this case the harness is simply holding the nosecone on so the stress on the rope and knots would be minimal, plus the CA is only on the end of the knot holding it together and not through the rope so it still flexes as it did without any CA.
 
A moon burner has an asymmetric propellant mass distribution until the motor burns out and even if your rocket is perfectly balanced without the motor, you will always have a turning moment due to the asymmetric mass distribution which is effectively the same as an asymmetric thrust. This is unavoidable and unless you spin the rocket at a rather high rate to average out the asymmetry, the rocket will turn it the direction of the extra mass. If you have a 6 DOF flight simulation program you could simulate the effect but I don't believe you can do it without modifying the program inputs. You should however be able to estimate it by running burnsim to get an idea how the Ix shifts as a function of time during the burn. I would guess without doing any calculations, the highest asymmetry occurs about halfway through the burn.

This rocket has an aspect ratio of ~35 to 40 to 1. While the zero angle of attack stability factor may be 2.5 in the y direction, it will be different and varying in the x direction. To further complicate the motion, due to the very large aspect ratio, the stability can rapidly disappear at relatively small angles of attack due to the large airframe surface area that simple stability calculations ignore.

Bob
 
bobkrech said:
A moon burner has an asymmetric propellant mass distribution until the motor burns out and even if your rocket is perfectly balanced without the motor, you will always have a turning moment due to the asymmetric mass distribution which is effectively the same as an asymmetric thrust. This is unavoidable and unless you spin the rocket at a rather high rate to average out the asymmetry, the rocket will turn it the direction of the extra mass. If you have a 6 DOF flight simulation program you could simulate the effect but I don't believe you can do it without modifying the program inputs. You should however be able to estimate it by running burnsim to get an idea how the Ix shifts as a function of time during the burn. I would guess without doing any calculations, the highest asymmetry occurs about halfway through the burn.

This rocket has an aspect ratio of ~35 to 40 to 1. While the zero angle of attack stability factor may be 2.5 in the y direction, it will be different and varying in the x direction. To further complicate the motion, due to the very large aspect ratio, the stability can rapidly disappear at relatively small angles of attack due to the large airframe surface area that simple stability calculations ignore.

Bob
Click to expand...

Hi Bob,

Thanks for your input. I understand most of what you say, at a fairly high level, however what I am not getting is a sense of whether you would recommend this motor or not. One theme that seems to be consistent with your post and the previous one as well as the two other comments I had offline is that there is no reasonable way to determine what will happen unless I launch it.
 
I strongly suspect your rocket doesn't have perfectly aligned fins. A little angle on one and you get some lift on that fin causing the rocket to spin. The spinning rocket will even out the difference in the moonburner weights as it ascends. You might get a little coning, but I would be surprise that you see any effect at all. If there is any effect from the moonburner weights, I don't think you will be able to tell what is from the motor mass and what is induced from construction variances or winds at the launch site. Unless the wind is less than 1 or 2 mph for the first 1000 ft or so I think wind effects on the flight will be much larger than any effect you get from motor mass even with perfect construction.

Kind of like worrying about how a large hood ornament on your car will effect MPG. It will, but it will be so minor you won't be able to tell what the effect is except in perfect test conditions.
 
Handeman said:
I strongly suspect your rocket doesn't have perfectly aligned fins.
Click to expand...

I am so offended that you are suggesting that my abilities would result in less than a perfect rocket :wink:

Handeman said:
A little angle on one and you get some lift on that fin causing the rocket to spin. The spinning rocket will even out the difference in the moonburner weights as it ascends. You might get a little coning, but I would be surprise that you see any effect at all. If there is any effect from the moonburner weights, I don't think you will be able to tell what is from the motor mass and what is induced from construction variances or winds at the launch site. Unless the wind is less than 1 or 2 mph for the first 1000 ft or so I think wind effects on the flight will be much larger than any effect you get from motor mass even with perfect construction.

Kind of like worrying about how a large hood ornament on your car will effect MPG. It will, but it will be so minor you won't be able to tell what the effect is except in perfect test conditions.
Click to expand...

You seem to be inline with everyone else, in that who knows what will happen, but likely nothing. Coning was one of the things I was worried about but more because it was over-stable, I was not thinking about the coning effect as a result of the center of gravity off-set.

I have only seen one video with the K-350 Moonburner which was in crazy Jim's drag race @ Southern Thunder 2007

[video=youtube;G73KrgB7ses]https://www.youtube.com/watch?v=G73KrgB7ses[/video]

I am not sure which rocket it was, likely the one in the end of the video that just keeps on going, but it would be nice to know more about the rocket. For that matter anyone that has flown the Loki K-350 I would really appreciate hearing about your experience.
 
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I think any instability from the offset core will be compensated by the awesomeness of the long burn. I have burned a few moonburners in a Mongoose 98, and I never really noticed any difference in the flight compared to standard cores. (anecdote alert!)

Dale
 
As tfish pointed out, your rocket is heavy and long -- it'll be fine. I've had trouble with moonburners only in shorter/lighter rockets (2# dry and as long as a Pro54 6XL case with a few extra inches for chute and nose cone).
 
I believe these are two good examples of what Bob was saying can happen with a moonburner

[video=youtube;cIYDYSBlAkw]https://www.youtube.com/watch?v=cIYDYSBlAkw[/video]

[video=youtube;V7BjSZFCPqE]https://www.youtube.com/watch?v=V7BjSZFCPqE[/video]
 
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jimzcatz said:
The nozzle is still in the middle, why would an offset core effect anything?
Click to expand...

Hi Jim,

Below is some information from another site which should put it into perspective. The picture shows two moonburners, #2 is what we are talking about in this thread.

I am not sure if it is technically correct, but it provides a good illustration.

1) Standard Core grains
2) Moonburner (Tangent Port) grains
3) Moonburner (Offset Port) grains
4) C Slot grains (Slot depth = Grain Radius)
4) C Slot grains (Slot Depth < Grain Radius)
5) Pellet grains
6) Cored End Burner grains
7) Star Grains

View attachment 264340
 
I think this aspect has not received enough attention (even though it's not part of the original question):

bobkrech: To further complicate the motion, due to the very large aspect ratio, the stability can rapidly disappear at relatively small angles of attack due to the large airframe surface area that simple stability calculations ignore.
Click to expand...

6.8 % of the rocket length is a small stability margin. What Bob means is that this can result in instability with sufficient side wind at lift off (CP would move forward of CG). Also, it gives your fins only a short lever to correct the trajectory.

10-15 % of rocket's length distance between CP and CG is more reasonable.

Oliver
 
Raketenolli said:
I think this aspect has not received enough attention (even though it's not part of the original question):
Click to expand...

I whipped up a quick and dirty 54mm X 81" long rocket and simmed it using Rocksim on a AT K185. With breezy conditions (15-25mph) and a 6' launch rail, the initial angle of attack was over 25 degrees and the CP shifted by almost 5 calibers. Both Rocksim and Openrocket will plot the CP location during flight so this sort of thing is fairly easy to check.
 
UhClem said:
I whipped up a quick and dirty 54mm X 81" long rocket and simmed it using Rocksim on a AT K185. With breezy conditions (15-25mph) and a 6' launch rail, the initial angle of attack was over 25 degrees and the CP shifted by almost 5 calibers. Both Rocksim and Openrocket will plot the CP location during flight so this sort of thing is fairly easy to check.
Click to expand...
Thanks Dave. Just another example of an "over-stable" rocket becoming unstable under certain not so unusual conditions.

Bob
 
A few months ago a group of us burned a prototype full M C-slot as a sub-scale test for a larger motor. Unfortunately we didn't keep track of orientation of the slot with respect to the video, though we had intended to... But it appeared there may have been on the order of half a degree of offset thrust in the first few seconds of the 8 second burn, even though the back of the grain was adjusted to minimize asymmetric thrust. So that's another slight asymmetry. It MIGHT be that this asymmetry in thrust is such that the thrust vector points more closely towards the offset center of mass of the rocket.

Gerald
 

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