- Joined
- Mar 27, 2013
- Messages
- 22,537
- Reaction score
- 14,956
But only by a nose...
Most nose weight ever?
- Thread starter gary7
- Start date
Help Support The Rocketry Forum:
But only by a nose...
I have debunked the Apogee newsletter base drag "cone" thing more than once here already. My claim is the rockets in question are already stable without this adjustment. For the rockets in question, .6 cal stability is a greater proportion of the rocket's length than 2 cals of a long skinny rocket. The use of calibers is not that stability margin requirements are particularly affected by the rocket's diameter, but that it provides a handy form of measurement in the field. Further, I flew a finless rocket last year that was long and very un-saucerlike with a large but smooth transition with substantial base drag at the rear. The rocket was far less stable than all sims and adding a simulation of base drag would have made the simulation even worse. It ended up with a sharp, pointy BT55 cone almost completely filled with lead.
I found a way of improving recovery issues due to nose cone weight: place the chute in the middle of the shock cord and extend the nose cone end so it hits the ground first and the rocket can descend the last few feet with only the body supported by the 'chute. Unfortunately the rocket in question demonstrated the dangers of nose weight on its last flight as it had a shock cord failure. Seperating 2 heavy objects at high speed increase the stresses, especially with excessive ejection charge. As the airframe is draggy with no nose cone and has the shorter end of the cord, I believe the airframe was all but stopped as well as the 'chute opened when the nose cone reached the end of its rope. I don't think the cord had a weak spot, the stresses were just that high it snapped. Further, the nose cone by itself was highly stable so fell at full speed. Nose weight could be put towards the back of a NC so it tumbles or falls blunt end first, but in this case the NC formed 2/3 of the length of the entire rocket, so that wasn't a real option. In light of recent events, if this rocket is rebuilt at all I will probably completely change the design (the airframe survived after a long drift but the long complex nose cone was integral to the design and was damaged beyond repair).
I found a way of improving recovery issues due to nose cone weight: place the chute in the middle of the shock cord and extend the nose cone end so it hits the ground first and the rocket can descend the last few feet with only the body supported by the 'chute. Unfortunately the rocket in question demonstrated the dangers of nose weight on its last flight as it had a shock cord failure. Seperating 2 heavy objects at high speed increase the stresses, especially with excessive ejection charge. As the airframe is draggy with no nose cone and has the shorter end of the cord, I believe the airframe was all but stopped as well as the 'chute opened when the nose cone reached the end of its rope. I don't think the cord had a weak spot, the stresses were just that high it snapped. Further, the nose cone by itself was highly stable so fell at full speed. Nose weight could be put towards the back of a NC so it tumbles or falls blunt end first, but in this case the NC formed 2/3 of the length of the entire rocket, so that wasn't a real option. In light of recent events, if this rocket is rebuilt at all I will probably completely change the design (the airframe survived after a long drift but the long complex nose cone was integral to the design and was damaged beyond repair).
Last edited:
dixontj93060
Well-Known Member
- Joined
- Feb 19, 2009
- Messages
- 13,083
- Reaction score
- 45
How can adding base drag per the Apogee cone method adversely affect your stability margin? Base drag moves the CP further aft.
dixontj93060
Well-Known Member
- Joined
- Feb 19, 2009
- Messages
- 13,083
- Reaction score
- 45
It would have increased the simulation error. OR showed the CP about 2/3 of the way back already, but actually it was almost at the middle of the rocket, at least at certain small angles of attack, as verified by wind testing and flight. I have another rocket with no regular fins but more of a defined "cone fin" which is a draggy shape like a funnel or saucer, but this is also larger compared to the airframe. That one has a CP near the rear. But the rocket in question used a plastic quart milk bottle (cut off) which is a very low drag shape except for the base drag and it had much less effect on stability. This was a case of a rocket that is not short but would be expected to have a substantial base drag effect anyway because the thick part of the rocket was short and at the rear.
My point is if you look at needed stability margin in terms of rocket length rather than diameter, this correction is not necessary. The Apogee paper doesn't consider any rockets with a negative stability margin until base drag is compensated for. I think it's this nonsense with "calibers" of stability that is responsible for the idea these rockets weren't stable in the first place.
Or perhaps the needed margin is a function of both length and diameter. For example:
.6 cals x 3" dia. = 1.8", is 1/10 of 18" length OR 1/10 of 3" dia. + 15" long
2 cals x 1.6" dia. = 3.2", is 1/10 of 32" length OR 1/10 of 1.6" dia. + 30.4" long
Add: I also did some angle of attack comparisons in OR of a typical stubby rocket vs. a long and thin one and the thin one did need a lot higher stability margin in cals. The stubby one had little CP shift with AOA.
Last edited:
dixontj93060
Well-Known Member
- Joined
- Feb 19, 2009
- Messages
- 13,083
- Reaction score
- 45
The Apogee newsletter and its calculations are, by title and content, specifically for short, wide rockets. From what I can tell you are describing long rockets with base drag at the aft end. If so, I think your supposition that the work done by Bruce Levinson is not valid is really not applicable at all.
^^-- I think it's interesting but the examples they're using simulate as already stable without it. What would prove it is a rocket that simulates unstable until base drag is simulated, that flies stable. You're right my example is not conclusive it's a just a data point and definitely not a case of "all else equal". But if base drag stabilization is real, it applies to some extent to all rockets with base drag.
If you have a design that is 1" diameter and has a 1 cal (1") stability margin, and then change nothing but double the diameter so that 1" margin is now only .5 cal, is the rocket less stable? Not much if any. If instead the change was make it half as long, with the same motor and fins, with the usual designs both CP and CG become relatively closer to the center of the rocket. Even if the margin is now only .5 cal, the rocket is probably more stable than before because this CG/CP shift results in less movement of CP with AoA and the margin is still the same fraction of the rocket's length.
If I'm in the field and the rocket has CP marked on it and I balance it on my finger, it's a lot easier to eyeball the margin in calibers than any other unit, and would be very innaccurate to eyeball it in a fraction of the rocket's length unit. But calibers is not a fixed unit, nor a natural unit in terms of what matters. It is accepted that shorter rockets need less than long rockets, but people get nervous when the number is less than 1.
If you have a design that is 1" diameter and has a 1 cal (1") stability margin, and then change nothing but double the diameter so that 1" margin is now only .5 cal, is the rocket less stable? Not much if any. If instead the change was make it half as long, with the same motor and fins, with the usual designs both CP and CG become relatively closer to the center of the rocket. Even if the margin is now only .5 cal, the rocket is probably more stable than before because this CG/CP shift results in less movement of CP with AoA and the margin is still the same fraction of the rocket's length.
If I'm in the field and the rocket has CP marked on it and I balance it on my finger, it's a lot easier to eyeball the margin in calibers than any other unit, and would be very innaccurate to eyeball it in a fraction of the rocket's length unit. But calibers is not a fixed unit, nor a natural unit in terms of what matters. It is accepted that shorter rockets need less than long rockets, but people get nervous when the number is less than 1.
Last edited:
dixontj93060
Well-Known Member
- Joined
- Feb 19, 2009
- Messages
- 13,083
- Reaction score
- 45
The saucers that they show in the Apogee article are not stable in the least. Below is an example of a large saucer of mine. Negative stability, but it will fly due to the drag which is simulated and shown in the second attachment.
^^^ Best example I've seen. Perfectly to the point of the Apogee article. Sometimes, with "stubby" rockets a straight up simulation will make the rocket appear to be unstable, and therefore requiring excessive noseweight. This "trick" seems to help people get past that. I don't see how it was "debunked" by bill_s.
gary7
Well-Known Member
Attached is a file of my proposed rocket. That nose weight is 128 oz (8 pounds) giving the rocket a margin of about 2.2. It is barely "stable" at about 1.15 with an L2200 loaded. But if I remove the nose weight, the margin goes to -0.03 with that L2200 in the tube. What will happen then with trajectory? I need more concrete information based on facts, not opinion. Besides for the safety involved, I may want to use this as an L3 qualifying rocket someday. My original question about nose weight was just to get the conversation going but it has become more interesting now with the current discussion. Ergo my current question: Is this a stubby rocket and do I or not need to make some adjustment in Rocksim with that invisible transition on the aft end?? This file is not based on the final design by the way, but it's getting close.
dixontj93060
Well-Known Member
- Joined
- Feb 19, 2009
- Messages
- 13,083
- Reaction score
- 45
Please post the Rocksim file.
Saucers are another subject and I don't claim to understand them much. I had forgotten they were also included in the article, however, the example they gave of a merely short rocket started out simming at .65 margin. I probably shouldn't have claimed to "debunk" the article I should have said I "respectfully disagree" or "question it" or whatever. It's great actually, just not the end of discussion.
I have to wonder if the case of saucers really has to do with base drag, or the frontal drag somehow results in an effective CP behind it. The cone method suggested also came up with the saucers being rather absurdly overstable and looks to me would not simulate properly angle of attack. Meanwhile more conventional rockets don't seem to need this, experience is that .65 is Ok on a Fat Boy design and 0 is not. Math says that .65 is actually not that small.
Relating more to the original subject, nose weight usually isn't needed until you put a motor in the back end of the rocket, and then the weight sort of ends up being a fraction of the motor weight, ratio depending on the design.
Last edited:
gary7
Well-Known Member
View attachment Fat Brother 7.5 in.rkt
This rocket is in a very early stage of development as is the Rocksim file.
dixontj93060
Well-Known Member
- Joined
- Feb 19, 2009
- Messages
- 13,083
- Reaction score
- 45
No problem. I'll look at it tonight.
