Nose Weight - LOC Big Nuke 3E

I was taking a poll of LOC Big Nuke 3E owners, curious how much nose weight you have added. Its a fairly heavily built kit as I have fiberglassed both body tubes, 2x wrap of 6 oz and 1x wrap of 3 oz, and then a layer of 6oz and one layer of 3oz tip to tip on the fins. It is built to take a beating.

I made the changes in OpenRocket to the stock design to emulate what I have done with this kit. I added 10oz of metal scrap and epoxy to the nose this evening, which OpenRocket shows gets it to a 1.05 cal stability on a K550W, but only .96 cal stability on a K700W.

I want to make sure this thing flies stable, but I'm also interested in keeping it light as I can at this point so I don't sacrifice too much altitude. If I need to add additional weight I can add a few ounces of foam at a time until I hit the desired weight.

Before going that route I would suggest checking several different methods of stability prior to adding nose weight. I would also suggest that adding nose weight be one of the last things you do especially since you still have epoxy setting up. I would take that big beast with all the recovery gear in it as if it's going to fly and get some real world CG before assuming the CG in the 'puter is correct.

DAllen said:

Before going that route I would suggest checking several different methods of stability prior to adding nose weight. I would also suggest that adding nose weight be one of the last things you do especially since you still have epoxy setting up. I would take that big beast with all the recovery gear in it as if it's going to fly and get some real world CG before assuming the CG in the 'puter is correct.

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Best advice yet. I usually design 3fnc rockets in VCP and don't bother with CG. As long as I know the CP, the CG can get adjusted later.

Handeman said:

Best advice yet. I usually design 3fnc rockets in VCP and don't bother with CG. As long as I know the CP, the CG can get adjusted later.

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That sounds reasonable. Can I assume the CP in OpenRocket is correct, but the CG is suspect? I can load up the recovery gear and motor then balance it. Hope I didn't over-do it already with the 10 oz in the nose......

neond7 said:

Just to clarify regarding the epoxy, I wasn't referring to the body tubes still curing. Its been finished and painted for weeks and this was the final step. This evening I added some scrap metal nuts to the tip of the nose and used the epoxy (and a dowel rod) to keep them in the tip of the nose. After it cures, if I need to add additional weight, I would pour in foam a few oz at a time.

I had posted this because I'm sometimes dubious about the numbers the software generates and wanted some real world experience people have had with this kit.

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My experience is that the Barrowman equation is conservative and both OR and RS calculate it accurately. You can pretty much hang your hat on it. My opinion is that any rocket that has the CG in front of the CP is stable - even if it is 0.01 calibers. Now, do you want to risk your $200+ project on that small of a margin especially if all it takes is a parachute to shift a few inches out of place to make it unstable? I don't so there's the 1.0 caliber rule of thumb. Frankly, I'm a little surprised that a mid sized K puts the margin that close given it's relative length to body diameter which is why I suggested getting actual weights and the actual CG and plugging that information in prior to adding nose weight.

I didn't add any nose weight to either my LOC Big Nuke or my Big Nuke 3E. The 3E has flown twice on M1500Gs. The Big Nuke has flown on a variety of J motors and one K motor.

No nose weight is needed, unless you throw a gold brick in the tail as mounting points for your rail guides.
I built a Nuke that used only the nose cone as a stock part. The overall dimensions were still pretty much the same.
It came in a 27 lbs loaded with a 3 grain 76 mm motor. It flew nice a straight all three times.
The smallest motor being a 2 grain 76 mm.



JD

I assembled my L3 rocket using my wife's hand weights to simulated the motor weight. This allowed me to check for real world CG at launch and at motor burn out.
I ended up building a spacer that held my parachute just under the nose cone so I didn't need to add nose weight.
Sims are great for finding CP and really help with CG but you really need to do a flight ready test too. :2:

Wish I had asked this question BEFORE I epoxied in the 10 oz..... no way to get it out now. Besides losing some altitude, would there be any negative to having the weight in the nose? Excessive weathercocking?

I'll fly it and see how it goes. I guess I could always order a new nose cone, although installing new brass shims for the shear pins in the correct spots would be a bit tricky, but not impossible.

I can't think of anything aside from a small loss in altitude. On the plus side, you can use larger motors.

Just make sure your recovery attachment to the NC can handle it.
Those Plastic Eyelets can and will break .


JD

neond7 said:

Wish I had asked this question BEFORE I epoxied in the 10 oz..... no way to get it out now. Besides losing some altitude, would there be any negative to having the weight in the nose? Excessive weathercocking?

I'll fly it and see how it goes. I guess I could always order a new nose cone, although installing new brass shims for the shear pins in the correct spots would be a bit tricky, but not impossible.

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DAllen said:

My opinion is that any rocket that has the CG in front of the CP is stable - even if it is 0.01 calibers. Now, do you want to risk your $200+ project on that small of a margin especially if all it takes is a parachute to shift a few inches out of place to make it unstable? I don't so there's the 1.0 caliber rule of thumb.

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You need more than 0.01 calibers of stability to deal with the moment of rail exit. At this point, the rocket is very susceptible to crosswinds due to its low velocity -- not only do the fins not provide very much corrective moment, but the crosswind also makes body lift important, causing a CP shift and potentially resulting in instability. (cite)

That being said, in this case, it's a LOC Big Nuke. It's got plenty of fin area -- you should have no problems, noseweight or not, unless you're flying a 5" one grain motor or something

Not to mention any crosswinds that moves the Cp forward....

Bob

I added 10 ozs as well to my Big Nuke 3E. I loaded everything except the motor and found the CG and changed it in Rocksim. I am coming out at a 1.08 now with an M1500 loaded in it. I tend to add a little nose weight to my rockets to make fly straight up. I am not an altitude guy. I also put a forge eyebolt with Epoxy into the nose to prevent the dreaded plastic snap. The only drawback to extra nose weight is the tendency for weathercocking. I had my Gizmo DD turn into the wind aggressively. That turned out to be a good as it came in ballistic after an apogee deployment failure.

daveyfire said:

You need more than 0.01 calibers of stability to deal with the moment of rail exit. At this point, the rocket is very susceptible to crosswinds due to its low velocity -- not only do the fins not provide very much corrective moment, but the crosswind also makes body lift important, causing a CP shift and potentially resulting in instability. (cite)

That being said, in this case, it's a LOC Big Nuke. It's got plenty of fin area -- you should have no problems, noseweight or not, unless you're flying a 5" one grain motor or something

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I think he is referring to the fact that if at any given time the actual cp is behind the cg, the rocket is then stable. Not necessarily straight flying due to mass and thrust asymmetry or other factors, but stable.

CarVac said:

I think he is referring to the fact that if at any given time the actual cp is behind the cg, the rocket is then stable. Not necessarily straight flying due to mass and thrust asymmetry or other factors, but stable.

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That's in a static calculation.

In the real world, dynamics control the situation, and the CP moves forward as a function of angle of attack. A statically marginally stable rocket will become dramatically unstable in a good crosswind. Launch any underpowered marginally stable rocket in a 15-20 mph crosswind and watch it pinwheel at 20'-50' above the pad.....

So you don't have to think too hard, the 1 caliber stability rule of thumb was formulated, but it doesn't apply to long thin rockets where the CP can shift more than 10 calibers in a strong wind, or for short stubby rockets which have extra stability from base drag, neither of which are accounted for in a simple Barrowman CP calculation.

Bob

bobkrech said:

That's in a static calculation.

In the real world, dynamics control the situation, and the CP moves forward as a function of angle of attack. A statically marginally stable rocket will become dramatically unstable in a good crosswind. Launch any underpowered marginally stable rocket in a 15-20 mph crosswind and watch it pinwheel at 20'-50' above the pad.....

So you don't have to think too hard, the 1 caliber stability rule of thumb was formulated, but it doesn't apply to long thin rockets where the CP can shift more than 10 calibers in a strong wind, or for short stubby rockets which have extra stability from base drag, neither of which are accounted for in a simple Barrowman CP calculation.

Bob

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When I said actual cp, I was referring to the real CP at that particular AoA and airspeed, accounting for CP shift from all the various influences. If that is even the slightest bit behind the CG, it'll be stable, if not necessarily good-flying.

In your example, the marginally statically stable rocket's actual cp stops being behind its CG, and thus it becomes no longer stable.

bobkrech said:

In the real world, dynamics control the situation, and the CP moves forward as a function of angle of attack. A statically marginally stable rocket will become dramatically unstable in a good crosswind. Launch any underpowered marginally stable rocket in a 15-20 mph crosswind and watch it pinwheel at 20'-50' above the pad.....

So you don't have to think too hard, the 1 caliber stability rule of thumb was formulated, but it doesn't apply to long thin rockets where the CP can shift more than 10 calibers in a strong wind, or for short stubby rockets which have extra stability from base drag, neither of which are accounted for in a simple Barrowman CP calculation.

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Yah! That's exactly what I was trying to say, just put a little bit more concisely