Another nose cone question

[Dane Ronnow]

Concerning the Aerotech HV Arcas nose cone, I was adjusting the fit of the cone in the upper BT when I noticed the tip of the cone is off center. The cone fits the BT fine, but when I rotate the cone in the BT, the tip traces a circle about .25" in diameter. So, it's off axis by about .125".

I know this will make the rocket arc in flight to some degree. I'm just wondering how bad the arc would be, and if it's something I should be concerned about.

 

[Dane Ronnow]

Okay, I'll take that as a maybe.

 

[neil_w]

We’ll, there of lots of (low-power) rockets with all manner of asymmetric nose cones, and it never seems to be a problem.

Make of that what you will, but I can’t imagine that a very slightly off-center ogive will make any visible difference.

 

[lakeroadster]

That's a pretty big nose cone. I wouldn't be concerned.

 

[jmasterj]

Not going to hurt anything. My Aerotech nose cones are also off-center, flights are fine. Just try not to look at it.

 

[fyrwrxz]

Oh, I can just feel my CDO acting up. That's the correct order you know...

 

[Dane Ronnow]

Just try not to look at it.

Easier said than done.

 

[Dane Ronnow]

Oh, I can just feel my CDO acting up.

I pain your feel.

 

[Dane Ronnow]

I guess I'll just live with it. (And hope the rocket does the same.)

 

[jmasterj]

Easier said than done.

Agreed.

 

[David Schwantz]

Built the same kit 25 years ago, still flies straight as an arrow. I did replace the shock cord though.

 

[Dane Ronnow]

I can’t imagine that a very slightly off-center ogive will make any visible difference.

I guess that depends on your definition of slightly. I just took delivery of an NCR SA-14 Archer, and that nose cone, which is the same one as in the Aerotech kits with a 13-inch cone, is off by nearly 3/8 inch—11/16 inch total movement in a 180-degree turn.

These PP nose cones are a joke. I know it's not North Coast Rocketry's fault, or Aerotech's. It's the supplier. I suggest they go back to throwing pottery. Meanwhile, I'll be replacing this piece of crap with an Apogee PNC-66A. It's 4 inches shorter, but at least it's straight.

 

[crossfire]

No reason the cones can't be manufactured straight.

 

[Dane Ronnow]

No reason the cones can't be manufactured straight.

Part of the problem is in the shoulder. It's not square with the edge that meets the body tube.

 

[Dane Ronnow]

I guess all of this begs the question, is there such a thing as a 13-inch ogive, styrene nose cone? I can't find one anywhere.

 

[Initiator001]

Concerning the Aerotech HV Arcas nose cone, I was adjusting the fit of the cone in the upper BT when I noticed the tip of the cone is off center. The cone fits the BT fine, but when I rotate the cone in the BT, the tip traces a circle about .25" in diameter. So, it's off axis by about .125".

I know this will make the rocket arc in flight to some degree. I'm just wondering how bad the arc would be, and if it's something I should be concerned about.

No problem.
In over thirty years of flying AeroTech 2.6" diameter kits I've never experienced a change in flight direction due to a nose cone mold issue.
This is not an issue.

 

[Initiator001]

I guess that depends on your definition of slightly. I just took delivery of an NCR SA-14 Archer, and that nose cone, which is the same one as in the Aerotech kits with a 13-inch cone, is off by nearly 3/8 inch—11/16 inch total movement in a 180-degree turn.

These PP nose cones are a joke. I know it's not North Coast Rocketry's fault, or Aerotech's. It's the supplier. I suggest they go back to throwing pottery. Meanwhile, I'll be replacing this piece of crap with an Apogee PNC-66A. It's 4 inches shorter, but at least it's straight.

The nose cone mold was made by AeroTech back in 1988.
Due to the way blow-molding works there will always be a small accumulation of plastic at the tip. This plastic will harden at a different rate than the rest of the nose cone often resulting in a small change in the shape of the tip. We worked on resolving the issue but could never get a consistent 'fix' for the issue.

 

[Initiator001]

I guess all of this begs the question, is there such a thing as a 13-inch ogive, styrene nose cone? I can't find one anywhere.

Polystyrene does not work well in blow molding. It would not work in a nose cone mold with a 13" draw.
Polystyrene would work in an injection mold but the cost would be many times that of a blow mold.

 

[Dane Ronnow]

@Initiator001 - I wouldn't argue with someone of your expertise and experience. You forget more than I'll ever know if I live to be a hundred. But just so I'm clear on this issue: The tip of the nose cone pointed slightly off—and as I mentioned to @neil_w above, the word slightly is subjective—won't 'steer' the rocket in that direction, to whatever degree? And if it does, at what point does it become a problem? I unpacked the same nose cone today in an NCR SA-14 Archer kit, and it's off by nearly 3/8 inch. It strikes me that the increased camber on one side can't help but respond to air moving across the surface more like a wing, creating lift.

That said, if you're saying that that amount of off-axis—and now I'm speaking of the Archer nose cone and not the Arcas, with twice as much—does not respond that way to moving through the air, then I'll quit with the nervousness, and just put up with how it looks.

I appreciate your input, initiator. Thanks.

 

[Dane Ronnow]

Polystyrene does not work well in blow molding. It would not work in a nose cone mold with a 13" draw.

The 9" PS cone I have on my scratch build is straight and true. Shorter, yes, and harder. But absolutely straight. I can glue to it. And I can paint it without major prep. What's not to like? JMO.

 

[jmasterj]

That said, if you're saying that that amount of off-axis—and now I'm speaking of the Archer nose cone and not the Arcas, with twice as much—does not respond that way to moving through the air, then I'll quit with the nervousness, and just put up with how it looks.

You're overthinking things. The only thing it might affect enough to matter is the visual from up close (and any OCD you may have about symmetry, which for me is the hardest part to tamp down).

 

[bjphoenix]

If you are adding tape to build up the shoulder on the cone, I think you could build up a little bit on the back part of the shoulder on one side and a little bit on the front part of the other side to tilt the axis of the nose cone a little bit.

 

[Off Grid Gecko]

@Initiator001 - I wouldn't argue with someone of your expertise and experience. You forget more than I'll ever know if I live to be a hundred. But just so I'm clear on this issue: The tip of the nose cone pointed slightly off—and as I mentioned to @neil_w above, the word slightly is subjective—won't 'steer' the rocket in that direction, to whatever degree? And if it does, at what point does it become a problem? I unpacked the same nose cone today in an NCR SA-14 Archer kit, and it's off by nearly 3/8 inch. It strikes me that the increased camber on one side can't help but respond to air moving across the surface more like a wing, creating lift.

That said, if you're saying that that amount of off-axis—and now I'm speaking of the Archer nose cone and not the Arcas, with twice as much—does not respond that way to moving through the air, then I'll quit with the nervousness, and just put up with how it looks.

I appreciate your input, initiator. Thanks.

direction of flight is mostly dependent ... okay it's dependent on a lot, but think of it less like a rocket and more like a dart.
It's inherently stable (as long as the CG is leading the CP by a good margin, right?)
Spears, arrows, darts, shuttlecocks, all operate on this same principal. The weighty part of the rocket relative to the combined drag force is going to steer it. Worst case, you will have a slight angle of attack during flight, but prolly not noticeable at the acceleration rate of that rocket with any decent motor in it.
If the motor is centered and the thrust is vectored through the CG is prolly more important than the NC being dead straight.
When it comes to CP, it's not just a dot on the rocket. The pressure coefficient relies on how the whole rocket responds to a certain angle of attack at a certain velocity.

The OG nosecone that came with my 4" DX3 was way off, like an inch.. I couldn't stand to look at it. Before I finally ended up getting the new one I was able to adjust it a bit by tossing it on the dash of my truck in direct sunlight for a while and then carefully bending it over, but there's also a risk that you will crease the plastic, so I would not recommend that on anything smaller or thinner than a 3" nose cone and even then you need to be super careful and super patient. It can take several cycles.

The cause in my case was the molding process. one whole side of that cone is 2-3x thicker than the other side. I think you can imagine how that affects the curing process.

In any case, it's a cosmetic issue at best. it'll be safe. String spin it if you are still unsure and you'll see it lines up straight to the direction of motion, but that rocket might be getting just a tad big for that exercise.

 

[Dane Ronnow]

Thanks, @Off Grid Gecko. I appreciate your response. Short of asking Matt for a replacement cone—and I don't want to do that—this nose cone will end up on the rocket, for better or worse. If I don't like the way it flies, I won't fly it. Pretty simple.

Anyway, your post expresses what I need to know. Thanks again.