Aerodynamics Question

[cornbeef007]

So.....this is my first fiberglass build and I have a question regarding the thrust vector. I'm using a manufactured kit for a 1.6" diameter and a 42.6" length. Surprisingly, many of the components had loose tolerances such as the fit between the motor mount, motor mount centering rings and inner body diameter. Once the rocket was complete, I inserted a 360mm case and noticed that the case was not perfectly centered when looking down into the open sustainer. Basically, the thrust line is not entirely parallel to the length (longitudinal axis) of the rocket. It's not off by much, I can stand the rocket up on its motor retention tube and it stays upright and stable. i.e. it doesn't fall over or even try to.

Regarding stability, would the rocket be seriously hindered by this off tolerance situation or will the stability of the fins negate this issue. I have an upcoming launch and won't have time to build another lower fuselage section. But if it's unsafe it's unsafe.

I've been known to let perfection be a barrier to "good enough".

Thanks much!

 

[SolarYellow]

I think it will come down to what is the offset of your thrust vector relative to the center of mass vs. the radial symmetry of your aero drag vs. the restoring force of the fins leading to the rocket's overall stability. Another factor is that the actual thrust vector angle is known to wander as nozzles erode randomly and progressively through a burn.

I can't tell you it's safe, but there's certainly a possibility of it not being a problem.

 

[Funkworks]

If you want to measure the angle:
- stand the rocket on the motor mount as you say
- turn it such as to see the greatest lean
- take a picture exactly from that side
- draw a line in the picture along the table/floor surface
- draw a line in the picture along the rocket's longitudital axis
- measure the angle in the picture with a protractor.

 

[cornbeef007]

I think it will come down to what is the offset of your thrust vector relative to the center of mass vs. the radial symmetry of your aero drag vs. the restoring force of the fins leading to the rocket's overall stability. Another factor is that the actual thrust vector angle is known to wander as nozzles erode randomly and progressively through a burn.

I can't tell you it's safe, but there's certainly a possibility of it not being a problem.

Thanks, I appreciate the quick response.

 

[cornbeef007]

If you want to measure the angle:
- stand the rocket on the motor mount as you say
- turn it such as to see the greatest lean
- take a picture exactly from that side
- draw a line in the picture along the table/floor surface
- draw a line in the picture along the rocket's longitudital axis
- measure the angle in the picture with a protractor.

That’s a good idea..I’m traveling at the moment but when I get home I’ll measure the angle and reply.

 

[boatgeek]

A recent article in Sport Rocketry made a good point that if the rocket spins, any off-axis thrust gets evened out. You don't need much fin cant to develop a nice spin. Even with perfectly straight fins, you could probably get it spinning well enough with differential bevels on the leading and trailing edges.

 

[cornbeef007]

Finally was able to measure the angle. After looking at it, I think the safest course is to rebuild the booster. I just feel the angle was just too much. I learned a lot about fiberglass construction which I’m incorporating into the new build.

Thanks for all the good responses!