Need help figuring out how to take an accurate measurement.

All,

For reasons documented in another thread, I need to measure my rocket from the tip of the nose cone to the top of each fin root. It is seven feet tall. I need an accurate measurement down to 1/16".

I could use string, but string stretches. I could use a sewing tape measure, but they tend to wrinkle. I could use a carpentry tape measure, but they down curve well.

I really can't figure out how to get a good, accurate measurement on a rocket that big. Any ideas?

(the rocket corkscrewed and I am looking to see if the prefabbed fin can is on straight)

Thanks!

use a masking tape , mark it, remove it and stick it to something strait that you can measure

Try measuring the distance between each fin. If they aren't within a 1/16" or so then you might have found your problem.

agrippo said:

Try measuring the distance between each fin. If they aren't within a 1/16" or so then you might have found your problem.

Click to expand...

Ditto!
Sounds like one or more of your fins are NOT aligned properly. Try measuring between the fins at the leading and tailing edges that should show up any mis-alignment.

I hear ya ... but that shouldn't be possible with a factory-made fin can, right? If so, then GLR has a major production defect, and it should be affecting every rocket they sell that uses this fin can.

As I see, it, most likely the problem is either B or C below, with A being the perfect build.


B = canted motor tube inside airframe: no way to tell, no way to fix
C = canted fin can inside the airframe: this seems unlikely since the airframe slides over the assembled fin can and CRs
D = what you guys are saying: is this possible with GLR fin can?

Lay the rocket on a bench so the fin can hangs over the edge; you can clamp something for a straight edge on the bench to ensure a square fit. Put the tip of the nose cone up against a wall. Measure from the wall to each fin leading edge letting the table support the metal tape measure or adjust the table edge to touch one of the fin leading edges - by rotating the rocket, you'd be able to see if any of the edges are a different distance to the wall (or table edge).

Is this the ACME style can or another GLR product? I didn't read the 'other thread' but could some sort of thrust misalignment in the motor be to blame?

John,
C- Is possible, there can be variances in thickness of adhesive along the long axis. ie: if its built up at one end when it was inserted.
D- Is possible, if there was a maunufacturing defect that was not caught by QA. Most production runs of anything take a % of the total made to test.
Even though it is a pre made fin can one cannot automatically assume its "perfect" as we all know nose cones and other parts are subject to slight variances, if those variances are large enough it can cause all sorts of flight anomalies.

Assuming that the MMT is centered WRT the airframe:

You could get a piece of paper and mark where it comes together, then make a mark at the 1/3 point and the 2/3rd point and place it back on the rocket to see how the fin LE's line up.

If you have a machinists square, you can check the fins for how square they are.

Might be able to do something too if you found a big enough "Lazy Susan".

Greg

Remove the nosecone and measure it separately. From there, either do as KennB suggests or, with the rocket upright, lay a scrap of wood (with a straight edge) on top of the tube and measure from it down to the fin with a metal tape measure.

Those are some great suggestions for measurements. I will try them. If I find nothing out of the ordinary, I am going to have to assume that for a 7' tall rocket with three small fins, maybe I simply had an underpowered motor. If the measurements look good, I will try it on a higher thrust, lower impulse motor at the next launch.

If I find measurement issues, I'll report back and seek opinions for the next step.

Thanks. You all are the best.