Openrocket vs Judgement

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spence

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At yesterdays launch a flyer presented his rocket for an l2 cert. it was 80inches tall, 4 inch diameter, 3 fins , each 4 inch square.

The OR sim had a 2.4 caliper stability, with the cp 26 inches from the bottom. 13:1 thrust ratio.

On paper it looked ok...but the collective experience of the l2 cert team was this was a dangerous design with Such small fins on a tall rocket.

We scrubbed his flight.

Question is...have others seen sims that look fine, but refused to allow it to fly...just based on design judgement?
 
One should perform an OR sim flown with a realistic amount of wind. The default IIRC is pretty low. Many long rockets can end up being unstable due to wind. Of course OR is not perfect and there are many effects it doesn't model at all.

Refusing to fly it outright seems a little extreme to me, but I didn't see the rocket.
 
Long rockets are usually well over stable. And a 4" rocker with 4" fin span sounds fine.

What did the CG measure compared to the SIM displayed CP? Was there really two calibers of stability?

I would be a bit heated if I was denied to fly after presenting a SIM showing good thrust and stability numbers.

Certification flights are a test. Not an application. If it didn't look completely unsafe then someone's opinion should not have came into effect. I'm assuming since he scratch built this rocket, that it was not his first go at a bigger airframe. It should have just been called "on your feet and heads-up" flight. And flown further out.
 
If those fins were squares four inches on a side, assuming reasonable balance it would likely have been fine.

If those fins were four square inches of surface area, what was the planform, material, and motor choice?

Regardless, the RSO/LCO may refuse to allow the launch or static testing of any rocket motor or rocket that he/she deems to be unsafe.
 
With a 13:1 thrust weight, and sufficient forward mass to give 2.6 calibers (13% vehicle length separation between cg and cp), that's not the riskiest flight I've seen.

RSO has final say, but I wouldnt call that an unsafe flight.
 
Doesn’t sound unsafe here and if the flier had taken the time to ensure he had a simulation to back up his decision and it has a stability over 2 then I’d call it safe. 80 inches tall on a 4” airframe is not extra long at all. That’s within a foot of the majority of 4” dual deployment kits from retailers all over.
 
You say "we" but if you have a designated RSO than it would be his/her call. I had an RSO tell me no, on a kit I had flown before, he wanted me to prove stability which I was unprepared for. After that I always bring rocsim data at least showing CP/CG.
 
At yesterdays launch a flyer presented his rocket for an l2 cert. it was 80inches tall, 4 inch diameter, 3 fins , each 4 inch square.

The OR sim had a 2.4 caliper stability, with the cp 26 inches from the bottom. 13:1 thrust ratio.

On paper it looked ok...but the collective experience of the l2 cert team was this was a dangerous design with Such small fins on a tall rocket.

We scrubbed his flight.

Question is...have others seen sims that look fine, but refused to allow it to fly...just based on design judgement?

It’s not unknown to have a sim disagree with onsite judgement. Something that is allowed at one field might not be at another. We all have a responsibility to make sure that unstable flights or flights that might result in a waiver violation don’t fly. That doesn’t mean simply moving it further out and calling heads-up. Without seeing the rocket and talking to the certification team I don’t know what it was that made them deny the flight, and without knowing all the facts I would not second guess the cert team or the RSO. It sounds like more than one person had concerns.
 
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