OR predicted time to apogee way off....

[Jay Hart]

I launched my L1 rocket on an Aerotech reloadable H130W-14

I did not have any electronics except for JLCR for the main at 300'.​

I created a model in OR and it predicted a time to apogee/delay of 7 seconds.

Video shows it reaching apogee at about 14 seconds.
Good thing my delay adjustment was off.

I weighed the rocket on an accurate scale and its about 43 oz with motor
The model is quite close.

So I'm sure my rocket was not modeled way off (dimensions +/- 0.05 for example.)​

I played with increasing the weight by 1-2 lbs and altitude/time to apogee only changed 10-15%
as expected.

I want to fly it next on an I impulse but am concerned about getting the delay right.

I've attached my .ork file if you are curious.

 

[Jay Hart]

I retract part of my post that a 2 lb change did not produce a sizeable change as being expected.

 

[Nytrunner]

Was your rocket painted and sanded? Finish work reduces the skin drag of your rocket, increasing attitude.

Fin profiles have an effect as well

 

[Jay Hart]

I ran 6 variations of the simulation and forced/over road the weight from 20 - 60 oz where actual
weight is 36 oz.
As weight increases altitude goes down and max accel goes down....makes sense
but the time to apogee stays roughly unchanged (about 8 seconds). Perhaps that is just the way it is....
but explain why my video shows apogee at about 14 seconds.
attachment shows 6 simulation runs named according to weight.

 

[Jay Hart]

painted/sanded

 

[Jay Hart]

I looked more closely at my model and I had square fin edges but in fact were rounded.
simulation bumped up altitude by over 300' and time to apogee from 8.9 to 9.7

 

[Nytrunner]

Now what is the finish of each part set to?

Rough? Smooth? Polished?

 

[Jay Hart]

I chose "Regular paint"......not sure how to rate spray paint

 

[kuririn]

1. Time to max apogee = burn time PLUS optimum delay.
2. What did you set the delay to?
3. Can you post the video?
Laters.

 

[Jay Hart]

I chose smooth on all surface components giving a small change from regular.

 

[Jay Hart]

I modified delay from 14 to 8 seconds in theory. Video may be too big to load. I will try posting a link. Something doesn't add up....
Video near apogee/ejection is tough to make out....gonna get a
JL ALT3.

 

[Jay Hart]

video not available but it showed ignition at 14 sec and apogee at 28.
thanks to all who scratched their head and got me closer in my understanding

 

[neil_w]

What's the burn time on that motor?

 

[mikec]

OR's optimum delay is not reported independent of the actual motor delay being used, as long as your recovery deployment is relative to motor eject. If you set the delay to 7 seconds and that's too short, then the optimum delay will be computed relative to that deployment. Try setting the delay to 14 seconds and rerunning your various cases.

 

[Jay Hart]

About 1.6 sec burn

 

[Jay Hart]

I set the motor delay to close to the optimum delay so as to get a low velocity at deployment. I will try using a 14 second delay tomorrow.
Thanks to all of you.
Jay

 

[Jay Hart]

so I ran 2 simulations with motor having a 7 vs 14 second delay (no other changes).
Max Velocity and Max Acceleration are the same (as expected).
Time to apogee increased by 0.5 seconds (?).

 

[neil_w]

so I ran 2 simulations with motor having a 7 vs 14 second delay (no other changes).
Max Velocity and Max Acceleration are the same (as expected).
Time to apogee increased by 0.5 seconds (?).

If your 7 second delay was too short, that would have terminated the ascent a bit early.

 

[Jay Hart]

Thanks Neil,

It is starting to "make sense".