Slower than the sim

Discussions on high/extreme altitude and mach busting rockets.

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camacd
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Slower than the sim

Postby camacd » Sun Nov 11, 2018 3:09 pm

Hi Guys.
Today I did my L2 cert flight using a J400 Ciseroni. The flight went pretty well, and the altitude was very close to what I had expected in the Openrocket simulation - within 2%.
However the velocity was well down- 225m/s against a predicted Mach 1 - 330m/s approx.
Why would this be - if the sim was wrong, surely I'd see lower altitude as well?

Cam
"He who says it can't be done is liable to be interrupted by somebody doing it". - My Mum

Sirbrad
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Re: Slower than the sim

Postby Sirbrad » Sun Nov 11, 2018 6:40 pm

Did you weigh the rocket with the motor and check against the weight in the sim.

I have weighed a J400 and found it to be heavier than the sim suggested by 120g.

From playing with the Sim, a heavier rocket will go higher and slower.




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CATO
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Re: Slower than the sim

Postby CATO » Mon Nov 12, 2018 9:05 am

camacd wrote:Today I did my L2 cert flight


Congratulations. :D :D :D
"In thrust we trust"

AMRS 21 L3
TRA 07459 L3

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2018: 14,767 Ns (44% N)
Ns 17: 5,973; 16: 34,558; 15: 35,955; 14: 6,016; 13: 10,208
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SpaceManMat
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Re: Slower than the sim

Postby SpaceManMat » Mon Nov 12, 2018 10:55 am

Nice work on the certification Cam.

With velocity I usually see slower speeds than simmed. I believe this is a result of the sampling rate of the accelerometer.
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snrkl
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Re: Slower than the sim

Postby snrkl » Fri Nov 16, 2018 9:07 pm

SpaceManMat wrote:Nice work on the certification Cam.

With velocity I usually see slower speeds than simmed. I believe this is a result of the sampling rate of the accelerometer.


I’ve seen the same thing. I’ve had flights where I’ve flown two JLA3s side by side (testing one of them for accuracy after it spend a month and a half in the corn field) and recently I’ve been flying an A3 with my RRC3...

The A3s together usually agreed on most stats (although the baro readings were 15ft apart pretty consistently, despite being 3cm apart in the nose cone), but I’ve never seen the A3 and the RRC3 agree completely on max velocity or altitude.

My guess is that the altimeters are not 100% aligned generally, and like all things, implementation differences are where the margins of error lay...


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