OpenRocket: negative accelleration at T=0?

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Jul 22, 2020
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I will repeat the disclaimer that we can agree this doesn't really affect anything, more of a philosophical discussion at this point.
In this case, I believe rather strongly that most users would expect that in the plots, a = dv/dt, where v is w/respect to the ground, and therefore a stationary rocket has acceleration=0 until it starts moving. It's as simple as that. If the internal calculations naturally lead to anything different, then it is the developer's job to modify the presentation of the data to make more sense.
And in this case we all definitely have the right to demand our money back until it's fixed. :)
I'm going to file an issue with OpenRocket to fix this, sometime in the future (who knows when).
IMO, a perfectly acceptable fix would be to simply not plot anything before a>0, v>0, alt>0 — which I've been referring to as "liftoff", but OR seems to wait about 0.1s after the rocket starts to move before labeling the "liftoff" event — it probably has some altitude minimum > 0 before it declares liftoff.

John Beans

Founder, Jolly Logic
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Jun 5, 2010
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... This makes the acceleration of -9.8m/s/s correct even when fixed on the ground in the pure calculations of Physics...
...If you have an Accerometer chip in your rocket then it will measure an acceration of -9.8m/s/s when the rocket is sitting on the pad. Another proof this thinking is correct...
What it really means is that accelerometers can be tricky to use because they inescapably measure TWO things all the time:

1) motion
2) the force of gravity

We want to use an accelerometer JUST to measure motion, but we have to figure out how to isolate and remove the effect of gravity on the sensor to do so. Otherwise, the accelerometer seems to indicate 1G of acceleration on the pad when it's NOT accelerating, and 0G of acceleration when it begins to free fall and is actually accelerating downward at 1G.

But subtracting gravity is tricky because as the rocket arcs the relative direction of gravity changes relative to the sensor. So it's not like we can just always subtract 1G from the reading. We need to use the angle of the rocket relative to the ground to calculate the component of gravity that's aligned with the direction of flight, but unfortunately the accelerometer itself is no help in figuring out that angle during flight. This is the reason that "accelerometer-only" altimeters aren't that great at measuring altitude—flights are rarely perfectly vertical, and an arcing trajectory can lower the apogee by as much as 40% in a typical flight.


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Jan 23, 2009
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