OpenRocket: altitude at "ground hit" is -12ft

[Grant_Edwards]

I still can't figure out what's wrong with my BT20 spool rocket.

When the simulation is run with an A10T-3, about half the time the "ground hit" happens at altitude of -12ft, and the velocity at that point is about twice the normal value. That doesn't seem to happen with other motors (where ground hit always happens at altitude of 0ft with consistent velocity).

The .ork file is attached.

 

[waltr]

Interesting...When I ran the sim with your ORK file did get the -12 Ground hit.

Added a configuration with an A6T motor and got Zero for ground hit. Also noted that the Stability margin calibers (added to plot) oscillated with the A6T but was flat with the A10T.

Ran the A10T again and now the Stability oscillated and got zero for ground hit.
Altitude, etc did not change.
OR does plot some odd things sometimes but the important info (velocity off rod, altitude, Optimum delay, etc) always seems correct.

 

[gtg738w]

This is pure speculation but I would assume OR just continues the simulation steps until it gets an altitude less than 0. It changes the time step at some point during the recovery phase (much bigger steps than boost) so you may see something like this if the previous step was already close to the ground. For example if it has the rocket at 1ft above the ground descending at 26fps it would say it is still flying but then give an altitude of -12ft 0.5 sec later and end the sim there.

 

[Grant_Edwards]

This is pure speculation but I would assume OR just continues the simulation steps until it gets an altitude less than 0. It changes the time step at some point during the recovery phase (much bigger steps than boost) so you may see something like this if the previous step was already close to the ground. [...]

Yes, that appears to be what's happening. Here's the last chunk of the exported data. Columns are time, altitude, vertical_velocity

[...]
1.6232,54.279,-3.6349
1.6332,54.224,-3.8209
1.6432,54.166,-4.0036
1.6532,54.106,-4.1829
1.6632,54.044,-4.3586
1.6732,53.979,-4.5305
1.6832,53.911,-4.6987
# Event TUMBLE occurred at t=1.6932 seconds
1.7479,53.406,-5.9815
1.8045,52.863,-7.0999
1.8637,52.198,-8.2183
1.9264,51.391,-9.3365
1.9934,50.418,-10.455
2.0663,49.241,-11.573
2.1468,47.808,-12.691
2.238,46.036,-13.809
2.3447,43.789,-14.927
2.4752,40.824,-16.044
2.6474,36.631,-17.162
2.909,29.832,-18.279
3.409,16.082,-19.222
# Event EJECTION_CHARGE occurred at t=3.8532 seconds
3.909,2.0298,-19.101
# Event GROUND_HIT occurred at t=4.409 seconds
# Event SIMULATION_END occurred at t=4.409 seconds
4.409,-11.984,-19.119

At the start of the file (not shown), the step size is 2ms. When the rocket clears the launch rod, the step size drops to 0.57ms, then varies a bit for while before it starts to increase. At the start of the data shown above, the step size is 10ms for the first few lines, then starts increasing. At T=2s, the step size is 67ms, at T=3s, the step size is 500ms where it stays until the end of the simulation.

This is presumably done to save on CPU time and reduce the size of data sets.

FWIW, here's the complete set of step times for the above simulation (in ms, with comments and events shown). Obviously there's code in OR that dynamically adapts the time step size based on conditions. It looks like the "tumble" event might be what causes the simulation step size to start increasing to 500ms.

# Simulation 6 (Up to date)
# 377 data points written for 3 variables.
#
# Time (s),Altitude (ft),Vertical velocity (mph)
# Event LAUNCH occurred at t=0 seconds
# Event IGNITION occurred at t=0 seconds
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
# Event LIFTOFF occurred at t=0.054 seconds
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.00
# Event LAUNCHROD occurred at t=0.144 seconds
2.00
0.57
0.56
0.57
0.57
0.56
0.57
0.56
0.56
0.56
0.57
0.58
0.57
0.58
0.59
0.58
0.59
0.59
0.59
0.60
0.62
0.64
0.64
0.65
0.66
0.68
0.69
0.71
0.75
0.75
0.82
0.85
0.88
0.99
1.05
1.20
1.38
1.80
2.60
3.90
2.81
1.39
1.15
1.05
0.90
0.82
0.83
0.73
0.73
0.67
0.69
0.65
0.64
0.62
0.59
0.59
0.61
0.58
0.58
0.57
0.58
0.55
0.55
0.55
0.56
0.57
0.54
0.55
0.54
0.57
0.55
0.57
0.55
0.56
0.59
0.60
0.61
0.58
0.60
0.64
0.64
0.63
0.66
0.69
0.70
0.75
0.77
0.78
0.85
0.85
0.93
0.95
1.04
1.15
1.27
1.39
1.74
2.17
2.61
3.92
5.88
3.43
2.18
2.10
1.82
1.63
1.57
1.53
1.44
1.46
1.47
1.43
1.43
1.39
1.48
1.46
1.51
1.51
1.55
1.56
1.59
1.68
1.67
1.71
1.79
1.91
2.06
2.09
2.28
2.49
2.77
3.19
3.71
4.50
6.74
10.00
10.00
9.10
6.10
5.03
4.48
4.38
4.00
3.97
3.88
3.89
4.04
3.81
4.08
4.36
4.36
4.77
5.33
5.54
6.47
8.76
10.00
10.00
10.00
10.00
8.88
6.69
5.42
5.30
5.00
4.97
4.64
4.71
5.02
5.17
5.32
5.35
5.87
6.44
7.90
9.84
10.00
10.00
10.00
10.00
10.00
9.64
8.25
7.41
6.92
7.15
7.29
7.38
8.51
9.55
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
9.98
10.00
10.00
10.00
10.00
10.00
10.00
10.00
# Event BURNOUT occurred at t=0.85324 seconds
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
9.96
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
# Event APOGEE occurred at t=1.5032 seconds
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
# Event TUMBLE occurred at t=1.6932 seconds
64.70
56.60
59.20
62.70
67.00
72.90
80.50
91.20
106.70
130.50
172.20
261.60
500.00
# Event EJECTION_CHARGE occurred at t=3.8532 seconds
500.00
# Event GROUND_HIT occurred at t=4.409 seconds
# Event SIMULATION_END occurred at t=4.409 seconds
500.00