Open Rocket Sim Charts

[dnl2]

I just started to use Open Rocket and I don't understand a few of the plotted lines on the charts.

If any one could help explain I would appreciate it.

Both charts are for a 5 lb. 2.1" dual deploy rocket with no drogue chute. CTI I180 38mm motor.

In the "vertical motion vs. time" chart, I don't understand the vertical acceleration line. It starts out with a low acceleration, stay constant, then spikes when the main chute deploys. Is that because its going from a high speed with no chute, to a main chute?

The "Stability" chart also confuses me, If anyone could help explain. Also I'm assuming ( due to the charts name) that this is going to show me if my flight is stable.

Thanks for any help.

 

[Rex R]

well I suppose that 250ft/sec^2 for the 1st 1.5?sec could be considered low...then it goes somewhat negative untill the main deploys...looks about right to me(use the zoom feature to examine various areas of interest). not having used the stability plot, I have no clew, sry.
rex

 

[El Cheapo]

Do you have the flight set up as a dual event?

 

[chuck5395]

Do you have the flight set up as a dual event?

How does one do that?

 

[DJDeadParrot]

Its an attribute of the parachutes, themselves. But you have to have two parachutes in the model. Kinda doesn't matter where [as far as I've been able to tell]. On the General tab, in the lower right corner, you tell it that it Deploy at: >Specific altitude during descent< from the dropdown menu. Then you can set the altitude.

 

[KerryQuinn]

Hi Dave -

The plots look correct.
As stated above, the acceleration you see at the beginning of the flight is the boost coming from the motor. Then the rocket coasts - coasting = no change in acceleration, hence the ~0 value of acceleration. When the chute pops out, (particularly as you are doing it so late in the flight, there is a huge DE-acceleration (ie it jams on the "brakes") this is a quick reduction in velocity which is a way of saying that it is a huge amount of acceleration (acceleration = change in velocity). That's why you see a spike at that point. (By the way, the magnitude of that spike corresponds to the amount of force pulling on your shroud lines). If you add a drouge (or deploy before you have so much downward velocity, you would see less acceleration from the chute - but you WOULD have to walk further

As for the stability plot, it is showing you the distance from the top of your rocket to the CG and CP. Since the CG is closer to the tip, this is a good thing. The CG changes a bit as the engine burns, but overall the distance from the CG to CP is around 6 times your diameter tube, so OR is showing you that the stability margin, in "units" of body tube calibers is around 6. Depending upon who you ask, the "target" is around 1.5-2.0 for "stability". At 6, you are "over stable" which is not necessarily bad, but can lead to weather-cocking after launch. Near apogee, the rocket is going very slow (and is tipping over) so the CP location calculation starts to be a bit crazy. That's why the plot is jagged after that point. Don't worry about that part - what you want for stability is a stability margin above 1.5 or so during launch and coast.

-Kerry

 

[dnl2]

First off, thanks for the help.

I adjusted the chutes for dual dep. and adjusted the chute size until the warnings went away.

The graphs make more sense now.

There is a lot to learn with O.R.

 

[Sampo]

Hi,

The description by Kerry pretty much covers it. I'll just add that the "jitter" in the stability during flight is caused by the rocket pitching back and forth. Typically the CP of a rocket is calculated at zero angle of attack, but when the AOA increases the CP typically moves forward, reducing the stability. As the rocket pitches back and forth the AOA changes all the time, as does the static stability.

This phenomenon is often called wind instability, since a large AOA is normally caused by wind at launch. This affects especially long and slim rockets, which can lose many calibers of stability at even a few degrees AOA. Therefore the rule of thumb for 1-2 cal stability definitely does not apply to unusually slim rockets, and similarly short and thick rockets can easily do with less stability.

You can read more in Robert Galejs's great article "Wind instability - What Barrowman left out"
https://www.if.sc.usp.br/~projetosulfos/artigos/sentinel39-galejs.pdf

Cheers,
Sampo N.