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rklapp

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Aloha,
I made some nice rkt files because the one's in the Rocksim Library were awful. The V2 is tricky because can't attach fins to a transition. Anyways, I like the Flight Side simulation to determine how far it will travel, especially in the recent trade winds we've been having. What's up with the Ground Track sim? My rockets seem to go in circles.
 

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neil_w

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If you're using OpenRocket, you should save these as ORK files. The files above contain no motors or simulations to look at.
 

neil_w

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OK, better. Now, is your question regarding the behavior of the Lateral Distance line on the sim plot? If so, here's the Hi Flier XL plot for the D12:
1588967095410.png


The lateral launches into the wind, and distance peaks at about 100' (apogee). Then the rocket starts drifting back towards the launch point under parachute. At about T+14s, it passes overhead and then continues to drift to the other side. The lateral distance plot turns back upwards because the distance is increasing again, albeit in the other direction. In these plots, lateral distance is always a positive number; there is no direction factored in.

If we add "lateral direction" to the plot, we can see it in yellow:
1588967426187.png


Well, we can see that you have a direction change at T=14 as the rocket passes overhead (goes from 0 deg to 180 deg. The flip at T=18 from roughly +180 to -180 is meaningless, an artifact of the plotting code apparently mapping the angle to a value between -180 and +180.

Does this cover it?
 

rklapp

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Cool, when the trade winds stop (maybe next month :(), I'm curious how accurate the sims are.

I was using position north of launch instead of east of launch which is why it was going in circles. Most winds come from the windward side (east) of the island.

Is there a rocketry reason why the rockets might travel into the wind after launch? This happened to my Saturn V then the parachute failed and landed on someone's roof. I've been using talc lately on the parachute to keep them from getting stuck.
 

neil_w

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Yes, it's called weathercocking and is extremely common. It results from overstability and/or low speed off the launch rod. For the Saturn V it would presumably be the latter.
 

rklapp

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Interesting. (have to be careful how to search that on the internet)

I noticed the V2 goes upwind at 10mph but mostly straight up at 20mph. Science is awesome...
 

Steve Shannon

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Interesting. (have to be careful how to search that on the internet)

I noticed the V2 goes upwind at 10mph but mostly straight up at 20mph. Science is awesome...
When unconstrained, objects always rotate around their center of mass (also known as center of gravity or CG).
Whenever wind acts evenly upon an object, it can be simplified to a point force acting through a location we call the Center of Pressure (CP) for that object.
As long as the center of gravity is ahead of the center of pressure, a torque will result which tries to turn the object towards the direction the wind comes from. That’s called a restorative force and it’s the basis of stable flight for everything from arrows to darts to fin stabilized rockets. The greater the distance between the CG and the CP, the greater the torque. As the object turns into the airflow, there’s less and less side forces causing torque and the object stops trying to turn.
If the center of pressure is ahead of the center of gravity, the object tries to turn away from the source of pressure, which exposes even more area for the crosswind to act upon and so the object will literally constantly try to turn around. That’s what aerodynamic instability is and that’s why it’s so important to always build with the CG ahead of the CP.
 

rklapp

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So is there more or less restorative force at 20mph than at 10mph?

V2/D12-3 @ 10mph...
582010.png


20mph...
1588995821480.png
 

Steve Shannon

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So is there more or less restorative force at 20mph than at 10mph?

V2/D12-3 @ 10mph...
View attachment 415652

20mph...
View attachment 415654
The greater the wind speed, the more torque turning the rocket. A stable rocket is always trying to minimize sideways airspeed. The greater the vertical speed is the less effect the wind component has. Also, the shorter the distance between CG and CP, the less the lever arm and so less torque.
 

rklapp

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Great results today. I added clay to the nose cones of two of my unstable rockets which worked well.

The sim said the Hi-flier XL with the C6-3 was to go 40m into the wind then come back to the launch point. It landed 20m short because the parachute inverted again. This happened last week with my Honest John. I'm using swivels on the parachute lately. Could this be the reason or is it the way I'm packing the parachute into the tube? I'm also using talc on the parachutes to help them not get stuck inside (which is what happened to my Saturn V).

0509 1.png


 

Steve Shannon

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You might try a nylon parachute, like Top Flight Recovery sells. They don’t take a set like the plastic chutes.
 

Nytrunner

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It landed 20m short
Also keep in mind the landing and ground track sims are really just mathematical guesses. Theres so much variation and change with wind speed and intensity throughout the flight. Dont be disappointed when it doesnt land "where its supposed to"
 

rklapp

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I realized I was using too long of a launch rod. When I set it to the correct 35.5", it shows what actually happened.

1589143068992.png
 

rklapp

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Also keep in mind the landing and ground track sims are really just mathematical guesses. Theres so much variation and change with wind speed and intensity throughout the flight. Dont be disappointed when it doesnt land "where its supposed to"
I'm noticing that the sims are way off with the smaller rockets. I guess it's hard to model something like a Mini Fat Boy.
 

neil_w

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I'm noticing that the sims are way off with the smaller rockets. I guess it's hard to model something like a Mini Fat Boy.
Hmm, shouldn't be. Anything like a 3FNC or 4FNC should work pretty well, if the model is constructed correctly.
 

rklapp

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Continuing my thread from the Watering Hole.

https://www.rocketryforum.com/threads/what-did-you-do-rocket-wise-today.48649/page-870#post-2005359

This simulation said 200ft and altimeter said 166ft so close enough for me. When I sim east, it was mostly accurate except the rocket went north instead of east. When I sim north, it's a straight up and down. Again, good enough for me. I think the underpowered rocket is weather cocking but can't fight the wind and goes sideways.

What's weird is if I'm reading the plot correctly, the launch and landing icons are switched. Seems to happen with these underpowered flights. Maybe I'm reading it wrong...

1590785698876.png


This is for the Hi-flier XL in the video.

 
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