Nose cone weight HPR

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David Schwantz

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If you apply simple Barrowman CP calculation, where is 1/2 the projected area, the CP doesn't change. However, the faster you go, the further back the CP goes. Run RAS Aero on your rocket and it will plot the CP for various Mach's. The Mach shockwave prevents the air from seeing some of the frontal area.
So CP is related to velocity and the velocity is related to mass and thrust and drag.
Sorry but OP was asking about weight, not speed. CP does not change with weight.
 

BABAR

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I am L-0. I seem to recall however that if you don’t do the drilled crossbar technique, that it’s a good idea to rough the heck out of the inside of the cone before trying to glue any weight inside it. Without proper rinsing out and roughing up, the inside surface of the cone doesn’t have the best natural adhesion surface in the world.
 

dhbarr

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I am L-0. I seem to recall however that if you don’t do the drilled crossbar technique, that it’s a good idea to rough the heck out of the inside of the cone before trying to glue any weight inside it. Without proper rinsing out and roughing up, the inside surface of the cone doesn’t have the best natural adhesion surface in the world.
JB Plastic Weld is supposed to be about the least terrible vs. PE, PP, Nylon etc. AFAICT ?
 

David Schwantz

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Sorry, try using one of the sim programs and add 20lbs to a NC. CP, never changes. Have a nice day. Oh, by the way, does it snow down there??
 

RocketScientistAustralia

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1627355900076.png

1627355938238.png

As evidence was requested.......
RASAero sim for one of my fleet. At the transonic point CP moves back 5 inches. At Mach5 it's forward 15 inches from the Barrowman calculated position.So that's a 20 inch variation in the CP position which is velocity dependant. Where the CP moves for your rocket depends on your design.
The velocity you get to is dependant on total weight, motor NS, thrust and drag.
I'm not planning going to M5, but, I'm aware of where the CP will be. Certainly a lot of us get to the transonic region. To the best of my knowledge neither Rocksim or OpenRocket sim this. RAS Aero is the program for flight simulation for anything trans and supersonic. And it's free.
It's also the program used for class3 launch evaluation.

This does not take into account the CG variation as the N2O tank empties from the top down and it gets heavier toward the rear..... There's a juggling act.
Just to round this out, what's the relevance of the angles of attack? At Mach, 4deg is roughly a 50MPH side wind. Perfectly possible at altitude and you'll note that the CP just moved forward of Barrowman calculated, by 3 inches for this rocket. Or with that gust of wind, CP just moved forward 7 inches from where it was before the gust.

Regards from down under.

https://www.thredbo.com.au/weather/snow-cams/
3 hour drive from Sydney.......
 
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RocketScientistAustralia

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1627367851502.png

Here is the Openrocket sim for the MadCow 4" Patriot with a J510. Gets to 800 MPH. Just nicely in that CP transonic stability danger zone that you now know about. I'll leave entering it into RAS Aero for the fliers to find out where the CP moves to for this design.

Ok couldn't wait
1627373209879.png

You can see a 1" variation depending on the angle of attack.
 
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David Schwantz

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You know I will not argue about this, but your sims show exactly what I said. CP does NOT move with weight, only speed. enjoy your snow, I would.
 

RocketScientistAustralia

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Force =Mass x Acceleration (the force comes from the thrust of the same motor being used and is constant for the example)
Velocity=acceleration x time + initial velocity

Therefore final velocity, all things being equal, IS related to mass. Does that actually matter? The answer is, it depends..... :)

For the 4 " Patriot example all 3 angle of attack graphs show the CP moving back in the transonic region and becoming more stable. For my example at a 4 deg angle of attack the rocket becomes significantly less stable and the CP moves forward. Stability is not just dependant on nose weight added. If your rocket has marginal stability and is going to go into the transonic region it's important to know where that stability is at all times.

What is the answer for your rocket, I don't know. But at least run the RAS Aero sim and find out if you're going to be launching a J510 in a 4 " Patriot.
(no-one here would use the largest motor that fits and launch it... :) ) And don't let the sim program you are using let you think that CP is a constant.

Finally, just because a design has flown successfully on a lower power motor, does not mean it will work on a higher power motor. It probably will..... But.....All of the above.
Good luck and clear skies...

Now , how ARE you going to glue that weight in?
 

RocketScientistAustralia

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I owned a half share in a hovercraft at one time. If Sydney was IN a snow bound area, I'd consider one. I'm sure they are a lot of fun.
Australia as a land mass is the same size as the US and has a diverse range of climate zones. New South Wales (Sydney is it's capital) has a land mass 1.15 x Texas. Mount Kosciuszko is the tallest mountain peak in mainland Australia at 2,228 meters (7,310 feet) tall and is in NSW
Australia has ski fields and snow trails in winter in the mountain regions, but most Aussies would go to New Zealand. It's cheaper and has higher mountains. After that Japan or US.

The simple answer to your question is..
No.

It's up to you whether you want to learn something or just get a simple answer. :)
 

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