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Drag coefficient of my rocket


Azerty

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Hello everyone, i'm new here.

So my question is simple : How could i get the drag coefficient of my rocket ?

I want to have the equation that gives me the drag force of my rocket. I found the equation but dont know how to get the coefficient.

Thanks for your replies :wink:

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2 hours ago, Azerty said:

Hello everyone, i'm new here.

So my question is simple : How could i get the drag coefficient of my rocket ?

I want to have the equation that gives me the drag force of my rocket. I found the equation but dont know how to get the coefficient.

The drag coefficient of any solid body is a fancy way of putting a number on the question, "How easily does this shape push through air?"

Unfortunately, this is not an easy question to answer. A shape which pushes through air very easily at low speeds may push through the air much more poorly at high speeds. A shape which has a boat tail at the back will have a lower drag coefficient than a shape which terminates as a cylinder. A shape which has a rocket exhaust plume at the back will have a different drag coefficient than a shape which has no plume.

In actual practice, every rocket design is placed inside a supersonic wind tunnel and tested from Mach 0 to Mach 5 to determine drag coefficient. There is no reliable way to calculate drag coefficient other than by using a wind tunnel.

In theory, you can compare the shape of your rocket to the shapes of objects with known drag coefficients and work accordingly.

If you really want to skimp on effort, you can just estimate drag coefficient at 0.21. Go with 0.23 if your rocket has fins.

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So if i want to estimate the drag force of my rocket i'll just take a Cx of 0.21 and put it into this formula 915290ad0717091fb5b824135eaeb94c44b7e883 in excel and that's it ? 

EDIT : I just realized that i don't have the surface area either.....

Edited by Azerty
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1 hour ago, Azerty said:

So if i want to estimate the drag force of my rocket i'll just take a Cx of 0.21 and put it into this formula 915290ad0717091fb5b824135eaeb94c44b7e883 in excel and that's it ? 

EDIT : I just realized that i don't have the surface area either.....

Area is a reference. For cars, it is the frontal area (ie. looking at it from the front, how big is the total outline of the car). For airplanes it is the wing area (ie. looking down on the airplane, what it the area of the wings, including the bit that goes through the fuselage).

I don't know what area the previous poster assumed when he suggested 0.21, but my guess is that a rocket would use frontal area like a car. So probably take the widest part of the rocket and calculate the cross-sectional area.

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ok thank you

I'm using a protective shell so i'll just assume that it's a cone of x meters height ?

https://prnt.sc/jlum0j

Actually i'm trying to get the drag force because i'm coding a hoverPID in kos, i just go to 500m and get stable for example. I made the modele in excel and everything is ok as long as i don't need to go too high otherwise the drag force is too high and i have a big difference for the first overshoot in my step response for instance. So i need to include the drag force in the modele.

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14 hours ago, mikegarrison said:

For cars, it is the frontal area (ie. looking at it from the front, how big is the total outline of the car). For airplanes it is the wing area (ie. looking down on the airplane, what it the area of the wings, including the bit that goes through the fuselage).

Both are used. One is skin drag (friction of the passing air) and the other is form drag (the ramming effect that the object makes). For cars, form drag is much more apparent; on airplanes, skin drag dominates.

Probably for rockets other forms of drag would prevail, ie. wave drag (supersonic).

Edited by YNM
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The reason that drag works the way it does is this: in order to push air out of the way, a rocket must (notionally) accelerate that air to the same speed it is going. By Newton's second law, acceleration is force divided by mass. By Newton's third law, the force applied to the air is equal to the force the air applies to the rocket.

So the coefficient of friction is defined for a shape with the reference area being the cross-sectional area of the entire rocket at its widest point.

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So what do i do ? I'm a little confused. The widest point being the protective shell, can this works ? 

12 hours ago, Azerty said:

I'm using a protective shell so i'll just assume that it's a cone of x meters height ?

And i take a Cx of 0.21 ?

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Ok i'll try this thank you. I presume this will also work for airbrake.

Last question : is there a way to have the equation of the athmosphere density ? I found the table on the wiki but no equation. I can manage to do something with the table but i would rather have the equation.

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1 hour ago, Azerty said:

Ok i'll try this thank you. I presume this will also work for airbrake.

Airbrake will be designed to, well, brake. So its drag coefficient will be on the order of 1.5-2. Times its cross-sectional area.

1 hour ago, Azerty said:

Last question : is there a way to have the equation of the athmosphere density ? I found the table on the wiki but no equation. I can manage to do something with the table but i would rather have the equation.

Atmospheric density depends on so many factors that it's much easier to just use a reference table. 

You can calculate density based on pressure very easily, but the pressure equation is more difficult. This might help: http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/barfor.html

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Sorry to add a whole extra thing to think about, but the Drag Coefficient will also change depending on Mach number.  It usually increases towards M = 1, then decreases again after and levels out after M = 1.2 or so.  And the speed of sound itself changes with temperature, which varies depending on where in the atmosphere you are (decreases, then increases).

 

 

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Keep in mind that he's trying to do this for KSP. We're telling him how it works in real life, but the game may be a little different. In particular, looking up air density v. height for the Earth is not likely to give the right answer for KSP.

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I think i'll go with the table then, thanks. I know, with speed comes heat, so local pressure will vary, so do locale atmosphere density and i think i wont be able to include that in excel lol :D. What i want to do is an approximation of this while being as precise as i can in the limit of my knowledges.

Yes i want to know how it works in KSP in order to make my "simulation" the more accurate possible. My goal is to recover my rocket, SpaceX style, using kOS.

(Sorry if my English isn't good)

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1 hour ago, Azerty said:

I think i'll go with the table then, thanks. I know, with speed comes heat, so local pressure will vary, so do locale atmosphere density and i think i wont be able to include that in excel lol :D. What i want to do is an approximation of this while being as precise as i can in the limit of my knowledges.

Yes i want to know how it works in KSP in order to make my "simulation" the more accurate possible. My goal is to recover my rocket, SpaceX style, using kOS.

(Sorry if my English isn't good)

FYI, heating-related speed won't come into play here. It's looking for the density of the atmosphere before you get to it, not after.

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Maybe this helps:
in-flight Alt-F12/Physics/Aero/(set all checkboxes on)

Also you can switch drag on/off in Alt-F12/Physics/Drag, launch your rocket into orbit and compare the remaining fuel with drag on/off.

Or launch a rocket vertically and compare max altitude with drag/on/off.
As absolute value of potential energy / mass= GM/(R+h), and GM = gR2, so |E/m| = gR2/(R+h),
you can estimate drag speed loss dV= sqrt(2gR* ( 1/(R+hon) - 1/(R+hoff) ) ).

Say, your rocket climbs up to 16200 with drag on, and 33880 with drag off.
dV= sqrt(2*9.81*600000*(1/(600000+16200)-1/(600000+33880))) = 565 m/s.

For non-vertical launch loss +~25%.

Edited by kerbiloid
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Thanks for your replies

I didn't know that

17 hours ago, sevenperforce said:

FYI, heating-related speed won't come into play here. It's looking for the density of the atmosphere before you get to it, not after.

Maybe this will help you to understand what i wanna do :

https://prnt.sc/jmj8l8

In this exemple i start at the launch pad which is at 80 meters and i want to stabilize at 1000m. Here the drag isn't really important but it is if i want to start at 40.000m and stabilize at 110m for example.

I have to tune the K,Kp,Ki and Kd values in order do what i want. This "simulation" helps me to do that. So if i don't include the drag i might start the burn too early because i'll be assuming that nothing is slowing me.

I know there are other (smarter) ways to recover rocket with kOS but i want to try this.

Edited by Azerty
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FAR mod has a very detailed GUI for aerodynamic debugging (though still 1.3.1)

and here is some basic tutorial

 

Spoiler

 

Maybe this can be useful.

P.S.
But FAR updates/replaces the aerodynamics with its own formulas (much more detailed), so unlikely its calculations are enough accurate for the simplified stock aerodynamics.

Edited by kerbiloid
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6 hours ago, kerbiloid said:

P.S.
But FAR updates/replaces the aerodynamics with its own formulas (much more detailed), so unlikely its calculations are enough accurate for the simplified stock aerodynamics.

I have heard of this mod and it looks cool but i think it might be even more difficult to simulate it in Excel lol :D I'll give it a shot anyway and see what i can do.

12 hours ago, sevenperforce said:

I haven't used KOS, but keep in mind that assuming a decent amount of thrust, you will likely begin your landing burn after reaching terminal velocity. So drag is not a significant part of the equation at that point.

If i find the terminal velocity, i may be able to do something like this in excel :

If velocity>terminal velocity and altitude>X
use terminal velocity

This may be working when trying to recover the rocket even though it probably won't work in others case scenario.

EDIT :I tried from 40000 m and i never really reach terminal velocity, i indeed lose a big amount of speed at around 10000m to reach something like 200m/s and then i continually slow... 

Edited by Azerty
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11 hours ago, Azerty said:

EDIT :I tried from 40000 m and i never really reach terminal velocity, i indeed lose a big amount of speed at around 10000m to reach something like 200m/s and then i continually slow... 

That's terminal velocity. Because the scale height of Kerbin is so low, air density increases pretty consistently as you drop below 10 km and so terminal velocity will drop as well. But it's a pretty narrow window.

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Sorry for late response.

thanks for your explanation, i will try to find an average terminal velocity and see if it works ;) 

On 5/26/2018 at 3:55 AM, Lego_Prodigy said:

Heres my very not math answer: streamline the heck out of the rocket and see how well it cuts thru the air, and if not it needs more streamlining :P MORE STREAMLINING!

That's a typical kerbonauts response but not what i'm looking for lol :D 

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