# atmosphere drag and velocity question?

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I have been wanting to ask this question for a while now.. I have read a lot of people complain the atmosphere in KSP2 to be very "Soupy", which has kept me from asking.. as I just excused it as being "a bug". Some of my crafts flip when I didn't expect them to, others don't... and some I think should flip if I'm not "on the ball" with the gravity turn don't.. So I need ask this, to some extend dumb, question to know if my understanding is correct or wrong.

In my understanding when something that is long and cylindrical moves fast, I want to travel the direction with the least resistance... the air wants to press it to the smallest surface forward, which to me should mean that it is hard to turn because the craft wants to remain straight.  Not the other way around that as soon as its not straight, it will present the biggest surface towards the wind? Is it me who got it mixed up?

I feel when ever I make a rocket that is not straight up no bull excrements.. I will have to make a extremely inefficient orbit.. by flying all the way out of the atmosphere before doing the gravity turn... unless I load the craft with reaction wheels. Any way.. I guess I just need to know if its me or the game not balanced out proper.

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• Solution
6 hours ago, BechMeister said:

In my understanding when something that is long and cylindrical moves fast, I want to travel the direction with the least resistance... the air wants to press it to the smallest surface forward, which to me should mean that it is hard to turn because the craft wants to remain straight.  Not the other way around that as soon as its not straight, it will present the biggest surface towards the wind? Is it me who got it mixed up?

You're touching on what's colloquially known as The Golden Rule Of Rocketry. Which is: heavy stuff at the front, draggy stuff at the back. Think of an arrow shot from a bow. It has the heavy metal arrowhead at the front, and the fletching feathers at the back.

In technical terms, you're asking after the relationship between the center of pressure (the sum of all aerodynamic forces acting on the rocket) and the center of mass (the point on which the entire rocket's mass is balanced).

Because a given body pivots around its balance point, you can quite literally think of the center of mass as a hole through which you drive a nail to pin a model rocket to a wall. The model rocket will hang there, until an external force makes it pivot around the nail. An external force like, say, you blowing a strong wind at the dangling rocket from the side.

Imagine your model rocket has a set of big fins at the back, and nowhere else. The strong wind will push against all parts of the rocket. Because the fins are big surfaces that catch the wind, there are a lot more aerodynamic forces acting on the bottom end of the rocket than on the top. As a result, the fin equipped bottom end will be pushed away from the source of the wind. The model rocket pivots around its balance point, turning its nose into the wind. This is a configuration where the center of pressure is below the center of mass, and as you can see, this rocket really wants to fly straight into the airstream. The harder the oncoming wind blows, the harder the rocket sticks to flying straight as an arrow. We call this rocket passively stable, and we need to use active steering devices like thrust vectoring to make it change directions.

Now, imagine instead that your model rocket has a set of big fins on the nose, and none on the bottom. Like SpaceX's Starship. Now, the wind coming from the side pushes the top end of the rocket more than the bottom end, and the model pivots around its balance point, going engine-first into the airstream. This is a configuration where the center of pressure is above the center of mass, and it creates a rocket that really wants to reverse itself at the first possible opportunity. The harder the oncoming wind, the more aggressively it tries to reverse itself. We call this rocket passively unstable, and to fly it, we need to constantly fight its urge to flip until we are out of the atmosphere. Starship's first flight test ended in the whole skyscraper-sized rocket doing cartwheels in midair in the most Kerbal fashion because it destroyed too many of its engines by shattering the launchpad, and thus had too little TWR, flew too shallow, started pointing away from prograde to try and correct, and subsequently reversed itself because it is passively unstable.

When building rockets, you want them to have their center of pressure below their center of mass at all times, even as its fuel tanks drain. Then your rockets will be passively stable, and the atmosphere suddenly becomes your friend instead of your enemy. You want a heavy front, and you want a draggy rear. Which may seem like a tall order, given that the front typically features draggy fairings, and the heaviest part of the rocket - the first stage tanks and engine section - are all the way at the rear. But that is the challenge you, as a rocket engineer, must solve in order to succeed.

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I am so glad you touched upon Starship and super heavy. I tried to make something similar.. although mine was a small spaceplane on top of a stage 1. that could return... (its easier for me to glide things to a run way, than land on a launch pad. SO far the kerbels have to content with a 5-20km drive to pick up the maybe landed stage x)

I did manage to fix my rocket so its stable. Any way... your answer made me realize i was half wrong, half right. I have a lot of experience with ballistics with bullets... and i assumed we spin the bullet to better fight air resistance. But thinking a bit harder.. i guess we use the centrifugal force to keep it straight? kind of how a bike will dont want to fall and tumble when its spinning. I guess that's how reaction wheels work too?

Any way. Thank you for your reply. I have another question - what then when i get out of the atmosphere? I have had rockets that flew fine.. and as soon as im out the atmosphere the rocket flips its engines front. Is that because the engines are heavier than the tip of the rocket now?

Also... Is there a way i can tell KSP2 what order i want the tanks filled?

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2 hours ago, BechMeister said:

i guess we use the centrifugal force to keep it straight? kind of how a bike will dont want to fall and tumble when its spinning. I guess that's how reaction wheels work too?

Yep, that's correct. (Although KSP doesn't model reaction wheels quite that precisely. In the game they're just a force acting on the rocket.)

2 hours ago, BechMeister said:

I have another question - what then when i get out of the atmosphere? I have had rockets that flew fine.. and as soon as im out the atmosphere the rocket flips its engines front. Is that because the engines are heavier than the tip of the rocket now?

There is no reason for your rocket to flip outside of the atmosphere, unless (a) it's a bug, or (b) some SAS mode is telling it to point at something behind it.

2 hours ago, BechMeister said:

Also... Is there a way i can tell KSP2 what order i want the tanks filled?

I assume you mean the order in which tanks drain... but nope, that's not (yet?) a feature. Best you can do is use the resource manager during flight to shift fuel around.

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Thank you for taking your time to explain all this. I guess the summersaulting rockets out of atmosphere is bugs then.. i was getting crazy about that xD

I will fiddle around with the resource manager.

Again, thank you for your time

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• 2 weeks later...

[snip]

I think were i was wrong is i thought the wind would always push the cylender back to the smallest point - not considering the point of mass being the axis of rotation.. and that the craft would have a tiping point. Which is a bit silly of me.

How ever.. I do think a lot of Rockets in KSP2 have a tendency to fly with the widest part in the wind, and that its really difficult to get the gravity roll right.

I mean.. i mostly dont even attempt a gravity roll before 10km (like in the tutorial) - so I guess I just wanted to know if it was bugs, my assumption or if it is expected behavior of a rocket to flip so much.

Edited by Vanamonde

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