"Keep Your Centre of Mass High" - What?

[NecessaryWeevil]

The most fundamental thing I thought I knew about building rockets was that you want the centre of mass low to avoid flipping. Yet suddenly, everyone is saying the opposite. Can someone explain WHY I want the centre of mass high? And how high is high?

[Naiba]

Think of darts or arrows. Metal at the front (top) Feathers at the back (bottom). The centre of mass is at the top to keep the thing flying stable.

If your rocket starts to tilt over, a high centre of mass means more air will be pushing the back half of the rocket than the front half. That means the airflow will end up pushing the back end of your rocket down and getting the nose to point into the wind again. It’ll keep you pointing in the direction you’re flying.

Short point made long; it gives you stability while flying out of the atmosphere by making the entire back half of the rocket act like one giant stabilising fin.

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As for how high, depends on a few things.

How much is your centre of mass changing as you.
How wide is the bottom of your rocket?*
*(How much more drag will it generate relative to the top?)
How much torque you have to turn your rocket with.*
*(If your rocket doesn't have much in the way of control you'll want to make the rocket LESS stable or you might find yourself unable to turn at all on the way up.)
Other thing or two I probably forgot goes here.

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Hope this helps! :)

[Ottomic]

Yeah, it's kind of weird to me as well. I think both cases work, but relating to stability in particular, a high center of mass will ensure your momentum is on the tip, and as such, that you are directing the maximum amount of energy towards the nose. As long as you have a proper transmission between them (ie, no wobble), the thrust is directed towards it all the same. If your CoM is lower, you are leaving rocket in front of the CoM on which the wind impacting it will have a fulcrum effect and divert you further.

Figures.

I don't find it to be nearly as strict as CoL-CoM balance, though. But it will be stabler going up.

[dunbaratu]

The reason isn't *directly* because you want it high, but becasue you want as long a lever arm as possible between the center of mass and the draggy bits at the tail end (the fins).

Remember that in a free standing object not anchored to anything, the center of mass of the object is the fulcrum point it will rotate around when a rotation is applied to it.

When the center of mass is not very far from where the fins are, then the angular force the fins provide by dragging in the wind isn't very strong because the lever arm from the fins to the fulcrum is short. Meanwhile the opposite becomes true of the draggy bits up at the top - because they are farther from the fulcrum when the CoM is lower, their influence is higher. Thus with a lower CoM you are making the drag of the top parts cause more torque than the drag of the bottom parts do, so it flips to put the "more draggy" top part at the tail end of the headwind.

[The Lonely Sand Person]

Yeah, it's because rockets don't balance on their engines like a guy balancing a baseball bat on his hand. As soon as it leaves contact with the ground, the rocket is basically in freefall -- or at least, it's in the same situation as a similar vehicle firing its engine in orbit. The location of the center of mass doesn't make any difference to the rocket execept when it interacts with all this "air" junk that it's immersed in.

Incidentally, this is also why the "pendulum rocket fallacy" is fallacious. It doesn't act like a pendulum any more than a thrown yo-yo does.

[Ottomic]

Okay, yeah, the CoM is the middle point of a fulcrum. On one side you have the fins (your control) and on the other the force the air imparts. The shorter a fulcrum you give to the air, the easier to govern the rocket will be, and the more control you will impart with fin movement.

Okay, makes sense now.

[dunbaratu]

Originally posted by Aubri:

Yeah, it's because rockets don't balance on their engines like a guy balancing a baseball bat on his hand. As soon as it leaves contact with the ground, the rocket is basically in freefall -- or at least, it's in the same situation as a similar vehicle firing its engine in orbit. The location of the center of mass doesn't make any difference to the rocket execept when it interacts with all this "air" junk that it's immersed in.

Incidentally, this is also why the "pendulum rocket fallacy" is fallacious. It doesn't act like a pendulum any more than a thrown yo-yo does.

The reason the pendulum rocket fallacy is fallicious is becaues there's no hinge or flexibility to the rocket. For it to behave like a pendulm, there has to be a fulcrum in between the part where the force is applied, and the center of mass. With a swinging weight hanging from, say, a table, the attachment point of the string to the "fixed earth" at the top of the pendulum makes such a fulcrum. The table is providing the upward force to hold the weight up, and there is a fulcrum in between where that force is applied and where the center of mass is. That point where a pivot can occur is essential. It causes the *direction* of the force of gravity and the *direction* of the force of the table opposing it to be able to differ, as the force the weight provides is transmitted via the string under tension, and therefore it isn't applied "straight down" to the table like it would be if the string was a fixed bar, and it was glued to the table at an angle such that it won't swing.

The reason it's a fallacy is because rockets can be modeled as one stiff piece. There is no fulcrum point between where the force of thrust applies to the rocket and where the center of mass is on which the rocket can swing. There's plenty of good engineering reasons not to make a rocket be bendy. But if you *could*, then it would make a difference to put the thrust at the top.

Stiffness is why it's a fallacy (no hinge on which to pendulate), not because it's in freefall. From the frame of reference of the rocket, hovering on a rocket thrust with TWR=1.0 is the same thing as resting on a table. That's not freefall.

[The Lonely Sand Person]

Originally posted by dunbaratu:

There's plenty of good engineering reasons not to make a rocket be bendy. But if you *could*, then it would make a difference to put the thrust at the top.

No... it wouldn't. Even with a flexible link in the middle, the top and bottom of the pendulum rocket accelerate under gravity at the same rate, so neglecting drag, there is no net force acting to push the counterweight back to the far side of the thrust line. Without the ground pushing up on the rocket to provide a fixed directional reference, you can't stabilize it relative to the ground.

Remember: every reference frame is correct. From inside the (windowless) rocket, as soon as you lose contact with the ground, the only acceleration you feel is due to your own engine. When you shut it off, you can't tell whether you're falling through the atmosphere, orbiting a planet, or drifting in deep space. So let's remove the atmosphere and ground to simplify the situation; picture this flexible-link rocket in orbit. The mass is off to the right when the engine starts firing. It's off-axis, so the rocket begins to spin to the right. The engine gets tugged around, but it's happy to rotate since it's not actively stabilized, and it continues to accelerate the spin. Centrifugal force will eventually drag the mass around, but at that point any concept of "stability" has gone out the airlock.

[dunbaratu]

Originally posted by Aubri:

Originally posted by dunbaratu:

There's plenty of good engineering reasons not to make a rocket be bendy. But if you *could*, then it would make a difference to put the thrust at the top.

No... it wouldn't. Even with a flexible link in the middle, the top and bottom of the pendulum rocket accelerate under gravity at the same rate, so neglecting drag

Your post became irrelevant the moment you said that phrase "neglecting drag". This is all about the drag. That's the whole point. The force of drag is what opposes the thrust. With the drag applied at a point behind the thrust, you get tension, with it in front of the thrust, you get compression. With stiff material there's no difference between the two. With flexible material there is a big difference between compressing it versus tensing it. With compressing force, the more force, the more bent it gets. With tension force, the more force, the more straight it gets.

And that bent shape will end up excacerbating the error, as it moves the center of mass even further off line, and also increases the drag on the top as it exposes more of the surface diagonally to the headwind.

When talking about why rockets flipover in the game, and how to design them to avoid this, drag cannot be ignored. Drag is the *primary* cause of the flipover. The only reason it's impossible to design a rocket with pull-thrust that works better than push-thrusting is that stiffening the rocket to avoid the bending caused by push-thrusting is a far easier engineering task than trying to design a pull-thrust configuration that doesn't end up also moving the drag higher up the craft because of all the engine bits sticking out the sides at the top. All the advantage of putting the point of thrust higher up is countered by the fact that the bulky volume at the top (instead of having the rocket taper to a point) produces more drag than the fins do, which sort of removes the advantage that otherwise would be there. Also, staging is easier if you can drop off the larger first stage engines from the bottom instead of from the top.

[Rhedd]

Originally posted by dunbaratu:

... When talking about why rockets flipover in the game, and how to design them to avoid this, drag cannot be ignored. Drag is the *primary* cause of the flipover. ...

Yep! That's why my #1 piece of advice on every thread titled "my rocket keeps flipping over!!!!" is to go slower. Get that launch TWR down to a reasonable level, don't push too hard, and you can do what I (and NASA) do and start your gravity turn 0.02km off the ground and still not flip.

[The Lonely Sand Person]

Originally posted by dunbaratu:

Your post became irrelevant the moment you said that phrase "neglecting drag". This is all about the drag. That's the whole point.

I was explaining to you why pendulum stabilization is not a thing. It's a sidebar to the original post, true, but I think other posters adequately explained why you need the CoM high. As soon as YOU said "pendulum rocket fallacy", you stopped talking about drag AT ALL.

[dunbaratu]

Originally posted by Aubri:

I was explaining to you why pendulum stabilization is not a thing. It's a sidebar to the original post, true, but I think other posters adequately explained why you need the CoM high. As soon as YOU said "pendulum rocket fallacy", you stopped talking about drag AT ALL.

You're projecting. You're the one who tried connecting the phrase "pendulum rocket fallacy" to thread talking about the problem caused by drag. I only mentioned it when arguing against your claim. The "fallacy" is only a fallacy (meaning that pushing and pulling are identical because you are in a frame of reference where the rocket doesn't have any retarding force pulling back against the thrust) when on a world with no atmosphere. Once you have atmosphere, there *is* a difference between pulling and pushing explicitly because there IS in fact a retarding force pulling back on the rocket - drag. Therefore it stops being a fallacy in that scenario. The reason for putting the mass as high as possible is NOT, as you claim, because the pendulum fallacy is a fallacy. When there's atmosphere, as is the case when trying to solve this tipping over problem, pulling is in fact more stable because the force pulling back on the rocket causing it to tip over isn't gravity, it's drag. And putting the thrust at the back puts the rocket body under compression agaisnt that drag, rather than tension against it as would happen if it pulled from the front. Thus it makes the rocket bend more, excacerbating the problem of the drag.

It's not a fallacy to treat a rocket IN ATMOSPHERE as a pendulum, as long as you remember that the center of mass doesn't correspond to the pendulum's bob. It corresponds to the pendulum's fulcrum. The effective center of drag is what makes the pendulum's bob in the analogy.

[CondorDrake]

If you think about what happens when you bevel your rocket within the atmosphere, it becomes quite intuitive. If you go 45 degrees and your COM is at the tail of your rocket, your front would always flip around because it offers less resistance to the atmosphere. When your COM is at the top, the higher mass will cause more resistance to the atmosphere, which makes it easier to 'lay the top in the wind' and just correct your direction by adjusting the back.

[I Like Your Style]

Originally posted by NecessaryWeevil:

The most fundamental thing I thought I knew about building rockets was that you want the centre of mass low to avoid flipping. Yet suddenly, everyone is saying the opposite. Can someone explain WHY I want the centre of mass high? And how high is high?

Imagine you are balancing a midieval morningstar on your fingertip to impress your friends (the spiky ball is on top/handle end on your finger). You can do this because the center of mass of the weapon is towards the spiky end. It only takes small movements to keep it balanced.

Now Imagine trying the same trick with it upside down. The center of mass is much closer to your finger. If you want to keep that thing balanced, you're going to have to move your hand AND take a step every so often. It is much much harder and less forgiving.

If you imagine your fingers are the thrusters, and the morningstar is a rocket, you see the principle.

But who cares? You own a morningstar!

[Chibbity]

Originally posted by I Like Your Style:

Originally posted by NecessaryWeevil:

The most fundamental thing I thought I knew about building rockets was that you want the centre of mass low to avoid flipping. Yet suddenly, everyone is saying the opposite. Can someone explain WHY I want the centre of mass high? And how high is high?

Imagine you are balancing a midieval morningstar on your fingertip to impress your friends (the spiky ball is on top/handle end on your finger). You can do this because the center of mass of the weapon is towards the spiky end. It only takes small movements to keep it balanced.

Now Imagine trying the same trick with it upside down. The center of mass is much closer to your finger. If you want to keep that thing balanced, you're going to have to move your hand AND take a step every so often. It is much much harder and less forgiving.

If you imagine your fingers are the thrusters, and the morningstar is a rocket, you see the principle.

But who cares? You own a morningstar!

Bit of necro here bub.

This thread last saw action back in 2015.

Good explanation though lol.

[Astronaut]

The center of mass is its (what I like to call) handling point. The rocket's height does not matter at this point. That is the center of gravity and that will be the point around which the rocket turns. It doesn't matter where your center of mass is, as long as (on a rocket) the center of mass is higher than the center of lift. That's why fins are placed at the bottom. I don't know about zero fins though. Rockets do fly without them but usually flip. In real life though, anything generates lift, but often not enough. Take a brick for example. Not enough lift to counteract its weight. Anyways, KSP probably takes into account the lift generated by parts because the forces on a rocket, shift f12, are the same forces on a plane.