Hovering Objects Drift West on Kerbal?

[sonaxaton]

This may be a bug or intentional, I'm not sure, but I think it's inaccurate and somewhat annoying.

I noticed one time when I was testing a moon base on KSC's launchpad that every time I used the engine and RCS to lift upward slightly, my base would always drift in the same direction. I was pretty damn sure it was staying perfectly vertical, so I had no idea why this was happening. You can see this pretty easily if you build a small, very stable rocket and throttle the engine just enough to float a few meters in the air for a few seconds. If you touch back down gently, you'll be slightly off the center of the launchpad. I realized this direction is always West, and I think I know why.

Kerbin's atmosphere applies drag to spaceships, slowing down their speed. Vertically this works fine, but I think what happens horizontally is that as the planet spins to the East, the atmosphere stays still on top of it. When your ship lifts off the ground, it had rotational momentum equal to Kerbin's and is "thrown" to the East along with the planet. But since the atmosphere is not turning, drag tries to "slow you down" relative to the atmosphere, and you drift slightly West compared to the surface.

In reality I'm pretty sure this doesn't happen, but I could be wrong. Kerbin is much smaller than Earth, and spins much faster, so it just might be more noticeable. But I think that the atmosphere turns with the Earth, so if you jump up in the air, it doesn't pull you back to the West since you're not moving horizontally relative to the atmosphere.

This effect is much more noticeable when you just launch a rocket into space directly up. You'll notice that you get a parabolic flight path even though you went straight up, since again the atmosphere is dragging you horizontally. This might be slightly more realistic since I think higher up the atmosphere doesn't spin as fast as the Earth.

I could be wrong about all this though, if anyone knows more physics than I do please share. If I am right though I think they should fix this, at least for the case where you're only a few meters off the ground, since it's also pretty annoying that craft hovering off the ground will drift horizontally.

[sal_vager]

The planet is turning under you

[The_Rocketeer]

The planet is turning under you

Actually, I think you both have this wrong. Kerbin's atmosphere is turning at the same speed as Kerbin's surface, so it's drag is 0 at (and near 0 close to) the surface and wouldn't pull you west. Consequently, the planet wouldn't turn under you, as you'd also be moving at very nearly the same speed.

This effect is to do with the direction of gravity.

Try leaving a plane at the end of the runway with the brakes off, and you'll notice it starts to roll towards the other end (note this is the opposite direction to what you described - we'll come to that). Why is it doing this?

The answer is, gravity is pulling the plane forwards.

Most people think of gravity as linear (straight down), but really it's spherical - gravity always pulls to the middle of the sphere. On a big enough sphere, like Earth, it's easy to think of the surface being flat and gravity being perpendicular to/through it, but on Kerbin, a much smaller sphere, weird things become more noticeable - partly cos the effect of things like friction are much smaller relative to the force and direction of gravity.

Consequently, on a surface that is perfectly flat, like the KSC, gravity will attract a free-rolling object towards the part of the surface that is closest to where it's force is perpendicular - the part of the flat surface that is closest to the centre of Kerbin. In other words, from the perspective of the force of gravity, the whole KSC is bowl-shaped and curves to a point near the middle.

So why would gravity pull a rocket from the launchpad west, but a plane on the runway east? Because this 'low point' is somewhere between the two. If you look at the KSC from overhead, you find that the middle of KSC does in fact lie between those two points - exactly as you would expect it to be.

So, why would KSC be perfectly flat when the surface of Kerbin isn't? Basically, because KSC is a 3d model made in a modelling program that works with perfectly flat surfaces, that's just been stuck to the side of a far, far bigger model that much more nearly approximates a real sphere. Also, Kerbin's gravity is actually a separate mathematically functional sphere from Kerbin's surface, which will be even closer to a real sphere than the surface (as is the case with Earth's gravitational field).

And what's the story with the rocket? Well, turns out your rocket isn't quite pointing perfectly straight up on the launchpad, because the launchpad doesn't point perfectly straight up. So, if u keep your inclination exactly what it was when you launched, you're just holding yourself at a tiny fraction of an angle off vertical. Consequently, 'down' is actually slightly 'west' of where you think it is.

Hope that makes sense.

Edited December 2, 2014 by The_Rocketeer

[sal_vager]

Yeah that effect exists as well The_Rocketeer, though that pulls you East down to the center of the runway, the effect seen by sonaxaton moves the craft West

Edited December 2, 2014 by sal_vager
Spelling

[SpaceJuice]

Whats the "delta phisics time on" and what computer are you using?

[The_Rocketeer]

Yeah that effect exists as well The_Rocketeer, though that pulls you East down to the center of the runway, the effect seen by sonaxaton moves the craft West

Yep. So read it through...

Consequently, on a surface that is perfectly flat, like the KSC, gravity will attract a free-rolling object towards the part of the surface that is closest to where it's force is perpendicular - the part of the flat surface that is closest to the centre of Kerbin. In other words, from the perspective of the force of gravity, the whole KSC is bowl-shaped and curves to a point near the middle.

So why would gravity pull a rocket from the launchpad west, but a plane on the runway east? Because this 'low point' is somewhere between the two. If you look at the KSC from overhead, you find that the middle of KSC does in fact lie between those two points - exactly as you would expect it to be.

...

And what's the story with the rocket? Well, turns out your rocket isn't quite pointing perfectly straight up on the launchpad, because the launchpad doesn't point perfectly straight up. So, if u keep your inclination exactly what it was when you launched, you're just holding yourself at a tiny fraction of an angle off vertical. Consequently, 'down' is actually slightly 'west' of where you think it is.

Edited December 2, 2014 by The_Rocketeer

[sal_vager]

Yeah I read it, so your rocket on the pad is slightly tilted to the west relative to the planet, and if you have SAS on like most people do it won't veer off course, meaning you're actually flying westward unless you do something about it.

The effect of gravity is negligible in this case as you'll very quickly have passed through the point where you are directly over the gravitational center and gravity will be pulling you East, but as you are pointing West and SAS is holding your trajectory you keep on heading that way.

If you launched the rocket from the Runway it'll fly East, then be pulled by gravity Westward.

Simple

[The_Rocketeer]

The effect of gravity is negligible in this case as you'll very quickly have passed through the point where you are directly over the gravitational center and gravity will be pulling you East, but as you are pointing West and SAS is holding your trajectory you keep on heading that way.

Erm, no. If you are simply hovering for a few seconds above the launch pad, as described in the OP, the force of gravity will be almost constant, almost linear, and will certainly not reverse horizontal direction.

On a launch to orbit, you'd be right, but that's not the scenario being described.

[sal_vager]

I wasn't talking about hovering though, I was describing an ascent and how the pull might be changing.

Gravity would be pulling you directly back down but as you were at an angle at launch you will be at an angle to vertical all the way up.

A craft just hovering is just going to keep moving sideways as it's tilted from vertical, that is the effect here and not gravity or the atmosphere

[MarvinKitFox]

I wasn't talking about hovering though,

So you are just ignoring the OP's question?

He very specifically asked about his craft's behavior when hovering over the launch pad.

Rocketeer, +1

Vager, -5

[sonaxaton]

Basically, because KSC is a 3d model made in a modelling program that works with perfectly flat surfaces, that's just been stuck to the side of a far, far bigger model that much more nearly approximates a real sphere. Also, Kerbin's gravity is actually a separate mathematically functional sphere from Kerbin's surface, which will be even closer to a real sphere than the surface (as is the case with Earth's gravitational field).

Thanks Rocketeer, this answer makes the most sense to me. So if they wanted to fix this they'd have to make a more accurate model, namely by bending it slightly so it was sphere-flat not plane-flat.

[pincushionman]

Consider this also - when you are on the pad, your horizontal ORBITAL speed is the same as the pad's surface. Once you lift off without applying horizontal thrust, your horizontal orbital speed is unchanged. But now you're on a circle with a larger radius, so your ANGULAR speed is lower than that of that point on the ground. And that point on the ground will thus appear to revolve eastward faster than you are.

Since the atmosphere is "anchored" to the surface, you'll also experience a small force pushing you eastward, but at these low relative speeds it'll be pretty negligible.

And snce the differences in angular speed are proportional to the differences in "orbital" radii, a smaller Kerbin means a more pronounced difference.

[BagelRabbit]

Imagine the Space Center really does conform to the terrain of Kerbin, and is thus at a constant altitude above sea level. Then, everything on both the East and West side of KSC would be pointing straight up.

Now, take a butter knife and lop off a bit of the surface of Kerbin, so that you have a flat plane where the center has a slightly lower altitude above sea level than the sides.

On the side of KSC facing the ocean, the angle of a rocket pointing "straight up" isn't 90 degrees, it's true. It is angled, westward. An easy way to visualize this is if you chopped Kerbin in half and then built the space center right at where Kerbin's center of gravity is. A rocket launched from the Launchpad would be at nearly a 90 degree angle to Kerbin's center of gravity. The rocket would fly westwards, towards the center of gravity, far more than up. It would likely crash into the VAB or some other building if the TWR wasn't high enough.

There's your explanation. See if the same effect takes place on another body with an atmosphere, such as Laythe or Eve. I'm betting that's not the case.

In other words, The_Rocketeer, you're right...

Edited December 2, 2014 by UpsilonAerospace
Gah. East and West.

[wasmic]

Ummm, guys, KSC isn't bowl-shaped. The runway is bowl shaped independently from the runway, so the middle of the pad would be the bottom part of the launchpad bowl. Thus, you're already at the bottom of the bowl when you launch a rocket. I personally think that it's due to the planet turning underneath.

[The_Rocketeer]

A craft just hovering is just going to keep moving sideways as it's tilted from vertical, that is the effect here and not gravity or the atmosphere

You're not wrong, but you're also not right.

The reason that the craft is 'tilted' is because the direction of gravity is different from the direction of thrust. This stems from confusion about the orientation of the launch pad, which should point straight up but actually points slightly west. It's true that your % of thrust that isn't being applied purely vertically will contribute to drift in this direction, but it's also true that even if you were to shut off all thrust, you would appear to fall slightly 'west' rather than 'straight down', using the launch pad as your frame of reference.

Ummm, guys, KSC isn't bowl-shaped. The runway is bowl shaped independently from the runway, so the middle of the pad would be the bottom part of the launchpad bowl. Thus, you're already at the bottom of the bowl when you launch a rocket. I personally think that it's due to the planet turning underneath.

I think you mean the runway independent of the launch pad.

This may be true, though I doubt it - there would be noticeable changes of plane between the various KSC modules, and although there ARE clipping/collision issues at the boundaries, I have never noticed any change in inclination between one side of the boundary and another.

Either way, I believe the launch pad's 'tile' extends considerably towards the VAB, which would still account for the 'bowl' effect.

As for the planet turning hypothesis, the differences in rotational velocity would be utterly negligible. The acceleration due to gravity, however, is compound - the longer it goes on, the more noticable it is.

It's simple to test anyway - try launching and hovering the same vehicle from the launch pad and from the runway. I bet you get opposite drift.

Edited December 2, 2014 by The_Rocketeer

[Alshain]

I think it's an issue of momentum. In the real world when a craft leaves the ground it still possess the momentum of the earth's rotation in space. It's like if you were driving riding* in a moving vehicle and threw a tenis ball one foot in the air. It doesn't fly to the back of the car, because it has the cars momentum.

I'm not sure if KSP simulates the same relative speed once you leave the planet.

On another note, someone said gravity is spherical which is incorrect, it's conical. The visible part of a black hole is created by a massive amounts of gravity, that is the shape of gravity.

And yes, the runway is not flat, it dips in toward the center.

* Please do not throw tennis balls while driving.

Edited December 2, 2014 by Alshain

[purpletarget]

I've generally attributed such things to Coriolis effect, which basically goes back to Sal's original explanation, the planet is turning underneath, plus the angular velocity delta as you gain altitude from the surface. The atmo is probably the main thing that helps keep the ship where it is as long as it does.

Check another experiment sometime, landing on an equatorial plane on Mun, Minimus or some other airless body. Check your hover experiment there, and see what drift you end up with. It'll be easier to see on larger/faster sidereal velocity planets though (Tylo perhaps?).

[The_Rocketeer]

I'm not sure if KSP simulates the same relative speed once you leave the planet.

On another note, someone said gravity is spherical which is incorrect, it's conical. The visible part of a black hole is created by a massive amounts of gravity, that is the shape of gravity.

And yes, the runway is not flat, it dips in toward the center.

Yes KSP does simulate this. If you click your speed display on the navball HUD it'll change between orbital and surface (and target if you have one). You'll notice that even when parked on the surface and not moving, you still have orbital speed. That's why it's easier to reach orbit heading east than west.

For the sake of argument, you're right about the 'shape' of gravity, but for the sake of understanding (and the facts of KSP's physics) it's spherical in the game/this issue.

Lastly, the runway is in fact completely, perfectly flat. The 'bowl-shape' to which I have referred is a figurative way to describe how the force of Kerbin's gravity is applied to an object sitting on such a surface. The runway is not literally curved, it just seems to be so because of the effect of gravity working on an object at rest against a tangential plane.

[Darren9]

I think it's just that the launchpad tilts to the west. You're not pointing straight up when sat on it. If you launch a true frictionless symmetrical hovercraft from a stationary start it makes it's way to the VAB slowly accelerating - it always goes the same way and can only stay still or roll down hill. By the same measure the runway (at least at the launch end) tilts somewhere around SE. The whole of KSC just isn't perfectly flat or perfectly horizontal.

[The_Rocketeer]

I think it's just that the launchpad tilts to the west. You're not pointing straight up when sat on it. If you launch a true frictionless symmetrical hovercraft from a stationary start it makes it's way to the VAB slowly accelerating - it always goes the same way and can only stay still or roll down hill. By the same measure the runway (at least at the launch end) tilts somewhere around SE. The whole of KSC just isn't perfectly flat or perfectly horizontal.

http://i.imgur.com/6fNqDzm.png

Good demonstration.

This pretty much supports my entire explanation.

[Alshain]

Lastly, the runway is in fact completely, perfectly flat. The 'bowl-shape' to which I have referred is a figurative way to describe how the force of Kerbin's gravity is applied to an object sitting on such a surface. The runway is not literally curved, it just seems to be so because of the effect of gravity working on an object at rest against a tangential plane.

Nope, I'm pretty sure it is not perfectly flat, you can't apply a flat model to a spherical planet.

[The_Rocketeer]

Nope, I'm pretty sure it is not perfectly flat, you can't apply a flat model to a spherical planet.

...why not?

[sal_vager]

Nope, I'm pretty sure it is not perfectly flat, you can't apply a flat model to a spherical planet.

Eeeh, yeah you can

[waterlubber]

Here, I drew some pretty pictures to help explain UpsilonAerospace and The_Rocketeer's explanation of what I call "Gravity Walking"

[Ival70]

I have noticed this effect as well. I believe it is caused by the fact that if you point a ship straight upwards and try to hover, the ship will appear to be rotating westward ever so slightly, when actually the planet is rotating eastward, and the ship is maintaining an absolute heading. (This effect is much more pronounced in low orbit). And as the ship tilts slightly westward (relative to Kerbin), its engine will start giving the ship a tiny westard velocity.

Theoretically, if you were to leave a ship hovering for 90 minutes without adjusting the heading, it would turn all the way to the horizon on a heading of 270 (west).

Edited December 2, 2014 by Ival70
Theoretically..