Hovering Objects Drift West on Kerbal?

[The_Rocketeer]

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).

Woah... I completely missed this point the first time around, but its actually massive to this issue.

Because SAS keeps your craft's orientation fixed, any rocket hovering on SAS alone will gradually tip over relative to Kerbin's surface, not due to the surface moving faster than your craft, but simply because the orientation of up/down is moving and your craft isn't moving with it. Since Kerbin's full rotation period is about 6 hours, that means your craft will tip over 1 degree every minute!

The implications of that are massive for a hovering craft that isn't being corrected for it. In 5 minutes you go from a craft hovering completely vertically to one scudding towards the VAB at a 5 degree pitch - not remotely insignificant to anybody with any experience manoeuvreing VTOLs.

For the launch pad, this would be in cooperation with the effect I described earlier, but for the runway this effect would be working against it. It would be interesting to do a couple of experiments and see how long it took for Runway's the initial net east-bound drift to reverse due to SAS rotation.

+rep Ival, what a brilliant bit of brainpower...

[SRV Ron]

Put a rover on the pad and it will slowly roll off towards the VAB. The same rover placed on the runway will start rolling towards the sea. That is the slight off angle of gravity having an effect in the KSC complex.

Rockets launched straight up will land West of the pad, the distance determined by the time in flight and altitude achieved. Again, the cause is the slight angle of gravity at the pad.

[Empiro]

Hmm interesting, though in the case of hovering, it's due to your craft not pointed directly "up". If the launchpad isn't 0 degrees with respect to Kerbin (because the VAB and launchpad is one giant slab that doesn't follow the curvature of Kerbin), then it means that your rocket is actually pointed ever so slightly to the west, which will give your craft a bit of drift as you hover for long periods.

[Kryxal]

Additional effect, as the planet rotates under you, you're going to be angling MORE westwards...

[RizzoTheRat]

The space port is on the coast, its normal to have an inshore breeze. ;-)

[Kar]

The space port is on the coast, its normal to have an inshore breeze. ;-)

And this was best explanation here!

[The_Rocketeer]

@sal_vager necro-post, my bad, but I was reminded of this thread this evening and on re-reading realised I owed you a massive and long-overdue apology. Your first reply to this thread summed up in 6 words almost exactly the same effect that I credited Ival70 for later - when actually all he did was give flesh to what you'd said first!

I stand by my own description of another related process, but I'm sincerely sorry for my arrogance in failing to appreciate what you were getting at until someone dumbed it down for me! I shall atone by repping your reply too!

[Tyko]

On 12/2/2014 at 10:33 AM, pincushionman said:

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.

I never thought about this..it's just like the racecar track effect of object in lower orbits "gaining on" objects in higher orbits even though they are moving slower...crazy

[Broco]

On 6.8.2016 at 3:40 AM, The_Rocketeer said:

@sal_vager necro-post, my bad

I don't think it is so bad to revive this thread because this effect is still present in current versions and will be present in the future (I mean, why would you change it?).

But so just so I get this right: There are basically 3 effects for floating craft moving at the KSC (the first 2 apply for any place on Kerbin and all rotating planets/moons), right?

1. The planet rotates under a floating craft, so even if your craft floats perfectly still the planet moves under you
2. If you have SAS on your craft will tilt because your orientation changes as the planet rotates
3. the KSC is a flat model on a sphere so gravity pulls you to the lowest point of the model

Am I getting this right?

[cubinator]

2 minutes ago, Broco said:

Am I getting this right?

Yeah, pretty much.  

[Broco]

4 minutes ago, cubinator said:

Yeah, pretty much.  

I'm with this game since the early stages in 2011 and still amazed by the level of detail on the physics part and that I learn new things all the time