Clockwise / Counterclockwise atmospheric flight?

[Fyzzi]

Hello fellow space program CEOs

I've been just wondering (and this has been bugging me quite a long time) if it's more effecient (faster) to go around Kerbin with a plane going in the direction of Kerbin's rotation, against that direction or if it doesn't matter...

I know that it might me a dumb question, but I'm really unsure how the atmosphere reacts to the planets rotation...

So what do you guys think? I'm looking forward to see your ideas...

[Daishi]

Of course - although you'd get bigger gains the higher you fly And that caps at a certain point, because of the limits of air-breathing engines.

Edit - Flying with Kerbin's rotation, I mean.

Edited October 12, 2013 by Daishi

[Monkeh]

I guess you're questioning how the gaseous atmosphere spins relative to the planet below it and how that impacts aerodynamics?

I think it would be faster flying the opposite direction to planet spin. Faster to reach the same point on the surface, (or above that point), but slower to travel the distance of a full circumnavigation. If that makes sense.

The atmospheric affects requires someone with a much better grasp of fluid dynamics than myself to answer you though.

[Bacterius]

I guess you're questioning how the gaseous atmosphere spins relative to the planet below it and how that impacts aerodynamics?

I think it would be faster flying the opposite direction to planet spin. Faster to reach the same point on the surface, (or above that point), but slower to travel the distance of a full circumnavigation. If that makes sense.

The atmospheric affects requires someone with a much better grasp of fluid dynamics than myself to answer you though.

I doubt KSP actually models the atmosphere to this extent, though.

[FuzzyLlama]

On Earth, the jet streams typically travel in the same direction as the spin of the planet. This is because the primary control on jet stream motion is the rotation of the planet (factors such as differential heating of air over land and air over sea also affect the jet streams, and can even create easterly jet streams). So, if you were keen, you could coast on up to 15 km altitude on Kerbin, find a jet stream and ride against it. Your maximum surface velocity might be a little higher, as your engines would be having air rammed into them au naturale. Go above the jet streams and you lose some of this advantage, but you might be able to travel much faster (esp. if your operational ceiling is 30 km or greater). Traveling opposite to Kerbin's rotation will lower the around-Kerbin fly time because the ground moves under you while you fly. At such low pressure, drag hardly matters anymore anyhow, and the difference between 1500 m/s and 1600 m/s is much less than the difference between 100 and 200 m/s.

That all being said, I kind of imagine that until your navball reads 'Orbit' instead of 'Surface,' the effects of planet rotation beneath you are ignored. Maybe someone more familiar with Kerbics could enlighten us on this rule.

[Fyzzi]

I guess you're questioning how the gaseous atmosphere spins relative to the planet below it and how that impacts aerodynamics?

I think it would be faster flying the opposite direction to planet spin. Faster to reach the same point on the surface, (or above that point), but slower to travel the distance of a full circumnavigation. If that makes sense.

The atmospheric affects requires someone with a much better grasp of fluid dynamics than myself to answer you though.

Yeah, exactly what I meant the point is, that we should have more air intake if we went against the rotation of the planet/atmosphere, so that's why I'm asking dunno if the game counts with that, but it should give you some more intake, thus making you able to go higher... at least in my opinion...

[Fyzzi]

On Earth, the jet streams typically travel in the same direction as the spin of the planet. This is because the primary control on jet stream motion is the rotation of the planet (factors such as differential heating of air over land and air over sea also affect the jet streams, and can even create easterly jet streams). So, if you were keen, you could coast on up to 15 km altitude on Kerbin, find a jet stream and ride against it. Your maximum surface velocity might be a little higher, as your engines would be having air rammed into them au naturale. Go above the jet streams and you lose some of this advantage, but you might be able to travel much faster (esp. if your operational ceiling is 30 km or greater). Traveling opposite to Kerbin's rotation will lower the around-Kerbin fly time because the ground moves under you while you fly. At such low pressure, drag hardly matters anymore anyhow, and the difference between 1500 m/s and 1600 m/s is much less than the difference between 100 and 200 m/s.

That all being said, I kind of imagine that until your navball reads 'Orbit' instead of 'Surface,' the effects of planet rotation beneath you are ignored. Maybe someone more familiar with Kerbics could enlighten us on this rule.

Very interesting stuff mate thanks, I'd never imagine it worked like that in real life... it would be great to have some dev enlighten how it works in KSP though just to see if this really matters... anyway thanks for the comment and those facts

[Scarecrow88]

I recently did a circumnavigation flight flying westwards (against planet rotation) on the basis that the rotation would bring my starting point closer by the time I was all the way round. Flight time was 1 1/2 hours, so Kerbin would have rotated through 90 degrees in that time if my understanding is correct that Kerbin has a 6 hour day.

[sjwt]

That all being said, I kind of imagine that until your navball reads 'Orbit' instead of 'Surface,' the effects of planet rotation beneath you are ignored. Maybe someone more familiar with Kerbics could enlighten us on this rule.

I may be wrong, but the Orbit and Surface readouts are just a toggle that flips over at a specific point as I recall this being discussed before.

[Fyzzi]

I recently did a circumnavigation flight flying westwards (against planet rotation) on the basis that the rotation would bring my starting point closer by the time I was all the way round. Flight time was 1 1/2 hours, so Kerbin would have rotated through 90 degrees in that time if my understanding is correct that Kerbin has a 6 hour day.

Yeah indeed it does have 6 hour day... the rotation of the planet should help a lot so going agains Kerbin's rotation is surely faster and more efficient in that way... I guess now there's only one thing to solve - are you getting more air intake when going against planet's rotation? (that would be true if the atmosphere was rotating with the planet, 'cause you'd get air go agains your craft faster thus you'd get more air from the intakes and you'd be able to go higher and faster... ) I guess I'll do some testing as well to figure out

[Psycix]

In KSP the air is static to the ground.

In terms of wind and distance it does not matter which way you travel.

This is a frame-of-reference problem.

A circumnavigation is not faster because the planet rotated: You still have to make the same path over the same sphere. It is just from an outsiders point of view that looks at you making more or less than a full circle in order to do the circumnavigation. To you, it doesn't matter.

The only difference would be that if you go WITH the rotation, centrifugal forces will be stronger and you will have less gravity, which makes you able to travel at a shallower angle of attack with less drag.

Think of it as partial orbit supplemented by lift.

Of couse, this effect is minimal.

[Ralathon]

It'll matter very little. Kerbin's atmosphere is static in relation to the ground, so you'll get the same drag regardless of direction as long as your velocity is the same.

The only difference between going clockwise and anticlockwise is that when going anticlockwise (along kerbins rotation) you'll be moving closer to orbital speed, this means that the acceleration towards the ground will be lower and thus you can stay airborne with less lift. Less lift needed = more forward thrust which means you go faster/more efficient. But the effect will be very minor unless you're going at a significant fraction of orbital speed.

Edit: Damn, sniped by 1 minute

[adinfinitum]

Since the drag on the ship due to the atmosphere is calculated relative to the surface of Kerbin, it shouldn't have an affect on the time it takes to circumnavigate the planet. Because drag determines your max speed while you're in the atmosphere, it isn't going to differ going east or west.

In short, until you get out into space, Kerbin isn't going to rotate underneath you until you are out in space. One way to think about this is to imagine a blimp, hovering above the surface of Kerbin with zero surface velocity.In order to get 0 orbital velocity floating above the surface in your blimp, you'd have to be moving westward at about 170 m/s. However, once the force that is moving you at that speed stops, drag will slow you down until you are once again at 0 m/s surface velocity, and abou 170 m/s orbital velocity.

Now, if you were to compare a retrograde orbit to a prograde orbit of the same size and eccentricity, then a retrograde orbit would return to the same point over Kerbin faster than a prograde orbit because now the planet is moving relative to your orbit.

Edit: Looks like everyone decided to answer well I was typing my reply up haha

Edited October 12, 2013 by adinfinitum

[Scarecrow88]

In short, until you get out into space, Kerbin isn't going to rotate underneath you until you are out in space.

Kerbin definitely does rotate underneath you whilst you are still in the atmosphere. I've just done another circumnavigation flight, heading east. I took off in daylight and by the time I got back to KSC an hour later, it was dark. If Kerbin wasn't rotating beneath me while I was airborne, it would still have been daylight when I got back.

[Dispatcher]

Perhaps the reason why a plane starts out facing east on the runway (and why the runway runs on a W/E line) at the KSC is because Squad felt that there was some advantage in doing so. Of course this may or may not be the case. However, in doing so it doesn't seem to hurt a flight.

[adinfinitum]

Kerbin definitely does rotate underneath you whilst you are still in the atmosphere. I've just done another circumnavigation flight, heading east. I took off in daylight and by the time I got back to KSC an hour later, it was dark. If Kerbin wasn't rotating beneath me while I was airborne, it would still have been daylight when I got back.

Yes, Kerbin still rotates while you are flying, but it does not rotate in reference to the aircraft while you are still in the atmosphere, because your aircraft is also rotating with Kerbin, due to atmospheric drag forces.

Like Psycix and Ralathon said, whether you go east or west, it is not going to make a difference in the time it takes to make a circumnavigation of the planet.

If the atmosphere didn't rotate along with Kerbin, there would be a constant 170 m/s wind blowing to the west, and planes would be able to take off just by sitting still on the runway.

[Dispatcher]

I suppose the advantage of an eastward runway launch and flight is for a spaceplane, which by implication would achieve at least a suborbital flight or ideally an orbit.

[adinfinitum]

That's probably why they've got it facing east, that way we don't have to do a u-turn at the beginning of our flights.

Also, with the way my spaceplace test flights go, it's a lot better for everybody's safety that the runway launches over the ocean, otherwise the mountains west of the space center would be covered in craters.

[RoboRay]

A little off-topic, but a lot of people could simplify their questions and remove vagueness (like, "how do you define clockwise?") by simply saying East and West.

[bsalis]

Odd that this was only answered properly by the second page (by Psycix and Ralathon).

Would be interesting to see how weak the difference is due to partial orbital "lift". Perhaps someone could do an experiment. Same high speed high altitude plane, flown each direction. Needs to be done via MechJeb and or kOS or something at the edge of air intake threshold to be flown the same way.