Gravity turn. Do you need to do it?

[blar]

I'm wondering. If you want to get escape velocity and escape Kerbin does it make sense to just go straight up. full speed and get out of the atmosphere are fast as possible? Also when escaping Kerbin, dont bother orbiting just keep going up?

[rpayne88]

If you want it simple, go straight up. If you want the most efficient ascent, enter LKO at 70Km. Every second your rocket is vertical, it is paying a "gravity tax" of 9.8m/s^2. What this means is that a rocket that will be oriented vertically may only be accelerating at 0.2m/s while the same rocket oriented horizontaly would be accelerating at 10m/s (though, not to such an extreme.)

[maltesh]

When you fly straight up, you are spending delta-v fighting gravity head on the whole way.

When you do a proper gravity turn, you are allowing gravity to bend your path in the direction you want to go, so a larger fraction of the delta-V you spend thrusting goes into making your spacecraft go faster.

If your primary aim is efficiency, then yes, it always makes sense to do the gravity turn.

[Scrogdog]

These guys are right. You can, theoretically, go straight up. But then you still must go and chase speed to stay in orbit else you simply fall back down.

The best route is the most efficient one. That is always your task in KSP regardless of goal.

[patrickcanttype]

These guys are right. You can, theoretically, go straight up. But then you still must go and chase speed to stay in orbit else you simply fall back down.

I believe he was specifically asking about escape velocity.

As for which is more efficent, I would imagine a gravity turn would be the best option. This should have a gravity slingshot effect.

However, I am interested in testing this to confirm it. Even though one would be fighting gravity on a straight-up accent, I believe the delta-v spent on velocity in the verticle direction is more efficent than the same velocity in the horizontal direction. As rpayne said, you would be surrendering delta-v to gravity (which would decrease as one was further from the planet). The loss of delta-v may not be enough to make an orbital escape more efficient. I will do some testing tomorrow and report back.

[PirateAE]

/me gets think of doing some testing on this *evil grin* ill edit in a bit with results once completed.

well might take a while, just relized the ksp .19 i had with all my mods was borked and i hadnt gotten around to fixing it.

Edited June 2, 2013 by PirateAE

[Scrogdog]

I believe he was specifically asking about escape velocity.

As for which is more efficent, I would imagine a gravity turn would be the best option. This should have a gravity slingshot effect.

However, I am interested in testing this to confirm it. Even though one would be fighting gravity on a straight-up accent, I believe the delta-v spent on velocity in the verticle direction is more efficent than the same velocity in the horizontal direction. As rpayne said, you would be surrendering delta-v to gravity (which would decrease as one was further from the planet). The loss of delta-v may not be enough to make an orbital escape more efficient. I will do some testing tomorrow and report back.

I am interested. Don't forget, gravity also has useful effects. Perhaps we do not always wish to escape it.

Ok, you went straight up and escaped SOI. And? Now what? That is not directed towards you specifically, just wondering what one might gain from it other than in a theoretical fashion.

Edited June 2, 2013 by Scrogdog

[SalmonellaDingDong]

The most efficient way to reach escape velocity would be do use the gravity boost of a LKO. You need to do a gravity turn. You shouldn't go straight up. That's something that North Korea's rocket program would do.

[Dictator Dan]

The most efficient way to reach escape velocity would be do use the gravity boost of a LKO. You need to do a gravity turn. You shouldn't go straight up. That's something that North Korea's rocket program would do.

SHHHH! Dont tell them!

[magnemoe]

If you want it simple, go straight up. If you want the most efficient ascent, enter LKO at 70Km. Every second your rocket is vertical, it is paying a "gravity tax" of 9.8m/s^2. What this means is that a rocket that will be oriented vertically may only be accelerating at 0.2m/s while the same rocket oriented horizontaly would be accelerating at 10m/s (though, not to such an extreme.)

This is more relevant as you can use nuclear engines instead of mainsail after the gravity turn increasing efficiency.

[rodion_herrera]

I hope the following diagrams I made help in understanding why the "gravity turn" is important, if only to save on fuel/energy (also, I'm not really an expert in Physics, so those who are into the physics and math of this, if I make any error, kindly please correct or clarify my points, thanks). Pardon for the simplicity of my drawings--I could make better ones, but I was just after a clearer and "pictoral" explanation of the situation.

Imagine there was a shallow, cone-shaped hole (or crater) in the ground, and you were in the bottom of that hole...

To make this clearer, let's imagine creating a cross-section of this hole, along the green broken line...

...this is the profile we get, after cutting across the green line:

Now if you were on the bottom, and you climbed straight out of the hole, it would require some effort, due to the steep slope (around 30-35 degrees in my drawings)...

...but, there's an easier way...it's a bit longer, but at least, it requires lesser effort...you climb out the hole in a helix. So instead of 30-35 degrees slope, you only encounter perhaps 20-25 degree slope, so now the climb is easier on your knees!

..simple eh? That's why gravity turns make it easier for a rocket to get out of a planet.

NOW, an additional important note...remember our cross section diagram earlier?

Well, this isn't exactly the shape of the "hole" that gravity creates around Kerbin, the earth or any other planet. A more accurate depiction of that "gravity well" is like this...

And now it is clear, when you look at the above image, why you need staging, and why the first few stages need to have lots of thrust, because the "gravity slope" nearer the body is steeper, and thus you need more thrust, to get out of it...but the further and further you go away from the planet the "shallower" the "gravity slope" and this is why it takes only a few seconds of burn to change apoapsis or periapsis if you're in a higher altitude.

EDIT: Furthermore, if the planet you are trying to launch from has a thick atmosphere, this means if you do the gravity turn earlier, then you would encounter atmospheric resistance (drag) that would slow down your vert and horizontal velocities that your effort to do the turn would be wasted. This is why you need to have a good mental picture or data of the layers of atmosphere of the planet you're going to leave from (with data on the varying densities at different altitudes), so you can plan an efficient ascent.

I hope my pictures help you understand it better

Edited June 2, 2013 by rodion_herrera

[PirateAE]

Doing some testing so far, there is so far a differance in Total Dv for just excape between stright up and out and doing a gravity turn but not nearly as much as i was expecting. least in one solid burn. Will edit if no one else posted before i complete some more testing.

[haltux]

Doing some testing so far, there is so far a differance in Total Dv for just excape between stright up and out and doing a gravity turn but not nearly as much as i was expecting. least in one solid burn. Will edit if no one else posted before i complete some more testing.

I am curious about your results, and I'll try myself.

Because I am not sure enough to stand against everyone (and maybe I have some north Korean ancestors), but my intuition is that this "gravity turn" does not help at all (for the specific goal of escaping Kerbin). The so call "slingshot effect" provides only the energy that you have put in the sling shot, that is the energy used to reach orbital speed.

You get what you paid: to get potential energy you have to pay it in thrust, plus the price paid per second spend to fight gravity. The the best way is the quickest, straight to the sky.

Well, I am no physics major... I could have missed something.

Interesting question, anyway. I'll really check that.

[Scrogdog]

You get what you paid: to get potential energy you have to pay it in thrust, plus the price paid per second spend to fight gravity. The the best way is the quickest, straight to the sky.

http://www2.jpl.nasa.gov/basics/grav/primer.php

Check that out. As you can see, gravity can be used to increase or decrease momentum without "paying the price" as you put it.

[PirateAE]

Well heres my plimary testing after 2 tests of a .4 T payload to kerbin excape.

Testing was as follows, All probes are one solid burn, no pausing (save 2-4 secs switching for jeb help on auto accent). Gravity turn tests are layed out as follows and are auto flown to the target alt by mcjeb, XX / XY / YY / ZZ%.

XX = Start turn hight

XY = End turn hight

YY = Target Alt

ZZ% = Turn Shape (less is more horozontal)

Stright Up, No gravity turn.

Both these are right on the edge, not being active they wont excape according to tracking center, Base Line test

Result 1 5931 Dv

Result 2 5933 Dv

Gravity turn test, 10Km/80Km/100Km/40%

Base Line test

Result 1 5583

Result 2 ~5600 (lost focus for a sec)

Gravity turn test, 500Meters/80Km/100Km/40%

Expected more dv required

Result 1 5760

Result 2 5775

Gravity turn test, 20Km/80Km/100/Km/40% (testing)

Maybe Less Dv (Atually cost more Dv)

Result 1 5775

Result 2 5760

Will add more as i get them but overall its a defferance but not a large number.

Edited June 2, 2013 by PirateAE
New Results

[blar]

Thanks dudes!

@ rpayne88 whats LKO by the way?

I decided to play ksp at first without watching any tutorials or anything to make if more fun. So I have been going straight up till 70,000 every launch. The one good thing is that I've hence mastered building extremely powerful massive rockets that dont blow up

[Scrogdog]

Back to my earlier point, and not being an expert of course, but think about this.

I am in orbit while you have just escaped SOI. I can now use orbital velocity momentum to fling me somewhere. You, however, have no momentum. Right? Well, you do, but it has no goal in mind, if you see what I am saying. Now you must use additional energy to build up momentum to your next destination.

Is that flawed thinking?

Edited June 2, 2013 by Scrogdog

[PirateAE]

The Op as i understood it was just asing about excaping kerbin SOi, not headed to anywhere specfic im just testing kerbin excape, not heading somwhere (which this probe could handley do its got about 8 K Dv)

Edit updated my results post above

Edited June 2, 2013 by PirateAE

[tntristan12]

If you launch straight up at sunrise, you will be heading Kerbol-prograde, and if you launch at sunset you will be heading Kerbol-retrograde. If I want to go to Eve, is it more efficient to make orbit first and then execute an ejection burn, or to just keep launching up at sunset until my periapsis is low enough?

Something tells me the former is more efficient because less energy is wasted fighting gravity and you get to take advantage of Oberth... But it's an interesting setup for an experiment.

[PirateAE]

If you launch straight up at sunrise, you will be heading Kerbol-prograde, and if you launch at sunset you will be heading Kerbol-retrograde. If I want to go to Eve, is it more efficient to make orbit first and then execute an ejection burn, or to just keep launching up at sunset until my periapsis is low enough?

Something tells me the former is more efficient because less energy is wasted fighting gravity and you get to take advantage of Oberth... But it's an interesting setup for an experiment.

Ill add those to my testing, Do you want total dv expended till i reach the closest neibours orbit hight?

[PirateAE]

Thanks dudes!

@ rpayne88 whats LKO by the way?

LKO is Low Kerbin Orbit, Give or take 70-75 Km

[Jaedinator]

Just did a little testing myself with this rocket:

1. Straight up until i reached 100km AP then at Apoapsis rest dV on circularization burn

Ended: AP:100km Pe:-420km

2. Gravity turn at 10km standard curve (no mj so just flew the way i thought)

Ended: AP:100km Pe:-160km

Gravity turn brought me 260km more on circularization. (not sure how much dV those 260km are)

[PirateAE]

BTW heres my testing craft

http://postimg.org/image/501wzguod/full/

[rodion_herrera]

I think the gain could indeed be not that significant, but if we were talking about earth, then it perhaps it would be. Then you might then say, "But why do you even mention the earth, when we're talking about Kerbin?" Well, most of the people who get attracted to the idea of KSP is because, in one way or another, it DOES mimic, real-world rocket physics, and thus, if it did that, then logically, one would follow the general conventions used by rocket engineers and scientists in the real world, and APPLY that to KSP. I mean, isn't it logical to think that, if rockets can just as easily go to the other planets by just "pointing it straight up", then perhaps half of those who got attracted to the KSP idea, would not really get interested much anymore on trying it out, or playing it, simply because "...that's not how real rockets fly..." or "...that's how cartoon rockets fly...I don't want to play that kind of game..." ?

[PirateAE]

If you launch straight up at sunrise, you will be heading Kerbol-prograde, and if you launch at sunset you will be heading Kerbol-retrograde. If I want to go to Eve, is it more efficient to make orbit first and then execute an ejection burn, or to just keep launching up at sunset until my periapsis is low enough?

Something tells me the former is more efficient because less energy is wasted fighting gravity and you get to take advantage of Oberth... But it's an interesting setup for an experiment.

Well starting to test this, gonna toss these number here as well just for comparasation.

Stright up to kerbin excape, 5930 Dv

Gravity turn (10/80/100/40%), 5600 Dv

NOT waiting till planetary trasfer aligment, just an orbit that crosses the orbit of the planet; All one burn for stright up, orbit then insert will be a 100 KM parking orbit.

First up, Duna

Stright up,

Result 1 6100 Dv (sun was 1/2 On horizion)

Result 2 6100 Dv (sun was up about 7 Mins)

Orbit burn for all 4 below was 4730

Insert burn

Result 1 1081 (5810 Dv total)

Result 2 1068 (5798 Dv total)

Eve

Stright up

Result 1 6100

Result 2 6100

Insert Burn

Result 1 1046 (5776 Dv total)

Result 2 give or take the same:/ missed writing a number

Edited June 2, 2013 by PirateAE
finshed testing