Tutorial: How to Quick Orbit Kerbin Without Worrying (too much) in Gravity Turns

[Gustavo6046]

Burn your engines at full power with SAS on. Try to turn a bit. When apoapsis is at desired height, stop. Timewarp until apoapsis and burn prograde. Until the periapsis is at desired height. I learnt observing MechJeb movements, only it worries a bit too much with g-turns.

[Alshain]

Make sure you pack tons and tons of fuel because you are going to be throwing away that money with a near-vertical launch.

Edited August 1, 2015 by Alshain

[Gustavo6046]

Make sure you pack tons and tons of fuel because you are going to be throwing away that money with a near-vertical launch.

Yeah that's it. *glup* What??

[Alshain]

Yeah that's it. *glup* What??

If you launch straight up, gravity's pull will counter some of your engine's power, meaning you need more fuel to escape the atmosphere. Horizontal speed, on the other hand, stays with you.

If you launch straight up you are raising your Ap. When you turn horizontal you are raising both your Ap and Pe at the same time.

[Xannari Ferrows]

Burn just before apoapsis, and make the burn at a 0 degree angle, and between your anti-radial and prograde vectors.. This is to eliminate any fuel potentially wasted on burning radially.

[Alshain]

Burn just before apoapsis, and make the burn at a 0 degree angle, and between your anti-radial and prograde vectors.. This is to eliminate any fuel potentially wasted on burning radially.

But you've still wasted fuel getting to Apoapsis.

[Xannari Ferrows]

But you've still wasted fuel getting to Apoapsis.

You said it yourself; You to keep horizontal velocity.

[Alshain]

You said it yourself; You to keep horizontal velocity.

Correct, you keep horizontal velocity. But if you launch vertical and don't go horizontal till right before your Ap, then you've wasted fuel on vertical velocity. The sooner you turn, the less fuel you waste going vertical.

Edited August 1, 2015 by Alshain

[Xannari Ferrows]

Correct, you keep horizontal velocity. But if you launch vertical and don't go horizontal till right before your Ap, then you've wasted fuel on vertical velocity. The sooner you turn, the less fuel you waste going vertical.

My point exactly. Burning at 0 degrees gives you nothing BUT horizontal velocity, so how is it wasting fuel?

[Alshain]

My point exactly. Burning at 0 degrees gives you nothing BUT horizontal velocity, so how is it wasting fuel?

Because you burned vertically to get to Apoapsis, that is where you waste fuel.

[Xannari Ferrows]

Because you burned vertically to get to Apoapsis, that is where you waste fuel.

I'm not sure I follow that logic...

If you burned early to gain the vertical speed needed for a high apoapsis, you save fuel due to the Oberth effect. If it takes 1 minute from there to gain enough velocity to circularize your orbit, you want to burn at 0 degrees, which is parallel to your trajectory at apoapsis. If you do this, it should more or less remain the same while you gain your horizontal velocity, which you don't lose any of to gravity. This is because you're decelerating upward for the first half, and accelerating downward for the second half all from gravity, so the two cancel out. If you burn starting at apoapsis, you're accelerating downward for the whole burn, so it takes fuel to counteract your vertical acceleration, while you're also trying to burn horizontally.

[Streetwind]

The Oberth effect doesn't help you much here because the Oberth effect runs off of being fast (kinectic energy). At launch you are incredibly slow. You gain just about nothing. In fact, because you are increasing the distance from the planet faster than when flying a flat trajectory, you are gaining less from the Oberth effect when you do finally get around to speeding up to orbital velocity. And speeding up sideways is by far the bigger slice of the cake.

At the same time, every second that your engines are running in a vertical climb, you are throwing 10 m/s dV out the window. Yes, 10 m/s dV per second, simply lost. That's the cost of hovering, of maintaining the status quo against the acceleration of gravity. Only the excess you can put out beyond that actually goes into accelerating your rocket. Burn for one minute, and you lost 600 m/s - you'll have increased the cost of getting into LKO by a whopping 15%. And that's before taking into account the lessened Oberth effect mentioned above.

Gravity is the ripped jock at school, and he's NOT your friend. Better outrun him fast!

That said, good gravity turns are hard. Newcomers are always going to start pushing their apoapsis high and coasting, because it's easier. This is not a bad thing. It means that they have plenty of room to improve, and many new things to discover about the game and about physics. That's what makes KSP fun!

Edited August 1, 2015 by Streetwind

[Alshain]

I'm not sure I follow that logic...

If you burned early to gain the vertical speed needed for a high apoapsis, you save fuel due to the Oberth effect. If it takes 1 minute from there to gain enough velocity to circularize your orbit, you want to burn at 0 degrees, which is parallel to your trajectory at apoapsis. If you do this, it should more or less remain the same while you gain your horizontal velocity, which you don't lose any of to gravity. This is because you're decelerating upward for the first half, and accelerating downward for the second half all from gravity, so the two cancel out. If you burn starting at apoapsis, you're accelerating downward for the whole burn, so it takes fuel to counteract your vertical acceleration, while you're also trying to burn horizontally.

If you burn early to get get a high apoapsis you are not using the Oberth effect to it's full potential. Using that method, you raise your Pe out of the center of the planet by something just slightly less than 600km (Kerbin's equatorial radius) while already at 70km.

The Oberth effect says that the closer you are to a gravity well, the more efficient the maneuver will be. So if you raise that Pe BEFORE hitting 70km, it's still more efficient because you are closer to Kerbin's center of gravity. In fact the closer you are to the center of the gravity while burning horizontally, the more fuel you save. So you want to turn as soon as possible while allowing that aerodynamics will prevent you from turning right away. On the Mun, on the other hand, there are no aerodynamics, but there is terrain so you want to turn just as soon as you clear the local mountains, don't wait.

[maceemiller]

I have to admit, I can see his logic in going vertical but a decent g-turn is the best way to go. As for "the worry" in doing one I find that part of the fun......I never get bored launching as on some vessels ive built I can almost feel the craft creaking and wanting to break so I adapt my flying accordingly. Until my mission is over, tension, worry and adrenaline consumes me.....man I love KSP! ;p

[Xannari Ferrows]

Yeah, I am in no way saying this is the most efficient thing you can do, just the most efficient when it comes to not worrying about gravity turns low in the atmosphere, and pretty much only applying to atmospheres.

[Alshain]

Proper gravity turns just take practice. If you never do them you will never get better at them and you will always have to build larger rockets. If you say "I'd rather not worry about them" then you will be stuck in that rut forever.

[Stargate525]

Yeah, I am in no way saying this is the most efficient thing you can do, just the most efficient when it comes to not worrying about gravity turns low in the atmosphere, and pretty much only applying to atmospheres.

I think you're confusing 'efficient' with 'easy.' This isn't efficient in any form of the word, but yes, it is theoretically easier if you want to spend the extra 400-800dV. Almost as easy is turning to 45 degrees at 2k height and just holding that.

[Sharpy]

Every second you're in the air and not orbiting, you're wasting fuel.

How much? Launch your rocket and throttle it to a level that will let you float above the launchpad but not ascend. That's your wastage.

If you burn early then switch engines off, rising to the apoapsis - you've lost the fuel early on. If you rise to apoapsis and keep the rocket angled not to descend - the vertical component is your wastage. Whatever you do, until you orbit you waste fuel - and in direct relation to two factors:

- time spent until orbit

- differemce between your horizontal and orbital speed (squared, I think).

Therefore in your best interest is to start gaining horizontal speed ASAP - not at apoapsis, not at 10,000m. As soon as that doesn't mean your rocket flipping over and crashing and burning. Usually that moment is when you've gained about 40m/s.

The earlier you gain orbital speed, the less fuel you have wasted. That's fuel saved that allows you to lift more, further, in smaller, cheaper rocket.

The mantra is "Burn early" - as soon as factors preventing it are gone. The rocket tipping over. Excessive atmospheric drag. Hopeless atmospheric ISp. These are valid excuses to delay the turn. But to turn only at the apoapsis?

ps. what Oberth effect? Oberth effect is proportional to speed at the time of burn. If you burned straight up and reached the apoapsis, right at the apoapsis your speed is a flat zero against surface, only planet rotation speed orbital.

[Xannari Ferrows]

I think you're confusing 'efficient' with 'easy.' This isn't efficient in any form of the word, but yes, it is theoretically easier if you want to spend the extra 400-800dV. Almost as easy is turning to 45 degrees at 2k height and just holding that.

It's a relative term.

[cantab]

My generic ascent profile:

Vertical until reaching 100-150 m/s.

Pitchover. Pitching to 80 degrees is a good bet, but it does depend on the rocket.

Wait for prograde to "catch up" with your pitch.

Click Hold Prograde. If that's not available follow the prograde marker manually.

Cut engines when desired apoapsis achieved.

Set up manoeuvre to circularise and burn said manoeuvre. If manoeuvre nodes aren't available just guess when to start circularising.

[Kryxal]

My pitchover is 5 degrees at 50 m/s, and I just hold alignment till prograde catches up and turn SAS off (or use Mechjeb's SmartASS, far nicer to work with), but very similar.

[Senri-Sei]

By burning up and then across you're effectively accelerating your craft along the two shorter sides of a right angle triangle.

If you burn angular you're taking the shortest overall route to orbit by reducing the vertical ÃŽâ€V. The only hassle is that bothersome, life saving, atmosphere.

[Sirine]

Try my signature method. Its easy and working.

[lugge]

By burning up and then across you're effectively accelerating your craft along the two shorter sides of a right angle triangle.

If you burn angular you're taking the shortest overall route to orbit by reducing the vertical ÃŽâ€V. The only hassle is that bothersome, life saving, atmosphere.

Or, in other words: Pythagoras, isnt it?

Like, when you use one maneuver node to burn prograde and normal, you save fuel compared to 2 single burns?

That was my usual gravity turn explanation.

However, is it a good idea to burn till 10k to quickly leave the evil thick atmosphere?

[Superfluous J]

For very small probes (Flyby anything, or nail any orbit contract around Kerbin, Mun, or Minmus), I've started putting them on the biggest solid booster, with some fins at the back and a nice pointy fairing around the probe. Then I stick the whole thing on launch clamps and tilt it 5-15 degrees. To launch to (near) orbit, I just hit the spacebar on the launch pad (Do NOT turn on SAS!). By the time the solid burns out I'm usually only a couple hundred m/s from orbit, and was able to go get a fresh cup of coffee in the meantime.

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However, is it a good idea to burn till 10k to quickly leave the evil thick atmosphere?

Not since 1.0 came out.