enlighten me on reaching orbit.

[Dedjal]

I know how to reach orbit, gravity turn and how much DV that is needed.

But here is the question(s).

Engines have a better performance in vacuum, yet we still perform a gravity turn at "low" altitude, where the engines performe worse.

Why is it not feastable in ksp to burn upwards and then apply sideway velocity where the engines perform better?

Why is it feastable to stay as long as possible in a drag enviroment (pun not intended)?

[Taki117]

Because as you ascend you are also fighting gravity, not just air resistance. Perform an experiment. Take a small rocket, (One that is capable of reaching orbit) and burn straight up, then sideways. Take the same rocket and perform a Gravity turn. You will find that the rocket that does the gravity turn reaches orbit with more dV remaining. This is because it has built up enough horizontal velocity during ascent that once it finally reaches space it takes only a small amount of dV to circularize. With the other rocket, it has some initial horizontal velocity from the rotation of the planet, but it still needs to build up enough so that when it stops fighting gravity it misses the ground.

And orbit is basically (at it's most basic level) falling and missing the ground.

[Laie]

Engines have a better performance in vacuum, yet we still perform a gravity turn at "low" altitude, where the engines performe worse.

Right-click an engine on the pad, just so you can see it's data, and watch how quickly the ISP improves. From an engine efficiency point of view, 5km is by no means a low altitude, and 10km is almost space.

Why is it feastable to stay as long as possible in a drag enviroment (pun not intended)?

We don't want to stay there "as long as possible". We still want to get out of it reasonably quickly, with an emphasis on "reasonably". There comes a point where trying to avoid the atmosphere becomes more expensive than sucking it up.

[Dedjal]

Yes. hence why one performs a gravity turn.

But why is that so? Why is burning through drag more efficient than burn to space and then burn sideways?

You'd think that you would spend more fuel the longer you have to deal with drag, yet that does not seem to be the case. How so?

And another thing - To change your AP/PE, It is usually cheapest to do that at PE/AP, since less fuel is needed for bigger changes.

But this is not cheaper regarding burning right up, and then burn at AP and rise PE into a stable orbit.

- - - Updated - - -

Right-click an engine on the pad, just so you can see it's data, and watch how quickly the ISP improves. From an engine efficiency point of view, 5km is by no means a low altitude, and 10km is almost space.

I thought the engines scaled the ISP from the sea level and all the way to the 70k mark. This is not so?

[Jovus]

Basically, the answer is that the gravity losses of burning straight up are much greater than the atmospheric losses of not doing so. Gravity is pulling down on your rocket at ~10m/s^2, which means that in order to go up burning vertically you have to thrust such that your acceleration is >10m/s^2. However, if you're burning sideways, you get all of that thrust, instead of subtracting gravity. Anything in between falls off like the cosine of the angle.

It's actually even bigger a deal than that, since once you hit sufficient sideways velocity gravity actually helps you, which it never does when you're burning directly counter to it.

[Rokmonkey]

Dedjal, this is because the aerodynamic forces are less than those due to gravity.

EDIT- Ninja'd

If you go to a the Mun, you can burn up a little, then burn sideways, you just need to get high enough to clear the mountains.

As for the ISP, it is scaled based on pressure, not altitude. Although it is similar because pressure decreases with altitude, just not linearly.

[Laie]

I thought the engines scaled the ISP from the sea level and all the way to the 70k mark. This is not so?

They do, but it's not linear. Just watch the numbers, that will be better than anything I could write.

[mudkest]

At 70km you need to go "sideways"(or forward, whatever way you want to look at it) at just under 2300 m/s to stay in orbit. You *could* gp up for 70km, then speed up to 2300m/s sideways but you'd need to keep pointing some thrust down the whole time as well to stay at 70km otherwise you'd fall down again. Sorry for the horrible sesame-street like description

- - - Updated - - -

I thought the engines scaled the ISP from the sea level and all the way to the 70k mark. This is not so?

Theye do, but not lineary. ISP is not halfway between min and max at 35km, it'll be far better then halfway

[Drethon]

Not to mention the significantly greater distance of burning up and then sideways as opposed to using a curve to reach your target.

[EtherDragon]

Some great info in this thread, so I will steal everyone's excellent answers and summarize back to your questions:

I know how to reach orbit, gravity turn and how much DV that is needed.

But here is the question(s).

Engines have a better performance in vacuum, yet we still perform a gravity turn at "low" altitude, where the engines performe worse.

Why is it not feastable in ksp to burn upwards and then apply sideway velocity where the engines perform better?

The amount of energy lost counteracting gravity in this manner is much larger than the amount of energy lost to atmospheric drag. You will notice that most gravity turns start at roughly the 10-15km mark. At this point the atmosphere is starting to get pretty thin, so the actual drag on your rocket become quite low.

Why is it feastable to stay as long as possible in a drag enviroment (pun not intended)?

Your rocket efficiency grows exponentially toward it's vacuum Isp, because the atmospheric density falls off logarithmically. Even at 20km your Isp is pretty good.

The main point is, the faster you can get to a point where your rocket is pointing Orbital Prograde, the more efficient your burn will generally be. But you are correct, this needs to be done at sufficient altitude that the gains from this are not lost to atmospheric drag. For instance, pitching over at 5km and going full orbital prograde is a recipe for disaster. Conversely, going strait up to 70km, and pitching over there isn't very efficient either because you spend so much propellant counteracting gravity.

Once you are on the orbital prograde marker, you are no longer spending fuel just to overcome gravity, and that's a big deal - as G translates to 9.8m/s of Delta-V lost for every second used to fight gravity.

[Jovus]

You will notice that most gravity turns start at roughly the 10-15km mark. At this point the atmosphere is starting to get pretty thin, so the actual drag on your rocket become quite low.

Just to be clear, this isn't quite exactly true to real life. The stock (0.90 and earlier) simulation of atmosphere creates a lot more drag a lower altitudes than is realistic, so the turn to orbit tends to be much higher and more severe (also because the stock atmosphere is very unlikely to tear your rocket apart). When using a more realistic aerodynamic simulation on Kerbin, a pitchover for a gravity turn tends to be around 5 degrees when the rocket's moving at about 100m/s - which is usually below 2km altitude. In real life, a number of familiar rockets turn <1 degree at about 300 meters.^Source (This would probably be more efficient in FAR as well, but KSP's control scheme doesn't make that easy.)

5 degrees, you say? That's hardly a pitch-over at all! And you're right. The reason it's called a gravity turn is because after that initial deviation from the vertical, gravity actually tilts the rocket further, helping to set the optimal trajectory to orbit.

[Moonfrog]

At 10km, your isp is virtually at max. the same as a vaccum, difference only being 5isp. No need to go any higher to get better isp, the first 10km are the only part with poor performance of engines, the next 60km is pretty efficient for all engines with high isp in vaccuum.

[Superfluous J]

It's simply a matter of finding the sweet spot.

Burning straight up and turning in space is worse than turning 10 degrees 50km, which is worse than turning 20 degrees at 40km, which is worse than 30 degrees at 30km, which is worse than 40 degrees at 20km, which is worse than 45 degrees at 10km, which is better than 50 degrees at 5km, which is... wait that was better? Aha we've found a sweet spot.

Others have more specific reasons, but that's the thinking involved. We don't do it because of drag or gravity or blah blah blah. We do it because we reach orbit with the most fuel.

[EtherDragon]

Just to be clear, this isn't quite exactly true to real life. The stock (0.90 and earlier) simulation of atmosphere creates a lot more drag a lower altitudes than is realistic, so the turn to orbit tends to be much higher and more severe (also because the stock atmosphere is very unlikely to tear your rocket apart). When using a more realistic aerodynamic simulation on Kerbin, a pitchover for a gravity turn tends to be around 5 degrees when the rocket's moving at about 100m/s - which is usually below 2km altitude. In real life, a number of familiar rockets turn <1 degree at about 300 meters.^Source (This would probably be more efficient in FAR as well, but KSP's control scheme doesn't make that easy.)

5 degrees, you say? That's hardly a pitch-over at all! And you're right. The reason it's called a gravity turn is because after that initial deviation from the vertical, gravity actually tilts the rocket further, helping to set the optimal trajectory to orbit.

Yea, I was talking internal KSP terms. But you make a good points since all of this current optimal stuff will change in 1.0 with an updated aerodynamic model. I expect to see launch profiles much more like real life after 1.0 as being the most efficient.

[Jovus]

Yea, I was talking internal KSP terms. But you make a good points since all of this current optimal stuff will change in 1.0 with an updated aerodynamic model. I expect to see launch profiles much more like real life after 1.0 as being the most efficient.

Yep, I know everyone helping out with the thread probably already knew that; I just figured I'd go the extra step to try to help a newbie out. Sorry if it came off like a correction of what you said.

[Laie]

The reason it's called a gravity turn is because after that initial deviation from the vertical, gravity actually tilts the rocket further, helping to set the optimal trajectory to orbit.

Nitpick: gravity bends your trajectory. The rocket only follows and doesn't (cannot) point anywhere but prograde.

[Jovus]

Laie, I can't see what distinction you're making between what you wrote and I wrote. Can you expand that further for me? (I suspect we're in agreement, but I'm curious if I'm thinking about something incorrectly.)

[Laie]

Hmmm. Let me see. Maybe I should start with this:

The maneuvers in that picture are all prograde, yet the trajectory is bent noticably. I don't think I have to explain why this is so. The very same effect is at work during the gravity turn at launch. Launch also is a long prograde burn, after all.

It's not as if gravity would turn the rocket, and then the rocket would change it's trajectory, but the other way round. Remember: once things really get going, the rocket cannot deviate from prograde or it would be ripped to shreds. If you want the rocket to describe an arc, you have to make it so that prograde moves to the side. That's what the gravity turn does.

Now, what happens once you tip the rocket slightly? The sideways velocity accumulates, while gravity nibbles at the vertical speed. Over time, you get a trajectory that is ever more sideways and ever less up. The rocket has to follow that trajectory, like an arrow. But unlike an arrow, it's also thrusting all the way, and as it starts to turn it's thrust will also be ever more sideways and ever less up, accelerating the turn.

Gravity is shaping the trajectory; aerodynamics mandate that the rocket has to follow this path with only the most minute deviation.

One might say that this is a distinction without difference; I don't think so, but also don't believe it's worth making a lot of fuzz about it. That's why I prepended my previous statement with "Nitpick".

Edited February 24, 2015 by Laie

[Dedjal]

Thank you all.

I have now learned something new about ISP in KSP.

So armed with the knowledge that KSP engines is nearly at their top peak only 10Km up in the air and the "quote" underneath, Things now makes sense again.

"Once you are on the orbital prograde marker, you are no longer spending fuel just to overcome gravity, and that's a big deal - as G translates to 9.8m/s of Delta-V lost for every second used to fight gravity. "

[mpk10]

I think the short answer to this question is that you don't get to keep any of your vertical dV. Ever. So why use a bunch that you don't get to keep just to go straight up.

[thadisjones]

The only reason go upwards is so not go downwards while wanting to get more sideways. The more up you are, the less upwards is needed to not go downwards, then just sideways only! Then get orbit which is sideways, forever. More orbit wanted? Sideways prograde! Less orbit? Sideways retrograde! Always sideways. Not burn towards or away from planet in orbit. Is wasteful like spill drink! Only do rarely, like only rarely throw drink at commissar on purpose instead of drinking. Or if bring lots of drinks, can waste few on ups and downs. Inclination change is being drunk in public. Do it only moving slow at apoapse like walking park, not while speeding in car like periapsis.

[WhiteKnuckle]

The only reason go upwards is so not go downwards while wanting to get more sideways. The more up you are, the less upwards is needed to not go downwards, then just sideways only! Then get orbit which is sideways, forever. More orbit wanted? Sideways prograde! Less orbit? Sideways retrograde! Always sideways. Not burn towards or away from planet in orbit. Is wasteful like spill drink! Only do rarely, like only rarely throw drink at commissar on purpose instead of drinking. Or if bring lots of drinks, can waste few on ups and downs. Inclination change is being drunk in public. Do it only moving slow at apoapse like walking park, not while speeding in car like periapsis.

This may be the most amazing thing I've ever read on this forum.

[Zargg]

The only reason go upwards is so not go downwards while wanting to get more sideways. The more up you are, the less upwards is needed to not go downwards, then just sideways only! Then get orbit which is sideways, forever. More orbit wanted? Sideways prograde! Less orbit? Sideways retrograde! Always sideways. Not burn towards or away from planet in orbit. Is wasteful like spill drink! Only do rarely, like only rarely throw drink at commissar on purpose instead of drinking. Or if bring lots of drinks, can waste few on ups and downs. Inclination change is being drunk in public. Do it only moving slow at apoapse like walking park, not while speeding in car like periapsis.

Seriously, you win the internet sir.

Along the same lines, I believe I read a little saying on this forum once, it went kind of like "vertical delta v will be lost to gravity, but horizontal delta v is yours to keep."

[Branjoman]

The key thing is orbiting is less about altitude, and more about Speed. The point of being in an orbit is you are going so fast sideways that you essentially "miss the ground". It is much easier to obtain enough speed when you are gaining sideways velocity through most of the flight, rather than all of it in orbit.

[RW-1]

How was that Gin that afternoon?

Nice post.