Ascent paths discussion

[GalaxyGryphon]

so, lets assume gray is our wanted orbit height, blue is atmosphere, and green is sea level, using my horridly drawn picture here, what is in your opinion the 'best' ascent path regarding fuel, time, type of ship, etc?

the first ascent definitely gets you into a circular orbit fastest but i cant tell if it uses more fuel

also, when it comes to much larger ships, one or 2 usually a good choice, ive realized that its usually good to be out of the atmosphere by the time your trying to turn, so you have more control

[dogon11]

I use the middle, unless I have a very fast lifter. Then I use the third path.

[jayther]

The title reminds me of a Stargate-esque ascension. x]

The first ascent is not good, since you need to use a lot of fuel just to counteract the up movement, and more fuel to go in the direction of the orbit.

The third isn\'t good either, since you\'re using fuel to fight against atmospheric drag for a longer distance. Instead of fighting atmospheric drag for 70 km when going straight up or at a slight angle, you\'re fighting 90km of atmospheric drag. Plus, going at that angle, you still have to go upwards.

I\'m not sure if the first one is worse than the third one, but my experience is that the first one is worse because you need quite a bit of sideways speed to go in an orbit. At least with the third ascent, you already have a direction going.

The second one is probably the best one, since you\'re at a slight angle going up, so you already have some sideways direction going. Also, the gradual climb to the height of the orbit is better than a sharp turn because you gain momentum to go sideways already. Getting up to orbital height is easy; it\'s actually getting into an orbit is a little harder.

I\'ve learned that going at a slight angle at launch is good, then start to have a more dramatic angle between 10-15km (I usually start going to 45 degrees at 10km with SAS so it changes the angle slowly). Drag is less of a factor the higher you go up, so you should take advantage of this fact and start going sideways, even if you\'re not completely out of the atmosphere yet.

[Ph4ntom S33ker]

I was thinking of this earlier. I generally use the first one, as my logic is the less time you spend in the atmosphere the less fuel you use going through it. Then again, I\'m just getting re-acquainted to the game, so I\'m probably not much use.

[UmbralRaptor]

I\'d go with somewhere between 2 and 3, actually. I typically start pitchover once I get past 8 km, though some ships might be better later. It\'s unclear to me as to whether it makes more sense to get into a 70 km orbit and transfer to a higher desired one, or go directly and circularize. That said, here\'s my reasoning for such flat ascent profiles:

1) Mechjeb\'s drag results. Yeah, I was shocked that gravity drag was so much larger than aerodynamic drag for most profiles, but there it is. And the reduced gravity drag outweighs the increased aerodynamic drag.

2) Getting marginal rockets into orbit. Pitching over early seems to help quite a bit for those 1-2 tank ultra-minimalist designs. (Especially when MechJeb is doing the flying.)

I expect that this is related to the exponential falloff of the atmosphere.

edit: choice 1 is likely the best for large unweildy ships with control issues. But I build small, so...

[SteevyT]

Back before the new atmosphere the first one used to be the most efficient. But the atmosphere was more like swimming then.

[Endeavour]

I was also surprised by this. I used Mechjeb for the first time yesterday, and I thought the ascent info was cool. I was not ready to see that I lost more to gravity than to atmosphere.

[deadshot462]

I use something between the 1st and 2nd paths. Once I clear the lower atmosphere levels, I only use 50-70% throttle while i tilt forward so energy isn\'t wasted so fast as I 'move' my pro grade vector towards the horizon on the gimbal.

Once my prograde nears the horizon, I\'ll speed up the throttle and finish circularizing my orbit

[cardgame]

I think beginning a pitchover of approximately 30 degrees at 30km and then gradually turning more and more toward the horizon as you get closer to the exoshere is pretty efficient.

[Tim_Barrett]

Back before the new atmosphere the first one used to be the most efficient. But the atmosphere was more like swimming then.

I remember that brick wall... good times!

I usually take the second one, and have realized that full throttle isn\'t always the best option since there is a terminal velocity-like result that happens throughout the flight in the lower atmosphere.

[NovaSilisko]

The best path is a roughly logarithmic curve shape turned upside down:

As atmospheric density decreases, your angle increases at roughly the same rate, if you do it right. Usually I\'m over the ocean by 8 km, and over the next continent by 80.

[Aerzee]

I just wing it every rocket, but more or less like silsko\'s flight plan.

[flaillomanz]

The best path is a roughly logarithmic curve shape turned upside down:

As atmospheric density decreases, your angle increases at roughly the same rate, if you do it right. Usually I\'m over the ocean by 8 km, and over the next continent by 80.

Wow, even your MS paint rockets wobble.

I\'ve been doing it wrong all this time

[Fifflethecat]

This seems to be my flight path. I like this one because it makes Jebediah happy!

[Ronox]

Wow, even your MS paint rockets wobble.

I\'ve been doing it wrong all this time

I looked at the picture again after reading this, and couldn\'t stop laughing. It is however impressive that novas 3 pixel wide drawing of a rocket path still has staging. And wobbling.

[Talisman]

I find this plan works the best for me.

[ManaaniWanderer]

I normally start my roll at 40-50km, and by the time my pe is >80km my ae is 200km+. It works though as most of my rockets are overkill...or at least the launch stages are.

[closette]

Someone with a background in optimal control theory should be able to solve this problem, but from what I\'ve learned, the optimal ascent path to orbit is very rocket dependent. With the atmosphere\'s density halving every ~3500 meters in altitude, most people stay pretty much vertical until 10000m and a bit more.

My own rule of thumb is to stay vertical as long as the craft can keep up with the local terminal speed in the atmosphere. As soon as it falls behind, I start a gravity turn and try to time it so that I pop out of the atmosphere going ~ horizontal at 75 km. Then a short coast followed by a circularization burn. I don\'t always succeed and so prefer rockets which have some fuel margin built in.

You might also want to check out this thread in the Challenges section of the forum:

http://kerbalspaceprogram.com/forum/index.php?topic=13350

[duckunlimited2]

Most of the time I just go straight up, full throttle, with extremely excessive amounts of thrust (like 5 Bertha engines) to clear the atmosphere and get me going. Then I start to fine tune the orbit.

Or If I\'m using a space plane, I full throttle its engine at 45 degrees until apoapsis reaches my orbit, wait till I hit apoapsis, then circularize it.

[Richy teh space man]

For me I go straight up til around 12/13km then start banking slowly til I\'m horizontal at around 70/80km

[SunJumper]

I go straight up, then turn to the side.

[Max Grant]

i launch straight up, then watch the navball. When it switches from surface to orbit, the yellow circle always moves over a couple of degrees, and I point my ship into the circle. Depending on how much fuel my first liquid stage has left, I often watch my apopsis(apokerb?) until it reaches the desired altitude. Usually the way i build rockets, I try to be out of fuel in the first stage when that happens, and I then eject the lower stage, and wait until I\'m almost at apopsis to fire the orbit burn.

My mileage often varies, as I am a terrible pilot.

[Ted]

I tend to have a 5 degree inclination until ~10km and then I start banking until 70km at which point I coast to the apoapsis and then circularise.

[khyron42]

For most launches where I\'m trying for Kerbin orbit, my method is similar to what others have said. 5 degrees as soon as I clear the tower (more out of a wish to see first stage/wreckage not hit the base than because it\'s efficient), 20 degrees at 10 km, 30 at 20 km, 60 at 30, then switch to make view and go horizontal once my Ap is up above 70 km. Get the Ap near the intended orbit altitude, coast to Ap, and circularize.

Certain very unstable rockets I stay with vertical until 30 km before turning, because tipping even slightly at 20 km causes out of control spirals.

For launches towards the moon or Minmus, I launch to align the Ap to about the point where I want to do the TMI burn without worrying about what altitude the Ap ends up at, and then just do the TMI burn instead of circularize. This only works when I\'m less than half an orbit from the TMI point, though.

[Markus Reese]

For ease of simplicity, I like to bank 22.5 off vertical upon entering stratosphere, and then I will bank over to 45 after entering the Mesophere. After clearing into the ionosphere, I boost to nearer to my ideal peak altitude and apply fully horizontal thrust to accelerate up to orbital velocity. inertia and pitch angle to carry me up to the altitude. Ideally it would be a gradual bank over so that you are hitting optimal altitude the same time you achieve orbit, but difficult controls makes a staged set of turns better.