Ascent paths discussion

[nibula]

The last on the right is bad because you must have more thrust even if you have your apopsis up to 80 already because you are going low and orbit decays

[vexx32]

I usually thrust until about 40km, then start turning. The majority of the horizontal thrust is around 70km or so, though.

[PakledHostage]

I have been meaning to post a challenge that would help us learn something about this topic for quite some time. I finally decided to take the plunge since there seems to be some interest in this thread.

Please have a look at the Optimal Ascent Profile for this spacecraft challenge and give it a try.

[noneyabidnis]

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

I\'ve been trying to look back through older posts to see what you mean my 'local terminal speed in the atmosphere', but am having a hard time making sense of it as it relates to how quickly a gravity turn should go from straight up and down to completely horizontal.

Are you able to lay it out in a way my feeble mind can digest?

[closette]

Sorry my phrasing was a bit obscure. I was referring to the vertical ascent phase, with no pitchover at all. We were able to establish that for this case only (known as the 'Goddard Problem' in rocket science), the most fuel-efficient ascent speed is one which matches the terminal speed (which increases with altitude). See for example:

http://kerbalspaceprogram.com/forum/index.php?topic=7161.msg200489#msg200489 with PDFs attached.

Once you go from a 1D ascent to a 2D ascent to orbit, things get much more complicated, but since Kerbin\'s atmosphere is so thick up to about 10 km, almost every ascent path I have seen tried begins with this purely vertical ascent.

After a vertical or nearly vertical ascent phase, the initial pitchover angle and its rate of change is still up for discussion, and of course experimental testing.

[NovaSilisko]

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.

The joy of drawing with a mouse!