Why can I achieve higher altitudes when flying in Orbit than straight up?

[K^2]

4 hours ago, YNM said:

But before and after you still have the same energy.

I'm not sure what you're trying to refute. This has been established several posts ago. You specifically asked if dV results in same velocity difference in inclination change. THEN you brought up energy and angular momentum. The only claim so far has been that application of same amount of dV as an instantaneous impulse always produces the same change in velocity.

[YNM]

4 hours ago, K^2 said:

I'm not sure what you're trying to refute.

... I think I really am ! I was bloody clueless !

 

I've just realized : There's two way to achieve a 1000000 km Ap from surface :

- Burn prograde until you have that Ap

- Burn radially out until you have that Ap

Radial burns are "less efficient" than prograde burns, somewhat, thanks to vectors.

Launching straight "up" rather than doing a gravity turn means you're only doing a radial burn rather than a prograde burn, which as said before is less efficient.

Is this the answer why ?

Someone needs to calculate something on this :

1. The periapsis speed on a (radius) 600x1000600 km kerbin orbit

2. The velocity vector of an orbit that has a "surface plane" speed equal to the rotational speed of Kerbin, and also has a 1000600 km apoapsis radius.

Then subtract them (vector wise) to the surface velocity.

Edited August 19, 2018 by YNM

[wumpus]

14 minutes ago, YNM said:

Launching straight "up" rather than doing a gravity turn means you're only doing a radial burn rather than a prograde burn, which as said before is less efficient.

If you click the "stay prograde" icon without a small lurch at the start, you will continue burning prograde all the way up.  That isn't an efficient means to extreme altitude.  The issue is gravity will dragging you antigrade if you fire straight up and radially if you follow a pitchover (gravity turn in KSP-lingo).  Your rocket should burn prograde in either situation.  Having gravity pull radially will chart a vector (on a polar graph) at a higher altitude (over someplace towards prograde) while having gravity pull antigrade will simply slow you down going straight up.

"Prograde" simply means following your current velocity.  It doesn't assume you are heading into orbit.

[YNM]

37 minutes ago, wumpus said:

If you click the "stay prograde" icon without a small lurch at the start, you will continue burning prograde all the way up.

Orbital velocity wise. Not surface velocity. Click once on the velocity indicator.

Edited August 19, 2018 by YNM

[K^2]

7 hours ago, YNM said:

Radial burns are "less efficient" than prograde burns, somewhat, thanks to vectors.

Launching straight "up" rather than doing a gravity turn means you're only doing a radial burn rather than a prograde burn, which as said before is less efficient.

Is this the answer why ?

This is incomplete. Radial burn isn't inherently inefficient. Energy-efficiency of the burn is proportional to cosine of the angle between your prograde direction and direction of the burn. If you're in circular orbit, instantaneous efficiency of radial burn is zero. Of course, as soon as you start burning, the orbit stops being circular, and things start improving, but you'll never have that 100% energy efficiency that you would with a prograde burn.

Vertical ascent to altitude is a prograde burn, even though it's also radial. That bit is as efficient as it can be. Circularization will now take a lot of fuel, because you start from stand-still, but if you have no intention of making orbit, that's irrelevant.

This is further complicated by the fact that have you had very high TWR and started from a vacuum world, burning straight up would be the best way to gain altitude. Again, given that you do not plan to establish orbit. It's when your TWR is comparable to 1 that you start getting benefits of gravity turn when shooting for altitude. So it's not that simple.