Reaching orbit, Mechjeb vs IRL

[Fadly]

When using mechjeb's ascent guidance, the rocket will burn until they reach the desired apoapsis. Then they coast until they reach a point where they burn again to circularized. But in all the video of the real launch that i saw, the rocket seems to burn all the way to circular orbit, only stopping for short time for stage separationn. Can anyone explain?

[bewing]

It's because mechjeb isn't perfectly efficient. A continuous burn is slightly theoretically better.

 

[Foxster]

2 hours ago, Fadly said:

When using mechjeb's ascent guidance, the rocket will burn until they reach the desired apoapsis. Then they coast until they reach a point where they burn again to circularized. But in all the video of the real launch that i saw, the rocket seems to burn all the way to circular orbit, only stopping for short time for stage separationn. Can anyone explain?

That's only with MJ's default settings. Lots of tweaks possible. 

With real rockets you want to fire and shutdown the engines as few times as possible. This not being a restriction in KSP. 

[blowfish]

It really has nothing to do with Mechjeb and everything to do with the height vs delta-v required to achieve orbit and the sorts of rockets that tend to be built.  You need much, much more horizontal velocity to achieve orbit in real life.

That being said, there are partially and fully solid-fueled rockets in real life that do coast to apogee, and it is possible to build a rocket in KSP with a long-burning 2nd stage that will burn continuously into orbit.

[Gargamel]

Even with MJ, with high mass, "low" TWR second stages, I've had ascents that are nearly continuous burns.  They weren't intentionally designed to do that, it's just the way the ship flew. 

[Pand5461]

Kerbin's atmosphere is 70 km, which is about 11.67% of the planet's radius.

On Earth, that would translate to about 750 km of atmosphere.

In reality, rocket go to the low-Earth parking orbit at about 250-350 km with a continuous burn. If the payload needs to get to 750 km altitude, a series of transfers from a parking orbit is more efficient.

Space Shuttle didn't typically go even that high by a continuous thrusting. It went first to ~110×250 km orbit at about 150 km altitude, dropped the orange tank so that it reentered on the next orbit, and the orbiter itself circularized at apoapsis about a third an orbit after SSME cutoff.

In fact, going full gas until you get wanted apoapsis altitude and then coasting to apoapsis is in fact the most fuel-efficient strategy in KSP for most crafts, except very-low-TWR upper stages. A typical KSP rocket is powerful enough to orbital velocity way before it can jump out of atmosphere.

The idea that continuous burns at non-maximal thrust are more fuel-efficient than burn-coast-burn sequence is a common misconception here on forums. If you have to throttle down to optimize fuel consumption - the best way is to throttle down all the way to zero! Because in that case, you spend less time thrusting above horizon and having gravity losses because of that.

If the atmosphere thickness was about 25 km, then continuous burns would be the way to go.

[foamyesque]

I've hand-flown continuous-burn to orbit without throttle-down, but a coast-reignite is a.tremendously simpler and b. enabled by the low orbital velocities (and consequently gargantuan upper stage TWRs) in KSP.