Gravity turn: small Force turn or throttle for adjustment mid-turn?

[ssd21345]

I have seen some guides say throttle down if it the turn goes too fast, but in someone else says no, do 5 degree force turn for adjustment.  But which is actually better?

[Loskene]

Your gravity turn is highly dependent on the craft's aero profile and thrust to weight ratio. We're gonna need more details like a VAB screenshot with stats visible to go on. Throttling on ascent is generally to limit G-forces and provide finer control over your time to apoapsis, as well as to avoid wasting fuel on draggy launchers in the lower atmosphere. Some launchers you can just let rip, leave mostly hands off and have them fly like sounding rockets, others require much more babysitting for an efficient trajectory. There isn't really a one size fits all approach to gravity turns, at least if you care about efficiency.

[MechBFP]

Despite what others on here might tell you, throttling down, or forcing your tip down, both have roughly the same effect. 

What I do is force the tip a bit east once I hit about 60 m/s full throttle, and then continue full throttle and nudging a bit more east until my “time to apoapsis” is about 50 seconds. Then I lock to prograde and lower my throttle so that my “time to apoapsis” stays locked to about 50 seconds.  (You need KER or MechJeb to see this stat on the flying screen.)

Using this method I will usually get an apoapsis of 100KM and a perispsis of like 40KM, so I only need around 40 to 100 m/s to circularize to an orbit.

Edited March 5, 2019 by MechBFP

[Snark]

Moving to Gameplay Questions.

8 hours ago, ssd21345 said:

I have seen some guides say throttle down if it the turn goes too fast, but in someone else says no, do 5 degree force turn for adjustment.  But which is actually better?

Depends a lot on the details of your rocket, but I would guess that for most circumstances, a slight force-nudge would be better than throttling down.  Rationale:  The force-nudge is worse for aero; the throttling-down is worse for gravity losses.  My observation has been that for most "reasonably" designed rockets (i.e. tall, narrow, pointy, streamlined), gravity losses are a much greater bane than aero ones, so I'd guess that the latter would be preferable.

Personally, my own solution to this?  Revert to launch and adjust how much I tip the rocket immediately on takeoff.  Do that until I've got it nailed just right.  That way, I never have to do any sort of throttling down or force-turning and it "just works".

That may sound frustratingly trial-and-error to you, but actually it's not as bad as it sounds-- in particular, I generally design my rockets so that I'm religiously consistent about their TWR off the launchpad, which means that the amount of initial "tip" tends to be almost identical for all of them.  Which means I get pretty good at estimating the amount I need to get the gravity turn that I want.  Also, on those occasions that I do discover that I messed up and need to revert... the feedback's pretty rapid, because I generally know "okay or not" by the time I get to 300 m/s or thereabouts.  So I'm only throwing away a few seconds of ascent if I revert to launch.

[Tyko]

I've done the "restart a billion times" thing before. I've found that the DV savings between "ideal" and "pretty darn close" to be around 1-3%   i.e. less than 100DV

My current strategy based on lots and lots of launches is to get pretty darn close on my initial tip over. I then avoid any throttling until I'm well past 45 degree angle to the horizon (which should occur around 10-12km up). At that point the the gravity losses are less of an issue. I let time to AP get to around 50-60 seconds and use throttle control to optimize my flight path.

Edited March 5, 2019 by Tyko

[Laie]

14 hours ago, ssd21345 said:

I have seen some guides say throttle down if it the turn goes too fast, but in someone else says no, do 5 degree force turn for adjustment.  But which is actually better?

Provided that you're high enough already, so that drag plays only a small role, forcing down (within reason) is bound to be more efficient. Though I can't tell by how much; probably very little.

IMO, if you have to throttle or point down, the original launch angle was too steep to begin with.

That said, I'm with Tyko above: if you're not on a horribly wrong track, just muddle through. The savings between good and perfect are minimal. Being able to steer by pitch is a good skill to have if you ever play realism overhaul (where most engines can't throttle, so pitch is the only means of control). Otherwise, throttling is just so bleeping convenient that I hardly ever consider anything else.

 

[Superfluous J]

Also are you 10km up or 50? That makes a big difference as well.

If you're 50 km up you don't need to go full throttle as you're not really fighting much gravity anymore. Or, if you are, you have worse things to worry about than which direction your ship is pointing.

If you're 10km up, then I vote crank it over and keep it burning. But @Snark's right... you shouldn't be in that predicament in the first place. You should have risen through the first 10km better.

[herbal space program]

If we're talking about Kerbin, I would concur with others that taking the drag hit is generally better than throttling down, especially if you have good TWR and it's before you're going too fast. Eve is a somewhat different movie though, especially down near sea level. Drag losses there can be pretty extreme. Of course what I always do in those situations is hit the F5 key and try again with a little more initial tilt. I usually start a few pixels off of straight up, then lock to prograde when the edge of the marker touches my heading, usually going 100-150 m/s. If you get it just right, you can often not touch anything until you hit your target initial AP.  On Kerbin, I'm usually doing pretty well if I'm already a bit south of 45 degrees by 10 km, but that does of course depend on TWR. My rockets usually have pretty high values.

Edited March 6, 2019 by herbal space program

[ssd21345]

Thanks for the reply! So the consensus is that force turn is generally better, but it doesn't matter whether throttle or force turn if you're at high alt. I generally full throttle all the right up, but sometimes weird rocket design need some nudge to force turn.

Edited March 6, 2019 by ssd21345

[Aegolius13]

22 hours ago, Snark said:

Depends a lot on the details of your rocket, but I would guess that for most circumstances, a slight force-nudge would be better than throttling down.  Rationale:  The force-nudge is worse for aero; the throttling-down is worse for gravity losses.  My observation has been that for most "reasonably" designed rockets (i.e. tall, narrow, pointy, streamlined), gravity losses are a much greater bane than aero ones, so I'd guess that the latter would be preferable

Obligatory mention for this excellent thread coming to the same conclusion: