Ideal Ascent Profile

[SiliconPyro]

I know it can vary greatly from one rocket to the next, but are there some general guidelines? I've seen some who start their turn gradually at 12km and gradually turn toward the horizon as they rise while others wait until much later, make a 45 degree turn and hold that until AP reaches 85+ km. Is one vastly more efficient than the other? What about ship design can affect which profile is best?

[BioSehnsucht]

I noticed watching the Falcon 9 launch of CRS-1 Sunday night that they started gravity turn around 25 seconds or so (I think before, after they said it I thought to look at the time), which is much sooner than I think the default MJ auto ascent profile uses... which made me wonder..

[LaydeeDem]

I genrally begin turning at 10km and tilt to 45the degrees when I reach 500m/s.

[AncientAstronaut]

This is what I do. I never let my Apoapsis get within about 3km of my altitude. Even 5km is too close. And ideally, I want to end up with a Periapsis that is positive when MECO comes around.

You can keep your Apoapsis ahead of you if you have a higher Thrust to Weight Ratio. But anything above -150km for a periapsis at MECO is fine enough.

Just keep those things in mind and you can actually control your attitude and thrust as you ascend based on that.

[Elios]

they also want to get the thing out over the water as soon as they can from the cape which part of why they turn so soon

[Aescwulf]

I start doing a 45 degree turn along the 90 degree mark when I reach around 17,000m that's when my first stage is roughly gone and then keep on that 45 until my Apoapsis is at 100,000m then I cut engines and turn horizontal and then doing a burn at 93,000m to establish orbit and generally my orbits are circular but most of the time about 10,000 difference between Periapsis and Apoapsis

[SiliconPyro]

This is what I do. I never let my Apoapsis get within about 3km of my altitude. Even 5km is too close. And ideally, I want to end up with a Periapsis that is positive when MECO comes around.

You can keep your Apoapsis ahead of you if you have a higher Thrust to Weight Ratio. But anything above -150km for a periapsis at MECO is fine enough.

Just keep those things in mind and you can actually control your attitude and thrust as you ascend based on that.

I'm not really sure what you mean by letting the "apoapsis get within about 3km". As you rise your apoapsis constantly moves away from you.

[Beowolf]

Coincidentally, I was testing this exact thing last night, using a simple, 3-piece rocket (Mechjeb pod, 3200L tank, C7 aerospike engine). Using Mechjeb's ascent autopilot, I changed only the ascent profile under "Edit Path". For that ship, the most efficient profile I found used the default turn altitudes, but with the slider moved left to about the 33% position.

Once earlier, with a far heavier rocket (61t to 100km orbit), I'd tested and found for that rocket ascent was most efficient with the slider moved right to about 75%. So it clearly varies widely for different rockets.

I was disappointed, as I thought it would be most efficient with the turn delayed a few km. Nope. Not for those two vehicles, at least.

[AncientAstronaut]

I'm not really sure what you mean by letting the "apoapsis get within about 3km". As you rise your apoapsis constantly moves away from you.

Yes, but your thrust to weight ratio and your attitude of applied thrust determines how quickly it moves away from you. It is possible to surpass your Apoapsis while launching, you know.

[notasurgeon]

I'm not really sure what you mean by letting the "apoapsis get within about 3km". As you rise your apoapsis constantly moves away from you.

It has to do with how fast you're going. If you accelerate too quickly, your apoapsis will overshoot your target orbit long before you've had a chance to build up some horizontal velocity. I think this is fairly inefficient, because the sharper your ascent profile is the less time you'll spend near your apoapsis (the longer you're up there, the more of your circularizing burn will be while you're facing prograde). If you accelerate too slowly, more of your fuel is spent fighting gravity and not building orbital momentum. So there has to be a happy medium in there somewhere.

[thorfinn]

Compared to Earth, launches on Kerbin have higher gravity losses, and also quite high aerodynamic resistance losses in the first few kilometers. The optimal trajectories differ significantly.

[silent_prtoagonist]

My standard ascent profile is pitching over 15 degrees every 7500 m, putting you level to the horizon at 45 km. It seems to work pretty reliably for most designs, although occasionally I'll modify it to 15 degrees every 6000 or 10,000 m, for unusually high- or low-thrust designs, respectively.