FAR/RSS Gravity Turn

[dlrk]

How does TWR affect the pitch rate of a gravity turn? Is there a way to determine the best time and angle to start the turn, and/or the optimum TWR?

[Starman4308]

How does TWR affect the pitch rate of a gravity turn? Is there a way to determine the best time and angle to start the turn, and/or the optimum TWR?

The answer to this more or less boils down to "make some rules of thumb, and then experiment with each new launcher". As I understand, once out of the drag zone, you want to maintain a time-to-apoapsis of ~10 seconds (possibly a bit more if you're about to stage into a low TWR stage): that keeps you mostly horizontal, building up orbital velocity while minimizing gravity losses. It also has the advantage of generally keeping your rocket in a good orientation for body lift: body lift will help preserve your altitude, letting you spend less dV fighting gravity.

A higher first-stage TWR means you should start pitching over sooner, because otherwise you shoot out and upwards too quickly, having wasted an unnecessary amount of delta-V going upwards. A lower first-stage TWR means a later pitch-over, because it'll take you longer to get out of the drag zone. If your second/third/n'th stages are low-TWR, you want a more vertical initial ascent, because you need more of an upwards "kick" to buy you time to circularize; high TWR upper stages let you burn more horizontally.

EDIT: You can optimize ascent profile computationally, by what amounts to simulating different ascent paths until you get the best one. The KSP equivalent is endless revert-to-launch until you get it right.

[Motokid600]

Surface info is your friend. You can go by your vertical speed or as Starman said the time to apoapsis. I like to refer to both during an ascent. But 10 seconds? Wow.. that might explain why ive been having trouble getting a circular orbit after meco... Ive been shooting for an apo about 60 seconds ahead of me. As far as when your turn should start... general rule of thumb everyone states is 100m/s is a good time to start the turn. Apply that to every rocket and adjust accordingly. The more TWR the earlier your turn, the less the later. I have MJ set to start at 1km with a TWR of 1.3. works out nice, but.. a question if I may. One that may relate to the OP...

Later in the launch.. when your burning horizontal towards orbital velocity I almost ALWAYS have to pitch up to fight gravity once I see my vertical speed get below a 100m/s or so... I want to know how much pitch is too much? The most I ever needed was a 45 degree pitch back up to keep from falling. I still made orbit, but I couldn't help, but think If I could've done it better. So at what point would I say that "Okay.. this is wrong. I need to ascend more steeply or turn later."