Secrets for initial circular Kerbin orbit?

[strider3]

One of the hardest things I've tried to learn is how to launch to a circular orbit around Kerbin...at approximately 80 km. I always seem to end up with a highly elliptical orbit when trying to get my peri above 70 km. I end up with an apo way out there...150 - 200 km. The result, to my thinking, is a huge waste of fuel? I'll make my gravity turn to 10* and on the way to an 80 km apoapsis I turn slowly towards 90* to maintain my final apoapsis around 80 km. The problem I've found is that I must turn past 90* to maintain my final apo at 80 km...and as finally get my periapsis rising, I end up with a highly elliptical orbit. Maybe I'm not turning past 10* soon enough? I'm not sure. Any hints on how to achieve this?

[Tex_NL]

Don't burn your engines all the way until your PE clears atmo. Once your AP hits the desired altitude cut your engines and coast. At AP burn prograde to raise your PE. Easy as pie.

Edited November 26, 2016 by Tex_NL

[strider3]

Thanks! And I've tried to bake pies...nothing easy about it!

[bewing]

It's not really a huge waste of fuel. If you don't have enough TWR when you get to your Ap, you won't have enough dV to circularize without some inefficiency, so it's not always avoidable.

And once you're above 50km or so -- just go to a straight horizontal burn. That'll slow down your Ap rising, while gaining you as much orbital velocity as possible.

[Clipperride]

Once above 20,000m (with about 30°-45° tilt to the East, depending on TWR) I fly the rest of the ascent in Map view.  You want to tilt over as much as you can whilst still seeing the time to Apo slowly increasing, or at least remaining constan.  I don't like to let the time to Apo drop much below about 1 minute.  

You'll see the time to Apo start to increase quickly, by which time you should be horizontal.  As soon as the Apo reaches the desired height, cut the engines and coast up to 70,000m.  Depending on how low in the atmosphere you were at engine cut off, you may need a short burn to put your Apo back to the desired altitude (and with practice you'll get the hang of how much extra altitude you need to compensate for the drag).  As soon as you are out of the atmosphere, you can set a maneuvere node on the Apo marker and pull the prograde symbol until your orbit is circular.  Automated Mods may be able to fly an ascent more efficiently, but the above should at least give you a nice even orbit

EDIT - one other thing to remember, use the thrust tweakables on your engines to make the final adjustments more accurate.

Edited November 27, 2016 by Clipperride
Added PS

[GoSlash27]

Victor3,

 Low thrust is your friend when it comes to establishing a precise orbit.

 I like to keep my apoapsis 45 seconds ahead throughout my burn. I'll nose down below the horizon if that helps maintain the time and altitude I need. Once I establish a periapsis, I let the apoapsis close to within 5 seconds and burn prograde just enough to keep it there. This allows me to establish a perfectly circular orbit every time.

I should add that once I've transitioned to second stage, I switch to map view. There's nothing useful to see from external view at that point.

Best,
-Slashy

Edited November 27, 2016 by GoSlash27

[strider3]

3 hours ago, GoSlash27 said:

Victor3,

 Low thrust is your friend when it comes to establishing a precise orbit.

 I like to keep my apoapsis 45 seconds ahead throughout my burn. I'll nose down below the horizon if that helps maintain the time and altitude I need. Once I establish a periapsis, I let the apoapsis close to within 5 seconds and burn prograde just enough to keep it there. This allows me to establish a perfectly circular orbit every time.

I should add that once I've transitioned to second stage, I switch to map view. There's nothing useful to see from external view at that point.

Best,
-Slashy

I've wondered about that...and you have restated my question on that regard. I have always used direction to keep the Apo ahead of me, thrust usage I have not tried to keep my Apo ahead of me. I'm going to try that.

Thank you!

[Aethon]

@GoSlash27 has the best answer here so far.  TWR on the pad around 1.58.  Start your turn when velocity reaches 100 m/sec.  I try to reach 45 degrees as my time to AP reaches 40 seconds.  Map view ( or KER )- use throttle control or flatten your trajectory to keep your time to AP between 40-60 seconds.  

Usually by the time the game projects my AP as crossing 50km, my nose is on the horizon, and starting to dip below.  Burning slightly down holds down the AP while allowing PE to rise.

Once AP reaches 76 km or so MECO ( I like to stay low ), and coast to AP.  Circularize there..

 

Good luck.

Edited November 27, 2016 by Aethon

[Snark]

21 hours ago, Aethon said:

TWR on the pad around 1.58.  Start your turn when velocity reaches 100 m/sec.

Bearing in mind that for a streamlined rocket, gravity losses greatly outweigh aerodynamic losses, which favors having a higher launchpad TWR.  I've found that a TWR of up to 2 or so works pretty well, if the rocket is sleek (and especially if the rocket is big-- the bigger the rocket, the more the square-cube law tips the balance in favor of higher TWR, since drag matters less, relatively speaking).

These days I tend to aim for a TWR in the 1.8-2.0 range, and start the gravity turn right off the pad.  My benchmark for judging whether I'm too steep or too shallow is to look at my ascent angle when I'm ~8-10 km altitude; as long as I'm around 45 degrees at that point, it's good.

The hallmark of a well-executed gravity turn is that it requires very little actual "steering" from the player (ideally, none at all) after that initial eastward nudge-- the trajectory takes care of itself.

My standard path-to-LKO looks like this:

1. launchpad TWR in the 1.8-2.0 range
2. start the turn right off the pad (this takes some practice to get the amount right; it's very sensitive, a degree one way or the other makes a big difference)
3. sanity check at 10 km; if it's a lot steeper or shallower than 45 degrees, I've foozled it; try again
4. continue burning full throttle (staging as needed) until my Ap reaches the desired altitude
5. cut throttle and coast
6. burn prograde at Ap to circularize

Note that the high TWR is of greatest importance on the pad-- once your rocket gets to the point in its ascent where it's traveling mostly horizontal, TWR becomes much less important, and saving weight (i.e. not wasting it on having more engine than you need) becomes more important.  My usual formula is:  launchpad TWR of 1.8-2.0 (usually all SRBs), second stage (which activates when I'm ascending at 30 degrees or less) with a TWR in the 1.0-1.5 range, orbital insertion stage at fairly low TWR of around 0.5 or so.

 

[Tyko]

I'm pretty close to @Snark but go for lower TWR. 1.5 for my first stage (shoot for approx 1700DV), just around 1.1 for my second stage (approx 1700DV). I tilt to 10 degrees as soon as I clear the launch gantries but leave SAS on Hold Course. When the rocket hits 140-160m/s I switch SAS to Hold Prograde. this launch profile usually gets the rocket to a 45 degree angle between 10 and 15Km

After that, it's the same as Snark

 

[Empiro]

Having the proper TWR is good for efficiency, but getting a circular orbit is all about technique and practice.

Having a mod that displays your time to AP helps a lot but isn't 100% necessary.

The trick is to keep your AP ahead of you at just the right amount. 

Generally speaking, launch and do your gravity turn and keep burning until your AP is where you want it. 

Once your time to AP is about 30 seconds, start firing your engines again to circularize. Initially, point at the prograde marker, if this causes your time to AP to increase, point slightly "down" toward the ground, or wait a bit more time before firing your engines. If your AP is decreasing too quickly, point "up" a bit (and in the future, fire your engines earlier).

Ideally, your time to AP slowly decreases in a controlled manner.  As you get closer and closer to AP, and your orbital velocity approaches 2200 m/s, this will become more and more sensitive, so decrease your thrust as you're doing this. At the final part of the burn, your throttle should be close to the minimum, and your time to AP should be around 5 seconds.

Edited November 28, 2016 by Empiro