Orbit circularization?

[rocketman544]

So here I am, freshly blasted off from KSC, ready to dump my first stage and burn into orbit. I set a manuever node at my apoapsis for a circular orbit of about 100,000 m altitude. The problem is, even though I'm angled toward the manuever vector and achieve sufficient delta-v, my periapsis and apoapsis are both way off--a periapsis at 56 km and apoapsis at 130, for instance. What am I doing wrong?

[Ankou]

When you are in Map- Mode you can show the Nav-Ball by clicking on the small Arrow on the button of the screen. Now you can control your ship in this view and you don't need a maneuver node for such simple tasks.
When you reach your apoapsis just burn into your flight direction(yellow marker on the navball).
You will see the periapsis rise.
At one point they start changing positions because the periapsis is becoming the new apoapsis.
That is the point to shut down your thrusters.
Now you have a circular orbit.

[[m00] insane_eon]

The burn is too long. With some orbital changes the manoeuvre node won't really help you much because you'll be far too far away from the target orbit for most of the burn.
Either make your ascent flatter at apoapsis (I personally like to get above 50km with the apoapsis at least a quarter of the way around the planet, more if my orbital rockets are really weak) or add a radial component to your burn that will take care of vertical acceleration while you perform the manoeuvre.

[Trehek]

Although the node system gives you a estimated burn time, it projects your planned trajectory assuming a instantaneous burn. If you start burning at the node you'll be making a lower burn and reach an uneven trajectory. To be able to achieve the same with non-zero burn time, you need to average your burn around the node, meaning that you have to start your burn half the burn time before the node.

30 second burn? start burning towards the blue marker at t-minus 15 seconds.

[Bomoo]

It sounds like you might be burning straight up without increasing your horizontal velocity. I suggest watching some footage of real world rocket launches to get an idea of what a more viable ascent profile looks like.

[Trehek]

Relevant: http://xkcd.com/1356/

[Tokfan]

Originally posted by Trehek:

Although the node system gives you a estimated burn time, it projects your planned trajectory assuming a instantaneous burn. If you start burning at the node you'll be making a lower burn and reach an uneven trajectory. To be able to achieve the same with non-zero burn time, you need to average your burn around the node, meaning that you have to start your burn half the burn time before the node.

30 second burn? start burning towards the blue marker at t-minus 15 seconds.

This is as easy and helpful it will ever be.
You are not a tennis ball being hit by a racket. You have a rocket with inertia and acceleration.
Dvide your EST burn by 2 and start burning at T-minus [result] and keep burning until T-plus [result].

-And you know what? The outcome will still not be exactly what you plotted, but a close approximation.

[Trehek]

Originally posted by rocketman544:

The problem is, even though I'm angled toward the manuever vector and achieve sufficient delta-v, my periapsis and apoapsis are both way off

Your maneuver node doesn't need to be much off the AP for your circularisation to become erratic. I thought of a few more tips, which I often use to fine tune:

When you burn, keep an eye on the AP marker. If it starts running away from you way before your burn is complete, turn down your thrust until it stays put. If it falls behind, increase thrust if you can.

Similarly, if the AP seems to increase its altitude, turn your nose slightly down towards the planet. If it decreases, raise your nose.

[A Fat, Angry Serval]

First off, when launching, you want to execute a gravity turn, though I'm sure you already know this. Once you're ready to circularize your orbit, burn prograde as you near your apoapsis. If your apoapsis starts to fall, pull the nose up. If it starts to rise, point down. If it starts to "run away" from you, you've started your burn a bit too early. If it moves behind you, you've started your burn too late and will need to point the nose up to force it forwards (which also increases it).

Second... do not trust the maneuver node progress meter unless you know you can be 100% accurate (not even Mechjeb can do this). If you are even one second off you will not match the maneuver node exactly. I actually don't use maneuver nodes during circularization and just do everything by eye, and I get better results from that. That said, if you do use maneuver nodes, you want to start burning when "time to node" is about half your burn time. The node expects an instant change in velocity, which as you know is impossible. Of course, if the node is more than 10 degrees below the horizon at that time, you will need to execute multiple burns, though you shouldn't have to do this for circularization unless your spacecraft is horibly underpowered.

This brings my to my third point. Don't worry too much about accuracy unless you're using remotetech or are setting up a wireless power grid with KSP Interstellar. While a perapsis under 70km is not sustainable (technically you can get it down to 69km, but then you do have to worry about accuracy), you don't have to worry to much about it. That said, you should try to get into a habit of making your orbits roughly circular. It makes things like rendezvous and transfers MUCH easier.

[Tokfan]

The amount of time spent in a game means jack crap. You cannot use it as a shield while being arrogant.
So what if he/she "only" has 80 hours on track on the STEAM VERSION? What does that mean? nothing. nothing at all.
80 hours of what exactly? 80 hours of tinkering in the VAB trying to even launch?
80 hours of just launching into orbit with various different crafts finetuning these to perfect efficiency?
Does those 80 hours include watching youtubers like Harv or Scott?
Does those 80 hours include sitting at his/her desk at work doing dV calculations in his/her head?
Does those 80 hours include watching KerbalTV on Twitch?
Or, did she/he just leave the game running at normal speed and never even time warped a single time?

Time spent in this game is a moot point and means nothing.
This was a thread to help one of our fellow aspiring rocket engineers and you turned into a pissing contest that you selfproclamed yourself a winner in just because of time spent.

What we lacked from the OP was if he/she gravity turned at all and how the circulization burn was made. But yeah. You got the biggestest E-peen. Now shoo and let us "kids" help each other

[Rene]

Originally posted by Trehek:

Although the node system gives you a estimated burn time, it projects your planned trajectory assuming a instantaneous burn. If you start burning at the node you'll be making a lower burn and reach an uneven trajectory. To be able to achieve the same with non-zero burn time, you need to average your burn around the node, meaning that you have to start your burn half the burn time before the node.

30 second burn? start burning towards the blue marker at t-minus 15 seconds.

Just quoting this to make sure it gets noted, as it is the best post in this topic, IMHO.

Rocketman, you're either starting your burn too late or too early. I you start too early, basically, you'll push your apoapsis away from you and consequently not lift your periapsis enough. If you start too late, well actually the same thing happens, but you're pushing your apoapsis away from you behind you.

Ideally, you'd have your change in velocity instantaneous, and this is what is assumed in the caculation of the predicted orbit after the maneuver node. But since you can't do that, as said before, by me and others, split the burn equally before and after the apoapsis.

Additionally, staging during a burn will likely change your acceleration, resulting in a different overall burn time than what the game initially gave you and making matters even more complicated.

[GAMING_Alligator]

Originally posted by Ankou:

When you are in Map- Mode you can show the Nav-Ball by clicking on the small Arrow on the button of the screen. Now you can control your ship in this view and you don't need a maneuver node for such simple tasks.
When you reach your apoapsis just burn into your flight direction(yellow marker on the navball).
You will see the periapsis rise.
At one point they start changing positions because the periapsis is becoming the new apoapsis.
That is the point to shut down your thrusters.
Now you have a circular orbit.

You probably want to start burning full throttle around 15 seconds ahead of your apoapsis, otherwise you're liable to not have sufficient thrust to achieve orbit.