Navigation nodes and burning

[Tokay Gris]

So I know that the nodes are a punctual representation of a velocity change and the real burn should be in the way that the middle of the burntime should be the actual node.

Usually this works pretty well, but what if the burn times are long? That way you get a hefty part of the burning not done pro/retrograde.

Especially my deep space vessels have pretty low TWR, so burntime can easily reach 20 minutes and more (for that reason, the "Colin Campbell should be in an orbit of about 200 to 400 km).

So... should I follow the prograde marker while burning or keep the attitude?

Not burning prograde/retrograde is a waste of fuel (at least that's the way I figure it), if you just want to raise your orbit.

And if so... how do I figure when to burn in another direction? Is there an easy solution to this or should I just live with the wastes delta-V?

(Wasting delta-V is not really a problem, just an annoyance)

Edited June 8, 2013 by Tokay Gris

[Vanamonde]

For one thing, the maneuver marker will actually move during a burn, trying to show you how to turn the ship to stay as close as possible to your original intent, so just keep your nose pointed at it. That is, however, less than ideally efficient, so something else you can do is make part of the burn on each of several orbits, only firing the engine while you are close to the optimal location/time.

[Zelnogrard]

One thing that helped me to make these long burns more precisely was performing them in several steps whenever possible. For example, when going from LKO to the Mun, you rise your apoapsis and circularize repeatedly until you need only a moderate burn to get the Mun encounter. That might not be an option when timing is crucial, though.

[RoboRay]

One thing that helped me to make these long burns more precisely was performing them in several steps whenever possible. For example, when going from LKO to the Mun, you rise your apoapsis and circularize repeatedly until you need only a moderate burn to get the Mun encounter. That might not be an option when timing is crucial, though.

Don't circularize at the Ap after each short burn! You're wasting a vast amount of fuel.

You only need to raise one side of your orbit to get a departure trajectory, not both. Just keep making a short prograde burn each time you pass your periapsis, pushing your Ap higher and higher. In addition to not wasting fuel raising both sides of your orbit, you also get a big boost from the Oberth effect because your fuel possesses greater kinetic energy during your burns due to your high speed as you pass Pe.

Your sequence of orbits should like this:

Any time your total injection burn exceeds five or six minutes, you should break your transfer up into a series of these "Pe kicks" where you burn for just a few minutes each time you pass the ejection angle (where you put the maneuver node). The key is to burn equal amounts on each side of the angle... So, if you start the burn 90 seconds before reaching the node, stop the burn 90 seconds after passing the node.

Edited June 6, 2013 by RoboRay

[Imaginer1]

I do a few things.

One- I quicksave and do a fast burn to make sure the navigation computer has the right time down. Sometimes it takes a split second or two for the ISP to 'warm up' in an engine and let the nav computer have the right time.

Two- I try to set burns 50/50 around the node. If I need to burn for 1 minute, I start the burn at 30 seconds to the node.

If you're doing a REALLY big burn, maybe it's worthwhile to try and gravity slingshot- which is REALLY hard and I've only once been able to use in a useful way, and that by accident.

What I recommend, though, is changing your rocket design. 20 minutes is enough time to finish the better part of an orbit around Kerbin. Care to take the trouble to send me one of your .craft files? I'm interested

[magnemoe]

I start around 70% before, only exception is then the burn take you to far inside the atmosphere. Do not aim for perfect accuracy but do an correction burn afterward, this you want to be accurate with, you might want to do an late adjustment burn, 3-5 times the distance to sphere of influence, this to get into either equator or polar orbit and and good attitude, this is rarely needed for mun or minmus.

Finaly inside SOI adjust for correct attitude and set up the burn to get into orbit.

[Rickenbacker]

What everyone else said. Also, realize that you'll never be 100% accurate, as the maneuver node assumes that the velocity change is instant. Thus, any burn longer than a second or two, will introduce some errors that have to be corrected once you're done.

[Zelnogrard]

Don't circularize at the Ap after each short burn! You're wasting a vast amount of fuel.

That's true, my bad. The reason I circularized was because I intended to setup an encounter with the Mun for a gravity assist out of Kerbin SOI, and didn't want to bother with synching orbital periods or whatnot. Not even sure if free d/vs from assist cover the circularizing burn, though they probably should.

So my previous post might have been misleading, TC should just go with the "Pe kicks" method.

[Uberick]

Don't circularize at the Ap after each short burn! You're wasting a vast amount of fuel.

You only need to raise one side of your orbit to get a departure trajectory, not both. Just keep making a short prograde burn each time you pass your periapsis, pushing your Ap higher and higher. In addition to not wasting fuel raising both sides of your orbit, you also get a big boost from the Oberth effect because your fuel possesses greater kinetic energy during your burns due to your high speed as you pass Pe.

Your sequence of orbits should like this:

Any time your total injection burn exceeds five or six minutes, you should break your transfer up into a series of these "Pe kicks" where you burn for just a few minutes each time you pass the ejection angle (where you put the maneuver node). The key is to burn equal amounts on each side of the angle... So, if you start the burn 90 seconds before reaching the node, stop the burn 90 seconds after passing the node.

THAT, is the best example of Pe kicks I have ever seen!

On that same note, yea, for long burns do everything in small steps. I have an Ion probe doing a grand tour right now, and it routinely has to do 3-5 hour burns. Splitting it up if possible will increase accuracy.

[Tokay Gris]

Care to take the trouble to send me one of your .craft files? I'm interested

As soon as this I-net connection does again what it is supposed to do, I will.

One of the designs is the "Colin Campbell":

[Djhcc26]

Are you going interstellar because you could get to another star with that much d/v lol

correct me if im wrong but thats got to be somewhere around 15000 d/v in that thing

[Tokay Gris]

Well, as concerning PE-kicks... Since I am talking about transfers to Duna and Jool, I am not quite sure how to aim with PE-kicks.

Of course, I can raise AE until I reach the orbit of Duna (or Jool) but since the times will get longer and longer, I have to start REALLY early with this... and the chances to miss the transfer window are huge.

If I do it in one burn (or two at max), I can get the window, but will waste delta-V and will have to do course corrections on the way.

Also, I can try to do a slingshot (aiming for the moon when it is behind Kerbin to get speed in the direction of Kerbins orbit when I come out of it and aim afterwards... Currently I am testing this with probes.

[Tokay Gris]

Hehe... No, I am trying to get the front half to Jool, because an equally big ship is going there without the massive fuel reserves.

The "Colin Campbell" is a tanker. Another design (but was abandoned, because it desintegrated with 0.20) is the "Dum Vivimus Vivamus":

[RoboRay]

THAT, is the best example of Pe kicks I have ever seen!

I didn't make them... just a good example I found on Google image search.

Well, as concerning PE-kicks... Since I am talking about transfers to Duna and Jool, I am not quite sure how to aim with PE-kicks.

Of course, I can raise AE until I reach the orbit of Duna (or Jool) but since the times will get longer and longer, I have to start REALLY early with this... and the chances to miss the transfer window are huge.

A near-optimal transfer window for Jool and even Duna is days wide. Your Pe kicks should take no more than a day, unless you push the last one all the way out to Minmus or beyond. Stop your last kick with Ap near the Mun's orbit, as there is minimal benefit in going past that point, and it greatly complicates your injection-window timing because your orbital period will take days, instead of hours.

Here's how I do it:

• Make a maneuver node for the full transfer.
  
• Burn for two minutes on each side of the node, aiming at the node.
  
• Delete the node.
  
• Aim prograde for each subsequent burn as I pass my Pe (which is right on the ejection angle, where the maneuver node was), until I have my Ap near the Mun's orbit.
  
• Set up a new maneuver node at Pe for the rest of the transfer.
  
• Execute the maneuver normally.
  

Edited June 7, 2013 by RoboRay

[RoboRay]

(What happened to the Delete Post button?)

[Tokay Gris]

ooookey..... So this didn't work the way as planned.... Rescue is underway.