Calculating 50/50, 70/30 and related burns.

[MR4Y]

I don't know if there is a tutorial on it already, but as I'm trying to do them for longer burns IU get confused. How you calculate them? I tried doing them, the maneuvers work, but it's not really a 50/50 burn as most of it is completed before the node passes.

[Francesco]

I would say this is one of those things where having some rule of thumb is useful, but then it's mostly about your intuition and experience.

as you already know, the actual burn time is always going to be shorter than the one estimated by the manevuer node system.

on Kerbin, I usually keep burns 4 or 5 minutes long at max, by fractionating longer ones in two or more parts.

so for a 7-8 minutes departure burn, the first one is going to be exactly 120 seconds before the node and 120 after.

for burns that are shorter than that, or for the "final" burn during periapsis kicks, I follow a sort of 3/7 + 4/7 rule: I create a maneuver node, cut the estimated time in half, and round it up to a lower number.

so let's say for a 3' 21" burn, I won't start 1' 40" before the node, but more like 1' 30"-ish.

this is where a number of other factors come into place - like, having a LV-909 pushing your ship instead of a LV-N means your TWR is probably going to rise quicker, and therefore your burn time is going to be even shorter.

other small things could be having to throttle up from 0 to 100% at the beginning (unless you use action groups), which is going to take a second or so, and having to throttle down while approaching the end of the burn to avoid overshooting.

but I would also say, don't get too OCD with this: as long as you did a fine job with planning a node in the beginning, and get a good ejection angle, you'll have a lot of time to make corrections while still close to Kerbin.

the only exception being Moho

I swear putting a ship inside its SOI is the most frustrating thing in this game.

[MR4Y]

I would say this is one of those things where having some rule of thumb is useful, but then it's mostly about your intuition and experience.

as you already know, the actual burn time is always going to be shorter than the one estimated by the manevuer node system.

on Kerbin, I usually keep burns 4 or 5 minutes long at max, by fractionating longer ones in two or more parts.

so for a 7-8 minutes departure burn, the first one is going to be exactly 120 seconds before the node and 120 after.

for burns that are shorter than that, or for the "final" burn during periapsis kicks, I follow a sort of 3/7 + 4/7 rule: I create a maneuver node, cut the estimated time in half, and round it up to a lower number.

so let's say for a 3' 21" burn, I won't start 1' 40" before the node, but more like 1' 30"-ish.

this is where a number of other factors come into place - like, having a LV-909 pushing your ship instead of a LV-N means your TWR is probably going to rise quicker, and therefore your burn time is going to be even shorter.

other small things could be having to throttle up from 0 to 100% at the beginning (unless you use action groups), which is going to take a second or so, and having to throttle down while approaching the end of the burn to avoid overshooting.

but I would also say, don't get too OCD with this: as long as you did a fine job with planning a node in the beginning, and get a good ejection angle, you'll have a lot of time to make corrections while still close to Kerbin.

the only exception being Moho

I swear putting a ship inside its SOI is the most frustrating thing in this game.

What I do atm is the following:

-I look at the estimated burn time.

-If it's possible to half it exactly, I start the burn at half the time before the node. (2min30sec for 5 min for example.)

-If the number ends up having decimal points when halved. (7.5sec for 15sec for example), I round it to the next lower number.

-In true fashion, I add 3 seconds to the time so I can thrust up to 100%.

The burns work, but theyre not really 50/50 time wise. They're more like 85/15 burn or less.

[tavert]

The estimated burn time indicator can read really strange inaccurate readings depending on whatever your last throttle setting was.

As a rough estimate of time, I take maneuver delta-V times spacecraft mass divided by thrust. This neglects the fact that your mass decreases as you burn fuel so it's only a first-order approximation, but quicker and easier to remember than inverting the rocket equation.

[AmpsterMan]

I burn when the burn time and est time are the same.

Does it always work? No.

Does it work half the time? No.

Does it work at all? No.

I just do it that way.

The maneuver nodes are really only meant to give you a rough ballpark estimation so that is all you will ever get from it.

[MockKnizzle]

For non-critical burns (out to the Mun, circularization, etc) I just briefly throttle up to reset the estimated burn timer and then roughly cut that time in half. For more critical interplanetary transfers (like out to Eeloo) it's a little more complicated. I take my maneuver dV, divide it by my maximum acceleration (I have a Mechjeb window that displays the relevant stats) and then multiply by a factor that depends on what proportion of my fuel I'll be burning and how massive my ship is. For a large burn that will take up a large portion of fuel and thus a large portion of my total mass, I'll multiply by a small fraction like 3/7. For a smaller burn, I'll multiply by something closer to 1/2, like 4/9. Then I just add about a second and a half of throttle-up time and I'm good to go.