Manoeuvre node timing

[HorusKol]

I was setting up a transfer from Kerbin to Duna - I managed to get an orbit projected for Duna capture using a manoeuvre node - roughly about a 4 minute burn.

I waited until 4 minutes before the node, and then fired up my engines - using SAS to keep steady. It all seemed to go pretty well, until the burn was almost complete the resulting transfer orbit around Kerbol was not the same circle as the one the manoeuvre node plotted, and I was going to be late getting to Duna. I think I've managed to tweak the orbit with a couple of transition burns on route - but I'm wondering if my approach to timing burns is correct? This approach has worked out with Mun and Minmus - probably because the capture windows are bigger, there's less dV, and shorter burn times.

So, question:

Do I start a 4 minute burn 4 minutes before the node? Or 2 minutes before? Or when I reach the node?

I guess my other option is to loft up more engines for the transition stage.

Edited August 14, 2014 by HorusKol

[Master Tao]

Predicted orbits after maneuver nodes assume that all the change in velocity is applied instantaneously, so you need to account for that. For burns up to about 4 minutes, usually you can split the difference and make small corrections after finishing the burn. For longer burns, you may need to chunk the burn across multiple orbits in order to minimize corrections.

[von Ziegendorf]

For relatively short burns (when compared with orbital period) it should be enough to start half time before, in your example 2 minutes before. For longer burns try this: Long Burn Calculator (self advertising )

[Murph]

As far as I'm aware, the best approach is to align to the blue node indicator, lock heading with SAS, then split the burn time equally before and after the node time. That's what MechJeb does, and it seems to work pretty well.

Interplanetary transfers, however, commonly do need a small corrective burn after leaving Kerbin's SoI, as a tiny error in the initial burn to leave Kerbin causes a huge error at the target planet. I tend to do the Kerbin escape burn, wait until leaving Kerbin's influence, then a small correction just outside Kerbin SoI, possibly another mid-transfer to the target, then the main one to tune arrival after entering the target SoI. The corrective burns are generally very small compared to the total dV involved in the transfer. Sometimes it's useful to either use RCS or thrust limiter for the corrective burns, as it's often just a few m/s needed.

[Jean Deaux]

Split the time equally, assuming you won't be staging during the burn. Granted while you're burning you're losing fuel and thus mass is being reduced, but when staging you're also losing physical mass properties which will affect your burn times and will either increase or decrease your burn times depending on whether a different type of engine or number of engines are now firing. For such complicated combinations, MechJeb handles these calculations on the fly pretty well at the cost of messing up the display of the required burn time. And there are also probably a few spreadsheets out there that can do the calculation for you if you feel like taking the time to fill out all the required data to figure that out (the true rocket scientist?).

But sure, as a general rule of thumb, split the burn evenly.

[HorusKol]

As far as I'm aware, the best approach is to align to the blue node indicator, lock heading with SAS, then split the burn time equally before and after the node time. That's what MechJeb does, and it seems to work pretty well.

Interplanetary transfers, however, commonly do need a small corrective burn after leaving Kerbin's SoI, as a tiny error in the initial burn to leave Kerbin causes a huge error at the target planet. I tend to do the Kerbin escape burn, wait until leaving Kerbin's influence, then a small correction just outside Kerbin SoI, possibly another mid-transfer to the target, then the main one to tune arrival after entering the target SoI. The corrective burns are generally very small compared to the total dV involved in the transfer. Sometimes it's useful to either use RCS or thrust limiter for the corrective burns, as it's often just a few m/s needed.

This seems to be the best approach based on the nodes I've plotted for once I'm out of Kerbin SoI - but the projected capture window is only about 2 hours (on a trip over a couple of hundred days!)

For relatively short burns (when compared with orbital period) it should be enough to start half time before, in your example 2 minutes before. For longer burns try this: Long Burn Calculator (self advertising )

How long is a "long burn"?

Edited August 14, 2014 by Specialist290
Merging sequential posts

[Xavven]

You burned a little too early and ended up with too much radial-out velocity. You can still save your mission by making a correction burn radial-in with respect to the Sun.

A 4 minute burn isn't bad. A lot of my interplanetary transfers take at least that long. Personally, I consider an 8 minute burn or higher to be long (and usually inaccurate) but I usually only get into that territory using LV-Ns, and the higher ISP is sometimes worth it.

[AeroAviation]

I usually split the time equally so in a 4 minute burn I would burn 2 minutes before I reach it

[juanml82]

This seems to be the best approach based on the nodes I've plotted for once I'm out of Kerbin SoI - but the projected capture window is only about 2 hours (on a trip over a couple of hundred days!)

How long is a "long burn"?

Something in the range of ten minutes perhaps. And if you have a low Pe to leverage the Oberth effect and try to make it in one burn, you may end up heading straight into Kerbin. Those kind of burns require correction and, more often than not, it's best to split it into two or three burns. And once you set the nodes up, let MJ handle them while you go do something else, specially because physics warp might cause your ship to wobble or even get destroyed if you're docking or you have an unstable design.