Best way to do a long burn?

[Soda Popinski]

I've got my first mission heading to Jool. I've got a 1963 m/s burn the maneuver node tells me will take 25 minutes. I'm in a 100km circular orbit over Kerbin. What I first tried was a 10 minute burn starting at -5 minutes from the node, leaving me with about 1175 m/s (I think), then looped around again with a new node set at the PE for 1175 m/s, and started burnin 5 minutes before hitting the PE. I've used up 47.83t of fuel, which comes up to 2351m/s of delta-V (which I guess isn't too bad).

Could I have been more efficient if I did 3 or 4 shorter burns closer to PE to take advantage of the Oberth Effect?

I just did some calculations that I hit escape velocity just after 15 minutes of burning (1176 m/s). So maybe doing 3x 5 minute burns at PE (-2.5 min to +2.5 min).

Edited January 3, 2014 by Soda Popinski

[KvickFlygarn87]

Well, I suppose that the most efficient way should be to do as short burns as possible at periapsis. However, I'd do it as follows:

1. Burn short burns at periapsis. This can be all from 2 burns to 20 burns.
   
2. When you are near escape velocity, set up the node to transfer to Jool, and execute as normal.
   
3. ???
   
4. PROFIT!
   

[Kasuha]

If you do short burns near periapsis and rise your apoapsis withing the SOI, you spend a lot of time orbiting and your launch window is shifting. After a few orbits you may want to go in a different direction than where your orbit points.

That can of course be remedied by calculating that beforehand, but I'm not sure anyone here has suitable tools for that.

My personal approach is to do it in a single long burn even if it means some wasted fuel. And maybe pack a few more nuclear engines to prevent having such long burns.

[Captain Sierra]

I build my interplanetary drive stage's launcher to have some fuel left over. 1 mainsail engine, as inefficient as it is, can really shave some time off and so long as I lugged the upper stage of the lifter up there, I might as well get some use out of it. my standard asparagus design can get a 40 ton payload to Duna without a drive stage, but a payload that light means I can switch out the center engine for a skipper for even higher efficiency. Basically, since you are near Kerbin, you don't have to burn with a nuke. You can, in theory, use a stage with moderate efficiency and good thrust for the ejection burn and carry just enough fuel on the high efficiency stage for mid-course corrections and your braking burn (and return if necessary).

[maccollo]

This is the approach I use. Works without fail.

1: plan a maneuver that will get a Jool encounter.

2: raise your apoapsis with number of burns, starting at t- 2-3 minutes, and ending at t+2-3 minutes. Doing this will make sure the periphrasis doesn't shift.

3: Stop raising it when you get it just above the moon. Make minor tweaks to make sure you wont intercect the moon on the escape if nessesary. At this point you have 90% of the escape velocity, but looping around will only take 6 hours or so, so it wont affect your launch window much.

4: Do one final loop and burn all the way. The timing of this burn can be tricky, but I've found it usually works best to wait a little bit longer than the 50% point, something like 40% of the burn at t- and 60% at t+ tends to eject me in line with the planned escape vector.

[SRV Ron]

I've got my first mission heading to Jool. I've got a 1963 m/s burn the maneuver node tells me will take 25 minutes. I'm in a 100km circular orbit over Kerbin. What I first tried was a 10 minute burn starting at -5 minutes from the node, leaving me with about 1175 m/s (I think), then looped around again with a new node set at the PE for 1175 m/s, and started burnin 5 minutes before hitting the PE. I've used up 47.83t of fuel, which comes up to 2351m/s of delta-V (which I guess isn't too bad).

Could I have been more efficient if I did 3 or 4 shorter burns closer to PE to take advantage of the Oberth Effect?

I just did some calculations that I hit escape velocity just after 15 minutes of burning (1176 m/s). So maybe doing 3x 5 minute burns at PE (-2.5 min to +2.5 min).

Are you doing the long burn time with your ship pointed at the Maneuver Mode marker? Point the ship at the prograde marker, start the burn, and keep adjusting your ship as the prograde marker changes with the burn. That will allow you to take full advantage of the Oberth effect.

It may take some trial and error to pinpoint the exact time to start the maneuver on the ever changing orbit during a long burn.

If you are doing the long burn from solar orbit, the prograde and maneuver mode marker will usually be the same.

[maccollo]

Always point towards prograde when you do major burns. Not pointing towards prograde is wasted acceleration.

Imagine you had a really long burn escape burn from Kirbin. One that was as long as your orbital period. If you pointed. By doing the 50/50 approach and pointing towards the maneuver vector, then you would begin this burn by pointing towards retrograde. Clearly this is not efficient, burning towards pro-grade is efficient, everything between in a gradient.

[Soda Popinski]

Always point towards prograde when you do major burns. Not pointing towards prograde is wasted acceleration.

Imagine you had a really long burn escape burn from Kirbin. One that was as long as your orbital period. If you pointed. By doing the 50/50 approach and pointing towards the maneuver vector, then you would begin this burn by pointing towards retrograde. Clearly this is not efficient, burning towards pro-grade is efficient, everything between in a gradient.

No I didn't. Looking into this before, there seemed to be two schools of thought on it. Clearly burning when you're on the opposite side of the maneuver node is a problem. On the other hand, I'm trying it again right now with more burns, and I'm on my last burn. I'm past escape velocity, yet the current prograde marker curves towards where the apopasis would be, and away from Kerbin's solar orbit, which is where I want to be. I can clearly see at this point if I aim towards prograde, I'll be wasting fuel, but if I point towards the maneuver node, I go slightly outside the orbital curve. I suspect the best place to aim would be somewhere in between. I'll see how this plays out and check my fuel usage. I might try it again pointing towards prograde the whole time.

[SRV Ron]

No I didn't. Looking into this before, there seemed to be two schools of thought on it. Clearly burning when you're on the opposite side of the maneuver node is a problem. On the other hand, I'm trying it again right now with more burns, and I'm on my last burn. I'm past escape velocity, yet the current prograde marker curves towards where the apopasis would be, and away from Kerbin's solar orbit, which is where I want to be. I can clearly see at this point if I aim towards prograde, I'll be wasting fuel, but if I point towards the maneuver node, I go slightly outside the orbital curve. I suspect the best place to aim would be somewhere in between. I'll see how this plays out and check my fuel usage. I might try it again pointing towards prograde the whole time.

With every maneuver resulting in a curve to the destination due to gravity, you may find that holding prograde on a long burn while in orbit is more efficient then burning the Maneuver Mode marker. Let us know the results of that testing.

[Soda Popinski]

I tried to follow the prograde mark, but my big lumbering craft wasn't really compatible with continually turning. There was a lot of wobbling / shimmying, so it spent way too much time off the prograde mark. At that point, it wouldn't have been a fair comparison, so I aborted. I might try all 3 with a smaller craft just to see.

What I did was perform the burn split into 5 minute chunks. Originally, I attempted to do 3x 5 minute burns to take me to JUST before escape velocity, then do the final burn on the 4th time around, but as warned above, my final orbit (took days), I missed my launch window. So I redid it with 2x 5 minute burns and a long final ejection burn. Ended up saving just a little over 3 tons of fuel...out of 48t. Something to be sure, I was just hoping for more.

Thanks for the help everyone.

[Fidelacchius]

Yeah, If you want max efficiency do many short burns.

You don't want to burn under your desired exit trajectory. You will be wasting fuel fighting gravity.

With Ion Engines i have done 5-10 burns before (painful). Once your orbit path is lined up more with the exit you want you can do a full longer burn to break orbit.