NERV Ejection Altitude

[More Boosters]

Hey,

Usually I depart from a 75x75 orbit with chemical engines but with NERV, I went for 200x200 to compensate for steering losses. However they are still far too great, so I'm wondering if they would be lower further up. Should I just give up on the Oberth effect and depart from a 500x500 orbit or higher? What's good altitude for interplanetary ejection when using Nerv?

[Tex_NL]

The lower the better. Period!

But that can make long burns dangerous as it might push you back into the atmosphere. Solution: don't make your burn all at once. Multiple passes will give you the same result at much higher efficiency.

[Superfluous J]

One trick is to split the burn. You have to do the last X m/s (where X is about 1000m/s less than your total burn) all at once because once you eject from Kerbin's SOI you can't swoop around for another try, but if your total burn is - say - 1500 m/s you can do one 500m/s burn, loop around, do another ~500m/s burn (just don't escape SOI), swoop around, and then do a 3rd one to eject.

You have to start early (multiple days in some cases considering you're talking about orbiting up past Minmus' orbit) and watch out for Mun (and to a lesser extent Minmus) encounters, but if you find that kind of thing fun (and let's face it we find that kind of thing fun) it's well worth at least doing once.

[Empiro]

It depends on your burn time. I think the general rule is that you don't want to burn more than 1/6 of your orbital period (of a circular orbit). As stated, you don't need to burn all at once, and you can raise your AP through several passes.

[jarmund]

My Jool 5 expedition's composition, summed up: 3 landers, 4 NERVs, and a bunch of MK3 liquid tanks. This resulted in a stupid amount of DeltaV (KER just said NaN.. it was literally off the charts), but on the downside was that it had a TWR of 0.05. I assembled it in a 75x75 orbit, and when it was time to leave I did multiple burns to raise my AP. I think I made maybe 10-15 orbits, each time burning at PE for about 2 minutes. In the end I ejected as planned and my PE never got above 80km.

Tips:
1. Start raising your AP early. After a while your orbital period will be so long that you run the risk of missing your transfer window while still working on raising your AP. And remember, a kerbin day is only 6 hours long. I spent around 2 weeks of ingame time raising my AP
2. Don't let the mun toss you around in the process
3. A low TWR ejection usually means a looooooong orbital insertion burn for when you arrive at your destination. My Jool5 expedition burned for 5 minutes continuously when arriving, including a triple reverse gravity assist by Laythe, Tylo and Vall

[More Boosters]

Maybe I'm just underestimating how wide transfer windows are then. Is the 300-400 dv you save with the Oberth effect worth the hassle though? I'm sure I lost at least 600 dV as steering losses during a 1.4k burn and splitting burns would only further reduce the accuracy.

Funnily enough I recently conducted my aforementioned ejection burn and Mun was on the way... It barely affected me and even gave me a boost. It would have even been somewhat useful if it hadn't turned out that I accidentally time warped past my Moho window and found out too late that Moho was going to outpace me by far. As luck would have it, Dres was in just the perfect position and even though I need to speed up from an orbit I purposefully slowed down to get to Moho, it looks like I'll still have 700 nuke DV to spare after getting a Dres parking orbit. The whole thing feels like a conspiracy by Dres to get a rover on it. :D

Edit: Funnily enough Alexmoon LW Planner mentions a slightly lower ejection DV (yes you still need to get up there I know) for a 500 by 500 orbit compared to a 100 by 100 one. Edited November 21, 2015 by More Boosters

[Superfluous J]

[quote name='More Boosters']Maybe I'm just underestimating how wide transfer windows are then. Is the 300-400 dv you save with the Oberth effect worth the hassle though? I'm sure I lost at least 600 dV as steering losses during a 1.4k burn and splitting burns would only further reduce the accuracy.[/quote]
If you had a 1.4k burn and actually burned 2k, you're not doing something right. Also, splitting burns doesn't reduce the accuracy any more than using a little draggable tool with pull-knobs on it to plan your burn. Just re-make the maneuver node in between each burn and you'll be fine. If you lose more than 100dV due to this I'd be shocked. If you lose less than a dozen I'd not be surprised. I've never bothered to do the math, or even check afterward. It WILL save a LOT of fuel compared to doing a 15-minute burn at periapsis in one go.

[quote name='More Boosters']Edit: Funnily enough Alexmoon LW Planner mentions a slightly lower ejection DV (yes you still need to get up there I know) for a 500 by 500 orbit compared to a 100 by 100 one.[/QUOTE]
There was a thread on here discussing... I don't remember the term. ________ orbits. I also don't remember the exact orbit to go for for the cheapest ejection, mostly because (as you stated) you have to get there, and it's still cheaper to just do the whole thing as low as possible. Note, though, that "as low as possible" may very well be 500km up if you are doing it all in one burn.

[More Boosters]

[quote name='5thHorseman']If you had a 1.4k burn and actually burned 2k, you're not doing something right. Also, splitting burns doesn't reduce the accuracy any more than using a little draggable tool with pull-knobs on it to plan your burn. Just re-make the maneuver node in between each burn and you'll be fine. If you lose more than 100dV due to this I'd be shocked. If you lose less than a dozen I'd not be surprised. I've never bothered to do the math, or even check afterward. It WILL save a LOT of fuel compared to doing a 15-minute burn at periapsis in one go.


There was a thread on here discussing... I don't remember the term. ________ orbits. I also don't remember the exact orbit to go for for the cheapest ejection, mostly because (as you stated) you have to get there, and it's still cheaper to just do the whole thing as low as possible. Note, though, that "as low as possible" may very well be 500km up if you are doing it all in one burn.[/QUOTE]

You're thinking of ballistic transfers I believe. I tested both "Optimal" and "Ballistic" transfers and the results were the same.

And yes, my mistake was to attempt a 12 minute burn at a 200 by 200 orbit.

[Red Iron Crown]

The tutorial linked in my signature provides a method to split up ejection burns and retain accuracy, worth a look if you're running low-TWR transfer stages.

[quote name='5thHorseman']There was a thread on here discussing... I don't remember the term. ________ orbits. I also don't remember the exact orbit to go for for the cheapest ejection, mostly because (as you stated) you have to get there, and it's still cheaper to just do the whole thing as low as possible. Note, though, that "as low as possible" may very well be 500km up if you are doing it all in one burn.[/QUOTE]
[URL="https://en.wikipedia.org/wiki/Gate_orbit"]Gate orbit.[/URL] Thread here: [url]http://forum.kerbalspaceprogram.com/threads/136036-Oberth-effect[/url]