Another Oberth-Effect Question?

[arkie87]

So let's say you are in Kerbin's SOI around Minmus's apoapsis/periapsis, and you want to escape Kerbin's SOI. Do you:

(a) Burn prograde (little Oberth effect)

( Burn retrograde to drop periapsis down to LKO, then burn prograde at periapsis at LKO (lot's of oberth effect)

I've done some calculations, and plotted V_inf (velocity after escaping Kerbin's SOI) vs. total deltaV budget:

It seems that for deltaV's less than ~600 m/s, option (a) is more efficient, whereas option ( becomes more efficient for higher deltaV's, so it seems that it might not always make sense to choose option ( i.e. Oberth.

Now, realizing that all interplanetary transfers in the game would require more than this 600 m/s deltaV budget, I was wondering if there are any practical situations in KSP where choosing option (a) is more efficient?

[von Ziegendorf]

What about coming back to Kerbin from Tylo (just an example)? If I remember correctly it's better to burn directly from Tylo's SOI.

[NecroBones]

In this case, it's almost always better to just burn prograde and do your escape. If you're out near Minmus's orbit already, you're almost at Kerbin's escape velocity anyway, and the burn will be small.

Secondly, the "burn retrograde to lower periapsis, then prograde at periapsis" maneuver, is called a "bi-elliptic transfer", and in order for it to save you fuel / delta-V, the ratio between the starting and ending "semi-major axis" of your orbits needs to be something like 12:1 or higher. So it's great for doing something like going from Kerbin, down to a low orbit over the sun. But not so good for an escape maneuver.

Wikipedia: http://en.wikipedia.org/wiki/Bi-elliptic_transfer

[magnemoe]

I found that burning from Minmus is more expensive than burning from LKO, at lest then going to Jool and more expensive places.

Rather than dropping from Minmus to an close Kerbin flyby I tend to do it in a more complex way.

1) set up the burn on an satellite in LKO, this is because I use mechjeb for the setup.

2) Find the time to drop from Minus so your Pe is at the node of the satellite but some weeks earlier, leaving Minmus SOI makes this easier.

3) adjust you Ap so you pass the burn point at the correct time, just an small adjustment here adds or substracts days from the burn time.

4) watch out for the Mun it can easy interrupt your orbit.

5) do the burn and enjoy the saving of 700-900 m/s

And yes this is only relevant if you refueled or build your ship at Minmus and have plenty of time before the burn, never done this for Eve or Duna but for the other planets it works real well. And yes remember to send you kerbals to minus to man the ship

[LethalDose]

I really don't follow what your graph is trying to show with "lots of oberth" or "no oberth", but I think

two years ago.

[LethalDose]

In this case, it's almost always better to just burn prograde and do your escape. If you're out near Minmus's orbit already, you're almost at Kerbin's escape velocity anyway, and the burn will be small.

[arkie87]

I really don't follow what your graph is trying to show with "lots of oberth" or "no oberth", but I think

two years ago.

The graph is supposed to show that (a) < ( when deltaV < ~600 m/s, such that this maneuver is not automatically the most efficient for all cases...

I wasn't trying to claim that i discovered this maneuver; I was merely asking if there are practical cases where this maneuver isn't the most efficient?

I also re-watched that Scott Manley video just in case, and he does not touch upon when this maneuver is not the most efficient.

[arkie87]

If what you mean is burning prograde from a circular orbit near Minmus' orbit (and I believe this is the situation intended to be described by the OP), then this statement is demonstrably false. See the video I linked.

I would be very wary about calling this a bi-elliptic transfer. I'm not going to say your wrong (what's described uses 2 ellipses) , but technically bi-elliptical transfers raise the Ap above the level of either the origin or target orbits. The article you linked does state the apoapsis is raised, and doesn't include a condition where the Pe is lowered.

Yes, thanks for replying to him. I agree with you.

I was very confused both from the fact that he implied that the maneuver was worse for most cases (which it isnt in the case of Minmus) and by his labeling of this maneuver a "bi-elitpic transfer", since, after checking out the wikipedia article, it doesnt seem like it is for the reasons you described.

[arkie87]

I really don't follow what your graph is trying to show with "lots of oberth" or "no oberth", but I think

two years ago.

Perhaps this graph will make more sense:

It is a graph of altitude above sea level vs. deltaV burn required to make (a) = (. Thus, if your intended deltaV burn is larger than the blue line, it is more efficient to do (; if it is below, (a) is more efficient.

Minmus is around 50e6 m, so reading the graph, deltaV ~= 600 m/s (noting the log-log scale)

Similarly, if you're at 200 km altitude, your intended deltaV burn would have to be around 8 km/s for dropping periapsis down to 100 km to be more fuel efficient... which makes sense

[GoSlash27]

I think you inadvertently answered your own question; there's nowhere you can go that makes the prograde burn worth it.

If you're just trying to escape SOI or make a small orbit change, then it's pretty obvious that buying into the Oberth effect isn't worth the DV investment. But if you're going somewhere, then it's in your interest in the long run to do that retrograde burn.

Best,

-Slashy

[arkie87]

I think you inadvertently answered your own question; there's nowhere you can go that makes the prograde burn worth it.

If you're just trying to escape SOI or make a small orbit change, then it's pretty obvious that buying into the Oberth effect isn't worth the DV investment. But if you're going somewhere, then it's in your interest in the long run to do that retrograde burn.

Best,

-Slashy

I wasnt only asking about transfers out of Kerbin SOI from minmus; thus, my question (still) stands-- are there any other practical transfer maneuvers in KSP where it doesn't pay to do (?

von Ziegendorf suggested that Tylo to Kerbin might be one example where it does not pay to fall towards Jool first.

I will run the numbers soon to see if he is correct.

Regardless, in theory, it is possible, and the graph suggests that the lower into the gravity well you are, the larger the burn you need to do to make it worth it.

[Superfluous J]

Aha. I misread your question as "Is there any transfers from Minmus that are worth it?" which was confusing considering right before then you said that all were worth it.

I think one of the big things about the Tylo/Jool thing is that at Tylo's orbit, you're actually pretty close to Jool (Compared to how close you are to Kerbin from Minmus' orbit) so you're actually getting help from the Oberth Effect from both Tylo AND Jool.

[GoSlash27]

I wasnt only asking about transfers out of Kerbin SOI from minmus; thus, my question (still) stands-- are there any other practical transfer maneuvers in KSP where it doesn't pay to do (?

von Ziegendorf suggested that Tylo to Kerbin might be one example where it does not pay to fall towards Jool first.

I will run the numbers soon to see if he is correct.

Regardless, in theory, it is possible, and the graph suggests that the lower into the gravity well you are, the larger the burn you need to do to make it worth it.

In that case, yes. There are all sorts of transfers that are worth doing without harnessing the Oberth effect. Specifically transfers within a single SOI where the new orbit is less than 12 times the radius of the old one (or vice-versa).

There's also the case where missing a transfer window costs more in DV than a retroburn for Oberth saves, which happens fairly often.

Best,

-Slashy

Edited November 27, 2014 by GoSlash27

[arkie87]

Aha. I misread your question as "Is there any transfers from Minmus that are worth it?" which was confusing considering right before then you said that all were worth it.

I think one of the big things about the Tylo/Jool thing is that at Tylo's orbit, you're actually pretty close to Jool (Compared to how close you are to Kerbin from Minmus' orbit) so you're actually getting help from the Oberth Effect from both Tylo AND Jool.

Exactly, and that's what my graph shows: once you are low enough in orbit, the cost of going lower to gain more velocity outweighs the benefits, unless you need a lot of deltaV...

And i imagine the balance point varies from planet to planet....

[arkie87]

In that case, yes. There are all sorts of transfers that are worth doing without harnessing the Oberth effect. Specifically transfers within a single SOI where the new orbit is less than 12 times the radius of the old one (or vice-versa).

There's also the case where missing a transfer window costs more in DV than a retroburn for Oberth saves, which happens fairly often.

Best,

-Slashy

I am more interested in where the balance point between burning retrograde and falling into LKO (or LXO if not around Kerbin) and then burning out vs. burning out from the original location. There definitely are more efficient (but more complicated) maneuvers, such as bi-elliptical transfer and gravity assists if the conditions are right.

Your point about missing the window costing more than assist is good though, especially given how many days it takes to fall back to Kerbin from minmus.

That said, I think i answered my own question with the second graph i posted, which states that the balance point depends upon altitude and intended deltaV. I would probably have to make this graph for each planet to fully answer my question, but i think knowing that in general, its probably better to burn down towards a planet if you are near SOI boundary is good enough

[Wanderfound]

I am more interested in where the balance point between burning retrograde and falling into LKO (or LXO if not around Kerbin) and then burning out vs. burning out from the original location. There definitely are more efficient (but more complicated) maneuvers, such as bi-elliptical transfer and gravity assists if the conditions are right.

Your point about missing the window costing more than assist is good though, especially given how many days it takes to fall back to Kerbin from minmus.

That said, I think i answered my own question with the second graph i posted, which states that the balance point depends upon altitude and intended deltaV. I would probably have to make this graph for each planet to fully answer my question, but i think knowing that in general, its probably better to burn down towards a planet if you are near SOI boundary is good enough

The missing question here is: why are you in a high circular orbit in the first place?

If you can, launch a rocket or spaceplane with a cargo bay into an orbit as elliptical as you can make it while still keeping enough ÃŽâ€V to drop your periapsis for an aerobrake. Drop the interplanetary probe when you cut thrust; kick off the probe's own engines when it hits periapsis.

Do this right and you can send a full collection of science instruments to any planet you choose with no on-board fuel apart from a small monoprop tank.

[arkie87]

The missing question here is: why are you in a high circular orbit in the first place?

If you can, launch a rocket or spaceplane with a cargo bay into an orbit as elliptical as you can make it while still keeping enough ÃŽâ€V to drop your periapsis for an aerobrake. Drop the interplanetary probe when you cut thrust; kick off the probe's own engines when it hits periapsis.

Do this right and you can send a full collection of science instruments to any planet you choose with no on-board fuel apart from a small monoprop tank.

Interesting. I assume you have to place the periapsis precisely though, since it wont be possible to adjust it later with monoprop alone, and then you would have to worry about transfer window expiring during the fall from HKO, as GoSlash27 pointed out

[Wanderfound]

Interesting. I assume you have to place the periapsis precisely though, since it wont be possible to adjust it later with monoprop alone, and then you would have to worry about transfer window expiring during the fall from HKO, as GoSlash27 pointed out

You're right that placement is tricky, but for an example of what a small probe can do from LKO:

Still had the FL-R10 tank half full. And that was from circular LKO, not an elliptical boost. Put the ellipse on, and you'll get comparable range from a Kerballed craft if you make it capsule + science and not much else.

The easiest way to set the ellipse involves first launching your carrier vessel into circular LKO. Then, set an interplanetary manoeuvre for it. This will let you know roughly where the periapsis needs to be. Then, cancel the manoeuvre and burn near the node to raise your opposing periapsis as high as possible and drop the probe. The required ejection angle will move somewhat while the probe is going to apoasis and back, but with a bit of experience and/or experimenting (very cheap if it's spaceplane-launched microprobes) you can compensate for that. You don't have to get the ejection angle perfect; the Oberth boost will more than compensate for any minor imperfections.

Edited November 27, 2014 by Wanderfound

[NecroBones]

If what you mean is burning prograde from a circular orbit near Minmus' orbit (and I believe this is the situation intended to be described by the OP), then this statement is demonstrably false. See the video I linked.

I would be very wary about calling this a bi-elliptic transfer. I'm not going to say your wrong (what's described uses 2 ellipses) , but technically bi-elliptical transfers raise the Ap above the level of either the origin or target orbits. The article you linked does state the apoapsis is raised, and doesn't include a condition where the Pe is lowered.

Yes, you're right, I misread the OP.

[arkie87]

Yes, you're right, I misread the OP.

well, thanks for posting about bi-elliptic transfers anyway

It's kinda counter-intuitive that they would be more fuel efficient under certain circumstances, and i enjoyed reading the wiki article about them.