Orbeth effect: how much does it matter?

[robopilot99]

I know the orbeth effect can save you tons of delta-v but how much and at different orbits?

[LethalDose]

I know the orbeth effect can save you tons of delta-v but how much and at different orbits?

Your question is really broad. In my experience, the Oberth effect is most useful when you're going from a satellite into interplanetary space by way of the satellite's primary (e.g. Minmus to interplanetary space by LKO or Tylo to interplanetary space by way of low Jool orbit).

I find that leaving Kerbin from Minmus can save ~ 400-700 m/s. But it's "saved" because I refueled at Minmus, I still had to spend dV to get to Minmus in the first place.

Edited April 29, 2014 by LethalDose
Screwed up your/you're... I hate that.

[Tex]

It depends on how far you're travelling. Oberthing can do wonders to your Delta-V budget when, say, travelling to Eeloo. However, you shouldn't worry about it too much for missions that stay in the Kerbin system, because then the effects aren't significant enough to affect anything. Maybe into the tens of units for Delta-V, but that's hardly anything you can't work around.

[Kasuha]

Oberth effect?

Starting at LKO (70x70 km)

- Hohmann transfer from LKO to just behind the SOI boundary: 930 m/s

- Hohmann transfer from Kerbin orbit to Jool orbit: 2700 m/s

- Direct transfer from LKO to Jool orbit: 1950 m/s

You cannot change anything on the 930 m/s you need to eject from LKO to interplanetary space, but for the rest of the transfer, you spend 1020 m/s if you do it at LKO, and 2700 m/s if you don't. So there.

[Vanamonde]

With regard to some removed posts, either comment on the subject of the thread or skip to the next one. There's no point in being unpleasant to each other.

[DrMonte]

I think about Oberth effect in the way - if you want to lower your orbit burn at apoapsis, if you want to put it higher burn at periapsis.

Transfers to other planets it's a bit too much math for a Kerbal. It's enough that I wait for a transfer window.

[rkman]

I find that leaving Kerbin from Minmus can save ~ 400-700 m/s. But it's "saved" because I refueled at Minmus, I still had to spend dV to get to Minmus in the first place.

You 'save' fuel not because of the Oberth effect but because Minmus is near the border of Kerbin SIO, far away from Kerbin's gravity so that you need only a little delta-v to reach Kerbin escape velocity.

The Oberth effect is stronger the faster you go, which is generally in low orbit. If you want to go from Kerbin's surface to planetary transfer it's best to first establish low Kebin orbit.

[LethalDose]

You 'save' fuel not because of the Oberth effect but because Minmus is near the border of Kerbin SIO, far away from Kerbin's gravity so that you need only a little delta-v to reach Kerbin escape velocity.

The Oberth effect is stronger the faster you go, which is generally in low orbit. If you want to go from Kerbin's surface to planetary transfer it's best to first establish low Kebin orbit.

Well, it's both, it' really is the Oberth effect is more than anything else because instead of burning at Minmus' orbit (47 Mm above Kerbin's surface), you drop down to LKO (100 km or 0.1 Mm above Kerbin's surface) and burn there, where your orbital velocity is much higher. This is actually more efficient than exiting Kerbin's SoI from Minmus directly.

Sorry that detail wasn't in my first response to the OP.

If you don't believe me, then maybe you'd believe Scott Manley. This exact procedure and it's efficiency are documented in his video here (links aren't working for me today:

) and are described in terms of the Oberth Effect.

[rkman]

Indeed it was not clear you first drop to Kerbin, but i'll admit i would not have guessed the Oberth effect would be that strong. To bad it is not the more efficient way if the starting point is Kerbin's surface. But probably good reason to establish a Kethane base on Minmus.

Should it not be possible to gain even more by also using Kerbin for gravity assist, by departing from Minmus when it is at the leading side of Kerbin, with periapsis behind Kerbin?

[LethalDose]

Indeed it was not clear you first drop to Kerbin, but i'll admit i would not have guessed the Oberth effect would be that strong. To bad it is not the more efficient way if the starting point is Kerbin's surface. But probably good reason to establish a Kethane base on Minmus.

Should it not be possible to gain even more by also using Kerbin for gravity assist, by departing from Minmus when it is at the leading side of Kerbin, with periapsis behind Kerbin?

So, first off, if you're burning, don't call it a gravity assist, because some ppl will get cranky. See this thread.

And yes, timing your escape from Minmus to coincide with the appropriate ejection angle from Kerbin is important. However I usually escape going outward into interplanetary space when Minmus is between the sun and Kerbin, and escape going inward when Minmus is furtherst from the sun. After some though, this timing may not be ideal, but it works very well in my hands. There's also the timing with your launch window to consider.

If you're interested in reading the conversation that's currently going on about using Minmus to escape Kerbin, I'd again refer you to that thread I linked above, instead of repeating the points here.

[Northstar1989]

Should it not be possible to gain even more by also using Kerbin for gravity assist, by departing from Minmus when it is at the leading side of Kerbin, with periapsis behind Kerbin?

What you want is for the the rocket to spend as much time falling inwards towards Kerbin (gaining speed, which will be useful for the Oberth effect) and as little time being slowed down by Kerbin's gravity on the way out as possible. It really doesn't matter the relative positioning of Minmus- only the final trajectory of the rocket as it exits Kerbin's SOI after Kerbin's gravity is done curving its path...

For best results, eject from a low Minmus orbit (Minmus' gravity is so weak the benefits from the Oberth effect on ejection will greatly outweigh the increased time its gravity slows you down during ejection) with a burn at a periapsis that will have the final trajectory when leaving Minmus' SOI pointed precisely retrograde from the direction of Minmus orbit around Kerbin. If you get it just right, you sohuld leave Minmus' SOI with enough velocity that you have almost no velocity relative to Kerbin upon SOI-transfer, and will consequentially fall almost straight in towards Kerbin.

You want your periapsis relative to Kerbin as low as possible without entering the atmosphere to achieve maximum benefit from the Oberth effect- though you also have to take into account the fuel cost of the burn near Minmus vs. the fuel savings at the burn near Kerbin...

Regards,

Northstar

[Red Iron Crown]

So, first off, if you're burning, don't call it a gravity assist, because some ppl will get cranky. See this thread.

Was that really necessary? I let it go after you posted, please do me the same courtesy.

To the OP, the further away from your orbit the destination orbit is, the more dV you will save by harnessing Oberth. If leaving from Kerbin, you can ignore the Oberth effect for transfers to Duna and Eve with only small efficiency penalties, but a transfer to Eeloo or Moho makes chasing Oberth much more beneficial.

Was there a specific case you were interested in, so we can give more concrete answers?

[Northstar1989]

Was that really necessary? I let it go after you posted, please do me the same courtesy.

To the OP, the further away from your orbit the destination orbit is, the more dV you will save by harnessing Oberth. If leaving from Kerbin, you can ignore the Oberth effect for transfers to Duna and Eve with only small efficiency penalties, but a transfer to Eeloo or Moho makes chasing Oberth much more beneficial.

Was there a specific case you were interested in, so we can give more concrete answers?

It's about the Delta-V of the transfer, not the distance to the destination, that determines the usefulness of the Oberth Effect.

I feel the need to point out this distinction because, again, you don't *have to* use the lowest-energy transfer possible to get to Eve or Duna.

If you're running a manned mission with a life-support mod, or playing for realism, it's very likely you'd want to eject with excess speed to get to your destination faster.

In that case, a fast transfer to Eve or Duna can require just as much Delta-V (and benefit just as much from the Oberth Effect) as a slow transfer to Jool or Moho...

In real life, NASA wants to wait on a Mars mission until it can develop propulsive systems that can get a manned mission to Mars in much less time than current chemical rockets allow. It's not desirable to make a minimum-energy transfer, and have a crew stuck in interstellar space (with the heavy radiation exposure that entails) for months at a time. That's a big part of why they're eying the VASIMR propulsive system so eagerly... (it could allow much faster transfers, thanks to its higher ISP, when scaled up to a multi-megawatt scale with sufficiently advanced nuclear reactors)

Regards,

Northstar

Edited May 6, 2014 by Northstar1989

[Red Iron Crown]

That's a clearer way to think of it, thanks. I was assuming minimum energy transfers, but your wording is more general. The greater the magnitude of the energy change desired, the more benefit Oberth provides.