Gravity braking with moons

[storm_soldier2377]

Here's something I've always pondered on for a bit. Using gravity assists to get captured on Jool is pretty straightforward as using Tylo and/or Laythe will get you captured around Jool. On Kerbin and Duna, however, it's not so clear. You might get a decrease in velocity if you get behind Ike or the Mun, but it's never quite enough for a capture. I've always wondered if it is better to use the moons for an assist and use a retrograde burn or if doing that just loses the Oberth effect of the larger body the moon is orbiting and ends up costing more d/V.

[Magzimum]

I am not familiar with a method to compare the two (slingshot vs. low Pe with Oberth), but who not try both?

If the moon of your choice is in the right position, it mayh be possible to get a slingshot, thereby slowing down a lot, and (through some adjustments) still achieve a low Pe around the central planet.

[The Aziz]

That is because Joolian moons are bigger, thus their gravity well is much deeper, so by doing a fly-by close to Tylo, it's more like you were doing it with Kerbin, rather than Mun.

[bewing]

Well, It depends a little on how fast you're coming in. The Mun is only moving at 540 m/s, so that's about all it can give you. So if you're coming in at 6000 m/s, you're not going to get much out of it. But Kerbin's Oberth is only 2300 m/s, so if you are coming in at 6000 m/s, you aren't going to get much out of that either. So if you are coming in slow enough for Kerbin's Oberth to matter, then you are also coming in slow enough that the Munar slingshot should be a significant fraction of your speed, I'd think.

[Snark]

I think the scale difference between Tylo/Laythe on the one hand, and Mun/Ike on the other, is just too big-- I've never found it worthwhile to use Mun or Ike for gravity assists.

Just to put things in perspective, here are the masses of the four moons, normalized so that Mun = 1.

• Tylo:  43.37
• Laythe:  30.12
• Mun:  1.00
• Ike:  0.29

There's just no comparison.  Laythe is over 30 times more massive than the Mun, Tylo over 43 times.  The mass disparity between Ike and Laythe is bigger than the disparity between Kerbin and Jool.

In short:  Tylo and Laythe are totally worth it, but as far as Mun and Ike are concerned... don't bother, IMHO.

[Superfluous J]

I find it's nice to use Mun to help equatorialize (that is TOO a word) my orbit upon returning from interplanetary, but not to slow down. Just fiddle with your interplanetary mid-course correction until leaving Mun's SOI puts you equatorial. If you can swing it, also have it send you in close to Kerbin for the slowdown burn or aerocapture, but that's trickier. Also tricky to do, you can get your Mun Pe really low and then do your Oberth burning there, but as @Snark points out, Oberth is a lot less useful there than at low Kerbin due to Mun's low mass.

[Aegolius13]

17 hours ago, Magzimum said:

 

I am not familiar with a method to compare the two (slingshot vs. low Pe with Oberth), but who not try both?

 

You should just be able to plot manuever nodes from way out and see the total delta-v needed to capture is with various options.... no need to actually test-fly to see what the actual maneuvers will be.  

I too have never found much use for gravity assist captures around Ike or the Mun.  In both cases, it's generally easier to aerobrake your way in unless your ship is really flimsy.  And I've never bothered with lunar gravity assists on ejection -- too hard to keep your course accurate.  

I did, however, find the Mun to be very helpful for getting an already-captured large asteroid much closer to the desired orbit around Kerbin.  Since pushing huge asteroids is so putzy, this saved a lot of time, annoyance and mineable ore.  

[Red Iron Crown]

On 11/29/2016 at 7:51 AM, bewing said:

The Mun is only moving at 540 m/s, so that's about all it can give you.

Minor point of order: The theoretical maximum change from a gravity assist is twice the orbital speed of the body. In practice it's always less than that, but a well timed one can easily exceed the body's orbital speed.

[bewing]

6 hours ago, Red Iron Crown said:

Minor point of order: The theoretical maximum change from a gravity assist is twice the orbital speed of the body. In practice it's always less than that, but a well timed one can easily exceed the body's orbital speed.

Theoretical maximum, yes. But that depends on whether the CB in question has either enough time to give your ship that much acceleration (ie. a very large SOI), or enough mass & density that you can get really really close to the center of the body for some really high Gs. And the Mun doesn't have either. The surface is low G, so it gets in the way and you crash before you can get a high G encounter -- and the SOI is only 4Mm across, so you tend to zip through it pretty quick (1 hr at 1km/s). So even with a retrograde Pe burn, in a practical sense your Munar slingshot is going to max out at about that 540 m/s number.

[Red Iron Crown]

3 minutes ago, bewing said:

Theoretical maximum, yes. But that depends on whether the CB in question has either enough time to give your ship that much acceleration (ie. a very large SOI), or enough mass & density that you can get really really close to the center of the body for some really high Gs. And the Mun doesn't have either. The surface is low G, so it gets in the way and you crash before you can get a high G encounter -- and the SOI is only 4Mm across, so you tend to zip through it pretty quick (1 hr at 1km/s). So even with a retrograde Pe burn, in a practical sense your Munar slingshot is going to max out at about that 540 m/s number.

All depends on your relative velocity with the Mun. If you can position yourself in front of it at a bit under its orbital velocity you can pick up a lot of velocity.

[bewing]

Very true, but this thread was about interplanetary gravity braking, not a gravity boost. If you start off going very slow WRT the Mun, then you can spend a very long time in the SOI and get a very nice boost indeed, agreed.

[Tex_NL]

20 hours ago, Snark said:

I think the scale difference between Tylo/Laythe on the one hand, and Mun/Ike on the other, is just too big-- I've never found it worthwhile to use Mun or Ike for gravity assists.

Just to put things in perspective, here are the masses of the four moons, normalized so that Mun = 1.

• Tylo:  43.37
• Laythe:  30.12
• Mun:  1.00
• Ike:  0.29

There's just no comparison.  Laythe is over 30 times more massive than the Mun, Tylo over 43 times.  The mass disparity between Ike and Laythe is bigger than the disparity between Kerbin and Jool.

In short:  Tylo and Laythe are totally worth it, but as far as Mun and Ike are concerned... don't bother, IMHO.

Ike might be the smallest on this list but it has a huge effect within Duna's SOI. Duna's gravity well is also very shallow and Ike's gravity is more than enough to completely flip your orbit around.
I clearly remember one time I was coming into Duna's SOI at the wrong angle. A most fortunate encounter with Ike did both flip my orbit and capture me in a Duna orbit.

[Snark]

33 minutes ago, Tex_NL said:

Ike might be the smallest on this list but it has a huge effect within Duna's SOI. Duna's gravity well is also very shallow and Ike's gravity is more than enough to completely flip your orbit around.

Oh, absolutely.  Ike can be such a pain when one is trying to perform various orbital maneuvering in near-Duna space.  Its SoI keeps getting in the way, and it has a huge effect on orbits, as you say.

But that's for a craft that's captured to Duna and therefore not moving very fast-- just a few hundred meters per second relative to Duna, at most-- so the effect is dramatic.  The context of this thread, though, is using it for interplanetary maneuvering-- and if a ship is coming into Duna's SoI at interplanetary-intercept speeds, it's really not going to be able to eke much dV out of an Ike flyby.

So mainly the point I was trying to make was:  Ike's really not very useful for gravity-braking on interplanetary maneuvers.

[Tex_NL]

2 minutes ago, Snark said:

Oh, absolutely.  Ike can be such a pain when one is trying to perform various orbital maneuvering in near-Duna space.  Its SoI keeps getting in the way, and it has a huge effect on orbits, as you say.

But that's for a craft that's captured to Duna and therefore not moving very fast-- just a few hundred meters per second relative to Duna, at most-- so the effect is dramatic.  The context of this thread, though, is using it for interplanetary maneuvering-- and if a ship is coming into Duna's SoI at interplanetary-intercept speeds, it's really not going to be able to eke much dV out of an Ike flyby.

So mainly the point I was trying to make was:  Ike's really not very useful for gravity-braking on interplanetary maneuvers.

In my example I came from a transfer from Kerbin to Duna. I still had all my interplanetary speed and by sheer luck I encountered Ike at just the right angle. By skimming Ike's surface I got enough assistance to completely flip my orbit and get captured.

[Snark]

1 minute ago, Tex_NL said:

In my example I came from a transfer from Kerbin to Duna. I still had all my interplanetary speed and by sheer luck I encountered Ike at just the right angle. By skimming Ike's surface I got enough assistance to completely flip my orbit and get captured.

Ah, good point.  I assume you must have had a really good transfer window, so your Duna-relative velocity upon Ike arrival was pretty low?  'Coz if you were coming in with tons of excess velocity, I suspect that wouldn't have made as much difference.

[Tex_NL]

5 minutes ago, Snark said:

Ah, good point.  I assume you must have had a really good transfer window, so your Duna-relative velocity upon Ike arrival was pretty low?  'Coz if you were coming in with tons of excess velocity, I suspect that wouldn't have made as much difference.

Yes. It was a near perfect transfer. The only real issue was coming in on the wrong side.

[juanml82]

Using the Mun to capture works for training missions which skim right outside Kerbin's SOI and head back home right away