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Gravity Assist Magic


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4 minutes ago, Spricigo said:

It depends more in how much time (both in game  and palyer time) you are willing to invest in the jorney. 

In-game time is no object. Player time...well, I don't want to spend TOO long.

By my numbers, I should be able to get to Duna (2,550 m/s) and from there to Ike, without needing a gravity assist. However, I wouldn't even know where to begin to try and do a semi-grand tour of anywhere else. With 2.9 km/s of dV, I know that Laythe, Tylo, and Moho are out of the question, but I don't know about Dres or Eeloo. I should be able to do Gilly easily enough, but I don't know whether I should try to use it for anything else. If I get into the Jool system, I can hit Pol, Bop, and Vall easily enough, but I don't know whether I can actually make it there.

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Jool should be easy enough if you set it up right ... you're going to want to use another craft to plan the burn, though.  The key is to drop back towards Kerbin at the right point in your orbit and make your burn at a much lower periapsis.

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27 minutes ago, Kryxal said:

Jool should be easy enough if you set it up right ... you're going to want to use another craft to plan the burn, though.  The key is to drop back towards Kerbin at the right point in your orbit and make your burn at a much lower periapsis.

Should I burn for Jool from Gilly, from Minmus, or from Ike? Or even from Dres?

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1 hour ago, sevenperforce said:

Should I burn for Jool from Gilly, from Minmus, or from Ike? Or even from Dres?

If you're heading for Jool, you can do my favorite move "the Minny drop". When Minmus is between Kerbin and the sun, right around the time of a Jool window (you can go a week or so late if you need to; Jool's huge SOI is very forgiving), burn retrograde to lower your Kerbin Pe to about 70-75km. Then, when you reach Pe, burn for Jool. I think it costs about 1,200m/s. Combine that with the free capture off of Laythe or Tylo and you've got plenty left for maneuvers.

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Something that is not so difficult and not so time consuming is, depart from minmus, tranfer to Eve and slingshot towards the outer planets. 

I mean, you said that you are 'not so great' with gravity assists but if you at least know the basics is quite doable. Specialy if you use some tool like flyby finder or TOT to help you. 

 

Gilly can be an usefull refuelling point in your way to moho, but I don't see much point if you destinatioin is somewhere

else.

 

 

 

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15 hours ago, sevenperforce said:

Should I burn for Jool from Gilly, from Minmus, or from Ike? Or even from Dres?

It's been said twice already, but wth... From Minmus, drop to a low PE near Kerbin, then do the transfer burn there. You want to use another craft in LKO to plan a bog-standard transfer, just so you know where your PE has to be.

That way, you spend about 400m/s to get from Minmus surface to Kerbin PE, and another ~1200m for the transfer burn to Jool. With more than 1km/s left in the tank, a straight Vall landing might be doable without any gravity assistery at all... barely... certainly if you pass Tylo and collect 500m/s. If your craft can refuel on Laythe, going there first might be easiest.

Not sure if 3km/s is enough to handle Moho even by way of Gilly, though. Anything else should be doable, though.

 

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3 minutes ago, Laie said:

It's been said twice already, but wth... From Minmus, drop to a low PE near Kerbin, then do the transfer burn there. You want to use another craft in LKO to plan a bog-standard transfer, just so you know where your PE has to be.

That way, you spend about 400m/s to get from Minmus surface to Kerbin PE, and another ~1200m for the transfer burn to Jool. With more than 1km/s left in the tank, a straight Vall landing might be doable without any gravity assistery at all... barely... certainly if you pass Tylo and collect 500m/s. If your craft can refuel on Laythe, going there first might be easiest.

Not sure if 3km/s is enough to handle Moho even by way of Gilly, though. Anything else should be doable, though.

Is Minmus-to-Jool-via-Kerbin a better option than Ike-to-Jool-via-Duna or Gilly-to-Jool-via-Eve? I plan on going to Gilly, Ike, and Duna anyway, so I'm not losing anything by picking the most efficient one.

Duna has less of a gravity well for Oberth assistance, and I lose more dV getting off Ike than I would getting off Minmus, but the actual Jool transfer burn is smaller. In contrast, Eve has the nicest gravity well of the three, and getting off Gilly is essentially free, but the Jool transfer burn is much larger there.

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5 minutes ago, sevenperforce said:

Is Minmus-to-Jool-via-Kerbin a better option than Ike-to-Jool-via-Duna or Gilly-to-Jool-via-Eve?

If you plan on visiting all these places anyway, and will refuel at every stop, I suppose that dV is not a suitable metric for efficiency. I'd say you want to line up your stops so that you can get to your next destination without worries, with frequency of launch windows being a secondary but worthwhile consideration.

Eve-Duna-Jool should work just fine. Or, if you want to touch all possible bases, perhaps Dres-Eeloo-Duna-Jool-Eve (with one  Eve gravity assist before you capture on the next round).

 

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8 minutes ago, sevenperforce said:

Duna has less of a gravity well for Oberth assistance, and I lose more dV getting off Ike than I would getting off Minmus, but the actual Jool transfer burn is smaller. In contrast, Eve has the nicest gravity well of the three, and getting off Gilly is essentially free, but the Jool transfer burn is much larger there.

sevenperforce,

 The gravity well is a double- edged sword, so don't rely on it as an indicator of which way to go.

The most DV efficient transfers aren't necessarily at the bottom of a strong gravity well. They are where the gravity losses and Oberth effect are balanced; the gate orbit.

Eve -> Kerbin gate 22 Mm
Eve -> Duna gate 4.4 Mm
Eve -> Jool gate 615 km

Gilly resides at 29.8 Mm, so it's not worth it to dive back from there when going to Kerbin.

Kerbin -> Eve gate 11 Mm (roughly Mun's altitude)
Kerbin -> Duna gate 7.8 Mm (roughly Mun's altitude)
Kerbin -> Jool gate 360 km (LKO)

Duna -> Eve gate -47 km
Duna -> Kerbin gate 56 km
Duna -> Jool gate -14 km

A drop is recommended for all destinations from Ike.

Jool -> Eve gate 127 Mm (closest moon is Bop)
Jool -> Kerbin gate 177 Mm (closest moon is Pol)
Jool -> Duna gate 319 Mm (well above all moons)

You should never dive down from Joolian moons.

Best,
-Slashy

 

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6 minutes ago, GoSlash27 said:

sevenperforce,

 The gravity well is a double- edged sword, so don't rely on it as an indicator of which way to go.

The most DV efficient transfers aren't necessarily at the bottom of a strong gravity well. They are where the gravity losses and Oberth effect are balanced; the gate orbit.

Eve -> Kerbin gate 22 Mm
Eve -> Duna gate 4.4 Mm
Eve -> Jool gate 615 km

Gilly resides at 29.8 Mm, so it's not worth it to dive back from there when going to Kerbin.

Kerbin -> Eve gate 11 Mm (roughly Mun's altitude)
Kerbin -> Duna gate 7.8 Mm (roughly Mun's altitude)
Kerbin -> Jool gate 360 km (LKO)

Duna -> Eve gate -47 km
Duna -> Kerbin gate 56 km
Duna -> Jool gate -14 km

A drop is recommended for all destinations from Ike.

Jool -> Eve gate 127 Mm (closest moon is Bop)
Jool -> Kerbin gate 177 Mm (closest moon is Pol)
Jool -> Duna gate 319 Mm (well above all moons)

You should never dive down from Joolian moons.
 

Perfect! This is what I needed.

Just so I understand it properly: if the gate orbit is near or above a given moon, you shouldn't drop; if the gate orbit is substantially below a given moon, you should drop?

What's going to give me the biggest margin for capture at Jool: Gilly-Eve-Jool, Minmus-Kerbin-Jool, or Ike-Duna-Jool? Given that all three recommend drops.

Also, what's the gate orbit for Jool to Eeloo? Should I try to go direct from a Joolian moon to Eeloo, or would I be better off leaving from Dres or Ike and getting a Jool assist?

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6 minutes ago, sevenperforce said:

Just so I understand it properly: if the gate orbit is near or above a given moon, you shouldn't drop; if the gate orbit is substantially below a given moon, you should drop?

sevenperforce,
 Exactly. And the DV savings from gate orbits can be amazing when you're able to utilize them. Especially from low orbit of a substantial moon.

9 minutes ago, sevenperforce said:

What's going to give me the biggest margin for capture at Jool: Gilly-Eve-Jool, Minmus-Kerbin-Jool, or Ike-Duna-Jool? Given that all three recommend drops.

Sorry, I don't understand what you mean by "biggest margin for capture".

 

 

10 minutes ago, sevenperforce said:

Also, what's the gate orbit for Jool to Eeloo? Should I try to go direct from a Joolian moon to Eeloo, or would I be better off leaving from Dres or Ike and getting a Jool assist?

gate orbit from Jool to Eeloo is 7.8 Gm; well outside it's SoI. The rest of it I can't answer, since gravity assists are too varied to calculate without a kinematic model.

Best,
-Slashy

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I'm currently thinking of going from Minmus to Kerbin flyby to Eve flyby to Gilly, then refuel. Gilly to Eve flyby to Duna, then refuel. Duna to Ike, then refuel. Ike to Duna flyby to Dres, then refuel. Dres to Jool system, hit all the moons I can, refuel. Jool to Eeloo, refuel, then head home. I can't do direct entry at Kerbin since I don't have chutes or heat shields, so that last bit might be tricky.

8 minutes ago, GoSlash27 said:
24 minutes ago, sevenperforce said:

What's going to give me the biggest margin for capture at Jool: Gilly-Eve-Jool, Minmus-Kerbin-Jool, or Ike-Duna-Jool? Given that all three recommend drops.

Sorry, I don't understand what you mean by "biggest margin for capture".

Most remaining dV. In case I fudge something up, or can't find a capture assist.

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47 minutes ago, sevenperforce said:

Most remaining dV.

Okay, I ran the numbers. Takes a while, since I have yet to make a single unified spreadsheet for this and have to do some of it by hand.
 Here's what the ejections to Jool would cost:

#1) Minmus->Kerbin->Jool  1,345 m/sec
#2) Ike->Duna->Jool  1,575 m/sec
#3)Gilly->Eve->Jool  1,672 m/sec

*EDIT* dammit, forgot to include to DV to orbit from the surface. Corrected now.

These do not take into account the DV for inclination correction at midcourse.
Best,
-Slashy

Edited by GoSlash27
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15 minutes ago, GoSlash27 said:

Okay, I ran the numbers. Takes a while, since I have yet to make a single unified spreadsheet for this and have to do some of it by hand.
 Here's what the ejections to Jool would cost:

#1) Minmus->Kerbin->Jool  1,345 m/sec
#2) Ike->Duna->Jool  1,575 m/sec
#3)Gilly->Eve->Jool  1,672 m/sec

*EDIT* dammit, forgot to include to DV to orbit from the surface. Corrected now.

These do not take into account the DV for inclination correction at midcourse.
Best,
-Slashy

I have not attempted midcourse inclination corrections before...I understand the principle, of course, but how do I plan the first maneuvering node without targeting an actual intercept?

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4 minutes ago, sevenperforce said:

I have not attempted midcourse inclination corrections before...I understand the principle, of course, but how do I plan the first maneuvering node without targeting an actual intercept?

Well, KSP allows you to place multiple maneuver nodes and twiddle them for the intercept. If you don't want to do the math yourself, you can get the transfer window and angles from AlexMoon. Find the transit time for the drop so you don't arrive late, and begin when the moon is optimally placed opposite the burn angle.
 This is why long drops are kind of a PITA :(

Best,
-Slashy

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