Kerbin-Kerbin Slingshot?

[eddiew]

So... I had a thought in the shower last night but I won't get chance to test it until tomorrow.

If you launched precisely 2 years ahead of your transfer window to Jool/Eeloo/modded-outer-planet, and put yourself on a solar orbit with exactly 2 years period, you should re-encounter Kerbin again at the time you should be launching. Moreover, you will be tangential to Kerbin's orbit, just like you would be when launching out of LKO.

Does this actually produce a gravity slingshot, or do you end up doing a flyby at LKO altitude at exactly the same speed you originally left it? My tiny mind cannot seem to grasp this one 

(I'm playing with OPM and Kerbol Plus Remade, so the distance to the outer solar system is considerable and I'm looking for any advantages I can gain to get low-tech probes out there, but ad-hoc flyby planning tools don't seem to exist.)

[bewing]

AFAIK, this will indeed provide a gravity slingshot -- but because there is a slingshot, your speed will increase and you won't be tangential anymore. So I think the orbit has to be a little less than two years, and hopefully the computer can figure out all the complexities of it for you, because I sure can't.

[GoSlash27]

eddiew,

 Unfortunately, it will give you a velocity change, but not the kind you want.

 It would *reduce* your speed in the inertial plane, bringing your Pe closer to the sun.

Think of it like bouncing a tennis ball off the back of a truck that's moving away from you. If you could get another encounter after that, it would have the opposite effect (like tossing a tennis ball out in front of a truck), but that would only return you to where you started.

Unless you pinball off of other planets, you've already got all of the velocity you can get from Kerbin.

Best,
-Slashy

Edited May 6, 2016 by GoSlash27

[eddiew]

Lol, well, that's two mutually exclusive answers, cheers chaps   Maybe I just need to try it out and see  

[GoSlash27]

12 minutes ago, eddiew said:

Lol, well, that's two mutually exclusive answers, cheers chaps   Maybe I just need to try it out and see  

eddiew,

 I'd recommend doing it from low Munar orbit. The Mun can be an analogue for Kerbin and Kerbin an analogue for the sun. It'd take a lot less time and effort.

Best,
-Slashy

 

[eddiew]

That's rather a good suggestion actually @GoSlash27... I will try. May already have some munar probes in place, I can just plot the manoeuvres on them  

I do feel like your truck analogy makes sense... Burning in LKO might result in 3500m/s half a minute later, but by the time you get to the edge of Kerbin's SoI it will be less. Two years later, entering the SoI again you'll do it at the same speed you left, gain some speed as you fall down to LKO, but only as much as you lost on the way up to the SoI boundry.

In my mind, that seems sensible ^^;

Edited May 9, 2016 by eddiew
was wrong

[Blaarkies]

What effects will a normal/anti-normal component introduce to this?
>Leave Kerbin with some normal component, so that the probe ends up in a Solar orbit with a resonance to Kerbin.
-Next encounter will be fom the top/bottom of Kerbin's SOI

I read a lot about Eve being exceptionally useful for slingshots because of it's inclined orbit. Is it just the inclination difference relative to probe and planet, or MUST the planet have an inclined orbit?

[SnakyLeVrai]

1 hour ago, GoSlash27 said:

eddiew,

 Unfortunately, it will give you a velocity change, but not the kind you want.

 It would *reduce* your speed in the inertial plane, bringing your Pe closer to the sun.

Think of it like bouncing a tennis ball off the back of a truck that's moving away from you. If you could get another encounter after that, it would have the opposite effect (like tossing a tennis ball out in front of a truck), but that would only return you to where you started.

Unless you pinball off of other planets, you've already got all of the velocity you can get from Kerbin.

Best,
-Slashy

 

Doesn't it depend on whether you get your assist from inside or outside Kerbin ? (inside being between Kerbol and Kerbin)

If you get your assist while being "higher than Kerbin" in respect with Kerbol, you should accelerate. Better yet, having your assist with Kerbin rather than with the Mun should have a stronger effect, as Kerbin's gravitational attraction is stronger, as long as you get it from very close (like between 75-100km).

Found this (look at Kerbin gravity assist 1 year after launch) :

Spoiler

 

 

[Plusck]

1 hour ago, SnakyLeVrai said:

 

Doesn't it depend on whether you get your assist from inside or outside Kerbin ? (inside being between Kerbol and Kerbin)

If you get your assist while being "higher than Kerbin" in respect with Kerbol, you should accelerate.

[...]

 

It doesn't matter so much whether you're inside or outside, but rather whether you first pass behind or in front of the planet after entering its SOI, and whether you leave the SOI with more or less of an angle relative to Kerbol.

If you're on a 2-year orbit, your velocity at Pe at Kerbin will be faster than Kerbin, so you will necessarily enter the SOI behind Kerbin. That means you either pass in front of Kerbin and are ejected behind, therefore resulting in less orbital velocity than Kerbin, or you pass first behind Kerbin then in front of it, and in this case you'll only gain velocity if your path deviates so little that you are ejected with less of an angle relative to prograde than when you entered the SOI.

So it's possible to get multiple slingshots from a planet, but only if your incoming angle is significant enough. Frankly, I don't know how they managed this for Rosetta.

 

[Red Iron Crown]

5 hours ago, GoSlash27 said:

eddiew,

 Unfortunately, it will give you a velocity change, but not the kind you want.

 It would *reduce* your speed in the inertial plane, bringing your Pe closer to the sun.

Think of it like bouncing a tennis ball off the back of a truck that's moving away from you. If you could get another encounter after that, it would have the opposite effect (like tossing a tennis ball out in front of a truck), but that would only return you to where you started.

Unless you pinball off of other planets, you've already got all of the velocity you can get from Kerbin.

Best,
-Slashy

I don't often disagree with you, Slashy, but I have to here. A gravity assist can add or remove energy from an orbit, depending on whether the craft passes in front of or behind the body. 

Further, I have seen multiple gravity assists routes used effectively in KSP, most notably @PLAD's 1km/s route from Kerbin to Jool. Going from memory it goes from Kerbin to Eve, then Kerbin, then Kerbin again on its way to Jool.

@eddiew Unfortunately, it's not as simple as arriving back at Kerbin when there would be a traditional transfer window. The math is a bit beyond me, but there is an excellent external tool for calculating such trajectories, again by PLAD. You can find it here: 

 

[eddiew]

Thanks @Red Iron Crown - I was aware of Flyby Finder, but I don't think it can help me with modded planets ^^

I seem to have sparked some contention here xD  

If I wasn't very clear in my opener, I'm not thinking of burn from Kerbin then leave it purely ballistic for 2 years. I'm thinking get to your AP around the sun, then make a small adjustment (probably only a few m/s) so that you come back to Kerbin from 'behind' as you would when using, say, Duna or Eve to push you outwards. What I can't wrap my mind round is whether this breaks conservation of momentum. You launched from Kerbin in the first place, it seems wrong that you can extract infinite velocity from looping back to it over and over...

[p1t1o]

The Juno probe apparently achieved a 3.9km/s slingshot around Earth on its way to Jupiter after travelling for 2 years, seems to fits the OP's question pretty well. For a minute there I thought it was due to arrive tomorrow, but its that blasted american dating system...

 

[Red Iron Crown]

@eddiew In real life, momentum is conserved because the planet's orbital speed is changed by an infinitesimal amount when a slingshot occurs. In KSP this tiny effect is ignored and the planet's orbit is unchanged (which is fine IMO, we're talking a very, very tiny effect). 

[eddiew]

23 minutes ago, p1t1o said:

The Juno probe apparently achieved a 3.9km/s slingshot around Earth on its way to Jupiter after travelling for 2 years, seems to fits the OP's question pretty well. For a minute there I thought it was due to arrive tomorrow, but its that blasted american dating system...

That's interesting, cheers. I hadn't found an example of Earth-Earth slingshotting before! Found this nice little video for confirmation:

https://www.missionjuno.swri.edu/earth-flyby

 

8 minutes ago, Red Iron Crown said:

@eddiew In real life, momentum is conserved because the planet's orbital speed is changed by an infinitesimal amount when a slingshot occurs. In KSP this tiny effect is ignored and the planet's orbit is unchanged (which is fine IMO, we're talking a very, very tiny effect). 

Ok, yeah... I guess several trillion tons of planet vs a 1 ton probe is going to have several trillion times the momentum  

[herbal space program]

Yes, this is totally doable OP. If you set it up so that you encounter Kerbin at your own PE, you will raise or lower your AP in the same spot, depending if you fly outside (lower) or inside (higher) of Kerbin. on your orbit. If you encounter it before or after your PE, you can also change the argument of periapsis of your orbit and raise/lower your own PE inside/outside of Kerbin's orbit. As RIC mentioned, PLAD's famous cheap route to Jool culminates in two sequential Kerbin encounters set up exactly as you said.

[Plusck]

3 hours ago, herbal space program said:

... depending if you fly outside (lower) or inside (higher) of Kerbin...

You're the second one to talk about "inside" or "outside", and I'm just not seeing where this idea comes from.

Checking PLAD's Jool-5 course (1 assist from Eve, 2 from Kerbin), PLAD approached Eve from the "inside" and Kerbin from the "outside" - apparently both times. However, it seems pretty irrelevant whether its inside or outside: what matters is that the angle on leaving the SOI is less than the angle on entering, and that you exit going as close to purely prograde as possible (which means passing behind the planet, wherever you come from).

It sounds a bit like this inside/outside question is a "thing" - so what am I missing here? Is there a thread about this? Is PLAD's Jool-5 route not what you're talking about?

[herbal space program]

2 hours ago, Plusck said:

You're the second one to talk about "inside" or "outside", and I'm just not seeing where this idea comes from.

Checking PLAD's Jool-5 course (1 assist from Eve, 2 from Kerbin), PLAD approached Eve from the "inside" and Kerbin from the "outside" - apparently both times. However, it seems pretty irrelevant whether its inside or outside: what matters is that the angle on leaving the SOI is less than the angle on entering, and that you exit going as close to purely prograde as possible (which means passing behind the planet, wherever you come from).

It sounds a bit like this inside/outside question is a "thing" - so what am I missing here? Is there a thread about this? Is PLAD's Jool-5 route not what you're talking about?

I think there are two things getting confused here and also a problem with what "behind" and "in front of" mean. What I'm talking about is if you are passing Kerbin on the side closer to the sun (inside) or farther from the sun (outside). If you are approaching the planet from retrograde, as you would be in a 2:1 resonant orbit you established from Kerbin in the first place, you want to pass it on the side closer to the sun, deviating your orbital path outwards. In the case of the PLAD K-E-K-K-J route, you are encountering Kerbin on an orbit that goes inside of Kerbin's, before your PE. In that case, the equivalent maneuver has you coming from the outside and again passing on the inside of Kerbin's orbit so that you get ejected essentially prograde on Kerbin's orbit. In both cases the result is to raise your Apoapsis, but in different places relative to where you eject from Kerbin.

Edited May 7, 2016 by herbal space program

[ShotgunNinja]

It is simpler to imagine a gravity assist as a change of frame of reference, so if you pass inside Kerbin SOI 'from the side its rotation is in your direction' you will accelerate, likewise you will decelerate if you pass 'on the side its rotation is opposite to your direction'.

 

EDIT:

Imagine a big whirpool at the center of a pool, and a smaller one rotating around it. Both are rotating on themselves as whirpool does.
Now imagine a dot in the pool, carried by the circular current of the big whirpool. Try to figure out what happen if the dot approach the small whirpool:

- if it pass on the side the small whirpool is rotating, it will accelerate
- if it pass on the opposite side instead, it will decelerate
- the nearest it pass to the center of the small whirpool, the bigger the effect

To summarize, space-time is water and gravity is the current.

Edited May 7, 2016 by ShotgunNinja

[Plusck]

5 hours ago, herbal space program said:

I think there are two things getting confused here and also a problem with what "behind" and "in front of" mean. What I'm talking about is if you are passing Kerbin on the side closer to the sun (inside) or farther from the sun (outside). If you are approaching the planet from retrograde, as you would be in a 2:1 resonant orbit you established from Kerbin in the first place, you want to pass it on the side closer to the sun, deviating your orbital path outwards. In the case of the PLAD K-E-K-K-J route, you are encountering Kerbin on an orbit that goes inside of Kerbin's, before your PE. In that case, the equivalent maneuver has you coming from the outside and again passing on the inside of Kerbin's orbit so that you get ejected essentially prograde on Kerbin's orbit. In both cases the result is to raise your Apoapsis, but in different places relative to where you eject from Kerbin.

OK, I'm gonna have to go sandbox a few runs because I'm pretty sure the inside/outside question is incorrect. You don't want to deviate your path outwards because that would be equivalent to a radial burn. It seems to me that you want to hit the SOI off-centre and exit prograde, and that's it...

2 hours ago, ShotgunNinja said:

It is simpler to imagine a gravity assist as a change of frame of reference, so if you pass inside Kerbin SOI 'from the side its rotation is in your direction' you will accelerate, likewise you will decelerate if you pass 'on the side its rotation is opposite to your direction'.

 

EDIT:

Imagine a big whirpool at the center of a pool, and a smaller one rotating around it. Both are rotating on themselves as whirpool does.
Now imagine a dot in the pool, carried by the circular current of the big whirpool. Try to figure out what happen if the dot approach the small whirpool:

- if it pass on the side the small whirpool is rotating, it will accelerate
- if it pass on the opposite side instead, it will decelerate
- the nearest it pass to the center of the small whirpool, the bigger the effect

To summarize, space-time is water and gravity is the current.

Wait no. There is no aether here.

Again, I'm gonna have to go sandbox this and come back to you.

[FullMetalMachinist]

@Plusck I was able to wrap my head around gravity assists thanks to this article/blog post.  It's a little long, but a good read.

Basically, if your orbit after the assist is more like (i.e. you approach with your orbit lines at an angle and leave closer to tangent) the orbit of the assisting body after the assist, it will increase your velocity.  If your orbit is less like the assisting body's orbit after the assist, it will reduce your velocity.  

Edited May 7, 2016 by FullMetalMachinist

[GoSlash27]

 I've been trying to wrap my head around how this is working, and I'm stumped.

My understanding of gravity assists:

The most velocity I would expect to see from overrunning a body is exactly the same as when you started (as if it never happened). Anything else gives a radial change and a reduction in total velocity. My understanding is that you have to come in from the front or side to actually see a velocity boost.

Paging @OhioBob ... What am I missing here?

I can see how if you came in a little crooked and left precisely prograde how you could get a *very* minor boost, but other than that...?

Best,
-Slashy

 

*edit*

40 minutes ago, FullMetalMachinist said:

Basically, if your orbit after the assist is more like (i.e. you approach with your orbit lines at an angle and leave closer to tangent) the orbit of the assisting body after the assist, it will increase your velocity.  If your orbit is less like the assisting body's orbit after the assist, it will reduce your velocity.  

Yeah, that's consistent with my understanding. So for a Kerbin/ Kerbin slingshot, I would expect the ship to arrive precisely tangent (because it left precisely tangent), and thus only a velocity reduction is possible. So how are people gaining velocity from a single body in multiple passes?

 

Edited May 7, 2016 by GoSlash27

[Red Iron Crown]

@GoSlash27 The trick is to not have your velocity vector be parallel to the planet's when you encounter it. I dug up the album of PLAD's Jool route, have a look here: http://imgur.com/a/13tuj

[Plusck]

43 minutes ago, FullMetalMachinist said:

@Plusck I was able to wrap my hear around gravity assists thanks to this article/blog post.  It's a little long, but a good read.

Basically, if your orbit after the assist is more like (i.e. you approach with your orbit lines at an angle and leave closer to tangent) the orbit of the assisting body after the assist, it will increase your velocity.  If your orbit is less like the assisting body's orbit after the assist, it will reduce your velocity.  

Absolutely - that's what I've been saying all along. No problem with that.

I have a problem with saying that you have to pass "inside" or "outside" - to me the only thing that counts is that you end up more prograde on exit. Passing inside or outside is irrelevant, passing behind or in front of the planet (because this will determine whether you end up more prograde or less) is the only thing that counts.

[FullMetalMachinist]

8 minutes ago, Plusck said:

Absolutely - that's what I've been saying all along. No problem with that.

I have a problem with saying that you have to pass "inside" or "outside" - to me the only thing that counts is that you end up more prograde on exit. Passing inside or outside is irrelevant, passing behind or in front of the planet (because this will determine whether you end up more prograde or less) is the only thing that counts.

Ah, yes, then I misunderstood what you were having difficulties with. And I agree, there is little point to using "inside" or "outside" to describe where or how you get an assist. 

[GoSlash27]

3 minutes ago, Red Iron Crown said:

@GoSlash27 The trick is to not have your velocity vector be parallel to the planet's when you encounter it. I dug up the album of PLAD's Jool route, have a look here: http://imgur.com/a/13tuj

RIC,

 The way I see it, the second Kerbin encounter is actually correcting the inefficiency of the first Kerbin encounter. I understand why it would have to be done that way in order to to sync the encounters with the bodies (no sense in boosting for Jool when Jool isn't there yet), but he would've entered the same exact orbit had he left the first Kerbin flyby tangent... right? So in that sense, the second flyby didn't "gain" anything, it just restored what could've been gained in the first pass, but wasn't.

 Or am I missing something?

Best,

-Slashy