Slingshots for decelerating

[aluc24]

Hi,

I am a bit confused on how to use slingshots to slow the craft down. Example: I played around with the Jool Aerobrake scenario from the stock game + Outer Planets addon. Using slingshots and just under 1000m/s dV, I visited all the planets in the Kerbol system. Now, I want to get back to Kerbin. I've got an intercept, but the relative velocity is too big for aerocapture. So I'm trying to use multiple slingshots past Kerbin to slow down before finally performing said aerocapture.

The problem is, my vessel orbit around Kerbol isn't just touching Kerbin's orbit, but crossing it an a steep angle. No matter which side of Kerbin I slingshot past, I cannot make this angle less. I already performed 8 slingshots past Kerbin, but I can't seem to slow down enough for aerocapture. Either I raise/lower periapsis and apoapsis, but the angle remains the same.

So my question is, what is the correct procedure to use the same planet and multiple slingshots past it to slow the craft down in relation of said planet? How do I minimize the orbit intersection angle?

Edited October 22, 2016 by aluc24

[JadeOfMaar]

Howdy. The simplest part of gravity assists is that: If you pass a planet, relative to it (with its spin vector) you'll accept its energy and accelerate. If you pass to it (against its spin vector) you'll fight its energy and decelerate. Also, the closer you approach during the encounter, the more you use the Oberth effect and your gravity assist is more powerful.

A very small or burn somewhere far before the encounter should help with that steep angle of approach.

Edited October 22, 2016 by JadeOfMaar

[bewing]

I think most of the answer is: if at all possible, use a different CB from your target for the slingshot. Eve, for example. Or the Mun. It's easier to get a plane change and a better angle if you've got more than one body to work with.

[aluc24]

Thanks for  quick answer, guys. The problem is not the inclination: it is the angle of the orbits on the same plane. For example, when you do the Hohmann transfer, upon arrival, the angle between your craft orbit and the target planet's orbit is zero on the intersect path (in other words, both elipses are tangent to each other at that point). When I do my slingshots, they are never tangent; instead, the angle is big, and that increases relative velocity.

The more slingshots I perform, the larger this angle becomes. Again, I'm not talking about inclination, but the angle between vectors on intersect point.

[aluc24]

Here's two screenshots to better illustrate my meaning: first one is before braking slingshot around Kerbin. Notice how the orbits are almost tangent to each other.The second screenshot is after braking slingshot. The orbit has decreced, but the angle between craft orbit and Kerbin orbit has increased for the next intersection. If I do more slingshots, that angle will increase further, no matter which side of Kerbin I fly past (prograde or retrograde). I want to minimize that angle. How do I do that?

Edited October 22, 2016 by aluc24

[bewing]

You don't want to minimize that angle. You want to maximize it. You want to come in at a 80 degree angle, do a close flyby in front of Kerbin, and leave at an 89 degree angle. The closer to 90 degress on exit, the more energy you lose.

Edited October 22, 2016 by bewing

[aluc24]

Here's one more screenshot after another slingshot: notice how the orbit has reduced drastically, however, the periapsis has dropped well below Kerbin's orbit, and apoapis is still too high. Whatever angle I set for the next slingshot (even if it goes through the Kerbin's core) won't provide an orbit that would match Kerbin's. It appears that no matter how I slingshot, I cannot reduce relative velocity to Kerbin any more. I can move periapsis and apoapsis up and down, but I can't reduce eccentricity.

 

3 minutes ago, bewing said:

You don't want to minimize that angle. You want to maximize it. You want to come in at a 80 degree angle, do a close flyby in front of Kerbin, and leave at an 89 degree angle. The closer to 90 degress on exit, the more energy you lose.

I'm not sure I understand you... I may have mis-phrased what I meant before: I want to reduce my orbit's eccentricity during slingshots around Kerbin until the vessel orbit matches Kerbin's orbit.

[bewing]

That can never work. Once you suck the energy out of your orbit and get into an eccentric lower orbit, then you engineer a close approach and do an aerobraking pass to capture. But gravity slingshots work on the relative difference in velocity between you and the target CB. You are asking for a zero relative velocity, which would take an infinite number of passes.

[aluc24]

1 minute ago, bewing said:

That can never work. Once you suck the energy out of your orbit and get into an eccentric lower orbit, then you engineer a close approach and do an aerobraking pass to capture. But gravity slingshots work on the relative difference in velocity between you and the target CB. You are asking for a zero relative velocity, which would take an infinite number of passes.

Okay, I see. So what do I do if the velocity is still too high for aerocapture? How do I minimize periapsis velocity during next pass at Kerbin?

[bewing]

10 minutes ago, aluc24 said:

Okay, I see. So what do I do if the velocity is still too high for aerocapture? How do I minimize periapsis velocity during next pass at Kerbin?

That's what the Mun is for.  One last gravity slingshot past that will get you captured into Kerbin's SOI -- which should always be slow enough for an aerobraking pass.

[aluc24]

Just now, bewing said:

That's what the Mun is for.  One last gravity slingshot past that will get you captured into Kerbin's SOI -- which should always be slow enough for an aerobraking pass.

Just tried, and it worked! Thanks a lot. I know now what is to be and not to be expected from slingshots

[Aegolius13]

I did a bunch of same-body gravity assists on a recent SSTO mission - pics at: http://imgur.com/a/Eztxs.  There is also a Kerbin accelerating gravity assist that works on similar princple, but backwards.

In my experience, you can't get that perfectly tangent final approach for free - but it can be cost-effective to do some adjustment burns so you do get it.  This was the first mission where I was able to aerocapture at Laythe without burning up in a non-shielded craft. 

There seem to be three keys.  The first is that, as @bewing says, you dont' want to be perfectly tangent in your assist pass.  But you want to be reasonably close to tangent so your second pass will also be reasonably close to tangent.  Seems to be a trial-and-error thing to find a happy medium.

Second, you want to come out of the assist with a nearly resonant orbit to that of the body you're working with, so you will encounter it again at about the same place.  The simplest way to do this is to approximately double the orbital period.  So with Kerbin, you would want an orbital period of ~2 years.  Hopefully you don't need to do a full powered assist to achieve this - you should be able to tweak your gravity assist altitude, and hence the power of the assist, with small burns well in advance.  

Third, following the assist, you need to do a little burn at the -apsis of your solar orbit opposite Kerbin (if you're coming from Jool or somewhere else in the outer system, this will be your apoapsis).  This might cost a couple hundred delta-v, but it seemed worth it to slow down your relative speed at capture.  

Now the tricky part is combining the strength of the assist and the amount of the burn to get that perfect tangent intercept.  Again this just takes some trial and error.  Before you get to the first assist, I'd suggest laying both sets of maneuver nodes until you get something that works for you.

 

Edited October 23, 2016 by Aegolius13