Planning gravity assists

[Gravitas Shortfall]

So I understand the general principal behind gravity assists, and can more or less make use of moons to change my velocity in interesting ways, but I can't help but feel my efforts a bit pitiful when compared to what NASA has managed to come up with when faced with a limited budget and, by extension, limited delta-v. The Cassini mission utilized not one, but two gravity assists from Venus, followed by a gravity assist from Earth, and finally one more from Jupiter before continuing on to Saturn. I don't know how I'd even begin to start planning something like that. I'm curious if anyone here has managed anything that roundabout for their missions to the outer planets, and if so, if it was just for the challenge, or because their computer couldn't handle high part numbers for more delta-v, or whatever.

[Just_Testing]

While I unfortunately cannot help you with your quest, I am also extremely interested in how one would go about something like this, and I also very much approve of your username. Good day to you, sir.

[notfruit]

Hehe, you don't plan gravity assists.

[7499275]

You're not the only one... I am faced with a limited part count instead of a limited budget. So I wouldn't mind knowing my self how to plan for even one gravity assists but would love to know how to plan for multiple assists to help propel me to another planet and back home again.

[Vector]

The closest thing I've done is used gravity assist from Eve to change planes and then with a tiny correction get a second encounter with Eve to enter an equatorial orbit. The first encounter was highly inclined because I didn't match Eve's orbital plane (the purpose of the exercise), but the second encounter was equatorial.

The key is that small adjustments prior to the fly-by produce large changes after the fly-by, so I think the general method would be to

1. plan first encounter

2. half-orbit or quarter orbit prior to first encounter, play with maneuver nodes to see what you can reach via slingshot, just trying to hit the SOI for a second encounter or get in the neighborhood

3. After entering first SOI but prior to periapsis, further adjust maneuver nodes and orbit to see if you can hit the SOI for the second encounter

4. After first fly-by, after leaving SOI, further adjust maneuver nodes to see if you can minimize periapsis for your second encounter.

5. Half-orbit or quarter orbit prior to your second encounter, play with maneuver nodes to see what options you have for a third encounter. Repeat from step 2.

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[NeoMorph]

Planning gravity assists is EASY if you use MechJeb. Just load up the landing dialog and if you clip the atmo it shows whether you are going into a aerobrake or a landing manoeuvre and gives you the numbers. My favourite one is to aerobrake around Duna and then flip out to Ike. Use the engines to do coarse course changes and use the RCS for the fine course changes.

[Vector]

Aerobraking and gravity assist are two separate things entirely. But yeah MechJeb is great for predicting results from aerobraking.

[Specialist290]

You can actually use gravity assists to a limited degree for any planet which has a moon of significant mass. I've used a pass around Ike or two to help get an orbiter around Duna into a lower orbit in combination with aerobraking for far less fuel than it would have taken with just a single straight engine burn to change the orbit directly.

As for interplanetary gravity assists, I believe that Arrowstar's Trajectory Optimization Tool has a feature that can help you find a course that will get you from one planet to another using a third as your "slingshot."

[ArmchairGravy]

In this vein, is a Voyager-like alignment of the planets occurring in KSP at some point?

[Gravitas Shortfall]

You can actually use gravity assists to a limited degree for any planet which has a moon of significant mass. I've used a pass around Ike or two to help get an orbiter around Duna into a lower orbit in combination with aerobraking for far less fuel than it would have taken with just a single straight engine burn to change the orbit directly.

As for interplanetary gravity assists, I believe that Arrowstar's Trajectory Optimization Tool has a feature that can help you find a course that will get you from one planet to another using a third as your "slingshot."

Thanks, I'll definitely give that tool a look.

The impression I get from Google is that for truly crazy, Eve-Eve-Duna-Kerbin-Jool-esque multiple assist trajectories, you need to do a lot of really fancy, really time-consuming math that depends on having good candidate trajectories to begin with.

[NeoMorph]

Aerobraking and gravity assist are two separate things entirely. But yeah MechJeb is great for predicting results from aerobraking.

I meant that as you are diving into Duna you are using the gravity to suck you in and whip you out to Ike. As the aerobrake calculations kick in it can help with plotting the orbit to intercept Ike... but yeah, you are right in that they are two separate things.

[Martijn404]

I've used a Gravity Assist to go from Kerbin to Moho, using an Eve encounter along the way. As I wanted to test this out I took some notes on how much delta-V I was using along the way, including course corrections. The gravity assist saved about 2000m/s; only needed about 3000m/s instead of 5000m/s.

I found this launch window by first looking up the ideal Eve to Moho transfer time with the KSP Launch Window planner, and then using this as Eve -arrival time to find my Kerbin-departure time and ejection-angle and speed. I can look up the exact launch day, target eve-arrival day etc if required

[teebob21]

It turns out that in the real world, writing a brute-force algorithm to solve this for multiple planetary intercepts is good enough for both a baccalaureate AND a Master's thesis at Cal Poly: http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1011&context=aerosp

Perhaps a KSP genius can extrapolate from this method to build a multi-intercept mod.

[trevhunsaker]

Im working on a concurrent BS/MS in aerospace engineering at utah state and plan on writing a multi gravity assist solver for my thesis. Keep your eyes peeled. I will probably be distributing it as a mathematica CDF.

[Vector]

I was thinking about trying to generalize and automate the procedure I use manually. First get an intercept, any intercept. Then you have a very narrow range of intercept angles and velocities, but by changing the displacement (using a small adjustment far in advance) you end up with a wide variety (two degrees of freedom) of possible output angles and velocities. Some of these lead to subsequent intercepts, some do not. Right now I search for these by trial and error by tugging on the maneuver nodes for a while, but it would be possible to find all reachable intercepts within a given time horizon. Each of these subsequent intercepts will have an associated intercept angle and velocity and again displacement would be adjustable with small corrections (adjusting even before the first intercept if precision is high enough).

This would ultimately produce a tree of potential intercepts that are reachable with only tiny course corrections, exploiting the chaotic nature of orbits. If your initial intercept has enough velocity, and the body has enough mass, the choices multiply to the point you can get pretty much anywhere for free. What's missing is the program to compute the transport network, but it definitely has me interested.