Using gravity of Eve to get to Moho easier.

[thrashing mad]

I can perform Mun->Minmus gravity assist just fine, but is it possible/feasible to use this technique with Eve->Moho? Real life Mariner 10 probe, en route to Mercury, used Venus' gravity this way after all.

Any clues? It doesn't have to be precise, first approach maneuver since Kerbin, Eve and Moho would have to be in a specific position relative to each other I think. I'm just wondering how to lower my solar orbit to Moho level, by using Eve flyby. Or in other words - how to get to Moho cheaper by using Eve flyby?

[leax256]

This could work. It would need to be timed perfectly though.

[Jason Patterson]

You'd either need to calculate the right launch window to make it happen in one go, or you could launch and get your gravity brake at Eve and set your solar periapsis to the correct altitude. You'd then need to make an adjustment burn at periapsis to get your Moho encounter. It's that last bit that can be problematic, since the burn is going to be costly at those speeds.

I haven't done the math or anything, but I would be surprised if you could save much delta-v with a multiple burn approach. I've got no good idea for how to calculate the launch window times for a one shot mission.

In any case, if you want to play with it, you'd just need to set up an Eve encounter and play around with the maneuver node until you get an orbit (post Eve encounter) that is sufficiently low. If you do it early enough in the process there shouldn't be much difference between a gravity brake and gravity assist as far as your initial launch goes.

[thrashing mad]

OK, I've done some testing.

Launched same probe from Kerbin's orbit (using quickload)

First up - direct transfer to Moho:

Next - transfer to Eve first:

Almost there - performed maneuver to bring Pe closer to the planet, and at the correct side of it:

It appears that it lowered my new solar orbit with mere 100m/s delta V:

One more smallish burn planned at Periapsis:

Worked great - with tiny loss of fuel I have my orbit intersecting that of Moho:

And after two solar orbits, and another minor maneuver, I have Moho encounter:

So yeah - it seems to work. With direct transfer I had 79,9 fuel left, with gravity assist 91,8. Not a large amount of saved fuel, but I think that with more efficient maneuvering effects could be even better. Also, with larger lander/return missions this might be quite significant.

[tavert]

Where you hit Moho in its orbit makes a big difference. One thing I've seen a few people do to good effect is using the Eve gravity assist mostly for the plane change (best to hit Eve when it's at a node relative to Moho). And Moho's period is so quick, it likely won't take too long to get the Moho encounter. You also don't have to do all of the braking while you're in Moho's SoI (though you'll lose a bit of Moho-Oberth effect if you don't), you can bring your apoapsis down a bit with a node placed when you cross Moho's orbit even if Moho isn't there at the same time. You can easily get an encounter by tuning the intermediate orbit period, and this first phasing maneuver will save you on the Moho capture burn.

[maackey]

Ideally you'd have Eve be at/near Moho's descending/ascending node by the time you get captured, get into a polar orbit/periapse/flyby and boost yourself up/down depending on where you are relative to Moho's inclination. To time it, you could see how long it takes to get to eve from kerbin, measure the distance, and wait until Eve is approximately the same distance from one of Moho's inclination nodes.

I've never tried it, this is purely theoretical, but it should work.

edit: ninja'd to hell, I'm super slow at posting

[bigpapabearxx]

I can perform Mun->Minmus gravity assist just fine, but is it possible/feasible to use this technique with Eve->Moho? Real life Mariner 10 probe, en route to Mercury, used Venus' gravity this way after all.

Any clues? It doesn't have to be precise, first approach maneuver since Kerbin, Eve and Moho would have to be in a specific position relative to each other I think. I'm just wondering how to lower my solar orbit to Moho level, by using Eve flyby. Or in other words - how to get to Moho cheaper by using Eve flyby?

It is very inefficient to use the moons as a gravity assist. Scott made it to Jool with gravity assists and save about 1000m/s dv.

[Diche Bach]

ADDIT: watched the Scott video linked above and that did clarify a lot of my questions, but still would love to see more discussion of these issues.

How do the real life astrophysicists determine these things? For example, who figured out that the "Grand Tour" alignment that allowed Voyager 1 and 2 would exist and that the missions needed to when they did?

I understand the basic premise of a gravity assist and have managed to bumble my way through a few in game; a couple of which actually seemed to help my velocity quite a bit. However, the math for describing one of these interactions is beyond me.

1. Is it possible to 'master' the art of gravity assists using mostly intuition or is it really necessary to get into the maths?

2. Also, how does one use the in game map or other tools (mods? addons?) to judge when celestial bodies are in good alignment for a series of fly bys?

3. I've looked around for discussions on the concepts of planning and executing gravity assists in Kerbal and haven't found anything that clarifies what exactly are the good alignments. Any links to something like that would be much appreciated.

4. It would appear that approaching for an exact optimum angle (and not simply from behind and slightly orthogonal to the assisting bodies trajectory) can make a big difference to how these work out. Out of the six or so I've done (all using Mun) I would say two of them actually seemed to backfire and slow me down else throw me into some crazy escape trajectory that only made matters worse (and no I didn't approach from the braking angle [from in front of the approaching planet and slightly orthogonal] I don't think).

The ones that seem to have imparted the most delta-V are when I was actually yanked out of my approach trajectory and swung into a new trajectory. In one case, I literally was yanked around in front of the Mun and then approached it nearly head on and achieved about a 5000m periapsis.

Based on this I have a few working principles on how to perform these but would love to learn more short of getting into the math.

A. Use a periapsis burn to extend an apoapsis close to the planets orbit: not sure if this apoapsis should be inside its SOI band or just outside it or perhaps even beyond its orbital path? Also not sure if the exact time and place of the periapsis burn can be eyeballed based on the position of the assisting body?

B. Time warp and wait for an encounter to show on the map (this part seems really amateurish; would love to be able to launch on a specific time and day and know that a particular rendezvous was optimal). When an encounter is plotted, try to use some very minimal burns to get a suitable fly-by setup. I find the map a bit difficult to work with at this stage, and judging whether a particular encounter really is or is not suitable for a fly by seems difficult with the way the periapsis projection markers are shown and do not show up when you shift focus.

C. Try to shift orbit so that it is more circular around the apoapsis to see if this will improve the fly by, in the sense of reducing the periapsis at the assisting body and giving a good escape trajectory (short of entering the atmosphere I understand the most delta-V will come from the closest fly-by).

D. I have no idea what to make of the escape trajectories. Is it really possible to eyeball these and the positions of other farther away bodies and judge whether they are good enough or should be adjusted?

E. Burning at the periapsis, or thereabouts can further augment the 'sling shot effect.' However, in several of my fly bys, the trajectory that I would've expected based on the "escape Mun" and/or "escape Kerbin" marker seemed irrelevant and I rather quickly just clicked back into an orbit around the sun.

Edited August 1, 2013 by Diche Bach

[kiwiak]

ADDIT: watched the Scott video linked above and that did clarify a lot of my questions, but still would love to see more discussion of these issues.

How do the real life astrophysicists determine these things? For example, who figured out that the "Grand Tour" alignment that allowed Voyager 1 and 2 would exist and that the missions needed to when they did?

I understand the basic premise of a gravity assist and have managed to bumble my way through a few in game; a couple of which actually seemed to help my velocity quite a bit. However, the math for describing one of these interactions is beyond me.

1. Is it possible to 'master' the art of gravity assists using mostly intuition or is it really necessary to get into the maths?

2. Also, how does one use the in game map or other tools (mods? addons?) to judge when celestial bodies are in good alignment for a series of fly bys?

3. I've looked around for discussions on the concepts of planning and executing gravity assists in Kerbal and haven't found anything that clarifies what exactly are the good alignments. Any links to something like that would be much appreciated.

4. It would appear that approaching for an exact optimum angle (and not simply from behind and slightly orthogonal to the assisting bodies trajectory) can make a big difference to how these work out. Out of the six or so I've done (all using Mun) I would say two of them actually seemed to backfire and slow me down else throw me into some crazy escape trajectory that only made matters worse (and no I didn't approach from the braking angle [from in front of the approaching planet and slightly orthogonal] I don't think).

The ones that seem to have imparted the most delta-V are when I was actually yanked out of my approach trajectory and swung into a new trajectory. In one case, I literally was yanked around in front of the Mun and then approached it nearly head on and achieved about a 5000m periapsis.

Based on this I have a few working principles on how to perform these but would love to learn more short of getting into the math.

A. Use a periapsis burn to extend an apoapsis close to the planets orbit: not sure if this apoapsis should be inside its SOI band or just outside it or perhaps even beyond its orbital path? Also not sure if the exact time and place of the periapsis burn can be eyeballed based on the position of the assisting body?

B. Time warp and wait for an encounter to show on the map (this part seems really amateurish; would love to be able to launch on a specific time and day and know that a particular rendezvous was optimal). When an encounter is plotted, try to use some very minimal burns to get a suitable fly-by setup. I find the map a bit difficult to work with at this stage, and judging whether a particular encounter really is or is not suitable for a fly by seems difficult with the way the periapsis projection markers are shown and do not show up when you shift focus.

C. Try to shift orbit so that it is more circular around the apoapsis to see if this will improve the fly by, in the sense of reducing the periapsis at the assisting body and giving a good escape trajectory (short of entering the atmosphere I understand the most delta-V will come from the closest fly-by).

D. I have no idea what to make of the escape trajectories. Is it really possible to eyeball these and the positions of other farther away bodies and judge whether they are good enough or should be adjusted?

E. Burning at the periapsis, or thereabouts can further augment the 'sling shot effect.' However, in several of my fly bys, the trajectory that I would've expected based on the "escape Mun" and/or "escape Kerbin" marker seemed irrelevant and I rather quickly just clicked back into an orbit around the sun.

This are good questions.

[Jouni]

In the real world, gravity assists are planned with proper tools and a lot of iterative work. When a probe launch may cost hundreds of millions, it's really cheap to hire a bunch of PhDs for a few years to plan the mission. In KSP, things are much simpler, but planning the gravity assists still often requires a lot of work.

The basic idea is this: Given a delta-v budget for the launch, you have a bunch of launch windows, where you will end up with an encounter with one planet or another. For each launch window, you have a range of possible values for the time and the location of the encounter and the velocity at the encounter. Then, given a delta-v budget for course corrections and a combination of parameter values for the encounter, you have a departure window, which tells when and where you can depart from the planet, and with what velocity. Some of these departure windows contain encounters with some other planet, and these encounters transform into a number of possible departure windows from that planet. Intuition will help to guess, which branches are worth investigating first, but there is still a lot of exploratory work to be done.

[cantab]

It is very inefficient to use the moons as a gravity assist. Scott made it to Jool with gravity assists and save about 1000m/s dv.

Mun to Minmus is fine, albeit offering only modest savings, since you're not trying to leave Kerbin's SOI. Likewise using gravity assists to get around the Jool system.

ADDIT: watched the Scott video linked above and that did clarify a lot of my questions, but still would love to see more discussion of these issues.

<snip>

1: Master, probably not without complicated maths and planning. Get competent in, though, yes.

2: It helps to increase your conic patch limit in the configs - letting you see more encounters in advance. In particular, you can see what your orbit will be after the gravity assist.

4: Indeed the angle of entry is important, along with how close to the assisting body you get - generally you don't need to get super-close. Typically if you approach from behind and below a body you'll get a boost, hence why it's easy to get ejected from Kerbin's SOI with a Munar gravity assist. I believe if you pass in front of the assisting body you'll tend to get a drop. In any case, the aforementioned conic patch limit lets you work out these things more easily.

Other questions: If you're planning on a burn, you do indeed want a nice low periapsis; the faster you're moving, the more energy the burn gives (or takes from) you. That's why it's not helpful to use the Mun to leave Kerbin's SOI; you're going faster in low Kerbin orbit so you're better just doing a transfer burn there. If you aren't planning on a burn, a low periapsis isn't important.

[Kryxal]

I actually caught a gravity assist off Tylo coming up from Vall to help set up my inclination for Bop, it's probably one of the better things you can do with gravity assists when they happen to be set up right ... oh, and the Mun actually is of use on the way out from Kerbin, it's just that it's not a huge boost and makes things even more touchy in getting where you're going.

[Specialist290]

Normally I'd close something that's been raised up from the depths like this, but since it's still pretty relevant and has sparked a productive discussion, in the words of a popular image macro: "I'll allow it."

Getting back to the topic at hand: I've found the Flyby Maneuver Sequencer that comes with Arrowstar's Trajectory Optimization Tool to be extremely handy for plotting missions that involve using a gravity assist from one body to get to another. From what I gather, though, there's no easy solution to finding the optimal launch window, so the program has to "brute-force" a solution by calculating the equations for several different alignments and picking the best from the lot.

[Gravitas Shortfall]

How do the real life astrophysicists determine these things? For example, who figured out that the "Grand Tour" alignment that allowed Voyager 1 and 2 would exist and that the missions needed to when they did?

Computers and and lots of clever math: http://www.esa.int/gsp/ACT/inf/projects/gtop/gtop.html

I've looked briefly at using PaGMO to calculate trajectories in KSP, but ran into trouble getting things to compile and run on Windows. I'll probably give it another try at some point, but I wouldn't complain if someone with either a Linux machine or more experience with Visual Studio decided to beat me to it.

[Skorpychan]

How about the viability of setting up a fuel station in Eve orbit?

I'm planning a mission there anyway, and Eve is apparently easy to get to in terms of DeltaV from Kerbin. I figure that topping my Moho mission up there would be effective, and allow me to drag less weight in fuel tanks down into Moho orbit.

Plus, Gilly has wonderfully low gravity to get Kethane out of.

[Kasuha]

Other questions: If you're planning on a burn, you do indeed want a nice low periapsis; the faster you're moving, the more energy the burn gives (or takes from) you. That's why it's not helpful to use the Mun to leave Kerbin's SOI; you're going faster in low Kerbin orbit so you're better just doing a transfer burn there. If you aren't planning on a burn, a low periapsis isn't important.

Not really true.

If you're planning a burn, you need to set up your periapsis so you get deflected the correct way after the burn. Your burn will add kinetic energy to your craft, it will "open" the slingshot trajectory a little. So you need to set up the slingshot for slightly tighter angle (lower periapsis) than you really need, then open it with your burn.

How about the viability of setting up a fuel station in Eve orbit?

I'm planning a mission there anyway, and Eve is apparently easy to get to in terms of DeltaV from Kerbin. I figure that topping my Moho mission up there would be effective, and allow me to drag less weight in fuel tanks down into Moho orbit.

Plus, Gilly has wonderfully low gravity to get Kethane out of.

You're free to build fuel stations anywhere you want. But it does not count as gravity slingshot if you stop there.

[Kryxal]

A full refuel and relaunch from Eve MIGHT help you out ... it might not, too, it REALLY depends on the launch windows unless you're willing to aim for the best point then do an adjustment burn to get the right period to your orbit.

It does sound easier than plotting out the slingshot, just make sure you use the Oberth effect.

[juanml82]

How about the viability of setting up a fuel station in Eve orbit?

I'm planning a mission there anyway, and Eve is apparently easy to get to in terms of DeltaV from Kerbin. I figure that topping my Moho mission up there would be effective, and allow me to drag less weight in fuel tanks down into Moho orbit.

Plus, Gilly has wonderfully low gravity to get Kethane out of.

I'm going to set up a refuel station for my planned manned mission to Eve surface. I think the main issue is delta-v for the Eve-Gilly burn which, due Gilly's high inclination, is relatively high. So, if you're ferrying fuel from Gilly, the lower gravity is compensated by the fact that your tanker needs enough fuel to go back to Gilly from Eve's orbit.

Me thinks the best location for such a station might actually be in Gilly's orbit, not Eve's. And if you're planning to use it to refuel interplanetary flights, then you have the issue of getting to Gilly to refuel instead of using the fuel to go straight to your destination. So, it might not be worth it.

But maybe someone already did something like this and can answer better.

[Skorpychan]

I'm going to set up a refuel station for my planned manned mission to Eve surface. I think the main issue is delta-v for the Eve-Gilly burn which, due Gilly's high inclination, is relatively high. So, if you're ferrying fuel from Gilly, the lower gravity is compensated by the fact that your tanker needs enough fuel to go back to Gilly from Eve's orbit.

Me thinks the best location for such a station might actually be in Gilly's orbit, not Eve's. And if you're planning to use it to refuel interplanetary flights, then you have the issue of getting to Gilly to refuel instead of using the fuel to go straight to your destination. So, it might not be worth it.

But maybe someone already did something like this and can answer better.

Gilly's gravity is tiny, and any station would be close to Gilly's orbital path in order to minimise dV used getting to it. Otherwise, I'm not above simply refueling the interplanetary missions via tanker. They're usually complex trains of modules hauled by a space tug anyway, and induce a lot of lag BEFORE a station nears them. And they're a pig to turn.

[elmarko]

The next trick is to copy the MESSENGER probe and use repeated flybys of Moho to reduce your solar apoapsis gradually to reduce your needed dV to brake at Moho.

[kinnison]

My dad back in the 60s took a collage physics course and the final exam they had to calculate a Earth, Venus, Earth, mars trip with slide rulers.

But honestly, these days they just use a few astrophysicists and some time on a supercomputer.

[Iansoreta]

It is very inefficient to use the moons as a gravity assist. Scott made it to Jool with gravity assists and save about 1000m/s dv.

Yeah, its not a good IDEA to use moons, unfortunately moon encounter happened to me while I performed trans-Duna injection burn, but at least I performed course correction burn to correct that error, even performing plane change maneuver with less delta-v for low altitude approach.

[Sirine]

Check the Times. It take few more years to get to destinations. Compared with direct burn... I rather think of how to get more dv from launch than to save dv.

[cantab]

Well, travel time doesn't really matter in the game. You can send a Kerbal on a hundred-year trip in a 1-man pod if you want, and even if you'd rather not (or you run a life support mod) there are always unmanned missions.