Multiple gravity assists

[fireblade274]

So long story short ive been slaving over a new ship that will tug a lander and extra fuel to Eloo, then come back with the "important" part of the lander, drop it off in a kerbin orbit to be retrieved, and then either set up the tug to be re-used or land it in the water near KSC. Ive gotten to the point coming back from Eloo, but I underestimated how much aerobraking it could take. 

This is the ship on way to Eloo:  https://www.dropbox.com/s/9329qazijte7gid/20160309174615_1.jpg?dl=0

After Eloo, it drops the extra fuel tank on the back of it, and only keeps the important crew modal and science stuff from the lander.

It has a med. sized heat shield in the center, and 3 small heat shields around the 3 "pontoons" going around the vessel.

When coming in directly from Eloo, my velocity is about 5.5km per second, and even with the heat shielding I end up losing the pontoons and wings, which makes my center fuselage tilt out of the med. heat shields protection and nothing survives.

I think the best way to make my craft work the way I want it to is to both add mining capability of the Eloo lander so I can refuel my ship as much as I can, and then do multiple gravity assists to get my craft slowed down enough to do an aerocapture. Can someone explain to me how to best do this? I understand the concept, but when It comes to implementation I dont really know where to start; currently where i am is i came to a 67km perapsis of kerbin and survived (reached Mach 21.something lol) and now my solar orbits apoapsis is close to Duna and my perapsis is between Kerbin and Eve. I have 500dv left lol, I was planning on surviving a Kerbin aerobrake. What should I do

[Snark]

17 minutes ago, fireblade274 said:

When coming in directly from Eloo, my velocity is about 5.5km per second, and even with the heat shielding I end up losing the pontoons and wings, which makes my center fuselage tilt out of the med. heat shields protection and nothing survives.

I think the best way to make my craft work the way I want it to is to both add mining capability of the Eloo lander so I can refuel my ship as much as I can, and then do multiple gravity assists to get my craft slowed down enough to do an aerocapture. Can someone explain to me how to best do this? I understand the concept, but when It comes to implementation I dont really know where to start; currently where i am is i came to a 67km perapsis of kerbin and survived (reached Mach 21.something lol) and now my solar orbits apoapsis is close to Duna and my perapsis is between Kerbin and Eve. I have 500dv left lol, I was planning on surviving a Kerbin aerobrake. What should I do

Various options, depending on what your priorities are, and also what your current situation is.  I'm a little confused.  Is this a mission that you haven't launched yet and are still tweaking the design?  Or is it a mission that's already in progress and you're trying to figure out how to pull it out of the hole?

If it's the haven't-launched-yet case:  then this is easy.  Just add mining & refueling to the ship, so it can top off its tanks at Eeloo.  That way you'll have oodles of dV to spare for when you arrive at Kerbin, and can slow down a lot before hitting atmosphere.

If it's the already-on-the-way-home, have-only-500m/s case and you need to pull it out of the hole:  There are a lot of different options with different tradeoffs, depending on what your priorities are.

For example, there's the question of what you're trying to save.  If it were me, all I really care about is the crew and the science.  So the easy solution to that is a rescue ship.  Build something very lightweight that has lots of dV-- perhaps it could be ion-powered-- with enough crew capacity to take the whole crew.  Launch it shortly before the Eeloo ship arrives, and give it a running head-start so that when the Eeloo ship goes screaming past, you can match velocities with it fairly quickly.  (Maybe the Eeloo ship can do some aerobraking to slow down to the degree possible, even if it's not enough aerobraking to capture.  Convenient, but not necessary.)

The rescue ship does a rendezvous and matches velocity.  Transfer the kerbals across, taking the precious science with them.  Then just abandon the Eeloo ship.  The rescue ship brakes to a halt, then accelerates back the way it came to arrive triumphantly at Kerbin.  Eeloo ship goes flying off into the solar system.

Total dV needed on the rescue ship will be 2x the Kerbin-relative velocity of the Eeloo ship (once to match velocities, once to brake to a halt afterwards), plus however much dV you want for coming home afterwards (just a little will do, it depends on how much of a hurry you're in).  So giving it, say, 15 km/s of dV from LKO should be plenty, with lots of safety margin.  That may sound like a lot, but it's easily doable with a lightweight ion ship.

It's true that following that approach would mean you'd lose the Eeloo ship and wouldn't get the recovery cost.  But if funds are the issue here, you could pay for the loss by just doing an extra contract or two.  Frankly, it'll be a lot less time and hassle to just do an extra contract than to try to set up some multiple gravity encounter way to get home.

On the other hand, if you really don't want to do that-- for example, if you've got your heart set on bringing that ship home (either for role-playing reasons, or just for the fun challenge of it)-- then something a bit more elaborate would be in order, but it would be rather complicated and I'd like to hear what your thoughts are before going into any further explanations. 

[fireblade274]

What an informative response! thank you Snark

First to clear up the confusion: I did already start the mission and land on Eloo, and end up in the situation of the aforementioned solar orbit, but I also did a quicksave just before I started the mission where the main ship and its connected modules are docked with my spacestation in kerbin orbit, So I can chose to do either one of your options; start over or do rescue mission. I think to solve the issue at hand, I will try to do a rescue mission where I will abandon the rescue ship, and use the rescue ship to transfer a good amount of fuel to the Eloo mothership.

I could abandon the ship and make my life easier, but I have 24million$ and in addition to fancying bringing the ship back, Id like to test how much further past minmus I can bring it down.

So I guess what I'm asking is how can I efficiently and practically use multiple gravity assists on my next mission to Eloo? I know how to do maneuvers with out mechjeb but I do like the time mechjeb saves me when planning things; is there a way to have mechjeb plan multiple gravity assist maneuvers for me?

And I'd love to hear all your further explanations  

Edited March 10, 2016 by fireblade274

[Gojira1000]

I'm suspecting you can do a gravity assist off Eve to kill a lot of that velocity. But I'll defer to Snark, who is far smarter than I.

[FullMetalMachinist]

There's the Flyby finder mod that helps you plan several flybys of multiple planets to line up multiple gravity assists. 

It's basically a transfer window planner that takes into account more than two planets at a time. It looks pretty scary and complicated, but the instructions on how to use it are pretty good. 

[PLAD]

I recommend Flyby Finder for planning multiple gravity assists (disclaimer: I wrote it) but TOT can also plan them. Nether one can handle double flybys of the same planet or multiple orbits of the sun between encounters, but there is still a lot to be found with what we've got. I fine example of using flybys was this mission by Val some time ago, Val used flybys to get out to Jool and then to get back to Kerbin at a lower speed than was possible with a direct Jool-Kerbin flight. In the pre-1.0 days we all just came piling in to Kerbin's atmosphere at extreme speeds but since then we have to slow down first, in this mission Val goes Jool-Kerbin-Eve-Kerbin to arrive at a safe speed. Note that the trip back is easier to plan than the trip out, Val just winged the return. If you are coming back from Eeloo though you can save a lot if you flyby Jool on the way to the inner system, and that could use some planning.

  I once went from Kerbin to Eeloo landing and back to Kerbin with a 12-ton SSTO, that was many versions ago so the SSTO is obsolete but the path is still good.

[mrclucks]

Just my .02

Last save I was using the outer planets mod and just brute force for to the planets.  On returning I set a kerbal peripasis as low as moho, waited a bit( for a 50y peripasis time you kind of have to) and when I was reaching jools orbit I played with the manuver node so I could use Eve and/or kerbin as a gravity assit to slow down, repeated as needed til my apoapsis was as high as Duna before I tried aerobraking at kerbin.  The key to gravity assists is time