Eve gravity assist to Moho

[Space Nerd]

Hi, I want to try the method that @BeanThruster used in his/her Mohollo mission to get to moho, which includes a Eve gravity assist to lower the orbit, but I can't get a Moho orbit intersect(not with the planet) after the assist(PE too high), can anyone help me?

[TheFlyingKerman]

Burn prograde at Eve PE.

Also make a small correction burn to use Eve for plane changes, so that your orbit intersects Moho at AN/DN, and PE close to Moho altitude. Then burn retrograde at Moho intersect to rendezvous with the planet.

IME it takes about 4000 DV from LKO to Moho orbit.

[Space Nerd]

The thing is, burn prograde at Eve PE(1200) costs even more dv than burning at solar AP(840), which is more than I liked.(I have just under 8000 dv at lko, but I want to have a lot of margin).

Or should I just burn retrograde at solar AP?

Edited March 1, 2020 by Space Nerd

[BeanThruster]

I'm not sure if it actually deserves to be defined a method, I would call it an incomplete gravity assist. That is, I did it in that way after I got frustrated with not being able to achieve a good gravity assist (i.e. encounters with Moho not requiring an expensive burn at Eve or an insane deltav for inserting into orbit). 
In that sense, try to plan your Eve flyby as if you would plan a gravity assist to Moho. You should plan the flyby for losing velocity, that is you should be approaching Eve from behind and pass in front of it (see also https://wiki.kerbalspaceprogram.com/wiki/Tutorial:_Gravity_Assist).

I found this tool very useful:

http://ens-gijs.github.io/ksp/

It is still a little bit confusing, but you can rouglhy figure out your orbit after the flyby

I didn't burn in Eve's SOI. Rather, I tweaked the flyby during the journey to Eve in order to achieve an optimal final orbit, that is, I tryed to minimize the inclination difference with respect to Moho's orbit, and I tryed to set my periapsis as close as possible to Moho's periapsis.  

I hope this may help you

Edited March 1, 2020 by BeanThruster

[Space Nerd]

I tried to lower pe by as much as possible , but it's still too high(even when I got a eve collision course).

[BeanThruster]

3 hours ago, Space Nerd said:

I tried to lower pe by as much as possible , but it's still too high(even when I got a eve collision course).

You're probably approachin Eve at the wrong time. Not all of the flybys can conveniently be used for lowering your orbit. Conversely, a properly set flyby can lower your orbit without the need of a burn.

Just to give you an example, try http://ens-gijs.github.io/ksp/ and look for a transfer to Eve using year 1, day 1 as earliest departure time, and 100 km for initial and final orbits. You will probably obtain a 2,711 m/s optimal transfer to Eve, leaving Kerbin on year 2, day 151, with a flight time of 199 days. Well, that flyby is not a good one, since it is rising the orbit. You can tell it just looking at the resulting orbit (the green one) and the possible encounters afterthe flyby, which are listed in the box below the porkchop (just Kerbin and Piovessa).

A different solution, like the one below, is much better.

Spoiler

https://imgur.com/7vx4kGa

https://imgur.com/WVXiv9r

 

 

 

Edited March 1, 2020 by BeanThruster

[Zhetaan]

On 3/1/2020 at 1:30 AM, Space Nerd said:

Hi, I want to try the method that @BeanThruster used in his/her Mohollo mission to get to moho, which includes a Eve gravity assist to lower the orbit, but I can't get a Moho orbit intersect(not with the planet) after the assist(PE too high), can anyone help me?

You need a second Eve assist; one won't normally lower your periapsis enough.  The trick to K-E-E-M trajectories is to use the first Eve assist to set the exact correct time for the second.  Eve's orbital period is 65.5 days, so you need a solar orbit with that period exactly; you can afford a bit of error margin on the Kerbin and Moho ends of the path.

There's one path that begins at Kerbin on day 162, flies past Eve on day 210.6 and again on 276.1, and finally reaches Moho on day 296.1.