How to reach another planet

[guitounet]

Hi  guys,

 

i'm currently playing this great game and after a month of kerbin around exploration i try to get to another planet.

To achieve this painful job, I use mechjeb and KER.

For exemple I want to reach moho, here is my process, (and sure something is wrong because almost never reach it).

1. I check deltav maps to get deltav requirements : at least 13000 to a round trip to moho.
2. I build my ship, launch , set it a stable orbit around 80km - 400km
3. use mechjeb to find the best transfert time (using adv transfer to another planet) and trigger  it
4. sometimes i have to adjust the trajectory of my ship but's it's  just few deltaV
5. then time pass.... And i'm in target planet encounter. And there my only known option is to burn retrograde to get an elliptical orbit around my target but it cost few thousand of dv more than thoses expected (ie for moho requirements are above 1500dv needed)
6. and so, my round trip is aborded for insufficient fuel reasons....

Is there a way to optimise planet encounter/elliptical approach? Is burning retro the right method for interplanetary journey ?

Any advices will be appreciated.

GuitouNet

[IncongruousGoat]

Try going to Duna instead. Delta-V requirements for Moho are ridiculously high, especially if you're using an autopilot to plot your burns. Even if you're doing it manually and finding the best transfer possible, it's still really expensive. So, yeah. Duna. Of all the planets, it's arguably the easiest to go to and return from.

It might help in general to learn how to plot these kinds of burns by hand, but that's just a staunch manual pilot's opinion.

[bewing]

Burning retro is indeed the right method, but it looks like you haven't learned about the Oberth effect yet.

When you are at a very low Pe (or in a very low orbit in general) you will always be going faster than when you are at high altitude. Burning prograde or retrograde is much more effective when you are going fast in the first place. It is hard to launch a ship directly into a very low orbit by hand, but since you are using MechJeb, it should be easier for you. If you leave Kerbin from a 72km x 71km orbit, you will be going nice and fast when you burn, so you will have good Oberth. During your journey, as you make small burns to get to your target planet, make your burns very carefully so that you will have a very low Pe when you arrive. That will get you as much Oberth as possible when you do your retrograde burn.

Simply making sure to use the Oberth effect to its fullest will probably get you all of that 1500 m/s of dV that you lost the first time.

 

[Kryxal]

Moho in particular requires special treatment.  I sent a Moho trip recently, and left at the corresponding node of the orbit, aiming to arrive where the orbit's periapsis is.  Moho's in a rather inclined and eccentric orbit, and you're best off matching inclination as high as possible (so, your departure burn should have a BIG normal component) and meeting Moho when it's moving fastest to cut down your retro burn.

By the way, if you use the above method, use your first pass through periapsis to set up an encounter next time around (or the time after that, as the case may be).

[Foxster]

One thing is that MJ optimises the burn to get to the planet, not prioritising how much of a burn you will have to do once you get there. 

Look at the solution that MJ gives you. Is the orbit path above the orbit of Moho? If so then you will be encountering Moho at a higher velocity than the optimal encounter of just catching up with it by having the same Ap. 

[Hodari]

If you're trying to do your first interplanetary mission, Moho is definitely NOT the one to go to.  I'd say it is by far the hardest planet to land on(though there might be a couple of Jool's moons that are arguably harder).  I'd definitely go for Duna first.  If you can build a rocket capable of landing on and returning from Kerbin's moons, then that rocket will have enough dV for at least a one-way trip to Duna as well.

As for Moho, I've been playing for a while and only just did my first Moho landing.  Aside from all the advice people have given above, one thing worth considering would be rather than trying to go directly to Moho, instead first drop down into a much lower orbit(I went for a circular orbit somewhere between Moho and Eve's orbits) and then go for the Moho encounter from there.  Might not be the most efficient way to do get the encounter, but it does mean you won't have such a huge velocity difference once you do encounter it.  If you have a mining base set up on Gilly, you could even go there first, refuel, and then continue on to Moho from there.

[Zeiss Ikon]

On 2/28/2017 at 10:00 AM, Hodari said:

 If you can build a rocket capable of landing on and returning from Kerbin's moons, then that rocket will have enough dV for at least a one-way trip to Duna as well.

I'll confirm this.  When I was learning this level of doing things in sandbox, I found that all I had to do to get to Duna (a flyby and return, with a little help from Duna's gravity well to set up the return orbit) was delete the lander from my Mun/Minmus vessel (which used an Apollo-style, separate lander vs. command/service stage).  With the two-stage lander removed, the transfer stage (a large orange tank, a Poodle, a small RCS tank, and a Mk. 1, plus assorted decouplers, parachutes, heat shields, RCS blocks and Vernors, etc.) had enough delta-V even without waiting for a "most favorable" transfer window.

That same ship couldn't come anywhere near Moho, unless I managed to use a Duna or Eve gravity assist to get a flyby.

BTW, if you don't like Duna for some reason, Eve is only a little more expensive to get to than Duna -- but be warned, if you land there, you probably won't get back to orbit.  Getting a lander up from Duna is only about three times the dV as Mun (and the air's much thinner than Kerbin's).