Hello! Help on transfer from Eve to Moho

[tomek1ni]

Hello!

I've got some problem. My plan was to take almost all of my kerbals on a trip across the system to level them all up. The first point of the travel was Gilly (Eve's moon) because it so happened, that a transfer window coming right about then. From that place, the next closet window was to Moho. But what I didn't do, and what I should have done, was to check the amount of delta V , that is needed to perform that maneuver. Thus while I'm parked in the south pole of Gilly I have 3439 m/s delta V plus around 200 from emptying monopropellant tank and converting ore.

I figured that the best moment to get ejected from Eve's system comes In 8 days, because then I almost don't need to correct my trajectory for Moho's orbit inclination.

You can also see the way how I get out of Eve's system here. Opposite to the planet's direction of orbiting the Sun. That seems to be the most optimal.

That's like the transfer trajectory between planets looks like:

And this is the moment when I close the orbit around Moho (not even landing):

I reckon that I would need another thousand more to land on the surface. Concluding, in total I'm somewhere around 1500 m/s to short on delta V to pull that off. 

My question to you guys is... do you see any way of making it work? Maybe some better way of utilizing oberth effect or any tricks of that kind? Thought that there maybe are some space veterans who had some advices.

I know I could just refill the fuel in the mid flight with KLM, but I would rather wait for a window to other planet than using cheats.

Regards, T

[Snark]

Hello, and welcome to the forums!

Moving this to Gameplay Questions.

[Zhetaan]

@tomek1ni:

Welcome to the cosy little family.

When going from Kerbin to Moho, it is often preferable to make the transfer to Moho's ascending node and then worry about an intercept rather than trying to get directly there on a Hohmann transfer.  Going from Eve to Moho is arguably worse:  Moho's orbit is inclined at 7 degrees and Eve's is inclined at 2.1 degrees, but their longitudes of ascending node are 85 degrees from one another, which means that while both orbits are tilted with respect to the ecliptic, they are tilted essentially crosswise to one another.  What this means for you is that there are very specific places where you can hope to get a minimum-fuel encounter, but only those particular places will work on your fuel budget.  It's certainly doable but that 356 m/s correction burn is potentially avoidable; I'd seriously consider burning to intercept the higher node and then then trying to work for an encounter there on the next orbit.

From the look of your intercept, I think you're burning a bit much; you're on a fast intercept rather than a minimum-fuel transfer.  I can tell because your departure from Eve and arrival at Moho are fewer than 180 degrees apart.  Consider leaving earlier.

Also consider gravity assists:  Moho doesn't have much gravity, but it doesn't have an atmosphere either, so you can squeeze the most out of Moho's gravity well both from a close assist and from the Oberth effect if you treat it something like a combination of periapsis kick burns and phased orbits.  What I mean by that is that once you have an encounter, correct it to get a braking assist and then, once you're there, burn at your Moho periapsis not to capture on the first pass, but simply to reduce your solar orbit some low-value rational multiple of Moho's orbital period, such as 3/2.  Correct the encounter to get a close pass again, and keep repeating until you capture.  In infinite time, you can do this entirely with correction burns until you need to actually capture, but I'll assume that you don't have infinite time.

You're on year 1, day 312 or thereabouts, correct?  That is a window, but it's not the best one you have available.  If you're willing to wait most of a year, there's a window that opens on year 2, day 250 that can get you to Moho for only 3,099 m/s.  You need something on the order of 870 m/s to land on Moho, so if you have 3,438 on your vessel and can strain 200 more from careful fuel management, then that leaves you only approximately 330 m/s short of a landing.  You might be able to take that off of the 3,099 m/s transfer burn if you're willing to use multiple passes and assists to get the most out of Moho's meagre gravity well, but taking ten percent off of a minimum-fuel transfer is asking rather a lot.

I'd suggest landing on a high plateau, too.

P.S.:  Have a look at this.  The challenge is so old that you absolutely should not trust any of the numbers depicted, but the flights demonstrate a lot of the principles that I described.  Maybe something there will give you an idea.

Edited July 25, 2018 by Zhetaan

[tomek1ni]

Hello!

 

Thank you for that extensive answer. It is way more than i expected. I red it a couple of time and tried to decipher the meaning of it... I'm rather an amateur wen it comes to all that terminology. For instance what does "intercept the higher node" mean?

You might get the 356 m/s burn wrong. That one suppose to eject me from Eve's system while I'm in PA going right by Eve. Red somewhere that performing burns there may save me a little delta V.  Correction for Moho's inclination is that 3 m/s burn. Was thinking if I manage to intersect it at that point, when in almost aligned with Eve's orbit, would save even more delta V. But later I came to a conclusion - as you said - that is rather fast intercept. So it cost me more when I need to break instead. 

I is year 1 day 312. An the next window would cost me less fuel indeed. I experimented a little trying to land my vessel. Haven't succeeded yet, but will try again if have some more time.

Regards, T

Edited July 27, 2018 by tomek1ni

[Zhetaan]

1 hour ago, tomek1ni said:

For instance what does "intercept the higher node" mean?

Moho's orbit is both eccentric and inclined.  It happens that in Moho's case, the ascending node is closer to the apoapsis, so it is the higher node (actually, specifically for Moho, the ascending node is at the apoapsis).  If an inclination-change burn is too expensive, then you can instead burn to intercept the target at one of the nodes, and since you're coming in from a higher orbit, it costs less fuel to intercept the higher node.  Of course, in some cases, you pay for that with a more expensive capture burn, so that's why it's best to combine such a move with other fuel-saving trickery such as gravity assists and aerobraking.

Of course, that move has drawbacks, too:  when you do encounter the target, you are not moving in the same direction, so that limits the amount of braking gravity assist you can get from the target.  It works for Moho because Moho doesn't have much gravity in the first place, so it's not really a sacrifice.

With more complicated assists, you can use inclination to your advantage:  though an inclined encounter probably won't give you effective braking, it can give you an inclination change, which means that a subsequent encounter can be used for braking.  The cheapest assist to get from Kerbin to Moho that I've ever seen (it's in the link I gave you) involves two Eve flybys:  the first is to adjust inclination and the second is to lower the periapsis to something that can encounter Moho.

1 hour ago, tomek1ni said:

You might get the 356 m/s burn wrong. That one suppose to eject me from Eve's system while I'm in PA going right by Eve. Red somewhere that performing burns there may save me a little delta V.  Correction for Moho's inclination is that 3 m/s burn. Was thinking if I manage to intersect it at that point, when in almost aligned with Eve's orbit, would save even more delta V. 

Ah, I see what I misinterpreted.  I saw the 701 m/s burn as your kick out of Eve's orbit and the 356 m/s burn as the inclination change.  Instead, you're dropping to a low periapsis in order to squeeze the most that you can out of Oberth.

Have a look at this.  The idea is that if you're already beginning from a relatively high orbit and going to a nearby planet (such as the next planet over--the requirement is a low hyperbolic excess velocity, or V_∞, and V_∞ is at its lowest if you don't need to go far), then sometimes, it makes more sense to start from the higher orbit and not bother with the periapsis dive.  It only works if you start fully-fuelled at the higher orbit, which for a normal Kerbin launch is not the case, but there's always free fuel at Gilly's orbit if you have the correct equipment.  There is also a very intriguing table of values comparing the required total delta-V for a dive/periapsis burn and a direct ejection on page 2.  You'll have to look into the maths on your own, but the equations are there.  Alternatively, you can quicksave, muddle the manoeuvre nodes a bit, and decide whether to keep the result or reload.

That being said, you are still likely to need a few passes at Moho to keep your braking delta-V in the realm of the possible.

Edited July 27, 2018 by Zhetaan

[Kryxal]

There's a second reason for a dive to PE from a moon ... it allows you to hit the correct departure angle at a time of your choosing.

[tomek1ni]

I LANDED!!! 

 

When I have more time I'll describe exactly what I did. In general I performed all the most expensive maneuvers while I was still in Eve's sphere of influence. Including adjustment for Moho's inclination.

[Zhetaan]

1 hour ago, tomek1ni said:

I LANDED!!!

Nicely done.  I see that your fuel tanks are suspiciously completely dry--I look forward to an entertaining story.  You should consider posting a Mission Report!

Edited July 31, 2018 by Zhetaan

[tomek1ni]

Yeah  It was my fifth attempt of landing..

It was the last puff of engines witch saved me from ramming in to the surface. I had like... 7 m/s delta v left while I was right above the surface.. it wasn't moving that fast, but you don't need much to damage the ship. The other thing, approach wasn't too efficient. Think I started the suicide burn too early. When the vessel was in orbit of 10 km over the surface, I still had something around 900-1000 dela v left. So it should be totally sufficient amount to land, but I burnt it all out anyway.

This is how the maneuvers look like watching it from the top while I'm back near to Eve.

 

The second one shows form the back how much tilted is the trajectory witch suppose to push me back towards Eve as close as possible. It also shows the inclination at which the ship is being ejected from SOI of Eve. It was the most difficult part, because you need to synchronize everything (angles, burns, and timing). For instance when my trajectory inclination still doesn't align with Moho's orbit, and I needed to decrease inclination of second maneuver (purple dotted line), but then the angle  I leave the Eve's SOI at changes either. So I need to move second maneuver nod further along the first trajectory to match the angle. But then you perform the maneuver later. So I need to shorten first trajectory. But then the velocity of I pass by Eve with goes down a little. etc... I had tried to launch directly when I was in high orbit over Eve, but it didn't work that well.  I tried to do some math but it was difficult for me to draw any conclusions from those calculation. It was even had to say to me if I did them correctly. So I don't know how much of a role Oberth effect played here, but I can say that only this way I managed to pull it all off. When ship started falling towards Eve it was at an altitude of 44000 km over the planet.

Those 3 maneuvers are the most expensive (not counting breaking and landing). Or rather they could have been way more expensive if done later... further away from Eve. There are other 2 on my way between Eve and Moho, but these are fairly minor and are there only to get more accurate interception.

 

The other thing was breaking. I didn't quite realized before i red your post how much of a difference it makes, when the ship moves exactly along planet's orbit during breaking. It was whopping 1700 of delta v of difference. Maybe there were some other factors influencing this, but this is probably the main one.