MESSENGER-style mission to Moho

[MAFman]

Whenever I've attempted a MESSENGER-style Moho mission, using mostly gravity assists to lower my velocity relative to Moho, I've always come up way short on delta-V, always requiring about 7 km/s even after 10 years of gravity assists to circularize. What am I doing wrong?

[Zhetaan]

On 6/25/2022 at 4:30 PM, MAFman said:

What am I doing wrong?

You should probably look into those gravity assists.  Without seeing more of your mission profile, I can't say anything definite, but my main suggestions are to check to see whether your encounter inclination at Moho is too high, and to see where you're meeting Moho on its orbit.  The best way to Moho after a gravity assist is to try for one that encounters it at its periapsis, not apoapsis.  Encountering it at the periapsis makes your orbital ellipse more similar to Moho's, which cuts the cost for capture.

[king of nowhere]

in general, the major problem with moho is intercept deltaV. to minimize that, you want your orbit to be as similar as possible to moho's orbit. the closer the two orbits are, the less expensive it will be to switch from one to the other.

to tell what you are doing wrong i'd need to see your gravity assist, but without knowing specifics I can guess your mistake maybe is that you are not meeting moho at the periapsis of your own orbit? if your orbit and that of moho look like the olimpic rings, they look close, but they actually require a huge radial burn to overlap.

you already made a similar question, and I gave some more detailed answers there.

 

Oh, there's also a possible other thing you may be doing wrong now that I think about it. maybe you sent your mission to moho, and are now trying to get moho gravity assists to circularize. that doesn't work, it's impossible. when you make a gravity assist, you always leave the planet with the same speed you came in. so you can use a gravity assist on a planet to raise or lower your solar orbit, you can use it to change inclination, but the one thing you cannot do is use a planet to reduce intercept speed on itself.

what you can do is take gravity assists from eve. this way, instead of having to burn from kerbin all the way to moho, you only burn to eve, and you save fuel. and at moho, instead of having an intercept from an orbit going all the way up to kerbin, your orbit only goes up to eve, lower intercept speed.

post pictures of what you're trying to do if you want more detailed explanations.

[Lt_Duckweed]

9 hours ago, king of nowhere said:

and are now trying to get moho gravity assists to circularize. that doesn't work, it's impossible. when you make a gravity assist, you always leave the planet with the same speed you came in. so you can use a gravity assist on a planet to raise or lower your solar orbit, you can use it to change inclination, but the one thing you cannot do is use a planet to reduce intercept speed on itself.

While this is technically true, there is a way around it, using messenger assists, also known as a Vinf leveraging manuver.

on your first encounter with Moho you simply fly past, bending your orbit slightly radial in/out (if you are coming in tangent which way only matters in terms of lining up your future periapsis with Moho's pe). This reduces both your periapsis and apoapsis somewhat.  Then, at apoapsis, you raise your periapsis such that your orbit and Moho's orbit are once again tangent at their intersection.  Then a few orbits later you encounter Moho again.  The amount that your Moho relative velocity decreased by will be significantly greater than the cost of the pe raise.

From an Eve flyby, a fairly optimal set of flybys is 5 flybys with 5 dsms and then a final encounter and capture burn.

Done this way, you can reach low Moho orbit from an Eve flyby in under 600 m/s

[king of nowhere]

16 minutes ago, Lt_Duckweed said:

While this is technically true, there is a way around it, using messenger assists, also known as a Vinf leveraging manuver.

on your first encounter with Moho you simply fly past, bending your orbit slightly radial in/out (if you are coming in tangent which way only matters in terms of lining up your future periapsis with Moho's pe). This reduces both your periapsis and apoapsis somewhat.  Then, at apoapsis, you raise your periapsis such that your orbit and Moho's orbit are once again tangent at their intersection.  Then a few orbits later you encounter Moho again.  The amount that your Moho relative velocity decreased by will be significantly greater than the cost of the pe raise.

From an Eve flyby, a fairly optimal set of flybys is 5 flybys with 5 dsms and then a final encounter and capture burn.

Done this way, you can reach low Moho orbit from an Eve flyby in under 600 m/s

really?

and here I thought I knew everything about gravity assists...

[camacju]

This same style of gravity assist is useful for capturing around any body where your encounter velocity is greater than escape velocity and aerobraking isn't possible. This includes:

-Moho
-Gilly
-Mun and Minmus
-Ike
-Dres
-Bop and Pol
-Eeloo

Vall and Tylo aren't on this list because with proper use of gravity assists, it's possible to encounter them already on an elliptical orbit, so additional deep space maneuvers won't help.

Edit: It turns out that I hadn't reexamined my assumptions about Mun in a while, it's likely that you don't need deep space maneuvers. But they certainly help.

Edited July 22, 2022 by camacju