Are mun assists really worth it

[bamslamkabam]

So I just finished up one of my career mode mun missions and I made sure my stages would either get ejected out the system or destroyed. Turns out the mun assist had ejected it into a interplanetary orbit. But whenever I try to do that to a full size craft the mun slows me down. I usually come in at 5 km to the mun. Am I doing something wrong or is this just not worth my time?

Edited February 5, 2019 by bamslamkabam
Mis-typed

[Superfluous J]

In short: not really. You can get about 500 or so m/s but you need to time it perfectly and execute it without flaw, or you'll be spending fuel fixing your trajectory. Fuel that would have been much easier spent just ejecting in the first place.

[bamslamkabam]

1 hour ago, 5thHorseman said:

In short: not really. You can get about 500 or so m/s but you need to time it perfectly and execute it without flaw, or you'll be spending fuel fixing your trajectory. Fuel that would have been much easier spent just ejecting in the first place.

Thanks for the advice. 500 meters per second is basically nothing considering I can always just add more boosters.

[magnemoe]

7 hours ago, bamslamkabam said:

So I just finished up one of my career mode mun missions and I made sure my stages would either get ejected out the system or destroyed. Turns out the mun assist had ejected it into a interplanetary orbit. But whenever I try to do that to a full size craft the mun slows me down. I usually come in at 10 km to the mun. Am I doing something wrong or is this just not worth my time?

Note that refueling in Mun orbit and drop down to LKO for the burn is way better.
Duna mission in Minmus orbit, note that everything outside the base is because its an speed run who has to bleed 4km/s to enter Duna orbit.
Minimum cost and you could just use the base. An Duna or Eve payload at minimum cost dropped from Mun or Minmus is 500 d/v. 
Mun refueling require an larger and more powerful tanker because higher dV to reach orbit but its also easier to do the injection burns. 

[bewing]

Style points.

[GRS]

Yes, they worth, you may need them for Eve ascent Vehicle transports, or just acting like a Boss, and getting to Minmus when your rocket pushed your PC near its limit and you got insufficient delta v.

[bamslamkabam]

11 hours ago, magnemoe said:

Note that refueling in Mun orbit and drop down to LKO for the burn is way better.
Duna mission in Minmus orbit, note that everything outside the base is because its an speed run who has to bleed 4km/s to enter Duna orbit.
Minimum cost and you could just use the base. An Duna or Eve payload at minimum cost dropped from Mun or Minmus is 500 d/v. 
Mun refueling require an larger and more powerful tanker because higher dV to reach orbit but its also easier to do the injection burns. 

well I dont have the isru unlocked yet so it would be easier just to dock to a tank in lko and then use that as a fuel reserve for my interplanetary mssions.

[Laie]

20 hours ago, 5thHorseman said:

You can get about 500 or so m/s

Really? I recall that the theoretical maximum was supposed to be on the order of 130m/s, and typically consider myself lucky if I get about 100m/s. When I even try, that is... even on challenges my budget is hardly ever so tight that I really have to.

[bewing]

29 minutes ago, Laie said:

Really? I recall that the theoretical maximum was supposed to be on the order of 130m/s,

No, the theoretical maximum is twice the orbital velocity. ie. 1080 m/s. But 500 m/s is fairly typical, as Mr. Horseman says. Burning retrograde by few m/s at your Mun Pe can get you a lot closer to the theoretical max.

[OHara]

I'm sure you're both right, and I'm surprised I can't find any historical post here with the math all in one place. 

The most Δv you can get from a close encounter with the Mun, without hitting its surface, is about 500m/s.  But, that 500m/s is a vector difference, mostly deflecting your path.  The resulting boost is not going to be in exactly the right direction that you need, and is not as effective at boosting your energy as would be 500m/s in low-Kerbin orbit when you are moving faster (Oberth effect).

In an idealized case, suppose the gravity assist from the Mun ejects you exactly along its orbit at 300m/s, you have 300m/s + 540m/s = 840m/s relative to Kerbin, at the Mun's orbital radius.  The excess energy above that needed to escape Kerbin is E = v²/2 − μ/r  = 840²/2 − 540² = (250m/s)².
But first you have to get to the Mun from low-Kerbin orbit using roughly 840 m/s using fuel

You could get the same excess energy directly from low-Kerbin orbit burning Δv such that (2400m/s + Δv)² /2 − (2400m/s)² = (250m/s)².  
I get Δv = 1010 m/s, using fuel .  So 170m/s savings in the idealized case, mostly because Δv at the Mun's altitude is not worth as much as in Δv in LKO. 

I think @Laie's example here shows the minor benefit really well:

Gravity assists from moons, other than the Mun, start being very useful when you visit Jool.

Edited December 26, 2018 by OHara