Minmus Gravity Assist for return from Mun?

[Qubyte]

Hi.

So I was returning from a Mun landing, and I must have hit shift or z accidentally while in map view, because after I closed map view, I saw that my engine was thrusting and that 400 m/s of dV was gone, and I was on a collision course with the Mun. I managed to put the craft back into orbit, but by then I only had ~700 m/s of dV left, which is not enough to return from the Mun. However, apparently it would only take me ~250 m/s of dV to transfer to Minmus, and from there I could do some gravity assists and burns to try and return back to Kerbin. I tried messing around with manuever nodes, and got an encounter with Minmus, but I was unable to get a gravity assist that could lower my PE a meaningful amount. How can I get an gravity assist that will help lower my PE enough that I'll actually be able to make it with the remaining fuel (or a totally different way to make it back that does not involve the cheat menu)?e

Thanks! For reference, my craft is in a 80 by 80 Mun orbit with about 7 degrees inclination.

[Stamp20]

You can raise your AP to be far above Kerbin, near or above Minmus height, then burn retrograde at AP. It won't take that much fuel to get your PE into Kerbin's atmosphere. From there, the atmosphere will do the rest.

Edited July 31, 2020 by Stamp20

[king of nowhere]

with 700 m/s you can get back to kerbin from where you are twice comfortably, perhaps 3 times. it's just a matter of finding the right place to burn. you want to exit from mun orbit pointing in the direction opposite to the movement of mun around kerbin. this way, all your orbital velocity around mun will be converted into orbital velocity to lower your periapsis once you are in kerbin orbit. a single burn of 250 to 300 m/s from where you are should do the trick, properly aligned. you only need to get your periapsis below 70 kilometers, you will aerodrag into the atmosphere.

 

the minmus gravity assist you want... i doubt it's feasible. minmus is very small and slow. what you can get out of a gravity assist is proportional to how much gravity your body has, and how fast it is orbiting. so, you get a really small kick from minmus. mind you, it may still be enough to put you in a kerbin aerobrake trajectory. also, i'm sure you can get to minmus with less than 250 m/s. 200 should be enough, again if you get the proper orbital timing.

in fact, with 700 m/s, you should be able to get from mun to minmus, circularize orbit around minmus, and then get back to kerbin.

[magnemoe]

On 8/1/2020 at 12:26 AM, king of nowhere said:

with 700 m/s you can get back to kerbin from where you are twice comfortably, perhaps 3 times. it's just a matter of finding the right place to burn. you want to exit from mun orbit pointing in the direction opposite to the movement of mun around kerbin. this way, all your orbital velocity around mun will be converted into orbital velocity to lower your periapsis once you are in kerbin orbit. a single burn of 250 to 300 m/s from where you are should do the trick, properly aligned. you only need to get your periapsis below 70 kilometers, you will aerodrag into the atmosphere.

 

the minmus gravity assist you want... i doubt it's feasible. minmus is very small and slow. what you can get out of a gravity assist is proportional to how much gravity your body has, and how fast it is orbiting. so, you get a really small kick from minmus. mind you, it may still be enough to put you in a kerbin aerobrake trajectory. also, i'm sure you can get to minmus with less than 250 m/s. 200 should be enough, again if you get the proper orbital timing.

in fact, with 700 m/s, you should be able to get from mun to minmus, circularize orbit around minmus, and then get back to kerbin.

Yes, 700 is more than enough, now if you are in an highly inclined orbit like an polar one you might have to wait until your orbital plane matches up with mun direction of travel, at this point making an node who have you escape the mun and lower you Pe around kebin should send you back. 

Now if you had 200 m/s dV you could use the Mun for gravity assist, get into orbit around Kerbin and lower your Pe so you get an encounter with the Mun after 2-4 of your orbits. 
You now want to pass the mun on the outside and mun gravity will bend your trajectory and lower your Pe, this can be a bit complex to set up however. 

[Gargamel]

As mentioned, 700 is plenty enough to get back, it's not enough though to establish a LKO, and then do a re-entry burn, that'll run you about 1100, give or take.   Just adjust your Kerbin Pe to about 30km and hang on. 

[king of nowhere]

11 hours ago, Gargamel said:

As mentioned, 700 is plenty enough to get back, it's not enough though to establish a LKO, and then do a re-entry burn, that'll run you about 1100, give or take.   Just adjust your Kerbin Pe to about 30km and hang on. 

actually, it is also plenty enough to enter LKO. put your kerbin Pe at around 50 to 60 km, this way a single pass through atmosphere won't be enough to deorbit but it will slow you down. do this aerobraking manuever until you have a low enough apoapsis, then you can raise your periapsis out of the atmosphere with roughly 30 m/s

I once had a craft in mun orbit with less than 500 m/s of deltaV left, and i used it successfully to rendez-vous with another craft in low kerbin orbit that was without fuel.

that specific rescue mission failed because after i made the rendez-vous i realized one of the two crafts lacked a docking port. but the thing is, with 700 m/s on a mun orbit you can go anywhere within kerbin's SoI, with fuel to spare

[Qubyte]

Thanks for the help! I went with doing the "bi-elliptical" transfer (for lack of a better way to describe it, and it's kind of similar...), and managed to get back successfully. I wonder why my previous Mun ejections were so inefficient. Perhaps I ended up putting the maneuver on the wrong side of my orbit?