deltaV for going from minmus to elsewhere

[king of nowhere]

i'm looking at the deltaV map, and it says i can get from a kerbin intercept to an eve intercept with just 90 m/s. indeed, i verified it; from LKO i could get an eve intercept with a bit more than 1000 m/s. nice! so i have a ship orbiting minmus (it stopped there for refueling), and i figured, from there i need 150 m/s to escape minmus - actually i am in a high orbit, i can escape minmus with 25 m/s, and if i do it at the right point in the orbit, i am already in a kerbin escape trajectory. so, if i angle my escape from minmus correctly, i should be able to get to eve with 90 more m/s of deltaV. total 115 m/s. make it 150 if the orbit is not just right. make it 200 because from minmus i lose some oberth effect.

instead, i cannot get a eve intercept even with 400 m/s. do notice that for the sake of this experiment i'm not even trying to get an actual intercept, i'm just trying to see if i can get a periapsis with kerbol of 9,5M kilometers, which is eve's orbit and is what would be required for an intercept. my best effort goes to 9.9M km, with 400 m/s of deltaV.

I'm trying to figure out the exact mechanics. am i doing something wrong? i consider myself well versed in orbital mechanics, and i don't see how i could improve the trajectory. from everything i ever tried, the most efficient injection burn is made on the planet, so waiting until i am out of minmus orbit to make another manuever gains me nothing - indeed, simulations confirm this. i already am exiting from the kerbin SOI with the best inclination, straight backwards compared to kerbin orbit - or at least close enough to it that it should make little difference.

the only thing i can think of that could possibly explain why i can't get to eve is oberth effect: on minmus i am slower, i get less juice from my thrust. i am fully expecting that i'll need more for this reason. i was expecting those 90 m/s may even become 180. but i need 480 m/s! that's 5 times more than i would have needed from LKO. can oberth effect here explain a 5 to 1 change in fuel efficiency? i read that you can get similar effects with jupiter, but on an earth-sized planet? and if it's not obert effect eating all my deltaV, then what?

if this is normal and there is no way around it, would it be more convenient to send my ISRU fuel back to LKO and refuel ships there? i thought by refueling them on minmus i would save those 900 m/s needed to get there, but if from minmus the cost of moving anywhere is quintuplicated...

 

on a related note, how can i calculate reliably the cost to get from mun to minmus, or from one of the various moons of jool to another mun, or to somewhere else?

Edited July 23, 2020 by king of nowhere

[bewing]

Yes, the difference is entirely Oberth effect. In LKO you have over 2000 m/s of velocity. In Minmus orbit, you only have 72.9 m/s. Round that down to zero in your head.

The Oberth effect says that if you burn while you already have velocity, then you get to keep more of that velocity all the way to your destination. Burning is much more effective (yes, easily 5 times more effective) if you already have a decent velocity.

Edited July 23, 2020 by bewing

[king of nowhere]

damn. i will have to rethink all my refueling logistics then.

[mikegarrison]

59 minutes ago, king of nowhere said:

damn. i will have to rethink all my refueling logistics then.

I ISRU-refuel from Minmus because a little bit of delta-v on the interplanetary transfers is not a big deal compared to the cost of bringing fuel up from Kerbin. Also, the gameplay is simpler. It's easier to get into high timewarp orbits. And Mun never interferes.

[Superfluous J]

1 hour ago, bewing said:

In LKO you have over 2000 m/s of velocity.

Actually, while doing that last 90m/s to burn to Eve, you have over 3000m/s of velocity.

[Spricigo]

3 hours ago, king of nowhere said:

would it be more convenient to send my ISRU fuel back to LKO and refuel ships there? i

There is many ways to do the trick. It's up to you to figure out what is more convenient to you. 

Personally, I like the idea of combining the reffueler with the transfer stage. Work like this: 

1. Launch the interplanetary ship into a high elliptical orbit, that comes near Minmus without entering Minmus SoI

2.Launch refueller from Minmus into a rendezvous&docking with the interplanetary ship. 

3.Do the transfer burn at the periapsis to get the Oberth Effect. 

4 Release the interplanetary ship and burn retrograde with the reffueler to stay within Kerbin SoI. (eventually get it back to Minmus, no rush there) 

That method don't require to get the interplanetary ship all the way to minmus or the reffueler all the way to LKO. But getting the correct launch windows can be tricky (need to get right when launching from both Kerbin and Minmus) 

 

 

[Reactordrone]

Minmus to Kerbin to Eve (165m/s to Kerbin + 110m/s boost close to Kerbin) can shave off a couple of hundred m/s compared to a Minmus to Eve transfer. It does make timing more difficult though.

[Superfluous J]

Frankly if you're refueling at Minmus, fuel is free so I'd just launch from there directly to Eve. No sense saving 150-200m/s with a TON of work and the likelihood of wasting more than that fixing any mistakes in execution.

[king of nowhere]

7 hours ago, Superfluous J said:

Frankly if you're refueling at Minmus, fuel is free so I'd just launch from there directly to Eve. No sense saving 150-200m/s with a TON of work and the likelihood of wasting more than that fixing any mistakes in execution.

to eve, yes. i spend 400 m/s extra in getting out from minmus, but i save 930 m/s on getting out of kerbin SOI, so it's a net gain for me.

but as i move to other destinations, quintuplicating the cost of the interplanetary transfer orbit will soon outweight the 900 m/s i save in getting up to minmus. and it's not just a matter of getting more fuel - as you pointed out, it's free - but of needing bigger fuel tanks.

[bewing]

Well, it doesn't quintuplicate forever. The maximum Oberth dV that you can gain in LKO vs. Minmus is about 2000m/s, and the maximum you can gain vs. the Mun is about 1000m/s. So it's really a question of whether you can conveniently get free fuel to LKO vs. LMO as far as I'm concerned.

 

[Okhin]

And since Minmus can be a pretty good fuel producing body, it might be worth it to use it as a refuel pit stop before escaping. It allows you to send ship unfueled from Kerbin, or to burn through that fuel you would have saved to reach another planet, which makes for cheaper launch. That's basicaally why I setup the Space Bar at the edge of Kerbin SOI. Also having a space station that far out is fun

[king of nowhere]

2 hours ago, Okhin said:

And since Minmus can be a pretty good fuel producing body, it might be worth it to use it as a refuel pit stop before escaping. It allows you to send ship unfueled from Kerbin, or to burn through that fuel you would have saved to reach another planet, which makes for cheaper launch. That's basicaally why I setup the Space Bar at the edge of Kerbin SOI. Also having a space station that far out is fun

yes, i was doing the same, flying my ships with just enough fuel to get to minmus and refueling there - sometimes i even had insufficient fuel for capture, i had to send a tanker on a rendez-vous on intercept trajectory just to catch the ship.

but i'm wondering if it would be more convenient to send the fuel to LKO and refuel ships there. if for some destinations it is more convenient to launch from LKO than from minmus because of oberth effect, i want to resupply in LKO. To that purpose, i already sent up a giant space tank that i'm slowly going to fill.furthermore, it is much more convenient to make trajectories from there, as from minmus i need to have minmus itself in the right place along its orbit for maximum efficiency, while with LKO i just need the correct placement of kerbin and the target planet - and i can skip even that if i'm willing to go on an elliptical trajectory and make an intercept in two manuevers.

is there a way to calculate whether it is more convenient to launch from LKO or minmus to a given destination? besides using the debug console to place yourself in the various orbits and trying all the options, of course.

Edited July 24, 2020 by king of nowhere

[Spricigo]

1 hour ago, king of nowhere said:

is there a way to calculate whether it is more convenient to launch from LKO or minmus to a given destination?

no. Because "convenience" is subjective. Personally I find the whole ordel of setting up a minmus mining base very inconvenient, since I can just lift it from Kerbin. (by the time ISRU is available, Funds is not an issue anymore)

However if you want to understand more about the oberth effect and the maths behind it, this thread is a good start:

 

[Superfluous J]

10 hours ago, king of nowhere said:

but as i move to other destinations, quintuplicating the cost of the interplanetary transfer orbit will soon outweight the 900 m/s i save in getting up to minmus

As was said, it's not 5x for everything. In fact, IIRC it's actually cheaper to get to Duna this way.

It's been a long time since I've played done something like this but I don't recall the extra cost ever being a factor.

My favorite thing to do back then was to launch a ship that used its own fuel to do most of the trip to Minmus, so it arrived mostly empty and got fueled up for the interplanetary trip. It felt so efficient.

[Spricigo]

1 hour ago, Superfluous J said:

 It felt so efficient.

IMHO maximizing fun/gametime is the only efficiency that matters.

[Superfluous J]

34 minutes ago, Spricigo said:

IMHO maximizing fun/gametime is the only efficiency that matters.

Agreed! But to me, being efficient IS fun.

[mikegarrison]

On 7/24/2020 at 1:16 PM, Spricigo said:

"convenience" is subjective

Agreed, and for me that most convenient bit is sitting a ship in a high (like, higher than Mun) orbit and being able to fast time-warp to when I make the transfer burn without worrying about whether Mun is in the way. Let's say you are in an 80-km LKO. Sure, you get a nice boost from the orbital speed, but you can only do a 5x 50x timewarp. Since delta-V is actually super cheap (and even cheaper if coming from Minmus), those are the convenience issues for me.

Edited July 25, 2020 by mikegarrison

[Spricigo]

4 hours ago, Superfluous J said:

Agreed

 

2 hours ago, mikegarrison said:

Agreed

And I thought my condition was not contagious... 

 

[Superfluous J]

2 hours ago, mikegarrison said:

Let's say you are in an 80-km LKO. Sure, you get a nice boost from the orbital speed, but you can only do a 5x timewarp.

That's why I won't play without

 

[king of nowhere]

11 hours ago, Spricigo said:

IMHO maximizing fun/gametime is the only efficiency that matters.

of course; and in this case, i maximize fun by trying to maximize efficiency.

after all, this game is a big sandbox mode with no real objectives, so we have to make our own. otherwise everything is pointless. why even go farther than minmus? you can unlock the whole tech tree while staying on mun. why make space stations? they provide no tangible benefits.

by the way, the launch window planner has nothing for going from minmus to interplanetary.

[HansAcker]

16 hours ago, mikegarrison said:

Sure, you get a nice boost from the orbital speed, but you can only do a 5x timewarp.

The "warp to maneuver" button on the burn-time indicator conveniently ignores these limits.

[Fraktal]

18 hours ago, mikegarrison said:

Let's say you are in an 80-km LKO. Sure, you get a nice boost from the orbital speed, but you can only do a 5x timewarp.

Which is why I use 120 km as standard parking orbit rather than 80 km. The difference in Oberth is negligible as far as the community delta-V map is concerned.

[king of nowhere]

19 hours ago, mikegarrison said:

 Let's say you are in an 80-km LKO. Sure, you get a nice boost from the orbital speed, but you can only do a 5x timewarp.

wait, what?

in a 75 km LKO i can do a 1000x warp, at least. i only have limitations if my periapsis goes below 70,25 km. even then, it's 50x as long as i'm out of the atmosphere. and i have no mods installed. i do have the breaking ground and making history expansions.

why this difference?

Edited July 25, 2020 by king of nowhere

[mikegarrison]

48 minutes ago, king of nowhere said:

wait, what?

in a 75 km LKO i can do a 1000x warp, at least. i only have limitations if my periapsis goes below 70,25 km. even then, it's 50x as long as i'm out of the atmosphere. and i have no mods installed. i do have the breaking ground and making history expansions.

why this difference?

Sorry, I meant 50x, not 5x. By default, the timewarp limit under 120 km but above the atmosphere is 50x. Between 120 and 240 km it is 100x. 240-480 it is 1000x. 480-600 it is 10000x. You can't go 100000x until you are above 600 km. Also, the limits only apply if the ship is in focus. You can switch back to KSP and timewarp as fast as you like. (Note, if this recently changed, then maybe I'm wrong here, but these are the numbers in the KSP wiki and also the ones I remember from the last time I played a lot, about a year ago.)

So no, you don't need to go to Minmus for fast timewarp. An orbit of 700 km is enough. But I still prefer to go park my ships waiting for their transfer window out pretty high just so I can easily ignore them until their waiting time is up. Delta-V is so very cheap that I just don't worry much about minimizing it except really early on in a career.

Oh hey, looking through the notes, I think maybe they did change some timewarp behavior for version 1.9. So maybe my memories on this (and the wiki) are outdated?

Edited July 25, 2020 by mikegarrison