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Tacombel

Interplanetary from Minmus

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Hi all:

I have frequently read that it is easier to depart from Minmus orbit, as you have most of the delta-V already, but I don't now how to plan it, as Mechjeb or any other tool that I know doesn't provide a solution.

 

Thx.

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I've done it. I even had a set of steps to make it work every time. But I really don't recommend it as it's error prone (Mun tends to get in the way a lot) and really doesn't save you very much fuel. And it only saves you fuel if you're fueling up at a Minmus mining base.

The basic idea is to launch your ship from Kerbin to Minmus so it arrives almost empty but with enough fuel tanks to go to the eventual destination from Minmus, at least a full Minmus month (and preferably more) before your ejection burn.. Then fill the tanks and you're good to go.

Have a ship in LKO (a simple satellite will do), and plot the ejection with that. You're never going to do that burn but when you target that ship, you'll be able to see its maneuver node so you can aim your ship to hit it.

Leave Minmus half a month from the planned ejection, or even better 1.5 months*, so you come down and your periapsis touches your target ship's maneuver node. Then coast down and pass it, and then burn prograde or retrograde so your next periapsis occurs at the correct time (ie the time of the projected burn, give or take a day or so).

Coast out to Apoapsis, and correct your tilt at the farther out of the two An or Dn. It should be really cheap.

Make your maneuver node, and note it'll be 900+ dV cheaper than a normal ejection burn.

Burn at the right time and profit.

A very small profit, but hey it's profit.

*NOT IN TIME! Half a month in the orbit, so you and Minmus are on the opposite side of Kerbin from where you want to eject. You should be able to do it less than a Minmus month  before, but it's better to give yourself some more time especially when you're learning the process. Don't go TOO early, though, because every orbit is a potential Mun encounter.

Edited by 5thHorseman
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1 hour ago, bewing said:

There are many people who claim that it's easier just to have refuelable booster stages in LKO.

There is a lot to be said for this approach if you want to get the most out of refuelling but don't want to take a lot of unnecessary hardware with you to points out-system.  Provided that you're willing to establish the refuelling infrastructure and standardise your rocket design to be compatible with the booster, it pays for itself the second time you use it because it gives you the same principal advantage as an SSTO:  reusable hardware.  In some ways, it's even better (e.g.:  no wings in vacuum).

However, since you also ask about 

2 hours ago, Tacombel said:

[...] easier to depart from Minmus orbit [...]

and to that I will say that it is only easier if you start measuring after you get there, and only in terms of fuel use--planning such a departure is usually harder because of reasons already mentioned by others.  Keep in mind that without refuelling, you still have to spend the delta-V to get to Minmus's orbit, and that cost counters the savings you get from the Oberth dive (the 'easier' departure) because conservation of energy applies.  However, there is one 'exception':  if you split the interplanetary transfer burn so that the first burn establishes an apoapsis at Minmus's orbit but keeps your periapsis at LKO, then you get some of the advantage of the Oberth effect even though you're starting from low orbit, but this is actually a different technique called periapsis kicking.

I'll put the details in a spoiler:

Spoiler

 

The idea of that technique is that burns are more efficient if you make them at high speeds:  this is the basic notion behind the Oberth effect.  I'm actually skipping a lot of the underlying mathematical subtlety of the Oberth effect but let's assume that this is an approximation.  The important part is that when we're discussing orbits, an object in circular orbit at LKO has a lower speed than an object in an elliptical orbit passing through a periapsis at LKO--this may seem counter-intuitive, but remember that you need a braking burn to circularise at low orbit.

At the same time, a lot of interplanetary rockets use low-thrust vacuum stages, which means long interplanetary ejection burns.  When completing such burns, naturally the final speed is high--it has to reach escape velocity, after all--but after you pass the periapsis, you're climbing out of the gravity well, which means losing speed.  With a low-thrust transfer stage, the last seconds of that burn are usually so far from periapsis that the Oberth effect savings are minimal.

However, it is precisely the last seconds of the burn that finally kick the apoapsis out of the sphere of influence and to an interplanetary destination:  everything before that only sends your rocket to a progressively higher elliptical orbit, which means that if you stop your burn early, you'll come back to the same periapsis.  You can use this fact, you can split your burn over multiple passes, have each one centred on the most effective portion of the orbit at and very near to the periapsis, and the only cost is the time spent going round those high elliptical orbits.

 

The important bit is that you start at low circular orbit, but you limit the burn time at periapsis to about a minute (perhaps up to two minutes, if you have especially weak engines and don't want to leave completing the manoeuvre to your grandchildren) because anything not at the periapsis wastes the Oberth effect.  Then, on the next pass, you burn for a minute centred on the periapsis again, and again after that.  Each of these burns at the periapsis 'kicks' the apoapsis higher and higher until you raise it to nearly the boundary of the sphere of influence.  The next and final burn is the one that sets up the final interplanetary transfer.

Planning for this kind of departure is not difficult:  set up the interplanetary manoeuvre as you normally would, but set it up well in advance:  two 'minths' of lead time should be your minimum time and you may need more.  The node will be in the same place as your eventual periapsis (that should make sense), but remember that each kick will change the orbit and thus also change the positioning of the final burn node, but we can use that for timing later.  The second-to-last burn will be less than a minute because this is what we will call the 'timing burn':  burn so that your orbit brings the node and the periapsis together.  In other words, you want the time-to-periapsis and the time-to-node values to match.  This may leave your apoapsis a little short of Minmus's orbit, or you may get the timing slightly off, but that's okay:  transfer windows to other planets are within acceptable tolerances for a week or more, so don't fixate on the 'perfect' transfer.  You can use MechJeb's node editor to figure a lot of this part, but it isn't necessary.  There used to be a mod for that, but it hasn't been updated since 1.2.2.

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@Tacombel

I believe the phrase "From Minmus Orbit" means at Minmus's orbital level. I've never jumped to interplanetary space directly from Minmus's SOI; that would be quite annoying due to Minmus's inclination.

Minmus is still quite useful for interplanetary travel due to the possibility of refueling. It only takes a few puffs of fuel to escape and level out the orbit.

So, try going to Minmus, refueling, escaping, leveling the orbit, and then going interplanetary.

If you don't need to refuel, you're probably fine with a lower orbit.

Perhaps someone with more experience can clarify or show me the error of my ways, but that's the way it is in my experience.

 

Hope that helps, and enjoy!

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As said above, going interplanetary directly from minmus is very inefficient and difficult. Any of the sources you've read about this meant that you leave minmus, do an Oberth dive to a very low Pe on Kerbin, then do your interplanetary burn there.

It's still rather difficult to do, because minmus needs to be at the proper ejection angle from kerbin when you begin the Oberth dive. Which means you may need to execute the first maneuver up to one minmus orbit in advance.

There are many people who claim that it's easier just to have refuelable booster stages in LKO. That way, you don't have any of this preplanning to do -- you can leave during any 30 minute window you like. After using a booster, you put it in a Minmus intercepting orbit. When it gets to Minmus, you refuel it and send it back to LKO.

 

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Once you get to the Mun's orbit, you need to be careful of the Mun itself, but that's only an issue after applying about 800 m/s of the 1000 m/s needed to escape Kerbin's SoI.  Also, once you set up your last orbit before ejection, PLOT YOUR EJECTION NODE RIGHT AWAY and look for a Mun intercept if your timing isn't exactly on the pre-plotted node's.

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As others have noted, the only benefit to going to Minmus is for ISRU fueling. That way, you're burning a fraction of the fuel you would have to perform the eject burn to get to the orbital altitude of Minmus. Otherwise, it's pretty much worthless.

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6 hours ago, MaverickSawyer said:

As others have noted, the only benefit to going to Minmus is for ISRU fueling. That way, you're burning a fraction of the fuel you would have to perform the eject burn to get to the orbital altitude of Minmus. Otherwise, it's pretty much worthless.

That's exactly the idea, but so far there is no way to depart directly from Minmus orbit to interplanetary :-(

 

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If I have good reason to stop at Minmus before making the interplanetary journey - for example,  refuelling on there with ISRU - then I do so,  but I don't concern myself with fancy assist maneuvers in that case (after refuelling,  delta v is hardly likely to be an issue in any case.      I  just get back into orbit of Minmus from the surface,   then do a Minmus escape burn (for what its worth,  i try to time this so that we escape prograde to Kerbin orbit,  so this contributes to the escape from Kerbin SOI.   Once in orbit around Kerbin, you're looking at an under 100 m/s burn to escape into orbit around the Sun.    And from there, plan the interplanetary journey as a straight Hohmann transfer,  though if you want you could get fancy by assisting off other planets to get to the  final destination from that point. 

Minmus itself has such low gravity I don't think there's much Oberth effect to be worth the trouble it is to exploit. 

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2 hours ago, Tacombel said:

That's exactly the idea, but so far there is no way to depart directly from Minmus orbit to interplanetary :-(

 

Sure there is. Wait until Minmus is in the right spot around Kerbin so it's going the direction you want to go when you eject, and then wait for your ship to be going around Minmus in the direction you want to eject, and burn prograde until you escape Minmus, then Kerbin, then get your Sun periapsis or apoapsis to the desired planet's orbit.

It's not particularly efficient or easy, but it's most definitely a way to do it that I've also done in the past with success.

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My favorite Kerbin orbit altitude for starting interplanetary travel is 610km. Not for any practical reason other than Its just above where you can use maximum warp while waiting for your transfer window. 

Though it does also reduce the Kerbin orbit escape dV requirements for your xfer stage, whilst mostly avoiding the Mun. 

Edited by Foxster

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Following your advice now I have a pusher in LKO that I refuel with a spaceplane for a minimum cost. It is expensive in terms of fuel, because recovering the pusher cost around 1250 m/s, but with the spaceplane refuelling in LKo is easy.

As a secondary benefit, with the pusher it is easy to push the ion probes and leave the long burn for when they are in solar orbit.

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The problem with starting at the Mun or Minmus and diving for a Kerbin pe is that it's very easy to make an inaccurate burn, because your Kerbin pe isn't that likely to coincide with the best place to do an interplanetary burn. Optimistically, you'll save about 500-600 dV vs refueling in LKO. Pessimistically, you'll miss the best spots to burn and you'll end up using more fuel to correct the burn.

I think having a fuel depot in LKO, filled with tankers "based" on Minmus is more fuel efficient - though you really need to like docking to do that.

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On 11/1/2018 at 8:11 AM, 5thHorseman said:

Have a ship in LKO (a simple satellite will do), and plot the ejection with that. You're never going to do that burn but when you target that ship, you'll be able to see its maneuver node so you can aim your ship to hit it.

I'm on mobile and can't seem to move these two quotes, but the above is a way to ensure the below doesn't happen.

3 hours ago, juanml82 said:

The problem with starting at the Mun or Minmus and diving for a Kerbin pe is that it's very easy to make an inaccurate burn, because your Kerbin pe isn't that likely to coincide with the best place to do an interplanetary burn.

 

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I have this crazy way of doin thangs n stuff. Buuuut it requires some luck and some more patience.
It only worked twice, so it must be useful (I just don't have the patience to time it right every time.)

Landed and refueled on Minmus, I launched straight up from the trailing side of the equator +/- 6 degrees, right as Minmus crosses Mun's orbital plane.
Burnin burnin burnin, I would've smacked into Kerbin, but instead, my ship came in right behind Mun. No more burn or we all gon' die! 
♫Now I'm freeee~! Free-fallin!♫ Mun scoops me into its SoI and slings me on faster, thus safe from the boiling splat on Kerbin. Barely need a drop of go-juice for corrections (if any at all) and already headed for interplanetary space!
Set that Kerbin Pe burn to your target world and enjoy your cost-effective and/or thrilling joy-ride!

Afterword: don't come with me if you want to live. I don't math. I eyeball it all into submission with my wild-eyed stare and toothy, somewhat-disconcerting grin. :cool:

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On 11/2/2018 at 6:04 AM, 5thHorseman said:

Sure there is. Wait until Minmus is in the right spot around Kerbin so it's going the direction you want to go when you eject, and then wait for your ship to be going around Minmus in the direction you want to eject, and burn prograde until you escape Minmus, then Kerbin, then get your Sun periapsis or apoapsis to the desired planet's orbit.

It's not particularly efficient or easy, but it's most definitely a way to do it that I've also done in the past with success.

I've done this using the Mun, since it has a shorter orbit and thus can more conveniently set up such escape windows.  In the end, though, I wasn't sure how much it helped versus just doing it from LKO.  Assuming topped off fuel tanks from both locations (say 80km Kerbin/15km Mun), do you know the over/under on that one?

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5 minutes ago, FinalFan said:

I've done this using the Mun, since it has a shorter orbit and thus can more conveniently set up such escape windows.  In the end, though, I wasn't sure how much it helped versus just doing it from LKO.  Assuming topped off fuel tanks from both locations (say 80km Kerbin/15km Mun), do you know the over/under on that one?

No, but my guess is LKO's better for the ship but Mun's probably better overall, considering you have to get the fuel from Mun (or Minmus) orbit down to LKO.

I don't really run numbers :D

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30 minutes ago, FinalFan said:

I've done this using the Mun, since it has a shorter orbit and thus can more conveniently set up such escape windows.  In the end, though, I wasn't sure how much it helped versus just doing it from LKO.  Assuming topped off fuel tanks from both locations (say 80km Kerbin/15km Mun), do you know the over/under on that one?

You should do a search for "Gate Orbits".

Basically, for any given interplanetary transfer you need to escape Kerbin's SOI with a given excess volocity (V). For any given V there is a given altitude at which you need the least amount of dV to attain that excess velocity. 
For Eve and Duna, that altitude is around the altitude of the Mun, so it is definitely useful to start from there with full tanks.
From memory, the altitude for Jool is somewhere in the 200-300km range. And for Moho or Eeloo it is at or lower than the top of Kerbin's atmosphere. Therefore, starting at the Mun for those destinations is no benefit, other than the obvious benefit of having full tanks before you leave.

For example, for Duna you need to burn for about 1100 m/s from LKO, but only under 700 m/s from a 12,000 km circular orbit. Of course, from Mun orbit you need to escape the Mun's gravity, which will increase that figure a bit, but you also benefit from the Mun's contribution to the Oberth effect. So you should still save roughly 400 m/s or so going directly from the Mun.
 

For all of the above, I'm talking about making the escape/interplanetary burn directly from Mun orbit, so you leave the Mun's SOI going very fast and prograde. Good for Eve/Duna, not so good for anywhere else (especially Moho, since you really need to align with the inclination of Moho's orbit, making it doubly expensive).

On the other hand, diving down from the Mun to a low pass at Kerbin to make your ejection burn will always save you about 550-600 m/s (assuming full tanks). The 250m/s burn to escape the Mun and return to Kerbin is "wasted", but once you're at Kerbin you'll already have the equivalent of a 850m/s burn under the belt, as it were. So it's definitely worth doing this for Jool (and for Moho, Dres or Eeloo if you can be bothered getting the inclined orbit right), while for Eve or Duna you just need to decide whether the additional planning time is worth the ≈150 m/s difference compared to just burning straight out from the Mun.

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On 11/1/2018 at 1:11 PM, 5thHorseman said:

I've done it. I even had a set of steps to make it work every time. But I really don't recommend it as it's error prone (Mun tends to get in the way a lot) and really doesn't save you very much fuel. And it only saves you fuel if you're fueling up at a Minmus mining base.

The basic idea is to launch your ship from Kerbin to Minmus so it arrives almost empty but with enough fuel tanks to go to the eventual destination from Minmus, at least a full Minmus month (and preferably more) before your ejection burn.. Then fill the tanks and you're good to go.

Have a ship in LKO (a simple satellite will do), and plot the ejection with that. You're never going to do that burn but when you target that ship, you'll be able to see its maneuver node so you can aim your ship to hit it.

 Leave Minmus half a month from the planned ejection, or even better 1.5 months*, so you come down and your periapsis touches your target ship's maneuver node. Then coast down and pass it, and then burn prograde or retrograde so your next periapsis occurs at the correct time (ie the time of the projected burn, give or take a day or so).

 Coast out to Apoapsis, and correct your tilt at the farther out of the two An or Dn. It should be really cheap.

Make your maneuver node, and note it'll be 900+ dV cheaper than a normal ejection burn.

Burn at the right time and profit.

A very small profit, but hey it's profit.

*NOT IN TIME! Half a month in the orbit, so you and Minmus are on the opposite side of Kerbin from where you want to eject. You should be able to do it less than a Minmus month  before, but it's better to give yourself some more time especially when you're learning the process. Don't go TOO early, though, because every orbit is a potential Mun encounter.

I have done this many times with an twist, I tend to enter high orbit around kerbin and then adjust
 here is an example of an trajectory. 
pzn1hQrl.png
First node is an adjustment to tune my orbit so i can line up my orbit so I match the satelite node. This require some fidling, recommend first making the lower Pe to low orbit first then adjust the adjustment node so timing is correct, yes its goes inward a bit and this makes the lowering of Pe a bit more expensive, think this was 200 something m/s. You should also align your orbital plane with the satelite. 
This method can be pulled off less than 1 minmus months ahead but is 150-200 m/s more expensive, you can also avoid the Mun easier. 

You ejection burn will be 8-900 m/s cheaper, it also seems like it has smaller errors than an burn from LKO.
Current planing an Jool mission  with window in around 44 days so i might try the direct from Minum orbit. 

Going to Duna or Eve you don't need boosters for this as the ejection burn will be very small for an lowest cost trajectory. 

Or you can reach Jool in an year. Plan doing this with having the booster lift an LV-N tug to orbit, the tug will then finish the burn, circulate bringing the booster, get some fuel in LKO, fly to minmus orbit for filling up all tanks, dock the base then drop this down to LKO for an 2200 m/s burn. using first the booster and then the tug as an second stage. Currently simulating the Layte aerocapture as I need to brake for 2-2.5 km/s in its atmosphere to end in an fitting orbit around Jool. 

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I started assembling my interplanetary crafts in High Mun Orbit.

Reasons are for refueling, and second for the easy Mun flybys from the Mun, just leave 2  Munar months ahead of the transfer window and leave the Mun's Soi at the point you want the flyby to happen . I tried doing a similar thing, but with leaving from Minmus, but the long minus month ruined my window as I was traveling in the wrong direction relative to my ejection. Ended up costing 1kDv more to get to Jool than the next mission that left from the Mun(although there were other issues with the Minmus send). My Mun->Jool trip cost around 2.1kDv, where as my Minmus->Jool was something like 3.4k, and is also not in the right inclination.

Edited by SlinkyMcman
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If you're building the craft in orbit then that changes every aspect of the calculation. The only thing you need to ever bring from Kerbin at that point is the Kerbals and they only need to be able to get to Mun or Minmus.

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Hello, I'm going through my first gameplay and I'm aiming for Duna. I've only been to minmus and mun. I'm playing stock and I found Docking and maneuvering kind of pleasant. I'm ignoring the deltaV calculations and I've been flying the same ships multiple times between minmus, mun and kerbin while refueling at minmus.

Up to this point, I'm using a crane to dock to the tail of my ship then undocking the fuel pod, carry it to my mining station and dock it on top of my miner to refill. Since I'm scaling up my ship to go to Duna (I feel like packing for vacations, I'm taking a LAB, a lander (obviously) a compact miner rig, 4 satellites to make a relay network and two fuel pods that I hope will suffice) I made an independent fuel carrier with more capacity, it's own engine and controls (so I don't need the crane). This time, the fuel transfer vehicle is too big to dock on top of the miner so the idea is to land it nearby and use a moving vehicle that would act as a hose or a bucket (coming and going) to fill it up. THAT's my headache right now...

The past two "gaming sessions", I was testing (at kerbin) different approaches to make a transfer cart. The ONLY moving part I was able to find (stock, I'm sure mods solve this, but I'm not ready for them) is the Advanced Grabbing Unit (using the "free pivot" option). Problems I'm facing is that if I want the pivot to move in a single axis, I need two AGUs and it's not working well. I'll keep you updated if I get to make a practical design! For now, my Duna exploration is being held by the refueling labor union.

Now, I can't properly recommend or encourage new players to adopt the minmus strategy because I yet haven't used it myself ! But I can say that minmus low gravity and inclination may train your skills and thanks to the ISRU refueling possibility, I'm less stressed with dV efficiency and more open to exploration. I'm sure I will end up checking the dV and maneuver efficiency in the end, but meanwhile I'm getting lots of fun this way. I can say that at least.

This thread is very interesting. Regards!

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