More efficient journey to Duna

[ThomasCZ]

Hello everybody,

I have a question for more skilled players. I'm assembling space ship for Duna journey in space and here is my question. Is it better to assembly it in Kerbin orbit or Minmus orbit?

Thanks for any ideas

[PringleMan]

That really depends on your launch vehicle. It will be much easier to escape the Kerbin system from Minmus, so if you can get everything out there, and you have a really large vessel, it will be easier.

However building a large ship would require quite a bit of fuel to get out that far, plus getting a tanker out there to gas it up before it heads for Duna. So unless you can reliably lift everything out to Minmus, it would be better from Kerbin Orbit.

[ThomasCZ]

Well I'm asking because I already assembled whole ship at Minmus Now when I'm trying to escape Kerbin system it seems to be difficult because everytime I am caught by Kerbin sphere of influence so my launching is something like this: Minmus Orbit -> Kerbin orbit (46M altitude) -> escape from Kerbin system. So I'm starting to think if it's not better just to assembly it on Kerbin orbit (150k altitude) and just escape Kerbin system.

PS: please excuse my english I'm not native english speaker

[regex]

I'm pretty sure you still have to expend the ~950m/s delta-V to escape Kerbin SOI even from Minmus orbit, so it's probably best to build your ship in Kerbin orbit for the ease and time savings.

[Motokid600]

What exactly is it you plan on taking to Duna that it needs to be constructed in orbit?

[ThomasCZ]

It's something like space station with 5+ astronauts, lander and enough fuel to get back to Kerbin. I't's inspired with real future missions to Mars and back. Well I think I'll try both. I tell you, it's interesting to build something at Minmus orbit Good for improving your transfering and docking skills

[Bunsen]

Building in Minmus orbit is probably not helpful. You have to haul all the same hardware to Kerbin orbit, then make an intermediate stop at Minmus (including capture and Minmus-escape burns). Also, you're doing your interplanetary transfer burn from further up Kerbin's gravity well, which means old Herr Oberth is not being your friend.

The one benefit I could see is that you could get a boost from Minmus's orbital velocity; sort of a slingshot that you pause in the middle by capturing into Minmus orbit. I don't think that would be enough help to outweigh the delta-v wasted on capture/escape burns, let alone the Oberth effect penalty. You also have Minmus's orbital inclination to contend with, which probably isn't going to be lined up with your desired interplanetary transfer orbit, so it will cost a little more delta-v to correct that.

[Francesco]

Also, you're doing your interplanetary transfer burn from further up Kerbin's gravity well, which means old Herr Oberth is not being your friend.

actually, he is: you have to escape Minmus as if you were going back to Kerbin, set a low Kerbin periapsis (70-75 km), and then boost those final 200 m/s (?) to escape Kerbin and head to Duna.

So you can actually leave the Kerbin system with more delta-v on board, as opposed to starting directly from LKO.

of course, this method requires Minmus to be at the right place in its orbit around Kerbin.

[Pugspaceprogram]

Generally, i would not assemble it in orbit, as it only has a few advantages (not many). But if i were you, i would just launch a ship with 9 liquid boosters in asparagus staging attached to an SRB with the payload on top.

[ThomasCZ]

And what about this: when I calculate both transfers (Kerb Orb vs Min Orb) here: http://ksp.olex.biz/ it's 1015 delta v from 150km Kerbin orbit and 664 delta v from 47000km Kerbin orbit (equivalent to starting from Minmus). It looks like it's more efficient to go from Minmus. Plus I'm starting there with full tanks.

[Francesco]

Generally, i would not assemble it in orbit, as it only has a few advantages (not many). But if i were you, i would just launch a ship with 9 liquid boosters in asparagus staging attached to an SRB with the payload on top.

you know that what you wrote doesn't mean anything without a proper context.

which SRB? which size of boosters?

[Bunsen]

And what about this: when I calculate both transfers (Kerb Orb vs Min Orb) here: http://ksp.olex.biz/ it's 1015 delta v from 150km Kerbin orbit and 664 delta v from 47000km Kerbin orbit (equivalent to starting from Minmus). It looks like it's more efficient to go from Minmus. Plus I'm starting there with full tanks.

If you don't care about the effort of getting things to Minmus orbit in the first place, then you're right. Maybe you've got a kethane refinery running up there or something, I dunno. But if lifting things from Kerbin is part of the cost, going from LKO to 47000km takes somewhere around 900m/s of delta-v and then there's a little more for the capture burn.

[ThomasCZ]

As I said, I already built that ship out there It started as normal space station and after 2 docked parts I said: "What about to build my first long range space ship here?" I admit it's not standard way how to go out of Kerbin system but it's not so hard to launch part of ship from kerbin and dock it at Minmus orbit. I'm building this ship like Kerbals "doing things first thinking later" So now I'm trying to figure out what is more efficient, if you have same ship with full tanks and with different orbits, to interplanetary transfers.

[Rickenbacker]

One reason I could think of for starting from Minmus (or Mun) orbit, is if you're using the Kethane mod to mine fuel for your ships. It's a LOT easier to get the fuel to orbit from the Mun or Minmus than it is from Kerbin.

[falofonos]

Put a ship in Kerbin orbit then calculate a Duna launch window for it and make the maneuver node.

Boost your Minmus ship to 150km periapsis coinciding with that maneuver node and time....

Herr Oberth gets out to give you a push.

Marvel as your high velocity ship requires far more deltaV for mid course correction to even get an intercept with Duna!!!

[tomf]

Without running the numbers I think that your best bet is to leave Minimus orbit and wait in a circular orbit around Kerbin with a radius = Minimus for your launch window. Dropping your PE to low Kerbin orbit might give a small saving but certainly not enough to be worth the hassle and I think it will probably end up slightly more.

If you were starting from scratch building at Minimus from components ferried from Kerbin would be very inefficient, the cost of getting them to Minimus is considerably more than the saving from starting from a higher orbit.