Refueling station: Moho or Gilly?

[Fwiffo]

I have what's proving to be quite a difficult mission in career mode to save a Kerbal who's in low orbit of the sun.  I designed a refueling station as part of the rescue operation, with the intent of landing it on Moho as a "last stop" gas station before heading inward.  However after studying the Delta-V subway maps a little further, I'm wondering if Gilly might be a better spot to invest the resources for a fuel stop (due to it having far less dV requirements to land / orbit / escape).

Any thoughts?

Edited May 14, 2016 by Fwiffo

[Alshain]

Gilly is also a lot easier to get to.  Moho moves so quickly and has such a small sphere of influence, not to mention it's inclination.

[Tig]

How low is low? 

I only ask because for the vast majority of missions, I agree with @Alshain and I'd say Gilly.

But Moho does have 2 advantages in your particular scenario, its closer to the sun and multiple transfer windows per year which might make a difference to you or not depending on mods.

Low Kerbol orbit does take a ton of dV to get in and out of... so I'm less concerned with the low dV cost of getting to Gilly when you'll have to design a rocket with a ton of dV anyway for low Kerbol.

Also, the orbit of your rescuee, if his inclination is positive to zero, Moho is an option, but if its negative I'd go Gilly for sure, the inclination dV cost that close (Moho) would be huge.

PS.  I'm not saying Moho is definitely better or anything - it typically wouldn't be...  just that for this purpose it could be an option.

 

Edited May 14, 2016 by Tig

[Scotius]

Gilly is easier, but you will need refuelling station near Moho more. So deep in the gravitation well every drop of fuel counts. But it only matters if you want to continue operations there. If you just want to grab your Kerbal and hightail it back to greener pastures, just send an automated tanker. Plonk it in a comfy parking orbit without inclination and stay away from Moho.

[DMSP]

Just wondering, you could use a gravity assist around Eve to slow down and/or kick you orbit to the altitude of that Kerbal. Especially if you aren't worried about time.

Otherwise, Gilly is the best option.

[Fwiffo]

Thanks everyone, I appreciate the diverse considerations pointed out.

3 hours ago, Tig said:

How low is low?

Rescuee's orbit is 2.3 billion meters, circular, no inclination.  Maybe 1/2 to 1/3 the radius of Moho's orbit.  Ok so I'm not sure if this qualifies as "low Kerbol" and maybe doesn't sound so bad to more experienced players.  But I got this mission very near the beginning of the game with barely any tech unlocked, and for gameplay reasons I want to pull it off without Ion engines.  I had a craft that started from Kerbin orbit with 17k dV which managed to rendezvous, but ran out of gas on the way back up before it even got its orbit as high as Moho.  The last stage was basically a command chair and Ant engine strapped to an Oscar tank.  Note I was using a fairly naive Hohmann maneuver and expensive "orbit synchronization corrections" near the sun; I realize there might be a more efficient approach (e.g. maybe a bi-elliptic transfer and better departure timing).  Open to suggestions.

Next attempt will be to start with a full tank from a fueling depot somewhere near Gilly or Moho.

Edited May 14, 2016 by Fwiffo

[cantab]

If your refuelling depot will have ISRU, then put it at Moho, it's closer to mid-way delta-V wise.

If you don't have ISRU, then what I might do is just drop fuel depots like breadcrumbs as it were. Ie:

Depart Kerbin and set periapsis level with the stranded Kerbal's orbit, and timed for rendezvous

During your burn at periapsis, drop full fuel depots. These will be in a series of elliptical orbits round the Sun.

Once you have the rescued Kerbal on board, use those fuel depots to refill. You might be able to chase them down right away, but otherwise just make extra orbits of the Sun and rendezvous with them at periapsis.

And one more thing to watch out for: Last time I went to Moho, the heat was enough to slowly vaporise my heatshield. If this is still the case then you may need to plan for a powered capture at Kerbin on your return, and then another rendezvous with a re-entry ship. (Or just a powered capture to LKO, and then you can re-enter a capsule without needing a heatshield).

[Jetski]

If using Gilly, make your refueling rig able to fly up and meet your ship in a highly elliptical eve orbit to save dV. Otherwise you could burn a lot of fuel leaving eve orbit after refueling ops. Make the refueler do the work instead!

[pandaman]

Off the top of my head I would try to do it without mining/refuelling bases.  The effort and expense you will need to go to to set all that up may as well go into just carrying more fuel and engines with you.  Moho is a tricky beast that just eats Dv if you don't get it right, why add extra risk and complications if you don't need to?

Build a decent sized ship for the round trip, top up the tanks in LKO before departure.  Maybe even send two or three to give you spare ships and back up fuel reserves, with care they can all end up in the same general area.

That said if you want an ISRU base for fun go for it anyway, on which ever takes your fancy, or both if you can't decide.

[Tig]

On 5/14/2016 at 5:32 AM, Fwiffo said:

Rescuee's orbit is 2.3 billion meters, circular, no inclination.  Maybe 1/2 to 1/3 the radius of Moho's orbit. [snip]

I had a craft that started from Kerbin orbit with 17k dV which managed to rendezvous, but ran out of gas on the way back up before it even got its orbit as high as Moho.

Okay, if you're that close on dV and the inclination is zero, the easiest solution might be like @pandaman suggested, just park the tankers in LKO.  I'd also add to not be afraid to launch with unfilled tanks and/or dock tanks in orbit to extend the range further if needed.

On 5/14/2016 at 5:32 AM, Fwiffo said:

Ok so I'm not sure if this qualifies as "low Kerbol" and maybe doesn't sound so bad to more experienced players. 

1) Don't worry about more "experienced" players.    Forum post count =/= skill.  Also, there's often not a "right" answer.  Depends on your goals...  would I setup a refueling station on/around Gilly for one rescue mission?  No, but who cares what I think?  It's your game to enjoy...     If I was planning other missions around Eve, then maybe I would.

2) This is somewhat opinion, but most of us would define "low X" in your context as the point at which science rewards transfer from high orbit to low orbit.  (250 km for Kerbin, 1000Mm for Kerbol, iirc).  Note that this is different than the dV "subway" maps that usually define low orbit as 10km above atmosphere/surface.

On 5/14/2016 at 5:32 AM, Fwiffo said:

Note I was using a fairly naive Hohmann maneuver and expensive "orbit synchronization corrections" near the sun; I realize there might be a more efficient approach (e.g. maybe a bi-elliptic transfer and better departure timing).  Open to suggestions.

I don't think Hohmann is naive, I'd say its the right choice here.  As to orbit synch the most common mistake people tend to make is with inefficient inclination changes - if your guy is at zero inclination, well, you really can't mess up that badly.   

Departure timing?  Meh, I probably wouldn't care myself, I mean at 2300Mm what's the orbital period like 30-35ish Kerbin days...?

As to bi-elliptic (fully admitting I haven't run your specific numbers) general rule of thumb is that your desired orbit needs to be about 12 times greater for bi-elliptic to make dV sense, and more like 13+ for any kind of reasonable time frame.  This goes back to mods as well, for example if you play with a life support mod, you generally wouldn't do manned bi-elliptic because of the *greatly* increased time.  Even if you got down to about 1000Mm above Kerbol where a bi-elliptic might make sense, you're talking about sending your Kerbals practically out to Dres and adding *years* to their trip.  Jeb doesn't approve. lol.

Edited May 19, 2016 by Tig