Refueling station in LKO questions

[Biggen]

I built a "small" refueling station in LKO.  It consists of two ST-14400s and another two MK3 LF Long tanks.  So, I have the ability to stockpile quite a bit of fuel when the time comes to start stretching my wings and head for interplanetary exploration.  I could add more tanks to this if desired as I have plenty of available docking ports.

As I was building out this station I was also building out my drilling rig on Minimus.  I decided to just build this as a fixed lander (e.g. not move it) and use another craft (called the fuel tanker) to dock with the drilling rig, transfer the fuel, take off, and head back to LKO to fill the station.  I didn't want to build a dedicated fuel rover to transfer the fuel from the rig to a "landing only tanker" so I added landing gear to my tanker and I'm able to flip it over and drive to the drill once landed on Minimus.  It's nice to skip using a dedicated fuel rover and have the tanker pull double duty as rover/lifter.

The tanker itself has four Nukes, a single ST-14400 tank (main body) and twin MK3 LF medium tanks (on the sides).  I also mounted twin MK3 LF short tanks on top of the MK3 LF medium tanks.  These short tanks are dedicated fuel for the 4 nukes.  I have them set at a higher priority so fuel pulls out of them and no fuel pulls from the medium tanks.  My reasoning was I wanted it to take no more than two round trips to fill the refueling station completely.  My hope was that the twin small MK3 tanks would hold enough fuel to lift off from Minimus, dock with refueling depot in LKO, and return to Minimus.

I was doing my 1st run back from Minimus to my refueling depot fully loaded with fuel this morning and I noticed it takes a hell of a lot more fuel to get from Minimus back to Kerbin than it did to go from Kerbin to Minimus.  I'm guessing its the crap ton of mass I now have loaded in the tanks.  I also had to do 3 - 4 aerobraking maneuvers at a Pe of about 38km at Kerbin in order to save as much fuel as possible so I didn't have to burn retrograde to bring my Ap down.

I guess my general question is, is it normal to have to aerobrake this much coming back from Minimus to Kerbin?  My speed is around 3200m/s at PE.  My fuel depot is in a 100km orbit around Kerbin as well.  I'm guessing there is no way around this and I should either plan to burn retrograde to slow down or just live with the 3-4 aerobraking orbits to bleed of speed.

Also, is there a more "efficient" way to do things than how I'm doing it?  Is it worth using a single tanker as a rover/lifter/tanker or do I need to break things up a bit between more vessels?

 

[renhanxue]

Well, once you've escaped from Minmus you're (presumably) in a highly elliptical orbit with a sub-100km periapsis and an apoapsis somewhere out around Minmus or beyond, kinda like the one you started your journey to Minmus from. Unless I've misunderstood something about orbital mechanics, lowering apoapsis and raising periapsis or vice versa is equivalent from an energy standpoint, so circularizing at periapsis is gonna cost you just as much dV as raising the apoapsis up to Minmus did. The only reason it may be cheaper is because of the Oberth effect. So, yes, it's normal, I think.

edit: this thread has some interesting discussion on the topic:

 

Edited February 2, 2017 by renhanxue

[Spricigo]

If you can survive the heat you may break harder by going deeper into atmosphere. this may help you to figure out how deeper you need to go for.

As for being more efficient: every extra link in the chain will add complexity and requires more time to manage, the mass reduction you may accomplish by taking the rover part out of the tanker its low enough to be deemed unnecessary IMHO.

[Cubfan]

8 hours ago, Biggen said:

is there a more "efficient" way to do things than how I'm doing it?

 

I removed one of the refueling steps too, but I did it by building a mining rover. It has 2 large drills, a converter, and a 4 star engineer to speed things along. I land my fuel shuttle, drive over to it, dock, drop my drills and start refueling. The shuttle itself holds 40,000 units of LF (plus the oxidizer, of course). On time warp it takes less than a minute. Minus what it needs for takeoff and landing, I'm able to transfer about 35,000 units of fuel to my ship in Minmus orbit. From there I drop back into the gravity well for some Oberth (usually about 75k) and shoot out into space. I don't use mods, so I don't know if what I'm doing is right (I eyeball transfer windows and guess at delta v), but I can tell you it works extremely well.

[Aegolius13]

9 hours ago, Biggen said:

I was doing my 1st run back from Minimus to my refueling depot fully loaded with fuel this morning and I noticed it takes a hell of a lot more fuel to get from Minimus back to Kerbin than it did to go from Kerbin to Minimus.  I'm guessing its the crap ton of mass I now have loaded in the tanks

As mentioned above, the delta-v cost should be the same going in either direction to/from LKO and a Minmus transfer orbit.  But in terms of fuel consumption, yeah, there's going to be an enormous difference between a full tanker and an empty one.  That's just the rocket equation for you.  

 

9 hours ago, Biggen said:

I guess my general question is, is it normal to have to aerobrake this much coming back from Minimus to Kerbin?  My speed is around 3200m/s at PE.  My fuel depot is in a 100km orbit around Kerbin as well.  I'm guessing there is no way around this and I should either plan to burn retrograde to slow down or just live with the 3-4 aerobraking orbits to bleed of speed.

Also, is there a more "efficient" way to do things than how I'm doing it?  

3200 sounds about right from Minmus, if you figure you're at about 2200 and change in LKO, and have to add 900 and change to get out to Minmus.  Basically, there is no magic bullet to getting around this - the closest thing to a free lunch here is aerobraking and you're already on to that.  

If you want to spend less time aerobraking, you have a few other options.  You can try to go deeper into Kerbin's atmosphere, but this can run the risk of either blowing up or getting stuck and ending up on the ground.  With some practice runs you should be able to figure out the lowest down you can go.  You may also want to design your craft for higher heat tolerance, for example, by eliminating or protecting parks with low heat thresholds.  

You can also try to design and/or fly so that your ship has more surface area showing.  "Pancake" is the common term around here.  This may require adjusting your ship's aerodynamics and weight balance.  

One solution might be put an inflatable heat shield on the front of your ship and just leave it there.  Those things slow you down better than pretty much any alternative, and they'll help with heat issues as well.  Just make sure your ship doesn't flip around - since these heat shields are draggy, they like to be in the back.  But note that the inflatables can occlude your solar panels and get in the way of docking, so you'd have to design around them.  May not work well with your rover concept, now that I think about it.    

As an entirely different idea to save some delta-v, you can try using a reverse gravity assist around the Mun on the way back from Minmus, which will lower your orbit.  But that probably won't save more than a couple hundred m/s, and it may be more trouble than it's worth to get the right intercept.  

 

9 hours ago, Biggen said:

Is it worth using a single tanker as a rover/lifter/tanker or do I need to break things up a bit between more vessels?

I think this mostly comes down to personal preference. I actually go the other direction and use standalone mining/refining/tanker/refueler rigs that do everything.  This keeps things simple and cuts down on putzy precision landings,  rendezvous and docking.  Yeah, you have to carry around your mining equipment all the time, but the weight is still fairly minor compared to all that fuel.  And I assume drills are draggy enough to help a bit with aerobraking.   

That said, I would not see a reason to separate the "lifter" and "tanker" functions.  The requirements of these two functions are pretty similar, and you've already gone with the best option in nukes, so no real reason to add more steps.  The one exception would be if you wanted to use some monstrously huge tanker to go from Minmus to Kerbin and back, and could not reasonably fill it on one go.  

[Foxster]

I think I'd be tempted to try having my refueling tanker parked in Kerbin orbit close to Minmus' orbit. That way you don't have to spend a lot of fuel going down the steep gravity well to LKO and back (2 journeys). 

It does mean getting whatever you lift from Kerbin out a little further (1 journey) but that really shouldn't be much of a challenge. 

Edited February 3, 2017 by Foxster

[Biggen]

Thanks for all the help guys!

I figured out that the shallowest Pe my tanker can handle for aerobraking is ~37km coming in around 3200m/s from Minimus.  This is coming in retrograde of course and letting the nukes handle most of the heat.  Any Pe shallower than that and things go boom.  It takes a few passes to get my Ap down to a manageable level of around ~500km.  From there it's another ~250m/s dV to come to a stop right next to my fuel depot so I guess it's not that bad.  The couple extra orbits around Kerbin for aerobraking also gives me an opportunity to match planes with the fuel depot so it's not totally wasted time.

I didn't design this tanker with heat shields as there is no good way to replace them once they are used up.  The abalator is a consumable and adds considerable mass.  The inflatable sounds good at first and I was going to use it, but then I discovered it can't be deflated and packed away again after use.  I had giant plans for a mothership/interstellar spacecraft that can travel to all the planets, aerobrake to achieve orbit, and house a lander to visit each surface, have the lander return to the mothership, and then have the mothership return to LKO via aerobraking but that plan may prove futile as aerobraking at Eve (among other planets) without a heatshield is a death wish.  I'm going to have to figure out a different solution...

I did redesign the tanker slightly and upped the dedicated nuke only tanks to MK3 LF Mediums from MK3 LF Smalls.  Filling the twin dedicated medium tanks now allows me to make a trip to the fuel depot with the other tanks fully loaded and then return to Minimus with nearly 1000m/s to spare.  I'll probably not fill them up completely during the next fuel run as I don't need to return to Minimus with that much spare dV.

It's nice to know I actually accomplished something in this game after 120 hrs of gameplay.  I kinda ready to start heading out into space as I'm getting tired of seeing the mint green landscape of Minimus!

[Tyko]

I tried this in my last game - shuttling tons of fuel to a central station. After it was loaded up, I realized that docking my big motherships with tons of parts and my big station with tons of parts put a real drag on my frame rates and required that my motherships had RCS so they could dock (more parts). Also, from an RP perspective, putting the two most expensive things in the solar system in close proximity was risky...So, I ended up just parking the mothership in orbit and flying tankers directly from my fuel sources to it and my station sits on a giant stock of unused fuel. This only works if you have plenty of time to fill your ship, but in the end it worked out better for me...just a though

[TheKorbinjer]

As an experienced KSP player ~1,500 ish hours combined over 4 computers I can say that the more your craft's mass is, the harder it is to aero-brake.

If you're worried about spending so much fuel, go for a more efficient craft. Ion engines have the power to land on and take off from  minmus without a doubt. (depending on the mass of your lander)

Get there with liquid fuel, retroburn/ prograde burn with ion engines. You only spend maybe 750-1000 m/s of dV getting there

(with liquid fuel/oxidizer).

 

 

Hope my suggestions help!

-TheKorbinger

Edited February 3, 2017 by TheKorbinger

[Cubfan]

2 hours ago, Biggen said:

I kinda ready to start heading out into space as I'm getting tired of seeing the mint green landscape of Minimus!

Nice work, but get used to seeing Minmus. I have refueling operations on Mun, Ike, Gilly, Pol, and even a little refueling rover on Tylo (mostly meant to refuel it's own braking engines, but can refuel a lander if I need it to). With all that, I don't really need them much. However, I feel like I spend half my life on or around Minmus.

[Lunar Sea]

Couple pics here would help just sayin'

[Biggen]

1 hour ago, Lunar Sea said:

Couple pics here would help just sayin'

Pics!  I built the drill before I knew what the heck I was doing and I didn't put any LF only tanks on it which was a mistake.  It also overheats like a mother even with radiators.  I may redesign it in the future.

All four tanks at the station are full.  The little tug hooked to the back did a great job building out the station.  Was easy once I learned proper docking and rendezvous techniques.  I'll keep the tanker on Minimus until needed again.  Those 5 kerbals (two in the drill rig and three in the tanker) will have to keep themselves busy on their own!

 

 

 

Edited February 3, 2017 by Biggen