Refueling

[JBMCW2010]

I am going to place a refueling station in Kerbin's orbit. The goal of which is to have a platform to host a debris tug/deorbit platform, refuel interplanetary missions and as a central hub of the Kerbin SOI.

I have 2 questions, first I am inclined to setup a mining outpost on Minmus, is there a good arguement against Minmus and in favor of Mun? My thought process is the easier takeoff and landing of mining barges to the surface of Minmus is more beneficial than the slightly easier act of getting to Mun.

My second question is the orbit of this station. At first I thought maybe a circular orbit at 250km. But am now more inclined to run it in an elliptical orbit of 150km-350km. I'm not efficient enough in my launches to reliably get to a 150km station in one shot. Is a circular orbit more preferable than the elliptical? Should I aim more towards a 250km or 350km circular or is elliptical ok?

Other notes: career play on normal/moderate setting with Comms Net enabled. I haven't just yet unlocked the mining equipment but will shortly.

Thanks in advance!

[mystifeid]

Why not have mining bases on both Minmus and the Mun? For sure you'll get mining contracts for both. These often have the requirement to put the ore in orbit of Kerbin and although just having the ore already on a Kerbin station is enough to satisfy this requirement you can use the opportunity to ferry ore to an ISRU on a Kerbin station from both the Mun and Minmus. To be able to reach Kerbin station from the surface of the Mun with my standard ore hauler - carries 4590 liquid fuel and 6000 ore - I need to put the station in a circular 650km orbit around Kerbin. An elliptical orbit with the same Ap might suffice I guess but would be harder and more time consuming to set up a rendezvous.

Stations around both Mun and Minmus serve as useful collection points for rescues using ships based at these stations. These ships can also perform a variety of other contracts. Mun and Minmus stations are resupplied from respective mining bases on the surface beneath them.

Edited June 27, 2019 by mystifeid

[bewing]

Welcome to the forums.

Minmus is nice because the deltaV cost to get into a return trajectory to Kerbin is so low. However, it takes a long time to get from Minmus to Kerbin. So the advantage to mining on the Mun and returning the fuel from there to Kerbin is that it's a 1-day trip. You can easily do six round trips to the Mun in the time it takes to do one to Minmus. If you aren't timewarping, then the travel time can become an important factor.

I prefer as low an orbit as possible for my LKO stations. Often, this means a 74km x 70.5km orbit. First, because I use it as a deorbit platform and I use passive deorbiting. That is, I lower my station's Pe to 65km, decouple the thing to be deorbited, and then raise my station's Pe back just over 70km again. This works best if I'm orbiting just above the atmosphere in the first place. Also, of course, the lower the orbit, the easier it is to reach. Also, my fuel tankers that come from Minmus use aerobraking to capture themselves at Kerbin. Since the aerobraking is free, it works best to make it as low as possible.

 

[JBMCW2010]

18 minutes ago, mystifeid said:

ferry ore to an ISRU on a Kerbin station

I'm not sure I understand that acronym, ISRU?

And is it more efficient to harvest on the surface and convert in orbit? Or would it work alright with harvesting and converting on the surface. As I said I've not unlocked the tech yet for harvesting so I haven't seen the scale of the parts required.

13 minutes ago, bewing said:

I prefer as low an orbit as possible for my LKO stations. Often, this means a 74km x 70.5km orbit. First, because I use it as a deorbit platform and I use passive deorbiting. That is, I lower my station's Pe to 65km, decouple the thing to be deorbited, and then raise my station's Pe back just over 70km again. This works best if I'm orbiting just above the atmosphere in the first place. Also, of course, the lower the orbit, the easier it is to reach. Also, my fuel tankers that come from Minmus use aerobraking to capture themselves at Kerbin. Since the aerobraking is free, it works best to make it as low as possible.

Yeah I get the low orbit and I like the idea of a low cost deorbit method but I'm not yet to a point where I can easily/consistently hit that 75km/85km initial launch orbit, I seem to always get my original Ap to about 250km before I circularize the orbit. I've also not tried to use an aerobrake and am not sure how to do it, I've not yet YouTube'd a tutorial on the basics. I understand the concept of using the atmo to help reduce your speed.

[mystifeid]

2 hours ago, JBMCW2010 said:

I'm not sure I understand that acronym, ISRU?

ISRU Converter -> Convert-o-tron. The things that convert ore to fuel. I forget what the acronym stands for - possibly the name was changed when the smaller convert-o-tron was added to the game. I'm not sure. Anyway, everyone still calls them ISRU's.

2 hours ago, JBMCW2010 said:

And is it more efficient to harvest on the surface and convert in orbit? Or would it work alright with harvesting and converting on the surface. As I said I've not unlocked the tech yet for harvesting so I haven't seen the scale of the parts required.

There is no change in mass when converting to fuel but there is apparently a small mass loss when converting to monoprop. But ore tanks are smaller and there are contracts to move ore from one place to another - often when you have not yet set up the means to do it the cheaty way - so it pays to have an ore tanker design.

And before launching, try opening 'Maneuver Mode' in the Mode Controls (lower left on the screen) so you can see your Ap while climbing and shut your engines off when it hits the desired altitude. Using aerobraking to help rendezvous with a LKO station - as suggested by @bewing - will help to make the ore/fuel (whichever way you decide to go) transport that much more profitable - ie less trips.

Edited June 27, 2019 by mystifeid

[Gargamel]

2 hours ago, mystifeid said:

I forget what the acronym stands for - possibly the name was changed when the smaller convert-o-tron was added to the game. I'm not sure. Anyway, everyone still calls them ISRU's.

Acronyms that are marked with a dotted underline are recognized by the forum software.  ISRU, VAB, SPH, etc..   Just mouse over them for a second for the full name to pop up. 

 

[Zhetaan]

@JBMCW2010:

Welcome to the party!  We don't have drinks, snacks, or even brightly-coloured hats, but you get to pick a profile picture, so that's something.

4 hours ago, JBMCW2010 said:

I'm not sure I understand that acronym, ISRU?

Others have hinted, and I will write it out:  In-Situ Resource Utilisation.  In other words, it means on-site mining and refining.  It comes from a NASA proposal.  For KSP users, ISRU typically refers to the refinery parts (these are known in-game as Convert-O-Trons) or to the act of mining itself; the drills have always been regarded separately, though technically ISRU refers to the entire process chain from prospecting all the way to storage prior to final use.

4 hours ago, JBMCW2010 said:

And is it more efficient to harvest on the surface and convert in orbit? Or would it work alright with harvesting and converting on the surface. As I said I've not unlocked the tech yet for harvesting so I haven't seen the scale of the parts required.

It is, on the balance, more efficient to harvest and convert on the surface.  The small Convert-O-Tron is ten percent efficient, which for you means that you spend fuel to haul ore to orbit (even if it's only Minus orbit) only for nine units of ore out of every ten to be wasted.  On the other hand, the small unit is not meant for large-scale refinery work anyway (it has a heat imbalance that makes it impossible to run continuously).

The large Convert-O-Tron converts one hundred percent, but it is only lossless when you use it to make liquid fuel and oxidiser.  Monopropellant manufacture results in a twenty percent mass loss anyway; this is because monopropellant has a density of 4 kg/L whereas liquid fuel and oxidiser have densities of 5 kg/L each.  In this case, if you plan on making no monopropellant, then you can leave the converter in orbit.  Personally, I never do, but that's only preference.

As to the scale, the small Convert-O-Tron is 1.25-metre scale (the same as the Reliant and Terrier engines and the bottom of the Mk. I Command Pod), and the large one is 2.5-metre scale (the same as the Rockomax fuel tanks and the Skipper engine).

4 hours ago, JBMCW2010 said:

Yeah I get the low orbit and I like the idea of a low cost deorbit method but I'm not yet to a point where I can easily/consistently hit that 75km/85km initial launch orbit, I seem to always get my original Ap to about 250km before I circularize the orbit.

If you keep going out to 250 km on launch, then I suspect that you are either not cutting your engines early enough or else flying an odd profile.  Try switching to map view once you get your rocket mostly horizontal (on most ascents, this should happen around approximately 30,000 km altitude) and watching the Ap marker (if you don't want to chase it with your mouse, right-click on it and it ought to stay visible).  When your apoapsis reaches 80 km, cut your engines.  You may wish to watch the 'Time Until' counter on the marker, as well, and try to keep the apoapsis roughly thirty seconds to one minute ahead of your rocket, which you can control by use of the throttle.  Done correctly, by the time you get to 75 km, you'll have most of the circularisation done already.

5 hours ago, JBMCW2010 said:

I've also not tried to use an aerobrake and am not sure how to do it, I've not yet YouTube'd a tutorial on the basics. I understand the concept of using the atmo to help reduce your speed.

You may be confusing aerobrake with airbrake.  Aerobraking is using atmo to help reduce your speed; airbrakes are parts available for the later planes.  Either way, why not try it and see whether you can figure out how to make it work?  You can use quicksave in case you guess incorrectly.

I won't tell.

Seriously, though, don't worry about a tutorial.  You don't need one to figure out how it works.  If you can re-enter and return a rocket to the surface, then you can aerobrake:  you still re-enter, but you stay high enough to miss the surface and avoid burning off any parts.  How high is something for you to discover:  science is driven by experimentation, and rocket science is the same way.

Also, the right altitude changes with the rocket.  A rocket with high drag may get a lot of braking with a shallow pass, but a needle rocket may need to dive into thicker air to get the same result.

Good luck, have fun, and do please tell us how it works out.  KSP is supposed to be hard, but it is also supposed to be fun, so if you should have any problems or particularly fun-destroying frustrations after trying to work it out from here, then please don't hesitate to ask about them.  We're here to help.

[JBMCW2010]

7 hours ago, Zhetaan said:

Try switching to map view once you get your rocket mostly horizontal (on most ascents, this should happen around approximately 30,000 km altitude)

I attempt the 90* turn but when I do I have been going very gradually as I've found if I go more than say 5-10* over prior to getting to the upper atmo I'll start to flip head over heels. My typical setup is a central Jumbo tank with mainsail and then I'll strap another jumbo and mainsail on the sides. My guess here is that the payloads I'm sending up is too heavy. Any tips on maybe a better method of ascent? Should I maybe cut back to a smaller fuel tank and make it 4 coupled tanks to lower the center of mass?

Sidenote: last night I successfully sent a biome hopping mission to Minmus and collected a boat load of science. Now I need to get a bit more cash to unlock the last lab upgrade.

[ajburges]

One practical use of a Munar fuel depot is that the Mun is approximately at the gateway orbit for Eve and Duna. That means Munar orbit is one of the most efficient locations to perform a Duna/Eve election burn from.

Don'tmix things up though: LKO election is still achieves a lower dV for mission, it's just that Munar orbit lowers the election burn cost (and you just refueled in orbit)

[JBMCW2010]

Is the idea of having a fuel mining operation on Minmus/Mun then having a freighter take it to LKO lose efficiency and I might as well just launch from Kerbin? I know I'd save on money spent, I can send rockets up that aren't fully fueled and simply use the "free" fuel off of the station.

[Freds]

I find it a tedious routine the mining/deliver around Kerbal. I have done it 2 or 3 times, then i just edited a part to "magically generates fuel" and installed into my 'kerbin space station'. Ships are refuelled there. Just need to imaginate that someone delivered the fuel when i was not there and it is ok. 

ISRU is very very very useful and essencial to the distant  missions. 
Almost all of my Kerbaled missions to Jool use ISRU in a way or another. I usually land a large ISRU barge in Pol. When a ship in need of refuel arraives, the barge just take off, do the refuel mission and land again.  Low gravity makes things easyer.

Edited June 27, 2019 by Freds

[JBMCW2010]

17 minutes ago, Freds said:

I find it a tedious routine the mining/deliver around Kerbal.

I played Eve for 8 years as the logistics director and freighter pilot for a few large coalition level alliances in their sov space, tedium is my game. Haha.

For a contract to get a station into Kerbin orbit I tried getting it up to 75km circular orbit and I managed to get it to a 75.0045km/74.9924km orbit. Now the question is, can I manage to rendezvous with it to complete the construction with some additional functionality.

[Zhetaan]

4 hours ago, JBMCW2010 said:

I attempt the 90* turn but when I do I have been going very gradually as I've found if I go more than say 5-10* over prior to getting to the upper atmo I'll start to flip head over heels.

That's the best way to do it; you want to turn gradually.  The best way to fly a rocket changes with the rocket, but for most rockets, going straight up for the first thousand metres and then pitching east by three degrees is good enough to start.  Continue the gradual pitchover a degree or two at a time, and so long as you aim to be at roughly forty-five degrees by 10 km and nearly horizontal anywhere between 30 and 45 km, you'll probably have no problem getting to orbit.  Some rockets prefer a much more specific ascent profile, but if you begin with that one, it'll work well enough in most cases and on the few times that it fails, you'll probably be in a good position to know why and how to correct it.  Eventually, you'll be able to build a rocket that requires you only to give the initial three-degree pitch at a thousand metres and it will fly itself to space--without setting SAS to prograde, without throttle work--just keep your hands off the controls until you shut down the engine, and perhaps you'll need to coast a bit before you circularise.

5 hours ago, JBMCW2010 said:

My guess here is that the payloads I'm sending up is too heavy. Any tips on maybe a better method of ascent? Should I maybe cut back to a smaller fuel tank and make it 4 coupled tanks to lower the center of mass?

If anything, I'd be concerned that your payload is actually too light; there isn't much that three Mainsails can't lift.  Relatively quick ways to check that would be to see whether you reach orbit with a lot of extra fuel (in your lifter stage--you obviously want your transfer and lander stages to have their fuel), and to see what your thrust-to-weight ratio is.  I don't think there's a TWR calculator in the latest version of KSP (I lag behind the most recent by a few versions), so know that the thrust of three Mainsail engines at sea level is 4137 kN; you'll need to take the mass of the fuelled rocket in tonnes (available in the VAB), multiply that value by 9.81 (Kerbin's gravitational acceleration at sea level), and divide 4137 by your answer.  Put in an equation:

TWR = Thrust / (FuelledRocketMass * 9.81)

Ideally, the answer is somewhere between 1.2 and 1.5, but I suspect that it's a lot higher for your rocket.  Being outside that range has drawbacks on either side.  Too low of a TWR means that you either cannot get off the pad at all (if it's less than 1) or else you ascend so slowly that a minor instability while you hover six metres above the pad causes you to tip over.  Too high of a TWR will get you off the pad, but it greatly reduces efficiency (which requires more rocket, which costs more), and more importantly, it makes the rocket difficult to control.  Imagine the difference between trying to control a garden hose versus a fire hose; each will water your plants; but one will water everything else.  Even better, imagine trying to make orbital rendezvous, or even trying to insert into a specific orbit altitude, when the most insignificant burp of your engines changes your apoapsis by ten kilometres.  It may be difficult to think of a hundred-tonne column that ascends to space like 'a crazed robot made out of explosions' as a precision machine, but consider it against the scale of the cosmos in which it operates and you'll get the right perspective.

5 hours ago, JBMCW2010 said:

Any tips on maybe a better method of ascent? Should I maybe cut back to a smaller fuel tank and make it 4 coupled tanks to lower the center of mass?

Don't lower the centre of mass.  If anything, you want it at the top of your rocket.  It seems counter-intuitive, but that's how it works.  Lowering the centre of mass is to fall victim to something called the pendulum fallacy, which is the idea that a rocket, like a pendulum, dangles from its centre of mass.  This notion has led to weird designs that do things like include the engines at the top of the rocket (the idea being to drag the rocket behind) or at the centre of mass.  This is wrong for two reasons.

First, a rocket is a rigid body.  Thrust from the engines acts on the entire rocket at once; when you light it, it all goes up at the same time.  A pendulum depends on the fulcrum, and thus on the relationship between itself and its pivot (the fulcrum cannot be rigidly attached to the bob of the pendulum); it thus is not a rigid body because it has that pivot point.

Second, a pendulum works because it is powered by gravity (or else some other force that acts as a 'restoring force' in physics parlance).  Rockets overcome gravity; that's how they work.  That's not to say that a rocket isn't affected by gravity, but simply to say that gravity does not stabilise a rocket's ascent.  If it did, then a rocket would become unstable as it ventured farther out of the gravity well, which does not happen; but a pendulum does become unstable and eventually swings round in circles.

The reason to put all the mass at the top of the rocket has to do with leverage.  If you can concentrate the mass as far away from the engines as possible, then you get the most effect out of gimballing and other attitude-control mechanisms.  This makes it so that you can respond to slight instabilities more easily, and thus it stabilises the rocket for ascent.

If your rocket suffers from too much engine, then I suggest that you cut back on the thrust.  You can do that with thrust limiters, but you can also do it by switching to Skippers on the outer boosters.  You ought to consider the Twin Boar, as well; it's one of the most cost-effective ways to get to space because it incorporates the fuel tank and the engine into one part.  It's best to put the fuel for a stage into as few tanks as possible; this isn't because it gives an advantage in physics, but rather because there are fewer parts and joints for the game to track, so it improves performance.

Do you have a screenshot of your rocket?

3 hours ago, JBMCW2010 said:

Is the idea of having a fuel mining operation on Minmus/Mun then having a freighter take it to LKO lose efficiency and I might as well just launch from Kerbin? I know I'd save on money spent, I can send rockets up that aren't fully fueled and simply use the "free" fuel off of the station.

Define efficiency.  The fuel requires a lot of time and an expensive infrastructure investment, but after that is free of cost.  The usual convenience of fuel in orbit is that it reduces launch costs by spreading the cost over several launches (at a minimum, one for the empty vessel and one for the fuel tank).  There are a lot of space stations that consist of a Jumbo tank with a docking port.

That being said, there are specific, desirable use cases for a fuel operation on the moons beyond 'I want to try it out'.  On the other hand, they are fewer than, say, the use cases for a fuel operation on the moons of Jool.  On the gripping hand, 'I want to try it out' is the reason you do anything in this game.

[mystifeid]

4 hours ago, JBMCW2010 said:

Is the idea of having a fuel mining operation on Minmus/Mun then having a freighter take it to LKO lose efficiency and I might as well just launch from Kerbin? I know I'd save on money spent, I can send rockets up that aren't fully fueled and simply use the "free" fuel off of the station.

For me it's more the amount of time it takes. For example - I'm going to Bop and my station is delivered in five parts (requiring five launches) and is then assembled in orbit around Bop. My base is delivered in two to four parts - more launches. A survey scanner is going, as well as a rover, an ore tanker and maybe one or two other specialist ships. If all of these ships are launched and then refuelled in LKO....well, I find the game becomes a lot more enjoyable if I just launch ships with the fuel they need to get where they are going. If I want to put a mining base on the surface of Tylo it means building a monster of a booster that's super expensive but that's part of the fun.

The only ships that are refuelled at my Kerbin station are ones that never return to the surface of Kerbin. Like the two shuttles that are continually going around an Ike - Duna - Gilly - Kerbin circuit or ones that handle out and return trips to other places. Or ships that aggregate rescued Kerbals at my stations. And mostly I try to refuel these ships on the surface of Minmus (or the Mun) in order to reduce the number of resupply trips to Kerbin station.