Refueling Stations, do they make sense in terms of D/V and/or ship complexity?

[Mitchz95]

For what it's worth, Scott Manley thinks a Minmus refueling station is worth it. Watch his 34th Interstellar Quest video, it's at the end.

[SgìobairOg]

I try and design all of my mission modules and some rocket segments to be capable of refueling.

I do this because I have a tendency to over-do my rockets and I end up dumping a nearly full stack of fuel in a place where it would be useful if I mess up and use more dV than I intended on a landing or something. Won't always work out to be useful but it has also already allowed me to salvage a mission gone wrong by launching a simple light fuel probe to get enough fuel out to a mission to get it home. First response nailed it though, imo; it will all depend on the circumstances. If you can build, without question a rocket with enough fuel for a round trip without overloading your part count, may as well do it.

[Jouni]

Interplanetary launches from LKO are theoretically more efficient, but it's very hard to hit anything with a 10 to 15-minute burn from there. Most of my interplanetary ships use nuclear engines, and unless you use a ridiculous number of them, the TWR of a fully fueled ship at Kerbin is usually around 0.2. This leads to very long burn times.

I see two practical alternatives. The harder, more tedious, and more efficient choice would be to first raise the apoapsis with shorter burns during one or more passes at the periapsis, and then do the final burn during the last pass. The easier and less efficient alternative is to refuel at Minmus, use a negligible amount of fuel to get to a circular orbit between Mun and Minmus, and do the launch with one long burn from there. With kethane, Minmus becomes the only reasonable option, as it takes much more work to refuel the ship at LKO than at Minmus.

[SgìobairOg]

Interplanetary launches from LKO are theoretically more efficient, but it's very hard to hit anything with a 10 to 15-minute burn from there. Most of my interplanetary ships use nuclear engines, and unless you use a ridiculous number of them, the TWR of a fully fueled ship at Kerbin is usually around 0.2. This leads to very long burn times.

I see two practical alternatives. The harder, more tedious, and more efficient choice would be to first raise the apoapsis with shorter burns during one or more passes at the periapsis, and then do the final burn during the last pass. The easier and less efficient alternative is to refuel at Minmus, use a negligible amount of fuel to get to a circular orbit between Mun and Minmus, and do the launch with one long burn from there. With kethane, Minmus becomes the only reasonable option, as it takes much more work to refuel the ship at LKO than at Minmus.

That's a good point.

[WhiteKnuckle]

For what it's worth, Scott Manley thinks a Minmus refueling station is worth it. Watch his 34th Interstellar Quest video, it's at the end.

Yeah I'm interested to see where he's going to go with that.

But, let's be clear, Scott seems to be doing a lot of things just because they're cool and impressive, not because they're efficient or necessary.

[LethalDose]

Yes, extra-kerbestrial refueling stations make an immense amount of sense. Minmus orbit is a great place to place a refueling station for leaving Kerbin's SoI to maximize the Oberth effect. Here's how I do it:

[*]Get ship and fuel to Minmus.

[*]Refuel at Minmus (you get to start your expedition with a full fuel tank)

[*]Escape Minmus going (Either infront of the planet to get a higher Apoapsis or behind the planet to get a lower periapsis around Kerbin. I'm not sure which is more efficient; I usually go forward)

[*]Burn retrograde at your new Apoapsis to drop your periapsis to ~ 80-100 km above kerbin's surface

[*]Burn prograde at periapsis. You'll need barely any fuel/dV to hit escape velocity.

[*]Profit

Now, refueling stations make even MORE sense when you watch

.

After you watch that, go back and take a look at your rockets. You start to see how much fuel you have to use to get payloads into orbit. If fuel came from other locations in the Kerbol system (like Dres or Duna), it would be WAY more efficient to fuel your rockets in outer space. IIRC, it takes more dV to go from the surface of Kerbin to low Kerbin orbit (LKO) (~2k dV) than to go from Kerbin to DUNA (~1.6K)!

Fuel production depots set up on planets with low gravity and little to no atmosphere (like Duna, or even better, Dres) that sent fuel back to Kerbin or out to Jool would be insanely efficient. Local fuel production would also allow (maybe even encourage) the point-to-point hopping described above with very small, efficient vessels.

But SQUAD can't figure out how make resource mining "fun", even when it's laid out for them. So anyone who might enjoy this kind of mechanic that belongs in the game is basically screwed until we do SQUAD's work for them.

And yeah, we're working on it.

Edited January 28, 2014 by LethalDose

[Northstar1989]

I'm assuming that having to get into a Duna orbit, and then meeting up with a refueling tug in orbit costs a lot of D/V in the long run, but would be offset by the ability to use smaller/lighter/simpler ships that don't want to either rip apart, or require 10 minute long burns (ran into that with my first manned mission to Duna, no fun)

There is a wide spectrum to the term "experienced player", and different players have different skill-sets and specialties. Speaking from the perspective of an experienced player who does a LOT of refueling, gravity-assists, and aerobraking; all to save fuel and Delta-V, let me try and clear this up for you.

TECHNICALLY, it costs more Delta-V to do a refueling stop, and then continue on your way to your destination, than just to make a straight transfer, all other things being equal- but there are several EXTREMELY important exceptions to this that can make it a lot more worthwhile:

(1) You can save a lot of fuel if the body you make your refueling stop around has an atmosphere. This may not seem intuitive, but aero-"braking" can actually be used to ACCELERATE your ship towards your destination. Let me explain this:

-Take the example of Duna, for instance. If you set up a fuel depot in low orbit around Duna, and send a spacecraft from low-altitude orbit around Kerbin to Duna via a low-energy transfer; Duna will be traveling a lot FASTER relative to the Sun than your spacecraft by the time it gets that far away from the Sun... Now, when you enter Duna's SOI, you will have a large velocity difference, and appear to be traveling very "fast" relative to the planet- but from the point of reference of the Sun, it is actually DUNA that is traveling very fast relative to YOU. If you dip into the atmosphere around Duna, it will exert acceleration on your spacecraft to bring it closer to Duna's speed. In essence, you are performing an aero-boost, so to speak, relative to your final target Jool.

- Now, if you got your fuel into Duna orbit in the same way (low-energy transfer and aerocapture) you will have saved quite a bit of fuel, as it is approximately 900 less Delta-V to make an ideal transfer to Duna than to make an ideal transfer to Jool (this ISN'T counting fuel-savings from a Munar gravity assist- which will save you close to the same amount of Delta-V on either trip if you perform it right). Now, it does cost approximately 1350 Delta-V to make a transfer from Duna to Jool- but you can make up quite a bit of that extra 450 Delta-V with a gravity-assist from Ike. Additionally, you can make a MUCH more precise burn from Duna than from Kerbin (the burn-time will be shorter), which will more than make up the rest of that Delta-V in terms of reduced steering losses and less need for course corrections if you are using NERVA engines... Of course, this is where (2) comes in...

- For BONUS fuel-savings, don't perform an aerocapture to Duna orbit- perform one to Ike. It's a bit tricky to pull off, but if you set you aerobrake such that your exit trajectory comes out with an apoapsis appropriately-oriented to Ike, you can perform an easy capture there, and save a lot more Delta-V then from capturing to Duna orbit- as it takes less Delta-V to make it from Ike orbit to Jool if you make appropriate use of the Oberth effect... (see below)

(2) Craft design and the Oberth Effect:

- Getting an accurate transfer burn to Jool from low Kerbin orbit is hard. More likely than not, you will need to perform a long, slow burn around Kerbin; followed by a shorter adjustment burn somewhere in interstellar space (MechJeb can help you find the optimal location for these adjustment burns, to minimize Delta-V costs. You could theoretically determine this yourself, but the mathematical equations required are nothing to sneeze at, and it will take a LONG time to locate the optimal adjustment points if you utilize trial-and-error course-plotting instead...) The fact is, the shorter a burn you have to make to get somewhere from Kerbin, the more accurate it is likely to be- simply by virtue of gravity having less time to curve you off-course during your transfer-burn. Additionally, there is something important you need to know about called the "Oberth Effect"...

- The Oberth Effect is a mathematical phenomenon that is EXTREMELY important to orbital mechanics. Without going into too much detail about it, you should know the basic result of it: the faster you are going when you burn your engines, the more energy you get out of it. What this basically means, in game-terms, is that it takes less Delta-V to get somewhere from a departure-point where you are already moving fast than in does to get there when you start out moving slowly: even less so than you might think. For this reason, you cannot simply find the difference between your current velocity and the velocity you need to be going at when you arrive at your destination to calculate Delta-V costs. The larger the Delta-V of a transfer burn from planetary orbit, the relatively less Delta-V it costs you relative to the final total kinetic and potential energy you will have at your final destination. If you make a transfer burn from low above Kerbin, it costs you less Delta-V than if you first raise your orbit to the edge of Kerbin's SOI, and then make the transfer from there. It is actually for this reason that an ideal straight transfer (with no steering-losses: though such burns are, in practice, impossible) to Jool from Kerbin theoretically costs you less than a burn to Duna and then from there to Jool, assuming you didn't make use of Duna's atmosphere to provide you with a portion of your velocity. However, the Oberth Effect leads to one other important point you need to know about in order to understand why refueling at Duna can actually make sense:

+ Due to the way the Oberth effect works, the higher your Thrust-to-Weight Ratio (TWR) is when you make a transfer burn, the less Delta-V it is going to cost you. This is actually for a rather simple reason, really- if your TWR is much lower, your burn will be much longer, and you will make a greater portion of it in a more distant orbit from the celestial body you are departing. This means that your average distance from the body you were orbiting was larger during the burn, and your orbital velocity was lower (the further you are from a body, the lower your orbital velocity). Put another way, you have already lost a portion of you velocity from the beginning of your burn to gravity by the time you are nearing the end of your burn. I don't know how to explain this better- the greater your TWR, the less Delta-V you will spend reaching your destina, even before you account for the greater steering-losses that a lower TWR also inevitably entails...

+ For obvious reasons (Duna's gravity is less than 1/3 Kerbin's), the same exact craft will have a much greater TWR around Duna than around Kerbin. What this means is, you will come a LOT closer to the idealized transfer to 1350 Delta-V from low Duna orbit to Jool's SOI than you will to the idealized transfer of 1915 Delta-V from low Kerbin orbit to Jool. Put simply, when you combine this with reduced steering-losses it's not 1915 Delta-v vs. 1060 + 1315 Delta-V (idealized transfers), it's more often 2700 Delta-V vs. 1250 + 1400 Delta-V. Just from this fact alone you will save Delta-V with the same craft design, even before you account for the additional gravity-assist that Ike offers you the opportunity to make (from Duna orbit), or the opportunity to aerobrake directly into an Ike-intecept, and refuel around Ike rather than around Duna, which will make your Delta-V savings even greater...

- I mentioned craft design earlier, and this is really another important factor to consider. In order to make a reasonably-accurate burn to another planetary system, you generally want to keep your burn-times to no more than 5-6 minutes (shorter if your departure orbit around Kerbin is REALLY low- I usually depart from at least 100 km or higher in order to make my transfer burns more accurate, and give my ships a little more breathing room to make rendezvous with fuel tankers or space stations for fuel, and sometimes crew, transfers before departure). Consider the following:

+ A craft with a TWR of 1 around Kerbin can accelerate at a little under 10 m/s^2. To make a transfer to Duna at that TWR would take a little over 2 minutes (assuming an expenditure of 1200 m/s Delta-V with steering-losses). For the same craft to make a transfer to Jool it would have to burn for at least 4 minutes (assuming an expenditure of almost 24000 m/s to Jool). This is ignoring the effect of staging and increasing TWR with fuel-losses of course, but in reality the TWR of NERVA upper-stages is also likely to be quite a bit lower...

+ Let's say your craft is of such mass as to have a TWR of 0.25 around Kerbin with a single LV-N (NERVA) engine. To raise that to 1.5 would take MORE than 6 LV-N's- as each additional engine adds a not-insignificant mass to the vessel. Additionally, each additional LV-N also reduces your available Delta-V budget for the same reason.

+ The factor of craft structural stability and design-felxibility also comes into play. To make a reasonably-accurate burn to Duna, you won't need as high a TWR as to make an accurate burn to Jool. This means your craft will be placed under fewer Kerbin-G's of acceleration for the transfer burns if you make the refueling stop at Duna (the aerobraking, on the other hand, might place your craft under quite a few G's- but step isn't strictly-required), or better yet, Ike (if you can, *ALWAYS* make the refueling stop at Ike, never make the stop at Duna- you will save a lot more Delta-V this way if you can still pull it off with an aerobrake, and especially, if you can't- say if your craft is too flimsy to survive aerobraking...) This gives you more options for craft-design: a craft that doesn't need to pull as many G's, and can get by with less thrust, has a lot more options for its design than one than needs to cram in 6 or 8 LN-N engines... You can even utilize some of the more thrust-anemic, but higher-ISP options than NERVA's this way, especially if you are using mods: for instance KSP-Interstellar's ATTILA engines, or NearFuture pack's HydrogenGas plasmodynamic thrusters (which are basically upsized ion engines that utilize a much less dense reactive propellant in the form of hydrogen gas, rather than Xenon, but benefit from improved ISP *and* TWR as a result- compared to stock ion engines...)

- Finally, it should be obvious to you that there is an upper-limit to the size of your spaceships. This is determined both by the limits of the game itself (i.e. part-count and strain on the physics engine), and by your ability to construct a structurally-sound vessel that isn't so complicated as to cause even you, its designer, to forget how to use it properly. Personally, I use mods like NovaPunch2 and StretchyTanks to scale up the maximum size of my craft (by providing larger-diameter parts, I can build larger vessels with reasonable part-counts), and sometimes I really push the limits of part-count sanity nonetheless; but I still make use of refueling stops to allow me to bring larger vessels to more distant destinations... (in fact, I often utilize refueling around the Mun or even Minmus just to reach Duna with many of my craft designs- usually because of one design constraint or another, such as their ability to act as a high-performance plane on Duna, or a frankly insane 240-ton payload size meant to help me establish a large colony on Duna that I simply couldn't build a big enough rocket to move to Duna without refueling...)

(3) Last, but not least, In-Situ Resource Utilization

- I'll be brief on this one. It (hopefully) goes without needing too much explanation that it is much lighter, and thus costs a LOT less fuel from Kerbin, to send a craft to another celestial body (such as the Mun or Duna) to produce and refine resources there (such as Kethane, or KSP-Interstellar's ability to perform electrolysis of Muna regoltih for Aluminum and Oxidizer), than it does to simply send all the fuel you need from Kerbin. In order to make use of these resources, you're going to either need to make refueling stops at the production location, or send fuel tankers from the production location to somewhere less out-of-the-way for your interplanetary vessel. Often, you'll make use of a combination of the two- for instance mining regolith on the Mun or Kethane on Duna, and then launching the resources to a fuel depot in orbit- which is the actual vessel your interplanetary ship will dock with and refuel.

- With enough mods installed, you can produce resources virtually anywhere. The four most important resource-utilization mods I know of are KSP-Interstellar, Kethane Pack, Extraplanetary Launchpads, and (indirectly) NearFuture mod. KSP-I offers a wide variety of resources in appropriate locations, ranging from Munar regolith to LFO mix (presumed to represent combustible ratios of cryogenically-stored Hydrogen and Oxygen gas) extracted from electrolyzing water from Vall's (presumed) ice deposits. Kethane offers, well, Kethane- wihch can be refined directly into LFO, Monopropellent, and even Xenon. Extraplanetary Launchpads offers a three-step resource system in which you mine Ore, then refine it into metal, then again into RocketParts, and finally use a craft-construction part on an existing base or vessel to construct additional ships like at the VAB or SPH, except off-world. Finally, there is NearFuture mod. It doesn't directly five you an ISRU capabilities- but its *HUGE* solar panels (ranging as large as the newest, 250 EC/s, "solar blankets") are the only *practical* way to make large-scale use of KSP-Interstellar's beamed microwave-power system to create solar electricity-farms in orbit.

- It also goes without saying that In-Situ Resource Utilization is the *ONLY* way you will ever set up sustainable long-term operations on other planets. It simply takes too many fuel tanker launches to send all the fuel you need to do all the things you want to do otherwise- and it's going to get even worse when they introduce in-game economy, and you have to pay for all these fuel tanker launches as well. I especially recommend Extraplanetary Launchpads and Orbital Construction mods for this purpose (please note, Orbital Construction does not provide a way to manufacture RocketParts on its own- you'll need to also install Extraplanetary Launchpads if you want this functionality- and in fact Extraplanetary Launchpads offers its own orbital-construction mechanic that is slowly making Orbital Construction mod obsolete, if not for the much larger RocketParts storage/transport modules it offers...)

I hope all this helps. I've gone on at great length- and I think I'm even going to post this in its own thread as an aid for new players.

Regards,

Northstar

[Northstar1989]

IIRC, it takes more dV to go from the surface of Kerbin to low Kerbin orbit (LKO) (~2k dV) than to go from Kerbin to DUNA (~1.6K)!

Fuel production depots set up on planets with low gravity and little to no atmosphere (like Duna, or even better, Dres) that sent fuel back to Kerbin or out to Jool would be insanely efficient. Local fuel production would also allow (maybe even encourage) the point-to-point hopping described above with very small, efficient vessels.

Actually, it takes over 4k Delta-V to get to Kerbin orbit. It takes 2k delta-V just to reach orbital velocity, but more than twice that thanks to drag and gravity-losses. It takes 600 Delta-V to make it from low Duna orbit to Kerbin, and even less from Ike, the Mun, or Minmus- so yes, if fuel consumption was all you were worried about, it would be a LOT more efficient to produce fuel on one of the moons of Kerbin or Duna, and ship it to low Kerbin orbit, than it would be to launch it from Kerbin. Of course, it still takes more than 1000 Delta-V to make it to orbit from Duna's surface... (drag from its atmosphere limits optimal TWR for ascent, thereby incurring gravity-losses as well as drag-losses)

Considering I've spent most of my past 2-3 months launching ships towards the Duna system to undertake a massive colonization effort there (you can see the Mission Reports thread HERE: http://forum.kerbalspaceprogram.com/threads/57509-Kerbin-and-Beyond-a-Maturing-Space-Program ), it'll be interesting to see if I can profit from launching fuel tankers BACK to Kerbin from Ike when the economy is released in a future update...

Regards,

Northstar

Edited February 4, 2014 by Northstar1989

[WhiteKnuckle]

I hope all this helps. I've gone on at great length- and I think I'm even going to post this in its own thread as an aid for new players.

Regards,

Northstar

I was going to say, this needs to be it's own thread/tutorial. Great read.

[nimbim]

I started out sending refueling ships to the planets where the craft was stranded in orbit. Then I started placing refueling stations before a mission. Then I realized that I can just build what's neccessary in LKO and save a lot of time.