• 0
Crillion

Refueling a Interplanetary MegaShip?

Question

I launched a $400,000 ship consisting of a lot of 3.75MM Fuel Tanks and such, with the intent to have a consistent vessel that could do an interplanetary trip and back. The fuel tanks were mostly drained to get the thing into orbit. I tried a refueling tug, but after about 40 minutes and a $160,000 tug launch, I only managed to maybe fill up a 10th of the tanks, if even. Any suggestions on how to refuel this expensive vessel, since it's now just chilling in low Kerbin Orbit (Or do I have to just make it into a new space station)? It does have enough fuel to get to say, the Mun or maybe Minmus if I have to.

Edited by Crillion

Share this post


Link to post
Share on other sites

21 answers to this question

Recommended Posts

  • 0

It will be a lot easier to refuel at Minmus than Kerbin. 

Share this post


Link to post
Share on other sites
  • 0

Short answer: check how much fuel you need, build a tanker that carries that amount of fuel and is able to dock with your vessel, build a launch system that can get that tanker with its full fuel payload into orbit, launch, RV, and refuel.

From what you're saying it sounds like you're using up a lot/most of your fuel payload just getting into orbit. If that's the case, you need a heavier launch system. If you're using a rocket, try making it twice as big for starters, and if you're not using it already, have a separate lifter stage and insertion stage. 

Since I don't know what your payload mass is, I can't be much more specific than that. But as a rough example, here's how I'd go about building a mid-range (rocket) launch system:

  1. Start with the core of the lifter stage: Orange Rockomax, Mainsail.
  2. Add an insertion stage: Oil-barrel Rockomax + second-smallest Rockomax, Skipper.
  3. Add a fairing (if necessary) and your payload.
  4. Check the thrust to weight ratio (TWR) and the delta-V (dV). For the lifter stage, minimum TWR in the atmosphere is 1.2. If it's over 2, then you're overbuilding. Your insertion stage can do with a little less TWR, especially if your lifter stage has a lot of dV. You need total dV of about 3400 to get into orbit; more is better as it gives you some slop.
  5. Add/remove tanks from the lifter and insertion stages to get the dV and TWR in the range you want. Typically you'll end up with somewhere north of 1000 dV for your lifter and the rest for your insertion stage.
  6. If you're a little short on TWR, add boosters. Thuds work well to give the lifter stage a little extra oomph. You can also strap on 2-4 solid fuel boosters but if you find yourself needing more than that, it's best to redesign. Generally speaking, when you notice that adding more tanks or more boosters only shifts your TWR or dV a little bit, it means you've hit the limits of your design and need to start over. 
  7. Check if after adjustments it meets your mission requirements. If not, start over but make it even bigger this time.

Naturally your maximum lift capacity will be limited by your tech level. If you don't have access to heavy-enough engines, there's only so much you can do by strapping together smaller ones.

Also, the KER (Kerbal Engineer Redux) mod is a lifesaver, as it will display your TWR and dV for each stage as you're building the rocket. I do not understand why this is not a standard feature as it's absolutely crucial for making remotely efficient designs.

28 minutes ago, Foxster said:

It will be a lot easier to refuel at Minmus than Kerbin. 

If he already had an ISRU operation at Minmus, I don't think he'd be asking this question.

Edited by Brikoleur

Share this post


Link to post
Share on other sites
  • 0
25 minutes ago, Brikoleur said:

Short answer: check how much fuel you need, build a tanker that carries that amount of fuel and is able to dock with your vessel, build a launch system that can get that tanker with its full fuel payload into orbit, launch, RV, and refuel.

From what you're saying it sounds like you're using up a lot/most of your fuel payload just getting into orbit. If that's the case, you need a heavier launch system. If you're using a rocket, try making it twice as big for starters, and if you're not using it already, have a separate lifter stage and insertion stage. 

Since I don't know what your payload mass is, I can't be much more specific than that. But as a rough example, here's how I'd go about building a mid-range (rocket) launch system:

  1. Start with the core of the lifter stage: Orange Rockomax, Mainsail.
  2. Add an insertion stage: Oil-barrel Rockomax + second-smallest Rockomax, Skipper.
  3. Add a fairing (if necessary) and your payload.
  4. Check the thrust to weight ratio (TWR) and the delta-V (dV). For the lifter stage, minimum TWR in the atmosphere is 1.2. If it's over 2, then you're overbuilding. Your insertion stage can do with a little less TWR, especially if your lifter stage has a lot of dV. You need total dV of about 3400 to get into orbit; more is better as it gives you some slop.
  5. Add/remove tanks from the lifter and insertion stages to get the dV and TWR in the range you want. Typically you'll end up with somewhere north of 1000 dV for your lifter and the rest for your insertion stage.
  6. If you're a little short on TWR, add boosters. Thuds work well to give the lifter stage a little extra oomph. You can also strap on 2-4 solid fuel boosters but if you find yourself needing more than that, it's best to redesign. Generally speaking, when you notice that adding more tanks or more boosters only shifts your TWR or dV a little bit, it means you've hit the limits of your design and need to start over. 
  7. Check if after adjustments it meets your mission requirements. If not, start over but make it even bigger this time.

Naturally your maximum lift capacity will be limited by your tech level. If you don't have access to heavy-enough engines, there's only so much you can do by strapping together smaller ones.

Also, the KER (Kerbal Engineer Redux) mod is a lifesaver, as it will display your TWR and dV for each stage as you're building the rocket. I do not understand why this is not a standard feature as it's absolutely crucial for making remotely efficient designs.

If he already had an ISRU operation at Minmus, I don't think he'd be asking this question.

 That makes sense, but given the size of the ship in question, that certainly makes it sound like lifting the needed fuel from Kerbin is going to be either nearly impossible, or require a ship so massive and so expensive it would be cheaper to just use my typical single use, less efficient rockets and just scale them up for interplanetary purposes. The goal here is a vessel that saves money by being able to repeatedly complete interplanetary journeys at the fraction of a cost of s normal vessel, but if I'm spending a million or more just to refuel the thing, it doesn't make sense.

I do have the mining stuff unlocked, so how would mining Minmus work? I can build an okay miner, but getting something that bulky there is the hard part, then figuring out a way to haul all the fuel up to the interplanetary ship and bringing the tug back down again seems fairly complex. Any suggestions on designs perhaps?

Edited by Crillion

Share this post


Link to post
Share on other sites
  • 0

Quite often the interplanetary fuel can be half the weight of weight of a ship, so one option is a 2 part launch, lift the ship with empty tanks to save on launch weight, and then lift the fuel in a separate launch.  Halving the payload weight means a massive reduction in the size of the lifter you need.  Powering your interplanetary ships with the nuclear engines saves a lot of fuel too, but does tend to mean a low TWR and long burn times.

How big and heavy is your ship?

For mining you either go for an all in one miner tanker which fills up on the surface and then lifts the fuel or orbit, or a mining base and separate tanker, possibly with a CLAW equipped rover to lug fuel from the miner to the tanker.  Don't forget to fit radiators though as the ISRU and drills need a fair bit of cooling to be efficient.

Edited by RizzoTheRat

Share this post


Link to post
Share on other sites
  • 0

i find it easiest to place a drill, smallest ore tank, and fuel converter on the ship and land on any moons nearby to fuel.

so launch - get to orbit - go to mun and fuel -get back into kerblin orbit - go to destination 

if the ship is massive it may be easier to fuel on minmus from near zero gravity tug

  • Like 1

Share this post


Link to post
Share on other sites
  • 0
15 minutes ago, Crillion said:

 That makes sense, but given the size of the ship in question, that certainly makes it sound like lifting the needed fuel from Kerbin is going to be either nearly impossible, or require a ship so massive and so expensive it would be cheaper to just use my typical single use, less efficient rockets and just scale them up for interplanetary purposes. The goal here is a vessel that saves money by being able to repeatedly complete interplanetary journeys at the fraction of a cost of s normal vessel, but if I'm spending a million or more just to refuel the thing, it doesn't make sense.

I do have the mining stuff unlocked, so how would mining Minmus work? I can build an okay miner, but getting something that bulky there is the hard part, then figuring out a way to haul all the fuel up to the interplanetary ship and bringing the tug back down again seems fairly complex. Any suggestions on designs perhaps?

It will always be more expensive and much more difficult to scale up lifters for interplanetary use, and in any case, the limiting factor is your launch capability. So if you're able to loft 20 tons to LKO, then the maximum practical mass for your fully-fuelled interplanetary craft is about 40 tons -- 20 tons for lofting the craft and another 20 tons for a refuelling launch. If you want to launch a bigger craft, then you need to develop a heavier launch system.

Reusable interplanetary craft are not easy. You need a lot of dV to insert yourself into a parking orbit for refuelling and relaunch. You can shave a lot off this by aerocapture, but you need to design your craft to survive it, which isn't all that simple, especially for Kerbin.

To get to Duna you need about 1000 m/s and to get back to Kerbin, about 600, so if you're a skilled pilot and your craft can survive Kerbin aerocapture, you can do the round trip from parking orbit to parking orbit with about 2000 m/s. If you don't do aerocapture, you're going to be pushing 4000 m/s, which is a lot harder. Or you can figure out how to mine and set up an ISRU operation at your destination: after that, you'll be able to do the round trips with 1500 m/s or so as you can refuel at both ends.

Basically... I'm afraid you may have bitten off more than you can chew. Interplanetary missions aren't easy to start with, and making your first attempt with a giant ship that you want to be reusable is... really hard. 

So, a recommendation. Park that giant ship of yours in LKO or do that Minmus or Mun round trip to get some Science, and consider it a valuable lesson learned in lifting stuff to orbit and moving it around. Then start a new Duna mission from scratch. Make this a simple round trip: one Mk 1 command pod with a heat shield, parachute, lightweight science experiments, and a storage can, carrying Jeb and staged to a propulsion unit running a Terrier and enough fuel to get there and back again, say 3500 m/s to keep it on the safe side. Get to Duna orbit, do some science, get back, re-enter and return with the pod, Jeb, and science only.

Then work your way up from there: do another one but this time pack only 2000 m/s, aerocapture on Duna, and hit the Kerbin atmosphere directly from your interplanetary transfer orbit.

Once you're comfortable with that, you can start figuring out how to make a reusable operation for the same. A reusable interplanetary stage is a good start: the hard part will be figuring out how to get back to a parking orbit around Kerbin (either make it survive Kerbin aerocapture from a transfer orbit, or pack enough dV that you can burn yourself there). From there on out, figure out how do the same for bigger payloads.

As to ISRU and mining, practice on Minmus. You will need one module with the converter, a drill, a power source (hint: fuel cells are good), and enough radiators to keep it cool, and another module that can dock to it, extract the fuel, and carry it to orbit. You can then use that as a refueling station for your interplanetary missions (although it won't work well for all of them as it costs more to get to some places from Minmus orbit than low Kerbin orbit). Test the setup first on Kerbin to make sure you're able to mine, refine, dock, and undock.

Share this post


Link to post
Share on other sites
  • 0

@Crillion, if your goal is to keep costs down, a Minmus-based refueling operation is definitely the way to go (at least IMO). I like the mining rover/fuel shuttle combo.

 

ULQ1qB3.pngYo

 

You just roll up to your fuel shuttle, drop the drills, and start refueling it.

 

hsnhaEf.png

 

After landing, takeoff, and rendezvous maneuvers, the fuel shuttle is able to transfer about 10,000 units of lf/o. This was long the backbone of my space program. They're old designs though, that I'd change a little bit (for instance, I have no idea why the shuttle has radiators; and the miner has the drills a bit too low; before 1.2, the large part of the drill could enter the ground; it no longer can, and it lifts up a bit when I drop the drills), but they still work great. Of course, you've gotta get 'em to Minmus in the first place.

 

ZdLe6rK.png

 

xrSh1fN.png

 

These are old as well, and can definitely be streamlined (as you can see, I thumb my nose at aerodynamics), but it'll give you some ideas at least. The initial payout is huge, but they're yours forever, and will pay off for decades to come. Hope this helps a bit.

 

 

Edited by Cpt Kerbalkrunch

Share this post


Link to post
Share on other sites
  • 0
11 minutes ago, Cpt Kerbalkrunch said:

a Minmus-based refueling operation is definitely the way to go (at least IMO).

It's easiest to set up on Minmus but it's no good if you're targeting Jool and beyond because it costs more dV to get there from Minmus than from LKO (~2500 as opposed to ~2000, enough to make a difference).

I don't think there is a definitive answer to this actually, it depends on your tech level, your design ability, your piloting ability, and where you're going:

Minmus pro: low gravity (easy/cheap to shuttle the fuel to/from orbit). 

Minmus con: inclined orbit, too high to get cheaply/easily to Jool and beyond.

Mun pro: easy orbit, better for some interplanetary launches than Minmus, resource-rich.

Mun con: deeper gravity well (take-offs cost more, landings are trickier; it takes some practice to reliably land close to your mining station).

Kerbin pro: unlimited fuel available on demand, low orbits are better suited for many interplanetary launches.

Kerbin con: you need a reusable, high-capacity launch system to do this cheaply.

I find it not worth the trouble to mine resources on the Mun or Minmus (except for practice), but then I like building reusable launch systems before I even start interplanetary missions. For me it makes more sense to build the ISRU at your destination instead: that way I only need the dV to get there, rather than the round trip.

Edited by Brikoleur

Share this post


Link to post
Share on other sites
  • 0
1 hour ago, Brikoleur said:

It's easiest to set up on Minmus but it's no good if you're targeting Jool and beyond because it costs more dV to get there from Minmus than from LKO (~2500 as opposed to ~2000, enough to make a difference).

I don't think there is a definitive answer to this actually, it depends on your tech level, your design ability, your piloting ability, and where you're going:

Minmus pro: low gravity (easy/cheap to shuttle the fuel to/from orbit). 

Minmus con: inclined orbit, too high to get cheaply/easily to Jool and beyond.

Mun pro: easy orbit, better for some interplanetary launches than Minmus, resource-rich.

Mun con: deeper gravity well (take-offs cost more, landings are trickier; it takes some practice to reliably land close to your mining station).

Kerbin pro: unlimited fuel available on demand, low orbits are better suited for many interplanetary launches.

Kerbin con: you need a reusable, high-capacity launch system to do this cheaply.

I find it not worth the trouble to mine resources on the Mun or Minmus (except for practice), but then I like building reusable launch systems before I even start interplanetary missions. For me it makes more sense to build the ISRU at your destination instead: that way I only need the dV to get there, rather than the round trip.

Well, it all depends on play style I think. A lot of guys like the reusable launch system like you do, and it seems to work pretty well. I just prefer the free fuel at Minmus.

And as for the transfer, it's actually cheaper. You just don't leave from Minmus itself. You drop back down to Kerbin. It costs about 150m/s to drop back to a Pe of about 70-75km. And you still have your Ap at Minmus's orbital height. It's a long way to fall. You build up a tremendous amount of energy. Then, when you reach Pe, you burn. You'll be amazed at how quickly your Ap rises to meet Jool. Definitely cheaper than a traditional burn from LKO. Timing is the critical part (and the most difficult to master). You have to wait for Minmus to be on the correct side of Kerbin before you drop down. There's also the transfer time to consider. It takes a bit of practice, but it's well worth it if you don't mind the extra real time you'll spend refueling. I got pretty good at timing the transfers (you usually need to go a little early or a little late; I prefer going later, as you can make up a bit of speed easily), and can make it work everywhere but Dres. It's just too small of a target (and too inclined). I always needed a correction burn. If you're wondering about Moho, it works there as well, but mostly because I always bounce off of Eve (which is easy to hit).

But Jool is by far the easiest. Its SOI is so huge you simply can't miss. Combine the Minmus refuel and drop down, with the free capture at Jool, and you can build tons of infrastructure there for dirt cheap. In turn, you can then use your mining operations around Jool to make things even easier on yourself later. It worked out really well for me when I was first trying to conquer the system. I highly recommend it but, I think we can both agree, it has to suit your play style.

Edited by Cpt Kerbalkrunch
  • Like 3

Share this post


Link to post
Share on other sites
  • 0
3 minutes ago, Cpt Kerbalkrunch said:

Timing is the critical part (and the most difficult to master).

Yep, there's the rub. You gain about 900 m/s minus what you burned getting out of Minmus which is indeed a major gain. But it is complicated. You're clearly a much better orbital mechanic than I am!

  • Like 1

Share this post


Link to post
Share on other sites
  • 0
3 minutes ago, Brikoleur said:

Yep, there's the rub. You gain about 900 m/s minus what you burned getting out of Minmus which is indeed a major gain. But it is complicated. You're clearly a much better orbital mechanic than I am!

Ha. I'm just a cheapskate. :)

  • Like 1

Share this post


Link to post
Share on other sites
  • 0

How much fuel do you need exactly? Someone else had a similar problem recently and I built this quick "monster rocket" as an example to show that it's not really hard to do huge lifters.

This carries 16,000 units of liquid fuel to orbit. Only like 70 parts. Only costs like 200k, which is nothing in Career mode.

So I guess my advice is to not overthink the problem and make it more complicated than it is. Just send a bigger refueling ship.

Spoiler

?interpolation=lanczos-none&output-forma

 

Share this post


Link to post
Share on other sites
  • 0
11 minutes ago, Brikoleur said:

Yep, there's the rub. You gain about 900 m/s minus what you burned getting out of Minmus which is indeed a major gain. But it is complicated. You're clearly a much better orbital mechanic than I am!

I rather like RedIronCrowns approach mentioned on the Oberth Effect thread.  Tanker launches from Minmus, docks with interplanetary ship in LKO, push the whole thing out to around Minmus orbit distance, top up the tanks on the interplanetary vessel, undock the tanker, and then either burn for the destination or drop back to Kerbin for a bit extra from Oberth, while the tug heads back to Minmus.  Take a bit of time but you lose the orbital inclination issue and some of the timing issues of launching from Minmus, and start from a fair way out with full tanks.

  • Like 1

Share this post


Link to post
Share on other sites
  • 0
7 minutes ago, Cpt Kerbalkrunch said:

Ha. I'm just a cheapskate. :)

Tangent... it's interesting how KSP takes people in different directions.

Some people get really into optimising transfers. Others get really into squeezing every last bit of efficiency out of their designs. Yet others just like to build absurd things and see if they can get them to work.

I like to design things that are fun to fly and work for some specific purpose, but find orbital mechanics something of a chore: I have attempted complex transfers such as those described here, but I don't ultimately get much satisfaction out of planning and executing them, and would rather spend that time tweaking my craft. So I generally just plug the numbers into alexmoon's calculator and run with that. Last time I tried something which worked in theory but required complicated transfers with precise timing I managed to execute it once and then got frustrated and bored and decided to brute-force it as usual.

(I do like aerocaptures though, because they're a design problem.)

Share this post


Link to post
Share on other sites
  • 0

Another approach that can work, which is easy and doesn't require virtuoso timing/navigation work, is to use Minmus as a fuel source for stocking up an LKO fuel depot.  Here are the components of a setup like that, which I tried in one of my games and worked pretty well:

  • Minmus miner.  Giant ship that shuttles between Minmus surface and Minmus orbit.  Has drills, ISRU, little bit of ore storage, and as much fuel tankage as you can cram on to it.  Engines don't need to be particularly high Isp, since it only needs enough dV to go between Minmus orbit and surface.  So they're typically something that has enough TWR to do reasonably efficient liftoff/landing.  A single Rhino can lift a gargantuan ship off Minmus.
  • Transfer tanker.  Giant ship that shuttles between Minmus orbit and LKO.  Completely reusable.  Very low TWR, very high Isp (typically powered by LV-Ns).  Uses the big 10m heatshields for aerobraking into LKO, so it needs very little dV to go from Minmus to LKO when it's heavily laden with hundreds of tons of fuel.  Going back from LKO to Minmus takes 900 m/s of dV, but it doesn't need much fuel for that because, 1. it has really efficient high-Isp engines, and 2. it does that when it's already offloaded its fuel, so the ship is fairly light at that point.
  • LKO fuel depot.  Storage facility for fuel.  It just sits in circular LKO and never goes anywhere.  Huge fuel storage, easily expandable as needed by docking more tanks to it.

Typical fuel transfer cycle looks like this.  Start with miner full on Minmus surface, tanker empty in Minmus orbit.

  1. Miner lifts to Minmus orbit, docks with tanker.
  2. Transfer nearly all of miner's fuel to tanker, leaving just enough to land again on Minmus.
  3. Miner undocks, deorbits, lands on Minmus with empty tanks.
  4. Miner starts up the drills and refinery so it'll have full tanks by the next time the tanker returns
  5. Tanker does a tiny burn to head back to Kerbin.
  6. Tanker aerobrakes into LKO, does tiny burn to circularize, docks with fuel depot.
  7. Transfer nearly all of tanker's fuel to depot, leaving just enough fuel for tanker to head back to Minmus.
  8. Tanker undocks, burns for Minmus, brakes into circular Minmus orbit.

Repeat the above steps as needed.  Each iteration of the cycle transfers several hundred tons of LFO to the LKO fuel depot.  Each iteration of the cycle involves one round trip from Minmus surface to orbit and back, and one transfer flight from Minmus orbit to LKO and back, so it doesn't take up much of your time.

The result is that you get lots and lots of fuel on your depot, which you can then use for whatever purposes you want.  If you've got a giant interplanetary mothership to send to Jool, all you need to be able to do is get it off the launchpad and to LKO with empty tanks.  Just dock it to the depot and top off the tanks before going on your way.

I might also add that if you like building big, you may wish to consider installing the SpaceY mod.  It has parts going up to 5m, and the biggest fuel tank holds several hundred tons of LFO.  Simplifies construction when you don't have to cobble together lots and lots of smaller parts, and also makes for less mouse-clicky hassle when you're transfering fuel among the various tanks.  Here's an example of a Minmus/LKO transfer tanker built around one of the big SpaceY tanks; it carries around 300+ tons of fuel at one whack, IIRC.  You could go a lot bigger if you use a few of the tanks.  (And if you want to go seriously large, you could add SpaceY Expanded, which goes up to 7.5m parts that are crazy big.  I've never run that one myself, since it's a bit too rich for my blood :wink: ... but I'm willing to trust that it's good, since everything NecroBones does is great.)

  • Like 1

Share this post


Link to post
Share on other sites
  • 0

1) Does your vehicle withstand at least 0.1g accel from one end?

No -> How were you planning to get anywhere?  Add moar boosters/struts, goto 1.

Yes -> Land it on Minmus, hook up a fuel hose to the refinery on the flats, do some other stuff for a bit, and come back to a full tank. 

Share this post


Link to post
Share on other sites
  • 0

This question about how to play the game has been moved to Gameplay Questions. 

Share this post


Link to post
Share on other sites
  • 0
12 hours ago, Snark said:

A single Rhino can lift a gargantuan ship off Minmus.

Here's my vote. 

Large?  Yes!      Practical?  No!        Delightful?  Maybe!?

Actually, this thing works pretty well once you get it to orbit.  Not the easiest thing to launch, though. 

S7wJKSh.png

 

Share this post


Link to post
Share on other sites
  • 0

Sorry about the late reply.

 

After some consideration, I have decided to turn this ship into a space station. It is still flyable, but the amount of refueling hardware needed wouldn't be viable. In this photo, You can see the first failed fuel tanker attached to the main ship, along with the crew shuttle I used to station an engineer on the station so he can fix anything that may go wrong due to Kerbal Mechanics. It has enough docking ports to be a good station!

 

GNrpmuY.jpg

  • Like 1

Share this post


Link to post
Share on other sites
  • 0

Whoa, that is a big ship!

Share this post


Link to post
Share on other sites
  • 0
On ‎27‎/‎01‎/‎2018 at 5:23 AM, Aegolius13 said:

Not the easiest thing to launch, though. 

I suspect that's a bit of an understatement :D

 

How much dV does @Crillion's ship have if fully fuelled?  That things a monster!

 

This is my current Minmus mining setup, miner on the left stays on the ground and the tanker with detachable tug lugs the fuel back to Kerbin orbit. I'm using Tweakscale to get decent sized LF tanks though.

e5Ij601.png?1

 I've just sent a Duna mission out on a slightly modified version of the method I described above.  I boosted it to an elliptical orbit out beyond the Mun with the tanker still attached, topped up the tanks, disconnected the tanker, and then burned for Duna at periapsis when it only needed something like 4-500m/s

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now