is it worth refueling before leaving SOI?

[Provisional Name 12]

I was watching a scott manly video today where he was trying to fly into the sun and he left the SOI of Earth by heading straight up when the planets petrograde vector intersected with the ship's. It occured to me, all that horizontal energy must be wasted, I have seen some skilled pilot line the mun up and catch an intercept using the same technique and was surprised at how much fule it saved compared to establishing an orbit and setting a trajectory. There is no way that in a delta v per delta v measurement it can be worth the extra fuel nevermind the time to circularize your orbit and intercept minimus.

Edited November 6, 2016 by SpaceCommunism

[I_Killed_Jeb]

It's not cost efficient in terms of pure dV but your source is limitless (mun/minmus miner) so why not top off?

 

I also have a feeling if you did the math out, it's cheaper to refuel in orbit from a minmus station than launching a big ship from Kerbin fully fueled

Edited November 6, 2016 by I_Killed_Jeb

[Snark]

3 hours ago, SpaceCommunism said:

I was watching a scott manly video today where he was trying to fly into the sun and he left the SOI of Earth by heading straight up when the planets petrograde vector intersected with the ship's.

Note that this is an inefficient way to leave the planet.  Regardless of whether you're going to the Mun, to Minmus, or to some other destination in the solar system, it's considerably more efficent in terms of dV to do a gravity turn so that you do most of your accelerating horizontally, than going straight up.  Going straight up all the way is wasteful because you pile on lots of gravity losses.

As for "do I launch a big ship straight from Kerbin, or refuel at Mun/Minmus first":  it's largely a matter of taste, and what you're optimizing for.  If you enjoy the added complexity of the two-step trip, doing the mining/refing, transferring the fuel, etc., then by all means do that 'coz it's fun!    On the other hand, if you're simply trying to save money... frankly, I think it's cheaper to do a direct launch from Kerbin.  Rationale:  It saves your time, which is the most precious commodity.  Sure, it costs more funds to do it that way... but with the time you save, you can dash off a quickie contract that pays way more cash than the amount you would have "saved" by doing the extra stop-over to refuel.

Basically, it totally comes down to your play style, and what it is that you want to get out of KSP.  Both ways work great, they just have somewhat different "payoffs".

[KerikBalm]

circularizing does not waste dV, because that velocity is stored, your ship goes in a circle, so you just add extra velocity at the right point in your orbit, and you get the velocity you need in the direction you need... but you also have your acceleration perpendicular to gravity, and thus minimize gravity drag losses. Excluding ISRU, refueling in orbit, or just launching a bigger ship and staging off empty tanks... doesn't really make a difference except for the dV required to dock the two ships

 

[Kertech]

Like @Snark said it depends on you.

 

we on this forum seem to get all caught up in efficiency and delta v budget! Squad have said (a lot of times) they're not gonna give us that data because the game is meant to be trial and error! 

 

Whether you you agree with this principle or not it does underpin a lot of the games decisions! 

 

So so if the question is ever "should I do X or Y" the answer is try it and find out!! 

[magnemoe]

7 hours ago, Snark said:

Note that this is an inefficient way to leave the planet.  Regardless of whether you're going to the Mun, to Minmus, or to some other destination in the solar system, it's considerably more efficent in terms of dV to do a gravity turn so that you do most of your accelerating horizontally, than going straight up.  Going straight up all the way is wasteful because you pile on lots of gravity losses.

As for "do I launch a big ship straight from Kerbin, or refuel at Mun/Minmus first":  it's largely a matter of taste, and what you're optimizing for.  If you enjoy the added complexity of the two-step trip, doing the mining/refing, transferring the fuel, etc., then by all means do that 'coz it's fun!    On the other hand, if you're simply trying to save money... frankly, I think it's cheaper to do a direct launch from Kerbin.  Rationale:  It saves your time, which is the most precious commodity.  Sure, it costs more funds to do it that way... but with the time you save, you can dash off a quickie contract that pays way more cash than the amount you would have "saved" by doing the extra stop-over to refuel.

Basically, it totally comes down to your play style, and what it is that you want to get out of KSP.  Both ways work great, they just have somewhat different "payoffs".

Yes, note that launching from Minmus makes you loose out the Oberth effect from launching from low orbit, found that going from an orbit around Kerbin around with an distance like Minmus and I used 2700 m/s, making it far more expensive than going from low orbit. 
Going directly from an minmus orbit would probably save some but not much and it makes making the node far harder.

Note that if you drop from Minmus down to close to kerbin and then do you burn you earn a lot, first you will move at 3 km/s so just an tiny burn of 100-200 m/s will send you to Duna or Eve, even better the going far as the Oberth effect will help a lot making it far easier to send an ship to Moho or Eeloo. 
Downside is that it take some practice to set up and you need to start well ahead of your launch window.
Smart to set up an satellite in low Kerbin orbit with the node you want to do like an Moho intercept. 
Send ship to Minmus to fill up, then enter high orbit around Kerbin outside Minmus orbit is perfect, but you need to lower your Pe so it intercept the satellite orbit at the burn node.
Timing should also match, either change your orbit around Kerbin to modify the time you lower Pe or modify Ap at low Pe, however this easy add 14 days. 
 

[adsii1970]

I've toyed around with the idea of having a fueling station (supplied by fuel from Minimus) in an orbit at the edge of Kerbin's SOI. The reasons why are quite simple:

• It justifies missions that originate from Minimus and sharpens my low-grav landing skills.
• It sharpens my intercept and docking skills.
• It puts more Kerbals in space - and it has a function (a crew of one pilot and two engineers for each fuel transport vessel).

Now is there actually any benefit to doing it that way? Probably not. However, there are some vessels that are simply cumbersome to land. I think this approach will be fun, will add another dynamic to the game, and anything that can improve my skills is worth it to me!

[ZooNamedGames]

Nasa and SpaceX thinks it is.

[magnemoe]

14 hours ago, ZooNamedGames said:

Nasa and SpaceX thinks it is.

Refueling in low orbit is another issue and makes obvious sense. Not only can you not only launch the upper stage dry but also use the payload as an upper stage if it has engines. The fuel is free once you have the infrastructure. 

Real world you have other issues, you can not get free fuel from mining in the near future but the stuff we do in KSP like strapping four liquid fuel boosters with crossfeed on an SLS, would cost serious money to design  
 

[Terwin]

10 minutes ago, magnemoe said:

Refueling in low orbit is another issue and makes obvious sense. Not only can you not only launch the upper stage dry but also use the payload as an upper stage if it has engines. The fuel is free once you have the infrastructure. 

Real world you have other issues, you can not get free fuel from mining in the near future but the stuff we do in KSP like strapping four liquid fuel boosters with crossfeed on an SLS, would cost serious money to design  
 

I use minmus refueling as a surrogate for orbital refueling on my interplanetary vessels with ISRU.

That way I can plan to have less then 2km/s left after making low orbit and still have a very comfortable safety-margin, regardless of my final destination.

(my non-isru probes often require almost as much of a launch stage as my interplanetary vehicles with life support, due to the need to be fully fueled during launch)

 

[Freds]

I never refuel in kerbins SOI, except when the ship is meant to be reusable to more missions.

For interplanetary missions, i usually create a 'mothership' which always stays in space, and use small ships to bring Kerbals and equipment to/from Kerbin.

What i do is to send dedicated mining/refuel ships to other planets, so i can refuel the mothership if needed. 

In Kerbin my refuel ship always land on mimus, in Jool its stays on Pol (but its capable of landing in Vall) , in Eve lands on Gilly.... But i got tired of mining Mimus to  refuel something in Kerbin's orbit, so i just made a Space Station with a 'cheat refueling unit', with give-me lots of fuel (LF, LFO, O ). I only use that in Kerbins, because it is not fun to have it in other places. 

Just need to use the imagination and belive that the Kerbal Space Agency contract someone to do refuel missions to the refueling Spacestation! .

[Curveball Anders]

1 minute ago, Freds said:

I never refuel in kerbins SOI, except when the ship is meant to be reusable to more missions.

For interplanetary missions, i usually create a 'mothership' which always stays in space, and use small ships to bring Kerbals and equipment to/from Kerbin.

Same here.

With the exception that I most often have all 'in-system' (regardless of system) stop over at the relevant station to either re-fuel or drop off excess fuel.

I don't use mining (yet) instead I have a standard un-manned tanker (called 'Progress', guess why) and a standard up-and-down craft (not called Soyus but Taxi ).

[magnemoe]

36 minutes ago, Terwin said:

I use minmus refueling as a surrogate for orbital refueling on my interplanetary vessels with ISRU.

That way I can plan to have less then 2km/s left after making low orbit and still have a very comfortable safety-margin, regardless of my final destination.

(my non-isru probes often require almost as much of a launch stage as my interplanetary vehicles with life support, due to the need to be fully fueled during launch)

 

Problem is that refueling on Minmus cost 1 km/s more than LKO so you need to bring more up, going directly from Minmus that is without doing the burn on an close Kerbin flyby is more expensive than burning from LKO as you loose the Oberth bonus

Now I have an systemliner, doing missions to Mun and Minmus, it refuel on Minmus as its part of its route. 

[Terwin]

11 minutes ago, magnemoe said:

Problem is that refueling on Minmus cost 1 km/s more than LKO so you need to bring more up, going directly from Minmus that is without doing the burn on an close Kerbin flyby is more expensive than burning from LKO as you loose the Oberth bonus

Now I have an systemliner, doing missions to Mun and Minmus, it refuel on Minmus as its part of its route. 

True, it is not as ideal as refueling in LKO, but it also requires no infrastructure and no docking.

Back when USI-MKS had a working orbital logistics system, I would always have a Kerbin fuel base that would fuel up anything that made it to orbit(usually including the liquid core portion of the launch vehicle), but as I no longer have a hands-off method of refueling in LKO, I refuel on Minmus as the minimum player-effort option for planning to reach orbit with a nearly dry upper stage.

[James Kerman]

Once I have an ISRU landed on Minmus I launch a refuelling station in Kerbin orbit that I dock to and de-fuel returning Minmus vessels before de-orbiting. 

[Kerbart]

From a fuel-only point of view it doesn't make sense to refuel in Mimnus orbit. It's always going to cost more fuel than going straight to your destination from the Kerbin surface.

From a mission point of view it might make perfect sense. You're starting at the edge of Kerbin's SOI, so you'll need to carry far less fuel at that point to burn for the intercept. This also means you can get away with a lighter ship, less fuel to launch it, etc. In the end you'll have to refuel and carry that up from the surface of Minmus, likely destroying your fuel savings but on the whole you can work with smaller, cheaper craft.

As others point out, there's a million factors that determine if it actually does make sense or not. If you already have a refueling station around Minmus it probably does. If you need to build the entire infrastructure for this one mission, it doesn't.

[wumpus]

18 minutes ago, Kerbart said:

From a fuel-only point of view it doesn't make sense to refuel in Mimnus orbit. It's always going to cost more fuel than going straight to your destination from the Kerbin surface.

From a mission point of view it might make perfect sense. You're starting at the edge of Kerbin's SOI, so you'll need to carry far less fuel at that point to burn for the intercept. This also means you can get away with a lighter ship, less fuel to launch it, etc. In the end you'll have to refuel and carry that up from the surface of Minmus, likely destroying your fuel savings but on the whole you can work with smaller, cheaper craft.

As others point out, there's a million factors that determine if it actually does make sense or not. If you already have a refueling station around Minmus it probably does. If you need to build the entire infrastructure for this one mission, it doesn't.

From a fuel-only perspective:

It makes sense if the fuel was delivered to your location via an engine more efficient than than the rocket fueling up (spaceplanes to LEO, ions>nukes>chemical for further out).  Note that you still want to burn at a minimal perigee to Kerbin, so refueling out by minmus makes scheduling a burn in the right direction much harder (this is probably less of an issue if your fuel dump is around 700m/s out).  

IRL this shows just what you can do with ion drives delivering chemical fuels for refueling, in KSP it isn't terribly useful (unless you really like flying spaceplanes).

It makes all sorts of sense to refuel fuel mined from Minmus, even if you move your depot closer to Kerbin.

From a rocket building perspective:

If you have a great launcher and issues scaling it up, there isn't a problem in delivering the rocket in sections (and possibly with empty fuel tanks if you hate docking wobble).

IRL it costs about a billion dollars to design a rocket.  So if you have a working rocket, there is a lot of motivation to use it and not design a bigger one.  It is even worse for a crew-rated rocket, so using an uncrewed rocket to bring up fuel makes even more sense (although I'm not sure if Spacex or Orbital is cheaper than Soyuz).

[Freds]

27 minutes ago, Kerbart said:

From a fuel-only point of view it doesn't make sense to refuel in Mimnus orbit. It's always going to cost more fuel than going straight to your destination from the Kerbin surface.

From a mission point of view it might make perfect sense. You're starting at the edge of Kerbin's SOI, so you'll need to carry far less fuel at that point to burn for the intercept. This also means you can get away with a lighter ship, less fuel to launch it, etc. In the end you'll have to refuel and carry that up from the surface of Minmus, likely destroying your fuel savings but on the whole you can work with smaller, cheaper craft.

As others point out, there's a million factors that determine if it actually does make sense or not. If you already have a refueling station around Minmus it probably does. If you need to build the entire infrastructure for this one mission, it doesn't.

It depends on the strategy adopted.

Sometimes, when i launch a heavy ship, it gets too heavy to launch with full tanks, but, if i launch  with just enough fuel to get to orbit, i can use a much smaller, less expensive, more simple and reliable launcher stage.

[Tyko]

I've tried Minmus fueling in previous saves. My current game has given me a plethora of big rocks flying into Kerbin SOI so I've decided to go with Asteroid mining instead.

Any advice on the best altitude to park the asteroids/refueling stations? I'm currently considering 3 options:

• 150Km - easy to get to from launch
• 500Km - a bit tougher to launch to, but a lot easier for craft in space to rendezvous with.
• 5000Km - Seems like a great place to launch interplanetary from. If I can time it right, I can lower my PE to 75Km and get a solid Oberth boost for my outbound trips.

Edited November 11, 2016 by tjt

[Nicias]

Assuming free fuel, the best departure orbit varies by destination:

So, if you want to have the smallest vehicle or use the least delta-v, you should park your craft at that altitude and have a fuel tanker come and fill it up.

Personally, I launched a mining rig/fuel tanker to minmus (two actually LF+Ox and LF only), Did a couple of fill-up runs, convinced myself that it could deliver fuel to LKO and return. Then I just launch empty to the apprioate gate orbit, and fuel up with hyperedit. 

 

[juanml82]

On 10/11/2016 at 1:20 PM, adsii1970 said:

I've toyed around with the idea of having a fueling station (supplied by fuel from Minimus) in an orbit at the edge of Kerbin's SOI. The reasons why are quite simple:

• It justifies missions that originate from Minimus and sharpens my low-grav landing skills.
• It sharpens my intercept and docking skills.
• It puts more Kerbals in space - and it has a function (a crew of one pilot and two engineers for each fuel transport vessel).

Now is there actually any benefit to doing it that way? Probably not. However, there are some vessels that are simply cumbersome to land. I think this approach will be fun, will add another dynamic to the game, and anything that can improve my skills is worth it to me!

As someone said, having a tanker in orbit allows you send the upper stages dry and save a lot in lower stages. But this only matters for very big ships.

However, going interplanetary from Minmus doesn't save you any dV than doing your interplanetary burn from LKO because you loose the Oberth effect. So, the most fuel efficient way to do it is to either:

Refuel at Minmus, leave Minmus in a highly elliptical Kerbin's orbit which is timed so the time for your interplanetary injection burn matches your periapsis. That way, you'll be leaving Kerbin' SOI with nearly full tanks, because you start your burn with your Ap already near the edge of Kerbin's SOI. But this is very difficult to time - you need to leave Minmus at a time that puts your periapsis in the correct side of Kerbin and also happens during the transfer window.

Or, you refuel a tanker in Minums, the tanker then goes back to Kerbin (possibly making several aerobrake passes) and you refuel your interplanetary mothership in LKO. That way, you don't need too much timing, since your orbital period is about 30 minutes instead of an entire week long and the tanker can go back to Minmus to refuel and serve another mission in the future. Such a mission profile is more complex as you have at least two ships you need to dock, aerobrake and send to different destinations. It also means you won't be leaving Kerbin' SOI with as much fuel as the previous way, as you'll need to burn some 900 dV just to leave Kerbin' SOI.

[suicidejunkie]

If your tanker is as large as it should be, and your docking ports are along the CoM, you can use the tanker as a booster stage.

Best of both worlds; fast orbit for making the window, and the interplanetary ship will still have full fuel at the edge of Kerbin's SoI, and the tanker needed to boost its AP anyways.

 

I did this for my Dres mission, but failed to return the booster to Minmus as pushing 10k LF through a pack of LV-Ns caused the control node to explode due to overheating.

The next version will include a radiator.

[Sneaky Bstard Sword]

On 11/6/2016 at 6:21 AM, KerikBalm said:

circularizing does not waste dV, because that velocity is stored, your ship goes in a circle, so you just add extra velocity at the right point in your orbit, and you get the velocity you need in the direction you need... but you also have your acceleration perpendicular to gravity, and thus minimize gravity drag losses. Excluding ISRU, refueling in orbit, or just launching a bigger ship and staging off empty tanks... doesn't really make a difference except for the dV required to dock the two ships

 

Yes your overall energy increases, but this can be offset by the Oberth effect, depending on the exact scenario. If your burn would be complete before you reach the altitude where you would circularize, a direct ascent is slightly more efficient. 

[KerikBalm]

No, not unless its an airless body and you've got an extreme TWR.

Every second you spend accelerating perpendicular to gravity is 10 m/s lost even with no atmosphere.

Even 2 seconds of that will likely make up for the dV difference due to any oberth effect - and even then, you can still launch into a gravity turn so that you see Mun coming over the ohrizon at about 30km where you can start accelerating as much as you want (or nearly so) without worrying about drag losses and heating.

Launching straight up with no gravity turn is simply less efficient, there's not really anything to debate here. You don't save anything, and you lose more to gravity drag.