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JPmAn

Interplanetary spacecraft question

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Would it be feasible to have a dockable engine section with an expendable, replaceable fuel section, or a reusable fuel section, with refueling spacecraft. What are your thoughts?

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Yep, can be done. Not sure if there are advantages to it, though. Flying a new fuel module to the thing would require you to have docking ports on each part. On the other hand, you could dock a tanker to a one-piece ship and transfer fuel through a single set of rings. 

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Thanks, should put up a poll, I am on my mobile phone right now

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You could use KAS/KIS to swap parts over rather than needing docking ports on everything, but there's a weight limit on what a kerbal can manipulate so you'd need to have several of them EVAing on the part at the same time.

Personally I go for the separate tanker approach and transfer fuel

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I'd opt for for the tanker option too.  You can be much more flexible in what ships you send  to deliver fuel.  As long as it has the same sized docking port you can send any vessels you like, and you will have a lot less shuffling about when you get there - just dock, transfer and go.

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On ‎1‎/‎28‎/‎2016 at 10:06 PM, JPmAn said:

Would it be feasible to have a dockable engine section with an expendable, replaceable fuel section, or a reusable fuel section, with refueling spacecraft. What are your thoughts?

In general if you're talking about large, crewed, interplanetary ships, it can be a long-term pay-off in funds and effort to build a ship that is modular like you are suggesting.  The reason being is that if it's a ship that is assembled in orbit, or is one-piece that requires a massive launch system, than it requires a lot of time, effort and/or career mode funds to replace the ship.  If you make a ship modular so you can change propulsion modules or fuel tanks, than you can refit your ship for follow-on missions.  Example: Let's say you assemble a ship in orbit capable of going to Duna and back, and after you return you want to go to Eve next.  The ship may not have enough delta-V and/or thrust-to-weight ratio to make a round-trip to Eve and back.  Instead of starting over and building a whole new ship, you just swap out a module or two, put some more fuel in it, and send it on it's next adventure.  Instead of doing a series of launches and rendezvouses, you may only have to do a few to refit the ship for the next mission.

The downside to keeping all the tankerage with your ship is that by the time you've finished your mission, you're going to be hauling around a lot of dead weight in the form of empty fuel tanks.  If you make portions of your fuel tanks jettisonable, you can save delta-V by loosing weight along the way, but it will of course cost funds to replace the hardware if you want to reuse that ship.  How much delta-V gain vs funds lost really comes down to the design of your ship and the mission profile.  My general rule of thumb is to save engines, crew compartments, utilities like batteries/solar panels, and comms/science equipement, since these are all either really expensive or have a lot of mass to put into orbit.  Fuel tanks in general I just toss away since they are relatively cheap compared to the others.

My most advanced design I roll with on interplanetary missions is in the picture below.  As you can see, it has multiple configurations depending on where it's going (ie Duna, Dres, Eve), and has reusability in mind.  Portions of the fuel tankerage are jettisonable, either from a 'Saddle' truss or from a 'Star' Truss.  In the case of the 'Star' truss, it only takes one launch to replace the four drop tanks with new tanks already full of fuel.  I found it's more economical to refill the larger 3.75m fuel tanks since launching new ones cost more money in the form of a heavy launch vehicle, versus several smaller launches with a refueling ship.  So sometimes I use both methods.  Bottom line: replacing tanks or refilling tanks depends on the mission profile and where you're going, how big the ship is, etc.  What is advantageous in my opinion is modularity to refit ships for multiple missions to different destinations.  Imagine having a fuel-efficient highway car that could rapidly be reconfigured into a muscle-car for high-speed trips.

EV-4%20Block%20Variants%20v2%20Small_zps

Edited by Raptor9

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