A reusable transfer stage: a good idea?

[Laie]

One way I use them is as q reuseable first stage for the first 1000 m/s

This reduce burn time a lot as the tug has a decent twr. Tug never leaves Kerbin soi

This, too, is neat. I've come to take a hybrid approach: my standard tug is an orange tank with two nervas on the sides, senior ports on either end. Any number can be combined, and after the transfer burn the "spent" ones turn around immediately.

Problems / downsides:

• with probe core, power source and RCS, these come to 16 parts apiece; a dumb throw-away thruster would only be 5 parts. Re-usable tugs are most interesting for large missions, but that's also where the part count will bite you the most.
  
• It's not advisable to link more than two, or three at most, because wobbliness. If a large mission requires more thrust and fuel (say Eve or Jool-5), the main vessel needs to be designed with enough attachment points to begin with.
  
• for any given mission, the tugs will always be too large or too small. Smaller units help, but also drive up the part count.
  
• the tugs will need the oddest amounts of fuel, and refueling each as needed will be difficult. So you also want a fuel depot in orbit to act as a buffer: the tugs draw whatever they need, whenever they need it; and you top off the depot every once in a while.
  
• of course, you need some means of transporting fuel to orbit. Preferably something you enjoy.
  
• lots of maneuvering required. Every new mission starts with a launch to rendezvous, returning tugs need to be brought back to the fuel depot.
  

One has to like this; especially the maneuvering part. A penny saved is a penny earned, they say; but in this case I suspect that the same effort that saves me one penny could as well earn me two. I prefer "meaningful" reusable tugs to "pointless" satellite contracts, but if that's not a matter of opinion, I don't know what is.

Edited January 12, 2015 by Laie

[NASAHireMe]

How does your pre-made transfer section reduce any of the interplanetary problems you mentioned: exit burn time, transfer time, injection burn time ... and back?

The point is you have more than one tractor vehicle, shuttling backwards and forwards so you resuse them and don't need one for every single mission.

You complain about burn and transfer time then say "RV and docking takes so much time..." !? Five minutes at the beginning and end of a two-year mission so you don't have to pay for, and launch, a whole new transfer section?

Not everyone can rendezvous and dock in 5 minutes.

Really.

Pecan, you're only partially right. Your statement of 'five minutes added to a two-year mission' is very disingenuous, though. Most of that time is spent time-accelerated at the maximum rate. But you CAN'T time-accelerate the parts you have to perform by hand: the exit burns (only up to 4x), fiddling with maneuver nodes and close-approach markers, waiting patiently for phasing orbits, mid-course corrections, final trajectory corrections, aerobraking (and all its' risks), repeatedly killing velocity-burning target target that RV requires, fiddling with RCS and docking, refueling, etc. All of that takes real-life time. A lot of it. Like at least 15 minutes per mission. Of which I would have to multiply to get a transfer tug back from Jool or Moho or wherever. And God help you if you don't have Kerbal Alarm Clock or MechJeb to help manage multiple interplanetary missions and crafts. You'd constantly be forgetting to do mid-course corrections or what not, and whoops, there goes your spacecraft.

The original question wasn't restricted to considering cost only. Real-life playing time is way more important to me, and to many other people, I imagine, b/c funds aren't that hard to get.

Second point is that since your one-off transfer stage doesn't need to have any dV to come back, you can often use a more powerful engine, which means your exit burns from Kerbin take less time.

Third point, which addresses your "don't have to pay for, and launch, a new transfer section". Obviously, I have to pay for that one-off transfer section, but I'm not launching it, then RV&Ding with my mission craft. It's launched together with the mission craft in one launch. So I don't pay a time penalty that you implied.

One way I use them is as q reuseable first stage for the first 1000 m/s

This reduce burn time a lot as the tug has a decent twr. Tug never leaves Kerbin soi

Magnemoe, what you suggested is exactly what I've been doing. The first 1000m/s burn, which gets you out of Kerbin SOI, is the most arduous part of the transfer burn, so I've been using an orange tank+Mainsail as an 'interplanetary booster'. But even then, I found that having to aerobrake, recircularize, then RV&D with my fueling station in LKO in preparation for the next mission took too much of my real-life time. I'd rather slap on that stage on my mission craft when I'm in the VAB and add a few cheap boosters to handle the extra mass at launch. I love interplanetary burns that take less than 3 minutes.

[Laie]

The original question wasn't restricted to considering cost only. Real-life playing time is way more important to me, and to many other people, I imagine, b/c funds aren't that hard to get.

Right you are. And I don't expect that the great rebalancing will substantially change this (the default at any rate; what people do with custom difficulty settings is their own business).

But even then, I found that having to aerobrake, recircularize, then RV&D with my fueling station in LKO in preparation for the next mission took too much of my real-life time. I'd rather slap on that stage on my mission craft when I'm in the VAB and add a few cheap boosters to handle the extra mass at launch. I love interplanetary burns that take less than 3 minutes.

Recovering tugs is a lot of hassle, and expensive too: in part count, because they need control and batteries and panels and stuff; in terms of fuel, especially when you do the "return immediately" thing, which again drives up overall mission size and part count; and not the least, in the player attention they demand.

That said, I think a point can be made in favor of an orbital refueling infrastructure. Reusable tankers can be dead cheap even when they are overpowered and fast; launching an empty mission saves a lot of funds (so much so that I don't care about using throw-away launchers). And the stage that took care of the second 2000m/s of the ascent can do that again, right away, after refueling. Still is a question of part count I guess; launching a significant amount of fuel in one go also requires a significant number of intakes and stuff.

[Pecan]

Pecan, you're only partially right...

Good points, well made :-)

I don't think I'd ever play KSP without KAC any more, especially.