The economics of KSP re-usability

[AeroGav]

I just saw 3Dprintingnut's  brilliant design on KerbalX for early career mode games -

https://kerbalx.com/3Dprintingnut/Sky-Shot-mk1

I downloaded the thing and yes, it really does work, and flies well.

It's got me thinking about the economics of re-usability, in KSP - assuming that's something you want to RP

(yes, i know, you can just ignore economics and fly a few more contracts to farm cash, and most of your cost is upgrading buildings not space vehicles)

 

In a standard rocket,  you have a lower stage, which you accelerate to about half of orbital velocity, then throw it away.    Then you have the upper stage, which is taken all the way in to orbit.     It consists of the upper stage engine, upper stage fuel tank, pod, and whatever stuff you need to survive re-entry and landing.  Typically,  the upper stage engine and tank are decoupled prior to re-entry, because it makes the process easier.  However, since you already accelerated these components to orbital velocity, there is no performance reason why you couldn't bring them back, provided you can solve the design issue.

To increase re-usability further, we have to start bringing more of the lower stage components up to orbit then home again.    At this point it becomes a tradeoff - there is only so much mass you can carry with you before performance declines too much.      

So it's helpful to start looking at the Dry mass vs Cost of these components to see which are most worth saving -

Reliant Engine   -  Mass  1.25T   Cost  1100     

Saving per Tonne when re-used = 880

Swivel Engine - Mass 1.5T Cost 1200

Saving per Tonne when re-used   = 800

FT400 tank  -  Dry mass 0.25T  Cost 316.4

Saving per Tonne when re-used   =  1265.6

Rather surprisingly, it is better to ditch your lower stage rocket engine but drag it's empty fuel tanks to orbit for re-use, assuming you don't have the delta V to bring the whole lot.

 

 

[bewing]

And of course, this all changes depending on exactly what you mean by "reuse" and "ditch". It takes a lot of kash to drag something up above the atmosphere. So stuff in orbit is potentially precious. ($10000 per pound IRL.) So ditching something suborbital is a total loss (without mods or heroic measures), while ditching it in orbit may make you a lot of money if you can just go around and gather it up later (and truly reuse it in space), while deorbiting and not landing it safely is a total loss, while deorbiting with a safe landing makes you back 50 to 100% of the original cost (which is fairly pathetic, but at least it's not a total loss and can actually be worthwhile for a very expensive upper stage).

Edited November 4, 2016 by bewing

[Tyko]

when I started my 1.2 save I thought about leaving all of my spent stages in orbit and then collecting them later for some use. I've also thought about docking each launcher at a local station and transferring any remaining gas before de-orbiting. The challenge I faced is that it usually required adding extra parts to all of those stages to make them recoverable/reusable and there wasn't a clear benefit to all the work. The biggest was having to add docking ports to everything so I could eventually connect them all. This was just wasted weight.

It's probably similar to the proposals to save the Space Shuttle giant orange tanks. I heard this was talked about, but I'm guessing they never came up with a use for all of them that out-weighed the cost of making them reusable.

Edited November 4, 2016 by tjt

[bewing]

You don't need to add docking ports if you use klaws on your station to dock with, so there is no extra cost. That's exactly what I do -- dock all my spent orbital tanks with my station using klaws, and suck them dry.

[WanderingKid]

"Deletes soapbox message"

Nevermind, you addressed this, briefly.   Sorry.  I just can't find any value in the time spent.

Edited November 5, 2016 by WanderingKid

[AndrewUpchurch]

Personally agree with completely, I always find out it is a waste of time trying to get empty orbiters and even small space stations to burn up or either land safely. So the solution would be to just leave them be and not really care only when traveling back from the mun and you run out of fuel when on target to crash into large empty space station. (already done) 

[bewing]

Yeah, it's a question of how much spare time you have to play -- and therefore how deeply you want to get into the RP aspects. But with KIS/KAS, the stuff in orbit can become very useful.

[GoSlash27]

I don't believe that this approach accurately reflects the economic impact.

 It's all about the cost per tonne of payload to orbit, and whether the savings of keeping a part vs ditching it is worth the player's added time and attention.

(Launch cost - payload cost - recovered value)/ payload mass.

 Personally, I look at how often I'm going to use a launcher to determine whether or not to go cheap. If it's something I do regularly (transferring crew, shipping fuel, etc) then I'll spend the time to make an economical way to do the job. Otherwise, if it's just a one-off launch I'll just make a disposable launcher and not worry about the additional cost. The savings of launch vehicles are cumulative; it doesn't work if you don't use it often.

 Going to other planets, the criteria change. It's not easy or convenient to replace parts that have been disposed of out there and I have a preference for logistic networks over disposable one-off missions. As such, I *always* make everything reusable in those situations to the point where I don't visit places that require staging.

Best,
-Slashy

[magnemoe]

14 hours ago, bewing said:

And of course, this all changes depending on exactly what you mean by "reuse" and "ditch". It takes a lot of kash to drag something up above the atmosphere. So stuff in orbit is potentially precious. ($10000 per pound IRL.) So ditching something suborbital is a total loss (without mods or heroic measures), while ditching it in orbit may make you a lot of money if you can just go around and gather it up later (and truly reuse it in space), while deorbiting and not landing it safely is a total loss, while deorbiting with a safe landing makes you back 50 to 100% of the original cost (which is fairly pathetic, but at least it's not a total loss and can actually be worthwhile for a very expensive upper stage).

Yes, the point of dropping the front fuel tanks is to lower weight for last part of gravity turn, circulation and return. Dropping the engine tend to reduce trust in my experience  
You also need to survive reentry. 
Now you could drop extra engines as your craft is lighter, one US rocket did this. 
Dropping SRB would be an good way to keep cost down. they are cheap and you basicaly launch an SSTO with low twr downside is an large return craft