On tank assignments

[Jestersage]

I been goign through KerbalX in order to improve my stock crafts, and one of the best reference designs I personally see is those by Raptor 9.

Two thing I noticed about his design:

1. It has one less stage than the real world counterpart. His "titan V" is the Saturn V analogue, and it has one less stage. Should this be consider the general rule of thumb?
2. Instead of using one large tank, he used two tanks that gives the same amount of fuel in the end (Once again, refernece his Titan-V.  Is this due to TWR? If so, is this the correct way to do so?

[Alshain]

1. No.  The only real reason to be concerned about number of stages in a stack is for cost effectiveness (by means of fuel efficiency).  Really you can use any combination of number of stages on a stack, but with carefully selected engines, you can tailor the fuel efficiency for the altitude your stage is at.  For that reason, you really only need 2 stages in a stack, which can be augmented with SRB's radially if you need a bit more thrust at sea level. 

Why only two?  Because we are really limited to 2 classifications of engines.  Those that work well at low altitudes (Reliant, Swivel, Mainsail, etc.), and those that work well at high altitudes/space (Spark, Terrier, Poodle, etc).  Adding a 3rd stage would mean adding an engine that duplicates of one of those two functions, if you are going to do that, just add more fuel to that stage and save the funding that way.  Some engines do work pretty well in either position.  For example the Skipper can be used on Stage 1 and paired with a Poodle, or used on Stage 2 and paired with a Mainsail depending on your payload mass.  Keep in mind, this 2 engine strategy is only for getting to space.  Once you get there you can of course have more engines on your payload.  If this is the craft you were referring, he is using that same strategy, a 2 stage stack to space with SRB, a command module and a lander.

What about mass?  Yes, dropping mass and making your rocket smaller improves fuel efficiency, but the bulk of the mass is from the engine, so that tank has a small impact once empty, but not significant enough to warrant the cost of the engine.

What about Asparagus/Onion Staging? (i.e. Radial LFO stages).  These almost always end up more costly on a launcher.  SRB are cheap, LFO engines are expensive.  I have personally lifted as much as 100 tonnes with only a 2 stage LFO stack and SRB.  If you really build to the extremes you can do it, but personally if my payload is any heavier than that I just break up the payload and send it on two rockets.

 

2. No.  2 tanks vs 1 tank makes no difference (unless you are discarding one) at all.  These tanks are equal mass and have equal fuel.  They scale perfectly.  The only room for pause is their tech tree node.  If you are targeting an earlier tech tree, the 2 tanks may appear before the 1 tank option.

 

One other thing I will note about the craft I linked.  He is building for the sake of replica.  However, the real world and KSP often differ.  For example, if the Apollo engineers had tried to put landing legs on the command modules, well that likely wouldn't have worked very well.  However, that would work just fine in KSP.  For that reason, a Apollo style lander separate from the command module is not going to be as cost effective as just putting legs on the command module with a little extra landing fuel.  That isn't to dissuade you from doing a true lander (it's fun!) but you should at least have all the facts.

Edited July 16, 2016 by Alshain

[Jestersage]

Thank you. I was trying to reduce the cost of my Lunex further before uploading it to KerbalX. It works... at the cost of 350k. Actually 3 stages and whatnot.

I suspect that 50k of that comes from using nuclear battery instead of the fuel cell.

[Alshain]

13 hours ago, Jestersage said:

Thank you. I was trying to reduce the cost of my Lunex further before uploading it to KerbalX. It works... at the cost of 350k. Actually 3 stages and whatnot.

I suspect that 50k of that comes from using nuclear battery instead of the fuel cell.

If you post some images I could give you pointers, if I have any.  Personally I like to classify the cost and mass capabilities of the lifter separate from payload, as payloads can vary in cost at the exact same mass.  The only thing you need to know about a lifter is how much mass it can lift, it doesn't matter how much that mass costs.

Edited July 16, 2016 by Alshain