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Poll: Stage-to-stage Mass Ratio


Cunjo Carl

Poll: Stage-to-stage Mass Ratio  

19 members have voted

  1. 1. What mass ratio do you like to use between your stages? How big is each stage relative to the one it pushes?

    • 1.15 : 1 - Asparagus for life!!
      3
    • 2 : 1
      4
    • 3 : 1
      6
    • 4 : 1
      4
    • 5+ : 1 - We'll be there when we get there.
      2


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Something I've been curious to ask for a while: What mass ratio do you like to use between your stages? There's often a hundred critical decisions for designing each rocket, but this one always seems to be the most fundamental for me when making the framework for a design.

Just for show, in the spoiler is a couple craft with a 3:1 and a 5:1 mass ratio. They both have decent specs and can get up to some serious mischief, but do so in quite different ways. You could alternatively have multiple stages firing at once to get into 2:1 territory at the cost of added complexity.... or even a 1.15:1 asparagus?

This isn't so much about raw efficiency (it's KSP, we can min/max 'till we're blue in the face). This is more about preference. What do you like?

Spoiler

 Three to One

Three_to_one_Top.png
Three_to_one_Bot2.png


6 Spiders
2 Sparks
1 Terrier
1 Poodle
1 Mainsail
1 Mamoth
1 Mamoth+SRBs

                                                                               

Typically:
Narrower TWR range
More expensive
More gangly

-----------------------------------------------------------

Five to OneFive_to_one_Top.png
Five_to_one_Bot.png

 

2 Spiders
1 Spark
1 Terrier
1 Skipper
1 Rhino
1 Rhino + lotsa SRBs

 

Typically:
Broader TWR range
Less expensive
More stout & sturdy

-------------------

 

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1 hour ago, Foxster said:

It's just never come up as a design consideration for me. Other things like dV, TWR, low drag, low payload and low overall mass are the things I'm thinking about.

This, in most cases. I often just design a stage by the amount of dV it is expected to expend. A Mun transfer stage doesn't need a 5:1 mass ratio, even if it does most of the landing burn too - I mean, that would be 5,445 m/s with a Terrier! I'd need 1,600 m/s at most. Admittedly you could have a single stage that finishes Kerbin ascent, transfers to the Mun, lands on the Mun, takes off again and returns to Kerbin... but such a thing is just extremely inconvenient to handle, because it is likely going to be tall and is going to fall over on the slightest of slopes.

Also, because of KSP's low TWR engines, high mass ratios are impractical on launch stages. Not impossible, mind you, just impractical in many situations. And then, I fly a lot with electric engines (I wonder if the banner in my sig gives it away?), and with those, you can't even get above a stage mass ratio of 2 due to the extremely bad tanks.

But ultimately, when it's an option, I try to get a reasonable amount of work out of my stages. Mass ratios of 3 or 4 get healthy numbers without running into strong diminishing returns.

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36 minutes ago, Ultimate Steve said:

*Comes in here not knowing what I'm going to answer because I don't calculate it*

*While the page loads thinks to vote for the most popular one*

*Sees that each option has exactly one vote*

I would guess that the most common answer for stock scale is probably a ratio of about 3:1. I don't really know though, and I'm making a guess by size rather than mass.

Edited by eloquentJane
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4 minutes ago, eloquentJane said:

I would guess that the most common answer for stock scale is probably a ratio of about 3:1. I don't really know though, and I'm making a guess by size rather than mass.

 

40 minutes ago, Ultimate Steve said:

*Comes in here not knowing what I'm going to answer because I don't calculate it*

*While the page loads thinks to vote for the most popular one*

*Sees that each option has exactly one vote*

That's totally what I'm talking about! :D When making a deltaV monster that spans 3.5m-0.5m this decision is something people have to make a lot (or have their design rules make for them). What I love about it is there's good choices right across the board, but all for different reasons. Thanks for taking a look!

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Two to three early game.

My handy mun lander used a 909 + FL-T400 + payload to get 2km/s on the ascent/return stage, then four tanks + 909 for the 2km/s transfer+lander stage.

The tourister booster's boosters have 3-way symmetry for 9 tanks vs 4 in the core, and it works good enough for me.

 

Later on the ratio is undefined, since it is SSTO or bust.

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I don't bother calculating that sort of thing. At a guess, I'd say something like 4~8:1 or so, in vanilla I tend to make the lower stages much bigger than I would in RO. One of the reasons for this is that the toy solar system is pretty much easy-mode; you really don't need much of a launcher to throw massive payloads in to LKO so I tend to use an upper stage purely to leave less garbage in space and to make circularization much easier (less mass to turn to the node and less mass to deorbit after payload separation).

And yes, I do make dedicated transfer stages (GTO, Mun, etc...) so that the two-stage launchers can be easily resused. I consider transfer stages to be part of the payload.

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19 hours ago, foamyesque said:

I actually like e, but since that's a little tricky to work out I tend to default to 3.

Keeping even delta-v is typically a better starting point than mass, and probably is rather close to e with stages with the same Isp.  I probably grab enough fuel tanks for roughly 2-1, but after that I'm looking at K.E. to optimize things.

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I don't balance my stages by mass, but rather DV. Boost stage is always 1,800 m/sec. After that, DV allocation is dependent on what I'm doing. 1,600 is good to get me just shy of low orbit if I'm incorporating a transfer stage. Otherwise I'll make it 1,700 to circ in LKO plus whatever DV I need to hit an intercept.

The 1,800 m/sec figure for the boost stage (calculated at 1/2 atmosphere) is important because it burns out at the point where aerodynamic control and streamlining are no longer a concern; roughly 27 km on a nominal gravity turn.

Best,
-Slashy

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8 hours ago, wumpus said:

Keeping even delta-v is typically a better starting point than mass, and probably is rather close to e with stages with the same Isp.  I probably grab enough fuel tanks for roughly 2-1, but after that I'm looking at K.E. to optimize things.

 

The thing about e is that if I proceed on that assumption I can work out the dV of stages in my head, since ln(e) = 1. 3's close enough to make it work and effectively gives me a smidge of margin on my calculations.

 

This lets me, e.g., design rockets while downhill skiing. :D

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