Difference of 1 ton

[Jestersage]

I am trying to make a mobile base to send to either Mun or Minmus (and possibly even Duna), and so far, I have made 2 version, with the mass difference being 1 ton.

While given a very powerful launcher (eg: Jupiter III) it would not be an issue, I am influenced a bit by raptor9's rocket design and want to keep the LV close to SLS or Saturn V, and thus won't be those crazy contraption. In that case, what difference does 1 ton overall? (Obviously will affect TWR, but does it matter?) To expand a bit, what about 2 ton... or even say 5 tons? (the mass of the X200-8 fuel tank)

[Streetwind]

The 1-ton difference reduces the thrust/weight ratio and the dV of any engine in its own stage and every stage below it. How much exactly? Impossible to answer without solving the rocket equation for every stage of your specific launch vehicle of choice. A mod like Kerbal Engineer or MechJeb will show you, of course.

What the mods don't show is that a lower TWR on your atmospheric stages increases your dV cost to orbit. Not only do you encounter increased gravity losses due to pitching over later and less aggressively, but you also take just a little bit longer to ascend, which means your engine Isp takes that little bit longer to grow to its vacuum numbers. How much extra dV exactly? Hoo boy. Here you're getting into trajectory analysis that's well beyond KSP and needs dedicated tools not available in the game. In other words: "go launch different configurations and write down the empiric test results, because you're not going to be calculating this". Though these effects are probably in the low double digits. The exact magnitude depends on how much of your payload the 1-ton difference is. If your whole base weighs 50 tons, then adding 1 ton would probably not cause a difference larger than measurement error. If your whole base only weighs 3 tons, then adding 1 ton will be quite noticable.

Edited February 22, 2017 by Streetwind

[Jestersage]

20 hours ago, Streetwind said:

The 1-ton difference reduces the thrust/weight ratio and the dV of any engine in its own stage and every stage below it. How much exactly? Impossible to answer without solving the rocket equation for every stage of your specific launch vehicle of choice. A mod like Kerbal Engineer or MechJeb will show you, of course.

Have KER of course, but have not mount the module onto the LV yet

20 hours ago, Streetwind said:

If your whole base only weighs 3 tons, then adding 1 ton will be quite noticable.

The lighter design is 6.6t and Heavier design is 8.7t, due to one using the LanderCan Mk1 and the other using Cupola.

Edited February 23, 2017 by Jestersage

[Benjamin Kerman]

Yes, the heavier your payload is, the amount of fuel needed to go anywhere increases exponentially due to the fact that you have to lift all the fuel you add on. I would go with the lighter design, but that's just me.