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In previous versions of KSP you could adjust the amount of fuel in your tanks so to get better TWR. You can still do it in KSP 0.24 but now the fuel tanks cost money.

My question is: if you reduce the amount of fuel in the vehicle assembly building (ei. before launch) will the ship be cheaper?

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Yes, you can save a bit of money by not completely filling your fuel tanks.

You can easily verify this in the VAB when you right-click on a fuel tank and watch how the vehicle cost in the lower left corner changes while you increase and decrease the fuel amount.

The prices for one unit of fuel are:

Liquid fuel: 0.8
Oxidizer:    0.16
Solid fuel:  0.6
Monoprop:    1.2
Xenon:       4.0

The price for fuel is already included in the list-prices of all tanks. The 850 funds price tag of a FL-T400 fuel tank, for example, consists of 666.4 funds for the tank itself, 149 funds for the liquid fuel and 30.6 funds for the oxidizer.

However, reducing the fuel amount is really only economical for solid fuel and xenon where fuel-cost is a very large fraction of total part-cost. Liquid fuel, oxidizer and monopropellant are so cheap compared to their tanks that you pay much less when you just substitute the tank with many smaller ones. Three FL-T100 tanks cost (and weight) less than one 75% full FL-T400.

Exception: When you have a manned command pod but no RCS thrusters on the ship, always remember to remove all the monopropellant from it to save both money and mass.

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  • One minor exception - sometime one may want a bigger tank to have enough surface to attach all external utilities, like antennas and science gatherers, he needs. And with FAR, three tanks may just break in atmosphere due to little variety in drag and lift forces.
    – Mołot
    Aug 1, 2014 at 7:18
  • As a tangent to this, e.g. if you use planes, and successfully land (and recover) the craft with all parts intact, you will have only paid for the fuel you used up. If you break a part off, you won't recover that part's cost when recovering the plane.
    – Flater
    Apr 27, 2015 at 14:46

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