JF Mezei wrote:
On 2016-09-21 14:42, Rob wrote:

I understood that it is weight of fuel that matters because the weight
determines the amount of energy you get from it, and the volume varies
by temperature.

Airplanes have limits on how heavy they can be at takeoff and landing.
Fuel represent a huge percentage of total weight. Every kilogram of
fuel you load means one less kilogram of cargo you can load.

I understand that the total weight of the aircraft is important, but
my understanding is that the weight of fuel is calculated for the trip
because the aircraft requires a specified weight of fuel to cover some
distance.
(this cannot be a linear function, though, because taking on more fuel
means the plane will become heavier and consume more fuel, requiring
even more fuel to make up for that)

So they know they need "10000 kg of fuel" (say 21000 pounds for americans)
for the trip, then they order that from the supplier and somewhere along
the line it has to be converted to liters or gallons. Error-prone.

I would say, make the fuel trucks deliver in kilograms, and all trouble
is over. Maybe wasn's to easy with an old mechanical counter, but
should be peanuts on a modern microcontroller based counter.

The density of fuel is needed because fuel pumps work with volume, and
the tanks on a plane measure volume. So they need the density to get
more accurate evaluation of it weight.

What I wonder about is how significant the varying density of fuel
will be relative to the total volume. When the density of the fuel
varies by 3% and then you add 10% at the end for safety margin, most
or all of the trouble to include density is wasted and you introduce
a new risk of introducing an error.

To get somewhat back on topic, SpaceX has made strides in getting
"supercooled" kerosene and I assume LOX in order to get more fuel loaded
in the same space.

Anyone know the rought weight difference between a normal launch and one
with super cooled fuel ?

Yes that is interesting, and in a space launch it is more interesting
because much more prediction, calculation and checking can be done on
a space launce than on an airliner trip.