The 100/10/1 Rule.

In article ,
Pat Flannery wrote:
The oxidizer is LOX -- cheap and dense. The fuel is probably propane --
slightly better performance than kerosene, less tendency to leave oily
residues and otherwise misbehave, and it's still liquid and quite dense at
LOX temperatures.

...Considering that I've seen propane cylinders for refueling lighters that
have very thin aluminum walls, is it even necessary to chill it?

It's desirable for several reasons.

For the same pressure, wall thickness rises as the tank gets bigger, so
big tanks won't be as thin as those little ones. The vapor pressure of
pure propane(*) is about 8.5atm at room temperature, and a rocket tank
pressurized solely to support structural loads probably needs less than
1atm overpressure, so we're talking an order-of-magnitude difference in
wall thickness.

(* Note also that commercial "propane" is a mix of light hydrocarbons, and
often has considerable butane in it to lower vapor pressure. In fact, my
recollection is that the fluid for those lighters is mostly butane. )

Then too, room-temperature tanks would be bigger, because propane at LOX
temperatures is about 50% denser than at room temperature.

Finally, chilled liquids make pump design easier -- might not even need a
boost pump for the propane -- because they're much less prone to cavitate.
(In fact, it might be worth chilling the LOX below its boiling point too;
Rockwell's X-33 design did that.)
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