What is the best propellant for a rocket? A good rocket propellant has high density and a high ISP. The pounds of thrust you get for a pound of propellant burned per second is called the ISP of the propellant. It is a measure of the efficiency of a rocket and is primarily determined by the choice of propellant. Some of the common rocket propellants and their ISPs are; (WITH LIQUID OXYGEN, LOX)
Propellant ISP DENSITY MIX RATIO 1# of LOX LOX 1.14 LOX/FUEL 1 RP1 250 0.8 2.3 0.43 Propane 260 0.53 2.9 0.34 Methane 268 0.44 2.4 0.41 hydrogen 360 0.071 6.4 0.15The list above would seem to indicate that hydrogen is the best fuel for a rocket based on it's high ISP, but this ignores the one great disadvantage of liquid hydrogen, it's very low density. Water is the standard for density and is 1. The list above shows that hydrogen has a density of only 0.071 which means that a gallon of liquid hydrogen only weights 0.56 pounds or 8.96 ounces. It hardly exists! The column labeled "1# of LOX" shows how much fuel (in pounds) you will need to burn with one pound of liquid oxygen. Two things are noticeable from this list, one, oxygen makes up most of the propellant mass of any rocket which is good because it is very dense, and two, you don't need a lot of liquid hydrogen. However, that small amount of liquid hydrogen takes up a lot of tank space, and that really raises the cost of a rocket.
After going through a lot of calculations (You design a small rocket with one pound of propellant for each fuel, then calculate the tank mass and the resulting deltaV/$) we come to a cost comparison of the above propellants, and guessing that the tanks cost four times what the propellant costs, I come to the conclusion that LOX/ RP1 is the cheapest propellant combination, and offers the best delta V per buck. LOX/ Propane is about 4% more expensive, and LOX/ Methane is about 8% more expensive. The combination of liquid oxygen and liquid hydrogen is 46% more expensive.
My choice of propane as a fuel was based on it's availability, it's ease of ignition, and it's lack of spill pollution hazard. In doing these calculations I wanted to determine how much of a penalty I was taking for that choice, and to determine if liquid hydrogen was as good a fuel as many say it is. It ain't.