Re: Why do fuels for rocket engines always a very low molececular weight?

The two most common types of bi-propellant (oxidizer + fuel) liquid rocket engines are: Liquid Oxygen (LOX), as the oxidizer, and highly refined Kerosene (RP-1), as the fuel; and LOX (oxidizer) and Liquid Hydrogen (fuel).  The use of super-cooled (below -150 degrees Celsius)is known as cryogenics; thus a LOX + LH2 engine is known as a cryogenic engine.  Hydrogen Peroxide is an oxidizer that is making a comeback.  It was used widely in early rocket design, and abandoned until recently due to its volatility.  The hydrogen peroxide found in household use is 3%, while  That needed for rocket engines is greater than 90%.  In concentrated form, it tends to react with air or the tank itself.  The results are catastrophic.  Today stabilizers are used to control the reactivity of hydrogen peroxide.  Note: many more types of oxidizers and fuels exist, many with higher performance; however, many of these are highly toxic as both products and reactants.  Bromine, Fluorine, Nitrogen Tetroxide, and Chlorine make excellent oxidizers due to their reactivity - this also makes them extremely dangerous.

A LOX-hydrogen system is far more efficient than a LOX-kerosene system; however it is a lower thrust system.

A hypergolic engine is one in which the fuel and oxidizer ignite upon contact - no ignition source is needed.  These propellants have a short storage life due to their intense reactivity.  Typical fuels include Nitrogen Tetroxide (NTO) as the oxidizer, and variations of Hydrazine as the fuel.

Solid propellants are also quite popular for their convenience and price.  Solid propellants can be stored for long periods of time, whereas, liquid oxygen and liquid hydrogen must be kept at very cold temperatures, below negative 150 degrees Celsius. Solid rocket motors (SRMs) typically contain ammonium perchlorate (a granular oxidizer), powdered aluminum (a fuel), and polybutadiene-acrylonitrile-acrylic acid (a fuel that is liquid during mixing and that polymerizes to a rubbery binder during curing).

Answer:

Low molecular weight is desirable due to its low total weight.  Rockets (launch vehicles) are designed to be as light as possible in order to boost the maximum amount of payload (mass-to-orbit).  Liquid Hydrogen has the advantage of being extremely light; however, it has the disadvantage of being sparse, thus requiring a large volume tank.  Kerosene does not require such a large tank; however, it is much heavier.  Lower mass propellants tend to be more efficient, but also, lower thrust (i.e., Xenon ion propulsion (XIPs, pronounced zips) have a specific impulse of approximately 2,500 seconds and a thrust of a fraction of a Newton).  

  This can be shown through the use of Newton’s Second Law of Motion (F=ma).  Here F equals the thrust of the rocket, m equals combustion product mass, and a equals acceleration.  Acceleration is the change in velocity with respect to time; thus, force is the net change of momentum with respect to time (momentum equals mass times velocity).  Normally, mass is a constant and velocity is the variable; however, in the case of a nozzle, the exit velocity is constant due to a fixed expansion ratio.  Through throttling, the mass flow rate can be changed.  This leads to the relationship F = mass flow rate (Mdot) times exit velocity (Ve), or F = (Mdot)(Ve).  This is not taking into consideration the effects of ‘back pressure’.  Back pressure is defined as the nozzle exit area (S) times the quantity (Pa-Pe); where Pa is atmospheric pressure and Pe is the nozzle exit pressure.  The new equation is F = (Mdot)(Ve)-s(Pa- pe).  Optimal expansion occurs when Pa equals Pe.

Related links: