Re: Why is it preferable to use hydrazine as rocket fuel instead of ethane?

Dear friend,

Before answering your question, I would like to give you an introduction about 
fuels and organic chemistry in general and then we will shift to your question. 
I hope you know from chemistry that organic compounds, or hydrocarbons are 
divided into two main groups:
1-Aliphatic compounds: under which there are three sub groups: Alkanes, Alkenes 
and Alkynes. These compounds are open chain compounds .
2-Aromatic compounds: These are characterized by the famous benzene ring. 
(honey comb like ring)
You mentioned ethane in your question. I guess you meant it to be the fuel used 
in your car. However this is not true. The fuel you use in the car is by 
mistake called benzene. Its actual name is gasoline. Surprisingly, this 
gasoline is composed of different (a variety of) long open chain aliphatic 
compounds, and no closed (aromatic compounds), despite its commercial name 
(benzene) gives that false idea.
There are some points one must consider when it is about to select a fuel:

1-Does the fuel have a high calorific value?
2-Is it poisonous ?
3-On combustion, what are the waste substances produced?
4-Does it contain impurities or not? If yes, then is it expensive to purify it?
5-After all of this, will the fuel remain with a reasonable price or not?

You are stating two different fuels to use; hydrazine and ethane. You are 
asking why do they use hydrazine in stead of ethane. Let me go though the above 
5 points one by one.
You know that oxidation or combustion of the fuel is the fast oxidation of the 
fuel accompanies by liberation of energy.
The petroleum oil, referred to as gasoline produced a calorific value of: 48 k 
Joule/gram. To be specific, the car fuel is called
 ISO-octane:  [(CH{3})3CCH{2}CH(CH{3}){2}].
 Its calorific value is 47.8 k .joule/gram. ISO-octane is an excellent smooth-
burning automotive fuel; it is given an octane number 100. 
On the other side, there is
 heptane [CH{3}(CH{2}){5}CH{3}]. 
It burns unevenly, and causes the motor to knock, and given octane number of 0. 
Thus by putting ratios of both chemicals, you form the fuel of certain octane 
number. 

Now what about ethane? Ethane (C2H6) is a gas at room temperature. It has a 
melting point at -183 C, and boiling point at -89 C. Its density is 0.546 
Kg/m^3 at -89 C. I tried to find the heat of combustion of it, however I was 
not able to find it. However, we can agree on a close compound to it. Heat of 
oxidation of methane (CH4) is 850 kJ/mole.

Let us shift to Hydrazine (N2H4). Hydrazine has a heat of combustion of 622.2 
kJ/mole. Hence, arises a great question mark. Why do the folks of NASA put in 
space shuttle Columbia that gas even if it has less calorific value. The point 
is that hydrazine is NOT their fuel!!
 Their fuel is methyl-hydrazine ( CH3-NH=NH2) and hydrazine as well . This gas 
(methyl-hydrazine) burns in air generating enormous quantities of heat; hence 
it is more beneficial to use in the space shuttle. The second point that adds 
to why they use this compound as their fuel is that because this compound is a 
liquid compound and not a gas one like ethane. You know to keep a gas, of such 
a low density, you need to keep it under large pressure in order to keep large 
amounts of it. (I guess you know that in order to keep in a container large 
amounts of  gas mass, you should keep it under large pressure. Hence, in space 
this is not easy)
 The third point that adds to hydrazine is its by-products of its combustion. 
It gives water vapor and Nitrogen. The fourth point that adds to hydrazine and 
its compounds is that it does not require ignition; when the hydrazine coming 
from its chamber meets the oxidizer at the combustion chamber, they ignite 
directly. On the other hand, ethane requires this ignition, and this ignition 
may create a lot of problems.

That’ all for now. Thanks for the great question.
References:
= = = = = =
  Denial, M.J.. Investigating Chemistry. Heinemann Educational Books. Great 
Britain 1982.

Petrucci, Ralph and Harwood, William. General Chemistry: Principles and Modern 
Applications. Prentice Hall International, USA. 1997 

Encyclopaedia Encarta. Microsoft Corporation 1997.

Moataz Attallah
The American University in Cairo-Egypt
Email:mizoa@aucegypt.edu
Undergraduate Mechanical Engineering Student
=======================================================
These are a group of Articles from Microsoft Encyclopaedia Encarta. I hope you 
will find them helpful

1-Hydrazine, colorless, oily liquid (H4N2), a powerful reducing agent, or 
electron donor. Derivatives of hydrazine are used (with strong oxidants, such 
as nitric acid) as rocket fuels, as corrosion inhibitors in boilers, in the 
synthesis of pharmaceuticals and agricultural chemicals, and in the rubber and 
plastic industries. Hydrazine is produced either by the reaction of chloramine 
(NH2Cl) with ammonia or by the reaction of sodium hypochlorite with urea. In 
both processes, gelatin or glue is used to prevent decomposition of hydrazine 
by unreacted oxidants.
Hydrazine melts at 2° C (35.6° F) and boils at 113.5° C (236.3° F). 
=======================================================
2-Liquid Propellants 
Although most of the scientists who pioneered in the field of liquid-propellant 
rockets used gasoline as a propellant, ethyl alcohol or refined kerosene has 
since been widely used. Ethyl alcohol, the fuel in such weapons as the V-2, 
Viking, and Redstone rockets, is burned with liquid oxygen, which, however, has 
the drawback of a boiling point so low that evaporation losses are considerable.
The search for a substitute for liquid oxygen has led, partly by accident, to 
another class of liquid fuels, known as the hypergols, and consisting usually 
of nitric acid as the oxidizer and either aniline or a hydrazine as the fuel. A 
hypergolic propellant does not require ignition, because the fuel and the 
oxidizer ignite spontaneously when brought together. The hydrazine called 
unsymmetrical dimethylhydrazine is particularly good at providing spontaneous 
ignition.
Liquid hydrogen is theoretically the most efficient fuel, but it is quite 
difficult and dangerous to handle. The problems of using liquid hydrogen, 
however, were successfully overcome by U.S. rocket engineers in the Centaur and 
Saturn 5 space launch vehicles and in the Space Transportation System, or Space 
Shuttle. 
========================================================
3-Liquid and Gaseous Fuels 
Common liquid fuels are fuel oils, gasoline, and naphthas derived from 
petroleum, and, to a lesser extent, coal tar, alcohol, and benzol obtained from 
coke manufacture. In stationary furnaces, less volatile fuel oils are sprayed 
through nozzles, with or without air or steam, into the combustion chamber. In 
an internal-combustion engine, volatile fuels such as gasoline or a gasoline 
and alcohol mixture (gasohol) are evaporated and the mixture admitted into the 
engine cylinder, where combustion is initiated by a spark. In these fuels, from 
16 to 23 kg of air are required for complete combustion of 1 kg of fuel. In 
diesel engines the fuel is injected as an atomized spray into the combustion 
chamber, where the temperature rise associated with the high compression ratio 
of diesel engines is sufficient to cause ignition.
Gaseous fuels such as natural gas, refinery gas, and manufactured gases such as 
producer gas are usually mixed with air before combustion to supply a maximum 
amount of oxygen to the fuel. The fuel-air mixture then issues from the burner 
ports at a velocity greater than the velocity of flame propagation to prevent 
flame flashback into the burner, but not so great a velocity as to blow the 
flame off the burner. If not premixed with air, these fuels usually burn with 
smoky, relatively cool flames. Natural gas burned with air can produce flame 
temperatures in excess of 1930° C (3500° F).
Rockets for space exploration may use liquid fuels such as kerosene and 
hydrazine, and carry an oxidizer such as liquid oxygen, nitric acid, or 
hydrogen peroxide.
======================================================= 
Fuel, substance that reacts chemically with another to produce heat, or that 
produces heat by nuclear processes. The term fuel is generally limited to those 
substances that burn readily in air or oxygen, emitting large quantities of 
heat. Fuels are used for heating, for the production of steam for heating and 
power purposes, for powering internal-combustion engines (see Internal-
Combustion Engine), and for a direct source of power in jet and rocket 
propulsion. In cases where a fuel must supply its own oxygen, as in many 
rockets and torpedoes, an oxidizing agent such as hydrogen peroxide or nitric 
acid is added to the fuel mixture (see Jet Propulsion; Rocket).
Chemical reactions in the combustion of all ordinary fuels involve the 
combination of oxygen with any carbon, hydrogen, or sulfur present in the 
fuels. The end products are carbon dioxide, water, and sulfur dioxide. Other 
substances present in fuels do not contribute to the combustion but either are 
driven off in the form of vapor or remain after combustion in the form of ash.
========================================================
Fuel efficiency or heating value of a fuel is usually measured in terms of the 
number of Btu (see British Thermal Unit) that are produced when a given amount 
of the fuel is burned under standard conditions. Heating values for solid and 
liquid fuels are stated in terms of Btu per lb, and values for gases in Btu per 
cu ft. A distinction is sometimes made between higher heating value, the entire 
heat evolved during combustion, and lower heating value, the net heat evolved, 
with allowance for the heat lost in the vaporization of the water produced by 
combustion. Approximate higher heating values of common fuels are: Solid fuels 
(Btu per lb): coal 12,000 to 15,000; lignite 6000 to 7400; coke 12,400; dry 
wood 8500. Liquid fuels: alcohol 11,000; fuel oil 19,000; gasoline 20,750; 
kerosene 19,800. Gaseous fuels (Btu per cu ft): acetylene 1480; blast-furnace 
gas 93; carbon monoxide 317; coke-oven gas or coal gas about 600; hydrogen 319; 
natural gas 1050 to 2220; oil gas 516; producer gas 136. See separate articles 
on most of these fuel