Variable Enthalpy,
The Fallacy of Fuel Rich Mixture Ratios
Dale Lawrence Jensen, P.E.
Executive Engineer, JENTEC
P. O. Box 694
Lawndale, Ca. 90260
jentec1 @juno.com
Introduction
• Fuel rich mixture ratios’ are routinely &
erroneously claimed to be the optimum for
rocket engine performance.
• This concept has prevented development of
efficient rocket engines since circa 1960.
• Other fallacies also confuse & mystify
rocket engine technology.
• This presentation is an attempt to clarify &
de-mystify rocket engine technology.
Introduction, Concluded
• History of fuel rich hypothesis
• Specific impulse of hydrogen fueled rocket
engines.
• Constant enthalpy versus variable enthalpy of
hydrogen.
• Specific impulse of kerosene fueled rocket engines.
• Fallacy of increasing specific impulse by changing
expansion ratio.
• Fallacies in the present United States space policy
The Big Bad Wolf Intimidates the
Three Little Rocket Piggies
Fuel Rich, Enthalpy, & Expansion Ratio
The Fallacy of Fuel Rich Mixture Ratios
• Rocketdyne book, Introduction to Rocket Missile
Propulsion, 1958.
Greater specific impulse for MR = 3.5 than for
MR = 8 is in error.
Erroniously claims fuel rich mixture ratios are
better.
• Rocket Propulsion Elements, 7th edition 2002,
continues the fallacy.
Claims fuel rich mixture ratios are better.
• These are incorrect and fallacious claims.
• This mis-information is routinely quoted by the
mis-informed.
Specific Impulse versus Mixture Ratio
Enthalpy of Hydrogen Combustion
Joseph Liston, Power Plants for Aircraft, McGraw-Hill, 1953
Specific Impulse, Kerosene Fueled
Rocket Engines
Enthalpy of kerosene is 18,800 British Thermal Units per pound of kerosene which is a
combination of carbon at 14,400 B.T.U. per pound & hydrogen at 43,000 B.T.U. per pound
Kerosene Fueled
Rocket Engine Deficiencies
• The extreme fuel rich mixture ratios make the
launch vehicle extremely in-efficient.
• It allows a cheap rocket engine to be built because
it does not have to be engineered to withstand high
temperatures or pressures.
• The engines operate so fuel rich, they dump eighty
percent of the kerosene overboard unused.
• We do not allow gasoline to be spilled into the
atmosphere, so, why do we allow kerosene to be
spilled into the atmosphere?
Other Fallacies
• Specific impulse can be increased by larger
expansion ratio nozzles
• For example it is claimed the RS-68 engines
impulse can be made equivalent to the Space
Shuttle main engines simply by changing the
nozzle to a larger expansion ratio nozzle.
• It is claimed its’ low specific impulse, 415
seconds, is optimized for low altitude flight.
Expansion Ratio versus Mach Number
• The Mach number required to realize specific impulse is
M = Isp * g / speed of sound.
• Speed of sound, a = 49 * Square Root (deg. R )
• Degrees Rankine is combustion chamber temperature.
• A typical speed of sound is 3133 feet per second
• Expansion ratio = (1 + 0.2 M2 / 1.2 )3 / M*
• Expansion Ratio = 21.7
• Why are rocket engine expansion ratios in excess of eighty
for the SSME engines & recently increased for the J2-X
engine?
* Alan Pope, Aerodynamics of Supersonic Flight, Pitman Publishing Corporation, 1950
United States Space Policy Errors
• Inefficient kerosene fueled rocket engines are to be used to re-supply
the space station.
• These inefficient rocket engines pollute the atmosphere with thousands
of pounds of unused kerosene which is an environmental disaster.
• Space launch system is being developed using obsolete & inefficient
rocket engines.
• These development are not sustainable because the launch costs cannot
be afforded.
• The NASA needs to correct these fallacies by development of advanced
performance rocket engines, operating at or near a stoichiometric
mixture ratio.
• We should not return to the Moon nor venture to Mars until these
engines are developed.
Conclusions
• The Congress is mandating the NASA develop a Space Launch System
using obsolete & inefficient rocket engines.
• The SLS is a system is a waste of public money & will be un-affordable
because of exhorbitant operating costs.
• The NASA is allowing development of in-effcient kerosene fueled
rocket engines because they are cheap to build.
• Launch vehicles using these engines will pollute the atmosphere with
thousands of pounds of un-used kerosene. The public/humanity cannot
afford this pollution.
Recommendations
• The united States needs to change its space
policy.
• Efficient non-polluting rocket engines need
to be developed before anything else.
• Write your Congressman