Garbage-In Garbage-Out (GIGO): The Use and Abuse of Combustion Modeling
and Recent U.S. Spacelaunch Environmental Impacts
Abstract
Although adequately detailed kerosene chemical-combustion Arrhenius
reaction-rate suites were not readily available for combustion modeling
until ca. the 1990’s (e.g., Marinov [1998]), it was already known
from mass-spectrometer measurements during the early Apollo era that
fuel-rich liquid oxygen + kerosene (RP-1) gas generators yield large
quantities (e.g., several percent of total fuel flows) of complex
hydrocarbons such as benzene, butadiene, toluene, anthracene,
fluoranthene, etc. (Thompson [1966]), which are formed concomitantly
with soot (Pugmire [2001]). By the 1960’s, virtually every
fuel-oxidizer combination for liquid-fueled rocket engines had been
tested, and the impact of gas phase combustion-efficiency governing the
rocket-nozzle efficiency factor had been empirically well-determined
(Clark [1972]). Up until relatively recently, spacelaunch and
orbital-transfer engines were increasingly designed for high efficiency,
to maximize orbital parameters while minimizing fuels and structural
masses: Preburners and high-energy atomization have been used to
pre-gasify fuels to increase (gas-phase) combustion efficiency,
decreasing the yield of complex/aromatic hydrocarbons (which limit
rocket-nozzle efficiency and overall engine efficiency) in
hydrocarbon-fueled engine exhausts, thereby maximizing system launch and
orbital-maneuver capability (Clark; Sutton; Sutton/Yang). The combustion
community has been aware that the choice of Arrhenius reaction-rate
suite is critical to computer engine-model outputs. Specific combustion
suites are required to estimate the yield of
high-molecular-weight/reactive/toxic hydrocarbons in the rocket engine
combustion chamber, nonetheless such GIGO errors can be seen in recent
documents. Low-efficiency launch vehicles also need larger fuels loads
to achieve the same launched mass, further increasing the yield of
complex hydrocarbons and radicals deposited by low-efficiency rocket
engines along launch trajectories and into the stratospheric ozone
layer, the mesosphere, and above. With increasing launch rates from
low-efficiency systems, these persistent (Ross/Sheaffer [2014];
Sheaffer [2016]), reactive chemical species must have a growing
impact on critical, poorly-understood upper-atmosphere chemistry
systems.