Combustion Instability in an Acid-Heptane Rocket with a Pressurized-Gas Propellant Pumping System Page: 3 of 53
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2 NACA RM E51Gl1
of instability may reduce specific impulse and is believed to be accom-
panied by changes in combustion efficiency. High-speed photographs of
this type of instability are shown in reference 2. A higher-frequency
mode of instability, which manifests itself in greatly increased heat-
transfer rates in the rocket combustion chamber and which generally
results in chamber burnouts, is called screaming (reference 3). Other
types of combustion instability have also been reported at a symposium
on liquid rocket instability at the Naval Research Laboratory, Wash-
ington, D.C., on December 7 and 8, 1950. Whether these combustion
instabilities are distinctly separate or are related phenomena in dif-
ferent frequency ranges has not been definitely established because the sa
origin and nature of the instabilities are unknown.
Chugging has been postulated to be a result of out-of-phase cou-
pling between the combustion-chamber pressure and the fluid flow in the
propellant feed system. Analyses of the chugging phenomenon in a rocket
engine with a pressurized-gas propellant pumping system have been made
on this basis. The significance of the ratio of feed-line pressure drop
to the absolute chamber pressure in determining whether chugging can
occur is discussed in reference 4. The stability range of operation of
a rocket engine is further defined in reference 5. This analysis
derived the following expression as a limit of stable rocket operation:
(Ap)cr 1
Pc t 2
2 1+(tc\
where
(6p)cr critical pressure drop in propellant feed system at which
chugging can occur
Pc absolute rocket combustion-chamber pressure
tc combustion-chamber time constant equal to twice characteristic
length divided by characteristic exhaust velocity
to period of oscillations, seconds per radian
The accelerations of fluid masses in the propellant feed lines during
transients were neglected.
A totally different explanation of chugging, based on the premise
that combustion instability must attune itself to the resonant fre-
quency of the combustion-chamber cavity, is advanced in reference 6. An
equation for the frequency f of the chugging oscillation for the
fundamental mode of the chamber vibration is given in reference 6.
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Tischler, Adelbert O. & Bellman, Donald R. Combustion Instability in an Acid-Heptane Rocket with a Pressurized-Gas Propellant Pumping System, report, September 27, 1951; (https://digital.library.unt.edu/ark:/67531/metadc64155/m1/3/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.