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Lessons Learned With Metallized Gelled Propellants
During testing of metallized gelled propellants in a rocket engine, many changes had to be made
to the normal test program for traditional liquid propellants. The lessons learned during the
testing and the solutions for many of the new operational conditions posed with gelled fuels will
help future programs run more smoothly. The major factors that influenced the success of the
testing were propellant settling, piston-cylinder tank operation, control of self pressurization,
capture of metal oxide particles, and a gelled-fuel protective layer.
In these ongoing rocket combustion experiments at the NASA Lewis Research Center,
metallized, gelled liquid propellants are used in a small modular engine that produces 30 to 40 lb
of thrust. Traditional liquid RP-1 and gelled RP-1 with 0-, 5-, and 55-wt % loadings of aluminum
are used with gaseous oxygen as the oxidizer (ref. 1). The figure compares the thrust chamber
efficiencies of different engines.
Comparison of rocket engine performance for four
fuels: RP-1, and 0-, 5-, and 55-wt % RP-1/Al.
Propellant Settling
After the gelled fuels are mixed and before this mixture is put in the propellant transfer tank, the
RP-1/Al must be stirred vigorously. During storage periods of 1 to 10 days, the metal particles in
the fuel begin to settle because of gravity, and a thin layer of clear RP-1 forms atop the fuel in its
storage can. The fluid layer is about 1-cm thick after about 10 days of storage. Ostwald forces
(refs. 2 and 3) promote clumping of the propellant during storage.
Using a Piston-Cylinder Tank
In previous testing of gelled fuels (ref. 4), there was some difficulty in feeding the metallized
gelled JP-10/Al into the piston-cylinder tank. A manual stirring process was used to reduce the
viscosity of the thixotropic fuel until a small pump could feed fuel into the cylinder. To speed up
this time-consuming filling process, we fabricated a pressurized transfer tank to fill the piston-cylinder tank. The transfer tank was charged with gelled fuel, and nitrogen pressurant was used
to flow the fuel to the cylinder.
Propellant Self Pressurization
Experiments were conducted after we had an unplanned pressurization of the piston-cylinder
tank. The propellant had been in the cylinder for several days to several weeks, and the tank
pressure had risen from zero to several hundred pounds per square inch (gravimetric). In our
testing operations we were unsure as to the fluid interactions, but high-pressure gas is generated
when RP-1/Al is exposed to water. To alleviate this problem for the short term, we replaced the
"pressurizing" fluid for the gelled propellant with hydraulic fluid. A series of tests that estimate
the gas-generation rate from a mixture of RP-1/Al metallized gelled fuel and a test fluid were
conducted. The test fluids were water, hydraulic fluid, and Solvent 140. Although Solvent 140
and hydraulic fluid generated little or no pressure, the water exposed to the RP-1/Al created
significant pressures in some cases.
Particle Capturing
Rocket testing in Lewis' Cell 21 uses a 9-ft-long tubular diffuser with a series of circumferential
water spray nozzles for cooling. This diffuser tube was augmented with an add-on tube (that
added 3 ft of length) and a 150-gal plastic tank to capture water from the cooling system and
particles from the rocket exhaust. During a firing, the top of the tank was removed and the
gaseous exhaust products were vented vertically away from the test cell. The water captured in
the tank was run through a 10-micrometer filter before the water was exhausted into the
laboratory area drain system. A small fraction of the metal particles were exhausted in this
manner, but the bulk of the Al2O3 and other solid combustion products were captured in the
cooling water flow. The filter did foul after a period of several days of testing, and when it
fouled, it had to be changed. During the entire 1-year test period, we changed the filter at least 8
to 10 times.
Gelled Propellant Protective Layer
During testing with the gelled RP-1 and the 5-wt % RP-1/Al, some residual propellant was found
in the rocket chamber, coating the entire injector face and all the chamber walls. This residual
propellant was actually a mix of unburned fuel (with a gray or clear pink color) and some black
or combustion products. Although none of the injector ports clogged, there was a potential for
the gel to obstruct the ports to a small degree. Once this thin layer was removed with a soft cloth,
the metal surfaces exhibited minimal erosion. An improved cooling technique might be derived
from this effect. The thickness of the layer is 1 to 2 mm for 0- and 5-wt % RP-1/Al. The layer is
easily wiped off with a soft cloth when the face is cleaned after disassembly. The gel layer also
coats the injector such that the O2 and fuel flow form holes in the layer.
References
- Palaszewski, B.; and Zakany, J.S.: Metallized Gelled Propellants: Oxygen/RP-1/Aluminum
Rocket Combustion Experiments. AIAA Paper 95-2435 (NASA TM-107025), 1995.
- Iler, R.K.: The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties,
and Biochemistry, John Wiley and Sons, New York, 1979.
- Selegny, E.: Charged Gels and Membranes--Part I. D. Reidel Publishing Company, Dordrecht,
Holland, 1976.
- Galecki, D.L.: Ignition and Combustion of Metallized Propellants. AIAA Paper 89-2883, 1989.
Lewis contact: Bryan A. Palaszewski, (216) 977-7493
Headquarters program office: OSAT
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Last updated April 17, 1996
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