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Countdown! NASA Launch Vehicles and Facilities
PMS 018-B 
October 1991
Section 2


Cryogenic Hypergolic Solid Next Page 


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Sir Isaac Newton stated in his Third Law of Motion that "every action is accompanied by an equal and opposite reaction." A rocket operates on this principle. The continuous ejection of a stream of hot gases in one direction causes a steady motion of the rocket in the opposite direction.

A jet aircraft operates on the same principle, using oxygen in the atmosphere to support combustion for its fuel. The rocket engine has to operate outside the atmosphere, and so must carry its own oxidizer. The gauge of efficiency for rocket propellants is specific impulse, stated in seconds. The higher the number, the "hotter" the propellant.

Specific impulse is the period in seconds for which a 1-pound (0.45-kilogram) mass of propellant (total of fuel and oxidizer) will produce a thrust of 1 pound (0.45- kilogram) of force. Although specific impulse is a characteristic of the propellant system, its exact value will vary to some extent with the operating conditions and design of the rocket engine. It is for this reason that different numbers are often quoted for a given propellant or combination of propellants.

NASA and commercial launch vehicles use four types of propellants: (1) petroleum; (2) cryogenics; (3) hypergolics; and (4) solids.



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Cryogenic propellants are liquid oxygen (LOX), which serves as an oxidizer, and liquid hydrogen (LH2), which is a fuel. The word cryogenic is a derivative of the Greek kyros, meaning "ice cold." LOX remains in a liquid state at temperatures of minus 298 degrees Fahrenheit (minus 183 degrees Celsius).

LH2 remains liquid at temperatures of minus 423 degrees Fahrenheit (minus 253 degrees Celsius). In gaseous form, oxygen and hydrogen have such low densities that extremely large tanks would be required to store them aboard a rocket. But cooling and compressing them into liquids vastly increases their density, making it possible to store them in large quantities in smaller tanks.

The distressing tendency of cryogenics to return to gaseous form unless kept super cool makes them difficult to store over long periods of time, and hence less satisfactory as propellants for military rockets, which must be kept launch-ready for months at a time.

But the high efficiency of the liquid hydrogen/liquid oxygen combination makes the low-temperature problem worth coping with when reaction time and storability are not too critical. Hydrogen has about 40 percent more "bounce to the ounce" than other rocket fuels, and is very light, weighing about one-half pound (0.45 kilogram) per gallon (3.8 liters). Oxygen is much heavier, weighing about 10 pounds (4.5 kilograms) per gallon (3.8 liters).

The RL-10 engines on the Centaur, the United States' first liquid-hydrogen/liquid-oxygen rocket stage, have a specific impulse of 444 seconds. The J-2 engines used on the Saturn V second and third stages, and on the second stage of the Saturn 1B, also burned the LOX/LH2 combination. They had specific impulse ratings of 425 seconds.

For comparison purposes, the liquid oxygen/kerosene combination used in the cluster of five F-1 engines in the Saturn V first stage had specific impulse ratings of 260 seconds. The same propellant combination used by the booster stages of the Atlas/Centaur rocket yielded 258 seconds in the booster engine and 220 seconds in the sustainer.

The high efficiency engines aboard the Space Shuttle orbiter use liquid hydrogen and oxygen and have a specific impulse rating of 455 seconds. The fuel cells in an orbiter use these two liquids to produce electrical power through a process best described as electrolysis in reverse. Liquid hydrogen and oxygen burn clean, leaving a by-product of water vapor.

The rewards for mastering LH2 are substantial. The ability to use hydrogen means that a given mission can be accomplished with a smaller quantity of propellants (and a smaller vehicle), or alternately, that the mission can be accomplished with a larger payload than is possible with the same mass of conventional propellants. In short, hydrogen yields more power per gallon.


Hypergolic propellants are fuels and oxidizers which ignite on contact with each other and need no ignition source. This easy start and restart capability makes them attractive for both manned and unmanned spacecraft maneuvering systems. Another plus is their storability they do not have the extreme temperature requirements of cryogenics.

The fuel is monomethyl hydrazine (MMH) and the oxidizer is nitrogen tetroxide (N2O4).

Hydrazine is a clear, nitrogen/hydrogen compound with a "fishy" smell. It is similar to ammonia. Nitrogen tetroxide is a reddish fluid. It has a pungent, sweetish smell. Both fluids are highly toxic, and are handled under the most stringent safety conditions. Hypergolic propellants are used in the core liquid propellant stages of the Titan family of launch vehicles, and on the second stage of the Delta.

The Space Shuttle orbiter uses hypergols in its Orbital Maneuvering Subsystem (OMS) for orbital insertion, major orbital maneuvers and deorbit. The Reaction Control System (RCS) uses hypergols for attitude control.

The efficiency of the MMH/N2O4 combination in the Space Shuttle orbiter ranges from 260 to 280 seconds in the RCS, to 313 seconds in the OMS. The higher efficiency of the OMS system is attributed to higher expansion ratios in the nozzles and higher pressures in the combustion chambers.



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The solid-propellant motor is the oldest and simplest of all forms of rocketry, dating back to the ancient Chinese. It's simply a casing, usually steel, filled with a mixture of solid-form chemicals (fuel and oxidizer) which burn at a rapid rate, expelling hot gases from a nozzle to achieve thrust.

Solids require no turbopumps or complex propellant-feed systems. A simple squib device at the top of the motor directs a high-temperature flame along the surface of the propellant grain, igniting it instantaneously.

Solid propellants are stable and easily storable. Unlike liquid-propellant engines, though, a solid- propellant motor cannot be shut down. Once ignited, it will burn until all the propellant is exhausted.

Solids have a variety of uses for space operations. Small solids often power the final stage of a launch vehicle, or attach to payload elements to boost satellites and spacecraft to higher orbits.

Medium solids such as the Payload Assist Module (PAM) and the Inertial Upper Stage (IUS) provide the  added boost to place satellites into geosynchronous orbit or on planetary trajectories.

The PAM-DII provides a boost for Delta and Space Shuttle payloads. The IUS goes on the Space Shuttle and the Titan III and Titan IV class of launch vehicles.

Only one of the nation's launch vehicles, the Scout, uses solids exclusively. This four-stage rocket launches small satellites to orbit.

Titan, Delta and Space Shuttle vehicles depend on solid rockets to provide added thrust at liftoff.

The Space Shuttle uses the largest solid rocket motors ever built and flown. Each reusable booster contains 1.1 million pounds (453,600 kilograms) of propellant, in the form of a hard, rubbery substance with a consistency like that of the eraser on a pencil. The four center segments are the ones containing propellant. The uppermost one has a star-shaped, hollow channel in the center, extending from the top to about two thirds of the way down, where it gradually rounds out until the channel assumes the form of a cylinder. This opening connects to a similar cylindrical hole through the center of the second through fourth segments. When ignited, the propellant burns on all exposed surfaces, from top to bottom of all four segments. Since the star-shaped channel provides more exposed surface than the simple cylinder in the lower three segments, the total thrust  is greatest at liftoff, and gradually decreases as the points of the star burn away, until that channel also becomes cylindrical in shape. The propellant in the star-shaped segment is also thicker than that in the other three.

A solid propellant always contains its own oxygen supply. The oxidizer in the Shuttle solids is ammonium perchlorate, which forms 69.93 percent of the mixture. The fuel is a form of powdered aluminum (16 percent), with an iron oxidizer powder (0.07) as a catalyst. The binder that holds the mixture together is polybutadiene acrylic acid acrylonitrile (12.04 percent). In addition, the mixture contains an epoxy-curing agent (1.96 percent). The binder and epoxy also burn as fuel, adding thrust.

The specific impulse of the Space Shuttle solid rocket booster propellant is 242 seconds at sea level and 268.6 seconds in a vacuum. 

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