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Liquid rocket propellant: Difference between revisions

Revision as of 01:20, 19 October 2004

During the 1950s there was a great burst of activity by propellant chemists to find high-energy liquid propellants better suited to the military. Military rockets need to sit in silos for many years, able to launch at a moment's notice. Propellants requiring continuous refrigeration, and which cause their rockets grow ever-thicker blankets of ice, are not practical. As the military is willing to handle and use hazardous materials, a great number of dangerous chemicals were brewed up in large batches, virtually all of which were dead ends.

Nitric acid is a particularly good story. The acid itself (HNO₃) is not stable, and gives off NO₂ fumes (hence the name white fuming nitric acid). Unlike N₂O fumes, which make people happy, NO₂ makes people die. The addition of large amounts of N₂O₄ makes the mixture red, but keeps it from changing composition, leaving the problem that nitric acid will eat any container it is placed in, releasing gases that can build up pressure in the process. The breakthrough was the addition of a little hydrofluoric acid (HF), which forms a self-healing metal fluoride on the interior of tank walls and makes Inhibited Red Fuming Nitric Acid storable. Although the development of military propellants was treated with the greatest secrecy, the trick to inhibiting nitric acid was published in 1954 and Russian rockets with the same fuel appeared shortly afterwards.

All the early rocket theorists noted that hydrogen would be an ideal propellant. Hydrogen gas is very bulky, however, and the only practical storage was as a liquid, requiring either immense pressures or extreme cold. Since immense pressure requires heavy tanks, cryogenic storage was pursued, and not mastered in rockets until the 1960s as part of the Apollo and Centaur upper-stage programs. The Space Shuttle uses liquid hydrogen and liquid oxygen for the orbiter's main engines.

The highest specific impulse chemistry ever test fired in a rocket engine was lithium, fluorine, and hydrogen (atripropellant), which was measured at 542 seconds specific impulse with a high-expansion nozzle in a vacuum. The combination is completely impractical: the hydrogen must be kept below -252 C (just 21 K), the lithium must be kept above 180 C, both lithium and fluorine are extremely corrosive, lithium ignites on contact with air, fluorine ignites on contact with nearly everything else, fluorine is very toxic, both lithium and fluorine are expensive and rare, and the rocket exhaust is both ionized (interfering with radio communication with the rocket), and toxic, which leads to questions about the effect on the environment.

The common liquid fuel combinations in use today are:

• LOX and kerosene (RP-1). Used for the lower stages of most Russian and Chinese boosters, and the first stage of the Saturn 5. Very similar to Robert Goddard's first rocket.

• LOX and liquid hydrogen, used in the Space Shuttle and the Centaur upper stages.

• Nitrogen tetroxide (N₂O₄) and hydrazine (N₂H₄). Used in military, orbital and deep space rockets, because both liquids are storable for long periods at reasonable temperatures and pressures. Hydrazine decomposes energetically to nitrogen and hydrogen, making it a fairly good monopropellant all by itself.

Propellant table

JANAF thermochemical data used throughout. Calculations performed by Rocketdyne, results appear in "Modern Engineering for Design of Liquid-Propellant Rocket Engines", Huzel and Huang. Units have been converted to metric. These are best-possible specific impulse calculations.

Assumptions:

• adiabatic combustion
• isentropic expansion
• one-dimensional expansion
• shifting equilibrium

Definitions

Definitions of some of the mixtures:

• IRFNA IIIa: 83.4% HNO₃, 14% NO₂, 2% H₂O, 0.6% HF
• IRFNA IV HDA: 54.3% HNO₃, 44% NO₂, 1% H₂O, 0.7% HF
• RP-1: see MIL-P-25576C
• MMH: CH₃NHNH₂