A hypergolic propellant is a rocket propellant whose components spontaneously ignite upon contact with one another. In contemporary usage, the term typically refers to the combination of dinitrogen tetroxide (an oxidizer) and one of the various forms of hydrazine (a fuel). Advantages of hypergolic propellants include their ability to be stored at room temperature and their ability to be reliably and repeatedly ignited without a separate ignition system. Unlike many liquid-fueled rockets that use cryogenic fuels or oxidizers stored at very low temperatures, hypergolic propellants can remain loaded in a vehicle for extended periods before launch. These characteristics led to their use in early intercontinental ballistic missiles and in upper stages of launch vehicles that require multiple engine restarts. However, hypergolic propellants are difficult to handle because of their high toxicity and corrosiveness. In later decades, many ICBMs transitioned to advanced solid-propellant rocket motors.
History
The fact that turpentine may spontaneously combust when mixed with nitric acid was discovered in the late 17th century by Frederick Slare, but it remained a scientific curiosity for centuries until it was proposed to use it for rocket-assisted take off during WWII. In 1935, Hellmuth Walter discovered that hydrazine hydrate was hypergolic with high-test peroxide of 80–83%. He was probably the first to discover this phenomenon, and set to work developing a fuel. Prof. Otto Lutz assisted the Walter Company with the development of C-Stoff, which contained 30% hydrazine hydrate, 57% methanol, and 13% water, and spontaneously ignited with high-strength hydrogen peroxide. BMW developed engines burning a hypergolic mix of nitric acid with various combinations of amines, xylidines, and anilines. Hypergolic propellants were discovered independently, for the second time, in the U.S. by GALCIT and Navy Annapolis researchers in 1940. They developed engines powered by aniline and red fuming nitric acid. Robert Goddard, Reaction Motors, and Curtiss-Wright worked on aniline/nitric acid engines in the early 1940s, for small missiles and jet assisted take-off (JATO). The project resulted in the successful JATO of several Martin PBM and PBY bombers, but the project was disliked because of the toxic properties of both fuel and oxidizer, as well as the high freezing point of aniline (−6.3 °C). The second problem was eventually solved by the addition of small quantities of furfuryl alcohol to the aniline. In Germany from the mid-1930s through World War II, rocket propellants were broadly classed as monergols, hypergols, nonhypergols and lithergols. The ending ergol is a combination of Greek ergon or work, and Latin oleum or oil, later influenced by the chemical suffix -ol from alcohol. Monergols were monopropellants, while nonhypergols were bipropellants that required external ignition, and lithergols were solid/liquid hybrids. Hypergolic propellants (or at least hypergolic ignition) were far less prone to hard starts than electric or pyrotechnic ignition. The "hypergole" terminology was coined by Dr. Wolfgang Nöggerath, at the Technical University of Braunschweig (Brunswick), Germany.
The only rocket-powered fighter ever deployed was the Messerschmitt Me 163B Komet, which had an HWK 109-509, a rocket motor which consumed methanol/hydrazine as fuel and high-test peroxide T-Stoff as oxidizer. The hypergolic rocket motor had the advantage of fast climb and quick-hitting tactics at the cost of being very volatile and capable of exploding with any degree of inattention. Other proposed combat rocket fighters such as the Heinkel Julia and reconnaissance aircraft like the DFS 228 were meant to use the Walter 509 series of rocket motors, but besides the Me 163, only the Bachem Ba 349 Natter vertical launch expendable fighter was ever flight-tested with the Walter rocket propulsion system as its primary sustaining thrust system for military-purpose aircraft. The earliest ballistic missiles, such as the Soviet R-7 that launched Sputnik 1 and the U.S. Atlas and Titan-1, used kerosene and liquid oxygen. Although they are preferred in space launchers, the difficulties of storing a cryogen such as liquid oxygen in a missile that had to be kept launch ready for months or years at a time led to a switch to hypergolic propellants in the U.S. Titan II and in most Soviet ICBMs such as the R-36, but the difficulties of such corrosive and toxic materials, including injury-causing leaks and the explosion of a Titan-II in its silo, led to their near universal replacement with solid-fuel boosters, first in Western submarine-launched ballistic missiles and then in land-based U.S. and Soviet ICBMs.
In the 1960s, late variants of French Véronique sounding rocket and the Vesta rocket, as well as the first stage of the first orbital SLV Diamant used the combination of nitric acid and turpentine discovered by Slare. It may also be used in amateur rocketry. The Apollo Lunar Module, used in the Moon landings, employed hypergolic fuels in both the descent and ascent rocket engines. The Apollo spacecraft used the same combination for the Service Propulsion System. Those spacecraft and the Space Shuttle (among others) used hypergolic propellants for their reaction control systems. The trend among Western space-launch agencies is away from large hypergolic rocket engines and toward hydrogen/oxygen engines or methane/oxygen and RP-1/oxygen engines for various advantages and disadvantages. Arianes 1 through 4, with their hypergolic first and second stages (and optional hypergolic boosters on the Ariane 3 and 4) have been retired and replaced with the Ariane 5, which uses a first stage fueled by liquid hydrogen and liquid oxygen. The Titan II, III, and IV, with their hypergolic first and second stages, have also been retired for the Atlas V (RP-1/oxygen) and Delta IV (hydrogen/oxygen). Hypergolic propellants are still used in upper stages, when multiple burn-coast periods are required, and in launch escape systems.
Characteristics
Advantages
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