Tonka is the name given to a German-designed rocket propellant first used in the Wasserfall missile; it has also more recently been used by North Korea but found its greatest applications in the Soviet Union, for example, in the propulsion projects of the A.M. Isayev Chemical EngineeringDesign Bureau. Its name is a reference to the tonka bean; as it was invented during the Second World War, it has no connection to the similarly named toys. Its composition is approximately 50% triethylamine and 50% xylidine, with the most common oxidizer used to burn it, nitric acid or its anhydrous nitric oxide derivatives. In a similar manner to the chemically related nitric acid /aniline propellant combination used in the USA's first indigenoussounding rockets, the WAC Corporal and Aerobee, this fuel-oxidizer combination is likewise hypergolic. The combination has a maximum practical Isp of approximately 216-248 at sea level, with the latter figure stated as a specification for the R-21 which was first fielded in 1963. The history of study into the hypergolic propellant in the Soviet Union, after the reverse-engineering of the GermanWasserfall missile, began in greatest part after its selection for the deeply studied but ultimately shelved Soviet Burya-intercontinental cruise missile project of the early 1950s, the knowledge base created in this project was then later put to use in the long serving, 1960s era, Kh-22, and the S5.4 engine - the original spacecraft propulsion braking or de-orbit engine used on the upper stages of the R-7 derived Vostok, Voskhod, and Zenit, which all burnt TG-02/AK-2x in low earth orbit and thus were notably responsible for bringing, the first human in space, Yuri Gagarin, back to earth in Vostok 1. The most globalized use of Tonka however is as an igniter in many Soviet Scud missile variants and its descendants, which are principally propelled by nitric acid and kerosene, a combination that in contrast does not spontaneously ignite when mixed and therefore necessitates an ignition source, a solution to this problem was the design of a co-injection system of a few kilograms of Tonka fuel beginning at the moment ignition is desired. The progenitor Scud engine, the S2.253, which has a Isp of 219 seconds. popularized this ignition arrangement. Triethylamine / xylidine mixtures composed the TX and TX2 fuels of the French SEPRrocket engines of the 1950s, used for auxiliary rocket power in the Mirage IIIC. In aircraft use, TX fuels were later replaced by non-toxic kerosene jet fuels, simplifying fuelling of the aircraft. Little change was required to the engines but as this was no longer hypergolic with nitric acid, a small tank of TX was retained for ignition. The use of Tonka by amateurs is not advised, as the exact proportions of ingredients necessary for the mixture to work as desired, rather than fail catastrophically, is a function both of the ingredients' purity, and of their temperature during use.
