Monopropellants are propellants consisting of chemicals that release energy through exothermic chemical decomposition. The molecular bond energy of the monopropellant is released usually through use of a catalyst. This can be contrasted with bipropellants that release energy through the chemical reaction between an oxidizer and a fuel. While stable under defined storage conditions, monopropellants decompose very rapidly under certain other conditions to produce a large volume of its own energetic (hot) gases for the performance of mechanical work. Although solid deflagrants such as nitrocellulose, the most commonly used propellant in firearms, could be thought of as monopropellants, the term is usually reserved for liquids in engineering literature.
Uses
The most common use of monopropellants is in low-impulse monopropellant rocket motors, such as reaction control thrusters, the usual propellant being hydrazine which is generally decomposed by exposure to an iridium catalyst bed (the hydrazine is pre-heated to keep the reactant liquid). This decomposition produces the desired jet of hot gas and thus thrust. Hydrogen peroxide has been used as a power source for propellant tank pumps in rockets like the German WWII V-2 and the American Redstone. The hydrogen peroxide is passed through a platinum catalyst mesh, ethylene oxide, hydrogen peroxide (especially in its German World War II form as T-Stoff), and nitromethane are common rocket monopropellants. As noted the specific impulse of monopropellants is lower than bipropellants and can be found with the Air Force Chemical Equilibrium Specific Impulse Code tool.
One newer monopropellant under development is nitrous oxide, both neat and in the form of nitrous oxide fuel blends. Nitrous oxide offers the advantages of being self-pressurizing and of being relatively non-toxic, with a specific impulse intermediate between hydrogen peroxide and hydrazine. Nitrous oxide generates oxygen upon decomposition, and it is possible to blend it with fuels to form a monopropellant mixture with a specific impulse up to 325 s, comparable to hypergolic bipropellants. In 2018 a new precious metal catalyst was invented for use with nitrous oxide - rhodium oxide on alumina spheres – which is more stable at higher temperatures than pure rhodium or iridium.
Direct comparison of physical properties, performance, cost, storability, toxicity, storage requirements and accidental release measures for hydrogen peroxide, hydroxylammonium nitrate (HAN), hydrazine and various cold gas monopropellants shows that hydrazine is the highest performing in terms of specific impulse. However, hydrazine is also the most expensive and toxic. In addition HAN and hydrogen peroxide have the highest density impulse (total impulse per given unit volume).
See also
- Monopropellant rocket
- Nitrous oxide fuel blend
- Green Propellant Infusion Mission
References
Further reading
- There is an entire chapter on the history of monopropellant development in the autobiography by
- The book "Germany's Secret Weapons In World War Two" by Roger Ford ( c.2000) contains some useful information on the surprising diversity of fuels and propellants employed by wartime Germany.
- "The Chemistry Of Powder And Explosives" by Tenney L. Davis is an outstanding, if outdated, source of information on a great many aspects of high enthalpy compounds. (This work originally published by MIT Press, 1943, as a textbook. Subsidy republication as late as 1995 by Pyrotek Inc., an amateur rocketry supply house. No catalog data given in this edition. Current publication status unknown.).
- 1999 Conference Paper on Historical Rocket Grade Hydrogen Peroxide Uses including monopropellant applications