Ultrafast Cold Reactions in the Bipropellant Monomethylhydrazine/Nitrogen Tetroxide: CPMD Simulations

Abstract

The bipropellant monomethylhydrazine/nitrogen tetroxide is used as a rocket fuel. Molecular dynamics simulations provide an explanation for the "cold prereaction", which yields methyldiazene and nitrous acid, and for the subsequent explosive reaction via dimethyltetrazane. I. Frank and co-workers give details in the following Communication. The bipropellant monomethylhydrazine (MMH)/nitrogen tetroxide (NTO) has been employed in astronautics for a long time, [1] for example, as fuel for the upper stage engine of the European launch vehicle Ariane 5 and in various U.S. spacecraft engines of the RS series (e.g. RS-21, RS-25, RS-28, RS-41, and RS-42). However, the mixture is unfortunately problematic in its handling and application. In 2001, flight 510 of the Ariane 5 project encountered serious problems due to complications within the upper stage engine.[2]On contact the components of the mixture react violently, explosively emitting heat and gas, and therefore the mixture is known as hypergolic. However, the mixture does not heat up during the reaction that precedes ignition ("cold prereaction"). Such a fast process is not easy to examine experimentally in full detail, as it is especially difficult to create a well-defined mixture of the reactants. Here theory provides an attractive alternative in terms of molecular dynamics simulations [5][6][7][8] within the Car-Parrinello approach, [3,4] where the extreme reactivity of the system is even advantageous. Car-Parrinello Molecular Dynamics (CPMD) allows the simulation of different types of reactions in a system through the description of the electronic structure with the density functional theory approach. [9,10] It is not necessary to predefine a reaction coordinate; rather, all degrees of freedom of the system can be considered. This study does not focus on highly precise computations of properties of isolated molecules. It aims rather at determining the relevant reaction schemes under varying reaction conditions like the ratio of starting materials, temperature, and impurities.As the first step, an MMH/NTO mixture with a ratio of 1:1 and a density of 0.9 g cm À3 was simulated (Scheme 1 and Figure 1). At temperatures above 300 K we initially observe electron transfer: an electron is transferred from an MMH molecule to an NTO molecule while the latter dissociates. The resulting ions are neutralized by a consecutive proton transfer. Afterwards a second electron is transferred, and again proton transfer yields neutral products (Scheme 1; details on the energetics are given in the Supporting Information). The oxidation state of the MMH molecule and its products during the molecular dynamics run can easily be determined from Scheme 1. Redox reaction leading from MMH to methyldiazene as observed in the CPMD simulations.

[Bis(imidazolyl)–BH2]+[Bis(triazolyl)–BH2]− Ionic Liquids with High Density and Energy Capacity

“…There were two major reasons to explain the ignition phenomena. First, the strongly reducing B−H groups as the hypergolic trigger were principally responsible for the enhanced hypergolic ignition properties observed, because a hypergolic reaction is essentially a redox reaction . To explain the contribution to hypergolicity of incorporating B−H bonds into the cation and anion, natural bond orbital (NBO) analysis was used to assess the electronic charge distribution.…”

“…Hypergolic bipropellants are commonly composed of a fuel and an oxidizer that can ignite spontaneously upon mixing without external ignition devices. Nowadays, the choice of fuel mainly focuses on hydrazine and its derivatives, such as monomethyl hydrazine (MMH) and unsymmetrical dimethylhydrazine (UDMH) . Hydrazine‐based fuels are acutely toxic and highly volatile so costly safety precautions and handling procedures are necessary.…”

“…Nowadays, the choice of fuel mainly focuses on hydrazine and itsd erivatives, such as monomethyl hydrazine (MMH) and unsymmetrical dimethylhydrazine (UDMH). [1][2][3][4] Hydrazine-based fuels are acutely toxic and highly volatile so costly safety precautions and handling procedures are necessary.T herefore, the developmento fa dvanced rocket fuels as alternative green fuels has received increasingi nterest from the materials and space sciences.…”

Ammonia, water, and hydrogen: Can nuclear motion be described classically?

“…Since the nuclear positions move classically, tunneling is always electron tunneling. The different reaction pathways that may be observed in different simulation runs are explained by deterministic chaos …”

Car–Parrinello Molecular Dynamics Study of the Thermal Decomposition of Sodium Fulminate

“…In previous work, we applied CPMD, for example, in the field of high-energetic materials to study the reaction between monomethylhydrazine and nitrogentetroxide (MMH/NTO), which is used as a propellant in rocket engines. [21,22] We found a variety of different reaction mechanisms, involving radical, acid-base, and redox chemistry. For sodium fulminate we again want to determine the most relevant reaction mechanisms, without specifying a certain reaction pathway or a specific type of chemical reactions in advance.…”