What is a hypergolic material?

A hypergolic material is defined as a substance that spontaneously ignites upon exposure to an oxidizer, which means it does not require an external ignition source, such as a spark or flame, to catch fire. This characteristic makes hypergolic substances particularly useful in certain aerospace applications, specifically in rocket propulsion systems, where they can ignite on contact with an oxidizer, facilitating controlled combustion without complex ignition systems.

By igniting spontaneously, hypergolic materials provide a reliable and efficient means of producing thrust, as they eliminate the need for ignition devices that could fail or complicate the engine design. The ability to ignite spontaneously simplifies the handling and operational procedures in environments like rocket launches.

In contrast, the other options do not accurately capture the essence of hypergolic materials. Non-flammable substances do not ignite at all, materials requiring heat to ignite need an external source of energy, and chemicals that can only burn in a vacuum are not inherently related to the definition of hypergolic substances. Thus, the chosen characterization as a substance that spontaneously ignites upon exposure to an oxidizer aligns perfectly with the true nature of hypergolic materials.