Editing Hybrid-propellant rocket (section)

===Common fuel choices===
A reverse-hybrid rocket, which is not very common, is one where the engine uses a solid oxidizer and a liquid fuel. Some liquid fuel options are [[kerosene]], [[hydrazine]], and [[Liquid hydrogen|LH<sub>2</sub>]]. Common fuels for a typical hybrid rocket engine include [[polymers]] such as [[acrylate polymer|acrylics]], [[polyethylene]] (PE), [[cross-link]]ed [[rubbers|rubber]], such as [[HTPB]], or liquefying fuels such as [[paraffin wax]]. [[Plexiglass]] was a common fuel, since the combustion could be visible through the transparent combustion chamber. [[Hydroxyl-terminated polybutadiene]] (HTPB) synthetic rubber is currently the most popular fuel for hybrid rocket engines, due to its energy, and due to how safe it is to handle. Tests have been performed in which [[HTPB]] was soaked in liquid oxygen, and it still did not become explosive. These fuels are generally not as dense as solid rocket motors, so they are often doped with aluminum to increase the density and therefore the rocket performance.<ref name=Humble_etal_1995/>{{rp|page=404}}