in the late 1990s offer a potential solution to this problem. Compared with liquid-based propulsion, re-fueling a partially or totally depleted hybrid rocket would present significant challenges, as the solid propellant cannot simply be pumped into a fuel tank. This may or may not be an issue, depending upon how the rocket is planned to be used. In general, much less development work has been completed with hybrids than liquids or solids and it is likely that some of these disadvantages could be rectified through further investment in research and development. One problem in designing large hybrid orbital rockets is that turbopumps become necessary to achieve high flow rates and pressurization of the oxidizer. This turbopump must be powered by something. In a traditional liquid-propellant rocket, the turbopump uses the same fuel and oxidizer as the rocket, since they are both liquid and can be fed to the pre-burner. But in a hybrid, the fuel is solid and cannot be fed to a turbopump's engine. Some hybrids use an oxidizer that can also be used as a monopropellant, such as nitromethane or hydrogen peroxide, and so a turbopump can run on it alone. But nitromethane and hydrogen peroxide are significantly less efficient than liquid oxygen, which cannot be used alone to run a turbopump. Another fuel would be needed, requiring its own tank and decreasing rocket performance. Fuel 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 LH2. Common fuels for a typical hybrid rocket engine include polymers such as acrylics, polyethylene (PE), cross-linked 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. Grain manufacturing methods Cast Hybrid rocket fuel grains can be manufactured via casting techniques, since they are typically a plastic or a