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A hydrogen rocket is a vehicle that uses hydrogen and oxygen as its primary propellant. By mixing the two gases and then igniting them, a reaction takes place that causes the release of a great amount of energy. This energy can launch a vehicle upward and, if large enough, into space. The widespread use of hydrogen rockets in the space industry over the years can be attributed to the fact that it uses one of the most efficient combustion reactions that can be created for propulsion.
The concept behind how and why a hydrogen rocket works has to do with the reaction between oxygen and hydrogen. When hydrogen is exposed to an ignition source, it will not ignite unless there also is a specific amount of oxygen present. A hydrogen rocket mixes hydrogen and oxygen in the correct ratio in a combustion chamber and then ignites the mixture, directing the force behind the rocket to make it move.
The amount of energy released from the reaction is very efficient compared to other fuels. Nearly all of the large rockets used to launch vehicles into space over time used hydrogen and oxygen as their fuel for this reason. One of the only drawbacks is that hydrogen is not as compact as other fuels and must be kept in liquid form, meaning it also must be kept very cold up until it is ignited.
The types of hydrogen rockets used in space carry both hydrogen and oxygen with them because neither gas is present there. On Earth, there are hydrogen-powered model rockets that use the same concept to achieve liftoff but do not have to carry oxygen with them because it is in the air. For a model rocket on Earth, oxygen will continue to enter the combustion chamber, where it can be ignited when it is mixed with hydrogen from a fuel tank inside the model.
The concept of a hydrogen rocket also has helped to create a number of science experiments to illustrate the core concepts of the engine. One of the simplest experiments involves filling a plastic bottle with hydrogen and oxygen and then placing a device inside that will cause a spark. The bottle is attached to a guide to drive it upward. The spark is ignited and the gas inside the bottle explodes, sending it skyward. Model rockets that are launched in this way can actually achieve a significant altitude in relation to their size, illustrating the power of the reaction.