Why Can’t Hydrogen Fuel a Rocket?

Hydrogen has long been used as rocket fuel, and its use in powerful rockets like the Saturn V Apollo rocket's stages, the US Space Shuttle main engines, and others, highlights its importance. However, despite its high specific impulse, hydrogen faces several challenges when used as rocket fuel.

The Behavior of Hydrogen

Hydrogen typically enters the combustion chamber as a gas, with liquid hydrogen preheated by the nozzle bell. Liquid hydrogen has a higher density than gaseous hydrogen at 700 bar. However, the main issue lies in the tank design. When compressed into a gas, the tank needs to be thick enough to withstand the high pressure, and the weight increases proportionally with the amount of energy it contains. In contrast, for liquid hydrogen, the tank's thickness does not increase proportionally with the scale, and thermal losses decrease with scale. This is due to the surface-volume ratio, making liquid hydrogen the better choice for rockets at scale.

Energy Density and Specific Impulse

One of the key reasons hydrogen is used in rockets is its high specific impulse (ISP). However, hydrogen's low density necessitates liquefaction, which requires extremely low temperatures of minus 253 degrees Celsius. These low temperatures cause numerous handling issues. For instance, materials used in tanks, pipes, and pumps can become brittle, and a full tank of liquid hydrogen left standing in humid conditions can form a thick layer of ice, complicating operations.

Other Rocket Fuels

While hydrogen excels in terms of ISP, other fuels like kerosene (RP-1) are easier to handle due to their liquid state at ambient temperatures. However, these fuels do not offer the same specific impulse as hydrogen. The decision on which fuel to use in a rocket involves a balance of performance, risk, and logistical challenges. Engineers must weigh these factors carefully during the design phase of a new rocket.

Notable Rocket Fuel Users

The Delta IV Heavy, the most powerful rocket in service as of December 2017, is a prime example of a rocket powered by liquid hydrogen. Other notable space vehicles that use hydrogen include the Saturn V's second and third stages, the US Space Shuttle main engines, and the Ariane V. This underscores the importance and long-standing use of hydrogen as a rocket fuel.

NASA's Perspective

A NASA article titled Liquid Hydrogen--the Fuel of Choice for Space Exploration provides more insight into the reasoning behind the choice of hydrogen as a rocket fuel. The article discusses the trade-offs involved in selecting rocket fuel, including the specific impulse, handling issues, and cost implications.

In conclusion, while hydrogen offers the highest specific impulse, its low density and the logistical challenges associated with its handling present significant challenges. Fuel selection for rockets, therefore, is a complex decision involving multiple factors. As space exploration continues, the importance and impact of these choices will only grow.