Liquid vs. Solid fuel rockets: what’s the difference?

Throughout the history of rocketry and spaceflight, rocket engines have always used two distinct types of fuel, each with its own advantages and disadvantages. Liquid fuel rockets store fuel and oxidizer separately and are brought together in a combustion chamber where combustion occurs. Solid fuel rockets are packaged in rocket cylinders and can be stored for long periods of time and will only burn when exposed to the heat of an igniter. While liquid fuel rockets can be stopped by stopping the flow of fuel, solid rockets cannot be stopped until the solid propellant is exhausted or the rocket is destroyed.

Solid rocket fuel was discovered in China around 220 BC in the form of black powder, which was used in many different forms to power smaller rockets. Powder rocket engines only began to be used on a large scale in various forms between 1936 and 1980, when Caltech and the Russian Institute of Chemical Physics conducted independent research on this type of propulsion. The most notable application of solid rocket fuel was its use on NASA’s Space Shuttle stack in the form of two semi-reusable solid rocket boosters.

The liquid-fueled rocket engine was pioneered by Robert H. Goddard, who wrote his first paper on the concept as early as 1909, and eventually launched the world’s first liquid-fueled rocket on March 16, 1926, on a farm in Auburn, Massachusetts. Liquid fuel rockets are by far the most widely used in spaceflight today. They have enjoyed success from NASA’s first rockets, to the Space Shuttle Orbiter, and all of SpaceX’s vehicles, including Starship.

Compared to liquid fuel rockets, solid fuel rockets have the advantage of being simple and powerful. Solid rocket boosters (SRBs) on NASA’s SLS moon rocket provide the majority of the thrust needed to get the rocket off the ground. On the other hand, solid fuel rockets tend to be difficult to throttle, and the fact that they can only be shut down when the fuel runs out presents a lot of risk. NASA’s short-lived Ares 1 prototype used a single repurposed Space Shuttle SRB and was the only NASA rocket that relied solely on an SRB at launch. These types of thrusters were the focus of the investigation into the Space Shuttle Challenger accident, where the seal rings on one of the SRBs broke, causing a burn that led to the loss of the vehicle.

Liquid-fueled rockets are the backbone of modern spaceflight, but they are complex and many years of development have gone into making them more powerful and reliable. Liquid fuel rockets have throttling and stopping capabilities essential for launch from Earth. Using liquid fuel also provides the ability to make more precise adjustments, essential during space maneuvers or planetary landings.

The most powerful liquid-fueled rocket on the planet is undoubtedly SpaceX’s Starship. The Starship stack is capable of carrying 110 tons of cargo into space in various configurations and is fully reusable. The 236-foot-tall Super Heavy booster carries a staggering 4,000 tons of sub-cooled liquid methane and liquid oxygen. The booster has already been the subject of two widely publicized tower captures, which will allow for much faster turnaround times between flights.

The Starship program is expected to be a pioneer in space refueling, as part of its strategy to take the vehicle to the Moon or Mars. This would never have been possible with the use of solid fuel rockets. Putting a spacecraft into orbit uses valuable fuel that will also be needed to perform landing and departure burns on the surfaces of other celestial bodies, and that’s why SpaceX is testing this refueling method. Transferring liquid fuel between two orbiting Starship vehicles is a key milestone that SpaceX teams will need to achieve before they can unlock crewed space exploration to other planets.