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Propellant makes up almost all of a launch vehicle’s mass. For any “burn,” the mass ratio is the ratio of the rocket’s initial mass to its final mass. If everything else is equal, a high mass-to-weight ratio is desirable for good performance because it indicates that the rocket is light and performs better, much like a low weight in sports cars is desirable.

The combination of rockets has the highest thrust-to-weight ratio of any engine; Additionally, this assists vehicles in achieving high mass-to-weight ratios, which enhances flight performance. The higher the proportion, the less motor mass is required to have been conveyed. This makes it possible to carry even more propellant, greatly increasing the delta-v.

Alternately, some rockets, such as those used in racing or rescue missions, only require a lightweight structure to achieve high accelerations and carry relatively little propellant and payload. The Soyuz escape system, for instance, can produce 20 g. A number of factors, including the type of propellant, the design of the vehicle’s engine, structural safety margins, and construction methods, greatly influence achievable mass ratios.

Liquid rockets typically achieve the highest mass-to-weight ratios and are typically utilized in orbital launch vehicles, which necessitates a high delta-v. With the notable exceptions of liquid hydrogen and liquid methane, liquid propellants are able to use lightweight, low-pressure tanks and typically utilize high-performance turbopumps to force the propellant into the combustion chamber. Liquid propellants typically have densities that are comparable to those of water.