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Almost all of a launch vehicle's mass consists of propellant. Mass ratio is, for any 'burn', the ratio between the rocket's initial mass and its final mass. Everything else being equal, a high mass ratio is desirable for good performance, since it indicates that the rocket is lightweight and hence performs better, for essentially the same reasons that low weight is desirable in sports cars.
Rockets as a group have the highest thrust-to-weight ratio of any type of engine; and this helps vehicles achieve high mass ratios, which improves the performance of flights. The higher the ratio, the less engine mass is needed to be carried. This permits the carrying of even more propellant, enormously improving the delta-v. Alternatively, some rockets such as for rescue scenarios or racing carry relatively little propellant and payload and thus need only a lightweight structure and instead achieve high accelerations. For example, the Soyuz escape system can produce 20 g.
Achievable mass ratios are highly dependent on many factors such as propellant type, the design of engine the vehicle uses, structural safety margins and construction techniques.
The highest mass ratios are generally achieved with liquid rockets, and these types are usually used for orbital launch vehicles, a situation which calls for a high delta-v. Liquid propellants generally have densities similar to water (with the notable exceptions of liquid hydrogen and liquid methane), and these types are able to use lightweight, low pressure tanks and typically run high-performance turbopumps to force the propellant into the combustion chamber.
Some notable mass fractions are found in the following table (some aircraft are included for comparison purposes):
|Vehicle||Takeoff mass||Final mass||Mass ratio||Mass fraction|
|Ariane 5 (vehicle + payload)||746,000 kg  (~1,645,000 lb)||2,700 kg + 16,000 kg (~6,000 lb + ~35,300 lb)||39.9||0.975|
|Titan 23G first stage||117,020 kg (258,000 lb)||4,760 kg (10,500 lb)||24.6||0.959|
|Saturn V||3,038,500 kg (~6,700,000 lb)||13,300 kg + 118,000 kg (~29,320 lb + ~260,150 lb)||23.1||0.957|
|Space Shuttle (vehicle + payload)||2,040,000 kg (~4,500,000 lb)||104,000 kg + 28,800 kg (~230,000 lb + ~63,500 lb)||15.4||0.935|
|Saturn 1B (stage only)||448,648 kg (989,100 lb)||41,594 kg (91,700 lb)||10.7||0.907|
|Virgin Atlantic GlobalFlyer||10,024.39 kg (22,100 lb)||1,678.3 kg (3,700 lb)||6.0||0.83|
|V-2||13,000 kg (~28,660 lb) (12.8 ton)||3.85||0.74 |
|X-15||15,420 kg (34,000 lb)||6,620 kg (14,600 lb)||2.3||0.57|
|Concorde||~181,000 kg (400,000 lb )||2||0.5|
|Boeing 747||~363,000 kg (800,000 lb)||2||0.5|