Thrust-to-weight ratio reflects the amount of forward momentum an engine can generate in comparison to its weight. Aircraft and rockets use thrust to overcome drag and move through the air. The higher the thrust-to-weight ratio, the quicker the craft will be able to accelerate, and the faster it can go. Engineers and other members of development teams use a variety of methods to control the weight of engines and the craft they power to compensate for weight and drag.
There are several ways to calculate this ratio. Some calculations just look at the weight of the engine, while others may consider the whole craft. In addition, the thrust-to-weight ratio can change depending on throttle speed and some other factors, like the role of gravity in craft that fly extremely high. For the purpose of technical specifications, developers may discuss the starting thrust and weight, noting that these can change in flight. This provides a general overview, and more specific data may be made available upon request.
Heavier engines tend to produce more power, but come with a thrust-to-weight tradeoff. Developers can use tactics like employing lightweight metals in engine construction and utilizing very light cowling to protect the engine. The same construction techniques can also be considered in the design of craft to reduce weight as much as possible. Designers also need to think about laden weight in fully fueled craft with a maximum payload of passengers and cargo.
Craft with a very high thrust-to-weight ratio can take off with a steeper angle, on shorter runways. Examples of this can be seen with military jets, many of which can safely take off and land on aircraft carriers, where there is little margin for error. These craft are surprisingly light, considering their design and payloads, and their engines are extremely powerful. This allows them to generate a high thrust-to-weight ratio.
Commercial aircraft, cargo jets, and other craft may have lower ratios, for a variety of reasons. Designing high ratios tends to be expensive, and can involve tradeoffs in safety and comfort, depending on the craft. Creators of aircraft do not want to design deliberately unsafe aircraft, but may be more comfortable with low margins of error in some settings, and not others.
Military pilots, for example, receive hours of training and constantly practice, which prepares them for a variety of incidents. Commercial pilots carry precious cargo and may be less experienced, which makes safety considerations very important. Cost-benefit analysis helps engineers determine which design features to implement, given the potential applications of an aircraft.