Spiral Model
The spiral model is a combination of waterfall and iterative development process with emphasizing on more risk analysis. Risk is essentially any adverse circumstance that might hamper the successful completion of a software project. For example, the risk involved in accessing data from a remote database can be that the data access rate might be too slow. The risk can be resolved by building a prototype of the data access subsystem. Thus, this model provides direct support for coping with the project risks.
Spiral Model in Software Engineering
It has planning, risk analysis, engineering and evaluation phase. Each phase in the spiral model begins with a design goal and ends with the client reviewing the progress. The development team in Spiral-SDLC model starts with a small set of requirements and goes through each development phase for those set of requirements. The development team adds functionality for the additional requirement in every-increasing spirals until the application is ready for the production phase.