Editing Solid-propellant rocket (section)

==Design==
Design begins with the total [[Impulse (physics)|impulse]] required, which determines the [[fuel]] and [[oxidizer]] mass. Grain geometry and chemistry are then chosen to satisfy the required motor characteristics.

The following are chosen or solved simultaneously. The results are exact dimensions for grain, nozzle, and case geometries:
* The grain burns at a predictable rate, given its surface area and chamber pressure.{{citation needed|date=June 2020}}<ref>{{Cite book |last1=Kosanke |first1=K. L. |url=https://books.google.com/books?id=AK8zAwAAQBAJ&dq=the+grain+burns+at+a+predictable+rate%2C+given+its+surface+area+and+chamber+pressure&pg=PA888 |title=Encyclopedic Dictionary of Pyrotechnics: (and Related Subjects) |last2=Sturman |first2=Barry T. |last3=Winokur |first3=Robert M. |last4=Kosanke |first4=B. J. |date=October 2012 |publisher=Journal of Pyrotechnics |isbn=978-1-889526-21-8 |language=en}}</ref>
* The chamber pressure is determined by the nozzle throat diameter and grain burn rate.
* Allowable chamber pressure is a function of casing design.
* The length of burn time is determined by the grain "web thickness".{{clarify|date=June 2020}}

The grain may or may not be bonded to the casing. Case-bonded motors are more difficult to design, since the deformation of the case and the grain under flight must be compatible.

Common modes of failure in solid rocket motors include fracture of the grain, failure of case bonding, and air pockets in the grain. All of these produce an instantaneous increase in burn surface area and a corresponding increase in exhaust gas production rate and pressure, which may rupture the casing.

Another failure mode is casing [[seal (mechanical)|seal]] failure. Seals are required in casings that have to be opened to load the grain. Once a seal fails, hot gas will erode the escape path and result in failure. This was the cause of the [[STS-51-L|Space Shuttle ''Challenger'' disaster]].