On this page there are pictures of actual liquid rocket engines at rest and in action. They show the internal view of what a liquid engine looks like, something you don't see very often. As you can see, they are very complex pieces of machinery and are susceptible to malfuntions and other accidents.

 By test-firing rockets, scientists can work out the bugs and test the thrust before committing them in a launch. This is required due to human life and millions of dollars on the line.

 Images provided by http://spaceboy.nasda.go.jp/note/rocket/E/Roc09.html