This post also goes for spacefire. Now that I have (from other threads and posts) discovered that you are NOT just being negative for the sake of being negative I will respond with as much information as I can give an honest seeker of knowledge!<br /><br />Just a few minor points first. ANY vehicle that intends to operate in space MUST have some sort of rocket engines for maneuvering. There is NO air or affective gravity (if you are going at least orbital velocity) in space. So a reaction type of engine that carries its own air with it, is the only type of engine that can allow someone to move in different directions from the initial direction that the vehicle entered its orbit with!<br /><br />This is why the orbiter has to have reaction engines on both the rear and the front of the orbiter. The Orbital Maneuvering engine (larger engines in the pods at the rear of the orbiter) are further required to slow the heavy vehicle down from orbital velocity to allow it to drop into the earths atmosphere for an eventual landing. As both of these systems are pure rocket systems, they carry their own fuel and oxidizers with them in tanks in the orbiter. Thus the need for the rocket engines and internal tanks that you refer to.<br /><br />From the pure technical side of things the reasons for the solids are indeed for cost purposes. However, they are not powerful enough to get the orbiter into orbit on their own. This is why the SSME's (Space Shuttle Main Engines) are also needed. The solid rocket motors are what are called boosters, and the liquid engines are what is called sustainers. The liquid engines use up far more liquid oxygen (the oxidizer) and liquid hydrogen (the fuel) than could possibly be carried internally in the orbiter. Hence, the use of the large external tank. <br /><br />The take off sequence is as follows. The SSME's are started initially, their performance must come up to a certain value in some four seconds (something so difficult it is truly amazing