Except that on a lot of the engines it's not - there's the much-maligned "tankbutt", then a boxy section that we can imagine has the combustion chamber and turbopumps, and then the nozzle. The aerospike doesn't obviously have that, and while chances are the machinery would be in a casing flush with the rest of the rocket, perhaps that casing should be part of the aerospike model. It's a bigger problem for the new Shuttle engine, which really does look like a nozzle stuck on the ship with no engine.As for the shape, well professional sources call it an aerospike http://web.csulb.edu/colleges/coe/ae/rockets/aerospike/ft-1/flight-1.htm http://www.dfrc.nasa.gov/Gallery/Photo/Aerospike_Rocket/HTML/EC04-0113-146.html