I agree that there's probably some young grad students in there, to counterbalance the engine. Can you imagine the noise inside of there if it was hollow? Inside of a massive cavity pushing atmosphere out of the way at Mach 5. LOL ouch. In fact, it's almost certainly full of some type of foam for structural integrity, probably even designed to break up the natural resonance modes. One time when we took a flight instrument to vibration testing, the tech goes "watch this", and takes a buncha coins out of his pocket when the QA guy had his back to us, and starts shaking them around during the test. QA almost pulled the plug before they saw us laughing our asses off. I miss that tech.
You're probably seen one of those in movies or something. It's a Rykote windjammer, a windscreen that's good for maybe 15mph winds. Put a softie on it and you can maybe handle 20mph. I know a guy who has been known to catch flies and put them in sound mixers' windjammers. His friends add porn movies to his IMDb credits when he's not looking.