AA guns. and Patriot missile Batteries.
Uhmm - are you claiming there were such things in place before 9/11? Because, fact is, there were SIX agents on the Personal Protection Detail trained in the use of the Stinger missile...and that was it.
Unless, you're one of those tinfoil hat freaks that claim the "gubbiment has all sorta secret stuff stashed away" - I gotta tell you, I was a logistics officer on the staff at 21AF.
Quite frankly, there is all sorts of stuff I know about the flow of people and materials all over the US at the time. simply put, unless these were some sort of magical AA type facilities that require neither manning, maintenance or material support, they weren't there. I know about the six agents at the White House, because I know about the transportation and time critical change out of the Stinger sets.
I can certainly tell you a lot about what was sent in AFTER 9/11 in the area of both AAA and SHORAD/THAAD, but there just wasn't snot-all there on 9/10.
So, let's say United 93 makes it to DC and hits the White House? Bush isn't home, and they kill some of the functionaries. As I said in your other post on the subject, so what? Relatively speaking, it is mostly a symbolic act. Well, OK, for the poor bastards that die, it's the end of the world, but for the rest of us? - purely symbolic.