Even some of the most staunch anarcho-capitalists I know believe in centralized national defense as one of the very few legitimate functions of the state.
Would not having planes invisible to radar (B-2), or those that could fly faster than missiles (SR-71) be nice secrets to keep from everyone who's thinking about invading you?
Really? You or they must be using the term "anarcho-capitalism" in a non-standard way. In the standard usage of the term, the whole point is to be distinguished from pro-government libertarians and minarchists who believe that there are a few legitimate functions of the state. I've never heard anyone use the term to refer to an ideology that accepts any legitimate functions of the state.
I'm not claiming any authority over the term "anarcho-capitalism," and you can use it however you like, but I have read a lot of works by people who are widely considered to be pivotal to anarcho-capitalism, and I've been somewhat active in several online communities that call themselves anarcho-capitalist, and I am reasonably confident that your usage is incredibly non-standard, and that very few people who are aware of the term would use it to describe minarchists.
I'm strongly against it. A lot of our breakthroughs are too expensive for dangerous countries to copy in any case.
> be nice secrets to keep from everyone who's thinking about invading you?
When is the last time the US was invaded by anyone? When is the last time any country even tried to invade the US? If all your justifications are about nightmare scenarios that haven't occurred in centuries they're probably not very good justifications.
Why do you think those answers are what they are?
Could it be that the latest public data we have is that we were building planes FIFTY YEARS AGO that could effortlessly outrun SAMs? Can you possibly imagine the shit that is flying out of Groom Lake these days?
The US' geographical position has had more to do with their safety than any secret they've ever had. But don't let me disrupt your "G.I. Joe" comic book fantasies.