First flights are basically never more than a basic functionality check of take-off and landing configurations. (Note that there were no pictures of it with the gear retracted, because that's also typically something you don't try on a first flight.)

Yes, I get it. It's just they buried the lede of how far they have to go.

I mean, if you trust the math, you can just YOLO it and see what happens. The FAA is the major hurdle there though, because they don't trust the math, nor do they have the ability to really analyze it. So how far they have to go is mostly up to what they can get approved for, and how long it'll take to provide the data the FAA needs to give them the go-ahead.

It's fairly likely they'll find things that need changing along the way, which may also result in the need for additional regulatory oversight, so, double impossible to know if this will be flying at design speeds in a year or ten years or ever.

I'm sure we'll get a HN post about it when it does however.

It's not really about trusting the math, it's trusting the math has been implemented on that real-life aircraft over there. The truism about not wanting to fly an aircraft design which relies on a function being Lebesgue-integrable but not Riemann-integrable is not questioning the mathematics.

Ya think? It's a test flight to make sure that the aircraft can take off, fly, and land. I doubt they even retracted the landing gear.

Flight testing will slowly and very methodically expand the flight envelope until it meets or fails its design objectives.

It's a 1/3 scale prototype so I guess through some creative maths the speed of sound may as well be 1/3?

No supersonic aircraft has ever demonstrated that on the first test flight. That you would write this comment is... something you should think about.

The Mirage F1 went to Mach 1.15 on its first flight in 1969. It wasn't unusual in that era.

I'm skeptical of that claim but I'll take your word for it.

It would very much be an unusual exception, in that era and now. First flights rarely even raise the landing gear, let alone push the performance envelope. Flight testing programs are designed to be incremental to manage risk as well as learn things in a deliberate vs yolo way.

Rockets are really where you see the opposite being the norm, because it's hard to do a rocket launch half way.

Just the first test flight, not a great idea to run anything at top speed before you’ve validated it won’t fall apart.

speed of sound changes with altitude right?

Effectively, yes.

Effectively no! It doesn't depend on pressure. Basically just temperature (Also composition, esp: water content and form).

That relies on assumptions (ideal gas, adiabatic compression) that might break down for supersonic jets, but it's pretty close for the speed of sound.