Comment by SideburnsOfDoom

> There wasn't much more to be learned about large passenger jets using their intended custom engines from a small GEJ85 powered platform,

This is key to me.

I'm a layman in Aviation, so I'll unpack that.

The Boom XB-1 demonstrator (1) uses GEJ85: the General Electric J85 engines, as seen on military jets (2).

This is not the desired production jet's "Symphony" engine (3), which at a guess has to be both larger and more efficient?

So whatever is to be learned from the demonstrator, it doesn't tell us much about the final engine design.

In fact, all I know about this desired engine, is that Rolls-Royce isn't making it. (4)

Are they still planning to design the engines in-house? If they're making good progress, why are we hearing about how they're replacing excel as a design tool.

As I said in the other comment:

I'm not an expert, but this seems like the engine is on the critical path to success, and also high chance of failure. i.e. Without engines, they have nothing but a glider.

And if Rolls-Royce thinks that it's either not technically or commercially feasible, then who can do it?

1) https://en.wikipedia.org/wiki/Boom_XB-1

2) https://en.wikipedia.org/wiki/General_Electric_J85

3) https://en.wikipedia.org/wiki/Boom_Symphony

4) https://www.space.com/boom-supersonic-rolls-royce-engine-spl...