It doesn't much matter what is/isn't possible, or what could/couldn't be economical, if the company & engineers who say they are working to develop it...are not actually capable of shipping even a puny nerfed toy version of their supposed product.
My guess: the RR engineers concluded "maybe it's possible, but these idiots will never achieve it". Then quietly shared that with their colleagues at other engine makers.
Haven't seen any explanation on why Boom can't just buy a few current fast jet engines for their demonstrator.
My impression is that Boom was not looking to RR for engines for their demonstrator, but for their full-size production model. Those engines would have to be carefully designed & optimized for economic long-haul supersonic operation. (Vs. existing supersonic military jet engines are basically cost-no-object gas-guzzling hot rods.)
Three GE J85-15 engines for the XB-1 demonstrator.
The "small supersonic aircraft" which is what the XB-1 is, is a solved problem. "Advanced composites" is irrelevant, any aerospace firm can make a supersonic-capable aircraft out of "advanced composites". Scaled Composites build SpaceShipOne in three years with about a hundred people in a hangar.
But Boom hasn't gotten the XB-1 into the air after five years and tons more cash.
Building a new supersonic airframe: easy
Building a new supersonic jet engine: hard
Literally and actually with no exceptions every single modest-sized aerospace firm can build a supersonic airframe. That problem is solved. Tens, if not hundreds, of thousands of supersonic aircraft have been built and many of them are capable of carrying passengers.
This is not an exaggeration. My employer isn't even in the supersonic aircraft business and we could build and fly a supersonic aircraft if some executive got a wild hair up his butt, in less time and money than Boom. Tiny and embargoed firms in a variety of foreign countries with small or non-existent aerospace industrial bases have done it.
The research into the aerodynamics and stresses on supersonic airframes is done.
Engines are hard. There are only a handful of companies with the expertise and cash needed to develop a supersonic jet engine, especially one that can super cruise.
Boom is saying "yah we're gonna build a supersonic passenger jet, uhhh, don't worry about the engine we'll figure that out later, give cash please".
What Boom is doing is functionally equivalent to an EV startup saying "Yeah we're going to design a car that can go 300mph (easy, individuals {Bob Dauernheim, for one} in garages have done it) that is powered by electric motors (hard) that don't exist yet and that someone else will design" and then getting hundreds of millions of dollars to design the chassis while negotiating with motor and battery suppliers to do the R&D into the hard bit at their own risk with the promise of profits from future sales to all of the people who want a 300mph EV.
But even if you assume there's a viable transatlantic market for supersonic flight, you probably can't charge all that huge a premium over business/first. If your premium has to be 2x to 3x, I doubt you have a market.
Handwaving away noise issues with supersonic flight over land gets you closer to something viable. But you still need economics that aren't that different from an all business-class subsonic. (And my understanding is that British Airways had trouble making all business work on the London to New York route.
I don't know - the all-business class A318 service from London City to JFK ran from 2009 to 2020 and was up to two flights a day for a significant part of that time. I guess it was really Covid that killed it.
https://en.wikipedia.org/wiki/SABRE_(rocket_engine)
Mind you, I've been hearing about that since I was at high school and I'm in my late 50s. :-)
And composite materials like on the A350 and 787 have been around, see the A350 and 787.
The plane doesn't exist either, it was a plan.
> And composite materials like on the A350 and 787 have been around, see the A350 and 787
Their point of comparison is the Concorde. If they can make a plane that is significantly cheaper than the Concorde, thanks to the great tech that is now mainstream (composites, turbofans, CAD) but wasn't back then, it can be similar in cost to modern day business class, while still being supersonic. At least that was their marketing pitch, i don't see it taking off without an engine.
The faster you go in an atmosphere the more the atmosphere resists. It gets less and less efficient the faster you go. That's just how it works.
Of course it would be less efficient than a subsonic jet, but they never claimed the opposite - the idea was for it to be faster, but not (much) more expensive than today's business class on subsonic jets.
This is actually not even approximately true; like many aerodynamic phenomena, drag is super nonlinear in airspeed and not even monotonic, with a bunch of weird outcomes around mach 1. The first chapter of every aero textbook probably includes a variation of this famous chart [0], showing drag coefficient or sometimes drag force (estimated for some famous/classic airframe) as a function of mach number.
This is why prolonged supercruise can exist for airplanes without giant comical fuel tanks etc.
That said, your main point about CAD/physics definitely stands and is a good one; imho it would be unlikely to design and build e.g. a 10x better airplane than the Concorde today, even with all the fancy computers simulations etc.
[0] https://www.researchgate.net/figure/Drag-coefficient-as-a-fu...
Computers are great for marginal gains and saving labor costs, but they can't give a 10x improvement unless there's already a 9x improvement in materials and techniques. Small nit, but CAE is simulations, while CAD is just drafting.
Even if your drag coeficcient goes down from 0.3 to 0.25, doubling your velocity will still more than triple your fuel consumption over constant distance. Then the tradeoffs in airframe to make it not fall out of the sky in the transonic regime, then the loss of ISP from much lower bypass engines, then the smaller cabins, then carrying the extra fuel... you can carry far fewer passengers. Sure your drag went down substantially from mk 0.98, but noone is proposing doing a transonic trip because that would be insane.
You wind up using many times more energy per passenger, meaning you pretty much only have first class due to noone on a budget even considering paying 10x as much. Throw in the cost of SAF and you're looking at the price of a house per ticket in order to save 5 hours in the air from a total 15 hours travel/boarding/waiting time.
You are making two assumptions, one explicit and one implicit. That the cross-section area is constant (it’s not, it depends on the angle of attack, which is smaller at higher speeds) and that the air density is constant (it’s not, supersonic planes fly at much higher altitudes, where the air is thinner).
You're making a similarly dishonest equivocation by carefully trying to frame the problem as if it were a mystery and the problem hadn't been reduced to lift/drag ratios 70 years ago. Which have an easily calculable upper bound for supersonic flight, by the way.
A supersonic craft has a much worse lift to drag ratio than a subsonic one. A supersonic craft even has a worse lift to drag ratio than the same supersonic craft at subsonic speed.
You can't even optimize your craft for that high altitude high speed regime because it still has to fly at lower speed and altitude and make it through the sound barrier without getting destroyed.
You will be using at least three times the fuel, so will need at least three times the ticket price. But that's not the only place a basic understanding of kinetic energy disqualifies the entire concept. Your engines/fans will also need at least double the effective exhaust velocity so their energy needs per unit of thrust will be doubled.
With this we can predict that all other things being equal, a supersonic craft will use about 6-10x the fuel per passenger mile as a subsonic one. A 747 has a bit over double the range and 4-6 times the passenger count as a concorde, so the naive approach got us in the ballpark.
Then for long haul flights (the only kind where supersonic will save time) existing airliners are almost half fuel by mass, which means you're in the regime of the rocket equation rather than having fuel scale linearly with distance. Then you can't offset business and first class with economy tickets and freight because noone is going to fly economy supersonic for 6-10x the cost or pay 6-10x the shipping for no perceptible time saving. With the previous in mind though, the supersonic craft is going to be limited to about 4 hours in the air before you start to lose passenger capacity to fuel. So it's not even great for long haul.
In short. Unless fuel prices drop dramatically ticket prices for a new supersonic craft would be commensurable with a new apartment for time savings of around 4 hours or at best 30% of the total door to door time.
The Concorde engine was efficient at cruise - and pretty awful everywhere else. At cruise it was about 10% more efficient than a GE90. But the Concorde had 4 of them, and a 777 has 2 GE90s.
This is also why you don't see 4 engined planes anymore, except for the A380 and most people were hell bent on retiring during the pandemic until they couldn't get more 787s/A350s and the 777X is delayed.
The other stuff is incremental efficiencies, not enough to fundamentally change the very challenging math of supersonic flight efficiency.