I shudder to think of the emissions per passenger mile that this jet will produce and so make worse our climate change problem.

Other than ego tripping why would anyone really want to pay what I suspect will be a very high ticket price?

We can do many meetings these days over the Internet, far better use of most people's time and far kinder to the environment.

Having said that: I expect to be ignored and NASA will try to build this.

Depends. Concorde was exceptionally efficient at 60,000 M2, and so was the SR71 at 90,000 M3. In fact, with aircraft of that sort, the faster they went the more efficient they got, because the ram-compression in their (very carefully managed) adjustable air inlets reduced the amount of power needed to be tapped from the exhaust by the turbine to run the compressor. Thus, the engines can be kept running using far less fuel than on the ground.

The trouble is getting up to height / speed in the first place. This is a compromise on all jet powered aircraft (turbojet, turbofan), that there is an optimum cruise height / speed for best efficiency, and the further from zero feet / zero knots that is the harder it is to get from runway to cruise. This is true for ordinary airliners. Some time ago, all the airlines started doing quite steep / quick climbs to altitude, because someone worked out that the fuel burned on such a sporty ascent was paid back by spending more of the journey at optimum cruise height. That in turn has had an impact on engine life / design, because by spending more time at a high power setting the engine wears out quicker. Considering that in the light of a new supersonic airliner, it could be that using a solid rocket to get to speed / height fast could pay off. Not that SRBs are particularly clean things...

But yes, it sounds expensive, fairly pointless, unlikely. The laws of physics haven't changed, nor has the operational experience of Concorde and the SR71. Both needed huge amounts of maintenance, and the SR71 (the wildest, most on-the-edge power plant ever in and aircraft) never solved the inlet un-start problem (Lockheed could only mitigate it), which would be even worse at M4. I suspect that there's some ailing US defence contractor in need of some gratuitous taxpayer funding to produce several lbs of printed paper that 1) says "can't be done" whilst 2) looking like it say "can be done, give us more money".

The laws of physics haven’t changed, but airport security has.

Concorde traded on the idea that reducing a 7 hour flight time to a 3 hour flight time was quite attractive. Including a 1 hour wait on the originating airport, and maybe 45 minutes at the destination with waiting for baggage, that reduces 8 hrs 45 down to 4hrs 45. Half. Now, for transatlantic, you are looking at 3 hours checkin to takeoff. So that’s 10hrs 45 down to 6hrs 45. Only 30% quicker.

But worse still, the *real* Concorde USP was “London to New York and back, for a meeting, without an overnight stay”. This just no longer works. The difference between 10hrs 45 and 6hrs 45 is basically nothing at all, since all you can do when you get there is go straight to the hotel to sleep. There’s literally nobody you can sell that proposition to, everybody who has loadsamoney will prefer Business (or even First) on a larger slower aircraft.

Or they have their own private jet.

"The difference between 10hrs 45 and 6hrs 45 is basically nothing at all, since all you can do when you get there is go straight to the hotel to sleep."

This demonstrates my concept of going faster by going slower. If you take a longer direct flight and can sleep on the way, the trip takes much less waking time. I also find it better to arrive at a convenient time when shifting time zones rather than going really fast depending on the direction. All the non-flight time it takes these days just getting through to the plane and the interminable layovers since fewer flights are direct just eats up a day. It pushes out the "just drive or take a train" radius. I keep hoping Amtrak will get some money and train advocate management and look at overnight trains more and also schedules that make some smaller station stops happen during daylight. I really don't want to arrive someplace at 2am, but I'm fine leaving really early or late.

Try - "this project should not have public funds wasted on it". It should be privately funded, just as development of regular passenger aircraft are funded with the end goal of making a profit. (Not just operating profit, but also recovering development costs many times over).

Supersonic large capacity passenger jets will never recover development costs(*). Finally, the result of following common sense economics would be the same - the project wouldn't happen.

(* Maybe one small enough to carry Jeff Bezos and a dozen hangers on and bodyguards - that's possibly profitable for a production run of 100, all pre-paid orders, 10~100 million a jet - but should be privately developed.)

"I expect to be ignored and NASA will try to build this."

NASA's mandate is science more than engineering and product development. The question they might be trying to answer is IF a hypersonic transport CAN be built large enough for passengers with enough range. So, it's a bit more engineering than typical, but maybe there will be technology spinoffs that apply to other aircraft.

I agree that supersonic flight is not that useful these days. A flight on Concorde was on my bucket list years ago, but I didn't have any business case for paying the premium and could afford the time to take a standard sub-sonic flight if I had to. Even Concorde had limitations that really held it back. If you wanted to go from Los Angeles to Sydney direct and had the $12,000 to pay for business class, maybe a supersonic flight would be worth the few more $$ it would take to shave off a bunch of time if one could make that hop but Concorde couldn't. If was just going NYC to LHR, I don't see enough of a time savings to be worth a price increase. I'd prefer to bump up a class. The Concorde wasn't all that fancy due to the size of the airframe.

Hmmmmm.

Above 40,000ft, or thereabouts, a human cannot survive even if breathing pure oxygen. The pressure they're breathing has to be higher (equivalent to a lower altitude) for enough oxygen to enter the bloodstream for survival.

This is why U2 / SR71 pilots wear / wore full pressure suits, in case of cabin depressurisation (and ejection). Pilots of jets like Typhoon can wear partial pressure suits, that can squeeze them around the chest to stop them exploding when breathing air at > ambient pressure in the event of depressurisation. On the odd occassion aircraft like the EE Lightning got up to U2 heights, they were effectively taking a chance that they'd not get a cabin depressurisation when up that high.

So hang on a mo. What did Concorde do about depressurisation? No special clothing was required, pilots and passengers just wore ordinary garb?

The answer is that, for Concorde, its cabin pressursation blowers and air con could keep the pressure inside high enough to be able to breath and survive, even if two whole windows had blown out at 60,000ft / Mach 2. Had that actually have happened, it would have been extremely noisy inside and very uncomfortable, but the occupants would survive long enough on oxygen masks for the aircraft to descend to a more accommodating altitude.

For a Mach 4 airliner, I can't see that flying below 90,000ft. To avoid a pressure suit it'd require a similar spec to Concorde's, but with at least another 30,000ft drop in pressure. That might not be so very different - there's barely any air at 60,000ft as it is. So perhaps that aspect of a Mach 4 airliner - ordinary cloting - is possible.

> So perhaps that aspect of a Mach 4 airliner - ordinary cloting - is possible.

What about TikTokers? Will they still be able to do their weird little dances between the isles?

This sounds like a modern popular cultural phenomenon of which I have hitherto been entirely ignorant. Is my already tenuous belief in the sanity of humanity best served by 1) not Googling it to see it, and 2) spending more (or less?) on airline tickets so as to be in a class less likely blighted by such shifts in self-loading-freight?

This is no different from regular airliners which also pressurized. The only difference is in the degree of pressurization, but the difference between one atmosphere and zero atmospheres is really not all that great. No one worries (well maybe some do now after the Titanic thing) about those small subs that take tourists down to 50 ft which is a much larger pressure change. If windows are shown to a weak spot there's an easy fix - no windows on those planes. If necessary they could install OLED "windows" that show the outside view from cameras along the outside of the plane and few would care they aren't "real" windows.

The big issues will remain noise and efficiency. The number of routes that could possibly benefit are a lot smaller if it is too loud to travel over populated areas, and airlines won't buy it if it requires so much fuel per passenger that they are taking a huge risk that there won't be enough rich people willing to fly it often enough that they make a profit. Then you have the risks of economic downturns, greater pressure from world governments against less efficient means of travel, or a black swan like finding that emissions at 90k ft are far worse in some important way than emissions at 35k ft.

There's not much difference in the degree of pressurisation, but there is a massive difference in air flow. On a regular airliner, it has to be able to maintain pressurisation whilst dribbling air out the exhaust vent at a slow rate, and if it fails the passengers survive because the oxygen from the masks is sufficient to keep them alive. On Concorde, it has to be able to maintain partial pressurisation whilst there's two windows blown out - a huge rate of air flow - because oxygen alone would not keep the passengers alive at 60,000ft air pressure. Without that partial pressurisation they would asphyxiate, even on 100% pure oxygen.

What's also not good is that the boiling point of water at 60,000ft is only 24C, so bodily fluids (such as is found lining lungs, blood, etc) start to boil if not kept at a higher pressure. At 40,000ft, boiling point is 54C, so no blood boiling there.

Concorde was affected by heat created by the air rushing across control surfaces. This problem was engineered out. The internal expansion joints were quite wide. M4 sounds so much harder for a large passenger jet.

Would I fly it? Definitely.

The Concorde's skin temperature was 110°C at Mach 2 cruise speed. The SR-71's skin temperature was about 250°C at Mach 3. So the main problem for a Mach 4 aircraft would indeed be heat and it's probably not solvable for a commercial aircraft. NASA cannot ignore this simple fact, which means that this project covers something else (where is the black helicopter icon ?)

The SR-71 had expansion gaps so large to allow for metal expansion that sitting on the runway it would be constantly leaking fuel until it got up to speed.

"The SR-71 had expansion gaps so large to allow for metal expansion that sitting on the runway it would be constantly leaking fuel until it got up to speed."

It would also be topped up via mid-air refueling which isn't an option for passenger flights.

SR-71 - loose fitting panels to allow for heat expansion...

[1]https://nodum.org/was-sr-71-blackbird-leaking-fuel/

[1] https://nodum.org/was-sr-71-blackbird-leaking-fuel/

So much so that the last pilot of Concorde famously left his cap jammed between the instruments ...

This kind of hypersonic passenger plane is half way between Concorde and SpaceShip Three. Very high altitudes are necessary for the air to be thin enough to obtain these speeds, so the flight path will likely be semi-ballistic. The payback is that, despite the humunguous fuel burn, flight time is so short that seat-mile costs and pollution can actually fall; you don't have to spend several hours holding the thing up in the air. And if SpaceShip Three can go even higher while meeting safety standards, I don't see why this thing can't.

The powerplant is likely to be a variable-cycle beast of some kind and, whether it's an airbreathing rocket/bypass-ramjet like SABRE or a turbo/ramjet like the SR-71, will feature Reaction Engines' revolutionary pre-cooler.

The real problem will be thermal management. The SR-71 leaked fuel like a sieve until it warmed up and the gaps closed, SpaceShip Three uses a rocket but lacks the extended burn time. The cruise time of the SR-71 meant that fuel had to be circulated round the undercarriage bays to keep the tyres from melting. It used its fuselage-full of fuel tanks as a huge heatsink. By contrast a hypersonic passenger jet has to cool the whole fuselage - while having twice the heat to get rid of unless it flies so far up that it struggles for air. At least the sonic boom will be no real problem up there.

I can't help wondering if this is all just a way to screw funding for black project hypersonic research out of the civilian budget.