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Comment by allenrb | Hacker News Reader

For a moment, reading only the first paragraph, I was hopeful that the problem of accelerating from subsonic to hypersonic had been solved. Alas, further down it makes clear that a booster was involved. Presumably a small solid rocket that accelerates to Mach 3-5(?) prior to scramjet ignition. Fine for a weapons system but will never be useful as transportation.

Is anyone aware of progress toward being able to reach these speeds without disposable boosters?

The SR-71 could make Mach 3 on its own but at incredible cost in fuel and complexity.

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Unless you're trying to go to space, there really isn't much reason to go this fast in the first place. Even relatively modest supersonic passenger flight has proven to be a commercial failure (good luck to Boom Tech inc... you guys will need it.)

If you're going to space, an SSTO with scramjets would be interesting. But I think if SpaceX's reusable Starship succeeds without that tech, it would be hard to justify the additional complexity and expense of an SSTO built with a class of engine which hardly exists at all right now.

> relatively modest supersonic passenger flight has proven to be a commercial failure

Tried it once in the 70s. Didn’t work. Everyone go home.

We tried with the British-French Concorde, the US Boeing 2707, and the Russian Tupolev Tu-144. That's three tries (or maybe two, the Boeing version was cancelled before prototype completion).

Maybe we can do better today with the lessons learnt and the advances in material science since then. There's several startups banking on that. But so far commercial supersonic flight has only worked out as prestige projects, not as commercial successes.

Wasn’t TU-144 just a pissing contest entry with retrofitted engines of another aircraft? And Boeing 2707 quickly scrapped? Only Concorde ever saw the light of commercial operations. It was truly limited by its technology, of that date. And post 9/11 situation sealed its fate, its had one fatality in 20 years, its already better than 747 max by that metric. There is no point in saying Supersonic is impossible, things changed a lot in last few years. Having computer monitors instead of a dedicated flight engineer with gazillion dials and Fly by wire, it can be much more optimised now. Boom just needs to make a working jet, and it would definitely revolutionise aviation again.

> Wasn’t TU-144 just a pissing contest entry with retrofitted engines of another aircraft?

Pissing contest, maybe, but the Tu-144 was its own aircraft. The airframe was designed from day one to be exactly what it was, a supersonic airliner (and contrary to popular myth, the design was not simply stolen from the Concorde although maybe the premise was..) I'm not sure if the original Tu-144 engines, the NK-144's, were originally intended for another plane, but as far as I know the only planes they were ever put in were the Tu-144. They were later replaced by RD-36 engines, which were designed for the Tu-144.

And let's not forget the Concorde's engines had a military background. They were derived from engines originally meant for the [cancelled] TSR-2.

The major problem I see for Boom is social. Environmental and class/economic concerns particularly; see the public backlash against Jeff Bezos' rocket hop for instance. Not to mention, the little experiment we've been running for the past ~2 years with remote work and teleconferencing has probably reduced demand for high speed business travel.

I always thought the problem was intractable and related to just ... moving through the atmosphere at that speed induces a lot of drag. Has there been any sort of major breakthrough that would change that?

I guess you could always fly at a much higher altitude (with its own problems) but it all seems motivated by, well, prestige as you put it. Like, I have a hard time believing it'll ever be more economical (as a commodity/for the market), environmentally friendly, profitable (for the operator), or just more practical than transsonic flight. The only advantage seems to be bragging rights, and the ability to cross great distances really quickly for an elite few.

> But so far commercial supersonic flight has only worked out as prestige projects, not as commercial successes.

I suspect that further optimizing the fast part of air travel doesn't offer as much gain as further optimizing the slow part, which is between the real origin and the plane door and between the plane door and the real destination.

Check out Hermeus, working on building a reusable hypersonic aircraft. The engine they're using is a proprietary TBCC engine based on the GE J85.

SpaceX Starship will do this and be fully reusable. Starship can fly long distance with a reusable booster and somewhat shorter distances without a booster.

Granted, its not really 'flying' but it does reach the speed needed.

I believe the descendant of HOTOL is intended to bridge that gap... https://www.reactionengines.co.uk/

https://en.wikipedia.org/wiki/SABRE_(rocket_engine)

Blackbird does 3.75 designed maximum on turbo-ramjet I think?

> The SR-71 could make Mach 3 on its own but at incredible cost in fuel and complexity.

the SR-71 never struck me as "incredibly complex", the space shuttle however...

The J58 powering the SR-71 is still one of the most complex engines ever built. The space shuttle is a simple rocket by comparison.

https://en.wikipedia.org/wiki/Pratt_%26_Whitney_J58

And the SR-71 is so simple the complexity falls off a cliff at the engine, hence your inclination to only care about that in comparing the two.

The space shuttle is like a reusable mobile space station, airlocks and all. Occupants of the SR-71 wore space suits the entire time, there's a lot of minimalism on display in the SR-71 which is a large part of what makes it so glorious.

You seem to be suggesting but not outright saying something that is a misconception: The SR-71 did in fact have a pressurized cockpit, which was also air conditioned as well (it had to be, because the aircraft would cook the crew otherwise.)

The SR-71 was sophisticated in other ways as well; for instance in having ejection seats. In fact the first four shuttle flights (all Columbia) had ejection seats too and guess where they got those seats from? The SR-71 of course. And on those flights with ejection seats, guess what the shuttle crew wore? They wore pressure suits, like those worn by SR-71 crew! They stopped wearing those starting with STS-5, when they removed the ejection seats. However the pressure suits (though not the ejection seats) came back after the demise of the Challenger.

What I am not saying: that the shuttle was simple.

What I am saying: That the SR-71 was more sophisticated than you've given it credit for.

I think the two of you are largely in agreement, but GP rightly points out that the SR-71 was less complex than it could have been and that this is in party why it was so successful. The ability to leave stuff out in engineering exercises can lead to substantial benefits and that's worth noticing.

The SR-71 was very complex. For example it had multiple heat exchangers to get rid of heat, to transfer it to the fuel before it was combusted. The titanium structure was complex to maintain. Welding needed to be done in "bubble" work stations under protective gas.

> The titanium structure was complex to maintain. Welding needed to be done in "bubble" work stations under protective gas

Operational complexity is orthogonal, and tends to be inversely proportional to simplicity in implementation...

For instance they needed to refuel the thing immediately upon reaching operating temperature in flight, since it leaked like a sieve on the runway until everything expanded. Rather than try fix that somehow with more engineering, they shifted the complexity into operations.

IIRC it couldn't even start its own engines cold, relying instead on hot-rod v8s setup on the runway to bootstrap the thing. More operational complexity in favor of leaving an entire subsystem out of the plane.

Don't get me wrong, I'm a huge fan of the SR-71, but so much of that appreciation stems from its ruthless pursuit of its narrow operational goals, for as much what it isn't as it is.

It's the polar opposite of stuff like the space shuttle or F-35 where the aggregate complexity is through the roof to accommodate a kitchen sink.

Oh come on. SR-71 was a complex military aircraft. Maybe it was not as complex as the space shuttle. Certainly much more complex than an F-16.

One can start here for example http://www.enginehistory.org/Convention/2014/SR-71Inlts/SR-7...

>Operational complexity is orthogonal, and tends to be inversely proportional to simplicity in implementation ...

I'm not sure what your point is with this. All complexity is operational complexity.

> All complexity is operational complexity.

Uh, what?

Example to illustrate the difference:

Automatic transmission in an automobile, operationally simpler than the manual constant-mesh trans it broadly replaced: Driver (AKA the Operator) selects drive or reverse when stationary, no need to participate in gear changing while in motion. Substantially more complex in implementation in exchange; go look at the hydraulic valve body of your average slushbox.

Manual trans: Simple implementation, single clutch assembly, sets of gears selected mechanically by axially sliding dogs on splined shafts. Operationally complex since the driver must continuously participate in the selecting of gears and clutch operation while in motion.

Oh I see it now, you mean at the time of operation by the end-user.

I was thinking about the operational complexity of developing/manufacturing such thing, hence why for me it seemed to be virtually the same thing.

Thanks for taking your time to clarify this.

The arguably most complex technology of Space Shuttle was SSME. Also, arguably the most important part of LEO rocket technology is engine - as soon as engines matured enough, space era started. Yes, there are gyros and superlight tanks, but Lambda-4S still does illustrate the importance. And hypersonic winged flight problems were greatly decreased in the Space Shuttle case by a "brute force" approach with ceramic tiles.

SSME was more or less repeated in results by RD-0120, and rather soon. I'd argue RD-0410 was more complex. SR-71 engine works in two different modes, and even today is not really repeated elsewhere. I think that shows how awesome the SR-71 design was - especially, but not only, for its time.

I'd appreciate a good analysis of design problems and solutions of SR-71 to decide which project was technically more complex.

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