This starts off as a discussion of APUs but turns into a Q/A with participation by Concorde pilots, designers, and even a flight attendant who all clearly pine for their favorite plane ever. I couldn't pick one of the many anecdotes to highlight, so I posted a link to the entire monster thread. It rewards digging.
On the Concorde, the APU did not exist. On both contemporary and modern planes, the APU is the Auxiliary Power Unit, which is a fancy way of saying electric generator. Basically a moderate-size internal combustion engine (relative to the plane; big planes have big APUs) with the usual arrangement to generate electricity.
Why? Well, it's handy if a plane can start its own engines, and big engines can't usually be started by batteries, so batteries start the APU and the APU starts the turbines. The APU is also used to drive climate and entertainment systems when on the ground (and not connected to the Ground Power Unit, also mentioned early in the thread).
Finally, the APU can provide fault tolerance: if the turbines are disabled in flight, the APU can provide hydraulic pressure for the control surfaces, as well as electricity for safety systems and the potential to restart the turbines. There is some correlation in turbine failures (flying through something, fire, one turbine blade flies out and destroys a second engine), but the APU is inside and less likely to fail at the same time.
Related is the Ram Air Turbine, which is basically a small wind generator that can pop out of the fuselage if needed to provide some power to critical system when really nothing is working.
Minor addition: engines need compressed air to start and APU provides that, too (used for engine starting, cabin pressurization and some anti-icing systems).
Enjoyable, nostalgic thread. Later on one member (age 78) of the core Concorde design team even appears. Here's an amusing anecdote from early in the thread:
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Whilst on the Concorde conversion course at Bristol, occasionally crews would have the privilege of meeting some of the original design engineers and draughtsmen who had worked on the Concorde project.
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Suffice to say that the senior fire officer who misread litres-per-minute as gallons-per-minute during an Olympus water ingestion test probably would not want any further publicity, likewise the apprentice who didn’t defrost the chicken before firing it into an engine running at full power in the bird ingestion test. My favourite was the supersonic hailstone story, fired as part of a hailstone ingestion test, but with uncertain results, the final resting place of said hailstone still being slightly obscure to this day. If anyone in the greater Bristol area got hit by a particularly hard snowball in the early sixties, the Filton test engineers are very sorry, and would like to apologise!
However, it is often the little insights into the past that amuse one the most and stick in one’s mind. During one such conversation, with a couple of thermodynamicists, I ventured to ask how they had settled on the (rather difficult to memorise) various temperature limits associated with Concorde.
For instance, why a nose temperature limit of +127°C, why not +130°C, much easier for a pilot to remember?
“Isn’t it obvious?” one replied politely, genuinely puzzled by my question.
“Computer generation,” replied his colleague to him, pointing his pipe stem at me.
“Ah yes,” said the first, “that would be it.”
They then went on to explain, in ever such a kindly manner, that, in thermodynamics, apparently the square, and the square root, of the absolute temperature of a material are terms used in many equations. Being armed mostly only with slide rules (and as they were in the vicinity of 120°C to 130°C as a limit anyway) it had been decided to make life easy and settle on +127°C as the limit, a temperature for which they could easily calculate the square and square root in their heads.
Noticing my bewilderment at the thought that anyone might be able to calculate the square or the square root of 127 in their heads, they proceeded to explain it to me still further, very slowly; in the manner that one would speak to an aged and rather deaf great aunt!
• Absolute zero = -273°C = Zero Kelvin = 0K
• Max Nose temp = +127°C equal to 400K
• √400 = 20
• 400² = 160,000.
These are the people with the amusing stories to tell!
If you're wondering why the air isn't full of supersonic passenger aircraft, one of the main reasons is that high bypass turbofan engines are much more efficient than turbojet engines like the Olympus engines in the concord. (And way quieter)
Turns out a lower volume or air in a gas turbine engine moving at supersonic speed is much less efficient than a large volume of air moving at lower speed.
From page 2 of the discussion:
"Anyway, back to some figues; at Mach 2, 50,000', the typical fuel burn per engine would be around 5 tonnes/hour, falling to around 4.2 tonnes/hour at 60,000'."
So at 60,000 feet, total fuel flow would be:
4.2 tonnes/hour * 4 engines = 16.8 tonnes/hour
16.8 tonnes/hour * 326 gallons per tonne = 5477 gallons/hour
5477 gallons/hour * 1/1320mph = 4.15 gallons/mile, 0.24 miles/gallon
0.24 miles/gallon * 120 passengers = 28.8 passenger miles/gallon
The 787 boasts around 120 passenger miles per gallon. So the Concorde uses 4x this much at highest efficiency. Not bad for 60's engineering at mach 2! I can see why it was not profitable to operate however, due to the increases in fuel cost since the aircraft was designed.
But even if fuel represented 100% of the ticket price, these supersonic flights could catch all of first- and business-class market. Rich people and businessmen value their time.
Each engine type has a range where it's superior. Turbojets are much more efficient at high speeds, while high-bypass turbofans are better at low speeds. The Concorde burned remarkably little fuel at Mach 2, but used up ludicrous amounts of fuel at idle, taxi and takeoff. Read the thread, it's all in there.
If you read further into the thread, a pilot says that the high fuel consumption during takeoff was a consequence of the aircraft's weight being higher, and higher air density at lower altitudes -- the engines were not more efficient at higher speeds as an earlier post suggested.
Note that the idle fuel consumption (1t/hour; not sure whether that figure was per engine or for the entire plane) was not higher than cruising (4-6t/hour) -- it was just high in absolute terms.
Thank you; I didn't mean to imply the plane used more fuel at idle than at supersonic cruise.
Fuel consumption during takeoff and climb was something like 25 tons an hour. Part of this is because the Concorde's wing has high drag at high angles of attack, most of it was due to the engines needing to be in full afterburner. The plane needed full afterburner to take off even without a full load of fuel on board.
At low speeds, most of the turbofans' power goes to creating a really high-speed stream of exhaust. That's what I mean about the engines being fundamentally more efficient (at converting fuel into forward motion) at higher speeds, where the exhaust speed more closely matches the speed of the plane.
Yes, someone (M2dude, I think) says that 65% of the thrust, at speed, was provided by the intakes. That said, at lower speed, all that work to bring air up to speed is accelerating the plane as well.
Even so, since drag tends to increase as the square of velocity, and in proportion with density -- you're spending a lot more effort to maintain velocity at higher speeds. The afterburners are mentioned as increasing fuel consumption by 80% and power output by 17%. The high fuel consumption comes down to the plane accelerating, climbing, being in denser and warmer air, and heavier -- all at once. While the engines might be marginally more efficient in some sense at high speed, this would be cancelled out by the vastly higher drag at low altitude/speed.
An enjoyable thread, so much good discussion, no trolls or stupid comments, mostly professionals and interested amateurs discussing a topic they love, and know lots about.
Another reason might be that supersonic flying doesn't really gain you all that much. You still have to spend time getting to the airport, being there a couple of hours in advance, go through all the security measures etc, and jump through similar hoops upon arrival. Cutting your flight time in half sounds nice, but the effect on your actual travel time is a lot smaller.
Please note that the Concorde requirement was set circa 1958-60, and the plane first flew in 1969; airlines were placing orders in the early 1970s.
Back in those days, airline security was a lot more relaxed: despite numerous hijackings nobody had yet figured out that a couple of hundred tons of airliner and jet fuel made a potent self-guided cruise missile, so the usual ending of a hijacking was on the ground with a negotiated hand-over. It was still possible to get from drop-off at the airport to the Concorde departure lounge in under 30 minutes; and for the trip from Arrivals at JFK to Wall Street ... well, that's what helicopters are for. (If you can afford a Concorde ticket you can afford a helicopter limo service!)
The immediate aftermath of 9/11 pushed the suggested minimum check-in time for trans-Atlantic flights up from 30-40 minutes to 3 hours, for everyone, VIP's included. So the folks who could afford a first class ticket -- the only kind of seat available on Concorde -- decamped for NetJets and other bizjet services, because the civil aviation security regulations didn't apply to private jets and you could still do the 30 minute from door-to-plane thing.
(And then Airbus refused to renew Concorde's type certification at the 30 year mark, allegedly just as they were discussing development of a new second-generation SST with a large Japanese corporation ...)
Actually, airport security -- at least for international flights -- in the late 60s and early 70s was not more relaxed. Having traveled quite a bit as a child in those days, most international travel involved having all your bags searched by hand in front of you. The difference was that air traffic volumes were much lower, you spent less time in queues, and Concorde passengers were first class and got preferential treatment.
The lightest airport security in my experience was in the mid-to-late 90s following deregulation. I flew from Sydney to Zurich in 1998 without breaking stride at either end (literally waved through every checkpoint).
Haha, yeah... in the early '90s I was late to the airport flying out of Berlin (to Edinburgh), arriving like 10 minutes before the scheduled time... I ran like nuts to the gate, and although they were clearly furious with me, and I got a ... very personal (but fast) ... security examination, and they still let me on the plane!! [Luckily I had no baggage to check.]
That was admittedly pretty ridiculous, but they were generally a bit lax back then. I thought it was actually rather odd because there had been a number of incidents, like Lockerbie, and the gulf war was a thing, and people actually were sort of worried about airplane terrorism. Not worried enough I guess...
9/11 only affected Concorde to the extent that there was a slump in the whole airline industry. It's not like people fled that plane in particular for bizjets due to waiting times.
The immediate impetus for retiring the plane seems to have been the prohibitive cost of rewiring on the whole fleet, to bring it into compliance with stricter modern requirements.
It didn't help matters when one of the Concordes briefly caught fire over the Atlantic due to an electrical short, something that was only discovered after the fact.
I flew in 2003. Car dropped me off in front of the "Concorde" sign at JFK. I walked in the door, checked in, went through security, and then sat in the Concorde room until departure. It was far less than 30 minutes!
Really, it had a great deal more to do with politics: specifically, the Europeans had a supersonic airliner and the US didn't, so there was no way in hell the US government was going to do anything other than make it as difficult to use as possible.
> the Europeans had a supersonic airliner and the US didn't
Technically no, but the 747 is fast, almost too fast in these days of fuel economy.
The 747 was designed by Boeing to do M.87 (Pan Am wanted M.9)
, though it now sits around M.855 (913 km/h) [1] due mainly to the requirement of fuel efficiency over raw speed. Almost Mach 1. How did Boeing designers get this speed? Hint: The hump and area rule (min drag > M.8) [2] Now a DC8 was pushed to M1.012 under special conditions. [3] And as a point of interest, Air Force One (VC-25A) is rated at M0.92 (1015 km/h). [4]
"... The fastest airplane out there today is the 747. Not one of them has been able to match the cruise speed of the 747. That indicates to me that it’s a high-technology airplane ..." Joe Sutter [0]
wing sweep alone doesn't explain why the 747 is the fastest subsonic transport to date. If it did, competitors would have copied the idea. I'm curious why? Boeing describe it as a combination of wing design, engine, electro-mechanical systems. Maybe I should ask Boeing.
Because the penalty for that speed is higher fuel burn. That's why Boeing is having trouble selling the 747-8, and why four-engine airliners in general are getting phased out.
My source for the wing sweep thing is also Joe Sutter, in his wonderful book. Also highly recommend Widebody, by Clive Irving, which is sadly out of print.
I take it you're referring to the "no sonic boom in my backyard" shenanigans that hampered flights to/from/over the US in the early days? These were sorted out for the key NY-Europe routes by 1977, just over a year from first commercial flight.
Do you have any specific reasons to believe that the USG ever stopped or even simply tried to discourage the Concorde from flying to airports on the U.S. East Coast?
I've been in a sonic boom under a navy plane in the continental USA, not adjacent to any military base. But it only happened once, so maybe it was a mistake.
I saw the Concorde at OshKosh Air Show. Doubt they had problems flying there, landing, and setting up for viewing plus several demonstration flights. After that, I recall they were set to fly out west coast for another show.
Don't think the USG had any issues with the plane.
Not to mention the flights from JFK to Heathrow and Paris.
I lived in London in the early 2000's before Concorde stopped flying; I was in Fulham so happened to be under one of the Heathrow approach flight paths.
I remember seeing and HEARING the Concorde coming in for landing at Heathrow. Absolutely the loudest thing in the sky! (apart from the occasional RAF flyover for Her Maj's birthday or some such...) You couldn't mistake it for anything else.
The Concorde engines were apparently based on the Vulcan Bomber's engines. There's still one of those that flies, and you can see it at airshows in Britain once in a while. I saw it at this year's Farnbourough which was an awesome experience (as was seeing the Airbus A380 take off at an extremely short distance -- when it's not loaded with fuel and cargo it's incredibly powerful and agile). The Vulcan was load, and has an incredible "howl": https://www.youtube.com/watch?v=H_ARSE8jEHQ
"The only thing that tells you that you're moving is that occasionally when you're flying over the subsonic aeroplanes you can see all these 747s 20,000 feet below you almost appearing to go backwards, I mean you are going 800 miles an hour or thereabouts faster than they are. The aeroplane was an absolute delight to fly, it handled beautifully. And remember we are talking about an aeroplane that was being designed in the late 1950s – mid 1960s. I think it's absolutely amazing and here we are, now in the 21st century, and it remains unique."
—John Hutchinson, Concorde Captain, "The World's Greatest Airliner" (2003)[114]
I really hope we get to fly in one of these, or a modern version, someday.
I noticed in the second post how the MEPU was developed by Sundstrand - the same company that made calculators in the 70s. (I, as a vintage calculator collector, actually have one.) On HN we hear about pivots all the time, typically not too drastic, but imagine going from manufacturing MEPUs for the Concorde to manufacturing calculators!
Here is a thought, Imagine if a Concorde got hijacked? An F16's top speed is approx 1320 mph and the Concorde's is 1350 mph per wikipedia. I feel like there might have been some government pressure to take this plane out of service with all the fear mongering post 9/11
A concorde going 1350 mph still has less kinetic energy than a 747 going 600mph. Also, I'm not positive, but I'd suspect that the concorde couldn't hit 1350mph at only a few hundred feet due to the really soupy atmosphere down here.
> A concorde going 1350 mph still has less kinetic energy than a 747 going 600mph.
Not really no. Remember that Ek grows with the square of speed, so you need 4.5g of 747 for each gram of Concorde, something you only get when closing in on a 747 at MTOW (400t for a -400, 440t for a -8i) and a completely empty Concorde (80t).
Comparing the 747-400 and Concorde using maximum takeoff weights, the 747 comes out at 15GJ[0] when Concorde is 33GJ[1]. Using empty weight has roughly the same ratio. Even swapping the -400 for a -8i yields only marginal improvements to 17GJ[2].
With a loaded but dry plane (simulating the arrival at the end of a flight, under the assumption that fuel is a much bigger part of a Concorde than a 747) and maximum fuel load at takeoff, Concorde comes out at 16GJ[3] and the 8i at 10GJ[4].
Even if we use the same route, halve the 8i's fuel load (to account for twice the range) and replace it all with additional payload (which I'm not sure makes any sense), it's still only 13GJ.
Going back to the first paragraph, fully loaded 747's 15~17GJ[0][2] would indeed beat a completely empty Concorde's 14GJ[5], but that's about it.
The Soviets could shoot down an SR-71 that invaded their airspace by 1980 (I can't find a reference ; it seems likely we could shoot down a Concorde if needed. (I say likely rather than definitely, since a large part of the USSR deterrence involved scrambling interceptors before the SR-71 reached their airspace, and by 9/11 we no longer had a plane capable of launching an AIM-54 which could almost trivially shoot down a Concorde).
They did shoot down a U2; a Mig(31?) was able to gain missile lock on an SR-71. In any event shortly after they had developed the capability we ceased to enter soviet airspace with an sr-71.
A Concorde could not outrun an F-15, which has a maximum speed of more than Mach 2.5 at the cruising altitude of a Concorde. And an F-16 would only need to get close to a Concorde in order to shoot it down with missiles.
Still, you'd be surprised how a fast target shrinks missile envelopes. I spoke with a guy who spent a few minutes chasing a MiG-21 over Vietnam in a Navy F-4. They were both supersonic -- don't know how fast, but they were in the weeds and IIRC even in the 80s the Tornado IDS doing M1.2 at sea level was about the fastest thing at that altitude -- and from 7 miles astern, his Sparrows just didn't have the range to do it. That from a missile that we think of as a 30+ mile range weapon.
Similar story with today's weapons. I've read about frustrated F-15 drivers saying all an F-22 has to do in a mock dogfight is change his heading by 10 or 20 degrees and the F-15's beyond-visual-range missile solution goes out the window, such is the F-22's usual speed advantage with supercruise.
I was thinking that in the hypothetical scenario an F-16 would intercept the Concorde head on. I don't think that a Concorde had the right kind of radar to detect a BVR missile launch so it would not be able to turn and outrun the missile. But even if it was able to do so, an additional pair of F-16s on each flank of the lead fighter could wait for the Concorde to turn and run towards them after the initial missile launch.
