So yesterday I was investigation the relation between thrust and horsepower, and ran across this interesting website. One thing it says is that although the engines on a Boeing 747-200 can produce, under optimal circumstances, 219,000 lbs. of thrust, when the plane is cruising at Mach 0.9 at 40,000 feet only 55,145 lbs. of thrust are needed to maintain that cruising speed. So basically, the jet is using 25% of its maximum thrust to cruise at Mach 0.9. This invites the question: if the throttle were opened wide up, could the 747 break the sound barrier? Or would it not have enough power? Or would it disintegrate before hitting Mach 1.0? What say you, Dopers?
Yes, once. 
Yeah, I figure if the plane succeeded in breaking the sound barrier the condensation cloud would be made up of equal parts water vapor and pieces falling off of the airplane. But I wonder if the plane would have the power and structural integrity to cross Mach 1.0 in the first place.
Of Boeing and Airbus on Wiki, few have a max speed of Mach .90. And even then, that’s a maximum speed rating, which I assume “maximum” to mean ‘not structurally safe to go beyond’. They likely may be capable of Mach 1.0 for a period of time, but not something the test pilots would have done to found out based on the engineers suggestions. But that is speculation, a more informed person may come along with more info on this.
True; I see that most have a maximum speed in the Mach .85 range. But there are some that go faster–the Boeing 747-400 has a maximum speed of Mach .92, the Airbus 380 has a maximum speed of Mach .89, etc.
I keep thinking there was an incident some years back when a commercial 747, diving almost vertically in an emergency, exceeded Mach 1 and survived. A casual search didn’t find mention of it, though.
I have a recollection of listening to a couple of people in the know about the big Boeings and Airbusses being able to exceed Mach 1 without a problem, it’s just that the engines can’t do it in level flight, you need a gravity assist. However, that’s completely annecdotal and I can’t recall further details.
I’d be a little surprised if they’d suddenly disassemble. Although an airplane exceeding Mach 1 does produce a shockwave, that doesn’t mean anyone aboard the airplane will notice.
Don’t forget, the air is pretty thin up there - even at full throttle the engines may not develop their maximum thrust due to the thinness of oxygen at that altitude.
Maybe, maybe not - airplanes may have more than one maximum speed. For example, maximum speed in level flight versus maximum safe speed in a descent. Maximum manuvering speed is the safest for rough air. “Never exceed speed” is another one, which is one you really don’t want to go past. Keep in mind that some of these have a safety factor calculated in, so if you exceed them by 1 knot or 5 knots or whatever the airplane won’t suddenly disintegrate.
Actually, since test pilots fly to the edges of the “envelope” as part of the testing process, it wouldn’t surprise me if they had done this.
Admittedly, my knowledge of the big airplanes is very limited - we need one of the Dopers who actually fly big iron to add to this discussion.
True; I hadn’t really considered that. But also, this thin air is being rammed into the engine at 600+ mph–it’s like having the world’s most powerful supercharger. So I would *think * (by which I mean, “I would make a totally ignorant and uneducated guess”) that the engines would have at least as much oxygen available for combustion at Mach .9 at 40,000 feet as they would at sea level sitting motionless on a test stand (which is how the website I cited in my first article says an engine’s maximum thrust is measured).
Former big Boeing & fighter pilot here …
At altitude the engines produce a LOT less thrust than they do at ground level.
In fact aviation has a concept called “service ceiling”, which is where at full throttle you have so little excess thrust that the airplane will not climb faster than 100 feet per minute while holding constant speed. Considering that a minute represents about 8 miles of horizontal travel, that’s a pretty shallow grade. When you can’t climb a hill that shallow, you’re flat out of extra engine power for any purpose, including more speed.
In general the relationship between air drag, engine power & the speed of sound gives best performance in the high teens. The factors above variously depend on air temperature, presure, and density in different ways and for a pilot’s ballpark mid teens seems to be about best. An aerodynamicist & a powerplant guy could explain why.
Even at best altitudes a typical airliner with a typical load is not gonna have the cojones to push through Mach 1 in level flight on engine power alone. A completely empty long range airplane like a 747 or 767-400 or 777 might be able to get close in level flight, say .97 or so.
Aerodynamically, the drag increment as you approach Mach 1 in horrific. It’s not exponential, but its close. And an airliner has a shape that is meant to push the knee in the curve up near .9 (or maybe .95 for a 747) so they can cruise just below that knee efficiently. Trying to push up past that knee just isn’t going to get you very far.
So much for engines. How about the biggest source of power available to a high altitude jet, all that potential energy stored as altitude …
You can dive any jet airliner through Mach 1 no sweat. The challenge is reusing the airplane.
A quick intro/caveat. Reading about supersoncic flight you hear the term shock or shockwave a lot. If you’re forming a mental picture like an explosion that damages the airplane, that’s 100% not it. A quick Google or Wiki search for supersonic can probably get you a good layman’s intro. Supersonic airflow isn’t inherently dangerous, just very different from subsonic flow & different designs are needed to cope with the very different operating regime. Sorta like the diference between living in water & air. An amphibious creature can live on both land & water versus a fish outta water that’s in a world of hurt. Anything operating outside its design envelope is gonna have prolems.
There are three issues w/ supersonic flight in airplanes not designed for it.
- Stuff starts breaking off: This is stricly a low altitude problem where the effective wind pressure (IAS to aviators) at Mach 1+ is enough to pop inspection panels off, break antennas & such. And like the space shuttle Columbia, when something breaks off at the front, it becomes a powerful missile by the time it gets to the back. If the debris goes into an engine or smacks the front of the tail that might trigger a chain reaction failure.
- Flutter: ever see a car on the freeway flying a sports team pennant or banner? it’s whipping madly in the slipstream. That’s flutter, a chotic & self-reinforcing vibration caused by the interaction between the flexibility of the material & it’s characteristc freqency versus the forces from the airstream.
Now imagine that was part of the tail of your airplane. How long do you think metal structure, or hinges for control surfaces (the moving parts of the wing & tail used to steer the aircraft) would last flapping like that?
A few seconds to maaybe a minute tops. A LOT of design effort & testing goes into assuring that parts & hinges & power steering units & such are stiff enough that nothing ever starts flapping in the breeze no matter how they move. But when subjected to a much stronger breeze, eventually you get beyond the design stiffness envelope & something somewhere starts to flutter, followed a few seconds later by it breaking off the airplane. If the broken part was your tail, or a big piece of it, the game is over. Control surfaces are particularly susceptible to this and will likely be the first things to go. Losing the ability to steer is not good for your longevity.
3. Compressibility: This is a catchall term for control problems brought on by Mach shock. To cut a large discussion down to a couple sentences … As the airflow over the tail passes through Mach 1, the loads on it change draastically and the result is the nose wants to drop into a steeper dive & simultaneously the effectiveness of the tail controls drops to just 1 or 2% of normal. In other words, yuo’ve passed a point of no return. All the pulling in the world will not raise the nose again & your dive is getting steeper & steeper & steeper. In just a few seconds the speed will be 1.2, then 1.4, then 1.5, then something breaks for sure. Or the ground arrives. You can’t recover control until you slow down & that isn’t going to happen unless you can shallow the dive & that isn’t going to happen because the controls are ineffective.
In fact in an airliner one of the easy ways to kill yourself is to let a routine descent get a little too steep. The overspeed warning noisemaker in the cockpit is enough to wake the dead for a reason. In all the “non-pilot flies a jetliner to a safe landing” fantasies we’ve discussed on this board, this is how I predict most of them will end.
All in all, I don’t recommend supersonic flight in a transport-type jet.
OTOH, I have done it in fighters a bunch. In a properly designed modern aircraft it is a total non-event. No change in control effectiveness, no noise change, just a momentary burble in the instruments as the Mach shock crosses their various sensor ports for the first time & we’re doing 1.1 or 1.2 or …
Very helpful response, LSLGuy. Thanks!
Just to add to what others have said. A commercial jet will not exceed the sound barrier under its own power in level flight because of the type of engines they use.
They use a turbofan engine. Because the thrust comes from the blades of the fan, supersonic flight is impossible.
Planes that can exceed mach 1 use a turbojet engine. The thrust, which can exceed mach1, comes from the exhaust gasses exiting the engine.
I know it was retired but wouldn’t the Concorde still qualify?
I mean, it’s no 747 but it covers all the qualifications. Commercial, jet and supersonic speed.
The P&W TF30 is a turbofan used to power the F-14A Tomcat, a supersonic aircraft.
The ability of a commercial jet engine to accelerate an aircraft to supersonic speeds would probably have more to do with the thrust it can deliver and the ability of its intake to handle supersonic airflow. Also, they are a high bypass turbofan engine. It’s not so much that they are turbofans, but a particular type of turbofan designed for efficient and quiet operation at high subsonic speeds.
Aircraft such as the F-14 and Concorde have engine intakes designed to slow the intake air to subsonic speeds.
LSLGuy has mentioned another important limitation: no jet engine can produce its maximum rated thrust at its highest altitude. At altitudes where maximum thrust is available, drag will limit the maximum speed to well below mach 1.
There have been commercial jets (e.g. 707) that used turbojet engines, but these would also not have been able to reach mach 1 in level flight.
And justrob is correct that the Concorde supplies a “Yes” answer to the OP.
When I was a young kid in the early 60’s I remember commonly hearing sonic booms. Then at some point they went away. What was likely to have been producing them? This would have been in Indianapolis.
This is from wikipedia, so make of it what you will:
This is the cite given for the China Airlines flight 006: ASN Aircraft accident Boeing 747SP-09 N4522V San Fransisco, CA, USA
There’s no mention of the top speed attained during the uncontrolled dive.
This may be the incident that Baldwin was thinking of.
This is the cite for the DC-8: http://www.dc8.org/library/supersonic/index.php
This one does specifically mention a top speed of mach 1.012.
There’s no cite given for the Boeing certification tests.
I figured my statement would be questioned since I left it pretty vague. True, the f-14 could go supersonic on a turbofan - but not without afterburners. Commercial airliners do not have afterburners. I should have said that the airline engines are not capable of supersonic flight - not implied that no turbofans are capable of supersonic flight.
As far as the Concorde, I’m pretty sure that they’re not flying anymore. That’s why I didn’t mention it.
While the Concord is (was) a commercial jet, I think the main thrust of the OP’s question was meant to be “Can a jet which was not specifically designed to break the sound barrier do so?”. A Concord is a commercial jet, and a B-52 is not, but I would be a lot more impressed by the B-52 going supersonic than the Concord.
I wonder how the BUFF would handle supersonic flight. It has swept wings, so it has a pretty good chance of breaking the sound barrier without ripping itself apart. I certainly wouldn’t want to be aboard when they tried though.
An excellent and very informative response.
Beyond the practical issues of structural integrity and engine thrust, drag in the near supersonic and beyond region (wave drag) requires a prohibitive increase in thrust for general shapes. An aeroshape that might be highly efficient at lower speeds at reducing parasitic form drag might be completely detrimental at transonic ranges. This is because at such speeds the air stagnating against the leading surfaces of the plane no longer behaves like a linearly compressible fluid; it suddenly gets “hard”, creating a shockwave in front of the plane that acts as a boundary; in effect, it creates a virtual aeroshape that is a big blunt cone with vastly more surface area than the plane itself. Designing a plane to flyi efficiently at supersonic speeds requires careful use of the area rule and anti-shock features to minimize the aspect of the shock wave, and keep the aft end of the body within the original shock wave, hence the move to delta wing configurations for supersonic craft like the Concorde, the F-14, or the SR-71. Airliners are optimized to reduce lift-induced drag (by getting more effective wing surface) to make them more fule efficient (i.e. less of total thrust is used to just keep them in the air) and to reduce shock at the low-transonic range at which they fly.
As also previously noted, high bypass designs are required on engines in order to allow them to fly at supersonic speeds, but they aren’t terribly efficient. Afterburners are also used on many craft to get additional impulse at the cost of essentially dumping fuel out the back, creating a very crude and inefficient sort of ramjet. This isn’t something you’d use on an airliner for any number or reasons, including cost and the effect on passengers.
Stranger