Suppose one were to take a normal modern business jet engine, and attempt to make a supersonic aircraft. I understand the bypass ratio is too high, and the fan is too large to be optimal for supersonic flight. But how bad is the penalty? How inefficient would this be compared to a purpose designed engine?

Wish I could got ask one of the Skunkworks guys but I can't. But consider this.

This is the fuel TFSC for the GE F101-GE-102 engine (without afterburner): 0.562 lb/(lbf h) (dry thrust) and a max dry thrust of 17,390 lb

And this is the same for the CFM56-7B18 (which was derived from the above): 0.5 lb/lbf/h (14 g/kN/s) with a max thrust of 19,500 lb

Is there a huge difference in TFSC? Of course but there is no other way to get supersonic than to have a small diameter engine with high exhaust velocity.

Andre3K wrote:

Wish I could got ask one of the Skunkworks guys but I can't. But consider this.

This is the fuel TFSC for the GE F101-GE-102 engine (without afterburner): 0.562 lb/(lbf h) (dry thrust) and a max dry thrust of 17,390 lb

And this is the same for the CFM56-7B18 (which was derived from the above): 0.5 lb/lbf/h (14 g/kN/s) with a max thrust of 19,500 lb

Is there a huge difference in TFSC? Of course but there is no other way to get supersonic than to have a small diameter engine with high exhaust velocity.

That's interesting. But it's backwards from what I want. Suppose you took the CFM as built and tried to fly Mach 1.5: would it make any thrust and any guess how much?

Wouldn’t work, way too much drag even if you get the inlet velocity below M1.0 at the face with a very special inlet design. The exhaust velocity would be much too low.

Supersonic flight is very dependent on inlet design recovering the pressure gradient and using it as thrust. Very rough explanation. You might look up Concorde or SR-71 inlet and combustion cycle.


Gf

GalaxyFlyer wrote:

Wouldn’t work, way too much drag even if you get the inlet velocity below M1.0 at the face with a very special inlet design. The exhaust velocity would be much too low.

Supersonic flight is very dependent on inlet design recovering the pressure gradient and using it as thrust. Very rough explanation. You might look up Concorde or SR-71 inlet and combustion cycle.


Gf

You can put any inlet you want. And I agree it would not work (I even said so in the original post). But the engine would produce some thrust .. I'm just wondering how inefficient it would be.

kitplane01 wrote:

GalaxyFlyer wrote:

Wouldn’t work, way too much drag even if you get the inlet velocity below M1.0 at the face with a very special inlet design. The exhaust velocity would be much too low.

Supersonic flight is very dependent on inlet design recovering the pressure gradient and using it as thrust. Very rough explanation. You might look up Concorde or SR-71 inlet and combustion cycle.


Gf

You can put any inlet you want. And I agree it would not work (I even said so in the original post). But the engine would produce some thrust .. I'm just wondering how inefficient it would be.

When it comes to atmospheric flight (not space flight) you have to have at least a unity of exhaust gas velocity if you want to travel at that speed and even then that's not a guarantee. So if the bypass air is moving at 550mph and the core air is moving at 700mph you could theoretically end up somewhere in between but never equal to the core exhaust due to it's relatively low thrust at altitude and the drag caused by the huge fan and other parts of the aircraft.

When it comes to space flight, it's not so much about the exhaust gas velocity, but more about how long it goes on and the mass loss. Space shuttle exhaust gas velocity is less than 9000mph yet it has no trouble reaching 17500mph in space. It just doesn't work like that in the atmosphere.

Andre3K wrote:

kitplane01 wrote:

GalaxyFlyer wrote:

Wouldn’t work, way too much drag even if you get the inlet velocity below M1.0 at the face with a very special inlet design. The exhaust velocity would be much too low.

Supersonic flight is very dependent on inlet design recovering the pressure gradient and using it as thrust. Very rough explanation. You might look up Concorde or SR-71 inlet and combustion cycle.


Gf

You can put any inlet you want. And I agree it would not work (I even said so in the original post). But the engine would produce some thrust .. I'm just wondering how inefficient it would be.

When it comes to atmospheric flight (not space flight) you have to have at least a unity of exhaust gas velocity if you want to travel at that speed and even then that's not a guarantee. So if the bypass air is moving at 550mph and the core air is moving at 700mph you could theoretically end up somewhere in between but never equal to the core exhaust due to it's relatively low thrust at altitude and the drag caused by the huge fan and other parts of the aircraft.

When it comes to space flight, it's not so much about the exhaust gas velocity, but more about how long it goes on and the mass loss. Space shuttle exhaust gas velocity is less than 9000mph yet it has no trouble reaching 17500mph in space. It just doesn't work like that in the atmosphere.

Lots of planes with turbofan engines can exceed Mach 1, even without the afteburner. The F-22 and F-35 can do so, and I think the Saab Gripen can too. I believe they all use an exhaust the accelerates the flow to the necessary speeds. Even the SR-71 had subsonic flow in the engine's themselves. No one (excepting things like ramjets) have a supersonic flow inside the engines.

I believe it's called a convergent-divergent nozzle.

By its nature a high bypass fan, like the CFM, accelerates a large mass by smaller velocity Delta, supersonic flight requires a higher velocity delta meaning a relatively lower mass. Yes, the flow in all the example is subsonic at the inlet, it’s the flow at the tail end and how the engine and nacelle handle the large pressure increase as the inlet flow is slowed from supersonic to subsonic for the engine to use it.

Yes, an F-22 can supercruise as can the Concorde for that matter, but they don’t use high bypass fans. The Concorde used turbojets.

GF

As Andre3K states, exhaust velocity must exceed the vehicle's velocity for atmospheric flight, as net thrust is roughly the velocity difference multiplied with the (air) mass flow. (Bleed air and fuel mass flow are not accounted for here, but they tend to be one order of magnitude smaller than the core's mass flow.)

kitplane01 is right that gas turbines typically operate at comfortable subsonic speeds which reduces losses tremendously. This is achieved through careful inlet design: For subsonic engines, the inlet is a "simple" diffuser, while for supersonic engines it arranges a cascade of oblique shock waves (where the flow remains supersonic, to recover as much of the ram air pressure as possible) concluded by a normal shock wave that slows the flow to subsonic levels. The total pressure of bypass and core air is then increased by the engine: The bypass's air's through the fan, the core's through the compressor stages, where a major share is extracted again by the turbine(s) in order to turn the compressor(s) and fan.

The remaining pressure difference is then transformed into velocity in a nozzle, where the air is accelerated through expansion to ambient pressure. For a subsonic-subsonic acceleration, this nozzle will be convergent, for supersonic-supersonic acceleration it is divergent, and for subsonic-supersonic acceleration it is a convergent-divergent design where Mach = 1 is achieved in the smallest cross section. Accordingly, convergent nozzles are of interest for subsonic aircraft, and convergent-divergent nozzles for supersonic aircraft, as the air exiting the turbine or fan will always be subsonic. (Ideally, the convergent-divergent nozzle is adjustable as the supersonic plane has to be able to fly subsonically, too.)

In an ideal environment, the exhaust velocity achievable is purely dependent on the pressure ratio of the nozzle. For example, the ambient pressure needs to deceed 0.528 (air, Poisson constant = 1.4) of the pressure in front of the nozzle for the flow to reach Ma = 1. Higher relative ambient pressure will prevent the flow from becoming supersonic, no matter the nozzle design. With fan pressure ratios typically below 1.5 (ambient / pre-nozzle pressure ratio = 0.66) a high-bypass turbofan engine's bypass mass flow cannot become supersonic. Core mass flows may leave the turbine with slightly higher pressures, and thus may be expanded to supersonic levels. The overall mass flow, however, cannot become supersonic. Note that turbofan engines feasible for supersonic flight typically are low-bypass (bypass ratio = 0.3 .. 0.6) and feature multiple fan stages in order to achieve the necessary pressure ratio.

Accordingly, a high-bypass turbofan engine designed for subsonic flight cannot go supersonic. At the very least, a redesigned variable intake would be necessary.


(Long time lurker, first post. Best regards from Dresden, Germany!)

The proposed Aerion supersonic bizjet has apparently signed a contract with GE to develop an engine specific to the aircraft. Even through Flexjet apparently has placed orders for 15 aircraft: I'd be very surprised if it ever goes beyond the prototype. They're also apparently working with Lockheed Skunkworks:

http://www.youtube.com/watch?v=25K2t-vrlaY

In a perfect system, you could accelerate a bit above the exhaust speed of an air breathing jet engine, do to the added mass of fuel to the total flow.
In practice, yeah, you need to accelerate the exhaust above the speed you want for the plane. And that's not going to happen with high bypass turbofans. Military supercruising jets powered by turbofans do it with low-bypass multi-stage fans. Witch is probably the way any supersonic civilian jet would go..

There were built quite a lot of subsonic jets with zero bypass. Yes, inefficient and loud.
JT8D has bypass ratio of 0,96. And it is installed on 727, 737 and DC-9 all the way to MD-80 (only MD-90 has higher bypass).
Kuznetsov NK-321 has bypass ratio of 1,4, and it is on Tu-160 and Tu-144, both of which exceed Mach 2
Tay has bypass ratio of 3,1, and it is on Gulfstream IV. Honda HF120 has 2,9, and is on Hondajet and Cessna CJ.

For a plane designed to cruise at Mach 1,5...1,7, would bypass ratio 2...3 be sensible?

chornedsnorkack wrote:

There were built quite a lot of subsonic jets with zero bypass. Yes, inefficient and loud.
JT8D has bypass ratio of 0,96. And it is installed on 727, 737 and DC-9 all the way to MD-80 (only MD-90 has higher bypass).
Kuznetsov NK-321 has bypass ratio of 1,4, and it is on Tu-160 and Tu-144, both of which exceed Mach 2
Tay has bypass ratio of 3,1, and it is on Gulfstream IV. Honda HF120 has 2,9, and is on Hondajet and Cessna CJ.

For a plane designed to cruise at Mach 1,5...1,7, would bypass ratio 2...3 be sensible?

The JT8D was discussed as a potential engine for new supersonic transport aircraft. That should give you an idea for a reasonable bypass ratio ... definitely less than 2.0. Modern fighter jet engines are in the 0.3 - 0.6 bypass ratio range but 1.2 - 1.6 should still work fine on larger aircraft.
Perhaps unexpectedly, increasing bypass ratio does not always improve fuel efficiency. The faster you go, the smaller the optimum bpr. At around M2 - M3 you start entering the area where ramjets (with subsonic combustion) are better than turbojets.

The Swedes turned the JT-8D into a supersonic fighter engine—powered the Viggen.

GF

I always thought of the EJ200, Snecma M88 and F119/YF120 as good basis for engines to power hypothetical new supersonic transport aircraft. The first two for business jets and the latter for airliners.
All of them are relatively modern designs and conceived with supercruise in mind from the get go.

rjsampson wrote:

The proposed Aerion supersonic bizjet has apparently signed a contract with GE to develop an engine specific to the aircraft. Even through Flexjet apparently has placed orders for 15 aircraft: I'd be very surprised if it ever goes beyond the prototype. They're also apparently working with Lockheed Skunkworks:

http://www.youtube.com/watch?v=25K2t-vrlaY

I can confirm they are working with Skunkworks. But I can also say the engine they are working on is nowhere near what would be called a "high bypass" turbofan.

kitplane01 wrote:

Andre3K wrote:

kitplane01 wrote:

You can put any inlet you want. And I agree it would not work (I even said so in the original post). But the engine would produce some thrust .. I'm just wondering how inefficient it would be.

When it comes to atmospheric flight (not space flight) you have to have at least a unity of exhaust gas velocity if you want to travel at that speed and even then that's not a guarantee. So if the bypass air is moving at 550mph and the core air is moving at 700mph you could theoretically end up somewhere in between but never equal to the core exhaust due to it's relatively low thrust at altitude and the drag caused by the huge fan and other parts of the aircraft.

When it comes to space flight, it's not so much about the exhaust gas velocity, but more about how long it goes on and the mass loss. Space shuttle exhaust gas velocity is less than 9000mph yet it has no trouble reaching 17500mph in space. It just doesn't work like that in the atmosphere.

Lots of planes with turbofan engines can exceed Mach 1, even without the afteburner. The F-22 and F-35 can do so, and I think the Saab Gripen can too. I believe they all use an exhaust the accelerates the flow to the necessary speeds. Even the SR-71 had subsonic flow in the engine's themselves. No one (excepting things like ramjets) have a supersonic flow inside the engines.

I believe it's called a convergent-divergent nozzle.

The difference's being 1. None of the engines you mentioned are high bypass and 2. The core exhaust on all the mentioned engines is blended with the bypass exhaust BEFORE the propelling nozzle (blended in the afterburner) exit thus resulting is supersonic exhaust via the C/D nozzle.

It doesn't matter what the flow velocity is before the nozzle, only the velocity on nozzle exit matters. Unless you get into Scramjets.

Andre3K wrote:

kitplane01 wrote:

Andre3K wrote:

When it comes to atmospheric flight (not space flight) you have to have at least a unity of exhaust gas velocity if you want to travel at that speed and even then that's not a guarantee. So if the bypass air is moving at 550mph and the core air is moving at 700mph you could theoretically end up somewhere in between but never equal to the core exhaust due to it's relatively low thrust at altitude and the drag caused by the huge fan and other parts of the aircraft.

When it comes to space flight, it's not so much about the exhaust gas velocity, but more about how long it goes on and the mass loss. Space shuttle exhaust gas velocity is less than 9000mph yet it has no trouble reaching 17500mph in space. It just doesn't work like that in the atmosphere.

Lots of planes with turbofan engines can exceed Mach 1, even without the afteburner. The F-22 and F-35 can do so, and I think the Saab Gripen can too. I believe they all use an exhaust the accelerates the flow to the necessary speeds. Even the SR-71 had subsonic flow in the engine's themselves. No one (excepting things like ramjets) have a supersonic flow inside the engines.

I believe it's called a convergent-divergent nozzle.

The difference's being 1. None of the engines you mentioned are high bypass and 2. The core exhaust on all the mentioned engines is blended with the bypass exhaust BEFORE the propelling nozzle (blended in the afterburner) exit thus resulting is supersonic exhaust via the C/D nozzle.

It doesn't matter what the flow velocity is before the nozzle, only the velocity on nozzle exit matters. Unless you get into Scramjets.

For #1 .. that's totally true. And the point of the question. How much does high bypass hurt efficiency for a supersonic aircraft.
For #2 that's true. One would need to have a different intake and exhaust for supersonic flight.

Given that .. how much does the high bypass hurt?

Been answered, hurts 100% supersonic flight is not possible for high bypass engines.

GF

Super Sonic engines will always be less efficient and require a variable geometry exhaust nozzle. That costs money. Otherwise, you lose thrust on takeoff.

Subsonic engines currently have fixed nozzles, but a variable nozzle will save 2% to 3% . Pratt originally was going to put them on the fan only on the GTFs, but that cuts the fuel savings and then they looked into the weight and maintenance of the nozzle and decided it ruined 1-hour mission economics.

Expect it to be proposed for the 797. Mixed core/fan is required for supersonic.

What wasn't mentioned is multi-stage fans. That is required for super-duper. Fighter engines usually have three stage fans. There are exceptions (e.g., JT-8D powered planes).

The CFM-56 is being optimized for supersonic flight. The big difference will be the low spool. It must be redone.

The nacelle is really the biggest challenge. Variable inlets are a must (it can be as simple as Douglas's boundry layer control).

Lightsaber

lightsaber wrote:

What wasn't mentioned is multi-stage fans.

I did mention them.

lightsaber wrote:

The nacelle is really the biggest challenge. Variable inlets are a must (it can be as simple as Douglas's boundry layer control).

Lightsaber

Concorde´s intake ramps were not actually moving until Mach 1,7.
A supersonic plane optimized to stay under Mach 1,7, cruising at Mach 1,5 or Mach 1,6, could well omit the variable intake and just optimize a fixed intake.

Indeed. A diverterless supersonic intake might be a more cost effective solution at lower supersonic speeds.

chornedsnorkack wrote:

lightsaber wrote:

The nacelle is really the biggest challenge. Variable inlets are a must (it can be as simple as Douglas's boundry layer control).

Lightsaber

Concorde´s intake ramps were not actually moving until Mach 1,7.
A supersonic plane optimized to stay under Mach 1,7, cruising at Mach 1,5 or Mach 1,6, could well omit the variable intake and just optimize a fixed intake.

That 1.7 Mach number looked familiar, now
I remember why, Concorde would use afterburners to accelerate through Mach one and would keep them on until reaching
1.7 M where you say the intake ramps would start moving, their sophisticated design contributed significantly to not only
slow incoming air to subsonic speed but also created a large proportion of thrust



So once the afterburners were off the variable intake ramps ‘took over’ augmenting available thrust and allowed for hours of ‘super cruise’ years before that term was invented

chornedsnorkack wrote:

lightsaber wrote:

The nacelle is really the biggest challenge. Variable inlets are a must (it can be as simple as Douglas's boundry layer control).

Lightsaber

Concorde´s intake ramps were not actually moving until Mach 1,7.
A supersonic plane optimized to stay under Mach 1,7, cruising at Mach 1,5 or Mach 1,6, could well omit the variable intake and just optimize a fixed intake.

They began moving at M1.3. M1.7 was reheats off at around roughly 43,000ft. AT also armed. Then keep climbing and accelerating until the "corner point" between climb and M2.0 cruise climb.

The future looks like a non moving intake. Most of the second generation SST aircraft went this way. Europeans did, so did Russia too with a planned cryogenic fuelled SST.