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kitplane01
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Supersonic Engine effeciency

Fri Jun 05, 2020 4:23 am

How much have supersonic engines become more efficient over time?

I'm thinking of supersonic commercial aircraft. Unlike fighters, such a plane would spend it's cruise in supersonic flight.

Obviously subsonic jet engines have gained efficiency, but the single biggest improvement (increased bypass ratio) doesn't work as well during supersonic flight.
 
flipdewaf
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Supersonic Engine effeciency

Fri Jun 05, 2020 6:18 am

kitplane01 wrote:
How much have supersonic engines become more efficient over time?

I'm thinking of supersonic commercial aircraft. Unlike fighters, such a plane would spend it's cruise in supersonic flight.

Obviously subsonic jet engines have gained efficiency, but the single biggest improvement (increased bypass ratio) doesn't work as well during supersonic flight.

Supersonics engines have a different trick though, they go fast. we are used to measuring the engines SFC (mass flow rate per unit thrust) but thrust is only useful if it’s helping you move so it’s good to compare this combined with SFC.

The Concorde Olympus engines had an SFC in the order of 1.195 lb/hr/lb which compared to a modern high bypass engine getting ~0.5lb/hr/lb doesn’t seem great however seeing as concord was going at Mach2.05 vs 0.83 for a modern jet or 2.46 times the speed then the useful work done (thrust x speed) by the engine was actually pretty good. Effectively 0.48 normalised to Mach 0.83.

So have supersonic engines been able to benefit from the ever increasing bypass ratios? Maybe not but then they were always better anyway....

Edit to add: Efficiency vs fuel consumption are very important terms. Efficiency is often referred to as the ratio of useful work out compared to the energy rate in. Bypass ratios don’t change the efficiency, they really change the effective production of thrust from a given amount of energy.

Fred


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LH707330
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Re: Supersonic Engine effeciency

Sat Jun 06, 2020 3:13 am

Concorde was great from a thermodynamic efficiency standpoint, what made it inefficient was the awful L/D that forced the engines to do so much more work.
 
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kitplane01
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Re: Supersonic Engine effeciency

Sat Jun 06, 2020 3:56 am

flipdewaf wrote:
kitplane01 wrote:
How much have supersonic engines become more efficient over time?

I'm thinking of supersonic commercial aircraft. Unlike fighters, such a plane would spend it's cruise in supersonic flight.

Obviously subsonic jet engines have gained efficiency, but the single biggest improvement (increased bypass ratio) doesn't work as well during supersonic flight.

Supersonics engines have a different trick though, they go fast. we are used to measuring the engines SFC (mass flow rate per unit thrust) but thrust is only useful if it’s helping you move so it’s good to compare this combined with SFC.

The Concorde Olympus engines had an SFC in the order of 1.195 lb/hr/lb which compared to a modern high bypass engine getting ~0.5lb/hr/lb doesn’t seem great however seeing as concord was going at Mach2.05 vs 0.83 for a modern jet or 2.46 times the speed then the useful work done (thrust x speed) by the engine was actually pretty good. Effectively 0.48 normalised to Mach 0.83.


Whoa!

I've never ever seen a speed normalizes SFC. Is that even a thing?

If a jet engine accelerates from 200 mph to 400 mph should I expect it's SFC to halve? If it goes from 1 mph to 0 mph should it's SFC go to infinity?
BTW, I totally get that work = thrust * speed.

flipdewaf wrote:
So have supersonic engines been able to benefit from the ever increasing bypass ratios? Maybe not but then they were always better anyway....

Edit to add: Efficiency vs fuel consumption are very important terms. Efficiency is often referred to as the ratio of useful work out compared to the energy rate in. Bypass ratios don’t change the efficiency, they really change the effective production of thrust from a given amount of energy.


Back to my now modified question, how have SFC at Mach 2 changed for engines between 1970 and 2020?
 
mxaxai
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Re: Supersonic Engine effeciency

Sat Jun 06, 2020 1:38 pm

kitplane01 wrote:
I've never ever seen a speed normalizes SFC. Is that even a thing?

If a jet engine accelerates from 200 mph to 400 mph should I expect it's SFC to halve? If it goes from 1 mph to 0 mph should it's SFC go to infinity?
BTW, I totally get that work = thrust * speed.

SFC stays (approximately*) constant. SFC = fuel / hour / thrust. If we compare two aircraft A and B with the same cruise thrust, we get the same fuel / hour for both. Now, if A flies faster than B for the same thrust, A uses less fuel / distance than B. Combined with the payload capability of A and B and non-fuel-related costs we could work out CASM. So SFC is just one of several factors that determine how efficient an aircraft is.

The only supersonic engines developed since the Concorde have been military, so up-to-date SFC numbers are a bit hard to find. This page (http://jet-engine.net/miltfspec.html) suggests that old turbojets like the GE J79 found on the F104, F4 and others had a SFC of ~0.84 lb / lbf h. More modern turbofans like the F100-PW-229 on the F15 & F16 achieve a SFC of ~0.73 lb / lbf h, and the B-1B is listed with a SFC of ~0.56 lb / lbf h. These are for static dry take-off thrust, though.

*SFC changes with environmental conditions (e. g. speed, air density) and thrust (take off SFC is different from cruise SFC).
 
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Starlionblue
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Re: Supersonic Engine effeciency

Sun Jun 07, 2020 12:55 am

Concorde B would have evolved Concorde into a much more sustainable product economically. Notably, it would have removed reheat entirely from the engines. A drooped leading edge would have dramatically improved L/D during takeoff and approach, plus there were numerous other aerodynamic tweaks.

http://www.concordesst.com/concordeb.html
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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kitplane01
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Re: Supersonic Engine effeciency

Sun Jun 07, 2020 5:43 am

mxaxai wrote:
kitplane01 wrote:
I've never ever seen a speed normalizes SFC. Is that even a thing?

If a jet engine accelerates from 200 mph to 400 mph should I expect it's SFC to halve? If it goes from 1 mph to 0 mph should it's SFC go to infinity?
BTW, I totally get that work = thrust * speed.

SFC stays (approximately*) constant. SFC = fuel / hour / thrust. If we compare two aircraft A and B with the same cruise thrust, we get the same fuel / hour for both. Now, if A flies faster than B for the same thrust, A uses less fuel / distance than B. Combined with the payload capability of A and B and non-fuel-related costs we could work out CASM. So SFC is just one of several factors that determine how efficient an aircraft is.

The only supersonic engines developed since the Concorde have been military, so up-to-date SFC numbers are a bit hard to find. This page (http://jet-engine.net/miltfspec.html) suggests that old turbojets like the GE J79 found on the F104, F4 and others had a SFC of ~0.84 lb / lbf h. More modern turbofans like the F100-PW-229 on the F15 & F16 achieve a SFC of ~0.73 lb / lbf h, and the B-1B is listed with a SFC of ~0.56 lb / lbf h. These are for static dry take-off thrust, though.

*SFC changes with environmental conditions (e. g. speed, air density) and thrust (take off SFC is different from cruise SFC).


I'm now so confused

I don't think one should multiply SFC * velocity to get a useful metric.

(fuel_used / thrust_produced ) * velocity
(kg / (kg * meters * seconds^2)) * (meters / seconds)
1/seconds

The units of the resulting product are just 1/seconds, which doesn't make sense (that I understand).

My understanding is that as a jet airplane's speed increases, the engines will either require more fuel or produce less thrust (or both). Also, there will be some changes in SFC, but not something linearly scaled with velocity.

There is a concept of efficiency that says an engine producing thrust at 0 mph has 0% efficiency, and that it's efficiency doubles as it goes from 1 mph to 2 mph. But I don't think that's an actionable engineering concept.
 
flipdewaf
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Re: Supersonic Engine effeciency

Sun Jun 07, 2020 9:19 am

kitplane01 wrote:
mxaxai wrote:
kitplane01 wrote:
I've never ever seen a speed normalizes SFC. Is that even a thing?

If a jet engine accelerates from 200 mph to 400 mph should I expect it's SFC to halve? If it goes from 1 mph to 0 mph should it's SFC go to infinity?
BTW, I totally get that work = thrust * speed.

SFC stays (approximately*) constant. SFC = fuel / hour / thrust. If we compare two aircraft A and B with the same cruise thrust, we get the same fuel / hour for both. Now, if A flies faster than B for the same thrust, A uses less fuel / distance than B. Combined with the payload capability of A and B and non-fuel-related costs we could work out CASM. So SFC is just one of several factors that determine how efficient an aircraft is.

The only supersonic engines developed since the Concorde have been military, so up-to-date SFC numbers are a bit hard to find. This page (http://jet-engine.net/miltfspec.html) suggests that old turbojets like the GE J79 found on the F104, F4 and others had a SFC of ~0.84 lb / lbf h. More modern turbofans like the F100-PW-229 on the F15 & F16 achieve a SFC of ~0.73 lb / lbf h, and the B-1B is listed with a SFC of ~0.56 lb / lbf h. These are for static dry take-off thrust, though.

*SFC changes with environmental conditions (e. g. speed, air density) and thrust (take off SFC is different from cruise SFC).


I'm now so confused

I don't think one should multiply SFC * velocity to get a useful metric.

kitplane01 wrote:
(fuel_used / thrust_produced ) * velocity
(kg / (kg * meters * seconds^2)) * (meters / seconds)
1/seconds

SFC is rate of fuel used per thrust produced so fuel used should be in kgs^-1
kitplane01 wrote:

The units of the resulting product are just 1/seconds, which doesn't make sense (that I understand).

My understanding is that as a jet airplane's speed increases, the engines will either require more fuel or produce less thrust (or both). Also, there will be some changes in SFC, but not something linearly scaled with velocity.

So if the SFC drops 10% with and increase of speed of 20% (Assuming the L/D remain as constant) then the fuel use per unit distance drops.
kitplane01 wrote:
There is a concept of efficiency that says an engine producing thrust at 0 mph has 0% efficiency, and that it's efficiency doubles as it goes from 1 mph to 2 mph. But I don't think that's an actionable engineering concept.


It’s a totally applicable engineering concept, it’s why my car turns itself off whilst doing 0mpg at the traffic lights.

Fred


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mxaxai
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Re: Supersonic Engine effeciency

Sun Jun 07, 2020 11:43 am

kitplane01 wrote:
I'm now so confused

I don't think one should multiply SFC * velocity to get a useful metric.

(fuel_used / thrust_produced ) * velocity
(kg / (kg * meters * seconds^2)) * (meters / seconds)
1/seconds

The units of the resulting product are just 1/seconds, which doesn't make sense (that I understand).

My understanding is that as a jet airplane's speed increases, the engines will either require more fuel or produce less thrust (or both). Also, there will be some changes in SFC, but not something linearly scaled with velocity.

There is a concept of efficiency that says an engine producing thrust at 0 mph has 0% efficiency, and that it's efficiency doubles as it goes from 1 mph to 2 mph. But I don't think that's an actionable engineering concept.

The 'unit' of SFC is ((fuel_mass / time) / thrust_produced), i. e. kg / (N * s) = kg / (kg * (m / s^2) * s) = [s / m] in SI units.

The interesting efficiency metric for a given aircraft is CASM, simplified to (fuel / (passenger * distance)).

If we multiply SFC with thrust, we get (fuel_mass / time / thrust_produced) * thrust_produced = fuel_mass / time, i. e. fuel flow in [kg / s].

Divide fuel flow by speed: (fuel_mass / time) / (distance / time) = (fuel_mass / distance) in [kg / m].

Divide this by the payload capacity and we get 'efficiency' as (fuel_mass / (distance * passenger)) in [kg / (m * passenger)].


So assuming two aircraft with the exact same engines - same thrust, same SFC - we can increase efficiency by increasing either speed or payload capacity. What we can change is the aerodynamic and structural efficiency of the aircraft. Looking purely at the engines, the Concorde has great engine efficiency compared to subsonic jets. However, the other aspects are not very efficient (especially the aerodynamics) so the overall aircraft becomes inefficient. We can see this in the fact that Concorde has almost as much thrust installed as the A330 despite carrying far fewer passengers over a far smaller distance.

For comparison, Concorde has a fuel flow at cruise of ~20,500 kg/h and the A332 has a fuel flow of ~5,500 kg/h. For a trip LHR-JFK @ M2.04 and @ M.82, respectively, the A330 can do the trip in 6:22 h using 35 tons of fuel, and Concorde can do the trip in 2:34 h using 52 tons of fuel. (Just a brief rule-of-thumb estimate, real world figures will vary).

So while engine efficiency definitely matters, future supersonic flight will also have to figure out how to improve L / D and structural weight.
 
mxaxai
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Re: Supersonic Engine effeciency

Sun Jun 07, 2020 3:26 pm

kitplane01 wrote:
My understanding is that as a jet airplane's speed increases, the engines will either require more fuel or produce less thrust (or both). Also, there will be some changes in SFC, but not something linearly scaled with velocity.

Addendum to the above:

SFC can be approximated as constant for a rough estimate but it does vary. You are correct that it does not scale linearly and the precise function depends on the engine in question. The variation is less than 10% typically. This study (http://www.srs.aero/wordpress/wp-conten ... alysis.pdf) contains a few nice graphs for the PW120 turboprop engine showcasing the variation.
 
flipdewaf
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Re: Supersonic Engine effeciency

Sun Jun 07, 2020 6:18 pm

The Breguet range equation:

R= (1/SFC)*U*(L/D) *ln(W1/W2)

You can see from here that the range for a given fuel use (W1 -W2) varies inversely with SFC and varies linearly with lift to drag ratio and velocity.

If you combine the U and SFC terms I.e. U/SFC you can get a useful comparative engine performance term for aircraft operating in somewhat different speed regimes.

It is in fact the same as velocity multiplied by impulse which is measured in seconds. So we would have time multiplied by velocity to get distance.

We don’t normally do this as we are comparing aircraft flying at relatively similar velocities, we normally apply the velocity along with the lift to drag ratios on a drag polar to demonstrate where minimum drag (maximum endurance) (L/Dmax) occurs and maximum specific range (UL/Dmax).

In terms of where the SFC improvements are happening in supersonic engines there are the same types of things happening in the core as in the commercial world, more blinks, ore single crystals, more efficient turbine cooling. The main changes coming (and already happening?) are around variable cycle tech which isn’t directly aimed at the SFC at high Mach directly but around being able to Taylor the operating conditions of the engine more clearly to the flight regime on the fly ( pun intended).

Fred


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CowAnon
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Re: Supersonic Engine effeciency

Tue Jun 09, 2020 7:41 am

kitplane01 wrote:
mxaxai wrote:
kitplane01 wrote:
I've never ever seen a speed normalizes SFC. Is that even a thing?

If a jet engine accelerates from 200 mph to 400 mph should I expect it's SFC to halve? If it goes from 1 mph to 0 mph should it's SFC go to infinity?
BTW, I totally get that work = thrust * speed.

SFC stays (approximately*) constant. SFC = fuel / hour / thrust. If we compare two aircraft A and B with the same cruise thrust, we get the same fuel / hour for both. Now, if A flies faster than B for the same thrust, A uses less fuel / distance than B. Combined with the payload capability of A and B and non-fuel-related costs we could work out CASM. So SFC is just one of several factors that determine how efficient an aircraft is.

The only supersonic engines developed since the Concorde have been military, so up-to-date SFC numbers are a bit hard to find. This page (http://jet-engine.net/miltfspec.html) suggests that old turbojets like the GE J79 found on the F104, F4 and others had a SFC of ~0.84 lb / lbf h. More modern turbofans like the F100-PW-229 on the F15 & F16 achieve a SFC of ~0.73 lb / lbf h, and the B-1B is listed with a SFC of ~0.56 lb / lbf h. These are for static dry take-off thrust, though.

*SFC changes with environmental conditions (e. g. speed, air density) and thrust (take off SFC is different from cruise SFC).


I'm now so confused

I don't think one should multiply SFC * velocity to get a useful metric.

(fuel_used / thrust_produced ) * velocity
(kg / (kg * meters * seconds^2)) * (meters / seconds)
1/seconds

The units of the resulting product are just 1/seconds, which doesn't make sense (that I understand).

My understanding is that as a jet airplane's speed increases, the engines will either require more fuel or produce less thrust (or both). Also, there will be some changes in SFC, but not something linearly scaled with velocity.

There is a concept of efficiency that says an engine producing thrust at 0 mph has 0% efficiency, and that it's efficiency doubles as it goes from 1 mph to 2 mph. But I don't think that's an actionable engineering concept.


kitplane01, there are a couple of mistakes:

  1. Work does not equal thrust * speed. Instead:
    power = rate of work per unit time = thrust * speed
  2. SFC * velocity does not result in a (1/time) unit of measure. When the amount of fuel used is expressed as mass:

    SFC = fuel usage / thrust
    = (mass / time) / (mass * distance / time^2)
    = time / distance

    SFC * speed = (time / distance) * (distance / time)
    = non-dimensional value

On the second point, I do agree that SFC * velocity doesn't seem like a useful metric. You want SFC to be low and velocity to be high, but if you multiply them, you get a medium value that you could duplicate by making the SFC high and the velocity low (a bad combination).

Also, SFC can be confusing since it's sometimes calculated with the fuel amount expressed as weight (downward force), where W=mg (g being 9.81 m/sec^2, the gravitational constant at or near the earth's surface), instead of being expressed as mass:

SFC = fuel usage / thrust
= (force / time) / (force)
= 1 / time

This form of SFC was used in the previous post about the Breguet equation. I find SFC easier to understand in this form, as lb/lb/hr is more intuitive to me than g/kN/s. Also, lb/lb/hr could also be written as kg/kg-force/hr without having to do any numerical conversion; that seems to be the preferred form of SFC in use in Russia and Ukraine.
 
kalvado
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Re: Supersonic Engine effeciency

Tue Jun 09, 2020 6:06 pm

CowAnon wrote:

On the second point, I do agree that SFC * velocity doesn't seem like a useful metric. You want SFC to be low and velocity to be high, but if you multiply them, you get a medium value that you could duplicate by making the SFC high and the velocity low (a bad combination).

Physics would tell work = force*distance, so there has to be some sense in all this. That force*distance has to transform into (thrust/velocity) - divided, not multiplied.
Another way to look at it is to talk about sfc in per-mile basis - how much fuel you need to maintain that thrust for 1 mile of flight. Increase of fuel burn in kg/(hour*lbf) ** can be offset by the shorter flight time at higher flight speed. After all, the amount of fuel spent on A to B flight is the ultimate metric in many aspects. In that sense, Concorde "poor" SFC is offset by shorter time of supersonic flight.

Now that would work great in some subsonic range, where lift, drag and velocity have linear dependance. Supersonic problem is that drag is much higher than in subsonic mode, and is not nicely proportional to velocity - so equal (lb fuel)/(mile of fixed thrust) doesn't do much good as more thrust is needed.


**I specifically mix different units from different systems to make things separate.
 
tommy1808
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Re: Supersonic Engine effeciency

Fri Jun 12, 2020 11:55 am

mxaxai wrote:
The only supersonic engines developed since the Concorde have been military, so up-to-date SFC numbers are a bit hard to find. This page (http://jet-engine.net/miltfspec.html) suggests that old turbojets like the GE J79 found on the F104, F4 and others had a SFC of ~0.84 lb / lbf h. More modern turbofans like the F100-PW-229 on the F15 & F16 achieve a SFC of ~0.73 lb / lbf h, and the B-1B is listed with a SFC of ~0.56 lb / lbf h. These are for static dry take-off thrust, though.


And at supersonic speed the fan in a turbofan becomes pretty much useless as far as producing thrust is concerned.

That is why the Aerion AS2 will only fly Mach 1.4. Needs to have some bypass to pass noise regulations, but even with a puny 1:3 bypass ratio flying faster becomes impractical. And that is in the super expansive high end business travel environment, where fuel cost are probably a smaller concern than for airlines.

Best regards
Thomas
Well, there is prophecy in the bible after all: 2 Timothy 3:1-6
 
mxaxai
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Re: Supersonic Engine effeciency

Fri Jun 12, 2020 1:12 pm

tommy1808 wrote:
mxaxai wrote:
The only supersonic engines developed since the Concorde have been military, so up-to-date SFC numbers are a bit hard to find. This page (http://jet-engine.net/miltfspec.html) suggests that old turbojets like the GE J79 found on the F104, F4 and others had a SFC of ~0.84 lb / lbf h. More modern turbofans like the F100-PW-229 on the F15 & F16 achieve a SFC of ~0.73 lb / lbf h, and the B-1B is listed with a SFC of ~0.56 lb / lbf h. These are for static dry take-off thrust, though.


And at supersonic speed the fan in a turbofan becomes pretty much useless as far as producing thrust is concerned.

That is why the Aerion AS2 will only fly Mach 1.4. Needs to have some bypass to pass noise regulations, but even with a puny 1:3 bypass ratio flying faster becomes impractical. And that is in the super expansive high end business travel environment, where fuel cost are probably a smaller concern than for airlines.

Best regards
Thomas

That is not entirely true, small bypass ratios (in the 0.2:1 - 1:1 range) still offer a benefit at supersonic speeds, especially for dry thrust. However, you can see the massive difference to subsonic turbofans with BPR of 10:1 and higher. It is also correct that the ideal BPR becomes smaller the faster you go, so at M2+ the difference to a pure turbojet becomes negligible and if you go faster a ramjet becomes preferable. There is also the added challenge that up to ~M1.5 a simple inlet such as found on the AS2, F35, F16 or Rafale is perfectly fine, but higher speeds profit significantly from a ramp style inlet like those found on Concorde, F15, MiG-31 or the Eurofighter, or a clean conical inlet like on the SR71 and MiG-21. This adds weight and complexity, though.
 
cpd
Posts: 6299
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Re: Supersonic Engine effeciency

Sat Jun 27, 2020 9:34 pm

mxaxai wrote:
kitplane01 wrote:
I'm now so confused

I don't think one should multiply SFC * velocity to get a useful metric.

(fuel_used / thrust_produced ) * velocity
(kg / (kg * meters * seconds^2)) * (meters / seconds)
1/seconds

The units of the resulting product are just 1/seconds, which doesn't make sense (that I understand).

My understanding is that as a jet airplane's speed increases, the engines will either require more fuel or produce less thrust (or both). Also, there will be some changes in SFC, but not something linearly scaled with velocity.

There is a concept of efficiency that says an engine producing thrust at 0 mph has 0% efficiency, and that it's efficiency doubles as it goes from 1 mph to 2 mph. But I don't think that's an actionable engineering concept.

The 'unit' of SFC is ((fuel_mass / time) / thrust_produced), i. e. kg / (N * s) = kg / (kg * (m / s^2) * s) = [s / m] in SI units.

The interesting efficiency metric for a given aircraft is CASM, simplified to (fuel / (passenger * distance)).

If we multiply SFC with thrust, we get (fuel_mass / time / thrust_produced) * thrust_produced = fuel_mass / time, i. e. fuel flow in [kg / s].

Divide fuel flow by speed: (fuel_mass / time) / (distance / time) = (fuel_mass / distance) in [kg / m].

Divide this by the payload capacity and we get 'efficiency' as (fuel_mass / (distance * passenger)) in [kg / (m * passenger)].


So assuming two aircraft with the exact same engines - same thrust, same SFC - we can increase efficiency by increasing either speed or payload capacity. What we can change is the aerodynamic and structural efficiency of the aircraft. Looking purely at the engines, the Concorde has great engine efficiency compared to subsonic jets. However, the other aspects are not very efficient (especially the aerodynamics) so the overall aircraft becomes inefficient. We can see this in the fact that Concorde has almost as much thrust installed as the A330 despite carrying far fewer passengers over a far smaller distance.

For comparison, Concorde has a fuel flow at cruise of ~20,500 kg/h and the A332 has a fuel flow of ~5,500 kg/h. For a trip LHR-JFK @ M2.04 and @ M.82, respectively, the A330 can do the trip in 6:22 h using 35 tons of fuel, and Concorde can do the trip in 2:34 h using 52 tons of fuel. (Just a brief rule-of-thumb estimate, real world figures will vary).

So while engine efficiency definitely matters, future supersonic flight will also have to figure out how to improve L / D and structural weight.


I’m guessing future supersonic aircraft will be a lot lighter using more modern materials. For the engines I’m not so sure as the proposed advanced engine designs were never built, such as those mid-tandem-fan designs and the like.

The Concorde-B Olympus engines were a pretty realistic proposition and not a huge stretch of technology.

I also wonder how engines like the GE-4 compared. That was a very powerful engine.

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