Forum: Technology

Jet engine power has been going up over the years. Compare the old F-105 with the new F-35. They are two similarly-sized airframes with a single engine, yet the Thunderchief puts out only 26,500 pounds of thrust compared to the F-35's 40,000+ pounds of thrust.

My question is; how much more can jet engine power go up? Could we expect to see single-engined fighter planes with 100,000 pounds of thrust or more in the distant future? What are the ultimate limiting factors in jet engine efficiency? Surely a jet engine can only extract so much energy from its fuel? Does anyone know what kind of efficiencies modern jet engines have (that is, the percent of energy in the jet fuel that is converted into useful thrust)? Is it near the theoretical upper limit?

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Kryptid wrote:

My question is; how much more can jet engine power go up?

Jet engine performance and development is directly proportional to the product of $$ and time.

"Jet Engine" power is unlimited, but I will venture to say "Turbine Engine" power will reach a limit someday. Jet engines are a "reaction" engine. Mass is expelled at a high velocity in one direction, while the engine is forced in the opposite direction. Rockets are also a reaction engine.

I'd bet on newer PDE (Pulse Detonation Engine) military engines in the future, and not so much on turbine based jets. PDE is a step over turbines, like turbines stepped past pistons. I think they will be the next leap in high-performance military aircraft propulsion. SCRAM jets may be in the future mix too, but they are not as efficient as the PDE when it comes to extracting energy from fuel.

Kryptid wrote:

Could we expect to see single-engined fighter planes with 100,000 pounds of thrust or more in the distant future?

The possibility is there, it was less than 60 years ago, in 1951 when the PW J57 was the first jet engine to make 10,000lbs thrust (dry) in flight only 10 years after Whittle's W.1 made 860lbs. The GE90-115B currently holds the world record for thrust at 127,900 lbs which was set in 2002. I'm not sure if we should "expect to see" a fighter engine of this rating any time soon, but 100 years after Whittle's first jet engine, it may be a reality. It may not be "turbine based" but my guess says it will be a "reaction" engine of some type.

Kryptid wrote:

What are the ultimate limiting factors in jet engine efficiency?

Temperature - The temperature the metals can stand in the combustion chamber and turbine.
Materials - The capability of modern (future) materials to operate under higher stress and temperatures.

Kryptid wrote:

Surely a jet engine can only extract so much energy from its fuel?

True - JP fuel will only give so much energy per unit, and at some point you will reach the maximum.
But - There are other fuels out there that could be used that yield more power per unit.

Kryptid wrote:

Does anyone know what kind of efficiencies modern jet engines have (that is, the percent of energy in the jet fuel that is converted into useful thrust)?

Umm... I have a killer head-ache, and I've had a couple drinks tonight.

You do the math: http://www.hq.nasa.gov/pao/History/SP-468/app-f.htm

Kryptid wrote:

Is it near the theoretical upper limit?

I highly doubt it. The F119 and F135 are both more efficient and much hotter running engines. The actual numbers are still classified, but I'm sure their efficiency is much better than the F100/F110 series engines. Back in the 1990s the DoD asked the industry/NASA/DARPA/etc to increase performance of "fighter sized engines." Even if they only achieved a 25-50% gain in SFC it would be amazing.

Lets also not forget NASA has achieved over MACH 10 with the X-43 and it's SCRAM jet engine. It is the fastest combustion engine ever. While it did eventually melt down and crash, the X-43 is a stepping stone to more power in the future.

All in all, that's my Two Cents

(Don't forget the fine print)
Cheers

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TEG

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Thanks for the information. Those equations could really help me if I take some time to understand them.

I'm perplexed by all this.

How can a more powerful engine be more fuel efficient than a less powerful one?

Or is it because turbojets like Jumo 004, J47, VK-1, R-25-300 etc. were gas guzzlers and supercruising turbofans like F119, 117S and AL-41 fuel misers?

skyhigh wrote:

I'm perplexed by all this.

How can a more powerful engine be more fuel efficient than a less powerful one?

Or is it because turbojets like Jumo 004, J47, VK-1, R-25-300 etc. were gas guzzlers and supercruising turbofans like F119, 117S and AL-41 fuel misers?

For the same reason modern car engines are more efficient than older ones: by operating at higher temperatures and pressures under tighter tolerances with better control systems, less friction, and better-optimized aerodynamics. Also, turbofans are generally more efficient at lower speeds than turbojets.

Elite 1K Joined: May 26, 2005 - 08:39 PM Posts: 1496

Quote:

Also, turbofans are generally more efficient at lower speeds than turbojets.

Which is why the F119's bypass ratio is so low, it's geared towards high-speed operation rather than subsonic loiter.

The math on these things is pretty simple, well until you to actually have to calculate the numbers for an actual production engine.

Thermodynamic (how much energy is extracted for the chemical reaction) efficiency can be defined for a gas turbine engine using only pressure ratio. As that goes up, efficiency goes up. Most current jets flying right now run somewhere around 30% efficient with comercial being a little higher and military being a little lower. Your entire power grid from the generator to your house is about 35% efficient. The turbine to generate the power is probably 99% efficient with everything downstream being less efficient.

Right now GE and Rolls are working on integrating pulse det technology into turbine engines making it constant volume combustion as opposed to constant pressure combustion. This will probably only net about 1-2% better SFC but that's still pretty big.

Kryptid wrote:

My question is; how much more can jet engine power go up?

More than that i suspect...

An atmosphere isn't required when using rocket propulsion, arc-jets, or other "non-air-breathing" technology! Cool

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TEG

Sorry TEG, arc jets are air breathing as you use an electrical arc to superheat the air and it shoots out the back really fast. I remember seeing something about a nuclear engine a while back which was self contained and shot discreet particles out the back near c as propulsion.

Elite 1K Joined: May 26, 2005 - 08:39 PM Posts: 1496

Are you thinking ion engines?

I think that might be it. I watch too much discovery channel at times. It may have been a variation on an ion engine but that sounds right.

No... They're similar, but still different in basic design I believe; you could add ION Engine to my list! Cheers

But there are arc jets - even Wikipedia has an entry on this one....

Quote:

Arcjets are a form of electric propulsion for spacecraft, whereby an electrical discharge (arc) is created in a flow of propellant (typically hydrazine or ammonia). This imparts additional energy to the propellant, so that one can extract more work out of each kilogram of propellant, at the expense of increased power consumption and (usually) higher cost. Also, the thrust levels available from typically used arcjet engines are very low compared with chemical engines.

They're not large engines, but they're not "air-breathing" They're typically used for thrusters on satellite due to their fuel efficiency over long periods of time. (Similar to Ion Engines)

But you never know..... Wink

There have been other, inefficient, low-power, expensive and heavy engines that have met with great success!

Edgar Buckingham - US National Bureau of Standard in 1923 wrote:

"there does not appear to be, at present, any prospect whatever that jet propulsion of the sort here considered will ever be of practical value, even for military purposes."

Who'd have thunk it? Shrug

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TEG

I guess I was thinking about things wrong and referecing something Kryptid brought up earlier which was an arc ramjet. It makes sense for satellites as they can use the solar panels to charge the capacitors so you can generate the arcs. For anything doing really heavy moving, arc jets are trash, but it's always good to create and understand different types of propulsion as sometimes they can find a niche.

Are you the official forum Necromancer?

Forum: Technology

Engine power keeps increasing...