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Rocket engine vs jet engine efficiency

by Noham1
Tags: efficiency, engine, rocket
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Noham1
#1
Jul31-14, 07:27 PM
P: 5
Which are more efficient, Rocket Engines or Jet engines, and why? It would make sense that rocket engines are more efficient because they aren't effected by air pressure but I have been told by some people that actually Jet engines are more efficient so I am confused. If you know of any link which can explain this all to me, please post it so I can take a look.
thanks
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russ_watters
#2
Jul31-14, 09:28 PM
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Welcome to PF!

You'll have to define "efficiency" for us -- there are lots of different ways to describe it.
Noham1
#3
Jul31-14, 10:03 PM
P: 5
Sorry I was unclear, I was referring to energy efficiency, as in how much of the energy is used for propulsion, and how much is wasted.

russ_watters
#4
Jul31-14, 11:18 PM
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Rocket engine vs jet engine efficiency

Propulsion engines don't output energy, they output thrust, so you can't have that as an efficiency measure. One common one though is specific thrust or specific impulse:
http://en.wikipedia.org/wiki/Specific_impulse

Specific impulse (usually abbreviated Isp) is a way to describe the efficiency of rocket and jet engines. It represents the force with respect to the amount of propellant used per unit time.
In this measure, jet engines benefit by not counting the air in the calculation, which is part of the reason they have higher specific thrust than rockets.
vemvare
#5
Aug1-14, 07:46 AM
P: 24
I think the calculating efficiency of turning energy into thrust in rockets should be as easy as the input energy (as in the chemical energy of each kg of fuel)/ (exhaust velocity2 /2)
Noham1
#6
Aug1-14, 12:04 PM
P: 5
You said per unit time. Is there a measure of efficiency based on distance and not time. It is understandable that it is more efficient based on time. But rockets are generally faster. Do you know which is more efficient based on time?
sday
#7
Aug1-14, 01:50 PM
P: 34
When u say efficiency over time do you mean just in quantity of fuel burnt, cost of fuel burnt or total cost of the most efficient jet engine vs. most efficient rocket engine. (Cost to operate) not to mention if speeds get high enough now the hull cost goes up for rocket due to heat not to mention cost of exotic material to go really fast. Etc
sday
#8
Aug1-14, 01:51 PM
P: 34
Like saying a Prius is more efficient than a traditional car, but once you start factoring the cost of batteries, manufacturing, etc the gap narrows
jackwhirl
#9
Aug1-14, 03:49 PM
P: 8
All rocket engines are jet engines, but not all jet engines are rocket engines.

Quote Quote by Noham1 View Post
It would make sense that rocket engines are more efficient because they aren't effected by air pressure...
Are you trying to compare the efficiency of a rocket in a (near) vacuum to that of a turbojet engine in atmosphere?

Reading the link russ_watters provided, it seems that jet engines are more fuel efficient.
http://en.wikipedia.org/wiki/Specifi...Larger_engines shows that rockets tend to use more mass of fuel per unit of thrust than non-rocket jet engines.

That said, you can't use an airbreathing jet engine in space, so they each have their strengths.
Noham1
#10
Aug1-14, 04:50 PM
P: 5
Quote Quote by jackwhirl View Post
Reading the link russ_watters provided, it seems that jet engines are more fuel efficient.
http://en.wikipedia.org/wiki/Specifi...Larger_engines shows that rockets tend to use more mass of fuel per unit of thrust than non-rocket jet engines.
What I am still confused about is why. Jet engines have to compress the air and require energy (fuel) to turn the turbines while rockets already have the compressed air, so they can skip the turbine compression part of the jet engine. Wouldn't that make rockets engines more fuel efficient per distance travelled (while traveling straight) no matter the altitude?
Noham1
#11
Aug1-14, 04:56 PM
P: 5
To be more specific about what I'm looking for, it is Thrust specific fuel consumption. As in mass of fuel used In a given period of time/thrust produced over that period of time.

http://en.m.wikipedia.org/wiki/Thrust_specific_fuel_consumption

Which has a lower TSFC? Rockets or Jets? And why?
CombiImp
#12
Aug1-14, 06:23 PM
P: 1
Quote Quote by Noham1 View Post
To be more specific about what I'm looking for, it is Thrust specific fuel consumption. As in mass of fuel used In a given period of time/thrust produced over that period of time.

http://en.m.wikipedia.org/wiki/Thrus...el_consumption

Which has a lower TSFC? Rockets or Jets? And why?
Jets. The reason is that a rocket must carry both its fuel and oxidizer. A jet, OTOH, gets it's oxidizer from the air around it, so it doesn't need the mass of oxidizer (that gets consumed) or the mass of tanks and piping for the oxidizer (which aren't part of the fuel consumption, but do figure in the overall power to weight ratio for the vehicle). Since the oxidizer doesn't count in the mass of "fuel" consumed for jets but does for rockets, the jet has the edge. Isp figures for chemical rockets are typically a few hundred seconds, whereas for jets you can get several thousand. This makes sense given that jets often need to cruise for several hours, to compare to this, a rocket will run through its fuel in a few minutes (admittedly, usually while putting out more thrust than its jet counterpart).
Chronos
#13
Aug1-14, 06:53 PM
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A practical way of defining engine efficiency is thrust to weight ratio. A rocket tends to be less efficient because it must carry its own oxidizer whereas a jet takes it from the atmosphere. For a more in depth discussion, see http://www.grc.nasa.gov/WWW/k-12/airplane/bgp.html
sday
#14
Aug2-14, 09:36 AM
P: 34
One thing to throw into the discussion is that when speaking of efficiency, we are talking about a high bypass ratio jet engine vs low bypass. All modern jet engines with high efficiencies are the high bypass ratio. Look at a B777 and the diameter of the engine nacelle is huge compared to the actual engine. I can't remember the exact ratio but I think it's something around 80% of the thrust is generated from the air going around the engine, not through it. A rocket would be like sitting in a boat filled with tennis balls, a gas powered tennis ball serving machine and the fuel to power it. So the boat has 1 ton of tennis balls, the gas and the machine. The faster the balls are served out the back of the boat, the greater the amount of thrust you generate. Or you connect the gas machine to a propeller, remove the weight of the tennis balls and expend less energy using the medium around you (the water) with a much lighter boat without the weight of propellent, the tennis balls. If you operate the propeller at the same fuel consumption rate as the tennis ball serving machine, the propeller would move the boat faster than throwing out the tennis balls. Especially initially with the weight of the balls in the boat. If you could compare the very last ball thrown vs the amount of thrust generated by the propellor during that same period of time... I'd say it is possible maybe even likely that the throw of the tennis ball on the last one generates more thrust per unit of time than the propellor, but during the majority of the test, the weight of the balls would cancel out that perfect conversion of potential energy to thrust. (just a gut feeling on that, maybe a seasoned physicist could chime in on that). I think the drag generated by the propellor moving through the water, the rotational drag of the propellor, turbulence around the propellor and the thrust vector being off axis would all contribute to inefficiencies that would not be present in converting mechanical power from gas engine to thrust. Throwing the tennis ball is like a rocket with more direct conversion to thrust.

No wonder spaceship one uses a Jet to get things started. A rocket at rest that must go vertical has an enormous task of lifting itself from a velocity of zero when it has 100% of the propellent at start. Or you take it to altitude with a jet, drop the rocket which is already traveling at say 400mph, simply modify the direction vector with the elevator, then ignite the rocket. You didn't need any of that heavy propellent to get you to altitude and speed. Think about the space shuttle. The majority of the fuel is used for the first 25% of the flight. A little off track, but I think looking at those two examples shows the strengths of each. Inside the atmosphere the Jet is more efficient, in space... the rocket wins. :)
Doug Huffman
#15
Aug2-14, 11:48 AM
P: 101
Quote Quote by Noham1 View Post
[ ... ]It would make sense that rocket engines are more efficient because they aren't effected by air pressure ... [ ... ]
Not so. See nozzle/bell design.
Lsos
#16
Aug6-14, 06:07 AM
P: 777
Quote Quote by Noham1 View Post
What I am still confused about is why. Jet engines have to compress the air and require energy (fuel) to turn the turbines while rockets already have the compressed air, so they can skip the turbine compression part of the jet engine. Wouldn't that make rockets engines more fuel efficient per distance travelled (while traveling straight) no matter the altitude?
Keep in mind that you do get back the energy from compressing the air in a turbojet engine. It gets expanded through the turbine, on top of the fact that now it's hotter and moving much faster.

Besides, a rocket engine has to also "compress" the fuel. Not in the same sense as an air-breathing engine, but a rocket engine has to push a ton (quite literally :P) of fuel into the combustion chamber, and it has to do so against the incredible pressure that is in there. Power requirements of many tens of thousands of horsepower is not uncommon. Probably not much compared to the total rocket output, or relative to a compressor in a turbojet engine, but it's still there. Perhaps it's not relevant to this discussion, but it's enough of an engineering challenge that I've heard launch vehicles being described as a "turbopump with a rocket attached".
Darryl
#17
Aug13-14, 07:55 PM
P: 109
I would say the efficiency of jet engines is highly depended on the medium it is travelling in, in air it has 'good' efficiency, as it does not have to carry around its own oxygen supply, less weight.

but in the vacuum of space on in water, jets are not so good, I would even say their thrust and efficiently would tend towards zero.
But I could imagine a jet engine to be highly efficient in a pure oxygen environment.


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