What is the difference between Boeing 777 aircraft engines & Apollo rocket engines.

Do these two types of engine work the same way?

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    $\begingroup$ Did you read the Wikipedia article on rocket engines, particularly the introductory portion and the section on a rocket engine's principle of operation? Compare for example the second paragraph in the introductory section of Wikipedia's article on jet engines, as well as the section on rockets. $\endgroup$ – a CVn Nov 7 '16 at 14:47
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    $\begingroup$ Before asking on this website, you should perform basic research so that your question is quite more specific. A quick search provides you the fact that a B777 uses jet engines (as for most -if not all- commercial long haul airliners) whereas Saturn V (the Apollo rocket) uses rocket engine (as for all rockets). $\endgroup$ – Manu H Nov 7 '16 at 16:45
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    $\begingroup$ One is a jet turbine engine, the other is a rocket engine, they are completely different. $\endgroup$ – Steve Kuo Nov 7 '16 at 17:03
  • $\begingroup$ Where does one begin to answer this, each first stage engine in their own right represented the single most powerful engine on the planet. First stage F1 engines were designed to operate up to 70-80KM, 777 turbofans cannot operate to even half this altitude. At MECO, those first stage engines are a throw away piece of trash... 170 seconds to climb to 70Km and nearly 100Km down range, 777 Engines.... 50K hours is possible. The rocket carries an oxidizer that has been compressed and cooled prior to fuelling the tanks a jet engine sucks in the air and compresses it then expands it, $\endgroup$ – jCisco Dec 6 '16 at 12:26
  • $\begingroup$ What is the difference between an AK-47 and an RPG? Do these two types of weapon work the same way? $\endgroup$ – Robert Columbia Sep 16 '17 at 2:13

There are two main differences.

A rocket engine does not need an external source of oxygen for combustion and has only one opening - the exhaust. It gets oxygen from its own fuel supply, typically using liquid oxygen or from another oxidiser.

A jet engine, such as those used on 777s, needs to draw in external air to get oxygen for its combustion and has two openings, the exhaust and the inlet for air.

Both use combustion of fuel to push hot expanding gases from the exhaust and Newtons 3rd law tells us that an opposite and equal reaction to the exhaust of these gases is thrust, propelling the engine forward.

Note. Some rocket engines, such as cold gas reaction engines, do not use combustion but the ones most people think of as "rocket engines" do.

Here is a diagram of the F1 engine used on the Apollo Saturn V launchers.

enter image description here

Source history.nasa.gov

Here is a diagram of a typical jet engine, the Rolls Royce Trent.

enter image description here Source: rolls-royce.com

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    $\begingroup$ Liquid oxygen (LOX) is an oxidizer, because oxygen is an oxidizer. It is one of several possible oxidizers with different properties. Another main contender is nitrogen tetroxide (N2 O4). See also. $\endgroup$ – a CVn Nov 7 '16 at 14:38

Sort of, but the comparison should be made with a turbojet (e.g. first Boeing aircraft, or military Bell X-14) rather than a turbofan.

  • A turbofan, in addition of producing gases (like a rocket engine and a turbojet) also accelerates cold air that is not used to burn fuel but produces most of the thrust (this air is called bypass flow or cold flow). We can say that in a turbofan, thrust is produced by pure cold air and hot gases are nearly only used to spin the big fan which accelerates cold air.

  • A turbojet, like a rocket engine, produces accelerated hot gases by combustion. This is the only source of thrust for both engines. I've added the turbojet principle at the end of this post for reference.

Rocket engine specificity

A notable difference is that rocket engines of this time were designed to be started only a few times, and for the first stage only once (the upper stage used for orbital maneuvers can be started a few times).

A rocket engine must eject gases at very large velocity to accelerate the payload up to 7 km/s, while a B777 travels at less than the speed of sound (0.3 km/s). This also creates a difference in the design and working at Mach 8 rather than Mach 0.8 is much more complex.

Saturn V used for Apollo missions had 3 stages, the first stage using rocket fuel and the other using liquid hydrogen as fuel. Let's look at the second stage engine. the J-2 worked with liquid hydrogen being oxidized with liquid oxygen, the result giving water and heat. Both propellants were drawn from cryogenic (low temperature) tanks and were hugely pressurized by turbo-pumps (8,450 kPa/1,225 psi)t rotating at 8,600 rpm.

enter image description here

Hot burnt gases were ejected like in an aircraft turbojet to produce thrust, albeit the ejection nozzle looked more like an enormous bell than a standard turbojet nozzle.

enter image description here
(Saturn V second stage J-2 engine. Source: Wikipedia)

There was no cold flow like in the B777 turbofan, air was not in sufficient quantity and no fan would have worked.

enter image description here
B777 turbofan principle, source

For reference:

enter image description here
Civil turbojet principle, source

  • $\begingroup$ I thought the Saturn V first stage (S-IC) used F-1 engines burning RP-1 and LOX. The second and third stages (S-II and S-IVB respectively) used LH2/LOX burning J-2 engines. $\endgroup$ – Fred Larson Nov 7 '16 at 14:46
  • $\begingroup$ Re "...rocket engines are designed to be started only a few times, and for the first stage only once...": Not universally true, e.g. SpaceX's first stage engines, or the Shutte main engines. $\endgroup$ – jamesqf Nov 7 '16 at 18:12
  • $\begingroup$ @FredLarson: Ah I stand corrected, this is indeed the second stage. That was a so long time ago, I had forgotten. $\endgroup$ – mins Nov 7 '16 at 21:14
  • $\begingroup$ @jamesqf: The main engines of the Orbiter are never restarted during the flight, it would be difficult as the external tank has been jettisoned at this end of the ascent. The orbital maneuvers are done with the OMS, including the deorbit burn, smaller attitude adjustments are done with thrusters on the RCS. $\endgroup$ – mins Nov 7 '16 at 21:20
  • $\begingroup$ @mins: I wasn't clear on what you meant. The engines are started once per launch, but are used for a number of launches, so restarted that number of times. $\endgroup$ – jamesqf Nov 8 '16 at 4:29

In rocket engines, the thrust is generated by the fuel particles - there is only combustion chamber and nozzle.

In jet engines (turbofan), the thrust is generated mainly by air flowing through fan never reaching the combustion chamber.


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