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I can't find the reason why rockets are able to produce so much thrust when compared to turbojets.

I do know that the rockets carry their own own supply of oxygen since there is no oxygen in space or the upper region of our atmosphere. And that the upper stages use hydrogen for longer range.

But given that RP-1 is kerosene basically. A highly refined form of kerosene that is used in jets but kerosene nevertheless. Is there something else that gives the first stage of the rocket it's unbelievable thrust or is it just all down to the RP-1 fuel?

Rodrigo de Azevedo
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    You'd not get many hours of use out of a jet engine that was outputting thrust similar to that of your average space fairing rocket, and even if you could, most passengers wouldn't be comfortable sitting ontop of the kind of fuels that could produce it. – Dan Dec 26 '15 at 18:51
  • Yes, But how is it that it's able to squeeze so much thrust even though it's only for a short time? is it the fuel or is it something else? –  Dec 26 '15 at 18:55
  • Well, it's a little of everything - I don't think there's a single answer to this. I mean, apart from both relying on the expulsion of gasses to create thrust, your average rocket engine doesn't really share anything in common with your average commercial jet engine. Different working principles, different use cases, different design etc. You may aswell ask why a 747 can't go into space in my opinion – Dan Dec 26 '15 at 19:10
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    Does it help that the fuel is burning with the oxidizer LOx instead of air where only 21% is oxygen? –  Dec 26 '15 at 19:55
  • @F.Sherrif that only helps from the standpoint that the rocket can work in environments without adequate oxygen and that the fuel/oxidizer ratio can be optimized for the entire burn. Your question could be improved by stating what you know about rocket engines so that the scope can be narrowed from writing an entire article on how rockets work. – casey Dec 26 '15 at 21:06
  • "As they push the Shuttle toward orbit, the engines consume liquid fuel at a rate that would drain an average family swimming pool in under 25 seconds generating over 37 million horsepower." (source). This mass is ejected to provides thrust (in addition of boosters thrust). Note that the specific impulse of SSME is very much lower than a turbofan, meaning they are less efficient. – mins Dec 26 '15 at 23:39
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    Compare the size of the fuel tank (LH2) of the Buran shuttle (source) with an aircraft, and imagine this is just for a 10 mn flight. The fuel rate explains why the thrust is so high, while the engines are less efficient. Very efficient turbo-pumps are used to provide this rate. – mins Dec 27 '15 at 00:02
  • It's the other way round: rocket first stages are required to accelerate very large fuel loads vertically, to much higher speeds than aircraft. Therefore they are required to produce a very large amount of power and built accordingly. – pjc50 Dec 27 '15 at 02:23
  • Rocket fuel is under very high pressure, allowing it to be consumed at a much faster rate - jet engines use propellers to create pressure from the intake, but there is a limit as to how much can actually be produced due to its spinning nature, whereas rockets don't have this limitation. Also as F.Sherrif mentioned, they are burning much purer fuels. – user2813274 Dec 27 '15 at 16:36

7 Answers7

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For one, don't only look at the engine, but at the whole propulsion system. This includes tanks, piping, controls, pumps and the actual engine. Now the rocket looks much less favorable, especially if you size the tanks for equal running times.

The rocket does not need any of the parts which are ahead of the combustion chamber of a jet and also does not need the turbine. Also, being designed for full thrust only, it does not need an adjustable nozzle. Please look below at the engine installation of a typical airliner (I tried but could not find a fitting cross section of a turbojet plus intake):

Jet engine and nacelle cutaway drawing

Jet engine and nacelle cutaway drawing (picture source). As @Talisker correctly observed in the comments, the labels "high speed jet" and "low speed jet" have to be swapped in order to be correct.

Only the part labeled "combustor" and the section aft of the turbine are actually comparable to a rocket engine - all else is needed to condition and compress air or drive the turbo machinery in front. A rocket enjoys the luxury of being fed propellant and oxidizer at just the right ratio, condition and at high pressure, and since the oxidizer is mostly pure liquid oxygen, the turbo pumps for compressing it can be much smaller than the turbo machinery of a jet which works with an 80% nitrogen - 20% oxygen mixture of gasses.

Peter Kämpf
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  • Thank you sir. But when you say the fuel and oxidizer are mixed at just the right ratio. Do you mean more oxygen can be added to extract more thrust rather than a turbojet would have to make do from the oxyen in the compressed air. –  Dec 26 '15 at 22:51
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    @RoryAlsop, turbojet will have much better ratio of oxygen to fuel, actually. Most run with plenty of excess oxygen to achieve good combustion. But since you can't extract more power from given amount of fuel by adding more oxygen (above the stoichiometric ratio) anyway, it does not matter! – Jan Hudec Dec 26 '15 at 23:28
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    @F.Sherrif Rockets use just enough oxygen to heat the mixture to a temperature before too much energy is consumed by ionization, resulting in a lower density of the flow and thus a higher specific impulse. Jets would prefer to have less oxygen, but need the mass flow for thrust. They accelerate the exhaust flow by much less than a rocket. – Peter Kämpf Dec 26 '15 at 23:31
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    @F.Sherrif, you can't extract more thrust from the same amount of fuel by adding more oxygen. However since liquid oxygen pump can provide much more oxygen than an air compressor of comparable size, a rocket engine can be built to burn much more fuel per unit of time. – Jan Hudec Dec 26 '15 at 23:34
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    @JanHudec That is the key point I missed: The pumps in rocket motors are fed liquids, not gasses. This allows them to be much smaller. – Peter Kämpf Dec 26 '15 at 23:52
  • Thank you very much Peter Kampf and Jan Hudec for your answers :) I do understand now. –  Dec 27 '15 at 00:23
  • @PeterKämpf Just out of curiosity, say, a 3-4 hour flight had to have all its oxidizing O2 carried along in a cryogenic tank how big would it have to be? – curious_cat Dec 27 '15 at 16:52
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    @curious_cat: Just the oxygen required for burning fuel? To burn 1 kg of Kerosene you need 15.6 kg of air or 3.59 kg of O$_2$. Its density as a liquid is 1.14 g/cc, so it will use 2.65 times the volume of fuel. A cryogenic tank with sufficient insulation is required, though. For running regular jet engines you would still need lots of process gas. To carry that along as well is impossible. – Peter Kämpf Dec 27 '15 at 17:19
  • @curious_cat - I think that would be an excellent follow up question. We've had question about why aircraft don't carry their own oxidizer, and the answer is always it takes to much to be practical, but I don't recall an answer stating just how much it would take, whether practical or not. – FreeMan Dec 28 '15 at 17:18
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    @FreeMan: Done! See here: http://aviation.stackexchange.com/questions/23988/commercial-jet-with-cryogenic-oxidizer-tank-back-of-the-envelope-feasibility-ca – curious_cat Dec 29 '15 at 07:58
  • A side point, but is it accurate in the diagram you used for the core flow to be labelled "Low-speed jet" and the fan flow to be "high-speed"? I know most of the thrust comes from the fan because of the huge mass flow, but I though the actual jet exhaust still had a higher specific thrust contribution. – Talisker May 26 '18 at 16:01
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    @Talisker: Well spotted! Of course the labels are wrong and need be swapped. Did not notice it myself! – Peter Kämpf May 27 '18 at 04:56
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Seems as though everyone has missed the simple, obvious answer: the rate at which the engine burns fuel. To take a concrete example, the Saturn V's first stage carried 205,400 gal/770,000l of kerosene fuel, which it burned in a bit less than 3 minutes: https://www.space.com/18422-apollo-saturn-v-moon-rocket-nasa-infographic.html

By contrast, a Boeing 747 carries about a quarter as much (48,445 gal/183,380 l), and burns it over perhaps 12 hours.

jamesqf
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The rocket engine produces the same thrust regardless of the speed it moves. Differently, the thrust of the jet engine depends on velocity and declines as velocity increases, because of the ram drag. It is largely useless if the engine speed approaches the exhaust velocity. The exact formula for efficiency can be found here:

$$ \eta_p = \frac{2}{1 + \frac{v_e}{v}} $$

As a result, the rocket engine can produce significantly more thrust if the speed is really high.

h22
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The first, and most significant reason, is because more fuel is pumped in and burned. Why does a car battery have more stored power than a AA battery? Because, it is designed to be bigger, because that it necessary for the design requirements. But, this is not always the case. The mercury-redstone rocket, that carried Alan Shepard had 78,000 lbs of thrust, while the Boeing 777 can have up to 115,000 lbs of thrust per engine!

Adam
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  • So are you saying a turbofan and a rocket of similar proportions could have similar outputs? –  Dec 27 '15 at 16:30
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    You didn't say anything about proportions. Your questions is poorly phrased. It's like saying "can a shovel or a backhoe move more dirt? " The answer is that it depends on the specific design of each...Generally the thrust of a gas-turbine is not limited by the amount of fuel that you can dump into it. – Adam Dec 27 '15 at 20:30
  • Seems an accurate answer, more mass ejected at a higher speed. One of the difficulties is to feed the engine, so the complexity of the turbo-pumps which must raise the pressure of liquid fuel or oxidant from 2 bars to something like 500 bars. The energy required to pump propellants is huge. See this question on SE. – mins Dec 28 '15 at 14:20
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The answer is simpler than others mention here. It is simply that the rocket operates at much higher temperatures than gas turbines do. This translates to more thrust at the greater temperatures. Turbine blades in turbojets would melt at such high temperatures. Rockets cool the outer lining of the rocket combustion chamber. This is done by using liquid hydrogen that is extremely cold in pipes encasing the hot combustion chamber. The higher temperature in rockets gives more thrust.

fooot
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M. F
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  • I'm not sure that I understand what you mean. Temperature could play a role (although I have no idea how much) but I would suspect that it has to do with the energy output and pressure/fuel rate in the engines. – dalearn Apr 09 '18 at 16:31
  • This is incorrect. Well, it's true that the temperatures are higher in rocket engines, but it's insignificant compared to simply the amount of fuel being burned per second (which was mentioned in other answers). The burn temperature is determined chemically for a given mixture; a car engine burning a (nearly) stoichiometric mixture has a comparable burn temperature to a rocket engine; it's just not continuous. Or, another example, the afterburner: typically it has 30-40% higher temperature than in the main chamber but triple the fuel flow (to roughly double the thrust). – Zeus Apr 11 '18 at 02:51
  • @dalearn Higher temperature means more potential energy, means higher pressure, means higher velocity of escaping gasses. – Alexus Apr 18 '18 at 22:13
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A couple of reasons.

  1. Atmospheric air is a relatively lousy oxidiser. Firstly it's only 20% oxygen, secondly it's a gas so you need to move huge volumes of it to the combustion chamber. Thirdly the available pressure and volume varies with the flight regime.
  2. Jet engines have a turbine downstream of the combustion chamber. This limits the chamber temperatures that can be used (granted you can get around that by using an afterburner).

Orbital launch rockets use liquid oxygen fed by separately powered turbo pumps, so they don't have these issues.

Of course the trade-off is that rockets burn through a lot of fuel and oxidiser, so they can only sustain high thrust for a relatively short time. For getting into orbit that is what you need, for flying a plane an engine with worse thrust to weight but lower thrust-specific fuel consumption is a better choice.

Peter Green
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The thrust generated by a rocket can be related mathematically to the quantity and specific energy of the propellant or propellants and the burn time. Unlike the thrust of a turbojet engine it is not limited by the mass of the air than can be compressed and mixed with fuel in a given time or by the concentration of oxygen in the atmosphere.

J. Southworth
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