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Knowing that rocket powered engines have higher speeds than jet engines, it raises the question: are jet engines more efficient than rocket powered engines?

This is of course in terms of fuel consumption per distance traveled, as I am curious whether a rocket engine allows an airplane to travel a distance more quickly compared to a jet engine when consuming the same amount of fuel.

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    $\begingroup$ The only chance a rocket has is a suborbital flight. Get ICBM data and compare it to a 747. The jet will have an advantage in fuel consumption per distance, the rocket much faster. $\endgroup$
    – Robert DiGiovanni
    Apr 29, 2020 at 0:03
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    $\begingroup$ No, rockets engines donnot have higher speed than jet engines. That one was not designed to exceed mach1 while that one easily exceed mach1 without using full power. $\endgroup$
    – Manu H
    Apr 29, 2020 at 6:13
  • $\begingroup$ @ManuH Super's formulation is a bit inexact, but it's not wrong. The highest speed attainable by a jet engine is in the region of Mach 3.5. Rockets can reach much higher speeds than that. $\endgroup$
    – Hobbes
    Apr 29, 2020 at 12:04
  • $\begingroup$ You should be more clear in the title that you are asking about jet powered airplane with rocket powered ballistic transport vehicle. $\endgroup$
    – user3528438
    Apr 29, 2020 at 15:27
  • $\begingroup$ @Hobbes I agree, and that's I think the formulation must be clarified. As is, it is unclear if it is an false assumption, a mistake, or something forgotten (or something else) $\endgroup$
    – Manu H
    Apr 29, 2020 at 15:35

3 Answers 3

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Jet engines are far more efficient. The metric to compare them is 'Thrust-specific fuel consumption', i.e. the amount of fuel used to produce one unit of thrust. Fuel consumption per distance traveled is proportional to this.

A few figures from that page:

Rocket engine: Space Shuttle main engine, 225 g/kN.s

Jet engine in afterburner: EJ-200: 48 g/kN.s

Jet engine, dry: CF-6: 17 g/kN.s

So a rocket uses ~10 times as much fuel to produce the same thrust. Even if you add afterburning, a jet is more efficient.

During and after World War 2, there was some interest in rocket-engined aircraft. The Me-163 saw service in the Luftwaffe.

The interest continued for a decade or so, until jet engines with afterburners were developed. One exponent of that interest was the Saunders-Roe SR-53, a short-range interceptor with both a jet engine and a rocket.

It was developed in response to an RAF requirement for an aircraft that could take off and get to high altitudes very quickly; more quickly than could be attained with the jet engines of the day. enter image description here

Jet: Armstrong Siddeley Viper, 1640 lb thrust. Rocket: De Havilland Spectre, 8000 lb thrust.

Fuel carried:
jet: 500 lbs
rocket: 10,500 lbs

Endurance:
jet: 45 minutes
rocket: up to 7 minutes

speed:
jet: 180 kts
rocket: accelerate to Mach 2 once, then the fuel ran out

(source: mostly 'British experimental jet aircraft', Barrie Hygate.)

It was superseded by the EE. Lightning, which had 2 afterburning jets. It had 4 times the range despite carrying less fuel and more payload.

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    $\begingroup$ A rocket carries its own oxidizer, an afterburning jet doesn't. $\endgroup$
    – Hobbes
    Apr 29, 2020 at 14:55
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    $\begingroup$ @user3528438 Yes, it does. Replace the jet engines on an aircraft with rockets of the same thrust, and fuel consumption (for traveling the same distance at the same speed) will increase by a factor of 10. $\endgroup$
    – Hobbes
    Apr 29, 2020 at 15:31
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    $\begingroup$ Have you included oxidiser in the weight? It seems like you have included oxidiser in the weight for rockets but not for jets? $\endgroup$
    – David Jonsson
    Jan 2, 2022 at 15:02
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    $\begingroup$ Yes, I included oxidiser in the weight for the rocket, and not for the jet. Because the rocket has to carry its own oxidiser, which means you have to provide space and weight allowance for it in your rocket/aircraft. You cold split it out to fuel and oxidiser to improve the answer, that requires knowing the fuel/oxidiser ratio of the rocket engine. $\endgroup$
    – Hobbes
    Jan 3, 2022 at 16:22
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    $\begingroup$ @Hobbes: Even more important, a rocket has to carry all its own reaction mass. An airbreathing jet engine gets huge amounts of reaction mass from the atmosphere for free (even if the engine used all the oxygen in its intake air for combustion - and even an afterburning turbojet uses up only a small fraction of said oxygen, since it has to run very lean in order to avoid consuming itself - the four-fifths of the air that's made up of nitrogen and argon still serves as dedicated reaction mass). $\endgroup$
    – Vikki
    Aug 3 at 1:47
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The key to why an air-breathing jet engine is "more efficient" is that it doesn't need to carry oxidizer, it can just use the atmosphere. Carrying oxidizer is very heavy, which will severely impact the range and speed of the aircraft.

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    $\begingroup$ Also, an airbreathing jet engine gets free reaction mass from the atmosphere (the vast majority of the air that goes into a jet engine serves as reaction mass, with only a tiny fraction being used as oxidizer), whereas a rocket's supply of reaction mass is limited to its onboard fuel and oxidizer. $\endgroup$
    – Vikki
    Aug 2 at 5:38
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"Ballpark" figures for an 8000 mile trip Atlas ICBM vs 747 jet airliner:

Atlas ICBM: 8000 lbs delivered using 75 tons of RP-1/225 tons LOX = 27 lbs/ton total propellant, or 108 lbs/ton on a hydrocarbon fuel basis.

747 airliner: 150,000 lbs passengers and luggage using 150 tons fuel = 1000 lbs/ton and a softer landing.

Biggest disadvantage of the rocket is having to carry it own oxidizer, as 1 lb kerosene requires more than 3 lbs of oxygen for combustion. Not using a wing also greatly reduces efficiency. The winged jet is around 40 times more efficient for this application.

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