What is a ramjet? Was it used on the SR-71 Blackbird?

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    $\begingroup$ relevant: aviation.stackexchange.com/questions/9066 $\endgroup$
    – Federico
    May 13, 2015 at 16:20
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    $\begingroup$ and the SR-71's engines are not ramjets: en.wikipedia.org/wiki/Pratt_%26_Whitney_J58 $\endgroup$
    – Federico
    May 13, 2015 at 16:24
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    $\begingroup$ If you want to show a ramjet, please pick the D-21. It used the Marquardt RJ-43 as its engine, a true ramjet. The SR-71 is a less fitting example. $\endgroup$ May 13, 2015 at 21:03
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    $\begingroup$ The SR-71's engines are not pure ramjets (which cannot function below a certain speed because there is no mechanical compression mechanism), but at top speed a feature called compressor bleed bypass diverts some of the compressor stage airflow directly to the afterburner instead of going through the turbojet. This cools the afterburner allowing for sustained operation, and increases the efficiency of the afterburner itself, making it the dominant thrust-producing portion of the engine. Because this bypassed air is directly fueling the afterburner, the J58 is referred to as a "partial ramjet". $\endgroup$
    – KeithS
    May 22, 2015 at 0:38

4 Answers 4


A jet engine compresses air, heats it by mixing it with fuel and burns it, and lets the heated air escape at the end, where it accelerates to more than its initial speed in a convergent-divergent nozzle because the density of the heated gas is lower, thus needing a higher volume at the same pressure.

By converting the kinetic energy of the flow into pressure (potential energy), the intake creates high-pressure air to feed the engine. This is called pressure recovery and increases with the square of flow speed. Please see below for a plot: This puts a pressure of 1 at Mach 0.5, which is on the high side for flow speed near the compressor face in a jet engine intake.

pressure recovery ratio over Mach number

Note that in static conditions the air needs to be accelerated, so the intake pressure is only 84% of ambient pressure, and at Mach 0.85, the maximum speed of airliners, the intake pressure is 1.37 times higher than ambient pressure. But at supersonic speed things take really off: Pressure recovery for the Concorde was already 6 at Mach 2.0, and for the SR-71 it was 40 at Mach 3.2. If you want a more mathematical approach, the equation for isentropic compression gives: $$p_0 = p_{\infty}\cdot\frac{(1.2\cdot Ma^2)^{3.5}}{\left(1+\frac{5}{6}\cdot(Ma^2-1)\right)^{2.5}}$$ The odd exponents have to do with the ratio of specific heats $\kappa$ of air. 3.5 is actually $\frac{\kappa}{\kappa-1}$ and 2.5 is $\frac{1}{\kappa-1}$. Real compression ratios are slightly below those of the ideal isentropic compression due to friction, but not by much.

The exact equation used for the plot above is produced by calculating the ratio to the intake Mach number directly, this time with $\kappa$ = 1.405:$$\frac{p_{intake}}{p_{\infty}} = \left(0.2025\cdot Ma^2 \cdot\left(1-\left(\frac{Ma_{intake}}{Ma_{\infty}}\right)^2\right) + 1\right)^{3.469}$$

Thus, you get already the compression ratio of a J-47, an early turbojet engine, at Mach 2 and that of a GE90, a modern turbofan engine, at Mach 3.2. Beyond that, it does not make much sense to complicate the engine with turbo machinery - just let the ram pressure give you the compression for thrust generation. You need, however, speed up the vehicle by other means first, because the possible thrust is proportional to the pressure recovery, or the square of airspeed. No speed, no thrust!

You might have read claims that the J-58 of the SR-71 was a ramjet. This is only half true. Below Mach 2 it worked as a regular turbojet, but it had bypass tubes which ducted some air from the fourth stage of the compressor around the later compressor stages, the combustion chambers and the turbine directly into the afterburner. Now some of the air was compressed in the intake and fed directly to a combustion area and through a convergent-divergent nozzle, so this part worked like a ramjet. Some of the air, however, was still passing through the core engine to keep it running, though.

A better example for a ramjet-powered plane is the Lockheed D-21 reconnaissance drone, which used a Marquardt RJ-43 ram jet for propulsion. Its cruise speed was Mach 3.7, back 50 years ago! See below for a picture (source).

Lockheed D-21 in static display on trolley

Note that the same trick which makes a ramjet possible can be used to reduce cooling drag for high-speed piston aircraft. A well-designed cooling duct is slowing and compressing incoming air and heats it by letting it flow through a radiator. The heated air has a higher exit speed, resulting in jet thrust which can compensate cooling drag at higher speeds. The Republic XF-12, a much underappreciated design, made exemplary use of this technique.

  • $\begingroup$ For the Poetry majors among us, could you include an explanation of what "pressure recovery" refers to? Thanks! $\endgroup$
    – Ralph J
    May 14, 2015 at 0:04
  • $\begingroup$ Thinking a bit more about your closing remark on heating air to generate thrust I was thinking about using the same approach on a turbofan. Do you know if there have been any experiments with exchanging heat from the air after the compressor stage to the bypass flow? This could lower the temperature at the inlet of the combustion chamber, thereby increasing the thermodynamic efficiency of the jet core. The extracted energy can generate additional thrust by heating the bypass stream. $\endgroup$
    – DeltaLima
    May 14, 2015 at 10:02
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    $\begingroup$ @DeltaLima: Interesting concept! All I remember are plans for doing something similar on the T-56 for the P-3. The mechanical complexity was too big in the end, so this concept was dropped. In your turbofan idea the hot and cold flows would be next to each other, so complexity might be manageable. You will need a big volume, however, because heat exchange is more efficient at lower flow speed. I guess the volume demand makes this impractical. $\endgroup$ May 14, 2015 at 10:14
  • $\begingroup$ Another reason to want lower speeds is that the cooling air will otherwise have and increasingly high Mach number. I am not the first to think about this concept though, I've found several papers. The words intercooled and recuperated are used to describe it. There has been some theoretical work on it in the past 5 years but I could find any practical experiments. $\endgroup$
    – DeltaLima
    May 14, 2015 at 10:54
  • $\begingroup$ So I take the simple conclusion is: ramjet = jet engine without compressor? $\endgroup$
    – kevin
    May 14, 2015 at 14:18

The ramjet is conceptually the simplest jet engine. It is a duct where air is burned creating a hot jet that provides thrust, it is known aero thermodynamic duct as it is no more than a duct where a thermodynamic cycle is performed. They are no more used for airplanes, as it can not provide thrust at zero airspeed -since he does not have any compressor within the diffuser- and modern turbofan is much more efficient. It is composed by a diffuser , a burner and a nozzle , it uses dynamic compression of ram air within its inlet and then the hot jet expand in a convergent-divergent nozzle as it is supersonic.

Airplanes like the Blackbird used a ramjet to reach higher mach speed by starting the ramjet when the where already subsonic.

  • $\begingroup$ the blackbird did not use ramjets $\endgroup$
    – Federico
    May 14, 2015 at 6:45
  • $\begingroup$ @Federico, the P&W J58 engine was essentially a ramjet wrapped around a turbojet. At high speeds, only enough air flowed through the turbojet core to keep it spinning; most of the air bypassed the core to the afterburner, which functioned as a ramjet. $\endgroup$
    – Mark
    May 14, 2015 at 6:58
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    $\begingroup$ @Davide, the ramjet is the conceptually simplest jet engine. Actually building one is a feat of advanced engineering. $\endgroup$
    – Mark
    May 14, 2015 at 6:59
  • $\begingroup$ "No more used"? This makes it sound as though they were quite popular in the early days of aviation and fell out of favor, which isn't even slightly correct; only a few aircraft have ever managed to make use of a ramjet, and most of those were pure research -- research that continues today, albeit usually on scramjets. $\endgroup$ Jul 3, 2015 at 15:40

Take a look here

but the short answer

A ramjet, sometimes referred to as a flying stovepipe or an athodyd (an abbreviation of aero thermodynamic duct), is a form of airbreathing jet engine that uses the engine's forward motion to compress incoming air without an axial compressor.

In other words it uses the thrust it generates to compress the on coming air by "ramming" it into the engine. Since they have no way of drawing air in they do not work in a static (not forward moving) situation. They often work best at supersonic speeds. Aside form the device used to pump fuel into the engine they essentially have no moving parts.

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    $\begingroup$ What makes them more efficient at higher speeds? $\endgroup$ May 13, 2015 at 16:21
  • $\begingroup$ I dont know enough about them to comment but that may warrant a question on its own. $\endgroup$
    – Dave
    May 13, 2015 at 16:26

A ramjet is a jet engine in which ram air pressure, generated by the forward motion of the air vehicle is employed to compress air before fuel is mixed with it and burned to produce thrust through an increase in temperature and pressure of the expanding gases. This means that a ramjet cannot generate static thrust or operate efficiently below a certain airspeed. Ramjet engines are usually designed to operate at supersonic speeds. A scramjet, or supersonic combustion ramjet is a ramjet in which the airflow through the engine combustion section is supersonic.


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