Does a rocket engine that combines air-breathing and non-air-breathing modes exist?

The advantage would be it would have to carry a smaller tank and less oxygen, leaving more room for payload on a weight basis.

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    $\begingroup$ Yes. $\endgroup$ – Sanchises Mar 2 '20 at 11:44
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    $\begingroup$ @Sanchises the SABRE is still in development. $\endgroup$ – Manu H Mar 2 '20 at 15:03
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    $\begingroup$ @securitydude5 by definition, rocket engine is not air breathing. You should speak of jet engine. $\endgroup$ – Manu H Mar 2 '20 at 15:04
  • $\begingroup$ @ManuH SABRE is described as an air-breathing rocket. That is exactly how its engine functions in airbreathing mode. It has no fans or through-duct, what it has is a rocket chamber fed by fuel and oxidant. $\endgroup$ – Guy Inchbald Mar 4 '20 at 19:21
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    $\begingroup$ I fail to see why this question is closed. Yes it is an overlap between space.SE and aviation.SE. Aircrafts with rocket engines exist (e.g. the Me163), thus rocket engines are on topic here. Why can't our community accept overlaps? $\endgroup$ – Manu H Mar 5 '20 at 6:10

There's a very recent claim for an engine called Fenris, which claims to be a no-moving-parts "rocket" that uses ambient air for its oxidizer.

On the face, that sounds like a ramjet, but the claims made by the inventors appear (to me) to be investor bait, rather than anything that could actually be true. Video on the entry page at the inventor's web site shows a burn more like that of a poorly designed torch, than a rocket or jet.

As noted in comments, there is also the SABRE engine concept -- this is not yet a working engine, as far as I know, but will/would be a hydrogen-fueled turbo-ramjet which uses the liquid hydrogen to liquefy intake air while in jet mod, for use as oxidizer when it switches to rocket mode. The intent is for use as a spaceplane-to-orbit.

  • $\begingroup$ A no moving parts rocket that uses ambient air, it sounds like a ramjet. $\endgroup$ – Manu H Mar 2 '20 at 15:09
  • $\begingroup$ @ManuH Go look up the Fenris "rocket" -- their claims aren't calling it a ramjet, and they seem to think they can convince investors it'll work. Physically, the engine they have looks much like a rocket thrust chamber and nozzle. $\endgroup$ – Zeiss Ikon Mar 2 '20 at 16:52
  • $\begingroup$ SABRE uses liquid helium to cool the air to near (just above) its liquefaction point. This is then turbo-compressed and fed as oxidant to a conventional hydrogen-fuelled rocket chamber. The intent is to first find sub-orbital applications, with orbital flight being a longer-term aspiration. Currently the individual engine subsystems have all been bench-tested, notably the pre-cooler, and development of a flight demonstrator is starting. $\endgroup$ – Guy Inchbald Mar 5 '20 at 12:37
  • $\begingroup$ @GuyInchbald Been a while since I looked at the SABRE. Now they want to carry a second cryogenic liquid? One they don't even burn off, so have to carry all the way? Seems like trying to save a concept that didn't work as expected... $\endgroup$ – Zeiss Ikon Mar 5 '20 at 13:07
  • $\begingroup$ @ZeissIkon The helium intermediary has been in the design for a long time now. It has a complex thermodynamic function in utilising the waste heat to help drive the rest of the system. More like making practical a concept that was not quite figured out in the early days. There are other significant changes too, but design details seem scarcer than they used to be. $\endgroup$ – Guy Inchbald Mar 5 '20 at 14:04

The concept of a launch system combining an airbreathing 1st stage and a nonairbreathing 2nd stage is as old as the X-1 rocket plane carried aloft by a modified B-29 bomber, and carries on a tradition which includes the B-52/X-15 pair and today's Virgin Knight/SpaceShip 2.

This approach has the advantage of weight savings for these reasons:

  1. Less carried oxidizer, more payload
  2. Lighter structural requirement from lower "max Q" aerodynamic stresses as a result of launching from a higher altitude

However, the vast majority of energy required for an orbital launch (or beyond) comes from the 90 to over 20,000 (geosynchronous orbit) miles of remaining vertical lift and acceleration to 18,000 mph outside the atmosphere, clearly the realm of the (non airbreathing) rocket.

Any mechanism for breathing air, while viable as a 1st stage, becomes dead weight for the remainder of the flight. This approach can be replaced with a rocket 1st stage for heavy lifters, such as the Saturn V, the Space Shuttle, and Falcon lineage. (Note the Shuttle did not need those big wings to bring payload to orbit either, but lowered cost by returning the main engines for re-use).

For aircraft, airbreathing rams, turbochargers, and centrifugal or axial compressors are the most cost effective way to increase power. Used in conjunction with lifting wings, they increase payload carrying capacity and range through greater efficiency. Since jets use excess air in proportion to fuel for combustion, more power can be generated by adding more fuel to the airflow. These "after burners" can be used to increase thrust significantly, but are less efficient.

While turbojets could be considered "airbreathing rockets", the very air they breathe becomes a drag/friction heat impediment at hypersonic speeds.

Research could continue with use of very cold fuels such as liquid hydrogen or a hybridized air/LOX intake in order to increase the density of fuel/air intake charge of a jet, but manufacturing LOX while in flight for rocket propulsion may be beyond reality.

See LACE and SABRE for more information.

  • $\begingroup$ For commercial applications, it boils down to safety, reliability, and cost. Although a hypersonic craft may use less energy than even a ballistic suborbital flight, simply hurling it with a reusable LOX/LNG rocket (which could be launched from an airbreathing carrier) may be the best alternative to date. Research with the SABRE approach remains of interest. $\endgroup$ – Robert DiGiovanni Mar 8 '20 at 9:46

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