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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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.
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:
- Less carried oxidizer, more payload
- 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.