
The rocket motor of the future “breathes” air like a jet engine - rbanffy
https://arstechnica.com/science/2020/06/the-rocket-motor-of-the-future-breathes-air-like-a-jet-engine/
======
jjk166
This is an example of what's known as a Rocket Based Combined Cycle engine
(RBCC), sometimes also referred to as Ducted Rockets or Air Augmented Rockets
or Ejector Ramjets.

It's essentially the chemical analogue of a jet engine, using the thrust of a
small rocket for compression instead of fan blades. While like a ramjet it
doesn't have any moving parts in the airflow, the rocket engine lets it
produce static thrust, so it can potentially take off from the ground. The
pressure rise from the rocket exhaust can be much higher than what a
mechanical system of the same weight could produce. The exhaust of the fuel
rich rocket engine can be combusted further downstream with the entrained air,
acting like an afterburner. The hot exhaust mixes with and heats the incoming
air very quickly which is important at high speeds where the air has a short
residency time in the engine. At higher altitudes where there isn't enough air
to support combustion, the duct acts as a large and thus high-efficiency
nozzle for the rocket engine. While not necessarily the best propulsion method
at any one stage of the flight, RBCCs are unique in that they beat rocket
performance at every point from ground to orbit.

That all being said, RBCCs have some limitations. The ducts add significant
weight and make the rocket more aerodynamically complex. For single stage to
orbit orbit, most trajectories spend little time in the range where RBCCs
provide their greatest advantage. For two stage to orbit, a Turbine Based
Combined Cycle (TBCC) engine outperforms the RBCC for the first stage and the
weight can't be justified over that of a pure rocket for the second stage.
Finally with the advent of propulsive landing, the value of an airplane-like
shuttle with a heavy structure has been called into question.

While interesting and promising, it's hardly a new technology, it's been in
development for decades. The Soviets were likely the first to seriously
explore the concept in the form of their GNOM ICBM which reduced the size
sufficiently for the ICBM to be transported easily on land[0], using Nasa
explored an RBCC powered SSTO shuttle in the early 90s known as GTX[1]. Solid
Propellant RBCCs have seen wide use in recent years for the latest generation
of air-to-air missiles from several countries.

[0]
[http://www.astronautix.com/g/gnom.html](http://www.astronautix.com/g/gnom.html)

[1]
[https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/199900...](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19990062726.pdf)

