Air-augmented rockets (also known as rocket-ejector,
ramrocket, ducted rocket, integral rocket/ramjets,
or ejector ramjets) use the supersonic exhaust
of some kind of rocket engine to further compress
air collected by ram effect during flight
to use as additional working mass, leading
to greater effective thrust for any given
amount of fuel than either the rocket or a
ramjet alone.
It represents a hybrid class of rocket/ramjet
engines, similar to a ramjet, but able to
give useful thrust from zero speed, and is
also able in some cases to operate outside
the atmosphere, with fuel efficiency not worse
than both a comparable ramjet or rocket at
every point.
== Operation ==
In a conventional chemical rocket engine,
the rocket carries both its fuel and its oxidizer
in its fuselage. The chemical reaction between
the fuel and the oxidizer produces reactant
products which are nominally gasses at the
pressures and temperatures in the rocket's
combustion chamber. The reaction is also highly
energetic (exothermic) releasing tremendous
energy in the form of heat; that is imparted
to the reactant products in the combustion
chamber giving this mass enormous internal
energy which, when expanded through a nozzle
is capable of producing very high exhaust
velocities. The exhaust is directed rearward
through the nozzle, thereby producing a thrust
forward.
In this conventional design, the fuel/oxidizer
mixture is both the working mass and energy
source that accelerates it. It is easy to
demonstrate that the best performance is had
if the working mass is as low as possible.
Hydrogen, by itself, is the theoretical best
rocket fuel. Mixing this with oxygen in order
to burn it lowers the overall performance
of the system by raising the mass of the exhaust,
as well as greatly increasing the mass that
has to be carried aloft – oxygen is much
heavier than hydrogen.
One potential method of increasing the overall
performance of the system is to collect either
the fuel or the oxidizer during flight. Fuel
is hard to come by in the atmosphere, but
oxidizer in the form of gaseous oxygen makes
up to 20% of the air. There are a number of
designs that take advantage of this fact.
These sorts of systems have been explored
in the liquid air cycle engine (LACE).
Another idea is to collect the working mass.
With an air-augmented rocket, an otherwise
conventional rocket engine is mounted in the
center of a long tube, open at the front.
As the rocket moves through the atmosphere
the air enters the front of the tube, where
it is compressed via the ram effect. As it
travels down the tube it is further compressed
and mixed with the fuel-rich exhaust from
the rocket engine, which heats the air much
as a combustor would in a ramjet. In this
way a fairly small rocket can be used to accelerate
a much larger working mass than normal, leading
to significantly higher thrust within the
atmosphere.
=== Advantages ===
The effectiveness of this simple method can
be dramatic. Typical solid rockets have a
specific impulse of about 260 seconds (2.5
kN·s/kg), but using the same fuel in an air-augmented
design can improve this to over 500 seconds
(4.9 kN·s/kg), a figure even the best hydrogen/oxygen
engines can't match. This design can even
be slightly more efficient than a ramjet,
as the exhaust from the rocket engine helps
compresses the air more than a ramjet normally
would; this raises the combustion efficiency
as a longer, more efficient nozzle can be
employed. Another advantage is that the rocket
works even at zero forward speed, whereas
a ramjet requires forward motion to feed air
into the engine.
=== Disadvantages ===
It might be envisaged that such an increase
in performance would be widely deployed, but
various issues frequently preclude this. The
intakes of high-speed engines are difficult
to design, and they can't simply be located
anywhere on the airframe whilst getting reasonable
performance – in general, the entire airframe
needs to be built around the intake design.
Another problem is that the air thins out
as the rocket climbs, so the amount of additional
thrust is limited by how fast the rocket climbs.
Finally, the air ducting weighs about 5×
to 10× more than an equivalent rocket that
gives the same thrust. This slows the vehicle
quite a bit towards the end of the burn.
== History ==
The first serious attempt to make a production
air-augmented rocket was the Soviet Gnom rocket
design, implemented by Decree 708-336 of the
Soviet Ministers of 2 July 1958. More recently
NASA has re-examined similar technology for
the GTX program as part of an effort to develop
SSTO spacecraft.
Air-augmented rockets finally entered mass
production in 2016 when the Meteor Air to
Air Missile was introduced into service.
== See also ==
Liquid air cycle engine – collecting oxidizer
instead of working mass
