Ramjets, traditionally, are inefficient below Mach ~1,
and produce little to no thrust (and so are not useful)
below
about Mach 0.5. This is partly because they receive too
little ram air at these relatively low airspeeds.
(Sufficient ram air is needed to allow compression in the
intake,
which is a diverging nozzle. A diverging nozzle
slows down the air, because there's the same mass
airflow across more cross-sectional area, and slower-
moving air
has higher static pressure for some reason. The
compressed air is then combusted with fuel, and the
exhaust is
blown out the back to produce thrust.) (The other reason
for inefficiency is the nonexistence of shockwaves at
subsonic speeds. Supersonic airspeed ramjets—not
scramjets—use shockwaves to contain the flame front
and shape
the internal airflow. Subsonic ramjets, IIUIC, need
draggier internal structures to do the same things.)
To increase the amount of air the ramjet receives, the
obvious thing to do is blow into its intake. This has been
done, in a variety of ways: with integrated electric fans,
with electric shop-vacs and leafblowers (as used for the
Maggie Muggs travel mug ramjet), and with pulsejets:
[links]. The problems with these are readily apparent:
integrated fans impede ram air at higher airspeeds and
introduce turbulence, and/or increase complexity; shop-
vacs and leafblowers are only suitable for test stand use;
pulsejets provide pulsatile blowing, and supply air that's
already had quite a bit of its oxygen used.
Blowing a ramjet with a pulsejet is effectively putting
two separate jet engines in series. My design effectively
puts one jet engine in series with itself.
A turbojet, essentially, is in series with itself: it takes
energy from its exhaust using a turbine, and uses it to
drive
a compressor at the intake. In fact, that's the difference
between a ramjet and a turbojet: a turbojet is a ramjet
with the addition of a compressor to blow into its intake,
and a turbine to power the compressor. My design
accomplishes the same thing, without the turbine or the
compressor.
Now, you might be thinking that I'm planning to simply
recirculate the ramjet's exhaust to become its blow air.
That would work even at zero airspeed, but only if the
air had unlimited oxygen, and no thrust was desired.
That's
not very practical (so you probably weren't thinking
that).
A common addition to hobby pulsejets is the augmentor.
This is a duct encircling the exhaust nozzle, somewhat
like an afterburner, but without the burning. It's
effectively an annular injector/ejector/eductor that,
using the
Venturi effect, entrains surrounding air into the exhaust
jet to increase thrust by increasing mass flow, at the
expense of exhaust velocity. (Actually, a surprising
amount of air is entrained even without any duct: [link].
So I
expect my design to work quite well with one.)
In my design, the augmentor is used only on a portion of
the ramjet's exhaust that is split off with a pipe from the
main exhaust nozzle (bleed air, basically, but taken post-
combustion). This pipe goes around to the front of the
engine, where the augmentor is mounted some distance
in front of the intake. The split-off exhaust coming
through the pipe is released through a nozzle into the
front of the augmentor. The augmentor then entrains a
lot
of ambient air from in front of the engine and blows it
into the intake, providing more air to the ramjet. The
use
of the augmentor minimizes the ratio of recirculated air
to fresh air, avoiding asphyxiating the flame with
exhaust
and avoiding taking too much of the exhaust away from
producing thrust.
This is expected to provide sufficient blowing that the
ramjet can stay running, and producing thrust, down to
much lower airspeeds, possibly even zero, after being
started with high airspeed or external blowing.
(This system could also be implemented with a
non-augmentor-shaped injector/ejector/eductor, as a
separate
device that could be mounted inside a wing or something
and would not have to be externally aerodynamic. This
would then be ducted to the ramjet's intake.)
The blow system should automatically turn off and get
out of the way of the actual ram air when the airspeed
gets
high enough that the ram air is sufficient to blow the
ramjet. Probably, it should turn off gradually with
increasing
speed, balancing artificial blowing with ram air to keep
the ramjet efficient.
As a bonus, this system can quite simply be combined
with other ramjet variants such as integrated rocket
ramjets
and air-augmented rockets.