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BLown rAMJET. A ramjet for low(er) airspeeds.
Ramjets, traditionally, are inefficient below Mach ~1,
and produce little to no thrust (and so are not useful)
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
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-
has higher static pressure for some reason. The
compressed air is then combusted with fuel, and the
blown out the back to produce thrust.) (The other reason
for inefficiency is the nonexistence of shockwaves at
subsonic speeds. Supersonic airspeed ramjetsnot
scramjetsuse shockwaves to contain the flame front
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
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
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.
not very practical (so you probably weren't thinking
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,
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].
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
of ambient air from in front of the engine and blows it
into the intake, providing more air to the ramjet. The
of the augmentor minimizes the ratio of recirculated air
to fresh air, avoiding asphyxiating the flame with
and avoiding taking too much of the exhaust away from
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
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
high enough that the ram air is sufficient to blow the
ramjet. Probably, it should turn off gradually with
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
and air-augmented rockets.
Wikpedia: Ramjet § Flight speed
"Ramjets generally give little or no thrust below about half the speed of sound, and they are inefficient (less than 600 seconds) until the airspeed exceeds 1,000 kilometres per hour (280 m/s; 620 mph) due to low compression ratios." [notexactly, Jan 14 2016]
Ramjet blown with integrated fan, intake-impeding/turbulence-causing version
What it looks like, anyway. [notexactly, Jan 14 2016]
Ramjet blown with integrated fan, complex version
[notexactly, Jan 14 2016]
Ramjet blown with external leafblower or shop-vac
Maggie Muggs travel mug ramjet [notexactly, Jan 14 2016]
Ramjet blown with pulsejet
[notexactly, Jan 14 2016]
Ramjet blown with some unclear ultrasonic mechanism
[notexactly, Jan 14 2016]
Unaugmented Venturi effect demonstration
by Matthias Wandel [notexactly, Jan 14 2016]
Look Around You: Water
What the title reminds me of. [notexactly, Jan 14 2016]
V-1 cutaway showing internal parts
Mentioned in my anno. [notexactly, Jan 14 2016]
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||I seem to recall being curious about the overall length
of the engine of a V1 buzz bomb in WW2. I always
thought it was rather longer than necessary. Perhaps it
has been explained here. Thanks.
||Which leads me to a suggestion. What if your engine
contained more than one air-flow compression-by-
restriction shape, and a small fuel injector associated
with each one (so as to leave plenty of oxygen for the
next one)? The last one might be dealing with
supersonic air, even while the first one gets air from
electric wheels taxi-ing the vehicle on the runway. I
can see that those restrictors need to be iris-able, and
the fuel injectors need to be individually controllable
(and more than one at each restrictor).
||How about having several pulse jets in parallel that pulse
at different times, a la a digital to analog sort of
||// I seem to recall being curious about the overall length of
the engine of a V1 buzz bomb in WW2. I always thought it
was rather longer than necessary. Perhaps it has been
explained here. Thanks. //
||All cruise missiles are pretty long and narrow, for
aerodynamics I guess.
||The V-1 used a pulsejet, and pulsejets are generally pretty
long, because they have to be acoustically tuned to pulse
properly. But the pulsejet was still only about half the
length of the fuselage.
||I found a diagram showing how the internal space was
||// How about having several pulse jets in parallel that
pulse at different times, a la a digital to analog sort of
||I might actually have been going to post such a thing soon,
once I get it properly designed. Not sure what you mean by
"digital to analog sort of arrangement", though. Something
like an R2R DAC?
||Essentially you're proposing to transfer a portion of the energy in the exhaust stream to the inlet in order to provide some pre-compression. This is theoretically a good idea and, as you point out, is thermodynamically analogous to a turbojet. The problem I think you'll encounter is that the transfer mechanism is via a viscous process (entrainment) which is fundamentally inefficient. As a consequence I expect that the engine will not be self-sustaining at very low speeds. It might slightly reduce the minimum operating speed relative to a conventional ramjet though, so you get a bun from me.
||I mean like using enough pulse jets to simulate one
continuous pressure rise. Cost could be a bit
||Ah yes, that makes sense. I was thinking of doing pretty
much that, but just to make a less annoying noise and to
reduce vibration, not to provide a more continuous blow for
a ramjet. My idea also has the pulsejets synchronized; IDK if
yours does. (Random phases can't be relied on, IMO, because
they'll be at slightly different frequencies and therefore
beat, producing worse flow and noiseover longer
timescalesthan a single pulsejet. But enough random
spaced-out frequencies could work okay.)