ramjet/scramjet: what makes expanded gas to go out in the back not the front?

In looking at a ramjet/scramjet diagram, it seems that the burnt gas would
want to go in both direction: towards both the inlet and the outlet. Since
there is no valve to prevent a back flow, why wouldn't this push the air out
of the inlet and stall the engine?

peter wrote:

In looking at a ramjet/scramjet diagram, it seems that the burnt
gas would want to go in both direction: towards both the inlet
and the outlet. Since there is no valve to prevent a back flow,
why wouldn't this push the air out of the inlet and stall the
engine?

The inlet raises the static pressure of the incoming air by slowing
it down. In a scramjet the inlet is a carefully designed converging
duct. A supersonic flow in a converging duct will slow down although
in a scramjet the flow never goes subsonic. In a ramjet the inlet is
a (also carefully designed) converging-diverging duct which slows the
flow to subsonic speeds.

Jim Davis

there is a combination of factors.

the combustion gas already has a considerable inerita towards the
outlet. plus the gas entering through the inlet creates higher pressure
there than at the outlet. The concept of path of least resistance
applies here.

peter wrote:
In looking at a ramjet/scramjet diagram, it seems that the burnt gas would
want to go in both direction: towards both the inlet and the outlet. Since
there is no valve to prevent a back flow, why wouldn't this push the air out
of the inlet and stall the engine?

"peter" wrote in message news:eaVmd.6978$m36.1500@trnddc02...
In looking at a ramjet/scramjet diagram, it seems that the burnt gas would
want to go in both direction: towards both the inlet and the outlet. Since
there is no valve to prevent a back flow, why wouldn't this push the air out
of the inlet and stall the engine?

Drive a car at 10mph. Stick a lit match out the window. Does the
match's flame expand evenly in all directions, or does the wind blow
the flame backwards?

The ramjet/scramjet have vast amounts of air piling in from the front,
a wall of air that the flames cannot blow back against. In fact, it
took a while to figure out how to keep the engines lit in the face of
that hurricane. When it is lit, the flames don't want to crawl forward
against all that incoming air - they want to go backward, in the
direction the wind is pushing it.

Mike Miller, Materials Engineer

"peter" wrote in message news:eaVmd.6978$m36.1500@trnddc02...
In looking at a ramjet/scramjet diagram, it seems that the burnt gas would
want to go in both direction: towards both the inlet and the outlet. Since
there is no valve to prevent a back flow, why wouldn't this push the air out
of the inlet and stall the engine?

One way to look at this is: if no fuel was added, you can easily see
why the gas would continue through the engine - it would even provide
a little bit of thrust (though less than the drag caused by the intake
duct). Adding combustion doesn't change the pressure (it can't
really, because as you mention there are openings on both ends!), it
just raises the temperature - that increase in temperature increases
the thrust to useful levels. But the temperature increase does not
really change the pressure through the engine - it just makes the
pressure more available for work. (To be fair, the pressure is
slightly higher in the exhuast because of energy available from the
higher temperature, but the effect is minor when considering the
direction of mass flow through the engine.

So far as I know, ramjets and scramjets are the same as all other
jet engines. The expanded gas goes out the back because that's the
direction of pressure drop.

In a turbojet, a compressor at the front of the engine raises the
pressure of the incoming air. In a ramjet or scramjet, the kinetic
energy of the incoming air is (partially) converted into pressure
by the shape of the intake.

Then, the air is mixed with fuel and burned. The pressure actually
*decreases* a bit across the flame, in order to accelerate the
airflow to accomodate the increased volume. This burning is totally
unlike what happens in a piston engine. Forget about that, and think
about the burners on a gas stove. Pressure drops a bit (not a lot),
but volume increases a lot due to the temperature increase.

The combustion products are then expanded out the rear nozzle,
which converts the pressure into kinetic energy. Now the rear
nozzle doesn't have as much of a pressure drop as the intake has
a pressure rise (because we lost a little pressure in between).
So why does the engine have thrust?

The answer is that there is a great deal more volume heading out
the rear nozzle than entered through the front. Though the exhaust
nozzle converts less pressure back into kinetic energy, each unit
mass of gas has more volume (or higher temperature, more energy, all
the same thing). So at any given pressure, the exhaust gas is
moving faster than the intake gas at the same pressure. By the
time the nozzle has expanded the exhaust gas back to the intake
pressure, the exhaust has much more velocity than the intake air.

The difference in velocity multiplied by the mass flow of air
through the engine is the thrust generated.

"peter" wrote:

In looking at a ramjet/scramjet diagram, it seems that the burnt gas would
want to go in both direction: towards both the inlet and the outlet. Since
there is no valve to prevent a back flow, why wouldn't this push the air out
of the inlet and stall the engine?

I don't think _any_ of you guys get it. These are all plausible-sounding
excuses that don't really explain anything.

You have a presurized chamber. Normally gas in a pressurized chamber is
equal in all directions, regardless of flow. If the inlet and exhaust ports
were the same size, total inlet and exhaust pressure would be equal, which
would seem to imply zero net thrust.

First point: so far as I understand, the exhaust port is a lot larger than
the inlet port, so net thrust is forward.

Second point: forward motion and duct design cause inlet pressure to be
greater than chamber pressure, so inlet air does not get blown back out the
front.

Third point: in hypersonic combusion, air flow is a matter of inertia, not
pressure. The air flows too fast for pressure to propagate.

These are all conjecture. I have no expertise in this field other than
having sketched a idea for a nuclear-powered ramjet engine in grade school.
Please explain where I'm wrong.

/kenw
Ken Wallewein
K&M Systems Integration
Phone (403)274-7848
Fax (403)275-4535

www.kmsi.net

wrote in message . ..

I don't think _any_ of you guys get it. These are all plausible-sounding
excuses that don't really explain anything.

That sets an excellent tone for the conversation.

I wasn't aiming for a college physics lecture, just some simple
analogies to get the idea across to Peter. When he asked for more
detail, I'd give it.

Second point: forward motion and duct design cause inlet pressure to be
greater than chamber pressure, so inlet air does not get blown back out the
front.

That sounds familiar:

"The ramjet/scramjet have vast amounts of air piling in from the
front,
a wall of air that the flames cannot blow back against. ...
When it is lit, the flames don't want to crawl forward
against all that incoming air - they want to go backward, in the
direction the wind is pushing it."

Mike Miller, Materials Engineer

wrote in message . ..
"peter" wrote:

In looking at a ramjet/scramjet diagram, it seems that the burnt gas would
want to go in both direction: towards both the inlet and the outlet. Since
there is no valve to prevent a back flow, why wouldn't this push the air out
of the inlet and stall the engine?

I don't think _any_ of you guys get it. These are all plausible-sounding
excuses that don't really explain anything.

You have a presurized chamber. Normally gas in a pressurized chamber is
equal in all directions, regardless of flow. If the inlet and exhaust ports
were the same size, total inlet and exhaust pressure would be equal, which
would seem to imply zero net thrust.

This would be true if the chamber had no flow through it. Pretend
that we have no gas going into the chamber. As the air flows in, it
is decellerated and increases in pressure (a pressure rise across the
intake). As the air flows out, the air is going from high pressure to
low pressure, so it increases in speed. The total energy at the exit
is obviously less, due to friction and turbulence, but the velocity
could be anything based on the relative size of the intake and outlet.
Burning gas doesn't change the pressures much, but it does change the
energy available, which increases the outlet velocity.

First point: so far as I understand, the exhaust port is a lot larger than
the inlet port, so net thrust is forward.

Not typically true. The inlet is normally larger than the exhaust
port, so that the exhaust will be moving much faster that the inlet.
Thrust is not caused by the relative sizes.

Second point: forward motion and duct design cause inlet pressure to be
greater than chamber pressure, so inlet air does not get blown back out the
front.

True.

Third point: in hypersonic combusion, air flow is a matter of inertia, not
pressure. The air flows too fast for pressure to propagate.

This is a little simplistic. You can think of it as a chicken and egg
problem - pressure cannot propogate up a hypervelocity stream, but a
hypervelocity stream cannot form without a pressure drop. As a simple
example, if you take a high pressure balloon and exhaust it through a
nozzle (so that the air is supersonic) - and then quickly connect the
nozzle to a pressure vessel - the nozzle stops exhuasting, it doesn't
flow into the pressure vessel above the original pressure.

These are all conjecture. I have no expertise in this field other than
having sketched a idea for a nuclear-powered ramjet engine in grade school.
Please explain where I'm wrong.

Just so you know, much of the time when you think someone is using
"plausible sounding excuses" what that really means is that you do not
have some of the basic principles involved correct. So keep asking
questions!

David Summers

David Summers wrote:
Not typically true. The inlet is normally larger than the exhaust
port,

Not necessarily.

so that the exhaust will be moving much faster that the inlet.

Although the exhaust velocity is higher (which, everything else being
equal gives a thinner stream), the exhaust gas is also much hotter (and
hence takes up more volume) than the inlet air, and contains more mass
(the fuel has been added to the airstream), and is the same pressure as
the ambient, wheras the inlet has been compressed by the ram effect.

I believe that overall, the inlet is usually somewhat smaller than the
exhaust- the exhaust usually takes up the full diameter of the engine,
the inlet often doesn't. Many jet engines have a facility to reject the
excess air that the engine cannot burn, particularly at high speed.

Ramjets usually have an inlet shape that rejects part of the flow.

David Summers