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Proposals for air breathing hypersonic craft. I

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Old May 26th 04, 04:59 PM
Robert Clark
external usenet poster
Posts: n/a
Default Proposals for air breathing hypersonic craft. I

I saw some of the discussions on this group involving the effects of
the boundary layer on the power produced by military jet engines. I
thought participants here might have some input on the idea I present
My idea is to use boundary layer air to provide oxidizer to the
engines at hypersonic speeds. What I've seen discussed on this group
is that you want to avoid the ingestion of the boundary layer air
since it is of low velocity and low pressure. Military jets have a
"boundary layer splitter" in the engine intakes for this purpose.
However, in the cases I've seen this boundary layer is not used to
increase propulsion. The advantage of the boundary layer air though is
that it does not produce large amounts of ram drag, especially at
hypersonic speeds. I was suggesting supplying this boundary layer air
to a high bypass engine to provide additional propulsion.
BTW, the idea would be to use the boundary layer air that enters the
engine intakes as well as the boundary layer that sticks to the
wings/body of the aircraft. In the case of a hypersonic craft, I
imagine the thickness of the boundary layer at the intakes would vary
with speed and air conditions. However, in the cases of boundary layer
splitters I've seen they don't seem to be variable. See for example
the images of the splitters in the intakes on this page:


Do variable boundary layer splitters exist?

Bob Clark

================================================== =======================
From: Robert Clark )
Subject: Proposals for air breathing hypersonic craft. I
Newsgroups: sci.astro, sci.space.policy, sci.physics,
sci.mech.fluids, sci.engr.mech
Date: 2004-05-06 18:12:39 PST

I've read that the main problem from using the surrounding air as
oxidizer at hypersonic speeds is the drag involved. There is
apparently a maximum speed that "air breathers" according to this
constraint can achieve: less than orbital velocity. The air hitting
the intakes at hypersonic speeds causes a tremendous amount of drag:

From: Steven Pietrobon )
Subject: Liquid Air Cycle Rocket Equation
Newsgroups: sci.space.tech
Date: 1994-12-19 13:31:55 PST

I was thinking then about ways you could get surrounding air that is
at the speed of the craft. It occurred to me possibly you could use
the boundary layer. This is the layer of air that sticks to the
aircraft as it flies so is moving along with the craft at the same
speed. Usually you want to get rid of this because of the drag it
causes. But if it's going to be there anyway, moving along with the
craft, why not use it to provide the oxidizer for the craft?
I saw there is research ongoing that involves sucking in this air in
the boundary layer but only for drag reduction purposes, a method
known as "boundary layer control" or "laminar flow control":

Boundary Layer Control.

Such research is ongoing even for supersonic craft:

F-16XL Laminar Flow Research Aircraft.

Then perhaps these already available systems can be adapted to
provide air for the engines. A problem is the boundary layer is
quite thin so you might need to use quite large wings or lifting body
get sufficient air for the engines. This could wind up producing just
much drag as what you are trying to get rid of. Another problem is
that according to this web page at low density conditions the boundary
layer air may not be moving at the same speed as the craft:

Hypersonic Waveriders - Flow Characteristics.
Low Density Flow:

Bob Clark

================================================== =======================

From: (Robert Clark)
sci.astro,sci.space.policy,sci.physics,sci.mech.fl uids,sci.engr.mech
Subject: Proposals for air breathing hypersonic craft. I

Your estimate of 20lbs. of boundary layer air compared to 200 lbs.
through engine intakes seems about right. The actual amount will
depend on aircraft configuration. For example most forms of hypersonic
craft now considered have minimal wings, consisting mostly of a
lifting body. The suction system would have to be along the entire
body in this case.
I saw an estimate that gave the net thrust (equal to: gross thrust -
ram drag) as only 10% of the gross thrust for scramjets. This is from
a passage in "Spacecraft Dynamics" by William E. Wiesel, McGraw-Hill
1989, pg. 213:

"If very high Mach numbers are to be obtained, the incoming airflow
be slowed down but must be allowed to proceed through the engine
unhindered. This occurs in the scramjet, or supersonically combusting
ramjet. However, this introduces new problems. Since the airflow
through the engine is supersonic, it does not "know" where the engine
located since it cannot communicate with the engine walls by pressure
waves. The injected fuel might burn within the engine or several
hundred meters behind the vehicle. In the later case, of course, it
would produce no thrust.
The dynamics of hypersonic flight are much different from those of
rocket flight. A rocket attains high speeds by exiting the atmosphere
quickly and rotating into a horizontal attitude. This completely
eliminates drag after the first two or three minutes of flight. As
an aerospace plane achieves high speeds, the net thrust (the
difference between the thrust and ram drag terms) becomes relatively
small, probubly only about 10% at Mach 10. This does not leave much
margin for external forces Fext. In particular, there cannot be any
extra drag in Fext, so the aerospace plane must be essentially all
inlet seen from its front. A sudden flameout of the engines would
remove the thrust term, but not the ram drag, and the vehicle would
undergo catastrophic deceleration. Combined with a very severe
heating environment, these problems may make achieving a practical
vehicle quite difficult."

I found this quoted in:

From: Andy Haber )
Subject: Air Breathing Spaceplanes?
Newsgroups: sci.space.tech
Date: 1996/08/07

The book is from 1989. IF this were still the case then you might be
able to get by with simply eliminating the scramjet intakes, and the
associated ram drag, and using entirely the boundary layer air.
However, some more recent refs. suggest the ram drag is not as bad
with more current designs:

UQ researchers signal major advance in flight efficiency
Monday , 26 March 2001
"Up to 40 percent of the drag in an aircraft like a 747 is due to skin
friction," Professor Stalker said. "This technique could allow smaller
engines to go from A to B using less engine thrust, achieving quite a
"These economies are extremely important in the hypersonics game where
we're developing models for space craft to operate at earth orbital
speeds. We've found skin friction drag on models in hypersonic
aircraft to be at least half the total drag."

Since NASA has demonstrated in flight positive acceleration at
scramjet speeds, it couldn't be that the ram drag takes up 90% of the
gross thrust with these later models.
In this case what might work is to have a mixed system with both
scramjet intakes and boundary layer suction. If you reduce the amount
of air taken in by the intakes by say 10% thereby reducing ram drag by
that amount, and if the suction method reduces friction drag by say
5%, you could reduce drag by 15%.
Conceivably this could go into the net thrust for the craft. What I
envision is an engine that would work as a scramjet and a turbojet at
the same time(!) The scramjet intake air would be directed around to
bypass turbines to the rear of the engine to be combusted as with a
scramjet, but the boundary layer air would be directed to the turbines
to produce thrust like a usual turbojet engine. The engines of the
SR-71 operated as both a turbojet and as a ramjet so it is possible to
have such combined cycle engine. What would be novel is that both
modes would be operating at the same time.

Bob Clark

"johnhare" wrote in message
"Robert Clark" wrote in message
The key fact is that the boundary layer air is not moving at the same
relative speed as the free stream air. It has close to zero relative
velocity with respect to the aircraft, depending on how close to the
aircraft surface it is. You'll note the ram drag term is dependent on
this relative velocity. So if the relative velocity for this air is
zero, the ram drag term due to this air will be zero.

The ram drag is from the air that is slowed by the ship itself. Air
will not slow to near zero velocity relative from mach? without
applying energy to it in the form of ship drag. The question you
seem to be working on is, "Is the added drag from the intake
more trouble than it is worth?".

Supersonic intake drag is only a fraction of the total ship drag.
For us space types, a rocket allows us to accelerate above
the atmospere with effectively zero drag. Flying in the atmosphere
with or without intakes generates drag. The main question
is whether putting up with the drag at all is worthwhile compared
to getting out of the air completely.

For engine effectiveness at mach 1+, a well designed intake is critical.
It can be fairly said that supersonic flight is the story of intake design
if ABEs are used. A example might illustrate the point.

At some point in supersonic flight, the intake will compress the air by
ten times before it reaches the engine. Your boundary layer injestion
does not. If your engine swallows 1,000 cubic feet of air per second,
then at some altitude your boundary layer unit will get about 20 pounds
of air per second to work with. The same engine behind an effective
intake, gets 1,000 cubic feet per second, that has been compressed
from 10,000 cubic feet per second by the intake. The engine behind
the effective intake has ten times the reaction mass to work with per
second. About 200 pounds This would mean ten times the
thrust per engine mass, minus the intake drag, and intake mass.

The numbers should be run anyway of course. The intake might be
a quarter of the total vehicle drag, and mass as much as the engine itself.
So with your boundary layer injestion, you need three quarters
of the thrust to reach the same acceleration, but you would need
over seven of the same engines to achieve this. You end up
with one intake and one engine vs seven engines, clear win
for the intake inclusion.

A second problem with skipping the intake is the reduced pressure
ratio in the exhaust. If the standard engine has a nozzle pressure
ratio of three, then your boundary layer injestion engine will retain
that ratio of three for thrust. The intake plus engine at the same
mach number considered above will have a pressure ratio of
thirty to ambient. Sombody here has the real numbers, but my
first guess is that thrust per unit of air at the higher ratio will be
more than three times that of the lower pressure ratio exhaust.

Depending on the accuracy of the last paragragh, you could
easily require 21 engines injesting boundary layer air to match
1 engine and 1 intake at whatever mach number gives an intake
compression ratio of ten. Even this does not address the issue
of engine number and performance when intake compression
is 3 or 30 at higher and lower mach numbers.

================================================== =======================
Old May 26th 04, 06:36 PM
external usenet poster
Posts: n/a

ameteurish stuff

External combustion, pitiful humanoids. *External Combustion* for
hypersonics. Sheesh! An external diffuser jetshroud and heat
exchange thru inner wall. Optimize for Temperature. Sheesh!


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