Air Breathing Spaceplanes?
Tom Simonds
Newsgroups: sci.space.tech
Subject: Air Breathing Spaceplanes?
Now...here's a question.
Rockets and shuttles which are shot into orbit always carry their
own oxidiser in some form or another, at considerable expense. Most
of the fuel a rocked requires is just to lift the considerable
weight of the fuel.
Now, why aren't they working on a jet that uses air as the oxidiser
on the way up, thus saving weight? Seems pretty obvious. When the
engine starts to conk out due to lack of air, you inject oxygen into
it and keep going, all the way into orbit.
There are a number of ways of doing this. Here's just one of them.
Put nozzles around the inside perimeter of the intake. Squirt oxygen
into the intake when the engine starts to conk out due to lack of
air. It may not sound like the most elegant approach, but an existing
design could be thus retrofitted. The X-15, for example, would have
gone into orbit with some modifications. I presume the SR-71 could be
sent into orbit, too.
Question is, what am I missing here? Why hasn't it already been
done? It seems like if you're going to build a plane that flies into
space, that would be the first thing you'd work on, assuming that
you approach these problems logically.
- Tom Simonds
Peter Bellini
Tom,
It _has_ been worked on, and still is. Remember, for every design like
that there are tradeoffs. Lets look at some of them.
First, most aircraft use turbomachinery (turbines, compressors, etc.).
Turbomachinery can only take you to, say, Mach 3-4 until the efficeincy gets
shot or the turbine blades get too stressed. Turbojets have great
efficiency, but they are heavy (T/W=~10) and complex and low thrust and you
can only use them until Mach 4.
So you need ramjets. To save weight, you combine them into one "pipe" so to
speak. You get a turbo-ramjet. This is what the SR-71 uses. It just
closes the duct with the blades, and dumps the air right into the
afterburner section. (That's all a ramjet is). This can take you to about
Mach 7 or so given hydrogen as a fuel. Then it gets too hot inside and the
efficiency gets shot due to shock losses. Ramjets also are very efficient
compared to rockets. But again, they can only be used until Mach 7. And
they're heavy.
So you modify the duct to combust supersonically (a scramjet). These don't
yet exist at speed and scale, so you can only extrapolate. They become more
efficient than ramjets at about Mach 6 and require (maybe) some modification
to the ramjet duct. They are also heavy, but not as heavy as you might
think, because you don't need as much variable geometry for an accelerator
(launch vehicle) as you do for a cruiser (transport).
Nobody knows how fast you can take scramjets. Some say Mach 12 some say
Mach 20. You still need a kick to orbit nonetheless. So you use some sort
of rocket. Or you inject oxidizer into a scramjet duct and call it an
oxygen enriched scramjet, which is a rocket.
Now.
These systems are heavy. They are very complex. They are very expensive,
and in my opinion will always be more heavy,complex, and expensive than
rockets.
By their nature, they make the airframe a complex shape. Because of the
scramjets, you need inlet and nozzle expansion surfaces on the vehicle to
get enough thrust. This forces the vehicle to look like a surfboard (like
NASP) or a cone-type shape with engines wraped all around. These are either
hard (costly) to manufacture or have other problems.
The big point in their favor is that thier efficiency allows much lower mass
ratios needed for launch. Depending on how far you can take scramjets,
could be as low as 4 for an SSTO aribreather, instead of 8-9 for SSTO
rockets. This allows you take the big weight penalty of the engines, no
problem.
But it's the complexity and cost that is prohibitive. Also, these vehciles
must fly a depressed trajectory, requiring more time in the atmosphere.
This causes a very small penalty from drag, but a penalty nonetheless. It
also increases heating tremendously. So much so that ascent rather than
reentry can dictate the thermal protection needed. This isn't too bad, but
you require more expensive, more maintenance intensive tiles. You can use
metallics, but only to a certain point (~1800 F) and then you need ceramic
tiles or heavy, very expensive advanced carbon-carbon.
So, the bottom line is: Airbreathers can probably work. There are design
and technology challenges than we can overcome. But will our efforts be
worth it? We may end up with a more complex, costly vehicle than a rocket.
--
Pete Bellini
Aerospace Engineer - Summer Intern Master's Candidate
NASA Langley Research Center Georgia Tech
Hampton, VA Atlanta, GA
Chuck Buckley
In article <Dvo5B...@world.std.com>,
Tom Simonds <tsim...@world.std.com> wrote:
>Newsgroups: sci.space.tech
>Subject: Air Breathing Spaceplanes?
>
> Now...here's a question.
>
> Rockets and shuttles which are shot into orbit always carry their
> own oxidiser in some form or another, at considerable expense. Most
> of the fuel a rocked requires is just to lift the considerable
> weight of the fuel.
>
> Now, why aren't they working on a jet that uses air as the oxidiser
> on the way up, thus saving weight? Seems pretty obvious. When the
> engine starts to conk out due to lack of air, you inject oxygen into
> it and keep going, all the way into orbit.
>
> There are a number of ways of doing this. Here's just one of them.
> Put nozzles around the inside perimeter of the intake. Squirt oxygen
> into the intake when the engine starts to conk out due to lack of
> air. It may not sound like the most elegant approach, but an existing
> design could be thus retrofitted. The X-15, for example, would have
> gone into orbit with some modifications. I presume the SR-71 could be
> sent into orbit, too.
>
> Question is, what am I missing here? Why hasn't it already been
> done? It seems like if you're going to build a plane that flies into
> space, that would be the first thing you'd work on, assuming that
> you approach these problems logically.
>
> - Tom Simonds
>
>
>
>
Well, you have just described NASP. (National Aerospace Plane).
The X-15 was not even close to being able to make it into orbit. It
was limitted to about Mach 6 due to it's materials and structures. The
SR-71 was even more limitted. Nor, do I believe that it would be a good idea
to inject LOX into the SR-71's engines. (Gut feeling here... input anyone?)
On the surface, an air breather does seem obvious. It has been thought of
for years. But, it becomes a non-trivial issue when trying to get the engines
to work in an airless environment. It could be manged by greatly increasing
the complexity of the systems (increasing cost and decreasing reliability).
Or, two separate working systems could be used. (air breathing to a certain
level, then rockets). That increases weight and complexity also. It is
not obvious that a cost benefit analysis of that sort of engine is better
than the other proposed designs. It does fail a risk assessment comparison
against most single engine systems.
--
Charles Buckley | I took a multiple choice test once:
cbuc...@swttools.fc.hp.com | I checked "All of the Above"...
(970) 229-7607 | "None of the Above"..
I definately do not speak for HP | Then I set the test on fire
Steve Linton
tsim...@world.std.com (Tom Simonds) writes:
>Newsgroups: sci.space.tech
>Subject: Air Breathing Spaceplanes?
> Now...here's a question.
> Rockets and shuttles which are shot into orbit always carry their
> own oxidiser in some form or another, at considerable expense. Most
> of the fuel a rocked requires is just to lift the considerable
> weight of the fuel.
> Now, why aren't they working on a jet that uses air as the oxidiser
> on the way up, thus saving weight? Seems pretty obvious. When the
> engine starts to conk out due to lack of air, you inject oxygen into
> it and keep going, all the way into orbit.
It does look attractive, but there are a number of problems
1) The air intake structure is heavy, and gets hot -- jet engines are both bigger and more
complex than rockets of similar thrust
2) You are forced to stay in the atmosphere for most of your acceleration this way,
which creates a whole lot of aerodynamic problems that you can otherwise avoid
3) No one has yet demonstrated a jet engine which will work over the
required range of speeds
Andy Haber
In article <Dvo5B...@world.std.com> tsim...@world.std.com (Tom Simonds) writes:
>
> Now, why aren't they working on a jet that uses air as the oxidiser
> on the way up, thus saving weight? Seems pretty obvious. When the
> engine starts to conk out due to lack of air, you inject oxygen into
> it and keep going, all the way into orbit.
>
> Put nozzles around the inside perimeter of the intake. Squirt oxygen
> into the intake when the engine starts to conk out due to lack of
> air. It may not sound like the most elegant approach, but an existing
> design could be thus retrofitted. The X-15, for example, would have
> gone into orbit with some modifications.
On it's best day, the X-15 got up to about 2 km/sec in speed. To make
orbit, the required speed is 7.7 km/sec. It would take a lot of modifications
to add another 5.7 km/sec to the top speed of an X-15.
> I presume the SR-71 could be sent into orbit, too.
On it's best day, the SR-71 gets up to about 1 km/sec in speed ....
> Question is, what am I missing here? Why hasn't it already been
> done? It seems like if you're going to build a plane that flies into
> space, that would be the first thing you'd work on, assuming that
> you approach these problems logically.
The short answer is it's a lot harder than it sounds. For a longer
answer I quote "Spacecraft Dynamics" by William E. Wiesel, McGraw-Hill
1989 pg 183
"The rocket is the symbol of space exploration. The sheer size and
power of a large satellite booster place it in a category by itself in
terms of human craft. The Saturn V, for example, was over eight times
more massive at lift-off than the heaviest aircraft ever flown, or about
the mass of a naval cruiser. Only a few fighter aircraft can fly
straight up, and fewer still can accelerate while doing it. However,
this is a necessary attribute of all rockets."
Skipping to pg 213 where Aerospace Planes are discussed:
"If very high Mach numbers are to be obtained, the incoming airflow cannot
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 is
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."
NASP grappled with these problems for a time, then when the cost to
solve all these problems became clear, Congress quickly pulled the plug.
--
Andy Haber (an...@mail.hcsc.com)
Concurrent Computer Corporation, Ft Lauderdale, FL
Andy Haber
In article <4u8bc0$5...@fcnews.fc.hp.com> cbuc...@hpfcla.fc.hp.com (Chuck Buckley) writes:
>
>Nor, do I believe that it would be a good idea
>to inject LOX into the SR-71's engines. (Gut feeling here... input anyone?)
told, that if you pipe oxygen into a piston engine, a dramatic increase in
power output may occur. I've also been told that if you persist in this
manner for any length of time you will burn holes through the tops of
all the pistons at which point a dramatic decrease in performance will
occur. Even if the fuel system knows about the extra oxygen, and adds
more fuel to stay near the correct proportions that avoid excess free
oxygen after combustion, every kg of fuel burned produces a given
amount of heat. Existing engines use the airflow through them to get
rid of most of this heat. If you start putting more oxygen, and more
fuel with as you climb to where the air is thinner and so there is less
of it to carry away heat, your likely to hit a meltdown point fairly quick.
Same goes for turbojets. Modifications required to prevent the first
few turbine wheels from melting are likely to be quite extensive.
Jeff Greason
In article <Dvo5B...@world.std.com>, tsim...@world.std.com (Tom Simonds) writes:
|> Now...here's a question.
This one *really* ought to be in the FAQ. Standard answer #357...
|>
|> Rockets and shuttles which are shot into orbit always carry their
|> own oxidiser in some form or another, at considerable expense. Most
|> of the fuel a rocked requires is just to lift the considerable
|> weight of the fuel.
The "expense" is all in the hardware, of course -- the fuel is cheap,
and the oxidizer (assuming LOX), is *extremely* cheap -- pennies per pound.
|>
|> Now, why aren't they working on a jet that uses air as the oxidiser
|> on the way up, thus saving weight? Seems pretty obvious. When the
|> engine starts to conk out due to lack of air, you inject oxygen into
|> it and keep going, all the way into orbit.
(one of the many, many such approaches deleted).
|>
|> Question is, what am I missing here? Why hasn't it already been
|> done? It seems like if you're going to build a plane that flies into
|> space, that would be the first thing you'd work on, assuming that
|> you approach these problems logically.
#1) They are working on it. They have been working on it (at a low level)
for 30 years or more. However, getting one that's an improvement on
pure rockets is hard, because...
#2) Airbreathing engines have abysmal thrust/weight. 10-20x worse than
rockets. So once you have a paper design of a mixed airbreathing/rocket
engine, try taking off the airbreathing part (and it's intakes!) and
just replacing it with rocket propellant. You buy back a lot of the
delta-V you wanted to get from airbreathing. And...
#3) To breathe air, you have to be *in the air*. To breathe air long enough
to buy a substantial fraction of orbital delta-V, you have to spend a good
chunk of your trajectory in the air. This adds drag, which means the
airbreather has to provide more total delta-V than the pure rocket to
reach orbit. Combined with #2, this makes the exercise very doubtful.
Bottom line: If you use existing airbreathers, it's a losing proposition
for a single-stage vehicle (if you drop the airbreathing engines, that's a
different story). There are ideas floating around out there for innovative
ways to do it, some of which I think have some promise ... but it's nowhere
near as simple as you suggest.
Disclaimer: While I am an Intel employee, all opinions expressed are my own,
and do not reflect the position of Intel, NETCOM, or Zippy the Pinhead.
============================================================================
Jeff Greason "We choose to go to the Moon in this decade,
<gre...@ptdcs2.intel.com> and do the other things, not because they
<gre...@ix.netcom.com> are easy, but because they are hard." -- JFK
Peter Bellini
Jeff Greason wrote:
>
> #2) Airbreathing engines have abysmal thrust/weight. 10-20x worse than
> rockets. So once you have a paper design of a mixed airbreathing/rocket
> engine, try taking off the airbreathing part (and it's intakes!) and
> just replacing it with rocket propellant. You buy back a lot of the
> delta-V you wanted to get from airbreathing. And...
Actually, you don't. Because of airbreathers' (ramjet,scramjet) high Isp. This
allows the mass ratio (M_liftoff/M_burnout) to come way down from 8-9 for pure
rocket to as low as 4 for scramjets for the same delta v.
Pete
Michael Walsh
You are missing the point. Things are not as simple for air-breathing
engines as they are for rockets. That specific impulse is paid for
by both the weight of inlets and propulsion equipment, and also the
necessity to fly a different trajectory to maximize capture of
air for combustion purposes, and modifying the flight path to
minimize drag and aero-thermal heating.
A conventional ramjet takes in its air subsonically after it passes
through a shock wave and internal combustion is accomplished at
subsonic speed before being expanded through the exit nozzle.
The ramjet is good up to about Mach 8.
Above Mach 8 the only known airbreather is the scramjet and that
is still under development.
As I said in an earlier post, you should be able to get some good
information about these trade-offs from someone at Langley.
Mike Walsh
mancus@@rtpnews.raleigh.ibm.com
In <4u8bc0$5...@fcnews.fc.hp.com>, cbuc...@hpfcla.fc.hp.com (Chuck Buckley) writes:
> The X-15 was not even close to being able to make it into orbit. It
>was limitted to about Mach 6 due to it's materials and structures. The
>to inject LOX into the SR-71's engines. (Gut feeling here... input anyone?)
I once read a science fiction story aimed at teen-agers that was
written back in the X-15 area. The idea was that the X-15 could carry
a two-stage rocket not unlike Pegasus, which gave them a quick
and easy way to launch small payloads. Intuitively, I suspect this
would work quite well, although there are issues to be solved.
Separation would occur in vacuum, which is easy. The biggest issues
I see would be keeping the mass of the rocket and its attachment
structure down, and making sure the payload doesn't mess up the
hypersonic aerodynamics of the X-15.
There is no reason to pursue this now with Pegasus flying, but
it seems like an interesting proposal in an era when X-15 is
actively flying.
--Cathy mancus <ca...@zorac.cary.nc.us> <-- note email is back
Henry Spencer
A great deal of work has been done on such concepts. They have very serious
problems, which so far have prevented their use in practical launchers.
For one thing, engines that use air are *heavy*, because they need
machinery for handling very large volumes of air. The best military jet
engines have a thrust/weight ratio of perhaps 9:1, while the best rocket
engines are 125:1 or thereabouts. Oxidizer in tanks may be heavy, but it
burns off on the way up -- the mass of the engines has to be carried all
the way into orbit.
For another, it is very difficult to effectively exploit air arriving in
your intakes at hypersonic speed. It is ferociously hot and corrosive, it
adds a great deal of drag, and using it effectively is a major technical
challenge. The practical schemes for air-breathing launchers generally
use jets only for initial acceleration, switching to rocket power fairly
early in ascent.
> Seems pretty obvious. When the
> engine starts to conk out due to lack of air, you inject oxygen into
> it and keep going, all the way into orbit.
Jet engines do not make good rockets, and such a vehicle will necessarily
do most of its accelerating as a rocket. What you need is an efficient
rocket that can also function as an inefficient jet, not vice versa.
> ...The X-15, for example, would have
> gone into orbit with some modifications. I presume the SR-71 could be
> sent into orbit, too.
Neither one had the slightest chance of being able to reach orbit, even
with such modifications. Please learn something about the actual problems
involved before making such statements. If it was that easy, it would
have been done long ago.
> Question is, what am I missing here? Why hasn't it already been
> done?
Because it's a lot harder than you seem to think. Rockets work a lot
better. Substituting expensive, technically-challenging, excessively
heavy air-breathing engines for lightweight tanks full of cheap liquid
oxygen just isn't sensible.
--
...the truly fundamental discoveries seldom | Henry Spencer
occur where we have decided to look. --B. Forman | he...@zoo.toronto.edu
Oleg Zabluda
Henry Spencer (he...@zoo.toronto.edu) wrote:
: Substituting expensive, technically-challenging, excessively
: heavy air-breathing engines for lightweight tanks full of cheap liquid
: oxygen just isn't sensible.
How about using air-breathing nuclear powered engines. Just heat the air
and use it for thrust. Will enviro-nazis ever allow development
of such beasts?
Oleg.
Henry Spencer
In article <320909...@vab01.larc.nasa.gov> Peter Bellini <pbel...@vab01.larc.nasa.gov> writes:
>> ...try taking off the airbreathing part (and it's intakes!) and
>> just replacing it with rocket propellant. You buy back a lot of the
>
>Actually, you don't. Because of airbreathers' (ramjet,scramjet) high Isp...
Isp is not everything; in some ways it's not even the most important
thing. (This is heresy, yes, but it's also true.) The point of high Isp
is low mass ratio, but low mass ratio does not automatically correlate
with ease of design and construction. It does *if other things are equal*.
But they aren't. For a given vehicle configuration (engine type, fuel
choice), you certainly want maximum Isp, but Isp is not a good metric for
comparing different configurations.
For example, if you're trying to build an SSTO, do you want to use liquid
hydrogen or a hydrocarbon fuel? The LH2 has much higher Isp. But the
goal is not high Isp, but ease of design and construction... and dense
fuels win big there, because high mass ratios are easier to achieve with
them. The US flew an SSTO-class rocket stage using dense fuels in 1961
(the Titan II first stage) but has never managed to build one with LH2
(the S-II achieved the necessary mass ratio, at great cost and pain, but
its engines are not up to the job).
Peter Bellini
Michael Walsh wrote:
>
> Peter Bellini wrote:
> >
> > Jeff Greason wrote:
> >
> > >
> > > #2) Airbreathing engines have abysmal thrust/weight. 10-20x worse than
> > > rockets. So once you have a paper design of a mixed airbreathing/rocket
> > > just replacing it with rocket propellant. You buy back a lot of the
> >
> > Actually, you don't. Because of airbreathers' (ramjet,scramjet) high Isp. This
> > allows the mass ratio (M_liftoff/M_burnout) to come way down from 8-9 for pure
> > rocket to as low as 4 for scramjets for the same delta v.
> >
>
>
> engines as they are for rockets. That specific impulse is paid for
> by both the weight of inlets and propulsion equipment, and also the
> necessity to fly a different trajectory to maximize capture of
> air for combustion purposes, and modifying the flight path to
> minimize drag and aero-thermal heating.
>
> A conventional ramjet takes in its air subsonically after it passes
> through a shock wave and internal combustion is accomplished at
> subsonic speed before being expanded through the exit nozzle.
> The ramjet is good up to about Mach 8.
>
> Above Mach 8 the only known airbreather is the scramjet and that
> is still under development.
>
> As I said in an earlier post, you should be able to get some good
> information about these trade-offs from someone at Langley.
>
No, Mike, I understand very well how airbreathing systems work. I _have_
talked to people at Langley and I am currently designing an airbreather in my
internship in NASA's launch vehicle design branch. I understand _very_ well
the tradeoffs with an airbreather. And I'm telling you you CANNOT make a
blanket statement that says just because an airbreather has heavier engines,
this outweighs the Isp benefit. You just cannot. Yes, there is a tradeoff,
but how much of a tradeoff? I can _show_ you a design where the gain was
high enough to offset the weight penalty and the depressed trajectory.
Michael Walsh
Peter Bellini wrote:
>
> No, Mike, I understand very well how airbreathing systems work. I _have_
> talked to people at Langley and I am currently designing an airbreather in my
> internship in NASA's launch vehicle design branch. I understand _very_ well
> the tradeoffs with an airbreather. And I'm telling you you CANNOT make a
> blanket statement that says just because an airbreather has heavier engines,
> this outweighs the Isp benefit.ctory.
>
> --
> Pete Bellini
O.K., just so we understand what we are talking about. I didn't
make the blanket statement that you rightly object to. I am sure
that the people who pushed the NASP concept didn't spend that much
time and money on a concept that could be eliminated by some simple
rule.
Mike Walsh
GCHudson
>Actually, you don't. Because of airbreathers' (ramjet,scramjet) high
Isp. This allows the mass ratio (M_liftoff/M_burnout) to come way down
from 8-9 for pure rocket to as low as 4 for scramjets for the same delta
v.<
BUT, you are burning a lot of LH2 to get that high Isp. Consider two
vehicles, one pure rocket, the other a NASP-like thing. Pick your average
Isp and propellant densities. Design the NASP first. Figure out the dry
mass. Then strip off those airbreathers and put real rockets on the tail,
and load real rocket propellant in the beast. I guarantee the rocket will
inject more mass and payload than the airbreather. The reason: tank
weight goes as the volume contained and is not proportional to the
propellant mass.
And I'm not even taking into account the increased weight of the
unobtanium thermal protection for the NASP.
Chuck Buckley
In article <4ue2em$6...@dodgson.math.psu.edu>,
No. That would not be allowed. Read back through the history of that type
of system and you will begin to see why. (Project Pluto was my favorite)
Besides, once you start adding the mass of the shielding, you are just in
another trade-off situation. You could make a nuclear turbine engine that
could sit in your lap, but I doubt that you would want to have it *in* your
lap while running it. You would want shielding, and adequate shielding is
going to be a huge mass penalty. (You will not get far, if you advertise for
pliots over child-bearing age).
burnside
: >
: > #2) Airbreathing engines have abysmal thrust/weight. 10-20x worse than
: > rockets. So once you have a paper design of a mixed airbreathing/rocket
: > engine, try taking off the airbreathing part (and it's intakes!) and
: > just replacing it with rocket propellant. You buy back a lot of the
: > delta-V you wanted to get from airbreathing. And...
: Actually, you don't. Because of airbreathers' (ramjet,scramjet) high Isp. This
: allows the mass ratio (M_liftoff/M_burnout) to come way down from 8-9 for pure
: rocket to as low as 4 for scramjets for the same delta v.
: Pete
: --
: Pete Bellini
: Aerospace Engineer - Summer Intern Master's Candidate
: NASA Langley Research Center Georgia Tech
: Hampton, VA Atlanta, GA
First of all, it's not the same delta-v, it's much higher. This
is because the engine needds to fly in air of it is to breathe air.
Typical delta-v numbers are 31,000 ft/s for rockets, and over 40,000
ft/s for airbreathers to orbit.
Then there is the thrust to weight issue. The best asertion of the
thrust to weight of an airbreather that I've heard is 22, for
aerojet's deceptively simple rocket based "Strutjet" engine. They
claim that they can get a propellant mass fraction of 0.78 in an
integrated vehicle design. Perhaps they can; it wouldn't surprise me.
But the installed thrust requirement means that they spend most of that
T/W advantage on the weight of the powerplant itself.
Sooner or later you have to turn on the rocket anyway.
We are working on airbreathing assist for spaceplanes, but it has
more to do with operability. We can't seem to justify it on pure
performance terms, even with tanker assist to reduce the installed
airbreathing thrust requirement.
Mitchell Burnside Clapp
Pioneer Rocketplane
Henry Spencer
In article <4ue2em$6...@dodgson.math.psu.edu> zab...@math.psu.edu (Oleg Zabluda) writes:
>: Substituting expensive, technically-challenging, excessively
>: heavy air-breathing engines for lightweight tanks full of cheap liquid
>: oxygen just isn't sensible.
>
>How about using air-breathing nuclear powered engines. Just heat the air
If you want expensive, technically-challenging, and excessively heavy
engines, it's hard to imagine a better way of doing that than by using
nuclear air-breathers. :-)
Ash R. J. Wyllie
>Michael Walsh wrote:
>>
>> Peter Bellini wrote:
>> >
>> > Jeff Greason wrote:
>> >
>> > >
>> > > #2) Airbreathing engines have abysmal thrust/weight. 10-20x worse than
>> > > rockets. So once you have a paper design of a mixed
>> > > airbreathing/rocket engine, try taking off the airbreathing part
>> > > (and it's intakes!) and just replacing it with rocket propellant.
>> > > You buy back a lot of the delta-V you wanted to get from
>> > > airbreathing. And...
>> >
>> > Actually, you don't. Because of airbreathers' (ramjet,scramjet) high
>> > Isp. This allows the mass ratio (M_liftoff/M_burnout) to come way down
>> > from 8-9 for pure rocket to as low as 4 for scramjets for the same delta
>> > v.
>> >
>>
>>
>> engines as they are for rockets. That specific impulse is paid for
>> by both the weight of inlets and propulsion equipment, and also the
>> necessity to fly a different trajectory to maximize capture of
>> air for combustion purposes, and modifying the flight path to
>> minimize drag and aero-thermal heating.
>>
>> A conventional ramjet takes in its air subsonically after it passes
>> through a shock wave and internal combustion is accomplished at
>> subsonic speed before being expanded through the exit nozzle.
>> The ramjet is good up to about Mach 8.
>>
>> Above Mach 8 the only known airbreather is the scramjet and that
>> is still under development.
>>
>> As I said in an earlier post, you should be able to get some good
>> information about these trade-offs from someone at Langley.
>>
>No, Mike, I understand very well how airbreathing systems work. I _have_
>talked to people at Langley and I am currently designing an airbreather in my
>internship in NASA's launch vehicle design branch. I understand _very_ well
>the tradeoffs with an airbreather. And I'm telling you you CANNOT make a
>blanket statement that says just because an airbreather has heavier engines,
>this outweighs the Isp benefit. You just cannot. Yes, there is a tradeoff,
>but how much of a tradeoff? I can _show_ you a design where the gain was
>high enough to offset the weight penalty and the depressed trajectory.
>--
>Pete Bellini
>Aerospace Engineer - Summer Intern Master's Candidate
>NASA Langley Research Center Georgia Tech
>Hampton, VA Atlanta, GA
Pete
How would an airbreathing engine effect the design of the Black Horse
proposal?
-ash
Jeff Greason
In article <320909...@vab01.larc.nasa.gov>, Peter Bellini
<pbel...@vab01.larc.nasa.gov> writes:
|> Jeff Greason wrote:
|>
|> >
|> > #2) Airbreathing engines have abysmal thrust/weight. 10-20x worse than
|> > rockets. So once you have a paper design of a mixed
|> airbreathing/rocket
|> > engine, try taking off the airbreathing part (and it's intakes!) and
|> > just replacing it with rocket propellant. You buy back a lot of the
|> > delta-V you wanted to get from airbreathing. And...
|>
|> Actually, you don't. Because of airbreathers' (ramjet,scramjet) high Isp.
|> This
|> allows the mass ratio (M_liftoff/M_burnout) to come way down from 8-9 for
|> pure
|> rocket to as low as 4 for scramjets for the same delta v.
|>
|> Pete
Pete,
Consider the following two notional, zero payload craft:
Pure Rocket: Airbreather+Rocket:
GLOW: 10 tons 10 tons
Structure: 0.3 tons 0.3 tons (assumes hydrocarbon fuel)
Rockets: 0.25 tons 0.1 tons
Airbreathing
Engine: 0 tons 1.2 tons (assumes vertical takeoff:
if horizontal, reduce to 0.6 tons
of engine, add 0.6 tons of wing &
gear)
Propellant: 9.45 tons 8.4 tons
Mass Ratio: 18 6.25
Delta-V @350s: 9913 m/s
Delta-V @ airbreathing
@2000s to 2800m/s,
then @350s (rockets)
(1000m/s in G & drag
losses). burn ~1.30 tons propellant airbreathing
leaves mass ratio of ~4.4 for rocket stage
so total effective delta-V is:
5082 + 2800 m/s = 7882 m/s
So, in fact, you do buy back a lot of the delta-V by replacing airbreathing
engines with rocket propellant (in this case, you buy back more than you
lose, but if you assumed heavier structures or some payload, it would go
back the other way, slightly).
These are just some ballpark numbers -- but the case for airbreathing is
a very hard one (and we haven't yet considered that *equal* delta-V's are
not even enough -- the airbreather needs *higher* delta-V to make up for
increased drag losses). To make airbreathing work, we need either:
* dramatically improved thrust/weight ratio
* airbreathing that works over a wider Mach range
or some combination of the two.
I'll say again -- I think airbreathing *can* be done in such a way that it's
a net win; but it's very tricky. Personally, I think HMX is on the right
track; use the air as *reaction mass*, but carry your own oxidizer.
Peter Bellini
GCHudson wrote:
>
> >Actually, you don't. Because of airbreathers' (ramjet,scramjet) high
> Isp. This allows the mass ratio (M_liftoff/M_burnout) to come way down
> from 8-9 for pure rocket to as low as 4 for scramjets for the same delta
> v.<
>
> vehicles, one pure rocket, the other a NASP-like thing. Pick your average
> Isp and propellant densities. Design the NASP first. Figure out the dry
> mass. Then strip off those airbreathers and put real rockets on the tail,
> and load real rocket propellant in the beast. I guarantee the rocket will
> inject more mass and payload than the airbreather. The reason: tank
> weight goes as the volume contained and is not proportional to the
> propellant mass.
The definition of Isp is:
Isp=T/(mass flow rate)
So if you get higher Isp, you get the same thrust for less mass flow.
--
Pete Bellini
Aerospace Engineer - Summer Intern Master's Candidate
NASA Langley Research Center Georgia Tech
Hampton, VA Atlanta, GA
Peter Bellini
Ash R. J. Wyllie wrote:
>
> How would an airbreathing engine effect the design of the Black Horse
> proposal?
>
> -ash
If you're referring to the RBCC engines I've been talking about, then it's very
hard to say. You get good Isp over some of your trajectory, but low T/W. With
airbreathers, you a usually right on the edge of either being practical or not.
It's very vehicle and mission specific. You really can't say whether they work
or not just with words (much to the dismay of many in this newsgroup). Many
designs come out worse than an equivalent rocket vehicle.
Black Horse (Black Colt, or whatever they call their current design) actually
uses an "airbreather". You may have heard of it. They use 2 Pratt and Whitney
F100 turbofans. The same engine as in the F-15 and -16. This gives it
excellent self-ferry capability and ability to use conventional airfields.
A good idea, if they can make the rest of it work. (Inflight LOX transfer with
the necessaey L/D, etc.)
Pete
GCHudson
>So if you get higher Isp, you get the same thrust for less mass flow.
What's your point?
You might look up density impulse in a textbook. And think about my post
again. Do the calculation. Design an airbreather and figure out the
propellant mass fraction, tank volume, etc. Then load it with rocket
propellant and recalculate the injected mass.
Henry Spencer
In article <321074...@vab01.larc.nasa.gov> Peter Bellini <pbel...@vab01.larc.nasa.gov> writes:
>GCHudson wrote:
>> BUT, you are burning a lot of LH2 to get that high Isp... tank
>> weight goes as the volume contained and is not proportional to the
>> propellant mass.
>
>The definition of Isp is:
I can guarantee that Gary Hudson knows the definition of Isp.
>So if you get higher Isp, you get the same thrust for less mass flow.
However, if you use LH2 to get that higher Isp, you pay a significant
price in higher tank mass. (Not only are LH2 tanks a lot bigger, but
in general they have to be insulated too.) The difficulty of actually
building a vehicle is not a simple function of its mass ratio.
Jeff Greason
In article <321075...@vab01.larc.nasa.gov>, Peter Bellini
<pbel...@vab01.larc.nasa.gov> writes:
|> If you're referring to the RBCC engines I've been talking about, then it's
|> very hard to say. You get good Isp over some of your trajectory, but low
|> T/W. With airbreathers, you a usually right on the edge of either being
|> practical or not. It's very vehicle and mission specific. You really can't
|> say whether they work or not just with words (much to the dismay of many in
|> this newsgroup). Many designs come out worse than an equivalent rocket
|> vehicle.
With this, I'm in complete agreement. Airbreathers are marginal vs. rocket
vehicles -- which one is better for a given application depends sensitively
on the details.
Note that "marginal" here means exactly what it says -- I've never claimed
that airbreathers are always worse -- the advantages are, however, small
positive numbers in some cases -- and small (or large) negative numbers in
others. They're just not (yet) the Holy Grail of space transport, though
I think they're promising.
Anonymous
Why does an Airbreather need to use Air all the way to orbit? It would
make more sense to use a two stage system. An Airbreather to about Mach
one or Mach two and then switch to rocket engines to orbit. Keep it
Simple!
Unless I am mistaken, something like 50% of the weight of a rocket
consists of fuel, burned to achieve a speed of about Mach one. If the
first Stage Airbreather is reuseable, I think you could have a very cost
efficient launch vehicle. Most of the statements of this thread are
focused on a solution to use an Airbreather all the way (except of course
the final kick to orbit), where all the weight savings are lost. The
flight path should be straight up just like a rocket. Using Air to lift or
support the weight of the craft isn't economical when flying at high
speeds because of drag and the need of extra thermal protection from drag.
Any realistic Airbreathing Engine that will be supplying thrust to
speeds greater than Mach two should be solid state (no turbofans). Ramjet
& ScamJets are solid state engines because they have no moving parts to
compress the air. However, they don't work at speeds below Mach One.
Turbofans are too complex, require high maintaince, they are Heavy, and
they begin to lose efficiency after Mach One (or less).
Therefore Technology to provide air compression using a solid state
mechinism is required. Such an Engine should be able to operate as a
turbofan engine does at low speeds, as well as a RamJet engine at high
speeds, and can be switched to work as a rocket engine. An Engine design,
that can operate throughout the flight would be very cost effective
because of its low weight.
It should be fairly simple to combine a RamJet and Rocket engine into a
single engine, but the real trick is designing a solid state compressor
that can provide the neccessary compression and not interfere when the
engine switches to RamJet or Rocket.
Has anyone given thought to solid state compression technology?
Solid State technologies and advanced material sciences are the way to
go. Unless these technologies are used it will never be cost effective to
reach orbit, because the craft will be too complex and too heavy. If we
are ever going to be successful in space we need to reduce the cost of
reaching orbit to about the same cost as flying a commerical airliner.
GCHudson
I hesitate to post this because it is a cheap shot, but since the original
poster was " Anon...@nowhere.org (Anonymous)", I guess anything goes.
Speaking in favor of air-breathers as the only way to go to orbit, anon.
said:
>Ramjet & ScamJets are solid state engines...
I couldn't have given scramjets (i.e., supersonic combustion ramjets) a
better name. ScamJets, indeed. This goes into the file with unobtanium,
wishalloy and that favorite of mine, rapidly solidified unobtanium.
Apologies of courage to all scramjet fans out there.
Tom Simonds
I admit I don't understand what all the obstacles are to making
workable air breathing engines. If you asked me to propose one, it
would go like this: A multi - mode, hydrogen fueled engine capable
of functioning as a turbo jet, ram jet, and rocket without having
all the components of all three.
Start with an air breathing turbo jet. Put the turbo jet inside a
ram jet, so that when the engine functions as a ram jet the air
enters the front intake, flows around the outside of the turbo jet.
\_______________________________
Intake >>>>>>>>>>>>>>> Ram Jet >>>>>>>>>>>>>>>>>> Exhaust
/-------------------------------
Turbo Jet
\_______________________________
>>>>>>>>>>>>>>> Ram Jet >>>>>>>>>>>>>>>>>>
/------------------------------
^ Movable Scoops Front and Rear Open and Close
Design the ram jet with shutters on both the front, intake side
and rear, exhaust side. To make the transition to rocket mode, the
engine closes off the front shutter, then oxygen is injected with
the hydrogen. I propose being able to close off the rear of the
rocket and open the front so the engine can produce reverse thrust
to slow the plane down on reentry.
This kind of engine could also function in simultaneous mode, so
that the turbo jet and rocket are used together. This allows for
fast acceleration on take off.
Henry Spencer
In article <Dwt0p...@world.std.com> tsim...@world.std.com (Tom Simonds) writes:
> I admit I don't understand what all the obstacles are to making
Making them powerful and lightweight. Remember, you are trying to compete
with rocket engines. Good rocket engines weigh about 1/15 as much as jet
engines with similar sea-level thrust... and rocket-engine performance
doesn't deteriorate as speed rises, while jet performance does.
The real question is, why would you bother with air breathing engines? The
idea that they are somehow superior for launcher applications is a myth.
> ...To make the transition to rocket mode, the
> engine closes off the front shutter, then oxygen is injected with
> the hydrogen...
You've turned a (presumably) efficient jet into an inefficient rocket.
As Alan Bond has pointed out, this is backwards. Ramjets give out at
Mach 6 or so, while orbital velocity is Mach 25, meaning that such a
system spends most of its time as a rocket, and must be *efficient*
when running as a rocket. You want to add air to a rocket engine, not
add oxygen to a jet engine.
> I propose being able to close off the rear of the
> rocket and open the front so the engine can produce reverse thrust
> to slow the plane down on reentry.
Slowing down on reentry requires no engine power; the drag that is always
present when flying through air suffices.
Lewis Goudy
On Aug 28, 1996 11:51:44 in article <Re: Air Breathing Spaceplanes?>,
Henry Spencer <he...@zoo.toronto.edu>' wrote:
>Ramjets give out at Mach 6 or so, while orbital velocity is Mach 25...
Is this limitation predicated on combustion as the energy source?
I have in mind augmenting or replacing it with an external drive,
eg ground or orbitally based masers.
Jeff Greason
In article <Dwt0p...@world.std.com>, tsim...@world.std.com (Tom Simonds) writes:
|> I admit I don't understand what all the obstacles are to making
|> workable air breathing engines. If you asked me to propose one, it
|> would go like this: A multi - mode, hydrogen fueled engine capable
|> of functioning as a turbo jet, ram jet, and rocket without having
|> all the components of all three.
|>
Unfortunately, while the resulting system does not have a weight equal
to the sum of the weights of all 3, it has a weight substantially greater
than the weight of the heaviest component (the turbojet). And the
turbojet is (1) already damned heavy, and (2) is your "sizing" engine
for thrust, generally, since that's your subsonic propulsion mode.
By the way, combining the rocket with the other modes probably (IMO)
isn't worth bothering with -- since for a given thrust, the rocket
weighs less than 1/10 as much, just stick it on seperately and keep
your life simple.
Now I'll admit -- one thing that puzzles me is why ramjets don't have
a reasonable T/W. I understand why turbojets are heavy (look at a cross
section diagram an it's obvious -- the things are practially solid nickel :-)
But a ramjet is mostly empty space. The only thing I can think of is
that it's the inlet and exhaust nozzle, which generally have to be
fairly complex, movable assemblies to get reasonable performance. But
does anyone have T/W numbers for the relatively simple ramjets you see
in some smaller missiles?
Niels Stchedroff
tsim...@world.std.com (Tom Simonds) wrote:
>
> I admit I don't understand what all the obstacles are to making
> workable air breathing engines. If you asked me to propose one, it
> would go like this: A multi - mode, hydrogen fueled engine capable
> of functioning as a turbo jet, ram jet, and rocket without having
> all the components of all three.
>
> ram jet, so that when the engine functions as a ram jet the air
> enters the front intake, flows around the outside of the turbo jet.
>
> \_______________________________
> Intake >>>>>>>>>>>>>>> Ram Jet >>>>>>>>>>>>>>>>>> Exhaust
> /-------------------------------
> Turbo Jet
> \_______________________________
> >>>>>>>>>>>>>>> Ram Jet >>>>>>>>>>>>>>>>>>
> /------------------------------
> ^ Movable Scoops Front and Rear Open and Close
>
> Design the ram jet with shutters on both the front, intake side
> engine closes off the front shutter, then oxygen is injected with
> rocket and open the front so the engine can produce reverse thrust
> to slow the plane down on reentry.
>
> that the turbo jet and rocket are used together. This allows for
> fast acceleration on take off.
Congratulations. You've just reinvented the J-58 engine. The SR-71
powerplant. Often spoken of as a turbo-ramjet engine. The actual fan
never shuts down, but at mach 3, it provides only 18% of total thrust.
John Schilling
lrg...@usa.pipeline.com (Lewis Goudy) writes:
"Ramjets" implies subsonic-combustion, or in more general terms, sunsonic
energy addition, ramjets. If the airflow is supersonic throughout, we
call them "scramjets", which have their own problems, which are best left
for another post.
With ramjets proper, then, the limitation comes with the need to decelerate
the incoming air to a relative mach number of less than one, before adding
fuel (or laser energy, or whatever). Decelerating fast-moving air involves
compressing said air, which in turn invariably *heats* the air.
If the velocity of the incoming air is about Mach 8, the compression
process with produce so much heat that the air *entering* the combustion
chamber will already be at the thermal limit for the best engine materials
we can conceive. Which means you can't add *any* heat, by fuel combustion
or lasers or whatever, without melting the engine.
If you can't add heat, you can't add energy, and you can't produce thrust.
The thrust of a (subsonic combustion) ramjet drops to zero as the velocity
reaches about Mach 8. Mach 6 is then a practical limit - you can still get
*some* thrust, just not enough to be worth bothering with.
--
*John Schilling * "You can have Peace, *
*Member:AIAA,NRA,ACLU,SAS,LP * or you can have Freedom. *
*University of Southern California * Don't ever count on having both *
*Aerospace Engineering Department * at the same time." *
*schi...@spock.usc.edu * - Robert A. Heinlein *
*(213)-740-5311 or 747-2527 * Finger for PGP public key *
Kelly St
One air breathing systems I heard of is a pulse scramjet. It uses a
nozel, or cylinder, mounted slightly away from base. Sort of like a trash
can with the bottom removed mounted and inch or three above an up ended
trashcan.
Anyway you spray fuel into this from the base, and set it off with a
spark. The blast goes out through the gap around the base and out through
the nozel. Most goes out the nozel and produces thrust. But the inertia
of all that mass going out the nozel, then causes a partial vacume around
base. That causes fresh air to be sucked in through the gap near the
base. Spray in more fuel and repeat.
The idea works (in computer sims according to an Aviation Week artical)
from a standing start up to fairly high mach numbers. (6-10 I think??)
If you flare the cylinder out to an expansion nozel and ad oxegen you
might get an effective rocket out of the deal. The system has few parts
(nozel assembly and fuel injectors) so it should be light and relyable.
(If you could get it to work.) It could power a craft from dead stop to
high speed, maybe orbital speed with a rocket cycle. So It could solve a
lot of launcher problems.
I hope someone is still researching it.
Kelly St
> From: gchu...@aol.com (GCHudson)
> Date: 27 Aug 1996 13:17:18 -0400
> >Ramjet & ScamJets are solid state engines...
> I couldn't have given scramjets (i.e., supersonic combustion ramjets) a
> better name. ScamJets, indeed. This goes into the file with
unobtanium,
> wishalloy and that favorite of mine, rapidly solidified unobtanium.
I thought the Auroras were assumed to be external burning pulse scramjets.
Their sighted Mach 6+ speeds would seem high for a ram jet. And their
external burning diamond hull cross sections seemed more scram then ram
compatable.