Air Breathing for VTVL

Until recently I have considered air breathing engines on VTVL to
not be worth consideration. Now I believe they *might* be worth
consideration if they can reach certain performance goals. The
performance goals I have in mind are,

Dry mass/payload ratio to remain similar if, and only if, some
substantial performance or operational gain is made.
or
Cost of construction, testing, and operations drop a considerable
amount for similar mission capability.

I would like to quantify the performance requirement for
the curve in which the addition of a subsonic air breathing
engine reaches break even vs an all rocket VTVL.

There are 3 places I am aware of to get the mass for the air breathing
engine I am interested in. Replacement of some rocket engine mass
from launch for a minute or so. Minor fuel savings during launch.
Elimination of some landing fuel required by all rocket landing.
For figuring purposes, a 160,000 lb GLOW vehicle with 10,000
lb dry mass including payload and landing fuel. This is a dense fuel
SSTO as I am not sold on hydrogen for various reasons.

If terminal velocity is 100 m/s and 3 gee deceleration is used with no
margin, then a 30k rocket engine will burn 500 lbs of fuel in 5 seconds
before
landing. To match this a 15k air breathing engine will have a 20 second
burn using 150 to 300 lbs of fuel at Isp=2,000 and Isp=1,000 respectively.
Those two points on the curve suggest that the engines could mass between
350 and 200 lbs to match the pure rocket performance. This requires a T/W
of 43 and 75 for the 2 cases.

Replacement of some rocket engine mass on launch can only be a contribution
to the available air breathing engine mass. If the orriginal rocket is 200k
at
sea level, then the thrust replaced can only be about half of the 15k as the
air breathing engine loses thrust during the climb and craps out entirely at
30,000 to 50,000 feet. The gravity losses after that cut the gains some.
Say 7.5k of rocket thrust traded at T/W 125 for a 60 lb savings applied to
the air breathing engine.

Fuel savings on launch are argueable. Say 60 seconds at 10k average thrust.
This is 300 to 600 lbs of fuel vs 2,000 lbs for the rocket. This minor
savings
on fuel is during the low slow part of the trajectory. The best I can guess
is
it translates to 100-150 lbs in orbit.

This suggests that there would be 550 lbs available to the 2,000 second
air breather and 360 lbs available for the 1,000 second air breather.
T/W ratios required would be 28 and 42 respectively for these systems
to match pure rocket performance on a VTVL.

Leaving aside whether it is worthwhile to use air breathing at all, are
these numbers accurate enough to start an honest decision curve?

johnhare wrote:

This suggests that there would be 550 lbs available to the
2,000 second air breather and 360 lbs available for the
1,000 second air breather. T/W ratios required would be 28 and
42 respectively for these systems to match pure rocket
performance on a VTVL.

What kind of airbreathing engines do you have in mind here? If
turbomachinery based your Isps are too low and your T/Ws are too
high.

More importantly, I think if you want your airbreathers to do
double duty during ascent and recovery you're going to have to
address the issue of increased installation weight. An installation
optimized for deceleration/hover may not work well while
accelerating and vice versa.

Jim Davis

==
I would like to quantify the performance requirement for
the curve in which the addition of a subsonic air breathing
engine reaches break even vs an all rocket VTVL.

There are 3 places I am aware of to get the mass for the air breathing
engine I am interested in. Replacement of some rocket engine mass
from launch for a minute or so. Minor fuel savings during launch.
Elimination of some landing fuel required by all rocket landing.
For figuring purposes, a 160,000 lb GLOW vehicle with 10,000
lb dry mass including payload and landing fuel. This is a dense fuel
SSTO as I am not sold on hydrogen for various reasons.

Agreed!

Coincidentaly I just finished up a article for the http://www.rocketmanblog.com
blog about a notianal HTHL SSTO using jets up to about 100,000 - 150,000 feet
using a jet engine modification Darpa is pushing for their TSTO Rascel program.
(You might find the spreadsheet usefull? E-mail me if you want a copy.)

The Rascal paperâÿóÿý™s a

http://cism.jpl.nasa.gov/events/work...ton_Carter.pdf

http://www.darpa.mil/TTO/rascal/RASCAL_PS_Final.pdf

http://hypersonic2002.aaaf.asso.fr/papers/17_5148.PDF

Darpa has been doing tests on the engines the craft would need, and is
designing the system around off the shelf parts. Their 0 to maybe up to Mach 6
jet engines are modified versions of the engines that have been flying in the
F-15's for 30 years. No need for new rocket based combined cycle engines
(though they might be nice). They just spray water into the engine intake to
cool the incoming air to save the turbojets from overheating, and spray some
liquid oxygen ahead of the burners to make up for the thin air. Its like hot
roding up a normal car engine for occasional drag racing. Its been increasing
the thrust, without hurting the engines, for short bursts up to Mach 6. It
will get the Rascel mother ship up to 200,000 feet or so (in a glide after
flame out) where you drop of the upper stages out off a bomb bay like chamber.
Obviously after reentering and restarting the engines, the winnged Rascel
simply flies back to base.

T/W on the best fighter turbojets on the market are about 10-1, but the papers
list "significant" increases in the thrust with the mass and oxegen
augmenntation, but I couldn't find more details.

T/W on airliner like turbofans is much higher, and I've head of designs using
them with ramjets in the ducts to get higher T/W and speed, though possibly
more speed limitations then the Rascel design.

Given vertical launch SSTOs seem to burn about half their fuel/LOx to get to
Mach 6 and out of the air, this seems very usable.


I found with a LOX Kerosene fueled DC-X style SSTO. the fuel/LOx weighs 13
times as much as the rest of the ship combined. If you use jets like Darpa is
working on to Mach 6, then boost to orbit with rockets. Instead of needing 13
times the unfueled weight of the vehicle to get into orbit, you only need about
6 times the dry weight.

For a Winged craft like I was connsidering you'ld need to mid air "refuel" with
LOx like the Pioneer / Blackhorse concept, or fill the LOx tanks from the air
in midflight like
http://www.andrews-space.com/en/corporate/Alchemist(200311).html. Liquid
hydrogen fueld concept is proposeing.



So adding it up and skipping over the spreadsheet and equations. Assuming a
craft with cargo and everything it needs in space, weighs about 50 ton's. My
concept would:

- Takes off with 90 tons of Kerosene. (A smaller fraction then the
1950's SR-71'a could. If you want you can take off with less kerosene and add
more in flight.)
- Its carrying 10-16 tones of modified F-15 jet engines (Depending on the
acceleration rate you want) which is about the weigh fraction of the engines in
a F-15.
- Under 2 tons of empty LOx tanks and less then a ton of kerosene tank.
Note the Kerosene "tank" is likely mostly the wing, so it will likely weigh
less.
- Under 5 tons of rocket engines like P&W's RD-180s. (Depending on your
assent trajectory. Shuttles engine thrust is less then its weight after SSRB
sep, but gets to orbit either way.)

In flight you add 190 tons of LOx (liquefied oxygen). Maybe with mid air
refueling. Maybe with mid-air oxygen mining and liquefaction. The plane can
fly with this much weight as long as its going several hundred miles per hour.
Though its probably handling like a pig.

You bring all your jet engines to full power and boost out for speed and
altitude. When the engines finally flame out your at Mach 6 and leaving all
but wisps of the atmosphere. You start the RD-180 ish rocket engines. They
consume the rest of that huge fuel reserve, bringing the 50 tons of craft and
cargo into orbit. Assuming heaviest assumptions for jet and rocket engine
weights, everything else now weighs 27 tons. You drop your cargo (likely 5
tons or less) or dock with a station. When you want to come down you use a
small burn to decelerate you, and you renter.


One of the oldest serious proposals to build a HTHL SSTO (though on a far
larger scales) was Star-Raker by Rockwell in the '70's.
http://www.abo.fi/~mlindroo/SpaceLVs/Slides/sld047.htm

It took off and landed from a runway with its fuel and oxygen load. Used
rocket engines and ramjets for flight and boost to orbit. Expected little
issue with reentry heating due to its large size and proportionally low reentry
weight.


Hope this helps.

Kelly
Kelly Starks


"Humans are a race of compassionate predators."

"Jim Davis" wrote in message
. 1.4...
johnhare wrote:

This suggests that there would be 550 lbs available to the
2,000 second air breather and 360 lbs available for the
1,000 second air breather. T/W ratios required would be 28 and
42 respectively for these systems to match pure rocket
performance on a VTVL.

What kind of airbreathing engines do you have in mind here? If
turbomachinery based your Isps are too low and your T/Ws are too
high.

I believe I have invented a new arraingement for the turbomachinery
that is much lighter than that in current equipment. The aero/thermodynamic
cycle is a hybrid turbojet and air-turborocket. I finally figured a way
to build test models on my budget. The key point was finding a way
to build a blade row that operates efficiently as compressor over part
of its cycle, and partial admission turbine over the other part. The
blade row is regeneratively cooled during the compressor portion.
This allows much higher turbine admission temperatures than normal
and eliminates the afterburner. I am looking for the target performance
curve where the airbreather does not have a performance penalty
vs a pure rocket system.

More importantly, I think if you want your airbreathers to do
double duty during ascent and recovery you're going to have to
address the issue of increased installation weight. An installation
optimized for deceleration/hover may not work well while
accelerating and vice versa.

Any increased installation weight will have to be charged off to the
airbreather, which makes the true curve possibly more difficult to
reach. I believe I have found an installation scheme for this particular
engine type that will operate somewhat effectively in both orientations.

There are three performance curves on the graph. The low one is
what many people shoot for. Airbreathing is so desirable that
substantial performance penalties are going to be overlooked in
order to incorporate them. The middle curve is where performances
just match. The most difficult one is where not including airbreathers
must be justified in terms of simplicity or cost. I am interested in the
middle curve.

Jim Davis

John Hare

"Kelly St" wrote in message
...
==
I would like to quantify the performance requirement for
the curve in which the addition of a subsonic air breathing
engine reaches break even vs an all rocket VTVL.

There are 3 places I am aware of to get the mass for the air breathing
engine I am interested in. Replacement of some rocket engine mass
from launch for a minute or so. Minor fuel savings during launch.
Elimination of some landing fuel required by all rocket landing.
For figuring purposes, a 160,000 lb GLOW vehicle with 10,000
lb dry mass including payload and landing fuel. This is a dense fuel
SSTO as I am not sold on hydrogen for various reasons.

Agreed!

Coincidentaly I just finished up a article for the
http://www.rocketmanblog.com
blog about a notianal HTHL SSTO using jets up to about 100,000 - 150,000
feet
using a jet engine modification Darpa is pushing for their TSTO Rascel
program.
(You might find the spreadsheet usefull? E-mail me if you want a copy.)

I couldn't find your article at that site. I have a bit of a problem
buying HTHL SSTO as being able to close the technical case.
The landing gear and aero surfaces are unavoidable mass that
has to be hauled in both directions. For suborbital or first stage
work HTHL is more competative.

The Rascal paper s a

http://cism.jpl.nasa.gov/events/work...ton_Carter.pdf

http://www.darpa.mil/TTO/rascal/RASCAL_PS_Final.pdf

http://hypersonic2002.aaaf.asso.fr/papers/17_5148.PDF

I have some familiarity with this.

Darpa has been doing tests on the engines the craft would need, and is
designing the system around off the shelf parts. Their 0 to maybe up to
Mach 6
jet engines are modified versions of the engines that have been flying in
the
F-15's for 30 years. No need for new rocket based combined cycle engines
(though they might be nice). They just spray water into the engine intake
to
cool the incoming air to save the turbojets from overheating, and spray
some
liquid oxygen ahead of the burners to make up for the thin air. Its like
hot
roding up a normal car engine for occasional drag racing. Its been
increasing
the thrust, without hurting the engines, for short bursts up to Mach 6.
It
will get the Rascel mother ship up to 200,000 feet or so (in a glide after
flame out) where you drop of the upper stages out off a bomb bay like
chamber.
Obviously after reentering and restarting the engines, the winnged Rascel
simply flies back to base.

For my purposes, modifications of existing hardware are somewhat
interesting.
I have been unable to locate a single paper, book, or proposal that makes a
convincing case for high mach modifications of existing equipment.
Everything
I have seen so far gets too complex too fast for the purported advantage
over mach 3 or so.

T/W on the best fighter turbojets on the market are about 10-1, but the
papers
list "significant" increases in the thrust with the mass and oxegen
augmenntation, but I couldn't find more details.

My interest is in finding the T/WxIsp curve that makes a compelling
arguement
for a new type engine. IMO T/W of 25+ is possible with 4 digit Isp.

T/W on airliner like turbofans is much higher, and I've head of designs
using
them with ramjets in the ducts to get higher T/W and speed, though
possibly
more speed limitations then the Rascel design.

T/W on turbofans is typically lower in exchange for much better fuel
economy.
The curve is very different for cruise. I have not seen a convincing paper
for using ramjets. By the time ramjets start working well, it's past time to
get out of the atmosphere.

Given vertical launch SSTOs seem to burn about half their fuel/LOx to get
to
Mach 6 and out of the air, this seems very usable.

Verticle launch runs out of useful propulsion air long before mach 6. LOX is
cheap, I am comparing against the dry mass.

I found with a LOX Kerosene fueled DC-X style SSTO. the fuel/LOx weighs 13
times as much as the rest of the ship combined. If you use jets like
Darpa is
working on to Mach 6, then boost to orbit with rockets. Instead of
needing 13
times the unfueled weight of the vehicle to get into orbit, you only need
about
6 times the dry weight.

The ratio is more like 16. The problem is that those engines to mach 6 have
to be carried to orbit and back in addition to the rockets required anyway.
The mass breakdown has not been shown to work that I am aware of.
You might get GLOW down some, usually at the expense of more hardware,
complexity, and flight path difficulties.

For a Winged craft like I was connsidering you'ld need to mid air "refuel"
with
LOx like the Pioneer / Blackhorse concept, or fill the LOx tanks from the
air
in midflight like
http://www.andrews-space.com/en/corporate/Alchemist(200311).html. Liquid
hydrogen fueld concept is proposeing.

Familiar with them. Not the question I'm working on.


So adding it up and skipping over the spreadsheet and equations. Assuming
a
craft with cargo and everything it needs in space, weighs about 50 ton's.
My
concept would:

- Takes off with 90 tons of Kerosene. (A smaller fraction then the
1950's SR-71'a could. If you want you can take off with less kerosene and
add
more in flight.)
- Its carrying 10-16 tones of modified F-15 jet engines (Depending on the
acceleration rate you want) which is about the weigh fraction of the
engines in
a F-15.
- Under 2 tons of empty LOx tanks and less then a ton of kerosene tank.
Note the Kerosene "tank" is likely mostly the wing, so it will likely
weigh
less.
- Under 5 tons of rocket engines like P&W's RD-180s. (Depending on your
assent trajectory. Shuttles engine thrust is less then its weight after
SSRB
sep, but gets to orbit either way.)

In flight you add 190 tons of LOx (liquefied oxygen). Maybe with mid air
refueling. Maybe with mid-air oxygen mining and liquefaction. The plane
can
fly with this much weight as long as its going several hundred miles per
hour.
Though its probably handling like a pig.

You bring all your jet engines to full power and boost out for speed and
altitude. When the engines finally flame out your at Mach 6 and leaving
all
but wisps of the atmosphere. You start the RD-180 ish rocket engines.
They
consume the rest of that huge fuel reserve, bringing the 50 tons of craft
and
cargo into orbit. Assuming heaviest assumptions for jet and rocket engine
weights, everything else now weighs 27 tons. You drop your cargo (likely
5
tons or less) or dock with a station. When you want to come down you use
a
small burn to decelerate you, and you renter.


One of the oldest serious proposals to build a HTHL SSTO (though on a far
larger scales) was Star-Raker by Rockwell in the '70's.
http://www.abo.fi/~mlindroo/SpaceLVs/Slides/sld047.htm

It took off and landed from a runway with its fuel and oxygen load. Used
rocket engines and ramjets for flight and boost to orbit. Expected little
issue with reentry heating due to its large size and proportionally low
reentry
weight.


Hope this helps.

My orriginal question is simply stated as, "what performance is required of
a new style air breathing engine that would win an ICH tee shirt?"

Kelly
Kelly Starks


"Humans are a race of compassionate predators."

"johnhare" writes:

Airbreathing is so desirable that substantial performance penalties
are going to be overlooked in order to incorporate them.

I very much question this claim --- especially for VTVL.

Air-breathing T/W ratios are so wimpy that they almost always force wings
and horizontal lift-off in the final analysis, since the engine cannot
lift the weight of the fully loaded vehicle. You can dream all you want
about air-breathing engines with a T/W ratio of "43 to 75," but I very much
doubt that you or _anyone_ will be shipping one any time soon !!!

Furthermore, you appear to have made the common false assumption that
air-breathing performance is independent of airspeed. In point of fact,
the effective I_sp of an air-breathing engine is roughly inversely
proportional to airspeed above roughly Mach 1, so that at Mach 6,
the effective I_sp of an air-breather is only a few times better
than a rocket burning the same fuel. And since you are also assuming
water _AND_ LOX injection, you must include these in your propellant input,
so that your effective I_sp is even further degraded. At this point,
your engine is starting to look more like a bad rocket than a
air-breather. And as Henry Spencer has pointed out many times
in this newsgroup, when a careful performance analysis is done,
one usually finds in the end that it is better to build a good rocket
that can double as a bad air-breather than an air-breather that can double
as a bad rocket.

Finally, since most of the propellant will still be consumed after
air-breathing has become useless, unless you go to two stages or otherwise
drop off your fancy air-breathing engines when you reach Mach 6 or so,
all that heavy turbomachinery becomes so much useless dead mass for most
of the trajectory to orbit.

In summary, I continue to remain unconvinced that air-breathing is even the
_least_ bit desirable for anything except possibly the first stage of TSTO.
Furthermore, the claim that air-breathers can achieve a T/W exceeding 40,
and will be useful for VTVL makes me fall down and roll on the floor,
laughing my head off...


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

"Gordon D. Pusch" wrote in message
...
"johnhare" writes:

Airbreathing is so desirable that substantial performance penalties
are going to be overlooked in order to incorporate them.

I very much question this claim --- especially for VTVL.

It is somewhat irritating that you snipped a single sentence from
the paragraph to make it seem that my position is far from where
it really is. I pasted the paragraph in below to show the difference.

There are three performance curves on the graph. The low one is
what many people shoot for. Airbreathing is so desirable that
substantial performance penalties are going to be overlooked in
order to incorporate them. The middle curve is where performances
just match. The most difficult one is where not including airbreathers
must be justified in terms of simplicity or cost. I am interested in the
middle curve.

This is my position. I am looking for useful answers to what airbreathers
have to achieve to match rockets. The many people that write papers
and advocate airbreathing at any cost should be keelhauled.

Air-breathing T/W ratios are so wimpy that they almost always force wings
and horizontal lift-off in the final analysis, since the engine cannot
lift the weight of the fully loaded vehicle. You can dream all you want
about air-breathing engines with a T/W ratio of "43 to 75," but I very
much
doubt that you or _anyone_ will be shipping one any time soon !!!

You should have noted that my post suggested that this would be the required
performance to match rockets, not that the 43-75 was feasable in the
forseeable planning horizon.

Furthermore, you appear to have made the common false assumption that
air-breathing performance is independent of airspeed. In point of fact,
the effective I_sp of an air-breathing engine is roughly inversely
proportional to airspeed above roughly Mach 1, so that at Mach 6,
the effective I_sp of an air-breather is only a few times better
than a rocket burning the same fuel. And since you are also assuming
water _AND_ LOX injection, you must include these in your propellant
input,
so that your effective I_sp is even further degraded. At this point,
your engine is starting to look more like a bad rocket than a
air-breather. And as Henry Spencer has pointed out many times
in this newsgroup, when a careful performance analysis is done,
one usually finds in the end that it is better to build a good rocket
that can double as a bad air-breather than an air-breather that can double
as a bad rocket.

Are you responding to my post or someone elses? I am comparing
performance usefullness of a subsonic airbreathing system. Supersonic
requires fairly heavy (by comparison) intakes.

Finally, since most of the propellant will still be consumed after
air-breathing has become useless, unless you go to two stages or otherwise
drop off your fancy air-breathing engines when you reach Mach 6 or so,
all that heavy turbomachinery becomes so much useless dead mass for most
of the trajectory to orbit.

This is definately in response to someone else.

In summary, I continue to remain unconvinced that air-breathing is even
the
_least_ bit desirable for anything except possibly the first stage of
TSTO.
Furthermore, the claim that air-breathers can achieve a T/W exceeding 40,
and will be useful for VTVL makes me fall down and roll on the floor,
laughing my head off...

My orriginal question stands unaddressed, What performance is required
of an air breathing engine in order to match an all rocket LV performance.
I questioned whether the numbers I have derived for a requirement are
accurate enough for reasonable decisions to be made.
You should note that I also said that T/W of 25 was in reach maybe.

While you are rolling on the floor laughing, care to make a small wager
on the capabilities of a concept demonstrater? Since you are so certain,
you should be willing to offer really good odds.



-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

Opps,
Sorry I didn't respond to your post earlier. I've been busy and hadn't checked
in.

My article isn't on the site yet. The site editor is busy and getting behind.


You said, "I have a bit of a problem buying HTHL SSTO as being able to close
the technical case.". I'm not clear what your questions were. For a Mach 3 to
6 airbreathing bird I was getting take off weights ( LOx Added later) of
170-200 tons. And crafte weight on orbit of 50. Weight on orbit minus all
tanks and engines of 23-29 tons. Even cuting out 5 tons for reentry systems,
and 4-5 tons for landing gear, that still leaves 13-20 tons for frame crew,
crago, and such. Which seemed doable from what I could see.


I think the main difference between us is goals:


My interest is in finding the T/WxIsp curve that makes a compelling
arguement
for a new type engine. IMO T/W of 25+ is possible with 4 digit Isp.

===

My orriginal question is simply stated as, "what performance is required
of a new style air breathing engine that would win an ICH tee shirt?"

I was interested in seeing if you could do it with current equipment, not
searching for a justification for a new one.

I'm not saying a new engine with better perfomance wouldn't make things, but
I'm not at all clear Its needed to get the job done. A first generation craft
that could drop cost to orbit by a order of magnitude or two, and greatly
increase flexibility, would get the ball runing and later bankroll the dev of
better engines.
Kelly Starks


"Humans are a race of compassionate predators."

"Kelly St" wrote in message
...
Opps,
Sorry I didn't respond to your post earlier. I've been busy and hadn't
checked
in.

My article isn't on the site yet. The site editor is busy and getting
behind.


You said, "I have a bit of a problem buying HTHL SSTO as being able to
close
the technical case.". I'm not clear what your questions were. For a Mach
3 to
6 airbreathing bird I was getting take off weights ( LOx Added later) of
170-200 tons. And crafte weight on orbit of 50. Weight on orbit minus
all
tanks and engines of 23-29 tons. Even cuting out 5 tons for reentry
systems,
and 4-5 tons for landing gear, that still leaves 13-20 tons for frame
crew,
crago, and such. Which seemed doable from what I could see.

I have read a lot of information on using existing airbreathing engines, on
paper, disk, and web. None of them have been able to make a good case.
Most of the ones I have looked at use GLOW as the standard to judge by.
I'm looking for dry mass as the standard. A turbojet costs orders of
magnitude more per launch pound than LOX, or even RP. So far,
I have not seen one source that has dropped dry mass with the use of
any current airbreathing engines.

I think the main difference between us is goals:


My interest is in finding the T/WxIsp curve that makes a compelling
arguement
for a new type engine. IMO T/W of 25+ is possible with 4 digit Isp.

===

My orriginal question is simply stated as, "what performance is required
of a new style air breathing engine that would win an ICH tee shirt?"

I was interested in seeing if you could do it with current equipment, not
searching for a justification for a new one.

I have concluded no on this question. I am trying to find the break point
where that answer becomes yes. Airbreathing engines offer certain
operational advantages. The target is to find the point where these
operational advantages can be had without a performance penalty
compared to pure rocket operation.

I'm not saying a new engine with better perfomance wouldn't make things,
but
I'm not at all clear Its needed to get the job done. A first generation
craft
that could drop cost to orbit by a order of magnitude or two, and greatly
increase flexibility, would get the ball runing and later bankroll the dev
of
better engines.
Kelly Starks


"Humans are a race of compassionate predators."

"Kelly St" wrote in message
...
I was interested in seeing if you could do it with current equipment, not
searching for a justification for a new one.

I'm not saying a new engine with better perfomance wouldn't make things,
but
I'm not at all clear Its needed to get the job done. A first generation
craft
that could drop cost to orbit by a order of magnitude or two, and greatly
increase flexibility, would get the ball runing and later bankroll the dev
of
better engines.

I should have put some numbers on my other reply to clarify my points a
bit better. An all rocket VTVL SSTO is probably feasible at this time.
The margins will be tight with no slack for extra systems that don't pull
their own weight. Adding any current airbreathing engine to the projected
VTVL SSTO will cut into the payload for a given dry mass.

Take a theoretical VTVL SSTO at 100,000 lbs GLOW. At mass ratio of
16, there will be 6,250 lbs mass in orbit. If you add some hypothetical jet
which cuts the GLOW to 80,000 lbs and increases the mass in orbit to
7,250 lbs for the same payload, you have lost. The 1,000 lb jet you added
could cost something on the order of $1M. That $1M will buy 20,000,000
lbs of the LOX you replaced. That is 1,000 launches to reach break even
all else being equal.

All else is seldom equal. That 1,000 lb jet must be accelerated all the way
to
orbit, reentered, and landed. Most studies would seem to indicate that
adding
the jet will prevent SSTO operation without seriously advanced tech across
the board. Increasing the reentry mass and landing mass also increases the
cost of the vehicle. 1 lb of hardware at $1,000 lb will buy 20,000 lbs
of LOX.

For HTHL, you are lifting wings and landing gear sized for GLOW all the
way to orbit and back. Add all the weights of HTHL and you don't reach
SSTO mass ratio requirements. Add a second type all up propulsion system to
the required rockets and it becomes more difficult yet. You are forced into
some form of air tow or refueling to even attempt making it to orbit. Most
of the papers I have read assume one or more advanced technologies to
make this happen.

If you go TSTO, then many possibilities open up. Air breathing becomes
more feasible in several of them. I am basing on SSTO because it is a useful
math model. If pure rocket SSTO is feasible, and adding jets requires an
additional stage, then the jets hamper performance. You shouldn't add
systems that hamper performance without a very good reason. I would
like to find the point where the operational flexibilities possible with an
airbreathing engine do not cost performance in real terms.


Kelly Starks


"Humans are a race of compassionate predators."