How Rockets Differ From Jets

Rockets are a much different propulsion system then jets. They look a
little bit similiar, but work on somewhat different principles and
perform much differently:

Note: I am writing this because rocket propelled spaceplanes aren't
getting the attention they should be. Everything seems to be focused
on vertical takeoff rockets which are currently in abundance.

* A jet engine uses oxygen in the air as the oxidizer. Therefore, a
jet engine cannot operate in space or near space where oxygen is either
non-existant or negligible.

* Jet engines are fairly 'heavy' because they have metal turbines that
spin on the inside, compressing the air before fuel is added. What the
turbines can take in temperature and centrifugal forces limits the
engine's performance.

* Jet engines economize on fuel relative to a rocket engine. They,
therefore, operate continuously from takeoff to landing. They also
have a much smaller fuel supply which is at best equal to the dry
weight of the jet airplane. The engines work for hours, up to half a
day on our larger commercial jets, without refueling.

* Jet engines are highly refined, smooth running, low maintenance
machines that are used in thousands of aircraft on a daily basis.

When we think of rocket planes, or spaceplanes, we think subconsciously
of them having jets on them because this is what we are used to seeing.
Sure, we know that they have to be scram jets or rockets, but our
experience is with jet aircraft -- not rocketships.

* A rocket engine does not use oxygen from the air. It carries
oxygen, or some form of oxidizer, along with whatever fuel it is using.
This adds significant fuel weight to a rocketplane relative to a jet
aircraft. But it also frees the rocketplane from the Earth's
atmosphere. Space and near space are no longer barriers to combustion.

* Though the fuel weight of a rocketplane is heavy the weight of the
SSME, for example, is about 7,800 pounds. In short, the rocket engine
is much lighter -- per pound of thrust -- than a jet engine. It
becomes possible, therefore, to use additional rocket engines for VTOL,
4 for example, adding only an additional 31,200 pounds. Those same
engines could be used for reverse thrust to rapidly slow reentry speed.

* Vertical rockets are extremely fuel intensive. Rising vertically
with no wings means that fuel and rocket thrust has to counteract the
force of gravity, then additional fuel and rocket thrust has to give
momentum to the vehicle, which by it's very nature is crammed to the
gills with . . . fuel. And, liquid fuels are heavy. Go grab a gallon
of water, milk, or gasoline then figure out what giant tanks of liquid
weigh. No wonder the space shuttle, with about 7.5 million pounds of
thrust, rises so slowly. It weighs nearly 7.5 million pounds!

* The SSME (Space Shuttle Main Engine) is an example of a highly
refined and extremely reliable rocket engine. It is good for about 50
uses. Note: A rocket engine is used once everytime it is turned on
and off. After 50 uses you replace the engine with a new one. This
might work out to about 5 - 10 missions.

* Also, rockets do not burn continuously. Burns are usually specified
in terms of minutes, or seconds, not hours as with jets. The Shuttle's
reentry retrofire, for example, is a 10 second burn. But, rockets have
enormous thrust compared to a jet. They can do more in a couple of
minutes than a jet can do in 10 or 12 hours!

So, are rocketplanes, or spaceplanes if you prefer, different from jet
planes? Yes. In fact there are even more differences to consider.

A rocketplane must endure hypersonic flight -- in the atmosphere --
everytime it goes into space or reenters from space. Hypersonic flight
has one extreme difficulty: blast furnace temperatures. Temperatures
so hot they can melt any known steel in a matter of seconds, as we
learned watching the Columbia breakup in the atmosphere. Is this
insurmountable? No!

Ceramics can take hypersonic skin temperatures -- easily. They can
protect, by reflecting the heat, the materials underneath. Fire brick
is used in blast furnaces. They are used again and again and are
replaced yearly. They (silica tiles) are not only used in blast
furnaces but are used on the Space Shuttle. They are extremely light
and extremely thermal reflective. Perfect, except that they are also
soft and brittle. Not good for a rocketplane in a hypersonic airflow.

The proven material for hypersonic airflow is Corelle ceramic. It was
made for ballistic missile nosecones, tested, and in use for decades.
A spinoff, we use Corelle for dinnerware. Great stuff! It weighs a
little more than fire brick and is not quite as reflective, but it is
tougher, can be cast in larger sections, and can take almost any amount
of heat. Thinly sliced Corelle can give a lot of protection. It could
even cover fire brick, and protect it, creating a ceramic composite of
sorts. So, proven technology, has part of the heating problem solved.

Add the cryogenic cooling of liquid hydrogen as it goes to the engine
and the Shuttle's vacuum bottle design where a vacuum space protects
the inner from the outer hull, and a shirt sleeve environment is
possible. Best to have the astronauts in spacesuits, though, just in
case.

A HTOL (Horizontal Take Off and Land) rocket is more efficient than a
VTOL (Vertical Take Off and Land) tubular rocket. You do not use the
same equations to determine range, because with a waverider body the
weight is being lifted by the shockwave, not by extra fuel. In short,
the vast majority of the fuel is used for forward thrust. The old B-29
could travel thousands of miles at 20,000+ feet with a thrust to weight
of 10%. That's right, the B-29's engines were only 1/10 as powerful as
the aircraft's weight. But it took off again and again loaded with
bombs.

So, you ask, why aren't we already in Outer Space with Spaceplanes?

Answer: I don't know, I really don't know! If anyone knows please
tell me. Please!! (NASA, can you tell me?)


tomcat

tomcat wrote:
snip

A HTOL (Horizontal Take Off and Land) rocket is more efficient than a
VTOL (Vertical Take Off and Land) tubular rocket. You do not use the
same equations to determine range, because with a waverider body the
weight is being lifted by the shockwave, not by extra fuel. In short,
the vast majority of the fuel is used for forward thrust. The old B-29
could travel thousands of miles at 20,000+ feet with a thrust to weight
of 10%. That's right, the B-29's engines were only 1/10 as powerful as
the aircraft's weight. But it took off again and again loaded with
bombs.

So, you ask, why aren't we already in Outer Space with Spaceplanes?

You're glossing over the numbers.
Yes, you only need weight/ (L/D ratio) to push something flying in order
to gain level.

But, the mission of a launcher is NOT to cruise for a long time, it's to
get into orbit.
More thrust with rockets is cheap,
The time you are in the atmosphere is time you are subjected to lots of drag.
This means you need to carry lots of fuel.

Ian Stirling wrote:
You're glossing over the numbers.
Yes, you only need weight/ (L/D ratio) to push something flying in order
to gain level.

But, the mission of a launcher is NOT to cruise for a long time, it's to
get into orbit.
More thrust with rockets is cheap,
The time you are in the atmosphere is time you are subjected to lots of drag.
This means you need to carry lots of fuel.


Yes, achieving level flight is easier than SSTO (Single Stage To
Orbit). Waveriders deal very effectively with drag, however.

There is a little 'trick' to drag as well. Slimming the wings, or at
least lessening the lift by reducing body/wing curvature, lessens drag
as well.

When this is done you also have to decide on the takeoff and landing
speeds that are reasonable and possible. Slimming the wings to a 300
knot takeoff means strong landing gear and a long runway. Ditto for
landings.

Hypersonic waveriding SSTO's have been referred to as 'flying gasoline
cans'. Though the fuel is unlikely to be gasoline, 95+ % of the
dryweight is going to be fuel tanks.

This means designing a SSTO waverider is actually . . . easy! It also
means that you are taking a crew into the hottest blast furnace
imaginable surrounded by and sitting on -- volatile fuel. Not so easy.

So far, preliminary calculations indicate that starting out with a 1:1
thrust to weight is probably best. This should give the takeoff and
early flight performance of a F-15 Eagle.

After a scant minute or so thrust to weight will have climbed to 2:1
giving enough push to slice through the hypersonic speeds and touch
near space. Another 1 to 1 1/2 minutes should put the spaceplane into
orbit. So, we are talking about 3 to 4 minutes of burn time.

It is best to have -- and keep for retrofire or reverse thrust -- an
extra minute of fuel on board. So, all in all it works out to about 5
minutes of fuel. For calculations, with the SSME (Space Shuttle Main
Engine) as engine of choice, figure 1035 pounds of fuel consumed at
full throttle each second.

Now you can figure the necessary wet weight of the spaceplane and add
that to the the dry weight. My ballpark figures, taking into
consideration new lightweight materials, are that 6 minutes of onboard
fuel is possible. Dry weight has to be next to nothing to do this.
This could mean borderline escape velocity. Probably best to think of
high orbit, instead.


tomcat

in article , tomcat at
wrote on 10/15/05 3:11 PM:

Ian Stirling wrote:

You're glossing over the numbers. Yes, you only need weight/ (L/D ratio) to
push something flying in order to gain level.

But, the mission of a launcher is NOT to cruise for a long time, it's to get
into orbit. More thrust with rockets is cheap, The time you are in the
atmosphere is time you are subjected to lots of drag. This means you need to
carry lots of fuel.

Yes, achieving level flight is easier than SSTO (Single Stage To Orbit).
Waveriders deal very effectively with drag, however.

There is a little 'trick' to drag as well. Slimming the wings, or at least
lessening the lift by reducing body/wing curvature, lessens drag as well.

When this is done you also have to decide on the takeoff and landing speeds
that are reasonable and possible. Slimming the wings to a 300 knot takeoff
means strong landing gear and a long runway. Ditto for landings.

Hypersonic waveriding SSTO's have been referred to as 'flying gasoline cans'.
Though the fuel is unlikely to be gasoline, 95+ % of the dryweight is going to
be fuel tanks.

This means designing a SSTO waverider is actually . . . easy! It also means
that you are taking a crew into the hottest blast furnace imaginable
surrounded by and sitting on -- volatile fuel. Not so easy.

So far, preliminary calculations indicate that starting out with a 1:1 thrust
to weight is probably best. This should give the takeoff and early flight
performance of a F-15 Eagle.

After a scant minute or so thrust to weight will have climbed to 2:1 giving
enough push to slice through the hypersonic speeds and touch near space.
Another 1 to 1 1/2 minutes should put the spaceplane into orbit. So, we are
talking about 3 to 4 minutes of burn time.

It is best to have -- and keep for retrofire or reverse thrust -- an extra
minute of fuel on board. So, all in all it works out to about 5 minutes of
fuel. For calculations, with the SSME (Space Shuttle Main Engine) as engine
of choice, figure 1035 pounds of fuel consumed at full throttle each second.

Now you can figure the necessary wet weight of the spaceplane and add that to
the the dry weight. My ballpark figures, taking into consideration new
lightweight materials, are that 6 minutes of onboard fuel is possible. Dry
weight has to be next to nothing to do this. This could mean borderline escape
velocity. Probably best to think of high orbit, instead.

Tomcat, something you are not comprehending is the magnitude of escape
velocity. If the earth were a perfect frictionless sphere with absolutely no
atmosphere so you didn't even have to worry about lift at all you still need
a 3g burn of about 4 1/3 minutes to achieve orbit. Starting from a 1:1
thrust to weight ratio would up that to about 6 3/4 minutes. Time spent in
the atmosphere, especially at hypersonic speeds, will increase this time
significantly so the trick is to get out of it as soon as possible. That's
about what the shuttle does--two minutes up and six minutes sideways.

BTW, the shuttle probably lifts off the ground faster than your car
accelerates horizontally.

George Evans

"George Evans" wrote in message
...
in article , tomcat at
wrote on 10/15/05 3:11 PM:

Ian Stirling wrote:

You're glossing over the numbers. Yes, you only need weight/ (L/D ratio)
to
push something flying in order to gain level.

But, the mission of a launcher is NOT to cruise for a long time, it's to
get
into orbit. More thrust with rockets is cheap, The time you are in the
atmosphere is time you are subjected to lots of drag. This means you
need to
carry lots of fuel.

Yes, achieving level flight is easier than SSTO (Single Stage To Orbit).
Waveriders deal very effectively with drag, however.

There is a little 'trick' to drag as well. Slimming the wings, or at
least
lessening the lift by reducing body/wing curvature, lessens drag as well.

When this is done you also have to decide on the takeoff and landing
speeds
that are reasonable and possible. Slimming the wings to a 300 knot
takeoff
means strong landing gear and a long runway. Ditto for landings.

Hypersonic waveriding SSTO's have been referred to as 'flying gasoline
cans'.
Though the fuel is unlikely to be gasoline, 95+ % of the dryweight is
going to
be fuel tanks.

This means designing a SSTO waverider is actually . . . easy! It also
means
that you are taking a crew into the hottest blast furnace imaginable
surrounded by and sitting on -- volatile fuel. Not so easy.

So far, preliminary calculations indicate that starting out with a 1:1
thrust
to weight is probably best. This should give the takeoff and early
flight
performance of a F-15 Eagle.

After a scant minute or so thrust to weight will have climbed to 2:1
giving
enough push to slice through the hypersonic speeds and touch near space.
Another 1 to 1 1/2 minutes should put the spaceplane into orbit. So, we
are
talking about 3 to 4 minutes of burn time.

It is best to have -- and keep for retrofire or reverse thrust -- an
extra
minute of fuel on board. So, all in all it works out to about 5 minutes
of
fuel. For calculations, with the SSME (Space Shuttle Main Engine) as
engine
of choice, figure 1035 pounds of fuel consumed at full throttle each
second.

Now you can figure the necessary wet weight of the spaceplane and add
that to
the the dry weight. My ballpark figures, taking into consideration new
lightweight materials, are that 6 minutes of onboard fuel is possible.
Dry
weight has to be next to nothing to do this. This could mean borderline
escape
velocity. Probably best to think of high orbit, instead.

Tomcat, something you are not comprehending is the magnitude of escape
velocity. If the earth were a perfect frictionless sphere with absolutely
no
atmosphere so you didn't even have to worry about lift at all you still
need
a 3g burn of about 4 1/3 minutes to achieve orbit. Starting from a 1:1
thrust to weight ratio would up that to about 6 3/4 minutes. Time spent in
the atmosphere, especially at hypersonic speeds, will increase this time
significantly so the trick is to get out of it as soon as possible. That's
about what the shuttle does--two minutes up and six minutes sideways.

BTW, the shuttle probably lifts off the ground faster than your car
accelerates horizontally.

Unless you feel it is your personal responsibility to educate everyone too
lazy to pick up a textbook, I wouldn't waste too much time on Tomcat. We're
not debating (or even discussing, really) the merits of some space
transportation concept or proposal, though Tomcat thinks we are. He tosses
out buzzwords like they're advanced things on the very cutting edge of our
collective knowledge here, not realizing this stuff is decades old news--and
pretty basic at that! But hey, it's you're time, I guess...

Mike Dennis wrote:


Tomcat, something you are not comprehending is the magnitude of escape
velocity. If the earth were a perfect frictionless sphere with absolutely
no
atmosphere so you didn't even have to worry about lift at all you still
need
a 3g burn of about 4 1/3 minutes to achieve orbit. Starting from a 1:1
thrust to weight ratio would up that to about 6 3/4 minutes. Time spent in
the atmosphere, especially at hypersonic speeds, will increase this time
significantly so the trick is to get out of it as soon as possible. That's
about what the shuttle does--two minutes up and six minutes sideways.

BTW, the shuttle probably lifts off the ground faster than your car
accelerates horizontally.



Unless you feel it is your personal responsibility to educate everyone too
lazy to pick up a textbook, I wouldn't waste too much time on Tomcat. We're
not debating (or even discussing, really) the merits of some space
transportation concept or proposal, though Tomcat thinks we are. He tosses
out buzzwords like they're advanced things on the very cutting edge of our
collective knowledge here, not realizing this stuff is decades old news--and
pretty basic at that! But hey, it's you're time, I guess...


Still interesting. Good discussion.

in article , Mike Dennis at
wrote on 10/16/05 5:54 AM:

"George Evans" wrote in message
...
in article , tomcat at
wrote on 10/15/05 3:11 PM:

Ian Stirling wrote:

You're glossing over the numbers. Yes, you only need weight/ (L/D ratio)
to
push something flying in order to gain level.

But, the mission of a launcher is NOT to cruise for a long time, it's to
get
into orbit. More thrust with rockets is cheap, The time you are in the
atmosphere is time you are subjected to lots of drag. This means you
need to
carry lots of fuel.

Yes, achieving level flight is easier than SSTO (Single Stage To Orbit).
Waveriders deal very effectively with drag, however.

There is a little 'trick' to drag as well. Slimming the wings, or at
least
lessening the lift by reducing body/wing curvature, lessens drag as well.

When this is done you also have to decide on the takeoff and landing
speeds
that are reasonable and possible. Slimming the wings to a 300 knot
takeoff
means strong landing gear and a long runway. Ditto for landings.

Hypersonic waveriding SSTO's have been referred to as 'flying gasoline
cans'.
Though the fuel is unlikely to be gasoline, 95+ % of the dryweight is
going to
be fuel tanks.

This means designing a SSTO waverider is actually . . . easy! It also
means
that you are taking a crew into the hottest blast furnace imaginable
surrounded by and sitting on -- volatile fuel. Not so easy.

So far, preliminary calculations indicate that starting out with a 1:1
thrust
to weight is probably best. This should give the takeoff and early
flight
performance of a F-15 Eagle.

After a scant minute or so thrust to weight will have climbed to 2:1
giving
enough push to slice through the hypersonic speeds and touch near space.
Another 1 to 1 1/2 minutes should put the spaceplane into orbit. So, we
are
talking about 3 to 4 minutes of burn time.

It is best to have -- and keep for retrofire or reverse thrust -- an
extra
minute of fuel on board. So, all in all it works out to about 5 minutes
of
fuel. For calculations, with the SSME (Space Shuttle Main Engine) as
engine
of choice, figure 1035 pounds of fuel consumed at full throttle each
second.

Now you can figure the necessary wet weight of the spaceplane and add
that to
the the dry weight. My ballpark figures, taking into consideration new
lightweight materials, are that 6 minutes of onboard fuel is possible.
Dry
weight has to be next to nothing to do this. This could mean borderline
escape
velocity. Probably best to think of high orbit, instead.

Tomcat, something you are not comprehending is the magnitude of escape
velocity. If the earth were a perfect frictionless sphere with absolutely
no
atmosphere so you didn't even have to worry about lift at all you still
need
a 3g burn of about 4 1/3 minutes to achieve orbit. Starting from a 1:1
thrust to weight ratio would up that to about 6 3/4 minutes. Time spent in
the atmosphere, especially at hypersonic speeds, will increase this time
significantly so the trick is to get out of it as soon as possible. That's
about what the shuttle does--two minutes up and six minutes sideways.

BTW, the shuttle probably lifts off the ground faster than your car
accelerates horizontally.

Unless you feel it is your personal responsibility to educate everyone too
lazy to pick up a textbook, I wouldn't waste too much time on Tomcat. We're
not debating (or even discussing, really) the merits of some space
transportation concept or proposal, though Tomcat thinks we are. He tosses
out buzzwords like they're advanced things on the very cutting edge of our
collective knowledge here, not realizing this stuff is decades old news--and
pretty basic at that! But hey, it's you're time, I guess...

What can I say, I am a teacher. And like a lot of teachers I try to keep
other people from embarrassing themselves too badly.

George Evans

"Mike Dennis" wrote in message
. ..
"George Evans" wrote in message
...
in article , tomcat
at
wrote on 10/15/05 3:11 PM:

Ian Stirling wrote:

You're glossing over the numbers. Yes, you only need weight/ (L/D
ratio)
to
push something flying in order to gain level.

But, the mission of a launcher is NOT to cruise for a long time, it's
to
get
into orbit. More thrust with rockets is cheap, The time you are in the
atmosphere is time you are subjected to lots of drag. This means you
need to
carry lots of fuel.

Yes, achieving level flight is easier than SSTO (Single Stage To
Orbit).
Waveriders deal very effectively with drag, however.

There is a little 'trick' to drag as well. Slimming the wings, or at
least
lessening the lift by reducing body/wing curvature, lessens drag as
well.

When this is done you also have to decide on the takeoff and landing
speeds
that are reasonable and possible. Slimming the wings to a 300 knot
takeoff
means strong landing gear and a long runway. Ditto for landings.

Hypersonic waveriding SSTO's have been referred to as 'flying gasoline
cans'.
Though the fuel is unlikely to be gasoline, 95+ % of the dryweight is
going to
be fuel tanks.

This means designing a SSTO waverider is actually . . . easy! It also
means
that you are taking a crew into the hottest blast furnace imaginable
surrounded by and sitting on -- volatile fuel. Not so easy.

So far, preliminary calculations indicate that starting out with a 1:1
thrust
to weight is probably best. This should give the takeoff and early
flight
performance of a F-15 Eagle.

After a scant minute or so thrust to weight will have climbed to 2:1
giving
enough push to slice through the hypersonic speeds and touch near
space.
Another 1 to 1 1/2 minutes should put the spaceplane into orbit. So,
we
are
talking about 3 to 4 minutes of burn time.

It is best to have -- and keep for retrofire or reverse thrust -- an
extra
minute of fuel on board. So, all in all it works out to about 5
minutes
of
fuel. For calculations, with the SSME (Space Shuttle Main Engine) as
engine
of choice, figure 1035 pounds of fuel consumed at full throttle each
second.

Now you can figure the necessary wet weight of the spaceplane and add
that to
the the dry weight. My ballpark figures, taking into consideration new
lightweight materials, are that 6 minutes of onboard fuel is possible.
Dry
weight has to be next to nothing to do this. This could mean borderline
escape
velocity. Probably best to think of high orbit, instead.

Tomcat, something you are not comprehending is the magnitude of escape
velocity. If the earth were a perfect frictionless sphere with
absolutely
no
atmosphere so you didn't even have to worry about lift at all you still
need
a 3g burn of about 4 1/3 minutes to achieve orbit. Starting from a 1:1
thrust to weight ratio would up that to about 6 3/4 minutes. Time spent
in
the atmosphere, especially at hypersonic speeds, will increase this time
significantly so the trick is to get out of it as soon as possible.
That's
about what the shuttle does--two minutes up and six minutes sideways.

BTW, the shuttle probably lifts off the ground faster than your car
accelerates horizontally.

Unless you feel it is your personal responsibility to educate everyone too
lazy to pick up a textbook, I wouldn't waste too much time on Tomcat.
We're
not debating (or even discussing, really) the merits of some space
transportation concept or proposal, though Tomcat thinks we are. He
tosses
out buzzwords like they're advanced things on the very cutting edge of our
collective knowledge here, not realizing this stuff is decades old
news--and
pretty basic at that! But hey, it's you're time, I guess...


I flatlined the uneducated punk weeks ago.

George Evans wrote:
Tomcat, something you are not comprehending is the magnitude of escape
velocity. If the earth were a perfect frictionless sphere with absolutely no
atmosphere so you didn't even have to worry about lift at all you still need
a 3g burn of about 4 1/3 minutes to achieve orbit. Starting from a 1:1
thrust to weight ratio would up that to about 6 3/4 minutes. Time spent in
the atmosphere, especially at hypersonic speeds, will increase this time
significantly so the trick is to get out of it as soon as possible. That's
about what the shuttle does--two minutes up and six minutes sideways.

BTW, the shuttle probably lifts off the ground faster than your car
accelerates horizontally.



The vertical rocket concept is to minimize drag and heat by minimizing
distance traveled in the atmosphere. The vertical rocket, however,
uses nearly half it's fuel to support it's weight -- which is primarily
fuel weight -- before we can even talk about X number of G's, or escape
velocity.

If the earth were a perfect frictionless sphere with absolutely no
atmosphere so you didn't even have to worry about lift at all you still need
a 3g burn of about 4 1/3 minutes to achieve orbit.

Lift is to counteract gravity, not air friction. So, you do have to
worry about lift. Drag can, today, be dealt with quite well by wave
riders.

When mass ratio yields a 2:1 thrust to weight, G force will
significantly exceed 3 G's. If your calculations are different I would
be interested in seeing them. 4 1/3 minutes to orbit sounds about
right.

Time spent in
the atmosphere, especially at hypersonic speeds, will increase this time
significantly so the trick is to get out of it as soon as possible.

Again, drag is very minimal with modern designs, including the design
of the Shuttle. Remember, too, that the atmosphere thins rapidly.

You can't breathe at 20,000 feet. 100,000 feet requires vehicles
designed with high altitude in mind. And, at 200,000 feet, about 40
miles high, only the tremendous speed of a hypersonic vehicle will
enable airfoils to work for either lift or control.

A 1:1, increasing ratio, will take you to 20,000 feet in the blink of
an eye, to 100,000 feet in a minute or so. A 30 degree climb seems to
maximize lift/climb for such purposes.

If getting out of the atmosphere 'as soon as possible' means going
tubular/vertical then a trade off has been made. The huge amount of
lift that airfoils give has been negated in favor of a very slow --
fuel expensive -- vertical launch.

That's
about what the shuttle does--two minutes up and six minutes sideways.

And it is spectacular! But the Shuttle is not a 'true SSTO' and
vertical launch does not do away with air friction heat on reentry.
Neither does parachutes.


tomcat

in article , tomcat at
wrote on 10/16/05 5:15 PM:

George Evans wrote:

Tomcat, something you are not comprehending is the magnitude of escape
velocity. If the earth were a perfect frictionless sphere with absolutely no
atmosphere so you didn't even have to worry about lift at all you still need
a 3g burn of about 4 1/3 minutes to achieve orbit. Starting from a 1:1 thrust
to weight ratio would up that to about 6 3/4 minutes. Time spent in the
atmosphere, especially at hypersonic speeds, will increase this time
significantly so the trick is to get out of it as soon as possible. That's
about what the shuttle does--two minutes up and six minutes sideways.

BTW, the shuttle probably lifts off the ground faster than your car
accelerates horizontally.

The vertical rocket concept is to minimize drag and heat by minimizing
distance traveled in the atmosphere. The vertical rocket, however, uses
nearly half it's fuel to support it's weight -- which is primarily fuel weight
-- before we can even talk about X number of G's, or escape velocity.

You aren't think correctly. You have to expend the same energy to raise a
given weight whether in climbing flight or straight vertical lift. So the
best way to do it is as straight up as practical. Toodling around in the
atmosphere is just going increase your starting fuel weight.

If the earth were a perfect frictionless sphere with absolutely no atmosphere
so you didn't even have to worry about lift at all you still need a 3g burn
of about 4 1/3 minutes to achieve orbit.

Lift is to counteract gravity, not air friction. So, you do have to worry
about lift. Drag can, today, be dealt with quite well by wave riders.

You don't need to worry about lift if there is no atmosphere, as in this
hypothetical situation. You would just slide on the frictionless surface
until kinetic energy exceeded the energy of a circular orbit of height 0.

When mass ratio yields a 2:1 thrust to weight, G force will significantly
exceed 3 G's. If your calculations are different I would be interested in
seeing them. 4 1/3 minutes to orbit sounds about right.

A thrust to weight ratio of 2:1 will give an acceleration of 2 G's. That's
what the 2 means in the ratio. There is no way you can "exceed 3 G's". And
notice that the 4 1/3 minutes assume a constant 3 G acceleration.

Time spent in the atmosphere, especially at hypersonic speeds, will increase
this time significantly so the trick is to get out of it as soon as possible.

Again, drag is very minimal with modern designs, including the design of the
Shuttle. Remember, too, that the atmosphere thins rapidly.

You can't breathe at 20,000 feet. 100,000 feet requires vehicles designed
with high altitude in mind. And, at 200,000 feet, about 40 miles high, only
the tremendous speed of a hypersonic vehicle will enable airfoils to work for
either lift or control.

A 1:1, increasing ratio, will take you to 20,000 feet in the blink of an eye,
to 100,000 feet in a minute or so. A 30 degree climb seems to maximize
lift/climb for such purposes.

If getting out of the atmosphere 'as soon as possible' means going
tubular/vertical then a trade off has been made. The huge amount of lift that
airfoils give has been negated in favor of a very slow -- fuel expensive --
vertical launch.

Airfoils don't magically create energy. The only source of energy are the
motors. A good airfoil can *minimize* the added energy necessary to achieve
orbit over that necessary for a vertical launch. But flying to orbit is
still going to cost you more.

snip

George Evans