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  #1  
Old July 29th 05, 06:45 PM
Mitch
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Default airplanes and space flight

Hi there,

One thing that I always wondered about space flight is why most
agencies (NASA, etc.) do not use a more efficient way of lifting into
space. The vertical rocket takeoff seems to use so much energy and does
not take advantage of the physics of regular flight. Why would one not
use something like a modified commercial airliner (make it airtight and
so forth) and then perform a regular take off and fly up to the
altitude where the air still supports the lift on the wings (using
plain old kerosene) and then once that barrier has been reached utilize
a rocket engine to make it the rest of the way. Should this not allow
for much greater payloads to be carried since less fuel is needed to
get up to 30,000 ft? Plus one could use established procedures such as
in-flight refuling at altitude to lessen the need for fuel at take off
even more.

Just imagine how much could be hauled into space and how much cheaper
it would be if one would modify a 747 and use the cargo capacity of
such a plane. I realize that this is a little simplistic in its
description (put a rocket motor on a 747 and have it lift off), but
nevertheless, why not take advantage of wing designs, etc. to get into
space.

I am sure that there is a very good reason why this has not been done
yet, since there are thousands of very smart people working on these
problems. I would just like to know what the negatives are to this idea
that would make it not feasible to implement. I can't imagine that it
would be cost, since they spend a boat load on the shuttle program as
it is.

Thanks for taking the time to answer this question.



Regards,
Mitch

  #2  
Old July 30th 05, 03:59 AM
Monte Davis
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"Mitch" wrote:

Why would one not
use something like a modified commercial airliner (make it airtight and
so forth) and then perform a regular take off and fly up to the
altitude where the air still supports the lift on the wings (using
plain old kerosene) and then once that barrier has been reached utilize
a rocket engine to make it the rest of the way.


I'm sure you'll get much more detailed answers, but the simplest one
is that an aircraft's ~10 miles isn't that much of a head start on the
100+ miles for orbit -- and much more importantly, an aircraft's ~500
mph *really* isn't much of a head start on the 17,000 mph needed for
orbit.

The idea is seductive, going back to Eugen Sanger in the 1930s; it
seems reasonable on the face of it to "evolve" gradually from air
travel to space travel. But subsonic, supersonic, and hypersonic
flight are very different aerodynamic regimes as far as design for
lift, drag and collection/compression of engine air is concerned. And
the turbojet that works well from subsonic to Mach ~3 is not the same
as the ramjet that might make more sense around Mach 5-6, or the
scramjet that looks best above Mach 8.

Bottom line: the tempting simplicity of the space plane idea starts to
get dauntingly complex as you get into the engineering.

  #3  
Old July 30th 05, 06:25 AM
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Mitch wrote:
Should this not allow for much greater payloads to be
carried since less fuel is needed to get up to 30,000 ft?


Not *much* greater payloads.

There is an improvement. The spacecraft can be smaller, or deliver a
slightly larger payload, or get into orbit in a single bound.

But 30,000ft and, say, 600mph isn't much of a boost. It's a long, long,
long way from orbit. The difficulty of getting to orbit is better
described by "17500 miles per hour" than any figure of altitude.

Mike Miller

  #4  
Old July 30th 05, 11:06 AM
Ian Stirling
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Mitch wrote:
Hi there,

One thing that I always wondered about space flight is why most
agencies (NASA, etc.) do not use a more efficient way of lifting into
space. The vertical rocket takeoff seems to use so much energy and does
not take advantage of the physics of regular flight. Why would one not
use something like a modified commercial airliner (make it airtight and
so forth) and then perform a regular take off and fly up to the
altitude where the air still supports the lift on the wings (using
plain old kerosene) and then once that barrier has been reached utilize
a rocket engine to make it the rest of the way. Should this not allow
for much greater payloads to be carried since less fuel is needed to
get up to 30,000 ft? Plus one could use established procedures such as
in-flight refuling at altitude to lessen the need for fuel at take off
even more.


Not really.
The amount of fuel saved by starting out at 10Km is quite small.
And now, you've got wings, tail, landing gear, jets, ... to carry too.
Just imagine how much could be hauled into space and how much cheaper
it would be if one would modify a 747 and use the cargo capacity of
such a plane. I realize that this is a little simplistic in its
description (put a rocket motor on a 747 and have it lift off), but
nevertheless, why not take advantage of wing designs, etc. to get into
space.


Because unfortunately when you do the numbers, it does not make sense.

There are arguments for launching stuff from modified aeroplanes, but
that's the only case that makes some sense.
At best, you need a dry mass of around 10%, in order to get into orbit,
with a single stage, and then you have little or no payload.

With todays technology, wings, jet engines/... are simply too heavy.

  #5  
Old August 1st 05, 02:52 PM
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Mitch wrote:
Hi there,

Why would one not
use something like a modified commercial airliner (make it airtight and
so forth) and then perform a regular take off and fly up to the
altitude where the air still supports the lift on the wings (using
plain old kerosene) and then once that barrier has been reached utilize
a rocket engine to make it the rest of the way.


Gaining altitude and actually getting outside of the atmosphere is
easy. The space shuttle does that in under a minute. The whole rest
of the flight is gaining speed. So when you see the shuttle sitting on
the launch pad and you look at that huge tank of fuel, don't make the
mistake of thinking, "wow, it takes all that fuel to get up into
space." The truth is, it doesn't take much fuel at all to get into
space. It takes A LOT of fuel to get up to speed.

Your 747 idea is a way of using wings to gain altitude. That would be
a good idea if altitude was the problem.

To actually use an aircraft to launch a spaceship (the way Burt Rutan's
Spaceship One worked), you need to have a mothership that can lift the
spaceship *and* the fuel needed to gain orbital speed. So, imagine an
airplane carrying a big heavy tank of fuel like the shuttle's external
tank. The shuttle's main fuel tank holds 800 tons of fuel. That's
pretty heavy. The largest aircraft in the world is the An-225, and it
can only lift 275 tons. You'd need a much larger airplane to carry
enough fuel to get up to orbital speed.

  #6  
Old August 1st 05, 05:04 PM
datacide
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Hello,

that's exactly what the X Prize Foundation was intended for:

http://www.xprizefoundation.com/default.asp

The problem with just converting say a plane is that you need a helluva
lot of energy to overcome the earths gravity well, the second is
security, hence the X Prize.

regards

dc

  #7  
Old August 2nd 05, 04:23 AM
Mitch
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wrote:
Mitch wrote:
Hi there,

Why would one not
use something like a modified commercial airliner (make it airtight and
so forth) and then perform a regular take off and fly up to the
altitude where the air still supports the lift on the wings (using
plain old kerosene) and then once that barrier has been reached utilize
a rocket engine to make it the rest of the way.


Gaining altitude and actually getting outside of the atmosphere is
easy. The space shuttle does that in under a minute. The whole rest
of the flight is gaining speed. So when you see the shuttle sitting on
the launch pad and you look at that huge tank of fuel, don't make the
mistake of thinking, "wow, it takes all that fuel to get up into
space." The truth is, it doesn't take much fuel at all to get into
space. It takes A LOT of fuel to get up to speed.

Your 747 idea is a way of using wings to gain altitude. That would be
a good idea if altitude was the problem.

To actually use an aircraft to launch a spaceship (the way Burt Rutan's
Spaceship One worked), you need to have a mothership that can lift the
spaceship *and* the fuel needed to gain orbital speed. So, imagine an
airplane carrying a big heavy tank of fuel like the shuttle's external
tank. The shuttle's main fuel tank holds 800 tons of fuel. That's
pretty heavy. The largest aircraft in the world is the An-225, and it
can only lift 275 tons. You'd need a much larger airplane to carry
enough fuel to get up to orbital speed.



Hi folks,

Thanks a lot for the replies from everyone. This was very informative
and gave me a much better understanding of the problem. Thanks a lot
folks.

Regards,
Mitch

  #8  
Old August 17th 05, 08:19 PM
Mikko
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kirjoitti:
Mitch wrote:

Should this not allow for much greater payloads to be
carried since less fuel is needed to get up to 30,000 ft?



Not *much* greater payloads.

There is an improvement. The spacecraft can be smaller, or deliver a
slightly larger payload, or get into orbit in a single bound.

But 30,000ft and, say, 600mph isn't much of a boost. It's a long, long,
long way from orbit. The difficulty of getting to orbit is better
described by "17500 miles per hour" than any figure of altitude.


Why is that speed needed?

Only thing I can think is, that since earths gravity effects the craft
whole time, the longer it takes, the more gravity will "drag back" the
craft?


But also gravity gets smaller when you get more away from the earth?
How high does one have to go to have only half of gravity?



What if someone built a 30,000 ft high tube, similar to magnetic trains
- electrical magnets around it. Then you could just put metallic
cargo inside - without any engine or fuel, and shoot it up. The tube
would have to be high enough that there is no air where the cargo comes
out, and maybe part of the tube would have to be a vacuum.

Propably not something to do today, but still lot shorter than the
"space-lift", and it would give near 100 % payload.

(you could mix magnetic cargo with-non magnetic)
And maybe design some kind of shell that can used as "package" to shoot
cargo up, and then used as walls for space station or building in other
planet.
  #9  
Old August 19th 05, 11:53 AM
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Mikko wrote:

Why is that speed needed?


So you don't hit the ground when gravity pulls you down.

Orbital velocity for a given altitude could be defined as, "Moving fast
enough so that when gravity pulls you down far enough to hit the
ground, you've moved far enough sideways that you miss the horizon."

When you're at an altitude of 200-300 miles (like the shuttle or
International Space Station), you only have minutes to scoot sideways
far enough to miss the horizon. Earth's gravity is only barely
diminished at an altitude of 200-300 miles, so you're falling quickly.

For example, it only takes 22-23 minutes for the shuttle or ISS to fall
far enough to hit the core of the Earth. The shuttle and ISS need to
get about 4000 miles sideways in 22-23 minutes to miss the core,
mantle, crust, and atmosphere, or they're going to leave a pile of
metallic confetti on the ground. That calls for a high velocity. In
fact, the magical number is 17500mph.

But also gravity gets smaller when you get more away from the earth?
How high does one have to go to have only half of gravity?


About 1650 miles, deep in the inner Van Allen radiation belt.

What if someone built a 30,000 ft high tube, similar to magnetic trains
- electrical magnets around it. Then you could just put metallic
cargo inside - without any engine or fuel, and shoot it up.


And then it would fall back down and land near the launch point.

Well, unless you fired it at escape velocity, in which case the cargo
would just drift away from Earth, never to return. Most commercial
launchers like to have their satellites stay near Earth, where the
customers are.

Mike Miller

  #10  
Old August 19th 05, 02:16 PM
nmp
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Op Wed, 17 Aug 2005 19:19:28 +0000, schreef Mikko:


Why is that speed needed?


In simple terms: because otherwise the whole damned spacecraft falls
down, back to earth. It needs speed to constantly "escape" the tug of
gravity. It needs more speed the closer it is to the source of gravity,
and less speed if it can fly an higher orbit.

Very nice explanation he http://en.wikipedia.org/wiki/Orbit


 




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