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

"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.

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

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.

Originally Posted by Mikko

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.

Why is the velocity needed? Well, the answer is quite simple...if you fly straight UP, you fall straight down. Orbiting is simply the fine art of falling and never hitting the ground. If you want to get into an actual orbit, you need to have sufficient velocity that you fall around the planet, not back into it.

With no atmosphere (say the moon) can be orbited at a relatively low altitude, and due to the low gravity, requires significantly less velocity to orbit it. to orbit the earth successfully, you need to get above the atmosphere (much more than 30,000 ft - that's only six miles high). The effects of atmospheric drag on a spacecraft taper off, the further you go from the surface, but appreciable atmospheric effects can still be felt at 100 kilometers altitude.

At 17,500 mph, you are successfully falling around the planet.

Now on to your other idea... A magnetic rail gun. A good idea for a number of reasons. For example, you can inject the acceleration thru externally applied thrust (rather than burning fuel carried with you) The down side to this is that if the rail gun is within the atmosphere, you have the friction of all that air. Not particularly useful for use on the Earth's surface, but a VERY good way to accelerate payloads from for example the surface of the Moon.

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

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.

This sounds like a particle-accelerator. Could particle-accelerators be used to shoot 'beams' of materials into space? I guess a proton beam is essentially hydrogen but could heavier atoms like oxygen, nitrogen, etc. be accelerated into space? If so, would the beams dissipate before they could be captured or could they be focussed enough to reach a receptacle in orbit?

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

In article ,
Mikko wrote:

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?

Because that's orbital velocity in low-Earth orbit. Any slower than
that, and you're falling to the ground.

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?

Hmm, I suppose that's one way of looking at it. Here's another way:
gravity is pulling you toward the ground at (roughly) 9.8 m/sec. But
the Earth is also round; if you are moving forward fast enough, then by
the time you fall to the Earth, it's no longer there -- it's curved away
and is now behind you a bit. At 17500 mph or so, you're moving so fast,
that the Earth is *always* behind you; you're going around it as fast as
you are falling towards it.

Hmm, I think that was a lousy explanation. Surf the web a bit and I bet
you'll find better ones.

In pithy form: orbiting is the trick of falling towards the ground and
missing.

But also gravity gets smaller when you get more away from the earth?

Yes, but not for a LONG way. You can pretty safely ignore that effect.

How high does one have to go to have only half of gravity?

Well, gravity (like pretty much anything else, due to basic geometry)
falls off with the square of the distance. So you can write g2/g1 =
(r1/r2)^2. The radius of the earth is 6400 km; call that r1, and you
want the r2 where g2/g1 is 0.5. 0.5 = (6400/r2)^2, do the algebra, r2
comes to about 9100 km, or about 2700 km altitude.

So this decrease of gravity with altitude isn't much help in reaching
orbit.

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.

Yes, this has been explored before (it's generally called a mass driver
-- try a google search). It would be a massive engineering project,
probably more so than you realize, if you want to be able to launch
anything other than bulk materials like water. You'd need a barrel
length of over 50 km to keep the acceleration tolerable for humans (say,
2 Gs or so).

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

By "space-lift" I assume you mean a space elevator. Yes, it's
dramatically shorter than that, but has a number of additional
operational complications, like keeping the thing up in the sky and
getting your payloads to and from it. It also doesn't help much with
getting stuff back down, which is itself a rather hard problem.

,------------------------------------------------------------------.
| Joseph J. Strout Check out the Mac Web Directory: |
| http://www.macwebdir.com |
`------------------------------------------------------------------'

Mikko wrote:
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?

The simplest way to think of orbit is to use the old line from the
Hitchiker's series: orbit is throwing yourself at the ground, and missing.

Consider you standing in mid air, 100 miles up, starting to fall
towards the earth. Simply consider how fast you'd have to be going
sideways so you wouldn't hit it, but end up in space "beside" it. The
earth is a little more than 3900 miles in radius, so in the time it
takes you to fall to the earth, about 15 minutes, you have to go 3900
miles "to the side". 3900 x 15 x 5 (periods of 15 in an hour) ~= 17,000
miles per hour.

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?

A long way. Gravity decreases with r^2, so if the earth is 3900 miles
in radius, then the key number is 3900 squared, or 15,210,000. To get
one-half the gravity we need that number to be times 2, or 30,420,000,
which is 5,515. So basically you need to be 2000 miles out before it
becomes 1/2 the gravity, which is a pretty high orbit.

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.

Think of you standing on top of that tower. If you jump off, what
will happen? Well, you'll slowly but surely end up a splat at the bottom
of the tower.

But also consider what the earth looks like from that point. 30k
miles is a long way, the earth is now longer that "big". If you jump to
the side hard enough, you'll miss the earth on the way down. That's
orbit, albiet a highly eliptical one.

Maury