Science Question #1 - Re-Entry Vehicles

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Let me preface this question by saying I have no real background in
any sciences including aerospace related physics so this question
might seem a bit lame.

However, I'll press onwards!

Hypothetically speaking, if a spacecraft, equipped with some sort of
anti-gravity propulsion, crossed from orbit and into the atmosphere
(of say... Earth) and descended at a leisurely speed toward the planet
surface (toward a specific landing zone, not just any random point),
would it require heat-shielding as real-space vehicles do? Would it's
descent be rough like the dropship landing in 'Aliens', or a smooth
transition like most 'Star Wars' aerospace craft seem to experience?

In my uneducated reasoning, the vehicle would have to match the speed
of the spinning planet in order to arrive at it's chosen destination
and would not be able to avoid friction with the atmosphere, thus
requiring heat-shielding.

Would an anti-gravity device even work out in orbit, or would you have
to drop for a while before it kicked in?

Anyway, thanks in advance for any help, it's been bugging me for a
while...

Now for my next question...

 

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can't believe its not buddha wrote:
> Hypothetically speaking, if a spacecraft, equipped with some sort of
> anti-gravity propulsion, crossed from orbit and into the atmosphere
> (of say... Earth) and descended at a leisurely speed toward the planet
> surface (toward a specific landing zone, not just any random point),
> would it require heat-shielding as real-space vehicles do? Would it's
> descent be rough like the dropship landing in 'Aliens', or a smooth
> transition like most 'Star Wars' aerospace craft seem to experience?
>
> In my uneducated reasoning, the vehicle would have to match the speed
> of the spinning planet in order to arrive at it's chosen destination
> and would not be able to avoid friction with the atmosphere, thus
> requiring heat-shielding.
>

In the real world, in order for two objects in space to meet, they must
match vectors and velocities, such that their motions relative to each
other is zero. What most people forget, is that the planet is in motion
around it's axis, in it's orbit, and the entire systen is in motion
towards the constellation Hurcules in the case of Sol/Earth.

Heat shields would be necessary, if the velocity of the ship was being
decreased rapidly during it's decent into an atmosphere. Obviously, the
planes that have achieved fringe of atmosphere, and have returned, have
not had such shields, so the factor is one of velocity reduction over time.

In Star Trek, there is mention of heat shields of several occasions,
particularly in the original series. They also mention the shielding
when using the "sling shot" maneuver around the sun during the time
travel event to 20th centruy Earth to recover some whales in the movie.











> Would an anti-gravity device even work out in orbit, or would you have
> to drop for a while before it kicked in?


As gravity is a measure of the attraction of two bodies, in a system,
there are multiple effects to consider (and cancel if using Anti-grav).
ie- Sun attracts both earth and moon, which are attacted by each other, etc.

As there is no current working anti-grav device, I would say model the
the effects in accordance with your favorite literary example and not
get too specific on detail.

>
> Anyway, thanks in advance for any help, it's been bugging me for a
> while...
>


You're welcome. Hope it helps.

 

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In article <jjs8q05gotps98pbpn4kcefg3vfg1v7h34@4ax.com>,
can't believe its not buddha <buddy@buddha.org> wrote:
>Let me preface this question by saying I have no real background in
>any sciences including aerospace related physics so this question
>might seem a bit lame.
>
>However, I'll press onwards!
>
>Hypothetically speaking, if a spacecraft, equipped with some sort of
>anti-gravity propulsion, crossed from orbit and into the atmosphere
>(of say... Earth) and descended at a leisurely speed toward the planet
>surface (toward a specific landing zone, not just any random point),
>would it require heat-shielding as real-space vehicles do? Would it's
>descent be rough like the dropship landing in 'Aliens', or a smooth
>transition like most 'Star Wars' aerospace craft seem to experience?
>
>In my uneducated reasoning, the vehicle would have to match the speed
>of the spinning planet in order to arrive at it's chosen destination
>and would not be able to avoid friction with the atmosphere, thus
>requiring heat-shielding.

If you're going slow relative to the air, then you don't need heat
shields. If you don't match speeds with the planet as you enter the
atmosphere, the air will probably be going pretty darn fast. But you
could match speeds if you wanted to.

>
>Would an anti-gravity device even work out in orbit, or would you have
>to drop for a while before it kicked in?

If you're the GM, anti-gravity devices work any way you want them to work.

>
>Anyway, thanks in advance for any help, it's been bugging me for a
>while...
>
>Now for my next question...

--
"And don't skimp on the mayonnaise!"

 

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On Wed, 24 Nov 2004 11:48:00 GMT, can't believe its not buddha
<buddy@buddha.org> wrote:

>Let me preface this question by saying I have no real background in
>any sciences including aerospace related physics so this question
>might seem a bit lame.
>
>However, I'll press onwards!
>
>Hypothetically speaking, if a spacecraft, equipped with some sort of
>anti-gravity propulsion, crossed from orbit and into the atmosphere
>(of say... Earth) and descended at a leisurely speed toward the planet
>surface (toward a specific landing zone, not just any random point),
>would it require heat-shielding as real-space vehicles do? Would it's
>descent be rough like the dropship landing in 'Aliens', or a smooth
>transition like most 'Star Wars' aerospace craft seem to experience?
>
>In my uneducated reasoning, the vehicle would have to match the speed
>of the spinning planet in order to arrive at it's chosen destination
>and would not be able to avoid friction with the atmosphere,

The atmosphere rotates more or less with the planet. If you match the
speed of the spinning planet, you match the speed of the atmosphere.
The roughness of the entry would depend on how much of a hurry you
were in.

>Would an anti-gravity device even work out in orbit, or would you have
>to drop for a while before it kicked in?

Uh...why wouldn't it? Just because you are in orbit doesn't mean you
aren't affected by gravity. The very fact that you are in orbit shows
you are.

 

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>In Star Trek, there is mention of heat shields of several occasions,
>particularly in the original series. They also mention the shielding
>when using the "sling shot" maneuver around the sun...

This actually reminds me of something else I've always wondered about
space travel...

For a spacecraft travelling at fast sublight speeds (fast enough to
get you from planet to planet or jump gate without breaking the laws
of physics) what sort of shielding do you need on your ship to protect
you from spaceborne particles like dust, micro-asteroids, debris and
the like? How do you know you're not about to scream right through a
cloud of ball-bearings some Planet Express ship ahead of you just
jettisoned? (I guess a cloud of ball-bearing would show up on sensors
pretty well, but you get my point). What protects the ship?

 

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can't believe its not buddha wrote:
> This actually reminds me of something else I've always wondered about
> space travel...
>
> For a spacecraft travelling at fast sublight speeds (fast enough to
> get you from planet to planet or jump gate without breaking the laws
> of physics) what sort of shielding do you need on your ship to protect
> you from spaceborne particles like dust, micro-asteroids, debris and
> the like? How do you know you're not about to scream right through a
> cloud of ball-bearings some Planet Express ship ahead of you just
> jettisoned? (I guess a cloud of ball-bearing would show up on sensors
> pretty well, but you get my point). What protects the ship?

I remember reading about two systems but I don't have the reference.

The first system simply send a large mass out in front of the spaceship
to intercept any particle that were in the way.

The second system used a laser to strip electrons from particles in
front of the spaceship. Then a large magnetic field deflected them away
from the flight path. In Star Trek, their deflector dish does a similar
thing but uses gravity pulses to deflect the particles. Apparently this
works at warp too.

--- Shawn

 

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In article <drkaq0d40eqcvr3jk36vobjm7j53l3phj0@4ax.com>,
can't believe its not buddha <buddy@buddha.org> wrote:
>>In Star Trek, there is mention of heat shields of several occasions,
>>particularly in the original series. They also mention the shielding
>>when using the "sling shot" maneuver around the sun...
>
>This actually reminds me of something else I've always wondered about
>space travel...
>
>For a spacecraft travelling at fast sublight speeds (fast enough to
>get you from planet to planet or jump gate without breaking the laws
>of physics) what sort of shielding do you need on your ship to protect
>you from spaceborne particles like dust, micro-asteroids, debris and
>the like? How do you know you're not about to scream right through a
>cloud of ball-bearings some Planet Express ship ahead of you just
>jettisoned? (I guess a cloud of ball-bearing would show up on sensors
>pretty well, but you get my point). What protects the ship?

Mainly it's the micrometeorites you have to worry about. Spacecraft
operate for many years with flimsy armor, and even solar panels. By the
time you protect yourself from the radiation, you'll be immune to
micrometeorites.

Larger stuff must be detected and avoided. If you're flying around
more-or-less at whim in the solar system, for a reasonably typical
maximum encounter speed figure the speed of orbit around the sun at that
point, and double it for things that might be orbitting in the opposite
direction.

The Earth goes about 30 km/s in its orbit. Experimental rail guns have
been pushing 2 or 3 km/s in a four pound dart, getting close to ten times
the speed of sound, and they can penetrate the turret of an M60 tank.
--
"Beer is living proof that God loves us and wants us to be happy."
-- Benjamin Franklin

 

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> Mainly it's the micrometeorites you have to worry about. Spacecraft
> operate for many years with flimsy armor, and even solar panels. By the
> time you protect yourself from the radiation, you'll be immune to
> micrometeorites.
>

Also, particles approaching the speed of light emit dangerous levels of
radiation. With the theory of relativity, we find out that it's not just a
particle that is moving at those speeds, but a particle that is moving
RELATIVE to your position at those speeds. Basically, if you are
approaching light speeds (within a planetary system as you use for example,
the particles in the system are denser) they all give off dangerous levels
of radiation.

At this point, it becomes a theoretical proposal, and something similar to
the hypothetical explanations Shawn forwarded are the, most likely, best
possibilities:

>The first system simply send a large mass out in front of the spaceship
>to intercept any particle that were in the way.
>
>The second system used a laser to strip electrons from particles in
>front of the spaceship. Then a large magnetic field deflected them away
>from the flight path. In Star Trek, their deflector dish does a similar
>thing but uses gravity pulses to deflect the particles. Apparently this
>works at warp too.
>
> --- Shawn

 

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> Hypothetically speaking, if a spacecraft, equipped with some sort of
> anti-gravity propulsion, crossed from orbit and into the atmosphere
> (of say... Earth) and descended at a leisurely speed toward the planet
> surface (toward a specific landing zone, not just any random point),
> would it require heat-shielding as real-space vehicles do?

I believe the answer is no, the spacecraft would not require any heat
shielding - although it would be a good idea to have it in case the
anti-grav (or inertialess drive, or whatever) goes out and suddenly you
need to do some aero-braking. The reason our real spacecraft have heat
shields is because they are travelling *very* fast in orbit - typically
around 18,000 - 20,000 mph. These "ships" don't have enough fuel to
slow down to a "leisurely speed" and make a full-power rocket burn all
the way to the surface. Instead they use heat shielding and use
friction with the atmosphere to slow down. But if one could enter the
atmosphere at a relative velocity of, say, less than 1,000 mph, it
conceivably could do so without heat shielding.

> In my uneducated reasoning, the vehicle would have to match the speed
> of the spinning planet in order to arrive at it's chosen destination
> and would not be able to avoid friction with the atmosphere, thus
> requiring heat-shielding.

Actually the atmosphere spins with the earth as well...if it didn't
there would be constant 1,000 mph winds at the equator (here on Earth,
that is). So, if your hypothetical ship is matching the planet's
rotational velocity, it is also matching the atmosphere's rotational
velocity - no "extra" friction.

> Would an anti-gravity device even work out in orbit, or would you have
> to drop for a while before it kicked in?

Yes, it would. Gravitational attraction between two objets is inversely
proportional to the square of the distance between the two centers of
mass. See these sites for a description of how gravitational attraction
is calculated:

http://regentsprep.org/Regents/physics/phys01/unigrav/default.htm

http://www.arachnoid.com/gravitation/gravitation_help.html

You can also Google for gravitation, gravity, gravitational attraction,
etc. and you will find lots of stuff that might help.

While there is gravitational attraction betwen any two objects, the
amount of attraction depends on the mass of the objects (a planet will
exert *much* more attraction on the ship than the ship will on the
planet), and the distance between the 2 masses. So, it might take huge
amounts of power to get the same effect when farther away than when up
close, as it were.

Hope this helps.


As to your other question, the Hubble spae Telescope recently (in the
last few years) provided evidence that the rate of expansion of the
universe is growing - that something is (perhaps) counteracting gravity
and making the universe expand at an increasing rate. Google
"gravitational repulsion" and you'll get some good info. A little
warning though - it gets pretty hairy dealing with some of these
questions. The equations can be murder!