Re: why not send old satellites out into space?

To orbit the earth requires we impart a certain amount of energy to the
satellite. A satellitre in low earth orbit must travel around 7
km/sec. This means that each kg of mass requires the addition of 25
mega-joules of energy. Given the inefficiencies of rocket operation,
the energy needed to fight gravity and air drag, this turns out to be
about 35 mega-joules of energy for each kg.

Now, there are 9 MJ per kg of chemical fuel and oxidizer - this number
can vary depending on the materials - gasoline/oxygen, hydrogen/oxygen,
etc.

Then, there is the problem of lifting the fuel/oxidizer part way until
you use it, and then the difficulty of lifting the tanks to hold the
propellant and the hardware to mix it burn it and get thrust from it.

When all this is said and done you have about 50 kg of material for
every kg put into space.


So, this is what it takes to put something into space.

Now, once in Earth orbit, satellites naturally decay as they lose speed
due to very very low air resistance, they naturally tend to fall down
over time, if nothing is done.

http://cosmos.phy.tufts.edu/~zirbel/laboratories/Satellite.pdf

You propose sending satellites into deep space. This entails escaping
the Earth's gravity altogether. This requires that the orbiting
satellite be accelerated to escape velocity - 11 km/sec. This requires
60 MJ per kg of payload, far more than needed to orbit it.

Of course,to impart this much energy to the satellite requires that the
rocket fuel and oxidizer be brought to orbit for this purpose and held
there until the satellite is sent on its way.

This is impractical, especially since most satellites will fall down of
their own accord anyway.

The way to figure out the difference in this task is to use the rocket
equation;

u = 1 - 1/EXP(Vf/ve)

Where u = propellant fraction
EXP= exponent function
Vf = final velocity (7000 m/s - orbit, 11,000 m/s escape)
Ve = exhaust velocity (4,000 m/s - better available)

EXP(7000/4000)=5.75

1/5.75 = 0.173 ~ 17.3% = payload fraction.
1- 0.173 = 0.827 = 82.7% = propellant fraction

Which means each kg of payload requires 5.75 kg of complete rocket. Of
course combined rocket includes tanks and so forth, and since we need
something like 20% of the rocket to be mechanics, this leaves a small
fraction for useful payload.

In fact you need to stage your rockets to get into space.

EXP(11,000/4,000) = 15.64 mass ratio
1/15.64 = 0.0639 = 6.39% payload fraction
1- 0.0639 = 0.9361 = 93.61% propellant fraction

Here you have far less capacity to send things into space, if you must
send a kick stage along with the payload to shoot it into escape
velocity.

But that's not all!

While space is infinite, its not accessible once you escape Earth.
Sure, you can escape Earth by projecting sometihng away from Earth at
11 km/sec, but then, when you escape to about a million kilometers the
spacecraft is cruising along in orbit around the Sun and is likely to
hit the Earth at some point in the distant future anyway, since their
in the same orbit and the Earth has a powerful gravity field.

To *really* escape the Earth requires that we add far more than 11
km/sec to the speed of the satellite

http://www.cs.bsu.edu/homepages/dathomas/SpaceGrant/Bsf/4/bsf4-1.html

Basically, we need to have a hyperbolic excess velocity of about 36
km/sec at Earth orbit to leave the solar system altogether. This means
that the rocket must impart a total of 47 km/sec to the payload to
achieve this feat. This is far more difficult to achieve than merely
orbiting the Earth!

Now, people have proposed that we use ion rockets powered by sunlight
to counteract the very very low air resistance present in orbit. Ion
rockets have exhaust speeds of 50 km/sec or more. So, the amount of
propellant needed is far less than when using chemical rockets. The
problem here is power source. Solar energy means that the bulk of the
spacecraft must be solar panels. THis increases resistance on orbit,
and there's a diminishing return.

Even so, that's one way, adding a solar powered ion rocket kick stage
to every satellite - which cuts the useful payload to about 1/3 for all
satellites - and increases their longevity (hopefully beyond 3x the
present lifespan) - and gives us a shot at shooting the satellite off
into interstellar space.

But this is a long way around the barn.

Its far more efficient to let the satellite decay and fall to Earth at
the end of its useful life.

.

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