|Would it be possible to enter the atmoshphere
safely without the use of a heat shield? How
could that be done?
|Question Date: 2007-06-19|
Heat shields are only needed to remove the
kinetic energy of an object that is descending
into gravity well -- and usually to also remove
the kinetic energy of its orbital velocity. There
are many alternatives, but currently they aren't
practical as they require too much fuel. For
example, one to simply fire rocket engines until
you are nearly at rest and then gently falls into
the atmosphere... In such a case, the terminal
velocity would be much lower, obviating the need
for a heat shield. Unfortunately, this would also
require a 'stopping' fuel load nearly as large as
that which got you to orbit.
There has been
work on nuclear powered rockets -- e.g. Minerva in
the late '60's and early 70's but the perception
of danger from accidents (i.e. dropping a reactor
on an inhabited area) stalled this. However, such
an engine could potentially have a large enough
specific impulse to simply 'fly' into space. I.e.
no need to orbit -- just keep lifting.
Recently there have been efforts to study the
feasibility of an orbit elevator -- i.e. a 25k
mile long cable with a weight on the far end,
anchored to the ground. Cars on the elevator could
be electrically powered and use the energy of
descending cars to power those on the ascent. The
cable would need to be long enough to reach beyond
the geosynchronous belt (the distance at which
orbital velocity is the same as the angular
rotation rate of the earth). The only know
material strong enough and light enough to make
such a cable is Carbon Nanotube. Currently these
cannot be made long enough to do this -- but many
people expect that the length problem can be
solved. Such a system could put objects into orbit
space for a very tiny fraction of the energy
expense (and real cost) now needed.
Yes - you need a series of several parachutes
that will reduce your earthward velocity as you
descend.There was a man during the 1950s who
jumped out of a balloon at 80,000 ft elevation,
essentially in vacuum,and that is how he did it.
Of course, he was at rest relative to the Earth at
that elevation, whereas something like a space
shuttle is coming in pretty fast, so needs more
than a parachute. You could also slow yourself
down with engines. It's more practical to go with
a heat shield, usually, or at least was during the
Yes, it's been done by SpaceShipOne. spaceshipone
This plane wasn't going as fast as an orbiting
spacecraft would be. It only flew up above most
of the atmosphere and back. But the same
principles could apply to a larger, faster craft.
Basically, you need to find a way to slow
yourself down without generating too much heat
from atmospheric resistance. SpaceShipOne did it
by tilting its wings 90 degrees so they were flat
to the atmosphere--like a falling leaf compared to
a falling dart.
If you have a long time
(weeks or years), you can also just use the
friction of the very thinnest, high atmosphere to
start you slowing down to a speed where a ship
like SpaceShipOne could then land safely. This is
done for most probes we send to other planets.
Instead of using a big rocket to slow down the
probe once it gets to its destination, we use the
planet's atmosphere to do most of the braking. We
couldn't use this trick if there were people
aboard, though, since nobody likes a flight delay
of several months. :-)
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