MadSci Network: Physics

Re: Why is interplanetary travel limited to ballistic trajectory flight?

Date: Sat Apr 2 21:19:32 2005
Posted By: Layne Johnson, Undergraduate
Area of science: Physics
ID: 1112130980.Ph

Hello William.

A couple months ago I found this article on future space travel -

You might find some of the ideas work for your story.

The reason we launch rockets the way we do is because we know it works.
It's a proven method. But it's certainly a slow one. Here are some random
thoughts on alternative methods, written in no particular order. Hopefully
some of them will be of help.

Imagine a giant gun, maybe a barrel length of 20 miles, buried deep in the
earth with only the tip exposed to the surface. A fireproof disk sits at
the bottom, and a spacecraft sits on the disk. Every quarter mile or so
inside this barrel is an explosive charge. When the explosive charge
beneath it detonates, the spacecraft is forced upwards. Each time the
spacecraft passes a charge embedded in the barrel, that charge detonates,
too. The back pressure becomes enormous as the spacecraft gets closer to
the surface. It is fired into orbit the same way a bullet is fired from a
gun. The big advantage - the spacecraft doesn't need to expend any fuel to
escape the earth's gravitational pull. The big drawback - the g forces
would kill any passengers and destroy all but the heartiest of spacecraft.

So let's modify the gun a bit. Instead of explosives powering the motion,
let's use magnetics. We'll need a longer barrel this time, maybe 200 miles
long. It would have to be built on the surface of the earth, not buried. We
can angle the last few miles of it up at a slight angle. And instead of
explosive charges every quarter mile, let's use an electromagnet every 2
inches. What we have now is kind of like a magnetic bullet train, except
we're using it to push something much lighter than a train so we can
accelerate it faster. The spacecraft starts at one end of the barrel, with
all the electromagnets pulling it. As it passes the electromagnets, they
switch polarity, pushing it away instead of pulling it forward. Over 200
miles the spacecraft could build up enough velocity to escape the earth's
gravity, and do it smoothly enough that the g forces are easily within
human tolerances. Such a device could be built near the equator and used to
launch spacecraft towards the moon. It wouldn't take much fuel to adjust
the direction of travel enough to slingshot around the moon and aim for
anywhere in the solar system we want to go. Again, the big benefit is not
having to lug around enough fuel to escape the earth's gravity. The
drawback is that it limits the size of the cargo. The mass of the cargo is
proportional to the length of the barrel.

Now let's combine the escape velocity of this system with a constant
thrust. Rockets don't use constant thrust. They use enough thrust to escape
the gravity pull of the planet they're leaving, then they coast to their
destination. But we want a speedy trip to Mars, so let's look at constant
thrust. Many different systems would work. Hydrogen fuel cells, atomic
reactors, solar collectors, just about any existing way to generate power
with low fuel requirements could provide a slight acceleration up to the
halfway point of the trip, then a slight deceleration on the other side.
Goodbye weightlessness, hello Guiness Book of speed records!

We could build such a launching barrel on the moon. From there we could
obtain some really staggering speeds because of the lower gravity and lack
of atmospheric drag. The spacecraft could be launched towards earth, then
use our planet's gravity to slingshot to its real destination. Of course,
the immediate drawback that comes to mind is a malfunction or sabotage that
causes the spacecraft to smash into the earth. Maybe having a moon based
gun pointing at earth isn't such a good idea after all. Scratch that.

There's some fascinating work being done with teleportation down in

What Dr. Lam and his collegues have done might someday lead to a Star Trek
transporter. It's certainly possible. But the task of identifying,
cataloguing, and replicating every single sub-atomic particle in an object
then reconstructing it somewhere else would probably take longer than a
traditional ballistic trajectory flight to Mars and back. We're already
approaching the upper end of computing speed due to that pesky little law
stating nothing can move faster than the speed of light. But if we somehow
can find a way to increase computer speeds a trillion times faster than
they work now, then maybe identifying and replicating all the components of
an object through quantum entanglement will give us useable teleportation.
This field is in its infancy, and it's as hard for me to envision what will
become of it as it would have been for Ben Franklin to envision modern
electronics after watching lightning strike his kite.

Anyway, let's get back to Mars. If we land our spacecraft on Mars, we're
stuck facing the same problems we've already solved getting here, i.e., how
do we all go home when we're done? NASA solved this one beautifully with
the Apollo missions. Leave the space craft in orbit, where it retains all
its velocity. And drop a lander to the surface instead. That way the lander
only needs to carry enough fuel to escape and rendevous with the orbiting
spacecraft, and on Mars, that's not a lot of fuel.

About the window of opportunity - sometimes Mars is pretty close to us.
Other times it's on the other side of the sun. So sometimes the trip is
going to take about five times as long as it would at other times. Nothing
except teleportation is going to change that.

Good luck with the story. If you have any more questions about any of these
alternative ideas or want to discuss one of two of them in depth, please
let us know.

Layne Johnson

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