
Ask HN: Is escape velocity required to get to outer space? - gremlinsinc
I was just reading about how Elon Musk is trying to make it so rockets are reusable since they&#x27;re one of the most expensive parts of space flight, and then I had a thought... -- What if instead of a rocket to get out of the atmosphere, you went to a specific location in the stratosphere and some sort of tow cable would be launched from a platform(maybe anchored to the moon if needed?), the tow package attaches to the spacecraft and pulls it slower out of orbit without the need for excessive speed.<p>I guess this is similar to the space elevator idea, but wouldn&#x27;t a space elevator need to at some point exceed the escape velocity for earth?<p>Curious what other possible theoretical methods of escape could be available.. like some sort of magnet that defies or pushes against gravitational forces essentially nulling the gravitational pull? --
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lutusp
Yes, escape velocity is required. Mere position won't do it.

Remember that velocity is what carries a spacecraft away from Earth's
gravitational field. Anything less than escape velocity and the spacecraft is
pulled back into the atmosphere.

> What if instead of a rocket to get out of the atmosphere, you went to a
> specific location in the stratosphere and some sort of tow cable would be
> launched from a platform(maybe anchored to the moon if needed?) ...

You need to think about how far away the moon is. The mass of the cable would
very far exceed the mass of the spacecraft, and it would certainly cost more
than a chemical rocket, however inefficient.

> ... but wouldn't a space elevator need to at some point exceed the escape
> velocity for earth?

The space elevator idea relies on tapping earth's own rotational velocity, by
having a counterweight located sufficiently far away that it is held
perpetually aloft by centrifugal force, countered by the tension on the
elevator cable. That means the counterweight must be substantially more than
about 22,300 miles above the earth's surface (the geostationary orbital
height). This is another idea that requires very ingenious engineering to
prevent the mass of the cable from ruining the overall plan.

I think eventually people will think of something that none of us have thought
of yet, using advanced and possibly future technologies, to solve this kind of
problem. But it won't depend on a long cable stretching to the moon.

~~~
chvvel843
This is completely wrong. Escape velocity is the speed, which is required to
break free of the gravitational pull, BUT in the case that no additional
propulsion is available. In other words, you shoot an object with that speed
from the surface of the earth and provide no additional force to accelerate it
or keep the speed constant. You can leave the earth's gravitational pull with
ANY speed. Assume that there was a magically (geostationary) floating platform
300 km above earth. There are absolutely no problems for someone on that
platform to pull you up with a rope. Imagine a 1km high skyscraper... Do you
need to reach certain speed if you were to be pulled to the top of the
skyscraper?! No, speed is irrelevant. What is important is to have a constant
force, which is in the opposite direction of the gravity which is equal or
greater than G.

~~~
lutusp
> This is completely wrong.

On the contrary, it's completely right.

> Escape velocity is the speed, which is required to break free of the
> gravitational pull, BUT in the case that no additional propulsion is
> available.

You just changed the subject, and we could always branch out into a discussion
of perpetual acceleration sources, but that would depart the realm of reality.
The OP's question asked about reality.

> You can leave the earth's gravitational pull with ANY speed.

As a matter of fact, no, that's not possible -- unless you have a continuous
energy source and don't care how much energy you expend. Which is your new
topic, not the topic of current propulsion methods, the topic of the OP's
post.

Without benefit of something exotic like a space elevator you can stay in
place, stationary, 100 kilometers above the earth's surface, motionless with
respect to the surface, in perpetuity, but only by a constant expenditure of
energy. But that's a different topic.

> No, speed is irrelevant.

Without invoking technologies that don't exist, speed is completely relevant.

> What is important is to have a constant force ...

Be careful that you don't confuse static forces and those created by a dynamic
source of acceleration that require an expenditure of energy. Static forces
require no energy, but most accelerations involving motion do (apart from the
class of friction-free orbits).

My reply answered the OP's inquiry, not the science-fiction idea that you
could have an infinite, inexhaustible energy source.

~~~
chvvel843
I am not talking about sci-fi either ... only giving a simple example about
how it actually works. Anyone with even basic knowledge in physics will know
that you are wrong. Please research a little bit to see why you are wrong. An
object can leave the earth's gravitational pull at ANY speed. I just googled
the topic, one of the first results:
[https://uk.answers.yahoo.com/question/index?qid=200709040459...](https://uk.answers.yahoo.com/question/index?qid=20070904045946AAzXoFb)

~~~
lutusp
> An object can leave the earth's gravitational pull at ANY speed.

Yes, but as I have already explained without effect, science fiction is not
the topic. You need to understand the topic under discussion before replying,
not after.

In a discussion like this, it seems someone must always suggest building a
beanstalk that reaches to orbital heights. Fine, but not constructive, and not
the topic.

> Anyone with even basic knowledge in physics will know that you are wrong.

Please troll somewhere else. The OP asked for practical suggestions given the
realities of present-day aerospace engineering, a field in which I am more
than qualified. I can also wave my hands in the air, but that doesn't get
hardware into space.

\-- Former NASA Space Shuttle engineer.

~~~
chvvel843
1\. The OP stated that he is interested in what theoretical solutions there
are + is it required to reach escape velocity.

2\. You stated: >Yes, escape velocity is required. Mere position won't do it.

> Remember that velocity is what carries a spacecraft away from Earth's
> gravitational field. Anything less than escape velocity and the spacecraft
> is pulled back into the atmosphere.

Which is wrong, regardless of the context. And you agreed to it after I
pointed out the first result from a google search. And let's not even start
with this statement:

> Remember that velocity is what carries a spacecraft away from Earth's
> gravitational field. Anything less than escape velocity and the spacecraft
> is pulled back into the atmosphere.

This is ... you guessed it ... wrong. I am not sure if I should explain you
why. Lets just say that an object can have no velocity (a.k.a. not moving at
all) and still not be "pulled back into the atmosphere".

You are one hell of a troll for a space shuttle engineer :)

~~~
lutusp
> The OP stated that he is interested in what theoretical solutions there are

No, the OP asked whether there were _practical solutions, not theoretical
ones_ , and he gave several examples to make the point. I replied in that
context.

> Which is wrong, regardless of the context.

The context is _practical_ ways to leave the earth and make one's way into
space. The OP's post makes that perfectly clear. Read his post -- he suggests
a tether cable leading to the moon -- how exactly is that a theoretical
argument?

> And you agreed to it after I pointed out the first result from a google
> search.

I was agreeing that, if magic carpets existed, or if Jack and his beanstalk
were a rational proposal for getting into orbit, then my way of describing a
departure from earth's surface would be unnecessarily complex.

>> Anything less than escape velocity and the spacecraft is pulled back into
the atmosphere.

> This is ... you guessed it ... wrong.

Look -- learn physics. Stop trolling about topics about which you know
nothing. Your magic carpet, Jack & the beanstalk objections to everyday
reality really are just as absurd as they sound to an educated reader.

And STOP TROLLING.

~~~
chvvel843
I am not trolling, but trying to lead a conversation based on arguments. You,
on the other side, do not have any arguments... your only argument is that I
am trolling. Look, check this video:
[http://www.youtube.com/watch?v=eGimzB5QM1M](http://www.youtube.com/watch?v=eGimzB5QM1M)

This is how it works - at one point the rocket has 0 velocity, but does not
fall back to earth. They can do that at any altitude. You made so many false
statements, that clearly show that YOU are the one that should learn about
physics Your statement: "Anything less than escape velocity and the spacecraft
is pulled back into the atmosphere." is simply ridiculous.

------
singold
As I understand it, you wouldn't need to reach escape velocity in a space
elevator.

Think about it this way, if instead of an elevator, you have a space ladder,
in theory (if you have unlimited energy) you could climb to whatever height
you want, without reaching escape velocity.

AFAIK escape velocity is the minimum required to "throw" something away from
earth such as it would never come back because of gravity.

