A question for you physics types.

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Senior Space Cadet

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A rocket needs massive horsepower and go thousands of miles and hour to get into space.
Yet a balloon can almost do it with no propulsion at all. Or can it? What's the record? Pretty darn high. How much farther to be, technically, in space?
Explain the apparent contradiction or discrepancy.
Is it wrong to say a balloon has no propulsion?
Then there are geosynchronous satellites, which, from one perspective, are motionless. Seems like they could almost drop a probe down to Earth, like in the Star Trek movie, minus the red matter, of course.
Sorry, I know this isn't a science class, but there are guys on this forum way smarter and better educated than me and I thought I'd take advantage. I'm getting old, I think I should know these things before I go.
 

rockladen

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An orbit achieving rocket needs massive power for it needs to reach 17,500 miles per hour. At that speed above the atmosphere you are in orbit, or you can think of it as you are "in free fall". A geosynchronous satellite is also in orbit around the Earth, but is much farther up and traveling at the same speed as Earth's rotation, which makes seem to be still. Balloons rely on thinner that air gas (helium, hot air, hydrogen, etc) that makes the balloon rise. Balloons cannot reach space for it would stop climbing when the air becomes too thin.
 

rockladen

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For a rocket to "escape" Earths gravitational pull, you must reach 25,000 miles per hour hence the massive size and power of the Saturn V multi stage rocket stack.
 

dr wogz

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remember as well, that you need the speed AND the altitude.

A balloon is buoyant, floating on the air / atmosphere. If the balloon was infinitely stretchy / expandable, it would rise quite a way up (they tend to burst as the gas inside expands to the breaking point of said balloon)
 

rockladen

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Forgot to mention that orbit achieving rockets fly up and towards the east which is same direction as Earth's rotation (you can notice this almost immediately when rocket leaves the launch pad). This requires less energy and speed than flying against Earths rotation.
 

Nytrunner

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Then there are geosynchronous satellites, which, from one perspective, are motionless. Seems like they could almost drop a probe down to Earth, like in the Star Trek movie, minus the red matter, of course.
If they simply released a probe/object/tungsten rod, it would continue at the same speed as the satellite around earth (you'd now have 2 objects in geo instead of just one).

In order to "Drop" an object, the satellite would have to give it a push. If the satellite pushed directly, it would impart the same force on itself (Newton's 3rd). The satellite would be pushed away from earth just a tad, and the pushed object would simply have its velocity vector adjusted slightly towards earth (whereas when in orbit, the velocity vector is tangential to the orbit). The effect on each object is proportional to their mass (newton's 2nd, and you can throw in impulse momentum too if you want). the object would now have an elliptical orbit, and depending on how hard you push it, that orbit may drag in the atmosphere and cause it to reenter.

A better scheme would be for the satellite to release the object with a small (or large depending on your intentions) rocket motor on it that would fire once it's free of the satellite.
 

Funkworks

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À balloon floats on air in exactly the same way a boat or block of wood floats on water. It’s only a matter of density.

Satellites fly just like a baseball would if it was batted past the horizon: trying to fall down but too fast to succeed.
 
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kuririn

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Yet a balloon can almost do it with no propulsion at all. Or can it? What's the record? Pretty darn high. How much farther to be, technically, in space?
I was curious myself so I looked it up.
The current record altitude for an unmanned balloon is 51.2 km.
The Karman line (threshold to outer space) is 100 km.
So a little over halfway there.
If you think about it, a balloon will never reach space.
That's because the lifting gas will always be denser than the vacuum of space.
 

OverTheTop

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Then there are geosynchronous satellites, which, from one perspective, are motionless.
The satellites are in freefall. It is just that they are falling at the same rate as the earth is curving away from underneath them. For geosynchronous satellites this just happens to be the same rotational rate as the Earth.

Think about throwing a stone. The harder you throw it the further it travels before hitting the ground. Now imagine you can throw a stone so fast that its curve downwards is the same as the earth. Ignoring air resistance you have just achieved orbit. The most efficient way to achieve orbit is to launch horizontal. We have a pesky atmosphere which is why they launch vertically, in order to clear the atmosphere quickly and get the horizontal velocity in an efficient way.

For a satellite to de-orbit something it needs to slow down, by one method or another.

As far as balloons lifting something up there has to be some form of energy applied. The potential energy of a mass at altitude will have to have received the energy from somewhere, be it balloon or rocket motor. That potential energy needs to be provided by something ;).
PE = mass x gravity x height ([kg] x 9.8 [m/s/s] x [m])
This is just the energy of an object at height, without any kinetic component.

As rockladen mentions. most launches are flown east, so that the approximately 1000km/h speed that the surface of the Earth already has assists with the energy of the launch. That is 1000kph that you get for free, without any rocket fuel being harmed :). It isn't actually a lot in the whole equation, but every little bit helps. That is fuel that is not needed to be lifted to altitude, so energy saved (refer to previous equation).
 

vcp

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Then there is the question that you almost asked: What if you built a tower down to the earth from a geosynchronous satellite? The answer is, you would have a tower to space! If you had a vehicle that could climb it, you could travel to space with little more energy than a tank of gasoline. The trick is in the materials needed to build it, and the many problems in maintaining it. There are many science fiction stories on this topic, the classic is "The Fountains of Paradise" by Arthur C. Clarke.
 

Senior Space Cadet

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I pretty much understand all of that, but here's my dilemma.
If I balloon with, say, a space capsule underneath, such as the Red Bull stunt, can reach over 100,000 ft. (not sure Red Bull got that high) and, obviously, going much slower than 17,500 miles an hour, why couldn't something, with, say, antigravity, keep going at a very slow speed until it was in space? I'm not understanding the need for the 17,500 miles an hour. Seems like one mile an hour would work. I'm picturing the balloon continuing up (switching to antigravity or some other form of lift) at a very slow pace until it is in space.
It is like an Einstein mind problem without the massive IQ. I'm picturing myself standing on Earth and watching something float up, slowly, till it is in space.
 

MarkB.

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SSC,

All bets are off with "antigravity". If you know where to get some, you are about to be a very wealthy man.

Gravity is related to mass; more mass = more gravity. that's why you need less power to escape the "mass" of the Moon or Mars than you do Earth, because they have less mass and therefore less gravity. But everything has mass, even a very, very thin balloon or a feather or whatever. "Antigravity" would imply a mass less than zero. It's a neat Sci-Fi storytelling cheat, but nothing more.
 

Zeus-cat

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Rockets are designed to carry cargos and move them from earth to space. Balloons will never reach space no matter how big you make the balloon.

Your idea of going slowly to get to space would work, but it takes a lot of fuel. Think of it this way; a rocket can produce enough thrust to hover in one place. It doesn't go up or down, it just hovers like a helicopter. You are using X amount if fuel to just hold the rocket in a hover. Now if you increase the thrust by a small amount it will start to climb. Let's say you are now going up by 10 feet per second. To reach the Karman Line (about 100 miles up) it will take you over nine hours. You are burning a lot of fuel. Nine hours of fuel at X plus a little to get you that 10 feet per second.

Now if you increase your climb rate to 100 feet per second you get to the Karman Line 10 times faster. You need X fuel for one tenth the time plus whatever it takes to get you to 100 feet per second climb rate. If you increase your climb rate to 1,000 feet per second you only need X fuel for 1% of the original time plus the fuel needed to get you 1,000 feet per second climb rate.

Of course this is a gross simplification, but I think you get the idea.
 

Steve Shannon

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I pretty much understand all of that, but here's my dilemma.
If I balloon with, say, a space capsule underneath, such as the Red Bull stunt, can reach over 100,000 ft. (not sure Red Bull got that high) and, obviously, going much slower than 17,500 miles an hour, why couldn't something, with, say, antigravity, keep going at a very slow speed until it was in space? I'm not understanding the need for the 17,500 miles an hour. Seems like one mile an hour would work. I'm picturing the balloon continuing up (switching to antigravity or some other form of lift) at a very slow pace until it is in space.
It is like an Einstein mind problem without the massive IQ. I'm picturing myself standing on Earth and watching something float up, slowly, till it is in space.
Everything is relative.
A balloon floats on top of any atmosphere that is denser than the average density of the balloon and its payload. That why balloons are limited in altitude to much lower than “space”. 100,000 feet is low compared to space. A balloon cannot possibly get to space. Helium, hydrogen, or hot air all weigh much more than vacuum.
Second, getting to “space” doesn’t mean something is in orbit. Everything within the earth’s gravitational field will fall to earth unless it has a horizontal velocity that causes it to miss the earth. That’s what orbital velocity does, allow something to miss the earth. At the altitude where space begins, that velocity is about 17,500 miles per hour. As you get closer to earth the velocity needed to miss the earth increases. If you’re far enough away that the velocity needed to miss the earth exactly matches the surface velocity of the earth, you’re in a geosynchronous orbit.
An understanding of physics provides a basic understanding of how this works, but whether it’s obvious or not, we are all “physics types”, bound by the same rules of physics.
 

rklapp

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The 17,500 mph is what's needed to leave Earth's orbit from the ground. If instead it slowly rose up then ignited, it would need a lot less. Virgin tried with a 747 but didn't work (yet).

Google is trying to compete against Spacex's Starlink. It doesn't seem to be viable (yet).

The Aeros has always been fascinating. The problem with a blimp is that you need a lot of helium to lift heavy loads. When you drop the load, the airship goes up really fast. The solution is to compress the helium before dropping the cargo.

Back to the OP, balloons can only get you part ways, then rocket propulsion is needed to get into higher orbit and beyond. ULA's Atlas V first stage gets it to near orbit, then the second stage Centaur gives the X-37B a push into orbit. I have no idea why the Centaur also has to be in the rocket's fairing perhaps due to assembly.
 

Nytrunner

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So in orbit, you're falling in a continuous circle.
Yep! You're falling "down" but going so fast that "down" has changed direction as you travel around the planet

"Down" is really just "the direction towards the earth's center of mass"
 

Steve Shannon

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A rocket needs massive horsepower and go thousands of miles and hour to get into space.
Yet a balloon can almost do it with no propulsion at all. Or can it?
No - heavier air propels the balloon upward by displacing it.

What's the record? Pretty darn high. How much farther to be, technically, in space?
The balloon record for NASA is about 160,000 feet. There’s not a true distance to space, but for treaties it’s agreed to be 100 kilometers or 328,084 feet, so the highest flown balloon has only made it half of the distance.

Explain the apparent contradiction or discrepancy.
What contradiction?

Is it wrong to say a balloon has no propulsion?
Yes, nothing accelerates without the application of force (Newton’s First Law). Balloons are propelled by outside forces, in this case buoyancy, just as a cork is propelled toward the surface of a bathtub by the weight of the water.

Then there are geosynchronous satellites, which, from one perspective, are motionless. Seems like they could almost drop a probe down to Earth, like in the Star Trek movie, minus the red matter, of course.
Sorry, I know this isn't a science class, but there are guys on this forum way smarter and better educated than me and I thought I'd take advantage. I'm getting old, I think I should know these things before I go.
 

John Beans

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Currently the cost to get a kilogram (2.2 lbs) into low earth orbit ranges from $14,600 (Delta IV Heavy) down to $1,700 (Falcon Heavy with recovered first stages).

But those costs are premised on being held captive to the tyranny of the rocket equation. The rocket equation guarantees that getting out of gravity wells will always be difficult and expensive (barring any anti-gravity tech, but since we don't even really understand gravity that seems to be out of the question right now).

Here is an interesting thing to think about: what would it mean to escape the rocket equation?

Space elevators turn leaving gravity wells into bus trips, with similar economics.

For instance, imagine if we had space elevators on the earth and moon. The transit from one elevator to another takes almost no energy.

The price to move a ton of cargo from earth to the moon (and back) becomes essentially free over time.

To me, that transition from difficult + dangerous + expensive to easy + safe + cheap means that once we CAN build space elevators they become inevitable.

Just something to think about.
 

Nytrunner

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A rare lapse for Munroe was missing the opportunity to label the top "Space (but not for very long)"
 

John Beans

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I'm wondering if it makes me a bad person that I don't count Spaceship One's brief crossing of the Kármán line as the first private flight to space...
 

jhein

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Not exactly off topic but our friends at xkcd.com explain a lot about basic orbital mechanics very nicely! Introducing the Earth-Moon Fire Pole: https://what-if.xkcd.com/

Jim
 

Joshua F Thomas

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Hi, physics major here.

1) Getting to space requires going up. Staying in space requires going sideways.

2) A rocket uses a chemical reaction to produce thrust, which creates acceleration. It is this acceleration which puts the rocket into space (up) and eventually staying in space (sideways).

3) A balloon works by the buoyancy principle: The gas inside the balloon is less dense than the outside air, and the gas is displacing a volume of outside air. A pressure difference exists between the bottom of the balloon and the top of the balloon, because the atmosphere is thicker at the bottom; even on very small scales like millimeters. Because the gas in the balloon is less dense, it has less mass for the same volume. As a result, there is an upward force on the balloon, and it accelerates upwards. [Note: This works for all fluids, including water, which is why a boat can float in water - the volume of the boat is mostly air!]

4) Because the force of the balloon depends on the pressure difference, once you have no more atmosphere (ie, vacuum of space), the balloon can't ascend any more. In practice, it happens much sooner because the balloon has mass which needs to be supported by upward pressure.

5) Therefore, no balloon ever makes it to true vacuum. Balloons are height-limited by atmospheric pressure. They remain at their maximum height until gas escapes from the balloon or other forces cause them to come down. They aren't spacecraft. They're atmospheric craft.

6) Rockets can’t get to "true" vacuum, and in order to stay in orbit, require enough energy to match the rotational speed of the earth. As others have noted in this thread, every rocket is falling back to earth, all the time. They just happen to be going so fast that their rate of fall matches or exceeds the rate at which is earth is curving away underneath them while they move at minimum 17,500 mph (relative to the earth).


I just did analytical mechanics this semester, so feel free to ask other questions.
 
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trialsguy

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If you compare 2 rocket space flights... #1 Goes straight up into space, and then falls back to earth (due to the gravitational pull of the earth) - and -
#2 Goes up and outward, at a horizontal velocity to achieve earth orbit. Case #2 requires 15 times the energy as case #1, due to the horizonal velocity required to achieve orbit.

The first manned American examples of this are when Alan Shepard (the first American in space) went up in a Mercury Redstone rocket on a sub orbital flight that lasted 15 minutes (it went up, into space, and then back down). Later, John Glen was the first American to achieve earth orbit, using a much more powerful Mercury Atlas rocket. The power required to achieve earth orbit was illustrated in this recent movie
 

Funkworks

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6) Rockets can get to "true" vacuum, and in order to stay in orbit, require enough energy to match the rotational speed of the earth.
I trust someone here can find the um ... "typo". (I'm one of those who've been paid to be much less polite).
 
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