Space Travel

[familyman207]

On Oct 26, 2000 a guest - Coulter, UCLA asked about rocket propulsion and how it works…. Sighting that, “as there is no atmosphere in space for the rocket to push against, what does it push against?”

This is a question that has tormented me personally since childhood.

Further, I ask, as flight controls also require atmosphere to react against and there is no atmosphere in space, logic would also demand that flaps, ailerons, spoilers and elevators would not affect the flight path of a ship in space.

You used the analogy that if you sit in a chair on wheels and throw a large heavy ball that the chair will then move and sight this as the principle of how thrust in space is accomplished.

That analogy would seem to be invalid, as the experiment is NOT conducted in the vacuum of space.

As you stated Newton’s 3rd law, … “What ever you push against pushes back….”, only explains why the chair would move in atmosphere.

Well in space there is no matter (as we know it) or atmosphere to push back against, so with all due respect, I don’t see how the law can logically be applied to propulsion and flight control in outer space or how space travel is true.

Realistically, if we are to attain propulsion in outer space, logic demands it will have to be done by pushing or pulling against something real that exist in outer space. If we are to attain flight control it will also have to be done by pushing or pulling against something that truly exist in outer space.

Lastly, and Please, if you choose to reply, I ask respectfully that you address the issue with logic, not diplomas or insults.
Thank you,

---

LINK TO COLUMN: How do rockets work in the vacuum of space? - The Straight Dope

[Mangetout]

A rocket pushes against its own exhaust.

[AK84]

The rocket does not need to push against anything in space. The thrust which issues from its engine/motor nozzles pushes down. The rocket goes up.

[bob_2]

Newton’s law works in a vacuum and away from external gravity. Why wouldn’t it.

[Mangetout]

@familyman207

You used the analogy that if you sit in a chair on wheels and throw a large heavy ball that the chair will then move and sight this as the principle of how thrust in space is accomplished.

That analogy would seem to be invalid, as the experiment is NOT conducted in the vacuum of space.

And no, the analogy is not invalid. Vacuum is largely irrelevant in the context of this experiment.

Have a think about what could possibly happen in this scenario - you and a heavy object are floating next to each other in space. You push hard on the heavy object. What happens next.

It’s obvious that you and the heavy object will get further apart, so the only possibilities are:
[ul]
[li]The object stays put (why would it?) and you move[/li][li]The object moves and you stay put (why? What’s keeping you there?)[/li][li]You and the object both move in opposite directions.[/li][/ul]

[Colophon]

@familyman207

You used the analogy that if you sit in a chair on wheels and throw a large heavy ball that the chair will then move and sight this as the principle of how thrust in space is accomplished.

That analogy would seem to be invalid, as the experiment is NOT conducted in the vacuum of space.

Do you really think that if you do this experiment, the only reason you move backwards is that the ball is pushing against the air? If that were the case, then why would it move forwards rather than staying in place, “held by the air”, while you got pushed backwards? And furthermore, why wouldn’t the air push on you just as much?

[74westy]

I thing you’re referring to this staff report. I’ve asked a moderator to move it to the “Comments on Cecil’s Columns/Staff Reports” forum.

[Ravenman]

@familyman207

Realistically, if we are to attain propulsion in outer space, logic demands it will have to be done by pushing or pulling against something real that exist in outer space. If we are to attain flight control it will also have to be done by pushing or pulling against something that truly exist in outer space.

The simple fact is that we have attained propulsion in space, and it is done all the time. Forget the rockets that launch satellites into space: every satellite that is launched has propulsion on it to maintain its orbit.

For example, the International Space Station requires propulsion so that it can maintain its orbit. Even satellites out at geostationary – that’s 23,000 miles away from earth – need a bit a propulsion to maintain their attitude in relation to our planet. Here is a Wikipedia article that explains station keeping maneuvers.

Now use some logic here: if we know that satellites (and rockets, of course) already use propulsion in space in a very successful manner, the only explanation is that your belief of rockets needing something other than a vacuum to push against must be in error. Right?

[DrFidelius]

Further, I ask, as flight controls also require atmosphere to react against and there is no atmosphere in space, logic would also demand that flaps, ailerons, spoilers and elevators would not affect the flight path of a ship in space.

Which is why space capsules and space stations do not use flaps for directional control while in vacuum. They use small thruster rockets.

You used the analogy that if you sit in a chair on wheels and throw a large heavy ball that the chair will then move and sight this as the principle of how thrust in space is accomplished.

That analogy would seem to be invalid, as the experiment is NOT conducted in the vacuum of space.

As you stated Newton’s 3rd law, … “What ever you push against pushes back….”, only explains why the chair would move in atmosphere.

What you are pushing against is the ball. Whether the ball is also pushing against air is irrelevant.

[Grey]

Wordy math explaination:

momentum = velocity * mass

change in momentum = change in velocity*mass + velocity * change in mass = force

now in space, with no external forces on a system

change in velocitymass + velocity * change in mass = 0
change in velocitymass = - velocity * change in mass

change in velocity = acceleration

so the acceleration * mass of the ship is in the opposite direction from the exhaust
Wordy summary.

A rocket is basically a continuous explosion with an outlet. The combustion pushes exhaust out in all direction and only allows escape in 1. The resulting pressure on the non-exhaust port parts of the ship moves it forward.

[beowulff]

OP -
Don’t feel bad.
It took theNew York Times 49 years to realize they made the same mistake.

[Frazzled]

@DrFidelius

What you are pushing against is the ball. Whether the ball is also pushing against air is irrelevant.

Proof of that is that if you don’t release the ball you won’t move. If you were only pushing against the air there wouldn’t be any change in momentum whether you released the ball or not.

[Machine_Elf]

@familyman207

Further, I ask, as flight controls also require atmosphere to react against and there is no atmosphere in space, logic would also demand that flaps, ailerons, spoilers and elevators would not affect the flight path of a ship in space.

You’re right: aerodynamic flight control surfaces don’t work in a vacuum. This is why the space shuttle had two different systems for attitude control. One of these systems consisted of aerodynamic flight control surfaces, for use when the orbiter was functioning as a conventional aircraft within Earth’s atmosphere (i.e. during the landing phase at the end of each mission). Because these controls don’t work in the vacuum of space, the orbiter was also equipped with a reaction control system, an array of small rocket engines positioned at various positions and orientations on the body of the orbiter. These were used to rotate or move the orbiter while it was orbiting the Earth above the atmosphere.

Realistically, if we are to attain propulsion in outer space, logic demands it will have to be done by pushing or pulling against something real that exist in outer space. If we are to attain flight control it will also have to be done by pushing or pulling against something that truly exist in outer space.

As Ravenman has noted, we already have achieved propulsion in outer space. To cite one easy example, the Apollo astronauts would not have been able to safely land on the moon (or launch from it) if this were not true. Likewise, orbiting objects that have returned to earth on command (e.g. the space shuttle orbiter, or capsules returning astronauts from the ISS) would not be able to do so if rocket propulsion did not work in space.

You are correct that in order for propulsion to work, it has to be done by pushing against something that actually exists in outer space. That thing is the rocket’s own exhaust plume. That exhaust goes shooting backward because the spacecraft is pushing very hard against it - and in return, the rocket’s exhaust plume pushes the spacecraft forward. It may seem like the mass of the rocket’s exhaust is tiny, but it’s not. During a space shuttle launch, its main engines burned 1.6 million pounds of hydrogen and oxygen, and its solid rocket motors burned 2.2 million pounds of APCP. That’s 3.8 million pounds of “something” that the space shuttle was pushing against. Admittedly most of that rocket activity took place within the Earth’s atmosphere, but a fair bit of it did not; the main engines continued to burn for several minutes after the orbiter was well above the Earth’s atmosphere, bringing it to its final orbit height a few hundred miles above ground.

[jtur88]

Which means there can never be a “light-weight” space station, nor one that would retain maneuverability forever, it would always need to carry sufficient reserve mass to expel and form a Newtonian thrust reaction. Somehow, all that mass has to be gotten out into space, overcoming Earth’s gravitation to get there. Right?

[leahcim]

@jtur88

Which means there can never be a “light-weight” space station, nor one that would retain maneuverability forever, it would always need to carry sufficient reserve mass to expel and form a Newtonian thrust reaction. Somehow, all that mass has to be gotten out into space, overcoming Earth’s gravitation to get there. Right?

In the inner solar system, you can do a great deal with solar sails, which extract their momentum changes from radiation pressure from the sun instead of the expulsion of on-board reaction mass (and reaction wheels for attitude adjustments).

[engineer_comp_geek]

Moderator Action

This appears to be a comment on this staff report.

How do rockets work in the vacuum of space?

Moving thread from General Questions to Comments on Cecil’s Columns/Staff Reports.

[Chronos]

Note that objects in orbit only need a very little propulsion. If space were a perfect vacuum, then the ISS would need no propulsion at all. But there’s still a tiny trace of atmosphere up there, which gradually decays the orbit, so the ISS needs a bit of propulsion to counteract that. By the time you get as high up as geosynchronous orbit, this effect is so small as to be irrelevant, and a geosynch satellite will stay up until the end of the world… but they still need a small amount of propellant, since the whole point of being in geosynchronous orbit is to stay mostly over a particular part of the Earth, and assorted perturbations (mostly due to the Moon and to the fact that the Earth isn’t perfectly spherical) will cause it to gradually drift out of position.

To change your orbit, however (to get up there in the first place, or to come back down, or to get on a path to more distant destinations), you do need significant amounts of propulsion. Which is, again, a solved problem.

[C_K_Dexter_Haven]

MODERATOR:
Sorry, Familyman207, I’m getting to this a few hours late. Anyhow, welcome to the Straight Dope Message Boards, we’re glad you found us!

First point of clarification: You were worried about insulting responses. Fear not! Personal insults are not allowed on these boards (except in the forum called “The Pit.”) So, y’all are free to discuss anything, from any perspective, and must be polite to each other. That doesn’t exclude sarcasm or satire or witticism, of course, but we do draw the line at insults. So, rest easy!

Second point: when one starts a thread, it’s helpful to other readers to provide a link to the column (or, in this case, staff report) in question. Saves readers searching time, and presumably keeps us on the same page. No big deal, 74westy found it for us, and I’ve edited the link to the bottom of your opening post (OP.) You’ll know for next time.

Anyhow, I see it’s generated some discussion already, so, as I say, welcome!

[cochrane]

@familyman207

On Oct 26, 2000 a guest - Coulter, UCLA asked about rocket propulsion and how it works…. Sighting that, “as there is no atmosphere in space for the rocket to push against, what does it push against?”

This is a question that has tormented me personally since childhood.

Further, I ask, as flight controls also require atmosphere to react against and there is no atmosphere in space, logic would also demand that flaps, ailerons, spoilers and elevators would not affect the flight path of a ship in space.

You used the analogy that if you sit in a chair on wheels and throw a large heavy ball that the chair will then move and sight this as the principle of how thrust in space is accomplished.

That analogy would seem to be invalid, as the experiment is NOT conducted in the vacuum of space.

As you stated Newton’s 3rd law, … “What ever you push against pushes back….”, only explains why the chair would move in atmosphere.

Well in space there is no matter (as we know it) or atmosphere to push back against, so with all due respect, I don’t see how the law can logically be applied to propulsion and flight control in outer space or how space travel is true.

Realistically, if we are to attain propulsion in outer space, logic demands it will have to be done by pushing or pulling against something real that exist in outer space. If we are to attain flight control it will also have to be done by pushing or pulling against something that truly exist in outer space.

Lastly, and Please, if you choose to reply, I ask respectfully that you address the issue with logic, not diplomas or insults.
Thank you,

---

LINK TO COLUMN: How do rockets work in the vacuum of space? - The Straight Dope

** familyman207**, with all due respect, and not to be nitpicky, I think the words you mean are “cite” and “citing”. The difference is important here because people tend to ask for cites from other posters quite frequently. Not to try to cause you embarrassment. Maybe your spellchecker threw those spellings in there without you noticing. But the word “cite” is one you’re bound to encounter here sooner or later.

[Killing_Time]

@DrFidelius

Which is why space capsules and space stations do not use flaps for directional control while in vacuum. They use small thruster rockets.

That brings up a question that I have - When you see a cheesy sci-fi movie or TV show where a spaceship makes a turn and it “banks” the turn, that’s nonsense, right? A curve in a road would be banked because that allows gravity to counteract the tendency of a car to continue moving in a straight line, and a plane banks a curve because it is using lift to assist in the change of direction, right? There wouldn’t be any reason to make a banked turn in space, would there?