Rocket Braking System: Applying Newton's I Law in Space

[Ajit Kumar]

(a) Newton's I law : a body will move without stopping unless any force acts.
(b) There is no air in space, hence no aerial friction.

From (a) and (b), how can a rocket stop in space? How it applies brakes?

 

[ShayanJ]

The same way it goes forward. Just apply the thrust in the opposite direction.

 

[CWatters]

You might be interested in the recent New Horizons probe to Pluto. It shot right past Pluto at something like 30,000 miles an hour. It couldn't slow down and enter an orbit around Pluto because of the large mass of fuel that it would have needed for braking. That mass would have affected how fast the probe could have been launched towards Pluto making the trip take much longer. As it was it still took 9 years to get there.

I believe this is also the reason it took Apollo 4 days to get to the moon. Go any faster and you have to take more fuel for the deceleration phase, which also means more mass to launch.

 

[Nidum]

Bringing a spacecraft to a dead stop in space is not something that is usually needed or desirable .

Bringing an Earth orbit spacecraft safely back to the surface requires a controlled reduction of it's velocity . Bringing it to a stop would just cause it to plummet to the ground .

Docking to another spacecraft means matching orbits and velocities . The two craft are not moving relative to one another but they are still both moving .

A spacecraft anywhere in our solar system will experience gravitational attraction from nearby bodies such as Earth , moon , sun , planets . If spacecraft where actually brought to a stop these gravitational attractions would start it moving again . Spacecraft would gain velocity and it would probably eventually crash into one of the bodies .

 

[rcgldr]

Generally to slow down, the rocket is turned around and then the main rocket is fired. This is how low Earth orbit spacecraft slow down enough to re-enter the Earth's atmosphere which does the rest of the braking. They flip over to face backwards, fire the rocket, then flip over again to face forwards for re-entry. Ever play Asteroids or Lunar Landar ?

 

[Ajit Kumar]

Bringing a spacecraft to a dead stop in space is not something that is usually needed or desirable .

Bringing an Earth orbit spacecraft safely back to the surface requires a controlled reduction of it's velocity . Bringing it to a stop would just cause it to plummet to the ground .

Docking to another spacecraft means matching orbits and velocities . The two craft are not moving relative to one another but they are still both moving .

A spacecraft anywhere in our solar system will experience gravitational attraction from nearby bodies such as Earth , moon , sun , planets . If spacecraft where actually brought to a stop these gravitational attractions would start it moving again . Spacecraft would gain velocity and it would probably eventually crash into one of the bodies .

But how its velocity can be reduced?

 

[rcgldr]

But how its velocity can be reduced?

Either by turning around and firing the rocket, or using a gravity assist to slow down a rocket, by passing by in front of a planet or moon rather than behind it. Using a gravity assist to slow down is common when sending spacecraft to Venus or Mercury which are closer to the sun than the earth.

 

[Ajit Kumar]

firing the rockes

means launching a missile in the forward direction, so as to reduce the velocity by applying a negative force? isn't it?

 

[mfb]

Bringing a spacecraft to a dead stop in space is not something that is usually needed or desirable

It is not even well-defined. At rest relative to what?

means launching a missile in the forward direction, so as to reduce the velocity by applying a negative force? isn't it?

Spacecrafts don't launch missiles, they use their engines that fire the exhaust gases away at high speed. The spacecraft accelerates in the opposite direction as result of this. The spacecraft can fire its engines in any direction. One direction can be perceived as "forwards" (e. g. as seen from earth), one as "backwards". Those labels are arbitrary - something that slows a rocket relative to Earth can make it faster relative to the sun, for example.

 

[russ_watters]

It is not even well-defined. At rest relative to what?

Most of our spacecraft are headed somewhere specific, so "at rest" would be relative to their destination. Yeah, no one would try to "stop" unless they knew what they were stopping relative to. What happens with real spacecraft is they either:
1. Stop (land).
2. Slow down and enter orbit.
3. Whiz on by.

 

[Ajit Kumar]

It is not even well-defined. At rest relative to what?
Spacecrafts don't launch missiles, they use their engines that fire the exhaust gases away at high speed. The spacecraft accelerates in the opposite direction as result of this. The spacecraft can fire its engines in any direction. One direction can be perceived as "forwards" (e. g. as seen from earth), one as "backwards". Those labels are arbitrary - something that slows a rocket relative to Earth can make it faster relative to the sun, for example.

But it must have some surface to exert pressure on. Like on Earth it exerts high pressure on ground.
But in space, nothing is there. No air.

 

[Ajit Kumar]

Most of our spacecraft are headed somewhere specific, so "at rest" would be relative to their destination. Yeah, no one would try to "stop" unless they knew what they were stopping relative to. What happens with real spacecraft is they either:
1. Stop (land).
2. Slow down and enter orbit.
3. Whiz on by.

What if near the moon, if I find a beautiful girl? Who wouldn't like to stop?! ;)

 

[russ_watters]

But it must have some surface to exert pressure on. Like on Earth it exerts high pressure on ground.
But in space, nothing is there. No air.

You mean the rockets themselves? The only surface they need to push on is the inside of the combustion chamber, which pushes the rocket away from the combustion gases. Being near a surface actually gets in the way of that: rockets are more efficient in space (a vacuum) than they are on earth/in the atmosphere. Again, a different way: thrust comes from the rocket and exhaust gases pushing on each other.

 

[SteamKing]

But it must have some surface to exert pressure on. Like on Earth it exerts high pressure on ground.
But in space, nothing is there. No air.

That's where you're confused. Rockets don't work by exerting pressure on anything. Rockets work perfectly fine, even in a vacuum.

http://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-a-rocket-58.html

Firing a rocket in space, in whatever direction, produces a thrust, and this thrust is what propels the rocket, according to F = ma.

 

[Ajit Kumar]

You mean the rockets themselves? The only surface they need to push on is the inside of the combustion chamber, which pushes the rocket away from the combustion gases. Being near a surface actually gets in the way of that: rockets are more efficient in space (a vacuum) than they are on earth/in the atmosphere. Again, a different way: thrust comes from the rocket and exhaust gases pushing on each other.

Please explain this in more detail.

 

[russ_watters]

Please explain this in more detail.

There are two ways to analyze how a rocket works - like pressure/force because thrust is force:

Consider a closed, inflated balloon: equal pressure everywhere and it doesn't move. Now open the stem. Now there is no surface for the air to push on in that direction, so the pressure is no longer balanced and pushes the balloon in the other direction.

The other way to look at it is conservation of momentum: if you throw something one way (exhausted gases), you must move in the other direction.
http://www.braeunig.us/space/propuls.htm

 

[mfb]

Rockets in space even work a bit better than in an atmosphere.

Imagine an explosion of a bomb - the rapid expansion of the produced gas pushes the fragments away. No atmosphere needed. The rocket just makes this process a bit more controlled - hot gas goes in one direction, the rocket in the opposite direction.
Alternatively, get on a bike, throw something heavy (representing the hot burnt fuel of a rocket) backwards: you move forwards. No atmosphere needed, just conservation of momentum.

 

[sophiecentaur]

Please explain this in more detail.

Have you read about this anywhere else? There are countless links that describe how rockets work and the basic ideas of momentum that are involved. The second post on this thread actually gave you a pretty good answer. Speeding up or slowing down by using a rocket are both the same thing, basically; it's just changing velocity. (As Newtons law tells you)

 

[CWatters]

Please explain this in more detail.

See Newtons third law..

https://en.wikipedia.org/wiki/Newton's_laws_of_motion

If you throw stuff (eg burnt rocket fuel) in one direction you get a force in the other. There is no need for any air for the rocket to "push against". The rocket is effectively pushing against it's own exhaust products.