Propulsion question: How do spaceships fly in space?

Question

How would a traditional rocket system work in space if there is no air to push against? Makes me wonder how they do it.

Thanks for your answers. I knew there would be a rational explanation, unfortunately I had a hard time with physics. I still don't get it completely - I'm still trying to wrap my mind around your answers.

Special thanks to mohit and topbakamuna. Good explanations. I've got it now.

Wow! Thanks Tarvold. We got an answer from a rocket scientist at Boeing. Check it out below. Sweet...

Answer 1

Propulsion works following Newton's 3rd Law: every reaction has an equal and opposite reaction.

Now with jet and propeller engines, they suck in the surrounding air and within the engine fuel is mixed and ignited with the fast moving air to produce thrust, which makes them move forward. But the downside is that there must be a significant concentration of air (and more importantly oxygen, since that combusts with fuel) in order to move at all. This is why airplane engines that work on Earth would not work on Mars (because the air density is too small) or Venus (because the air density is too much plus there's little if no oxygen). Aircraft engines carry an efficiency factor of 60%-95%, depending on whether you are working with a Cessna propeller engine or a scramjet engine.

Automobile engines differ from jet engines in that they are not "air breathing" engines but rather the reactions occur internally within the engines. However, since the thrust created has to transition from the engine to the wheels via hydraulics, gears, etc, the efficiency factor is not that great.

Rockets, like auto engines, are not "air breathing" and create their thrust from internal reactions. BUT they use very efficient fuel (gasoline, believe it or not, is not that efficient, but gives the most bang for the buck - pun intended) which reacts 100% (meaning all of the fuel is used) and the thrust produced varies but is very powerful. Rockets are unique in that they employ the "De Laval" nozzles, which in general terms multiplies the force of the reaction by compressing the exhaust gases, choking it at the minimum area (called the throat): choking the flow causes the speed of the rocket to become supersonic easily.

This is why rockets work both as a launch vehicle and in space: they are completely independent of air. They can reach fast speeds that even scramjets are incapable of reaching (try Mach 25+) because scramjets are (fast) air-breathers. And efficiency is at 100% always.

Answer 2

Rockets do not fly by pushing against the air.

When the Rocket fuel (solid or liquid) burns the exhaust gas is expelled at a very high velocity

This means the combustion gives the expelled gas extra momentum in a direction of the expelled gas.

Then as total momentum has to be conserved, the burn process will give a equal momentum change to the rocket body in the opposite direction. This additional momentum will manifest as a increase in velocity.

Another way to look at it is to visualize what happens in side the ignition chamber of the rocket.

The rocket is in outer space and there is no atmospheric pressure. When the fuel is burning. pressure builds up inside the ignition chamber and that pressure gives rise to a force on the chamber wall, pushing it in a direction opposite the nozzle of the chamber.As the chamber is fitted to the rocket the force acts to accelerate th chamber hence the rocket.

Answer 3

It is simple. To start with propellant in rockets[solid or liquid or both for stages]gives the necessary momentum for the rocket to set course in an orbit or on a path.During this stage system works due to Newtons 2nd and 3rd laws.Te principle involved in the 2nd law namely rate of change of momentum equals the force that is propelling. Also action and reaction are equal and opposite. It is due to these principles that the rocket is propelled.Later The propulsion is switched off. Then the rocket moves on its own due to its momentum due to Newtons first law in the same direction when propulsion was switched off. That is a body in motion will continue its motion till acted upon by an external force. In other words all the laws of Newton are in operation for rocket propulsion. Is it any clear or have I confused you more?

Answer 4

The engine exhaust pushes against the back of the rocket and the back of the rocket pushes against the exhaust. It is Newton's 3rd law; for every action there is an equal and opposite reaction. The rocket is actually pushing against its own exhaust gasses. Even when operating in the atmosphere a rocket does not push on the air, only against its own exhaust gasses.

Answer 5

Jet propulsion uses the theory behind conservation of momentum and intertial forces to work, not the fact that it's pushing off a given medium such as air. If you are on roller skates holding a basketball and you throw the basketball away from you, you will be thrust backwards due to the conservation in momentum. This same concept applies to jet propulsion systems.

Answer 6

Rocket propulsion is not acheived by the propellent pushing against the air.

The propulsion actually comes from the force of the gasses escaping away from the ship. It's one of those equal but oppsite reaction sort of things.

Answer 7

well the rocket moves by ejecting the gas out of its exaust
the more the gas ejected with more the speed the faster it'll move

it's like the same principle when you untie the knot of the opening of a balloon
and it flies off

Answer 8

Issac's third law of motion:for every action there is an equal opposite reaction.

Answer 9

Captain Kirk, says warp 9 Scottie. And Scottie says I cannie get warp 9 Captain, but he always does. Dont you watch it?