Rockets: How Do They Work?

[Jodo42]

I think that I "get" that the basic idea of a rocket is that accelerating a gas/gases, usually through combustion, produces a force in whatever direction the engine points, and that due to Newton's 3rd Law, an equal force of opposite magnitude is applied to the vehicle, causing it to accelerate in a manner inversely proportional to its mass. My problem, I believe, is conceptualizing/visualizing the nature of the thrust force.

How does thrust, in the case of a rocket, work? I can't seem to visualize how a force is created through the acceleration of a gas. I get that there has to be a force because there's a mass being accelerated, but I don't get the mechanics behind how that acceleration is produced. How does the gas push the rocket if it is being ejected directly away from the engine? As I understand it, thrust is a contact force, so a push is somehow occurring.

[henryrasia]

Thrust is NOT a contact force. Think about it this way:

If you're ice skating holding a bowling ball. then you throw it, you're going to fly backwards. You are pushing on the ball, yes, but the ball doesn't need to "push" on the air. Now imagine that instead of bowling balls, they're tiny post combustion molecules, lots of them, LOTS of them. And what's "pushing" them out is their final impact against the combustion chamber before flying out of the exhaust. You may think that it'd be impossible to calculate all of these interactions, but rocket science simplifies it into thrust and nozzle physics.

Now sure, that's for traditional gas exhaust rocketry, but all rockets more or less follow this principle (ion drives use ions pushed by electromagnetic fields, for instance).

Hope this helps

[sal_vager]

The way I see it, it's pressure, just like on the inside of a balloon.

If you had a sealed vessel and burned fuel and air in it, you'd raise the pressure until it burst, same with a balloon if you keep pumping it up, the pressure increases until it pops.

When you put a pin in a balloon the air escapes, but the moment that air leaves the balloon it cannot act on it anymore, what pushes the balloon is the pressure on the side opposite the hole.

Same with a rocket engine combustion chamber, the gasses that leave the engine are effectively no longer anything to do with the rocket, it's the pressure on the opposite wall that drives the rocket forwards.

The wall isn't very large though so the pressure doesn't have a large area to push on, and the hot gasses are still expanding even after leaving the rocket chamber, that would be wasted energy but clever people realized that energy was also usable by fitting an extension, the engine bell, so the expanding gasses that left the chamber could fill the bell and push on its walls.

Some of that push is just to the sides, but because of the sloped sides some of it pushes the bell forwards as well, and that pushes the rocket, and the bells are optimised so when the gas finally leaves it's expanded about as much as it's going to.

Steam engines actually work a bit like this, with bigger pistons able to extract useful work from the expanded steam that still has some expansion left to go.

Once it's gone past the end of the bell though it's gone, there's no further force in the exhaust pushing the rocket, it's no longer in contact with it to do any pushing.

Think about when you throw a stone, while the stone is in your hand you exert a force on it and it resists that force, that's why it takes effort to throw, but once its left your hand neither you nor the stone can affect each other.

And in a infinitesimal way, just as you put energy (inertia) into the stone, a little bit went into to, pushing you the other way.

[shynung]

Imagine a box, with one of the sides left open. Assume this open box is indestructible. Place an imaginary explosive inside the box, and detonate it.

The force of the expanding gases produced by the explosion will push on the sides of the box from the inside. In a closed box, the forces pushing on all 6 sides will exactly cancel out each other, resulting in the box not moving (if it doesn't disintegrate, but let's ignore that for now).

In a box where one side is open, the expanding gases pushes on the side directly opposite of the open side (in addition the the other 4 sides), but it cannot push on the missing side; there's nothing to push on. In effect, the force pushing on the side opposite of the open side isn't cancelled out, resulting in a net force in the direction directly opposite of the open side. Hence, thrust.

Now, replace the open box with a rocket combustion chamber, the open side with a rocket nozzle opening, and the explosion with reacting rocket propellant.

[Steel]

EDIT: Ninja'd, not once, not twice, but thrice

Edited September 26, 2015 by Steel

[lajoswinkler]

Imagine you're on a tiny boat on a perfectly flat and calm lake next to a fixed marker. You start throwing balls in one direction. The boat will go in the other direction.

[Motokid600]

I officially dub this thread.. "The Sea of Analogies."

Just think of it as a contained, directed explosion. All that pressure needs to go somewhere.

[Red Iron Crown]

How does the gas push the rocket if it is being ejected directly away from the engine? As I understand it, thrust is a contact force, so a push is somehow occurring.

The expanding gas pushes against the engine bell, which is carefully shaped to convert as much of that push as is practical into forward force.

[Jovus]

Are you interested in a technical explanation? Because I can give that. It's all a consequence of conservation of momentum. The system that involves the rocket and the fuel has the same momentum before the rocket tosses the fuel out the engine as it does afterward. So, given that the fuel afterward is travelling in one direction, the rocket (minus that amount of fuel) must be travelling in the other in order for the momentum of the whole to remain the same.

Yes, you can model this as a consequence of Newton's Third Law primarily, but it's easier to model it as conservation of momentum out of which Newton's Third Law emerges.

[lajoswinkler]

The expanding gas pushes against the engine bell, which is carefully shaped to convert as much of that push as is practical into forward force.

That accounts for a small part of the thrust. Most of the force results from the conservation of momentum by direct throwing out of little balls called gas.

[Bill Phil]

Pressure times surface area of choke. You get force. That's most of it, anyhow.

[Ten Key]

How does thrust, in the case of a rocket, work? I can't seem to visualize how a force is created through the acceleration of a gas.

A fire hose has enough recoil to knock a human over. The phase of the mass being accelerated (solid, liquid, gas, plasma) doesn't matter-- mass is mass.

[RainDreamer]

Think of how a bullet get propelled: explosion with gas expansion in a confined space with only 1 exit.

[Red Iron Crown]

That accounts for a small part of the thrust. Most of the force results from the conservation of momentum by direct throwing out of little balls called gas.

True, it's like a lens focusing the gas in the right direction. The expanding gas is pushing on the engine, though, which was the point I was trying to get across.

[Armchair Rocket Scientist]

Think about when you throw a stone, while the stone is in your hand you exert a force on it and it resists that force, that's why it takes effort to throw, but once its left your hand neither you nor the stone can affect each other.

And in a infinitesimal way, just as you put energy (inertia) into the stone, a little bit went into to, pushing you the other way.

Not even in an infinitesimal way. The only reason you don't move when you throw a stone is that normal and frictional forces between your feet and the ground transfer the momentum to the Earth, which is so heavy its mass can usually be treated as infinite for those calculations. If you threw a stone on roller skates, you'd start moving backwards: according to my calculations a 75 kg pitcher throwing a baseball at about 80 MPH (yes, I converted the units) would send himself backwards at about 7 cm/s. That isn't high, but you'd definitely notice it. Throwing a baseball in space would also probably cause you to start spinning because the throw won't be aligned with your center of mass.

If we use a device to throw the object backwards even faster, you can get greater accelerations. For example, with a total mass of 75 kg a .22 rimfire cartridge will only send you backwards at 1.3 cm/s. A 30-06 rifle will send you backward at 12 cm/s, a .38 special will give you 4.5 cm/s, and a .50 BMG will send you backward at over 0.5 m/s. You could potentially get a full meter per second of delta-V by using a double-barreled 12 gauge shotgun with appropriate ammunition. Beyond that, the guns get so heavy they'll be a significant portion of your mass, making the calculations more complicated. XKCD did a "what if" comic about using machine guns as jet packs, but that was more concerned with getting enough thrust to lift off the ground.

Unfortunately, chemically-powered firearms rarely have muzzle velocities above about 1 km/s, which translates to an Isp of 100 s (assuming we're firing hundreds of bullets, treating them as a continuous flow of mass is a decent approximation). This is barely better than an Estes rocket motor, and can be beaten by homemade "rocket candy" motors. Railguns are expected to have muzzle velocities of 2-3 km/s. At the low end, this is comparable to commercially available high-power rocket motors using solid fuels. At the high end, it's comparable to kerolox rocket engines.

Just for fun, let's say our major league pitcher from the first example has a large magazine of balls, and throws until his arm gets tired. Typical pitch counts are about 100 pitches per game, so our pitcher will throw that many balls, with a velocity of about 90 MPH for an ISP of 4 s (ha!). The balls have a total mass of about 15 kg, so with a "dry mass" of 75 kg, the pitcher's total dV is: 7.3 m/s. He could probably run faster than that, and that figure assumes no losses from friction.

[YNM]

Force can also be thought as change of momentum over finite moment of time, not just work divided by distance, or something that drags something along.

Think of a cannon. When a cannon shoots, the cannon ball moves forward from still - a change in momentum. But momentum must be preserved, right ? So you get recoiling, the cannon sliightly moves backward. During the acceleration of cannonball, someone certainly can measure a force being applied at the cannonball - and the same thing in reverse for the cannon itself.

Rockets ? They're cannons with armament of hot gases. Force is attained during expansion in the combustion chamber, gas goes down and so the "cannon" goes up. Think of gas is worthless ? Well, think of how party ballon can fly just from cold air escaping. Think of that you're now filling it with gasses hotter than steam...

Edited September 26, 2015 by YNM

[wizzlebippi]

Since everyone here is doing everything but explaining how a rocket works, here it is:

Force = Mass * Acceleration

Force = Thrust

Mass = Fuel and Oxidizer

Acceleration is caused by burning fuel with oxidizer, creating heat and causing the resulting exhaust gas to expand. This pressure is contained within the combustion chamber and forced out the nozzle. The exhaust gas is accelerated through the nozzle, reaching Mach 1 at the throat and continuing to accelerate until it reaches the end of the exhaust bell.

Newton's third law states that for every action there is an equal and opposite reaction. The rocket is accelerated with the same amount of force that expels the exhaust gas out the nozzle.

Nuclear rockets use the heat from a nuclear reactor to expand fuel within a pressure chamber and accelerate it out the nozzle. Ion engines use electricity to ionize a gas like xenon and use the resulting repulsive force to generate thrust.

[Chakat Firepaw]

You're all going about this the hard way.

Consider a rocket floating alone in empty space. Now, no matter what it does it cannot move its centre of mass, which means that if you move part of it something else has to move the other way.

The propellant is part of the rocket. If it goes left, everything else goes right.