# How do rockets mouve in a vaccum **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [January 29, 2014, 2:44pm UTC](https://boards.straightdope.com/t/how-do-rockets-mouve-in-a-vaccum/680037 "2014-01-29T14:44:00Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Narajuna](https://avatars.discourse-cdn.com/v4/letter/n/df705f/32.png) [@Narajuna](https://boards.straightdope.com/u/Narajuna)\ **Post date:** [January 29, 2014, 2:44pm UTC](https://boards.straightdope.com/t/how-do-rockets-mouve-in-a-vaccum/680037/1 "2014-01-29T14:44:00Z")
Or how to comment on this site…dont get it neither. Coulter, did’nt respond? Maybe He did get it, all I understood from that smart babble was how it works in our atmosphere. Karen would You have a link for failers to see, the logic, as a mini rocket in a vacuum chamber. Guess I’ll keep on looking… ---
**Author:** ![XT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/xt/32/456_2.png) [@XT](https://boards.straightdope.com/u/XT)\ **Post date:** [January 29, 2014, 3:16pm UTC](https://boards.straightdope.com/t/how-do-rockets-mouve-in-a-vaccum/680037/2 "2014-01-29T15:16:55Z")
Rockets don’t mouve in vaccum…they mauve in vacuum. It’s quite pretty really… I believe [this](http://www.straightdope.com/columns/read/1846/how-do-rockets-work-in-the-vacuum-of-space) is the article the OP is on about. Seems a good explanation to me so not sure what part confuses you. From the article: > [@](#): > > Newton’s Third Law is usually expressed as, “For every action there is an equal and opposite reaction,” and you can also think of it as “Forces always come in pairs.” While you are pushing on the medicine ball, Newton’s Third Law says that the medicine ball is also pushing on you. Thus, you are accelerated by the force acting (backward) on you by the medicine ball. Never mind that it was you who decided to start the pushing in the first place; you can’t push on the ball without having the ball push back. Forces always come in pairs. Basically, this is it…rockets can move in a vacuum because of the third law, for every action there is an equal an opposite reaction. The rocket thrust pushes out of the rocket motor at great speed and the equal and opposite reaction is to push the rocket in the opposite direction. Pretty simple really…not sure why adding a vacuum to the scenario adds confusion, since the only challenging aspect of using a rocket in a vacuum would be how do you ignite and sustain the burn of the rocket motor in a vacuum (answer…use an oxidizer, or to put it a different way carry the oxygen with you). > [@](#): > > Of course, rockets work on more sophisticated principles than just tossing fuel out the back. First, the fuel is burned and its hot exhaust gases are expelled at very high velocity (if you toss the medicine ball faster, your body experiences greater backward force). And the rocket’s exhaust nozzle has a narrowing so as to squirt the exhaust gasses out even faster, like putting your thumb over the end of a garden hose. Exhaust from chemical propulsion (i.e., fuel-burning propulsion) is typically expelled at 2 km/s (= 4500 mph), and your average rocket mass at launch is 80-85% propellant (fuel + oxidizer), most of which eventually gets squirted out. ---
**Author:** ![terentii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/terentii/32/212_2.png) [@terentii](https://boards.straightdope.com/u/terentii)\ **Post date:** [January 29, 2014, 3:34pm UTC](https://boards.straightdope.com/t/how-do-rockets-mouve-in-a-vaccum/680037/3 "2014-01-29T15:34:33Z")
The burning propellant doesn’t need air to push against; it pushes on the inside of the combustion chamber, propelling the rocket forward. What happens after it’s expelled is irrelevant. Veeeeeeeery simple! ---
**Author:** ![silenus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/silenus/32/166_2.png) [@silenus](https://boards.straightdope.com/u/silenus)\ **Post date:** [January 29, 2014, 3:48pm UTC](https://boards.straightdope.com/t/how-do-rockets-mouve-in-a-vaccum/680037/4 "2014-01-29T15:48:51Z")
Drugs and computers don’t mix, dude. ---
**Author:** ![Steve\_MB](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/steve_mb/32/5339_2.png) [@Steve\_MB](https://boards.straightdope.com/u/Steve_MB)\ **Post date:** [January 29, 2014, 4:14pm UTC](https://boards.straightdope.com/t/how-do-rockets-mouve-in-a-vaccum/680037/5 "2014-01-29T16:14:40Z")
> [@Narajuna](#): > > Coulter, did’nt respond? I wouldn’t expect Ann to come up with a coherent answer on any subject, much less rocket science. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [January 31, 2014, 12:39pm UTC](https://boards.straightdope.com/t/how-do-rockets-mouve-in-a-vaccum/680037/6 "2014-01-31T12:39:07Z")
When the fuel is burned, it creates gases. The gas molecules push against each other. In a sealed system, this would create increased pressure, but a rocket is not sealed - it has an exhaust through a nozzle. Thus the gas molecules push against each other and ultimately against the spacecraft engine interior on one side and an opening on the other, which is what sends the gases out the nozzle. The rocket pushes the gases, the gases push the rocket. The harder the rocket pushes the gases, the faster the gases get pushed out, and the harder the gases push on the rocket. ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [January 31, 2014, 6:06pm UTC](https://boards.straightdope.com/t/how-do-rockets-mouve-in-a-vaccum/680037/7 "2014-01-31T18:06:01Z")
> [@terentii](#): > > The burning propellant doesn’t need air to push against; it pushes on the inside of the combustion chamber, propelling the rocket forward. What happens after it’s expelled is irrelevant. Veeeeeeeery simple! It’s not a “push” as you’d conventionally think about it though- pressure or anything like that doesn’t come into it. It’s all about the velocity of the reaction mass (in this case the exhaust products of the engine)- the way I understand it, it’s basically kinetic energy. If you take say… 1 kg of reaction mass and shoot it out of the engine at 100 m/s, you’re looking at 5000 joules of kinetic energy that went out the nozzle AND due to Newton’s 3rd law, was applied to your ship as well. Since the kinetic energy formula is 0.5_M_V^2, raising the velocity has much greater effects than raising the mass. ---
**Author:** ![John\_W.Kennedy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_w.kennedy/32/1031_2.png) [@John\_W.Kennedy](https://boards.straightdope.com/u/John_W.Kennedy)\ **Post date:** [January 31, 2014, 11:18pm UTC](https://boards.straightdope.com/t/how-do-rockets-mouve-in-a-vaccum/680037/8 "2014-01-31T23:18:38Z")
No. Energy is nondirectional. Energy does have to balance, but the balance is achieved by some other form of energy being changed into an equal amount of kinetic energy; assuming an ordinary chemical rocket, it’s chemical potential energy that is lost (in the burning process). It’s momentum—mass times velocity—that has to balance to explain the third-law effect. So the momentum of the rocket (mass of the accelerating rocket times its velocity) has to be equal and opposite to the momentum of the exhaust (mass of the exhaust times its velocity) at all times. Most basic problems in Newtonian mechanics are solved by setting up two equations, one making the energy balance and one making the momentum balance, and finding the solution for both. ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [February 1, 2014, 12:28am UTC](https://boards.straightdope.com/t/how-do-rockets-mouve-in-a-vaccum/680037/9 "2014-02-01T00:28:44Z")
I was thinking of specific impulse I’m pretty sure. > **[Tsiolkovsky rocket equation](https://en.wikipedia.org/wiki/Rocket_equation)** > > The classical rocket equation, or ideal rocket equation is a mathematical equation that describes the motion of vehicles that follow the basic principle of a rocket: a device that can apply acceleration to itself using thrust by expelling part of its mass with high velocity can thereby move due to the conservation of momentum. > It is credited to the Russian scientist Konstantin Tsiolkovsky (Константи́н Эдуа́рдович Циолко́вский) who independently derived it and published it in 1903, although The m... ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [February 1, 2014, 2:58am UTC](https://boards.straightdope.com/t/how-do-rockets-mouve-in-a-vaccum/680037/10 "2014-02-01T02:58:10Z")
You can think of it in terms of momentum, or in terms of force, or in terms of pressure. All are exactly equivalent, and are valid ways of setting up the problem. ---
**Author:** ![John\_W.Kennedy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_w.kennedy/32/1031_2.png) [@John\_W.Kennedy](https://boards.straightdope.com/u/John_W.Kennedy)\ **Post date:** [February 2, 2014, 4:39am UTC](https://boards.straightdope.com/t/how-do-rockets-mouve-in-a-vaccum/680037/11 "2014-02-02T04:39:31Z")
Yes, specific impulse has the advantage of taking into account the fact that the rocket gets lighter and lighter as it burns fuel. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [February 2, 2014, 11:53pm UTC](https://boards.straightdope.com/t/how-do-rockets-mouve-in-a-vaccum/680037/12 "2014-02-02T23:53:51Z")
> [@bump](#): > > It’s not a “push” as you’d conventionally think about it though- pressure or anything like that doesn’t come into it. What causes that mass to eject out the back and thus transfer momentum? A push. With guns and rockets, that push is typically gas pressure. Yes, it is different than a conventional push, where you push off an object. Take jumping off the ground. Conventionally, you push with your legs and then leave the ground. Alternately, you could throw a really big mass toward the ground and that would make you go up by momentum. The trick there is (a) supporting the really big mass before you throw it, and (b) throwing the mass rather than just relying on gravity to bring it down when you remove support. Both use a push to make you move. One pushes on the stable, solid ground; the other pushes on a big mass that moves away from you\*. Rockets push on a smaller mass that moves away from you a lot faster. Still means momentum. * * * - Technically, the Earth is just a big mass that moves away from you, but it is such a much larger mass than you that it moves an insignificant amount compared to how much you move. But it’s the same thing, really. 😉