Thrusters in Space

Bart: Go, Dad, go!

Lisa: How doth the hero, strong and brave, a celestial path to the heavens paved!

(The family stares at her.)

Lisa (dejected): Go, Dad, go. -The Simpsons

Last week, I got a question from one of my online friends, cmgraves. His question was straightforward:

How do thrusters work in space? On Earth, when we want to speed up, slow down, change our direction, or to change our motion in general, we always have something to push against. This is true whether you’re a runner and have the ground to push against,

or a turbofan engine with the air to push against.

But in the vacuum of space, your attempts to fly a plane would fail just as surely as your attempts to run, since there’s nothing to push against! How, then, is it possible to change your motion in space?

In other words, how do thrusters work? Take a look at the image below.

This is what happens when you explode a little bit of rocket fuel inside that rocket. Note the big difference between what happens here, inside of a rocket, and what happens when you just blow up something up on its own.

If I just let something explode, the explosion is going to move spherically outwards. For every little bit that moves to the left, there’s a little corresponding chunk that moves to the right. For every gram that moves up with a certain velocity, there’s a corresponding amount that moves down with a certain velocity. This is what we call the conservation of momentum.

But if I have a controlled explosion, I can spew all of that combusted fuel out — at a very high speed — in one direction.

But now, all of that high-velocity, combusted fuel gets blown out in a controlled direction! And if I want to accelerate, all I have to do is point my thruster in the opposite direction I want to move, burn a little fuel, and I’m done!

This is the same reason why, when you fire a rifle, there’s a large kickback that throws you backwards. There’s an explosion that goes on inside of the gun, part of the energy propels the bullet forwards (out of the barrel) at a certain velocity, and part of the energy propels the gun and everything it’s attached to (including you) backwards at a different, smaller velocity.

So, to recap, there is something to push against, but it’s all going on inside of the spaceship. Something gets shot out of the back, and the rest of it gets accelerated forwards.

(Any bet as to how far we’ll get in the comments before the inevitable joke about farts in space?)

Comments

  1. #1 NewEnglandBob
    March 15, 2010

    Newton’s third law: law of reciprocal actions

    Lex III: Actioni contrariam semper et æqualem esse reactionem: sive corporum duorum actiones in se mutuo semper esse æquales et in partes contrarias dirigi.
    ”To every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions”.

    http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion

  2. #2 Jonesy
    March 16, 2010

    So when do we get one about Ion thrusters?

  3. #3 Me
    March 16, 2010

    In space, no one can hear you fart!
    There, I did it ;-)

  4. #4 Jodie
    March 16, 2010

    Did anybody notice that very last guy? The recoil knocks his pants off?

  5. #5 Jodie
    March 16, 2010

    Hey Mr “Starts with a Bang”

    What happened to the recoil of the Big Bang? How is conservation of momentum accounted for?

  6. #6 Sili
    March 16, 2010

    What happened to the recoil of the Big Bang? How is conservation of momentum accounted for?

    If I just let something explode, the explosion is going to move spherically outwards. For every little bit that moves to the left, there’s a little corresponding chunk that moves to the right. For every gram that moves up with a certain velocity, there’s a corresponding amount that moves down with a certain velocity. This is what we call the conservation of momentum.

    *BZZZZZT*

    –o–

    Sorta unrelated, but this just struck me that other day. On Earth the atmosphere is full of oxidiser, so we move by carrying reducable ‘fuel’ around. (In space we have to bring along both.) The atmosphere of Titan contains lossa reducable fuel. Would it be possible to explore that planet with a craft ‘fuelled’ by oxidiser?

  7. #7 Mu
    March 16, 2010

    orta unrelated, but this just struck me that other day. On Earth the atmosphere is full of oxidiser, so we move by carrying reducable ‘fuel’ around. (In space we have to bring along both.) The atmosphere of Titan contains lossa reducable fuel. Would it be possible to explore that planet with a craft ‘fuelled’ by oxidiser?
    You mean reducing, or oxidizable fuel, but yes, you could build a jet engine using oxidizer in the reducing atmosphere. So you probably need to develop a whole new set of materials to operate under the reversed conditions for your hot section, oxides are out, carbide formation is in.

  8. #8 Jodie
    March 16, 2010

    I’m sure that isn’t right.

    First of all, what exploded? It moved spherically into what? I need one of those ants on the surface of a space balloon explanations for where all the momentum is going.

  9. #9 crd2
    March 16, 2010

    The matter that exploded moved into the area the expansion had opened, because the expansion happened just before the big bang. The real question is where, when, what is happening on the other side.

  10. #10 Jesse P.
    March 16, 2010

    @Jodie-
    One can’t really assume the standard conservation of energy or momentum in general relativity, since there is energy associated with space-time itself. Conservation of energy requires time translation invariance (noether’s theorem). In an expanding space, time is not invariant (still true today, but the effect due to expanding space is so small as to be unnoticeable). I agree that it is hard to conceptualize, since our minds are designed to cope with what we see everyday, expanding space is strange. See this blog post for a great explanation:
    http://blogs.discovermagazine.com/cosmicvariance/2010/02/22/energy-is-not-conserved/

  11. #11 the backpacker
    March 16, 2010

    What was up with that video I mean I could fire those guns when I was five.

  12. #12 Jodie
    March 17, 2010

    Heavy bullets with lots of gun powder?

  13. #13 MadScientist
    March 18, 2010

    @backpacker: sure, you just need a little training. If you had the strength you could even lift and fire an M14 or Garand (but you can cheat and fire on a bipod). A 5-year-old can even fire the Browning 0.50 machine gun (though I doubt they have the strength to load it). I don’t see any claims that there is anything special about being able to fire those rifles though.

    Now back to propulsion in space – it’s not all rocket motors; the deep space probes tend to be equipped with mercury ion engines. Warm up the mercury to vaporize it, give it an electrical charge, then accelerate it with an electric field. It’s a very weak motor but over long periods of time it can sure get those deep space probes moving at a high speed.

  14. #14 MadScientist
    March 18, 2010

    @sili: Rockets carry both the oxidizer and fuel, otherwise it is difficult to get the necessary oxidizer from the air to maintain the desired operation – far more fuel is burned than there is air which can be rammed through in flight, not to mention you need to compress that air and pump it into the ignition chamber. (Small toy rockets have no problems though.) The Saturn’s F1 motors for example were supplied with oxygen and kerosene which were carried on board and the shuttle carries the external fuel tank which has oxygen and hydrogen. The oxidizer is not necessarily oxygen though; numerous chemicals can be used such as potassium perchlorate and there are some rather exotic chemicals which can be used in solid fuel rockets.

  15. #15 Sili
    March 22, 2010

    I understand how rockets work. I was thinking trains, planes and automobiles. Or rather, just planes. In Earth’s atmosphere Oxygen is available so we take on fuel to burn. On Titan the atmosphere is thick and rich in hydrocarbons. My question is, is it possible for an unmanned probe to fly around in Titan’s atmosphere with ‘standard’ motors and a tank full of oxidant?

  16. #16 WoW CD Key
    June 18, 2010

    But in the vacuum of space, your attempts to fly a plane would fail just as surely as your attempts to run, since there’s nothing to push against! How, then, is it possible to change your motion in space?

  17. #17 jibrans
    June 21, 2010

    If I just let something explode, the explosion is going to move spherically outwards. For every little bit that moves to the left, there’s a little corresponding chunk that moves to the right. For every gram that moves up with a certain velocity, there’s a corresponding amount that moves down with a certain velocity. This is what we call the conservation of momentum.

    Thanks and Regards Rising damp

  18. #18 instrumental music
    July 25, 2010

    So, to recap, there is something to push against, but it’s all going on inside of the spaceship. Something gets shot out of the back, and the rest of it gets accelerated forwards.

  19. #19 Signage Los Angeles
    July 30, 2010

    Definitely agree with what you stated. Your explanation was certainly the easiest to understand. I tell you, I usually get irked when folks discuss issues that they plainly do not know about.

  20. #20 car loan
    August 21, 2010

    There’s an explosion that goes on inside of the gun, part of the energy propels the bullet forwards (out of the barrel) at a certain velocity, and part of the energy propels the gun and everything it’s attached to (including you) backwards at a different, smaller velocity.

  21. There’s an explosion that goes on inside of the gun, part of the energy propels the bullet forwards (out of the barrel) at a certain velocity, and part of the energy propels the gun and everything it’s attached to (including you) backwards at a different, smaller velocity.

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    September 3, 2010

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  26. #26 RachaelTEBEAU
    September 20, 2010

    Could you upload some details on the recently released space shuttles by NASA? There were some spectacular shuttles among them. We have lots of things to study about the behaviour of those space crafts. prohormones

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  29. #29 figure competition
    September 29, 2010

    The matter that exploded moved into the area the expansion had opened, because the expansion happened just before the big bang. The real question is where, when, what is happening on the other side and how will we ever learn what that actually is!

  30. #30 Tai Chi in Melbourne
    October 10, 2010

    Heh I didn’t know that… Maybe next post you can explain how they create artificial gravity in space shuttles?

  31. #31 MLM Network Marketing Blog
    October 10, 2010

    Useful info you got here, some good stuff I hadn’t seen before. Hope you keep updating on this because I’m sure I don’t speak for myself when I say I’d definitely be interested in reading more. I’ve been reading similar related blogs but this one seems to be the most informative. I appreciate you making it available for us and the effort you put into the writing and screen shots, keep up the great work.

  32. #32 Ebay Templates
    October 10, 2010

    Whats all this about artificial gravity on spaceships? Sounds abit crazy if you ask me…

  33. #33 honeymoons
    October 10, 2010

    Thanks, pretty awesone pictures…love the one of him sprinting and outter space

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  35. #35 Raw Food Books
    October 10, 2010

    Wow, these photos and explanations are great for “explosion” newbies like myself. I didn’t know science could be so much fun. :)

  36. #36 Ron Stone
    October 11, 2010

    Great question and answer. I had never thought about what a spaceship pushes against. Thanks.

  37. #37 Backpack Vacuum Cleaners
    October 11, 2010

    So exactly what happens when you fart in space? :P

  38. #38 Flower Mound Real Estate
    October 11, 2010

    That video was awesome! I love the old guys trying to be tough with the guns.

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    Many thanks, the picture of the rocket and explosion are amazing…

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  40. #40 Get Him Back
    October 25, 2010

    Wonderful explanation of thrusters in space. You make it easy to understand. Too bad, science in school wasn’t explained as well, we might have more kids interested in science.

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  43. #43 jimm
    January 12, 2011
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    January 30, 2011

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  45. #45 Carly13
    February 1, 2011

    It’s important also to point out that even without friction there is still no perpetual motion, otherwise fuel would not be necessary in space. Motors and pumps still experience the same friction in space as they do on earth. Cheers!
    Carly S.
    http://www.inin.com/Pages/default.aspx

  46. #46 commercial blenders
    April 6, 2011

    There IS or IS NOT something to push against? All that is going on inside of that spaceship? Something gets shot out of the back and the thrust propels everything forward, as I understand it.

  47. #47 lacne pneumatiky
    April 18, 2011

    What happened to the recoil of the Big Bang?

  48. #48 Fred Joey
    September 3, 2011

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  49. #49 Some guy ace
    April 8, 2013

    Wait I call bullshit, what causes kickback is the resistance of the air being forced out of that little barrel at an incredible rate. It’s still reliant on pushing against something exerting pressure in a vacum didn’t push you period or maybe I’m just dense?

  50. #50 Wow
    April 8, 2013

    The air isn’t going much, if any, faster than the bullet.

    Which has more mass?

  51. #51 David
    United Kingdom
    May 20, 2014

    The turbofan, especially the low-bypass type you show on the fighter-bomber, produces most of its thrust by reaction like a rocket, not by pushing against the air. Propellers do this, but the fan of a jet engine is to compress the air for the burner/reaction stage.