Yes, Rockets CAN Fly in a Vacuum

By Amy Shira Teitel | February 25, 2018 4:56 pm
Artist concept of the S-IVB firing to send the crew to the Moon, aka the translunar injection burn. It fired in a vacuum. For real. NASA.

Artist concept of the S-IVB firing to send the crew to the Moon, aka the translunar injection burn. It fired in a vacuum. For real. NASA.

“Rockets can’t fly in space! There’s no air for the engine to burn in space! And there’s no air for the rocket to push against in space! WE’VE NEVER LEFT EARTH!”

So goes the cry of people who don’t believe we landed on the Moon — at least, this is one of their claims. But it’s wrong. In spite of what they think, rockets can and do fly in a vacuum. Fuel and oxidizer mix and ignite in a combustion chamber causing a controlled explosion that is directed out through the engine bell. This forceful expulsion of hot gas is thrust, and when that thrust is powerful enough to overcome the mass of the rocket, it lifts (ideally) smoothly off the ground.

So where does this misconception come from? Most likely, from cars.

Most of us interact with a car engine in our daily lives, so that’s the familiar benchmark. A car engine isn’t the same as a rocket engine (if they were identical our morning commutes would be much faster!) but the general principle is the same: fuel and an oxidizer mix, compress, and ignite with a spark. In this case, we have gasoline, oxygen taken from the air, and a spark in the engine’s mechanics.

With our car, that forceful expulsion of hot gas is exhaust that comes out through the tailpipe. Like exhaust on a rocket, exhaust from your car does produce thrust, just not enough to overcome the mass of the car or the friction of the tires against the road.

Instead, it’s that friction of the tires that make the car move. The engines drives the wheels to turn, and as they do they “push” the road back. The road, in turn, pushes the car forward. It’s the same equal and opposite reaction!

So using cars as the starting point, we see where the idea of rockets pushing off the Earth or air comes from, but the two vehicles aren’t the same. A car uses friction to gain movement via Newton’s third law and a rocket harnesses the raw power of thrust from a much bigger engine… or engines usually.

But there’s another problem. Engines, we’ve just said, need air to burn, and there’s no air in space… true. But that doesn’t mean rockets magically don’t need oxygen. They just have to take it with them! On board any rocket (a chemical rocket, that is) is a fuel tank, an oxidizer tank, and something to spark an explosive reaction in the combustion chamber. Or in the case of a rocket burning hypergols the propellant and oxidizer ignite on contact.

Other kids of propulsion harness the same law of physics through slightly different means. Take ion propulsion for example. An ion engine ionizes propellant by adding or removing electrons to produce ions. Ion engines typically use the gas xenon. The gas is ionized, which sends a steady stream of positively charged ions out the back of the engine. It’s not a lot, but it’s enough thrust to move the spacecraft through space — remember we don’t have air resistance or anything when we’re off the Earth.

So, yes, a rocket still works in space all thanks to Newton’s third law of motion!


Sources: LiveScience; Cornell; Northwestern; NASA; NASA again.

MORE ABOUT: History, NASA, Rockets, Space
  • Andrew Worth

    “So goes the cry of people who don’t believe we landed on the Moon”

    Aren’t you just creating a strawman there?
    I’m aware of that the claim that rockets can’t fly in space “because they have nothing to push against” was made on January 13, 1920 in an unsigned New York Times editorial attacking Robert Goddard (a retraction was published July 17, 1969). Since that retraction I’ve never heard the claim repeated until today – by Moon landing conspiracy theorists or anyone else.

    • OWilson

      Not quite Fake News, but a strawman nonetheless!

      Then there are the “Flat Earth Believers”, “Climate Deniers”, which are used to bolster bankrupt intellectual arguments.

      In my long life in the debate arena , exept for the lampooning semi serious folks, I have never actually met a Moon Landing Denier, a Flat Earth Believer, or a Climate Denier. Maybe it’s because I tend to hang with sensible folks, when I’m away from the blogosphere. :)

      Politicians and journalists using this kind of air brained false construct to justify public policy, or to attack their political opponents is positively scary! :)

      • Michael Cleveland

        Would that all had only good influenced to hang with. I have heard the no fly claim, but generally as a question from middle school kids ( or developmentally arrested equivalents) who have started amassing enough understanding to pose it, but have not sufficiently arrived at the fuller understanding that could answer it correctly. That level of reasoning inability in place, it’s a small step to a flat earth and failure to believe as the arctic ice cap dissolves beneath your feet.

      • George Sparks

        Your full of shit. Talking about flat earth believers discredits your IQ. Everyone knows the earth is round. Climate change has been around since the dawn of time. Global warming was traded for climate change. So now go flap your gums again.

        • Michael Cleveland

          Which doesn’t say much for your self-vaunted IQ. There is an entire society devoted to flat earth believers. That they are morons is beside the point: they are out there nonetheless. Your attitude, on the other hand, is hardly likely to put you with the intellectual elite, either.

  • Uncle Al

    WE’VE NEVER LEFT EARTH!” Give Starman driving Elon Musk’s cherry red Tesla Roadster an apb for automobile theft. Tell the Shire of Experance they were not bombarded by Skylab. Tell folks with satellite dishes they are insane. There is ISS FUBAR,


  • Bill Paradise

    No, I think the misnomer is the use of the “fly”, which implies lift and/or air movement over a wing. The last line corrects that precept “rocket still works in space”, but should have been sooner in the article.

    • Michael Cleveland

      Why would you limit the definition of “fly”? It can be legitimately defined as the ability to move freely in three dimensions. Air over wings, sitting on a tail of fire…it all gets you somewhere, and better than gravity-bound walking.

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