Why doesn't a cigarette lighter generate thrust?

Question

The way thrust is created by a rocket is discussed here: How does fire create thrust in rocket?

If you look at a typical rocket, say V2, it has two primary tanks — one which stores the fuel and the other which stores oxygen. Is it that the lighter flame doesn't use its own oxygen, the reason that it doesn't generate thrust? Or is it that the thrust is so small that I can't feel it? Or is it something more complicated like the shape of the nozzle?

score 56 · Accepted Answer · edited Mar 03 '15 at 15:09

56

For the average disposable lighter, when you press the fuel lever a pressurised liquefied gas is released which will create a very small thrust.

The combustion, however, will not generate thrust because, unlike a rocket engine, it is not occurring within a chamber.

edited Mar 03 '15 at 15:09

Lightness Races in Orbit

488

answered Mar 02 '15 at 10:31

lemon

13,220

In essence if a sealed chamber with just one opening to the outside is created, the lighter will develop thrust, correct? Of course such a setup includes some way to supply oxygen into the chamber as well. – Shashank Sawant Mar 02 '15 at 11:01
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@ShashankSawant That's correct. Get yourself enough lighters, add a chamber, valve and oxygen supply to each, and off to the moon we go. – lemon Mar 02 '15 at 11:12
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@lemon: Did you cause this mess? – dotancohen Mar 02 '15 at 12:22
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-1 This could be written as a comment. Could you perhaps add some estimates to your first paragraph? That would make the answer much more entertaining. – Physics_maths Mar 02 '15 at 14:21
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Answers have to be entertaining now? – OrangeDog Mar 02 '15 at 14:47
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The combustion will also generate some minimal thrust, since it will heat the gas, causing it to expand, and some of that expanding gas will push against the lighter. Still, the combustion geometry is very poor (and the amount of fuel minimal anyway), so you won't get much thrust out of it. – Ilmari Karonen Mar 02 '15 at 16:20
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@LoveLearning You want this completely valid answer to be written as a comment, and you also want to add commentary that's unrelated to the answer? – wchargin Mar 02 '15 at 16:26
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In defence of @LoveLearning, I happen to agree that some maths would be a nice addition. I will have a crack at doing so when I get some spare time (although I invite anyone else to edit my answer). – lemon Mar 02 '15 at 17:06
@OrangeDog I like estimates and equations. Anyone can answer a question with a simple yes/no with a short motivation, but a quantitative answer is often much harder to formulate and imho much more valuable. The latter can be considered an entertaining answer. Thanks lemon for understanding. – Physics_maths Mar 02 '15 at 17:50
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lemon and @LoveLearning: certainly some math would be a nice addition, and would improve the answer. But nevertheless, it is an answer as is. It should not be posted as a comment. (And I consider it wrong to downvote because of the lack of detail, though of course we cannot enforce any particular reason for voting.) – David Z Mar 03 '15 at 05:59
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@LoveLearning: Answers should absolutely not be written as comments! Ever!! No matter how entertaining or unentertaining they are. – Lightness Races in Orbit Mar 03 '15 at 14:52

score 12 · Answer 2 · edited Mar 02 '15 at 22:58

Newtons third law states that every action has an equal and opposite reaction. A cigarette lighter is venting gas from a pressurised container and igniting it. The act of venting upwards will create a small opposing downward force.

Additionally, the ignition of the vented gas by the striker wheel creates a small pressure wave in the air, similar in principle to the shockwave that precedes a bullet or supersonic aircraft. (You can actually feel this force on you finger as you strike the lighter.) This pressurewave can be photographed with a high-speed camera - I'm not sure if anyone has done it with a lighter but there are videos on youtube of bursting balloons and targets being shot with bullets, filmed at 100+ frames per second, which illustrate this principle nicely.

In other words Cigarette Lighters DO generate thrust, it's just an almost imperceptibly small amount of thrust.

score 9 · Answer 3 · edited Mar 02 '15 at 16:10

Thrust is the reaction force derived from a rocket nozzle. You don't have to have combustion or fire to create thrust as evidenced by simple soda bottle rockets that use water and compressed gas. But in spaceflight combustion is one of the more efficient ways to create thrust in terms of the energy produced and weight of material required to derive that energy.

The force of thrust is derived by momentum exchange. The rocket throws off many small particles of mass at high velocity in one direction and by Newton's third and second laws, the spacecraft is accelerated in the opposite direction.

For fuels that do combust, a thrust chamber is engineered which allows the combustion to build a high pressure, and at one end of the chamber a nozzle is fitted that retains the chamber pressure and increases the velocity of the thrust. The magnitude of the thrust force is roughly the chamber pressure times the nozzle cross sectional area.

For the butane lighter there is no thrust chamber, and the combustion takes place in the atmosphere where pressure can't really build up. If there is any thrust to consider it only comes from the expansion of the butane gas from the valve opening, before combustion takes place. And I'll wager its on the order of micro-Newtons.

Why doesn't a cigarette lighter generate thrust?

3 Answers3