The Illusion Of Rocket Thrust

A quick 10 minutes to explain what's really going on with rockets. It's not rocket science. Oh, wait.... Well, Enjoy!
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[@EEVblog]

Never really considered it that way before. Great explanation!

[@koliedrus2576]

Ditto! I would not have expected that understanding an aspect of rocket science would elevate my mood!

[@MichaelOfRohan]

Still convinced the majority of the thrust acts against the nozel walls, not the combustion chamber as it may logically seem as fran pointed out. Id be interested in knowing if theres a way to test that. Nozel v nozelless thrust test?

[@squidcaps4308]

Same, i thought it was all about the velocity and mass of the gas escaping. This does make more sense.

[@r0bst4rl1ng]

@@squidcaps4308 i think doing the math that way works out the same. That is, yes, there's conservation of momentum and the increase of momentum of (the rocket and its remaining fuel) has to match the axial component of the momentum of the escaping mass. I think this description is more about understanding "where the forces manifest" and how to most-efficiently convert the combustion energy into axially-aligned escape momentum.

[@kasra7907]

It was asume

[@miltonruas5368]

Congratulations for the simple and clear explanation without using maths. The rocket equation states clearly that the higher the external pressure is, less thrust we will get, and you explained it very well... In conclusion it's in the vacuum scenario where thrust will be more efficient.

[@davesvoboda2785]

Aerospace engineer, here. The nozzle extension is more about matching the static pressure between the exhaust and the external atmosphere, while maintaining attached laminar flow. That governs the shape. Another way to look at rockets is is F=MA. You get a force by accelerating massive exhaust out behind the rocket. That massive exhaust is most "pushy" when it's lined up with the direction of motion, less energy is wasted pushed to the sides. The nozzle is designed to decrease static pressure from 70 atmospheres on the F1 (and 300! for the Raptor) in the combustion chamber to 1 atmosphere (or less) as the gas escapes. As the nozzle decreases the static pressure, the energy of that static pressure is exchanged for dynamic pressure, accelerating the exhaust backwards. If the static pressure of the exhaust is too low for the current atmospheric pressure, you get a certain amount of choking of the flow, and lots of turbulent exhaust in the plume, shock diamonds, etc, which wastes energy. If the static pressure is higher than the atmospheric pressure then the exhaust will splay out, wasting energy by throwing the gas to the side, when it should be going directly backwards. Of course, that's not possible when the external pressure is a vacuum, so vacuum nozzles are just as large as structurally possible, to lower the static pretty down as much as possible, knowing that there will be a certain amount of wasted energy.

[@jarls5890]

You can actually see the pressure change around the exhaust cone when a rocket travels upwards - when it gets really high you can see the exhaust plume get much wider and spread out. To convince people that cannot comprehend "it got nothing to do with pushing against air" I tell them to sit on an office chair with wheels. Pick up an empty copy paper box and throw it as hard as they can while on the chair. They hardly move at all. Then I tell them to repeat it with a box that is full of paper (heavy) - and see them roll quite the distance. If it had anything to do with "pushing against air" - results should have been the same. Then I tell them to imagine a rocket shooting out trillions of tiny boxes at very high speed.

[@davesvoboda2785]

@@jarls5890 I was more explaining the point of the nozzle. It's a given that it's not about pushing about anything, but throwing the exhaust out the back as fast as possible.

[@davesvoboda2785]

@@jarls5890 If you want to see something REALLY nifty, look at the inside-out nozzle that is the Aerospike, that stays equally efficient from sea level to vacuum.

[@holdenmccrotch6485]

Thanks I never knew that and always wondered about gas escaping from the side as it exits the nozzle. I had always imagined the pressure of the escaping hot gasses was very high and therefore you'd send it out in every direction, which is very inefficient. So the nozzle actually gives the high-pressure gas a space to expand back to ambient pressure? What would some kind of idealized vacuum nozzle look like -- just really really long? Like if you built your rocket in a vacuum and never had to deal with atmosphere at all, what would it look like?

[@jocramkrispy305]

@@holdenmccrotch6485 If you could make it massless, then it could be huge, but the diminishing returns of adding increasingly large rings to the bottom of the nozzle compared to the decreasing increase in thrust gives you a limit -- if you double the weight of the vehicle to get a 10% increase in engine efficiency then you've actually reduced the efficiency of the system by 45%

[@TheNeonRabbit]

If you fire a gun, the kick is the same whether you hit something or not

[@MichaelOfRohan]

Right. Same principle as rockets. Just instantaneous. Notice an RPG is recoilless? Because both ends of the barrel are open.

[@wayneluyt4089]

Exactly the bullet has inertia so it exerts a backwards force before it gets on its way. A rocket is shifting a huge mass of material out the back

[@underthemoon-p7q]

I feel threatened

[@MichaelOfRohan]

@@underthemoon-p7q we all do. Its the human condition. I will not cater to your feelings any more than you cater to mine.

[@MichaelOfRohan]

@@TheEmbeddedHobbyist that I gotta google

[@seanbryant2848]

No one ever explained that to me before... I just figured... well, I figured incorrectly. Thanks again, Fran! Excellent video!

[@shawnvines2514]

It is one thing to explain Newton's laws of motion. It is another to understand the misconceptions of others and try to educate them. You are amazing!

[@musashi939]

The problem is most people you find on the internet with that misconception are flat earthers, they want the earth to be flat and they don't want rockets to work. You can't convince them with facts where they're wrong. Imho this is wasted effort by Fran.

[@BollocksUtwat]

I've had this argument with people who say rockets don't work in vacuum and I realized after a while they didn't understand Newton.

[@railspony]

@@musashi939 Flat Earthers don't claim the Earth is flat, they claim that you can't "prove" that it isn't. And they're right. You can believe things, but you can't "prove" anything. And most people don't even know why they believe the Earth is not flat other than, "people I trust told me so!" But most people misrepresent their trust in science for an understanding of science, and they misrepresent belief as proof.

[@seanwatts8342]

@@railspony No, they blatantly & adamantly say "The earth is flat."

[@musashi939]

@@pear7777 you spin me right round right, baby right round right like a pancake round..

[@vwestlife]

This *is* rocket science!

[@jonpardue]

Interesting use and timing of the word impedance. This explanation is linkworthy!

[@RA-wv4cc]

Well done

[@Paxmax]

The great philosopher Franlaba is believed to once have said: "Thrust, it's like a broken peanut, imbalance is the key to unlock the force"

[@jorgenskyt]

"Philosopher", are you kiddin'? She was a Jedi!

[@Paxmax]

@@papalegba6796 riiiight. You'd need to explain where the mistakes might be for me to take your remark serious.

[@jamesdezort8005]

Most excellent explanation without being patronizing or condescending. Thank you, Fran!

[@gunlokman]

I was lucky enough to have a real physics teacher at my school and he explained it by imagining a chap in a small boat in the water with a pile of bricks. Each time he throws a brick out of the back of the boat the boat moves forward. It's not the brick pushing against the air. It's the chap pushing against the brick.

[@nickacelvn]

yes because the brick has a reasonably high mass. what is the mass a rocket pushes against? There is no brick in space. Only the exhaust gas.

[@angelsachse9610]

@@nickacelvn The gas may be light, but is being ejected at very high velocity.

[@angelsachse9610]

@@papalegba6796 A rocket engine is not a heat engine though. All heat engines use thermal cycling, rocket engines do not.

[@angelsachse9610]

@@papalegba6796 My point is that even though most rocket engines utilize the heat produced by the combustion of propellants for maximum efficiency, combustion is NOT necessary for a rocket engine to work. Have you heard of a so-called cold gas thruster? It has impaired performance compaired to the classic bi-propellant engine but works perfectly fine without combustion.

[@angelsachse9610]

@@papalegba6796 Have you not read my comment? I was specifically referring to a COLD GAS THRUSTER! Some Rockets burn. Some do not.

[@markferrell5261]

Us model rocketeers have to know where our center of pressure is to insure stability in flight. This explanation is elegant and simply stated. Excellent teaching!

[@ernietech-101]

Homer Hickman's research showed how important the proper design of the nozzle is to achieving thrust. Turns out a little impedence is a necessary thing...at just the right place

[@BooBaddyBig]

Yes, the flow needs to choke at the throat, i.e. go sonic, otherwise it won't go supersonic in the expansion section.

[@Kandralla]

Nozzle VS orifice plate: It's not the pressure drop that is the problem it's how much of that pressure drop is recoverable... IE how much of that energy is converted into less useful forms of energy (like heat). You have fluid all moving in one direction as it enters the restriction and ideally you want it to all keep moving in that direction. An orifice plate creates a low pressure dead zone to the sides of the orifice exit that is going to cause some of the fluid to change direction in an attempt to equalize the pressure difference, you have to pay energy to change the flow direction... So you end up with kinetic energy in the axial direction turning into kinetic energy in the radial direction and turbulance... which eventually turns into energy spent not pushing forward or heat depending on if the downstream side is enclosed or not neither of which is desirable whether you're trying to push a rocket forward or trying to measure flow in a pipe. A nozzle/venturi let's the fluid expand slowly to minimize the dead zones so at the end of the nozzle as much fluid as possible is still headed in the direction it started in.

[@jorgenskyt]

On behalf of the Danish based non profit NGO organization "Copenhagen Suborbitals" I'd like to thank you for a simple and straight forward explanation of the illusive concept of reaction thrust. Your wonderful "peanut model" have inspired me. We might be able to demonstrate your concept with a tiny, contained explosion in a tiny chamber, connected to a larger chamber - and demonstrate the difference between the direction and amount of thrust in this experiment under pressure and in vacuum. That might be fun! :-)

[@ichemnutcracker]

I once had a guy tell me that there is no way humans went to the Moon because radiation from the Van Allen Belt would have killed them. My reply was "didn't you tell me last week that the Earth was flat? How can it have a belt?" Sometimes people are more interested in the self-satisfaction of being 'contrarian' than they are in being logically consistent.

[@jarls5890]

When they start talking about the Van Allen Belt - i tell them "BELT" - you ever seen a "BELT" - you can go around it (even if they did not).

[@PropaneWP]

I don't know much about the Flat Earth lore, but apparently they seem to treat Earth as a unique anomaly. So, they recognize that every planet in the universe is spherical (as this is evident for everyone using a telescope), EXCEPT for Earth - which is SOMEHOW a plane. And yeah, it seems Flat Earth is mostly about contrarianism at all costs. It's about: "you don't tell me - I tell YOU". It's about cutting off the nose to spite the face - which gives me some immediate associations to certain science-denying political views.

[@dsm5d723]

Most people who want to talk to you now, I find.

[@GamesFromSpace]

His argument isn't as stupid as it seems, because it's our premise that the Earth is round and has a belt. He's pointing out what he sees as an inconsistency in that scenario. Of course, you can then just point out that subzero temperatures are also deadly, but you can still take out the trash in the middle of winter without instantly dying.

[@dsm5d723]

@@GamesFromSpace I have ONLY seen stupid people identify with the other stupid people the internet connected so as not to be left out. Congratulations, we have as Ted Kaczynski warned, made a generation that is declining in cognitive ability WTH the advance of calculator technology. I do not mean that people who have their own paradigms are included, I finished ALL of academia's hard work and a chat at lunch until extinction is all education has to offer. $$$$ for attention, my gene drive and actual working Light Saber, not you so ignore the future.

[@samhenderson2947]

I'd never thought of this. Very cool explanation. Thanks .

[@kitesupreme8907]

It's basic maths dude

[@reclaimatorerebus6531]

I remember having this confusion in high school. It wasn't until I actually took rocket propulsion courses in college that I really understood what was going on. I wish I'd had this explanation in high school :D

[@reclaimatorerebus6531]

@@papalegba6796 You're assuming that anything like that was taught in high school at the time I attended. This sort of topic was not covered at all, so I was left to just guess at how rockets worked until I took the actual courses on the subject, and my peers and I came up with some odd ideas. One actually thought the space shuttle had to toss out sand in order to have something to "push against." SO as I happened to be interested in rocketry at the time, I would have liked an explanation of how they worked. That said, I don't really get your comment about w=PV being wrong in terms of Fran's explanation. All this really illustrates is that if the pressure outside the combustion vessel were higher than the internal pressure, a rocket wouldn't work. It relies on the internal pressure being higher so that the exhaust can be vented, thus creating the imbalance that propels the rocket.

[@Lethgar_Smith]

I like to explain to people how a rocket motor is like an explosion and the rocket is like the shrapnel that flies away when a bomb explodes. If a grenade explodes in space the shrapnel is still going to be pushed away from the explosion. It is the expanding gases inside the reaction (thrust) chamber of the motor that is pushing against the walls of the chamber and thereby pushing the rocket. Putting a nozzle on the end of the reaction chamber causes the forces to be applied in a direction opposite from where the gasses are escaping.

[@helianthropy]

I was having that realization looking at that sealed, pressurized container in this video.

[@stuartgray5877]

@@CaptainMagnus And "rocket Exhaust" is jus millions of bullets that all have MASS. Most of these morons will also claim that "gas does not have MASS" in the vacuum of space. Of course they are wrong.

[@fakestory1753]

cool analogy

[@bluenoodle29]

Ha, my dad and I were talking about this a few days ago. Great video!

[@greenbanana311]

Rocket science isn't something you can "debate."

[@earlydoomer346]

@@greenbanana311 It's also not "science". There was nothing scientific about this, only speculations. It's not even possible to test for "space", since any vacuum chamber will fill up with gases/smoke the moment something is ignited inside of it.

[@tedhaubrich]

Your a more patient, kinder soul than I to explain this to people that can't intuitively grasp that rather simple concept. Pretty sure I understood this basic concept at two or three when I could do thinks or throw my arms around etc. I did learn why rockets for use on a vaccum are designed different from ones used in the atmosphere. So thank you.

[@RinoaL]

I've never realized people would think of it this way. Really I just think of the engine as thrust being the opposite side of the opening. Have a 6" round exhaust opening, and you get force against the other 6" back side of the rocket. The nozzle is just there to make it flow better.

[@PistonAvatarGuy]

I just think of Bender, flying through space, then throwing objects forward to slow down. All a rocket really does is throw combustion products in one direction, while the combustion process itself is used to accelerate those combustion products. I think that the main benefit of the skirt extension is to force the combustion products to accelerate rearward, because they'll have a tendency to want to pop out of a shorter nozzle sideways when there's no surrounding pressure to keep them from doing so.

[@peraltarockets]

I just got my L1 certification from the NAR so this is a great vid explainer to point friends to.

[@JustWasted3HoursHere]

I had it explained to me like this once, long ago: Okay, so you've probably done the experiment where you stand on a skateboard and throw a heavy object like a bowling ball away from you as fast as you can and it causes you to roll in the opposite direction, right? Is it because the bowling ball is pushing on the air? No. Would this work if you were in a vacuum chamber? Of course, since the air has nothing to do with why you move in the opposite direction. Now, imagine that instead of throwing one large heavy object away from you at a modest velocity, you are throwing trillions upon trillions of very small particles away from you at extremely high velocity. It would have the same effect and it would also work in a vacuum. This is exactly what's happening with a rocket. For more confirmation, look at amateur rockets that have their velocity and altitude measured internally with altimeters and velocitymeters. Some of these rockets have reached more than 70 miles. What's interesting is that even though the air at 30 miles is a tiny fraction (less than 1%) of the density of the air at sea level, the rocket nevertheless continually increases its velocity and altitude even though there is no air to "push on". So it seems that air density or air are not a factor in how well rockets work except that they work BETTER when there is none!

[@synthetic240]

"Is it because the bowling ball is pushing on the air? No." That is literally what these dingdongs think. They cannot do basic math so they imagine a scenario in which they are correct and insist their imagination is correct.

[@dave99887]

Amazing teaching skills Fran!

[@bzakie2]

Newton’s third doesn’t require anything to push on. It just happens.

[@chaos.corner]

@@blackdove6713 And vice versa.

[@kenlogsdon7095]

AKA conservation of momentum.

[@kenmore01]

@@blackdove6713 Yes, but why burn it then?

[@adamrak7560]

@@kenmore01 only for efficiency: combustions gets you high temperatures. And gases have higher pressure (or volume) in higher temperatures (pV=nRT). Small inefficient cold gas thrusters are used in space for their simplicity. You can replace the combustion with a nuclear reactor to heat hydrogen gas. You can also use electricity to heat and accelerate ionized gas.

[@BollocksUtwat]

@@blackdove6713 I realized arguing with one of these weird people that they think the gases in the combustion chamber are part of the rocket, hence the "it can't push against itself" argument they always use. They think of those gasses the way Fran explained them when she had the totally enclosed nutshell. They don't recognize that by opening the chamber up to atmosphere the gasses are now not part of the enclosed system of the rocket and therefore can push the rocket.

[@JD-lk7im]

I like the Diagram explaining how thrust is achieved.. It is shaped like a Nutshell, thanks Fran

[@davethetaswegian]

Great explanation, but for those that still doubt there is a big hint in calling it rocket exhaust. Exhaust from a rocket no more moves the rocket than a car exhaust moves a car, it just looks a lot more spectacular.

[@pretzelogic2689]

People also think a helicopter blows air against the ground to develop lift.

[@kitesupreme8907]

It blows fumes from it's own exhaust wich is lighter than air, and because it is hot air it then pushes up against the propeller becausehot air rises. Also a spinning propeller creates more surface area increasing the lift potential of the air. It's really not that complicated

[@p_mouse8676]

@@kitesupreme8907 How does this work with helicopters without an exhaust, like electric helicopters?

[@cambridgemart2075]

@@kitesupreme8907 Wow, that was a lot of stupid!

[@kitesupreme8907]

@@p_mouse8676 electric static building around the propellers from friction against air particles causes the lift in drones. It's very basic aeroscience

[@kitesupreme8907]

@@blackdove6713 I didn't say rotary wings and stationary wings aren't using the same lift dynamics as I mentioned about the exhaust fumes and static energy colliding with air particles to create symbiotic lift across a horizontal plain

[@USWaterRockets]

Fran: Great explanation especially the detail about the vacuum engine nozzles. A lot of people struggle with this. One question: Did you intend to say that the F1 nozzle extension was added later, because I have never heard this before. Even the F1 under test in your image appears to have been designed for a nozzle extension that simply was not used for that test. If they weren't planning the nozzle extension, they wouldn't have bothered with the exhaust manifold that dumps the pre-burner exhaust around the perimeter of the nozzle. That manifold was specifically designed to use the pre-burner exhaust to cool the nozzle extension.

[@greenbanana311]

You are correct, that's why the exhaust is so dark.

[@fluffycritter]

It's kind of funny how even today people think rockets can't work because there's nothing to push against - that's something that Goddard himself always had trouble convincing people about too! I think an analogy for why "pushing against" isn't the case would be someone throwing a tennis ball at a wall a few feet away. It's not the ball hitting the wall that applies force to you, it's the throwing of the ball which is.

[@catsupchutney]

The flat earth society is alive and well. Remember, statistically half the population is below average and about 14 percent of the population has an IQ below 80. Eighty is a pretty bad number.

[@bkuker]

Goddard did fall for the pendulum fallacy at first though :)

[@jensgoerke3819]

@@catsupchutney I guess the global flerf community will dismiss the explanation with "nu uh" because it went right over their heads. I've seen videos of them believing that gases equalize instantly while ridiculing the actual explanation, so "rockets can't work in a vacuum, ergo space is fake, NASA is lying, flat!"

[@captnodge]

I was like that with boat propellers. Couldn't quite get my head around it .now they actually find it more efficient to pull rather than push

[@markc2643]

The tennis ball analogy is better for an Ion Engine.

[@heintmeyer2296]

Fran, thank you so much for all the work you do. Right up there with Carl Sagan and Richard Feynman...

[@stuartgray5877]

Think of it another way: Does "Exhaust gas" have mass? The exhaust gas from most rocket engines (kerosene/oxygen) is Carbon Dioxide or CO2. If I threw a block of solid CO2 at your head, would you duck? The exhaust gas from a Space Shuttle Main Engine (Hydrogen/Oxygen) is basically PURE WATER! If I threw a block of solid water at your head, would you duck? Yes! because CO2 and water have MASS which gives it INERTIA or the "resistance to being accelerated". If the exhaust gas from a rocket had NO MASS, then the chamber pressure could not build to well over 3000 PSI because the gas could move out of the way instantaneously and effortlessly. Pressure CAN build because the "stuff" leaving the nozzle has INERTIA and takes TIME and "FORCE" to accelerate. So, the rocket PUSHES on the MASS of the exhaust gas while it is Still inside the engine. Once this mas leaves the nozzle the rocket can no longer "push on it" and whatever happens to the gas then is irrelevant to the forward force on the rocket engine.

[@oliversmith9200]

That is very clearly described and illustrated rocket motor physics. It's the best of both worlds coming with the usual FranLab low pressure delivery.

[@frankowalker4662]

"That's it in a nut-shell.". So that's why you drew a peanut husk. :)

[@terryolsson4145]

Wow Fran...watching you demonstrate and explain all aspects of life, electronics, physics and machine here on earth as we know it makes me a wiser person, though im 69yrs old, i still dig it. Thanks

[@lmfaoitsallie]

It's just recoil. I didn't realize this was such a big problem for folks.

[@deltib]

It's probably one of those things where someone started it, and people started picking it up, and spreading it without really knowing any of the physics behind it themselves. Like the "according to science, bumble bees can't fly" thing.

[@greenbanana311]

I know, right? 🤦‍♂️ I don't think it is a big problem for people, really. These concepts, and the laws of physics have been understood for a very long time.

[@kenlogsdon7095]

@@greenbanana311 Tell that to all the flat earthers who continue to refuse to believe in rockets in space because of their need to deny that space exists.

[@BollocksUtwat]

Some people are very very dependent on their intuitive sense of physics. They can't accept the leap required to look past their dumb ape intuition.

[@TheNefastor]

I'm not sure it is a problem. Talking with flat-earthers, I've realized that they will gleefully play dumb in their efforts to claim that we're all sealed under a glass dome. My view is to never waste time with that kind of people. They aren't ignorant, so much as they are contrarians. They perfectly know what happens when air escapes from a balloon, they've known since they were little kids, just like anybody else.

[@chrispeoples4606]

Thanks Fran, I added your video to my forces playlist o torment my physics classes!

[@jasonjansen9831]

Throwing a basketball while standing on a skateboard was always the way it made sense to me

[@dustyroads834]

Wow. That just upended 40 years of me thinking I had everything figured out. Way cool. Thanks.

[@bkuker]

Although the pressure pushing on the front end provides thrust, the nozzle is responsible for even more. Ideally in the nozzle you "use up" all of the pressure, converting it into exhaust velocity. You can actually look at it from a different point of view where all of the pressures (in the chamber, the nozzle, and the exhaust plume) cancel each other out (gas pushing on gas pushing on gas asymptotically out to vacuum) so the pressure difference provides no thrust at all, but the exhaust velocity is maximized. Newton's 3rd law and away you go!

[@mikefochtman7164]

Quite so. This is why nozzles designed for operating in upper atmosphere or outer space are shaped more aggressively than sea level. By allowing the exhaust gasses to expand more while still within the nozzle, there is more thrust developed against the nozzle wall in the forward direction.

[@bkuker]

"there is more thrust developed against the nozzle wall in the forward direction" it's fun, but not surprising, that you can also totally ignore the pressure and the forces against the nozzle, look at it only in terms of the gas, how flowing through the nozzle accelerates the gas and it's speed (and mass) once it leaves the motor and presto you get the exact same thrust!

[@kpbarrow]

These things are fascinating - there are a bunch of scientific misconceptions which are fairly common and often surprising. Possibly the most common being "close the front door, or you'll let the cold [temperature] in" rather than letting heat [energy] out. A weirder one was switching electrical sockets off so the electricity doesn't fall out. Great explanation!

[@bueb8674]

Or that a vacuum cleaner doesn't suck, the atmosphere pushes air into the area of relatively lower pressure.

[@ShubhamBhushanCC]

And this is why we need basic physics

[@kitesupreme8907]

You spelled psychics wrong

[@tomterrific9459]

@@kitesupreme8907 lolool..........

[@earlydoomer346]

So people would learn about the 2nd law of thermodynamics, which proves that "space" can't be real...

[@BooBaddyBig]

There is a mistake in the video- there is NO backpressure on the jet because it's supersonic, the pressure influence can't travel up that jet. Instead, the backpressure issue is on the OUTSIDE of the NOZZLE. Because the jet is supersonic, it can be expanded out to above or below ambient pressure. If it's above then you need a longer nozzle. But if if it's below at exit, then the portion of the nozzle where it has expanded below ambient the difference in pressure between the inside and outside of the nozzle represents a backwards force, a net loss of thrust. So there's no point in having that bit of the nozzle, it adds weight and reduces thrust. Still, if you have it, it can help later at higher altitudes, so there's usually a compromise nozzle.

[@RodrigoFernandez-td9uk]

What I've never understood is why you would need "something to push on" other than the combustion gases themselves that you push back at high speed.

[@salliemorrill2671]

That was really helpful. I never thought about the imbalance being necessary. Thank you.

[@muha0644]

Interesting theory, but it's not completely correct. The force of a deLaval nozzle is produced mostly from escaping gases. The reason rockets have nozzles is because the pressure outside needs to match the pressure of the exhaust to increase efficiency. The constriction exchanges the large pressure and low velocity into low (approx. atmospheric) pressure and high velocity. This is why rocket exhaust looks like it goes outwards horizontally in a vacuum, it's an inefficiency. The same thing is responsible for "shock diamonds" that form when rockets take off from the launchpad. The exhaust pressure is lower than the atmospheric pressure, so it's pushed in and forms the diamond shape. So yes, the gas moving backwards is responsible for the thrust, which is proportional to the speed at which it leaves the rocket and the mass of the gas produced after combustion (ideally water and co2). Just like if you threw a ball in space, you would accelerate in the opposite direction faster if you throw a faster ball. You can increase the thrust if you can push the gas out faster (just by making the constriction bigger), but then materials become a problem... It's hard to contain so much pressure. And if the exhaust pressure is too low it can also cause problems. No one part alone causes the thrust, all features of the rocket work together, but the constriction is by far the most important.

[@nmccw3245]

tl;dr: F=M*A

[@sircsolerom764]

You are helping me understand thrust on the f1 so much! I've gotten so interested in the Saturn V. I think the technology is so fascinating

[@markandrew8461]

The diagram did actually look like a nutshell too.

[@HFamilyDad]

Thanks Fran, I've wondered about this since childhood, I'm 54 now! All answered!

[@TheDirty360]

There's a similar example with road tires that I found interesting. The inside top of the tire actually takes the weight of the vehicle (other than sidewall stiffness). It seems counter intuitive because the bottom of the tire looks like it's holding the car up, but the top of the tire needs to take the increased pressure generated by the negative pressure caused by the tire pressing against the road.

[@trainliker100]

This is somebody who could be great teaching "science" in something like junior high school. (Do they still do that?) I sometimes ask people how many teachers/professors really made an impact on them from their 12 to 16 years they were sentenced to school. Not just good teachers, but ones who still shape their thinking to this day. Mentors more than teachers. Most people can come up with maybe three names. I think Fran could easily be one of those.

[@NewbFixer]

Why dont they teach this in schools? It would blow a childs mind and cause them to think an entirely different way about things. If they are left to make up their own minds they would believe the illusion of how thrust works. Incredible Fran! Making me smarter every day! x

[@Dannosuke25]

They do

[@rblackwell2771]

I remember reading about rockets in science in the early 90s

[@Rocketman5442]

Thank you! I was completely misunderstanding the concept of thrust. Now I understand.

[@Martin-DL]

A great video that brings a misconception of the hobby scientists into the right light with very simple words. I look forward to the second part. On my bucket list: Meet a Rocketdyne F-1 in Orlando

[@CARLiCON]

awesome topic Frau Blanche..it's the same principle (3rd law) that propels a balloon when you let it go...the stretched rubber pushes the air out the blowhole (action) causing the opposite end to move forward (reaction). It's amazing what Newt was able to come up with having no slide rules, calculators, & computers. Can you imagine what he'd be capable of using the tools we have today?

[@ComradeJehannum]

Whenever I run into one of those people who say "rockets don't work in a vacuum" people I generally direct them to the thrust equation on the NASA webpage (because I have it bookmarked). I then ask them to show Me how in the equation how Po approaching zero turns off all thrust. I've yet to have an answer that didn't call Me a NASA shill.

[@kitcanyon658]

I like being a shill for NASA. I tell them I get paid more when they keep posting nonsense.

[@tekcomputers]

Don't make a flat-earther do math, it confuses them.

[@rubenflores1200]

Can I have the link?

[@earlydoomer346]

I've written on a paper that you had 3 apples minus 4 apples. Did I prove that you have -1 apple? In other words, numbers on a paper prove nothing.

[@ComradeJehannum]

@earlydoomer346 oh look it's another idiot.

[@KarlHamilton]

Wow. Thanks for explaining that. I never realised that's how it worked. Gonna impress all my friends with this one haha.

[@randycarter2001]

It's the fact that you're taking a mass and throwing it out the back at high speed is where the thrust comes from. Recoil from a gun is a prime example. The effect increases by increasing mass or by how hard you throw. Mass 3 tons per second, velocity 0 to 2980 meters per second. Newton's 3rd law in action.

[@JeffreyJonesRecumbentRider]

Good explanation, every atom of every gas molecule that is ejected out the back contributes to the energy to push the vehicle forward. Look at the ion engine, there isn't a thrust chamber perse. The thrust comes from accelerating xenon atoms to near relativistic speeds using an electrical charge.

[@Kandralla]

The confusion people have is what's pushing against what. The place where the hot gas is pushing the vehicle forward is the front surface of the combustion chamber/nozzle. In the ion engine it's the coils that are generating the field that accelerates the particles.

[@sMASHsound]

recoil from a garden hose more than a gun

[@swinde]

@@sMASHsound Does anyone actually think that the garden hose whips around like it does because of the water stream pushing against the air?

[@GregWallis]

I cannot believe I didn't know this. Thank you, Fran!

[@DiamondCutter423]

A friend who's a jet mechanic once told me "jets fly by theory", meaning Newton's law.

[@bodan1196]

This is strangely the first time I have seen someone explain this in the same way I try to do. Strange, because it is... intuative, is it not? I guess that I have just somehow missed all the numerous sources that do. Great vid, Fran.

[@Fredrik-iz4ou]

Amazing how the atmosphere is an asset to jet propulsion but a hindrance to rocket propulsion.

[@janossurman2899]

Because you can check out the first one yourself, but not the other.

[@earlydoomer346]

They're the same thing...

[@ppleberrynd]

@@earlydoomer346 Jet propulsion requires ambient oxygen, rockets do not as they bring their own oxidizer.

[@earlydoomer346]

@@ppleberrynd It's still jet propulsion, just using a different kind of fuel.

[@MikeBracewell]

Great video. In other words, a rocket motor is just a pressure vessel, fitted with a gas generator, with a bloody great hole at the back. In a rocket motor, the pressure from the gas generator (the combustion chamber) is generated in ALL DIRECTIONS, EQUALLY. Without a bloody great hole at the back, all that generated gas-pressure will build & build until your rocket-motor goes boom! Bore a bloody great hole into the back of your motor & the rearward gas pressure is released but the forward pressure, which is constantly generated, is trapped against the front of the motor, pushing it, & whatever is attached to it, forward. Similar idea for a jet engine, but the forward gas-pressure pushes against the compressed air coming in from the front of the engine, via a compressor and / or air rammed-in through forward motion. Simples!

[@rene0]

a mama octopus full of cute little alien octopus babies!

[@JamesAllredWriter]

Hooray, at last an explanation that correctly explains how rockets work. If only more people understood what you have explained here.

[@dougg1976]

Thx for explaing so clearly , i think its like releasing a balloon

[@CASHSEC]

Well done Fran, First correct clear explanation of rocket thrust. Now you can tackle the illusion of time dialation outlined in Einstein's theory of relativity. The error in the mis-interpretation comes from the way we are taught the third law with throwing rocks out of that back of a boat to get the boat to move forward. The issues of inertia of the rocks or force diagrams are never brought into the lesson.

[@mpart_woodlathe-stuff]

This blows my mind - why I never made it past freshman engineering.

[@garylovesbeer]

Great explanation! Now we need one about turbojet engines. Especially with afterburner or reheat.

[@GraemePayne1967Marine]

Same principle, really. Except air breathing. Suck air in the front and compress it, add fuel and ignite it, and let it leave out the back. Again the force is applied against the combustion chambers, which in turn are attached to the aeroplane.

[@garylovesbeer]

@@GraemePayne1967Marine so how does reheat affect the thrust? What are the reheated gases acting against?

[@GraemePayne1967Marine]

@@garylovesbeer Naw that I'm not sure about because I've never been around afterburners. But it is adding more fuel and therefore more energy which would produce more force.

[@punksci6879]

"This is a sealed chamber" You mean a bomb.

[@zefallafez]

The nazis that own this website would demonetize this channel if she had put it that way. Probably get a visit from the kgbi to boot.

[@hrodebertcoad9848]

@@zefallafez If "Nazis" owned this website, PragurU wouldn't have such an issue with it

[@Zebra_M]

Either that or an aerosol can, a scuba tank, a soda bottle... Not every pressurised sealed chamber needs to explode. Although I don't suggest sealing up rocket engines, no.

[@cosmicrider5898]

Its an angry peanut.

[@BrokenVideoProductions]

Thanks Fran. This is the best video you ever made. Probably obvious to you, but an eye opener for me.

[@simonmikkelsen]

Great explanation, Fran. I have never doubted that rockets work and got the Lego Saturn V, but you puts words on how the exactly works.

[@EpsilonZRho]

Good video, Fran! If I may, I'd like to extend this a little to explain propeller thrust (an airfoil) and by extension, wing lift. Typical explanations state that propellers generate a force via the action of "moving air back". While it's true that air is moved as a reaction to the functioning of a propeller, this is a by-product of an airfoil's operation, rather than it's cause. Because of the shape of an airfoil and the nature of a fluid such as air, pressure gradients are established across all surfaces of the moving prop/wing. In the case of a correctly shaped propeller, the sum of the pressure gradients on the rear facing surfaces will be higher than those on the forward side. Because of this, the propeller experiences a force in a direction away from the higher pressure and towards the lower (these pressure gradients also exert a force on air molecules themselves, causing them to move, but in simple terms, that's not important to the generation of thrust). Since the propeller is ultimately attached to an aircraft (or something else), the force must be transmitted to everything it's attached to as well. Of course, there are numerous differences between a rocket engine and a propeller, such as the fact that a propeller works on a surrounding fluid in the form of atmospheric air, while a rocket engine carries this fluid working mass in the form of on-board propellants, thrust ultimately comes from pressure gradients regardless of nature of machine that's creating them.

[@pureflix8086]

You hear that? Thats the sound of space deniers and flat earthers screaming in confusion.😡

[@kwcnasa]

Thx Fran, we learn something today.

[@DaIssimo]

"P" is for peanut. Sorry I couldn't resist..... Carry on.

[@mikestand5710]

I am worse than that - with the sound off I thought that when the little red swallows flew they were lifting the big blue peanut... :-/

[@mikestand5710]

OM and then Fran says "Well that's it in a nutshell..."

[@brucepickess8097]

Hi Fran, brilliant explanation of how to get the first peanut into space!!!!

[@thetruth45678]

Nothing to push against in space?? Um.. there doesn't need to be. Has everyone forgotten Newton's Laws of Motion?????

[@burtpanzer]

But in order to stop, you'd need to push against something...

[@thetruth45678]

@@burtpanzer SMH I don't want to live on this planet anymore.

[@burtpanzer]

@@thetruth45678 Dude, it doesn't mean everything is a lie or that she's right.

[@thetruth45678]

@@spaceflight1019 😓 Please stop breathing my air.

[@thetruth45678]

@@burtpanzer Just stop. You have no business speaking such ignorance. Fran is right. You are not. Go away.

[@kerryjlynch1]

This is awesome. Every kid should see it. It provides a visceral explanation of a law that I suspect many of us took on faith and applied to get through college and career, despite not really "getting it."

[@atlanticx100]

Newton for every action there is an equal but opposite reaction.

[@evangaudet]

Awesome video. It’s funny that people in your comments argue with you about how rockets work. I had no idea how rockets work either but this seems like basic rocket knowledge and I’m not arguing with you over it. I thought it was very intuitive of you to point out that illusion at the beginning because that’s exactly how my brain interprets it. After your explanation though my physics 1 and 2 courses from undergrad decided to fart up.

[@ASilentS]

Is that a cracked version of DaveCAD? 😂

[@EEVblog]

Cracked? Free download on github!

[@GraemePayne1967Marine]

Excellent!!!!! It seems that far too many people missed (or slept through) high school Physics. Otherwise they would know this. It's good to have the occasional refresher as well. Thanks!

[@PatrickPoet]

So the air rushing out of an untied balloon doesn't provide the thrust either!!!

[@alfakennywon]

The air pressure inside a tied balloon is equal on all surfaces and is higher then the air outside. When the balloon is untied the air pressure remains equal on all surfaces but since there is now a hole the inner and outer pressure will begin to equalize. This means that the force applied to the inner leading surface is higher than the inner nozzle surface because that surface doesn't exist. Thus you have imbalance of forces acting on the inside of the balloon and it is propelled forward because the air inside it pushing again the front harder than the non-existent back surface. So the air or the difference in pressure is providing thrust just not at the nozzle but at the opposite end of the container.

[@ivantuma7969]

now do the aerospike :) ... it's like trying to pinch a corner of a table with greasy fingers (only the fingers are rapidly expanding gasses)

[@kregger]

Thanks Fran. I definitely was believing the illusion. Learning something new everyday ensures that I don't fall for the illusion that I'm smart. :P

[@kregger]

@@papalegba6796 Chatbot?

[@joevignolor4u949]

There are several key points that should be added to your explanation. The thrust is actually created as the propellants are accelerated out through the nozzle against their own inertial mass. The highest pressure in the engine is present only at the injector plate. This is because the pressure constantly drops as the hot combustion gasses exchange pressure for velocity as they are accelerated rapidly through the nozzle. So by the time the propellants exit the engine at an extremely high velocity the pressure is actually very low. Below is the standard explanation that I send to people to explain how rocket engines work: "To fully understand how rocket engines work in space you have to incorporate Newton's first law of motion along with the other two. The propellants have inertia proportional to their mass, which makes them want to remain in their current state of motion (1st law). As such when you accelerate them you are pushing against their inertial mass. This creates a reaction force proportional to the mass times the acceleration, expressed as F=MA (2nd law). This reaction then produces an equal and opposite reaction in the opposing direction (3rd law). As such no outside medium such as air is required to push against for a rocket engine to work in a vacuum. Instead, the propulsive force is created inside the rocket engine's nozzle as the propellants are accelerated against their own inertia. What's going on behind the engine and outside the nozzle really has little influence over what's going on inside the engine. So rocket engines can work in space even if there is "nothing to push against" as is sometimes claimed." So in fact rocket engines do have something to push against. They push against the mass of the propellants. So in essence the engine accelerates the propellants in one direction while the propellants accelerate the engine in the opposite direction away from a common starting point. Now let's have some real fun. In the Saturn V F-1 engine about 1,500 lbs of propellant is injected into the engine every second. When all that massive amount of propellant combusts it produces a chamber pressure of about 1,000 psi right at the injector plate. And at the time of injection the propellants are moving at about the same speed as the engine. But all this changes very quickly. The propellants, which are now in the form of hot combustion gasses, are accelerated through the nozzle going from zero to about 1,000 mph over a distance of just 18.5 feet. When that massive amount of propellant gets accelerated to that high a speed over that short a distance it will produce a huge amount of force. So according to the formula F=MA you have 1,500 lbs of propellant going from zero to 1,000 mph and so you end up with a static thrust of 1.5 million pounds. Finally the F-1's engine bell was sized that way so that the exit pressure was just 14.9 psi, which exactly matches the atmospheric pressure at sea level. As such the atmosphere itself becomes a virtual extension of the engine bell. This allows for some addition acceleration of the combustion gasses to occur behind the engine, which produced a little extra thrust right at liftoff when the rocket is at its maximum weight and you need all the trust you can get to lift that huge machine off the ground. This explains why the exhaust plume is seen to go straight down at liftoff but then as the rocket ascends into the higher atmosphere the exhaust plume "fans out" sideways as the atmospheric pressure drops off. Hope this helps.

[@apolloxiii5574]

I bet it were flat earthers that say rockets don't work in space lmao.

[@kitcanyon658]

Oh course they were the first to start this nonsense. If people can’t see through the obvious avoidance of reality that flatters do then they are mere children mentally.

[@laazfreeze3982]

There are so much more reasons for flat earthers,… I think the thrust in rockets is one of the minor things. A much more interesting topic would be, how the vacuum of space can coexist with earths athmophere without a barrier inbetween. (And please without gravity)

[@earlydoomer346]

I mean they don't... How would they have even tested it? Even the largest vacuum chamber in the world would fill up with gases/smoke right away and therefore seize to be a vacuum chamber, so testing for "space" is simply not possible. Yet, we're supposed to believe that they went to the Moon in 1969 with no problem whatsoever, and even came back super easy - all on the first-ever try...

[@apolloxiii5574]

@@earlydoomer346 It's called scientific principles, they don't need vacuum chambers to test rockets, they know they will work in space, rockets don't push of the atmosphere, they work on the action and reaction principle.

[@earlydoomer346]

@@apolloxiii5574 Even if that was how they worked (which is an unprovable statement, as, again, there's no way to do tests in a large enough vacuum), you can't make a rocket function perfectly for the first time for an unknown environment, let alone put humans in it, let alone broadcast it live

[@BillPalmer]

I look at it by Newton’s other equation F=Ma. The force is equal to the mass (the mass of the fuel and oxidizer) multiplied by the acceleration applied to it. The combustion chamber and nozzle work to accelerate the fuel/oxidizer mass as much as possible. The F=Ma forma the basis for the formulas for lift on a wing, thrust from a propeller, rotor, or jet engine as well.

[@silverXnoise]

Today I learned that this was something anyone struggled to understand. It’s not obvious and intuitive? Jeez, no wonder there are so many people fighting over wearing masks. Super basic ideas are apparently exotic to some folks.

[@peacesalamonlyone]

Brilliant.... Never do they explain this in ATPL classes or books. Thank you so much.

[@visvivalaw]

You throw something one way and you move the other way. Equal and opposite. It's really not that hard.

[@PovlKvols]

Very nice explanation, and probably the best I've seen so far. Very nice work, Fran! 👍

[@UsEr-839djsjwo68]

In an atmosphere there is pressure against which the internal gasses created by the rocket motor can react against to cause a pressure to build up inside the combustion chamber. When in a vacuum, there is no pressure for the gasses to react against, therefore, the combustion process can do no work, pressure cannot build up inside the vessel as shown in the diagrams in this video and cannot push against the inside of the rocket nor can it push against the resistance of an atmosphere. Accordingly, Newtons law does not apply bacause the gas expands instantaneously and also exhausts instantaneously via the rocket motor nozzle and is instantaneously dispersed throughout the vacuum of space. So, a rocket cannot work in a vacuum. We did not go to the moon. We have not explored space. You have been conned

[@Cynthia_Cantrell]

Wow, making all of those drawings must have been... exhausting!

[@gregmark1688]

As I learned it, the opposite reaction comes from the mass of the ejected exhaust particles.The thrust of a rocket is directly proportional to that mass. But of course, the same is true of the force exerted against the leading surfaces of the chamber, in your model, so it still works.

I ALWAYS wondered how theses Rocket Engines could possibly transfer the amount of force in to the rocket above without folding itself together. This finally makes sense now!

[@JP-sw5ho]

That was wonderfully simple and easy to follow. Could you do the aero spike engine next please?

[@slordmo2263]

Yes, you are correct, I believe. I'm not a rocket scientist, and I've seen it mentioned already in the comments, but it is also the 'ejection' of mass (hot gasses) at high speed that will cause a 'force' in the opposite direction of the flow.... that's why they take such care in the 'throat' of the engine to convert the lower velocity gas to high velocity (Using the Bernoulli principal). A rocket engine uses both to generate thrust, and again.... Rocket engines use Newton's laws and don't need air to generate thrust.