Does Rocketry Work beyond Earth's atmosphere?

[patrix]

you are also confounding gas laws with mechanical laws, and so your explanation fails to be as solid as you premise.

I would say that it is you who are doing this The_White_Lodge, but you are in good company. In order for Newton to apply, there has to be an action-reaction. If you fire a loaded gun in space, you will get that because the expanding gas will be able to act against the bullet in the barrel, and the reaction will be a recoil that moves the gun in the opposite direction. But if you remove the bullet (the rocket in vacuum case) there is nothing the expanding gas can act against. Hence no reaction.

[The_White_Lodge]

Well said Patrix, as far as I can tell we are in total agreement.

[NotRappaport]

So, the idea is that work is done by pressure. In an atmosphere, the gas ejected from the rockets would build a pressure gradient, from a maximum near the exhaust, and gradually decreasing until it levels off with atmospheric pressure. So, it's the pressure buildup what pushes the rocket upwards. A nice way to model this would be to imagine a compressed air cilinder and then opening the valve.

But in a vacuum, the gradient 'endpoint' would be 0, so the gas molecules would have no opposing force when moving away from the rocket.

That is the key part of the argument that convinced me rocketry cannot work in a vacuum! Within an atmosphere where there is air pressure and expelled gasses encounter resistance when they are forced out of the nozzle above a certain velocity, thus creating a pressure gradient that pushes in the opposite direction against the nozzle (and, by extension, the rest of the rocket). But in a vacuum, gasses and particles can be ejected at any velocity at all, even at many kilometers per second, and no pressure gradient can form so no pressure against the nozzle can be created.

[The_White_Lodge]

NotRappaport,

You are close to having the physics correct on this, but alas, this talk of a pressure gradient creating thrust is entirely incorrect. Whether you or anyone else here actually cares to understand the physics of why a space rocket is not a practical reality is another matter, perhaps we should just move on.

The pressure gradient has nothing to do with the mechanics of an explosion creating thrust or recoil. It does however play a critical role in ensuring that a "controlled explosion" can be managed in the first place, which as I and Patrix have explained is the reason why a rocket in space is an impractical reality.

[NotRappaport]

NotRappaport,

You are close to having the physics correct on this, but alas, this talk of a pressure gradient creating thrust is entirely incorrect. Whether you or anyone else here actually cares to understand the physics of why a space rocket is not a practical reality is another matter, perhaps we should just move on.

The pressure gradient has nothing to do with the mechanics of an explosion creating thrust or recoil. It does however play a critical role in ensuring that a "controlled explosion" can be managed in the first place, which as I and Patrix have explained is the reason why a rocket in space is an impractical reality.

I know the argument about Newton's 3rd law, but this fails for the important reason that the only useful reaction force from it is for mass that is ejected at precisely the angle opposite the direction of travel. Only a very narrow stream of mass is expelled along this exact trajectory to cause rectilinear motion. The vast majority of mass will be expelled at some glancing angle that would produce a torque, causing rotational motion instead.

For example, in the pic below the green line represents the only exit trajectory that will cause a recoil resulting in rectilinear motion. All the red lines represent but a small fraction of the many other possible trajectories at which masses can exit the nozzle and whose reaction force will be a torque.



The net effect is that the 3rd law has practically no effect on rocket thrust at all. The only way to ensure all the masses eject along the green trajectory would be to have a nozzle as narrow as the width of the individual exit masses (molecules) which would obviously severely restrict the rate of flow to near zero. EDIT: This would make it analogous to the bullet in a gun example.

[simonshack]

@ The White Lodge :

I have moved this discussion you started over at our "Satellites, general discussion and musings" thread - to this more appropriate thread
titled "Does Rocketry work in the Vacuum".

Please be more careful in the future - when choosing where to post your thoughts on this forum.

By the way, I truly enjoyed this paragraph of yours:

To understand why this is a case, consider a basic law of pressure: when there is a pressure differential between the interior and exterior of a chamber, to reduce the exterior on the pressure of the chamber has the same effect as increasing the internal pressure of the chamber. Now, if these rocket fuel tanks are already highly pressurized to begin with on the surface of the Earth, how can these fuel tanks maintain functionality through radical change of external pressure, which are not claimed to be happening gradually I might add, but rapidly. The answer is that, they cannot, and were such a design practically implemented, the fuel tanks would explode due to the rapid drop in pressure before even reaching the upper atmosphere

In fact, no man-made tank could possibly withstand the pressure differential between a pressurized tank - and a surrounding vacuum. It has all to do with tensile strength, of the sort that we simply do not have on earth : not even a diamond tank would do.

As for those still nurturing the fanciful idea of rockets being propelled in space - let me know if you'll ever witness a fish jumping out of the water and starting to fly up, up, and up in the sky. That would be quite miraculous. Yet, if you witness such a miracle, let me know! And make sure to capture a video of the miraculous ascent! You may sell it to some churches who, nowadays, are running out of miracles with which to attract new converts. Jesus walked on water, didn't he?

[The_White_Lodge]

Thank you Simon, that argument really is the clincher, and there is a set of equally as irrefutable arguments against the satellite narrative which I plan on detailing shortly.

Nonrappaport,

I now understand your explanation and I agree, however I must warn you that phrases like: "the 3rd law has practically no effect on rocket thrust at all" are very liable to be misinterpreted, especially in this internet environment with flat-earthers, mandela-effecters, and rational trigonomitrists. When I read that kind of phrasing, I took you position to mean Newton's 3rd law of motion was itself suspended because of a vacuum, which is absurd. I still suggest framing this particular argument with the emphasis on gas laws as you just did and with minimal emphasis on the laws of motion since they are themselves unaffected by a vacuum, while in contrast, the gas laws we know on Earth virtually break down completely in a vacuum.

[Nathan Draco]

Interesting stuff The_White_Lodge,

Looking forward to the "floaters" explanation.

I always found it weird that people would "invent" some system that operates in an unpredictable place such as space instead of making a way more cost effective system on the ground here on earth, which I suspect they actually have as well.

[SacredCowSlayer]

Dear The_White_Lodge,

I sincerely appreciate your contributions here thus far, and I’m in agreement with you and Patrix about Newtonian Laws, and how they would (and wouldn’t) work in the vacuum.

For the more simple minded people whose eyes glaze over in these posts, have no fear.

I’ll break this down in such a way as to give the SpaceFanBoy clubs something to Aim for.

Okay, if you wish to get into the vacuum, I do have the following proposal (and there are NO guarantees- except death):

1. Build two Giant Towers (or very thick steel rods) about 250-300 miles tall;
2. Attach a NASA-grade bungie cord to each rod;
3. Strap yourself into a pod of some kind with as many billiard balls, coconuts, or other dense solids of your choice;
4. Have the cord pulled back to its maximum resistance at a 45 degree angle;
5. After your whiz past the twin towers you’ll face little (but still an unknown amount) resistance in the ionosphere as you make your way (perhaps) into the vacuum;
6. Should you meet unexpected resistance before going into orbit (or beyond?)- PAY ATTENTION! THIS IS KEY!
7. Begin to throw all billiard balls, fruit, and well. . . basically everything, as hard as possible towards earth. This will provide some much needed thrust in such ultra thin “air”.
8. Enjoy the ride as much as much as possible, because it will be your last.

Mods: Feel free to launch my Post into the vacuum of the Derailing Room if deemed appropriate.
I can’t help but have a little fun around here sometimes.

Note: The above Instructions are loosely based on a short story I’m writing entitled “Coconuts For Isaac”.

[patrix]

Dear SacredCowSlayer,

Great post. Laughed out loud I also find the "Don't you believe in Newton!?" argument utterly absurd. Although I still need to stay humble since I not long ago was firmly convinced rocketry had no problems with vacuum. To my only defense I can say that I never actually tried to understand the physics, but now that I have, I find them impossible.

I would love to see some "Newton rocketeers" prove their point by performing common space docking maneuvers sitting in easy rolling shopping carts using whatever mass they like and throw it out of the carts...

[SacredCowSlayer]

Dear SacredCowSlayer,

Great post. Laughed out loud I also find the "Don't you believe in Newton!?" argument utterly absurd. Although I still need to stay humble since I not long ago was firmly convinced rocketry had no problems with vacuum. To my only defense I can say that I never actually tried to understand the physics, but now that I have, I find them impossible.

I would love to see some "Newton rocketeers" prove their point by performing common space docking maneuvers sitting in easy rolling shopping carts using whatever mass they like and throw it out of the carts...

That’s an interesting thought. But it doesn’t sound dangerous enough.

I’ve seen the condescending and irrelevant points (to “prove” rocketry in the vacuum) by SpaceFanBoy clubs about sitting in chairs with wheels on a solid surface and throwing a bowling ball, thereby moving the chair in the opposite direction. There is no need for me to point out the numerous flaws with this flimsy “point”, as they have been addressed on this fine forum at length.

But I do think it’s high time they put their rockets where their mouths are. So, if my Orbital Suggestion (November 2, 2017, on this Topic) is just too much to ask, I’ll propose another “less risky” (and more cheap) experiment.

I will challenge any one of them to try the following:

1. Climb aboard a plane fit for skydiving with any and all solid objects (of your choice) that can most easily be thrown, fired (including a .50 caliber machine gun), or otherwise accelerated (including rocket boooster(s) affixed wherever you like);
2. Jump out at a mere 1,000 feet (or higher if you like) and use the items and/or devices to demonstrate how Newton’s law will save your ASS from suffering an Epic Flattening!;
3. No parachutes either. That would just be disingenuous.
4. Also, Absolutely No “landing” in an abandoned/hollowed out/ mineshaft filled with 200 feet of feathered pillows on top of 20 memory foam mattresses- or anything of the kind. In fact, just aim for a Walmart parking lot.
5. Let me know when and where so I can make certain that I send an independent photographer to capture the moment. We will extract the data (not the parts though) from there.

Poor Mr. Newton. I feel a little bad for the fella. Undoubtedly, he would have no way to know that his observations would be sullied by the government (NASA clowns) to sell space fantasies, and in turn flip (or attempt to) science on its head.

[dblitz]

As long as it is airtight, a container of gas or whatever under pressure is isolated from the pressure outside. I don't think an outside decrease will result in an inner increase in pressure. The only way to increase inner pressure is to increase the volume of whatever is inside.

[Flabbergasted]

I don't think an outside decrease will result in an inner increase in pressure.

Then why does a gas balloon burst in a vacuum chamber?

[The_White_Lodge]

dblitz,

How is a container which is actively releasing fuel going to be airtight?

[SacredCowSlayer]

As long as it is airtight, a container of gas or whatever under pressure is isolated from the pressure outside. I don't think an outside decrease will result in an inner increase in pressure. The only way to increase inner pressure is to increase the volume of whatever is inside.

I want to make sure there isn’t any confusion about the points made about pressurized tanks.
The atmospheric force exerted onto the exterior of the tank matters. . . a lot.

Consider a recent road trip of mine with the family that included an elevation change of roughly 950 feet above sea level to about 10,800 feet above sea level.

We took a small rubber basketball that was flat enough that it couldn’t bounce (at home)- but it did have some air in it (if it had been a bicycle tire, the rims would have been scraping pavement). Since we “home school” (I don’t even like that phrase, as few things disrupt meaningful Education like calling it “school”) our children, it was an opportunity along the way to teach something that could be observed, which is nice for a change. I just love it when that happens!

[A Side Note: It always strikes me as odd just how little science I really got to observe for myself during my tortuous years spent in rectangle, bell ringing, loud speaker blaring, sensory overloaded, environments (called school) jam packed with people, nearly all of whom were there under compulsion in one form or another. I’ll leave that be, lest this post be deemed more appropriate in the “Parenting In The Simulation Age” Topic.]

While driving into the mountains I was explaining to my kids why our ears hurt and have to “pop” to relieve the pressure. So I asked my son to grab the ball from the back to see if it felt any different.

He passed it up front and we were surprised at just how firm it was.

When we got back home it was sad and deflated again.

But, we had certainly Not injected any more volume into the ball on the way through the mountains.
The difference was purely due to atmospheric pressure.

With little nearly a 10,000 foot climb in elevation, the expansion was dramatic.

It’s simple. As the atmospheric pressure dropped, the ball had less force exerted on its exterior.

I asked my kids “so, if we placed this ball in a monkey’s lap at sea level, and launched it at full throttle straight into the atmosphere, what would eventually happen?”

We all agreed (after considerable laughter) that the ball would eventually fail at its weakest point and it would blow apart.

The point I’m trying to make is that yes, you are correct that volume isn’t being added to the tank.
However, the ever decreasing atmospheric pressure from the outside the tank will render the pre-existing contents incapable of being stored as such.