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Old 12-14-2019, 07:10 PM
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When your rocket ship lifts off and you're NOT on Earth. A question.


When the L.E.M.'s upper half blew its load of rocket fuel and took off from the surface of the Moon during Apollo 11, et al, it was firing in a zero-air/ zero-oxygen environment.

Now, I know it was also lower G, and that may have helped to answer my question:

Lacking air, does one need substantially more fuel to take off from a given mass ( planet ) than one needs if there's an Earth- like mixture of gasses in the air?

Or said another way, does X - thousand gallons of rocket fuel ( whatever it may be made up of ) have the same propulsive force on the surface of the Earth per gallon as it will on the surface of Mars?
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Old 12-14-2019, 07:15 PM
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No atmosphere means no drag to overcome, so less fuel needed.
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Old 12-14-2019, 07:29 PM
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Yep, air does provide a lot of drag, especially when you go fast. In fact, the drag increases with the cube of your velocity, so it takes quite a bit of fuel to maintain speed when trying to achieve orbital velocity. However, the atmosphere also gets less dense as you go up, so drag will eventually start to decrease. The point where these two curves meet is when the rocket will experience the maximum dynamic pressure on the craft. After that, it's usually relatively smooth sailing.
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Old 12-14-2019, 07:31 PM
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What about the (probably insignificant) help due to buoyancy?
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Old 12-14-2019, 07:55 PM
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IIRC rockets are more efficient in a vacuum than in air. Something about the maximum efficiency of the exhaust. I'm sure I got some info about this on the Dope, but search isn't getting it for me.

Last edited by TriPolar; 12-14-2019 at 07:59 PM.
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Old 12-14-2019, 08:03 PM
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IIRC rockets are more efficient in a vacuum than in air. Something about the maximum efficiency of the exhaust.
This is correct. An engine designed for vacuum use is more efficient than one in an atmosphere.

Rockets convert temperature and pressure into momentum by expanding their exhaust. It's just like any other heat engine in that respect: you start with a hot, compressed bit of gas, and let it expand, doing work in the process. For a rocket engine, the nozzle is what does the expansion.

But you can only expand the gas to ambient pressure. Beyond that, and you get no benefit, and in fact can damage the nozzle.

In a vacuum, you can expand as much as you want. You put a big nozzle that expands the exhaust as close to zero pressure as you can get away with, and this extra expansion corresponds to more work done and more efficiency.

If you look at a Falcon 9 rocket, for example, the upper stage has just one engine while the lower stage has 9. The engines are virtually identical, except that the nozzle on the upper stage fills up the entire diameter of the rocket (the lower ones are smaller, since they pack 9 of them in the same diameter). That's because the upper stage operates in a vacuum and gets a significant benefit from the giant nozzle.
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Old 12-14-2019, 08:05 PM
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Rockets contain all of the oxydizer needed within internal tanks. No outside air is used for combustion. So an Earth like mixture of gasses in the air does not help in this regard.
Also, rocket nozzel shape plays a big role in it's fuel efficiency and the thrust produced, so each rocket engine is designed differently depending on the environment it is designed for. And all else being equal (even ignoring drag) rocket engines designed for space will always be more efficient than those designed to operate in an atmosphere.
So when something takes off from the moon, it has a more efficient engine and it has no atmospheric drag to make it work harder. So it requires much less fuel.
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Old 12-14-2019, 08:14 PM
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In fact, the drag increases with the cube of your velocity,....
The Wikipedia page says that the drag (force) increases as the square of the velocity, but the power needed to overcome the drag increases as the cube.

https://en.m.wikipedia.org/wiki/Drag_(physics)

Can you please explain ?

Last edited by am77494; 12-14-2019 at 08:16 PM.
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Old 12-14-2019, 08:23 PM
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Can you please explain ?
Power is just force times velocity. So, the force is proportional to v squared, and power has an extra v term to make it cubed.

The energy required to go a given distance goes up with the square, since the extra v term gets cancelled out by the fact that it takes less time to get there at a higher velocity. But if you're looking at fuel burn rate, that's energy per unit time and thus power is the right metric.

More heuristically: when traveling through air, you're basically pushing yourself through blocks of air at some velocity. Each block of air has a kinetic energy proportional to v squared, and pushing each one aside is also proportional to v squared. But at high speed you're also encountering more blocks of air per second, so you end up with a v cubed term. Or, if you're talking a fixed distance, there is also a fixed number of blocks of air along that distance, so it's v squared.
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Old 12-14-2019, 08:25 PM
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Drag is a force. Work is force times distance. Divide both sides of that by time gives: power= drag times velocity. If drag increases with velocity squared then power increase with velocity cubed.
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Old 12-14-2019, 08:48 PM
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E.g., a Rocketdyne F-1 (chosen for no special reason) has a specific impulse of 263 seconds at sea level, but 304 s in vacuum, so there you go. It also produces more thrust in vacuum.

A first-gen Vulcan rocket produced 431 s in vacuum, compared to 326 s at sea level. Point is, atmospheric pressure reduces the effective exhaust velocity even though it's the same engine (carrying its own liquid oxygen of course).
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Old 12-14-2019, 09:10 PM
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This is correct. An engine designed for vacuum use is more efficient than one in an atmosphere.

Rockets convert temperature and pressure into momentum by expanding their exhaust. It's just like any other heat engine in that respect: you start with a hot, compressed bit of gas, and let it expand, doing work in the process. For a rocket engine, the nozzle is what does the expansion.

But you can only expand the gas to ambient pressure. Beyond that, and you get no benefit, and in fact can damage the nozzle.

In a vacuum, you can expand as much as you want. You put a big nozzle that expands the exhaust as close to zero pressure as you can get away with, and this extra expansion corresponds to more work done and more efficiency.

If you look at a Falcon 9 rocket, for example, the upper stage has just one engine while the lower stage has 9. The engines are virtually identical, except that the nozzle on the upper stage fills up the entire diameter of the rocket (the lower ones are smaller, since they pack 9 of them in the same diameter). That's because the upper stage operates in a vacuum and gets a significant benefit from the giant nozzle.
A bit of a nitpick the nozzle isnít doing work to expand the gas. The gas expands due to its higher pressure relative to the environment. The nozzle can control the rate and direction of expansion so that there isnít mass and therefore momentum being wasted going to the side of the rocket.
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Old 12-14-2019, 09:23 PM
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A bit of a nitpick the nozzle isnít doing work to expand the gas.
The gas absolutely is doing work on the nozzle. It's not completely wrong to say that the nozzle is there to keep gas from squirting out the sides, but that's just a kind of hand-waving explanation of it.

A perfect nozzle would take the hot, compressed, stationary gas inside the combustion chamber, and convert it to a stream of gas at ambient pressure and temperature, moving in a straight line opposite the direction of movement.

In a vacuum, you can expand more. The gas expands and cools further than it would in an atmosphere. The work is being one on the part of the nozzle beyond that where the gas was at one atmosphere. Ignoring engineering concerns like cooling, an atmospheric engine is the same as a vacuum engine with the bottom part of the nozzle snipped off.
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Old 12-14-2019, 10:08 PM
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The gas absolutely is doing work on the nozzle. It's not completely wrong to say that the nozzle is there to keep gas from squirting out the sides, but that's just a kind of hand-waving explanation of it.

A perfect nozzle would take the hot, compressed, stationary gas inside the combustion chamber, and convert it to a stream of gas at ambient pressure and temperature, moving in a straight line opposite the direction of movement.

In a vacuum, you can expand more. The gas expands and cools further than it would in an atmosphere. The work is being one on the part of the nozzle beyond that where the gas was at one atmosphere. Ignoring engineering concerns like cooling, an atmospheric engine is the same as a vacuum engine with the bottom part of the nozzle snipped off.
I said the nozzle doesn’t do work on the gas though. Take a test nozzle fixed in place. Is it moving when the rocket is tested? No. It’s stationary. The force on the surface of the nozzle times the displacement of the nozzle is 0. Therefore the nozzle isn’t doing work by definition.

Ok, I read your post more carefully. You were saying nozzle did the expansion not the work...

Last edited by octopus; 12-14-2019 at 10:12 PM.
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Old 12-14-2019, 10:17 PM
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I assume the engine was tested in a vacuum room on Earth at some point?
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Old 12-15-2019, 12:30 AM
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There seems to me a question in the OP that hasn't been answered. Does the exhaust push against the atmosphere to propel the rocket? Given that I went to school 40 years ago, this might be wrong, but as I understand it, no, the atmosphere has no effect on the rocket's acceleration. In air, or in an airless world, the effect is the same. With the caveats given above which I never knew before. You learn so much around here!
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Old 12-15-2019, 01:20 AM
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I assume the engine was tested in a vacuum room on Earth at some point?
Yup. Vacuum for rocket engine testing isn't exactly easy, and uses a large steam ejector to keep the vacuum. Tests for the LM engines was done at White Sands.

https://www.nasa.gov/Directorates/he...-facility.html

The pics of stands 401 and 403 clearly depict the ejectors.

They did 57 tests, to a total of about 3300 seconds.
Read all about the development and testing here.

https://ntrs.nasa.gov/archive/nasa/c...9730010173.pdf
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Old 12-15-2019, 01:21 AM
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There seems to me a question in the OP that hasn't been answered. Does the exhaust push against the atmosphere to propel the rocket? Given that I went to school 40 years ago, this might be wrong, but as I understand it, no, the atmosphere has no effect on the rocket's acceleration. In air, or in an airless world, the effect is the same. With the caveats given above which I never knew before. You learn so much around here!
A rocket does not need to push against anything, but for maximum thrust you do need to design your nozzle to expand the exhaust to atmospheric pressure and not any more or less. Also the exhaust velocity will be lower in atmosphere.
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Old 12-15-2019, 01:22 AM
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I assume the engine was tested in a vacuum room on Earth at some point?
In order to fire a live rocket engine in a vacuum chamber you have to be able to remove all the rocket exhaust as fast as it is produced while maintaining the vacuum. Our B1 test site at the Plum Brook Station in Sandusky, Ohio had the capability of firing large engines at high altitudes but not a vacuum. Looking through the site history I don't see any mention of testing the LEM.

Later we built another high altitude chamber at the Rocket Engine Test Facility (the B Stand) at Glenn Research Center. I was an operator of that facility and it could achieve a fairly good vacuum but only had the capability to test up to 1000 lbs thrust.

Dennis

Last edited by mixdenny; 12-15-2019 at 01:23 AM.
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Old 12-15-2019, 02:08 AM
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There seems to me a question in the OP that hasn't been answered. Does the exhaust push against the atmosphere to propel the rocket? Given that I went to school 40 years ago, this might be wrong, but as I understand it, no, the atmosphere has no effect on the rocket's acceleration. In air, or in an airless world, the effect is the same. With the caveats given above which I never knew before. You learn so much around here!
That is exactly the misconception in the famous NY Times editorial from 1920 saying Robert Goddard was nuts. Which they retracted July 20, 1969. First article I found.
So whoever taught you 40 years ago must have learned the misconception a bunch of years before that.
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Old 12-15-2019, 03:08 AM
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So in a sense the misconception has it exactly backwards; the pressure of the atmosphere reduces the thrust by trying to push the exhaust gases back in.
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Old 12-15-2019, 03:28 AM
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There seems to me a question in the OP that hasn't been answered. Does the exhaust push against the atmosphere to propel the rocket? Given that I went to school 40 years ago, this might be wrong, but as I understand it, no, the atmosphere has no effect on the rocket's acceleration. In air, or in an airless world, the effect is the same. With the caveats given above which I never knew before. You learn so much around here!
As a layperson, and defintely not a rocket-scientist, the best explanation I've heard for this is the office chair one, it also has the benefit of being replicable.

Sit on an office chair on wheels with a ream of paper on your knee and your feet off the floor. Throw the paper away from yourself, at speed, in one direction and you and the chair will go in the other. That is all that a rocket is doing, it is ejecting mass (the exhaust) at speed in one direction and the pointy bit travels in the other direction. The energy for that ejection comes from converting the fuel used (food for the office chair, chemical for the rocket).
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Old 12-15-2019, 04:35 AM
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You were saying nozzle did the expansion not the work...
Right. And you are correct in that the nozzle is controlling the rate and direction of expansion. Just like a piston in a cylinder: it contains the hot gas, which does work on the piston and then the crankshaft. The gas is doing the work of course, but it's through the nozzle (or piston, or turbine, etc.) that the expansion is channeled into work.

You can also look at it at the microscopic level. At the top of the nozzle, there is some very hot and slowly moving gas. Individual particles are moving in all directions at high speed, but the average velocity is low. The nozzle is close to horizontal here, and when a gas particle bounces off of it, it moves mostly in the down direction.

Farther down, the gas is moving faster toward the exit. It's also cooler. So the particles are sorta moving out and downward on average. The nozzle is at maybe a 45 degree angle here, so when a particle bounces off of it it also tends to be in the downward direction.

Near the exit of the nozzle, the particles are moving very quickly aft, and the gas is relatively cool. The walls of the nozzle are almost vertical now, and any particles hitting it are at a grazing angle, because they're mostly moving straight down. This last impact straightens them out even further.

When a particle hits the wall of the nozzle, it pushes in a perpendicular direction to the surface. But because it's symmetrical with the other side of the nozzle, the horizontal component cancels out, and only the vertical component is left. The vertical component is strong at the top of the nozzle and weaker at the bottom, but every little bit helps. All of it contributes to the net thrust of the nozzle (the remaining part is from the gas traveling through the throat of the combustion chamber).
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Old 12-15-2019, 04:48 AM
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A rocket does not need to push against anything
Well, it pushes on its own exhaust.

A car pushes against the Earth when it accelerates. The car goes forward, and the Earth is pushed back at a tiny velocity. It takes almost no energy to accelerate the Earth this way since the velocity is so low.

An airplane pushes against the air. To keep flying, it takes an enormous amount of ambient air and pushes it downward at some velocity. This is less efficient than the car, because the mass of that air is a lot less than the Earth, and so it has to be accelerated to a higher velocity, which takes more energy. It's still pretty good, though.

A rocket pushes against its own propellant. Since there's so little propellant available (at most, its own mass), it has to accelerate it to as high a velocity as possible to get as much momentum transfer as it can. Which makes it extremely energy inefficient, at least at low speed; that high velocity exhaust is sapping almost the entirety of the energy when the rocket has just taken off. It gets better at high speed, though.

You always have to push against something to accelerate. The NY Times apparently didn't realize that you can push off stuff that you carry with you, though.
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Old 12-15-2019, 08:03 AM
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Quoth octopus:

I said the nozzle doesn’t do work on the gas though. Take a test nozzle fixed in place. Is it moving when the rocket is tested? No. It’s stationary. The force on the surface of the nozzle times the displacement of the nozzle is 0. Therefore the nozzle isn’t doing work by definition.
Which means that nothing is doing work on the nozzle. But that doesn't mean the nozzle isn't doing work on anything else. The exhaust is still going from stationary to moving, as a result of a force, and so something is certainly doing work on the exhaust. What do you think it is?
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Old 12-15-2019, 08:22 AM
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Which means that nothing is doing work on the nozzle. But that doesn't mean the nozzle isn't doing work on anything else. The exhaust is still going from stationary to moving, as a result of a force, and so something is certainly doing work on the exhaust. What do you think it is?
Just a comment, it is not the exhaust that moves the rocket, that is the equal but opposite force, and really the rocket couldn't care less about it, but it is pressure from the gasses that are pushing it. It some ways it's like a balloon that is allowed to fly expelling air out the hole. The only propulsive force is the pressure imbalance due to the hole (more pressure in the forward direction then the rear). Since we have no way of creating that pressure imbalance, whcb creates a force, without the equal and opposite force of some sort, that's where the exhaust comes in. The nozzle and the combustion chamber are the parts of the rocket that experience this force on the rocket side, the exhaust gas is what experiences the equal but opposite force.
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Old 12-15-2019, 09:04 AM
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In the end how you describe it is more a matter of where you choose to stand when doing your analysis. Invoking the gas laws and how gasses flow and behave is just a convenient way of packaging the conservation laws, especially conservation of momentum. Whether you choose to keep the conservation laws nicely bundled up inside the gas laws and talk of pressure, or let them out and look at the manner in which all the forces act and momentum transfers, you end up with exactly the same thing. All that the reaction products know is that they have suddenly got a lot more energy and they keep banging into and bouncing off things - mostly other things like themselves - very occasionally some run into things like metal walls, or colder gas molecules that aren't reaction products. The job of the engine designer is to get as much of that banging around to result in stuff going out the back carrying as much momentum as possible. To do this he applies knowledge of useful derived laws that govern such banging around of stuff. All of these laws eventually derive from the base conservation laws. It is a matter of convenience what level you need to play.
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Old 12-15-2019, 12:56 PM
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In order to fire a live rocket engine in a vacuum chamber you have to be able to remove all the rocket exhaust as fast as it is produced while maintaining the vacuum. Our B1 test site at the Plum Brook Station in Sandusky, Ohio had the capability of firing large engines at high altitudes but not a vacuum. Looking through the site history I don't see any mention of testing the LEM...
The LM ascent and descent propulsion systems and service module SPS were tested in vacuum chambers at Arnold Engineering Development Center (AEDC) near Tullahoma, TN:

History: https://media.defense.gov/2019/Jul/1...%20PROGRAM.PDF

Summary of SPS altitude testing at AEDC: https://pdfs.semanticscholar.org/07f...c86cacc7d6.pdf

To answer the OP, a launch vehicle experiences two notable performance losses: air drag losses and "gravity" losses from the vertical ascent phase. Of these two gravity losses are much higher. On the shuttle drag losses were only about 1% of orbital delta V, whereas gravity losses were about 16%.

Gravity losses are one reason why all launch vehicles pitch over as soon as possible. In the earth's atmosphere, they can't pitch over too soon due to atmospheric dynamic pressure.

On the moon, the lack of air means drag losses are zero, but that would typically only be a few % anyway for a streamlined vehicle. However -- the lack of atmosphere means the LM can pitch over dramatically, soon after lunar liftoff to reduce gravity losses.

In the Apollo 17 lunar liftoff you can see the LM ascent stage pitch forward soon after liftoff, accompanied by the callout "pitchover": https://youtu.be/9HQfauGJaTs
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Old 12-15-2019, 03:39 PM
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Which means that nothing is doing work on the nozzle. But that doesn't mean the nozzle isn't doing work on anything else. The exhaust is still going from stationary to moving, as a result of a force, and so something is certainly doing work on the exhaust. What do you think it is?
Integrate f ds over a distance interval where d1=d2. You get 0. The work the nozzle does is 0. The gas has a velocity due to the pressure of the gas compared to the ambient pressure. The gas accelerates due to that pressure difference and the net force on the rocket is due to the pressure difference between the gas and the environment.

The nozzle serves to control and direct expansion but it does no work. Itís similar to an elbow in a pipe system. The water flowing past the elbow is accelerated. The elbow puts a force on the water. However the elbow does no work on the water. This is just by definition of what work is.

Is the nozzle supplying any energy to do this so-called work? No. All the energy comes from the chemical reaction of the rocket fuel and oxidizer. The gas is the source of work in a rocket. Not the nozzle.
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Old 12-15-2019, 04:07 PM
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It's been awhile since I read this and I can't really recall the source but wasn't the actual performance of the Lunar Rover much better then expected attributed to the fact that the lack of air resistance had not been really factored in?
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Old 12-16-2019, 07:41 AM
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Originally Posted by Dr. Strangelove View Post
This is correct. An engine designed for vacuum use is more efficient than one in an atmosphere.

Rockets convert temperature and pressure into momentum by expanding their exhaust. It's just like any other heat engine in that respect: you start with a hot, compressed bit of gas, and let it expand, doing work in the process. For a rocket engine, the nozzle is what does the expansion.

But you can only expand the gas to ambient pressure. Beyond that, and you get no benefit, and in fact can damage the nozzle.

In a vacuum, you can expand as much as you want. You put a big nozzle that expands the exhaust as close to zero pressure as you can get away with, and this extra expansion corresponds to more work done and more efficiency.
AerospaceWeb has a decent explanation of rocket exhaust overexpansion/underexpansion, including a diagram that shows what to look for on a rocket engine in flight. the diagram shows the rocket exhaust having been overexpanded inside the engine bell at low altitude (after which it gets pinched back in to a smaller diameter again), and underexpanded at high altitude (the exhaust plume swells to a much larger diameter after leaving the engine bell).

You can clearly see this on the Saturn V in this launch video. At 0:40, it's still at low altitude; the exhaust plumes are overexpanded. It's hard to see the atmosphere forcing the plumes to contract in again since there are five exhaust plumes in close proximity, but you can see that the exhaust definitely isn't expanding out beyond the outer perimeter of the whole vehicle. Contrast this with the rocket exhaust at 2:00 and beyond, when the vehicle is somewhere between 50,000 and 100,000 feet; the exhaust plumes, now relatively unconstrained by the thin atmosphere, expand to a huge diameter after escaping the confines of the engine bells. That expansion is wasted energy; it could have been harnessed to propel the vehicle forward if the designers had incorporated much longer/larger engine bells, but the weight penalty of larger bells wouldn't have been worth it.

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It's been awhile since I read this and I can't really recall the source but wasn't the actual performance of the Lunar Rover much better then expected attributed to the fact that the lack of air resistance had not been really factored in?
Hadn't heard that, but it seems unlikely. The designed top speed of the rover was only 8 MPH, at which speed aero drag (if there had been any) would have been pretty minimal compared to the rolling resistance of those tires and the drag imparted by driving around on soft powder.
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Old 12-16-2019, 08:36 AM
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....rocket exhaust overexpansion/underexpansion....
You can clearly see this on the Saturn V....
This is a good point. A lunar liftoff would not only eliminate drag losses and greatly reduce gravity losses due to enabling early pitch over, it would allow a vacuum-optimized nozzle.

However even launch vehicle first stages on earth do not always (or even generally) use sea-level-optimized nozzles. This can be seen from the Saturn V flight instrumentation, available for each mission in "Saturn V Launch Vehicle Flight Evaluation Report", e.g, Apollo 16: https://ntrs.nasa.gov/archive/nasa/c...9730025090.pdf

I don't know what altitude the Saturn V F1 nozzle was optimized for but it was obviously not sea level. The actual flight instrumentation showed first stage thrust increased from 7.6 million lbf at liftoff to over 9 million lbf at stage separation altitude of 37 miles:

https://photos.smugmug.com/photos/i-.../i-DzXRHVC.jpg

So the question is how much additional nozzle performance optimization was available between the F1 engine as desgined vs a pure vacuum-optimized nozzle? The relevance is on the LM ascent stage engine nozzle (which was vacuum optimized), how much performance benefit came from that vs a less-optimized (but not sea level) nozzle more typical of a terrestrial launch vehicle first stage.
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Old 12-16-2019, 09:39 AM
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I don't know what altitude the Saturn V F1 nozzle was optimized for but it was obviously not sea level. The actual flight instrumentation showed first stage thrust increased from 7.6 million lbf at liftoff to over 9 million lbf at stage separation altitude of 37 miles:

https://photos.smugmug.com/photos/i-.../i-DzXRHVC.jpg

So the question is how much additional nozzle performance optimization was available between the F1 engine as desgined vs a pure vacuum-optimized nozzle?
Fair question. Several plots of interest here. Check out the third plot, halfway down the page, which shows altitude versus time after launch. During first stage, due to acceleration, it's spending more time between 0-18.5 miles than between 18.5-37 miles, so you'd think they would want to optimize it for some altitude below 18.5 miles. But I think if we replace altitude with ambient pressure, we'd see that ambient pressure decreases very rapidly. At 20,000 feet for example, ambient pressure is already less than half that of sea level; the pressure at 35,000 feet is about 1/4 that of sea level. This means that first stage spends much more of its time at an ambient pressure closer to zero than to 14.7 psi. So you could optimize the engine for, say, 35,000 feet and it would only be significantly suboptimal for the first ~1/4 of its burn, and pretty darn good for the rest of it.

The thing is, even vacuum-optimized rocket nozzles don't extract that maximum possible thrust from the exhaust plume. At some point the benefit of greater thrust is offset by the penalty of carrying all that extra engine bell around. just eyeballing rockets that are vacuum-optimized, it doesn't look like they add much more bell length/diameter beyond what is seen on rockets that have to deliver decent sea-level performance. Here for example is the lunar lander ascent engine; its aspect ratio doesn't seem far different from the Saturn's F-1 engine.

I think that to have optimized the F-1 engine for operation in vacuum, they would have added a bit more bell length/diameter, but not much.

Quote:
Originally Posted by Dr. Strangelove
But you can only expand the gas to ambient pressure. Beyond that, and you get no benefit, and in fact can damage the nozzle.
Can you elaborate on the "can damage the nozzle" detail?
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Old 12-16-2019, 09:50 AM
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As a quick note, atmospheric pressure drop as follows

p=poe-k*h where k is a constant and h is the height from sea level. Basically at 5km, pressure is ~50% of sealevel and by 10km it's only 20%
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Old 12-16-2019, 12:09 PM
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Quote:
Originally Posted by Grey View Post
As a quick note, atmospheric pressure drop as follows

p=poe-k*h where k is a constant and h is the height from sea level. Basically at 5km, pressure is ~50% of sealevel and by 10km it's only 20%
Just to expand on that a little. k is a constant (called "scale height") for each planet - it depends on the composition of the atmosphere, the temperature and the gravity of the planet. For Earth, scale height is about 8 km (depending on the temperature), while on Mars it's about 11 km.
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Old 12-16-2019, 12:25 PM
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Quote:
Originally Posted by Machine Elf View Post
just eyeballing rockets that are vacuum-optimized, it doesn't look like they add much more bell length/diameter beyond what is seen on rockets that have to deliver decent sea-level performance. Here for example is the lunar lander ascent engine; its aspect ratio doesn't seem far different from the Saturn's F-1 engine.
SpaceX uses a significantly larger 2-piece (extendable) nozzle on the vacuum version of the Merlin engine, as seen/discussed here.

Same for the Blue Origin BE-3, though it's difficult to compare the two pictures because of the difference in configuration.
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Old 12-16-2019, 12:53 PM
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The solid rocket boosters for the space shuttle had a two stage bell to deal with the changing air pressure. A longer and wider bell would descend down over the short narrow one used at take off. The boosters weren't going to ascend into vacuum but it still made a big difference in efficiency. Any SSTO rocket will have to deal with that somehow, though I think sea level takeoff SSTO makes little sense anyway.
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Old 12-16-2019, 03:19 PM
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Quote:
Originally Posted by Machine Elf View Post

Can you elaborate on the "can damage the nozzle" detail?
NASA literature mentions "asymmetrical, oscillating forces" which can damage the engine mountings. Maybe there could be additional undesirable effects due to cavitation or analogous phenomena? Wouldn't want the nice rocket engine to explode....

I'm sure the engines are launched at sea level at least slightly overexpanded, though, as long as it is not too extreme. Engineers can also modify the nozzle with altitude-compensating features like annular/plug/spike nozzles.
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Old 12-16-2019, 05:59 PM
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Quote:
Originally Posted by Machine Elf View Post
Can you elaborate on the "can damage the nozzle" detail?
As noted above, this regime is called overexpanded. The nozzle is trying to expand the flow to a lower pressure than ambient.

The problem comes from flow detachment: where does the exhaust separate from the nozzle? Normally speaking, this happens right at the bottom edge of the nozzle.

If the flow is slightly overexpanded, the separation still takes place on this ring shape. You'll see the sides of the exhaust push in a bit, smoothly flowing in from the rim. In fact this is desirable, because once the rocket travels to a higher altitude, the flow will become optimally expanded and gain efficiency from that.

When the flow becomes highly overexpanded, though, the exhaust detaches from the nozzle rim, and instead detaches further up the inside of the nozzle. But because it's a smooth surface, and because the flow itself contains irregularities, it doesn't stay in a perfect circle, and instead moves around depending on the various random forces.

So for one, you get constantly changing forces on the inside of the nozzle. Worse, you might set up a resonance--something that causes the nozzle to bend in one direction, and then another. If this amplifies, it'll rip apart the nozzle. Nozzle walls are thin and don't have a lot of stiffness.

There are some solutions to the problem. One way is to have a kind of step partway up the inner surface. At high ambient pressure, the flow sticks to the upper edge; as the pressure goes down it attaches to the lower edge. But it's not a great solution since it causes friction and a hotspot.

Other designs, like aerospike engines, avoid the problem completely, but have their own issues.

Because the Space Shuttle main engines had to work from sea level to vacuum, they had to deal with an even larger range of pressures than most engines. Its nozzles have quite a high expansion ratio for sea level, which makes them significantly overexpanded. They dealt with that by tweaking the shape of the nozzle slightly and just mechanically strengthening the nozzle to cope with the extra forces.
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Old 12-17-2019, 05:38 AM
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When the L.E.M.'s upper half blew its load of rocket fuel and took off from the surface of the Moon during Apollo 11, et al, it was firing in a zero-air/ zero-oxygen environment.

Now, I know it was also lower G, and that may have helped to answer my question:

Lacking air, does one need substantially more fuel to take off from a given mass ( planet ) than one needs if there's an Earth- like mixture of gasses in the air?

Or said another way, does X - thousand gallons of rocket fuel ( whatever it may be made up of ) have the same propulsive force on the surface of the Earth per gallon as it will on the surface of Mars?
are you asking if the atmosphere is used in the combustion of the rocket fuel? If so, the answer is no.
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Old 12-17-2019, 09:29 AM
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Quote:
Originally Posted by Yakamaniac View Post
are you asking if the atmosphere is used in the combustion of the rocket fuel? If so, the answer is no.
Yes that was part of what I was asking.
Thank you.

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Old 12-17-2019, 09:31 AM
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There have been some experimental engines that breathe air while it's available, and then switch to an internal oxidizer once they're out of the atmosphere. They never turn out to be practical, though.
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Old 12-17-2019, 09:39 AM
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Quote:
Originally Posted by Chronos View Post
There have been some experimental engines that breathe air while it's available, and then switch to an internal oxidizer once they're out of the atmosphere. They never turn out to be practical, though.
Air augmented rockets are used for some missiles. They do not seem practical for achieving space flight though, one reason being obviously that all their advantages disappear once out of the dense part of the atmosphere.
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Old 12-17-2019, 01:12 PM
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We've got a decommissioned Soviet Kholod air-breathing missile inside an office building near us. Probably going cheap because the technology didn't work very well.
https://www.dezeen.com/2016/02/19/ma...soviet-rocket/
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Old 12-17-2019, 03:24 PM
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I get all this, what I don't get is how an aerospike engine uses the exhaust gas to optimize the ideal nozzle shape for a given velocity. I've tried a couple of Youtube vids but I'm not seeing how this works in reality. Can someone explain this like I'm 8?
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Old 12-17-2019, 06:28 PM
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Quote:
Originally Posted by swampspruce View Post
I get all this, what I don't get is how an aerospike engine uses the exhaust gas to optimize the ideal nozzle shape for a given velocity. I've tried a couple of Youtube vids but I'm not seeing how this works in reality. Can someone explain this like I'm 8?
According to the qualitative description in that FAQ, it is not changing the the velocity of the exhaust gas to optimize the expansion, rather the pressure (if any) of the outside atmosphere pushes on the exhaust and constrains the expansion. I do not see anything there about changing the nozzle geometry during flight.
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Old 12-17-2019, 07:15 PM
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Quote:
Originally Posted by swampspruce View Post
I get all this, what I don't get is how an aerospike engine uses the exhaust gas to optimize the ideal nozzle shape for a given velocity. I've tried a couple of Youtube vids but I'm not seeing how this works in reality. Can someone explain this like I'm 8?
Everyday Astronaut has a pretty good page/video on the subject.

You can get a pretty good hint as to what's going on with just this image, though. See how the ambient pressure presses the exhaust flow to the surface of the spike? That's how you achieve no flow separation at arbitrary pressure. At lower pressure, the exhaust stream starts to widen as it reaches the end of the spike, but it remains in contact. It widens just as much as it needs to in order to expand to ambient pressure.
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Old 12-18-2019, 01:51 AM
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Quote:
Originally Posted by Cartooniverse View Post
Or said another way, does X - thousand gallons of rocket fuel ( whatever it may be made up of ) have the same propulsive force on the surface of the Earth per gallon as it will on the surface of Mars?
Oh no.
It has significantly less usable propulsive force ON EARTH than on the moon or Mars.

Reason:
0) surrounding air is not used by a rocket motor. It is not fuel, nor something to push against, nor something to get lift from like an airplane. It is merely 'stuff' that happens to be around the rocket.

1) Rocket engine thrust is reduced by ambient pressure. In the case of the Apollo Lunar ascent module's rocket motor, that loss of thrust would be about 16%

2) Once your rocket is moving, the air(if any) starts dragging back on it. Obviously Earth's high-pressure atmosphere causes a LOT more drag than that of Mars (1/170th as dense) or that of the Moon (3 quadrillionths as dense)

In addition, while it does not at all affect the propulsive force, the local gravity does reduce the actual propulsion one *gets* from the propulsive force. With again Earth gravity impeding launch much more than weaker Mars or feeble Moon gravity.

Last edited by MarvinKitFox; 12-18-2019 at 01:54 AM. Reason: included reason zero. Which is not a reason but a needed disclamer
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Old 12-18-2019, 09:56 AM
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Side note: To get a couple of humans and their life support system into orbit around the Earth, you need something roughly like the Gemini rockets. To get a couple of humans and their life support systems off of the Moon, you need only something about the size of a car. Though granted, that's mostly because of the difference in gravity, not atmosphere.
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Old 12-18-2019, 10:19 AM
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Quote:
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Side note: To get a couple of humans and their life support system into orbit around the Earth, you need something roughly like the Gemini rockets. To get a couple of humans and their life support systems off of the Moon, you need only something about the size of a car. Though granted, that's mostly because of the difference in gravity, not atmosphere.
Wouldn't the low gravity also be the cause of the lack of atmosphere?
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