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#1 2020-04-08 09:36:09

tahanson43206
Moderator
Registered: 2018-04-27
Posts: 7,326

Physics Topics

For SpaceNut re this new topic ...

From time to time questions arise in topics on this forum which call for an answer from Physics.

Over time, various forum members have generously responded by posting explanations to meet the immediate need, but that knowledge is then lost in the forum archive, never to be seen again, unless in your constant searches, you uncover them and bring them back into view.

This new topic is offered for future responses to specific questions that have Physics at the heart.

The goal here is to provide a single topic within which such helpful posts can be found. 

To start things off, I will offer a collection of citations from Google, on the subject of Angular Momentum.

The subject arose in a topic created and managed by Terraformer.  In that topic, Terraformer has proposed (among other things) the use of a space elevator to support commerce between Ceres and Mars, the Moon and other suitable locations where the products of Ceres would be useful, and where the economics of taking delivery from Ceres are superior to any other option.

Edit#1: The forum active membership includes some who have deep reservoirs of knowledge and related experience.

This topic is available as a place where a small part of that accumulated knowledge can be saved and (hopefully) passed along to coming generations.

The hope I have is that this topic will fill a niche between doing nothing at all, and becoming a Wikipedia author. 

(th)

Last edited by tahanson43206 (2020-04-08 09:41:29)

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#2 2020-04-08 09:37:44

tahanson43206
Moderator
Registered: 2018-04-27
Posts: 7,326

Re: Physics Topics

Angular Momentum

Google has kindly provided a long list of citations where Angular Momentum can be studied.

I was hoping for animation in educational material, and found a number of citations which feature animation.

animation of angular momentum

About 1,070,000 results (0.48 seconds)
Search Results
Web results

Angular Momentum (With 3D animation) - YouTubewww.youtube.com › watch
Video for animation of angular momentum▶ 7:38
Nov 18, 2016 - Uploaded by Omega Open Course
To watch problems involving angular momentum to understand the concept better, watch the following playlist ...

Angular Momentum - YouTubewww.youtube.com › watch
Video for animation of angular momentum▶ 1:48
Apr 26, 2010 - Uploaded by TutorVista
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity ...

Angular Momentum - YouTubewww.youtube.com › watch
Video for animation of angular momentum▶ 7:28
Sep 28, 2014 - Uploaded by Bozeman Science
The angular momentum of an extended object is a product of the rotational inertia and the angular velocity. The ...

Angular Momentum V2: Physics Concept Trailer™ - YouTubewww.youtube.com › watch
Video for animation of angular momentum▶ 1:16
Sep 14, 2015 - Uploaded by OpenStax
This Concept Trailer covers Rotational Motion and Angular Momentum and corresponds to Chapter 10 in ...

Angular Momentum GIFs - Get the best GIF on GIPHYgiphy.com › explore › angular-momentum
Explore and share the best Angular Momentum GIFs and most popular animated GIFs here on GIPHY. Find Funny GIFs, Cute GIFs, Reaction GIFs and more.

Conservation of angular momentum - The physics of rotationwww.cleonis.nl › physics › phys256 › angular_momentum
Animation 1 depicts that when a rotating assembly contracts the angular velocity increases. Inversely, when the distance to the center of rotation increases the ...
People also ask

(th)

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#3 2020-04-08 11:19:22

tahanson43206
Moderator
Registered: 2018-04-27
Posts: 7,326

Re: Physics Topics

Review of Video on Angular Momentum

Of the many videos available, I found this one to have a particular value for addressing the question of how to manage momentum in a system consisting of a space elevator, with cable to synchronous orbit, cable to a counterweight, and a payload ascending the cable.

Hewitt-Drew-it! PHYSICS 43. Angular Momentum
YouTube · 56,000+ views · 10/16/2012 · by Marshall Ellenstein

The video concludes with a question for the student, which is directly applicable to the instance of a payload crawling up a space elevator ribbon.

(th)

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#4 2020-04-08 13:16:12

louis
Member
From: UK
Registered: 2008-03-24
Posts: 6,865

Re: Physics Topics

A few years back I was trying to improve my understanding of some basic concepts in physics. I looked at some physics primers (designed to prepare students for university really). There was frequent reference to "force". But even in a primer of maybe 500 pages there was no definition of what it meant!

It took a good deal of reading and cogitation to work out that it didn't really mean anything at all by itself, but really was standing in for one or more of the four fundamental forces: gravity, electro-magnetic, weak nuclear and strong nuclear. In nearly all cases in the primer it appeared that it was the first two, gravity and electro-magnetic  that were being referenced.

I think this use of "force" is very misleading really. When you come to something like Newton's Third Law of Motion I have a problem:

https://www.physicsclassroom.com/class/ … -Third-Law

What really does it mean to say there is "an equal and opposite" reaction? The example is given of someone sitting on a chair - the forces are supposedly "balanced". I don't get that.  If you sit on a chair and it collapses under you, where was the equal and opposite reaction?  Isn't it truer to say that the electromagnetic forces in the chair were unable to stop the gravitational force pulling you towards the centre of the Earth? And isn't it truer to say that when the chair doesn't collapse under you it's not because there is an equal and opposite reaction it's because one force (gravity) can't overcome the electromagnetic force in the chair...(this might be incomplete - I don't know if the nuclear forces are also involved in the structural integrity of the chair). And if you, weighing 90 kgs, sit on a chair and it doesn't collapse, but equally it wouldn't collapse if there was a 5000 Kg elephant sitting on the chair, doesn't that suggest the real opposing action is actually greater than an equal force?

Or imagine a chair with an incredibly elastic material for its seat, and imagine the chair's legs surround a hole in the ground, and you put a little puppy dog on the seat and the seat  goes down and down and down, down into the hole. The puppy dog is still sitting on the seat but the seat is lower than the bottom of the chair's legs. Presumably an engineer would say "I've designed a good chair. There is an equal and opposite reaction when you sit on the chair and it does not collapse."


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#5 2020-04-08 15:09:44

kbd512
Administrator
Registered: 2015-01-02
Posts: 4,615

Re: Physics Topics

Louis,

After 500 pages, no mention of the fact that force is defined as mass multiplied by acceleration?  I find that hard to believe.  If you're asking for a mathematical proof, I'm not sure there is one.  Without diving into quantum mechanics, "F = ma" is observationally true.  Meaning, the generated force is proportional to the quantity of mass and the acceleration vector quantity.  For all measured examples of a mass "m" accelerated at rate "a", force "F" is generated.  Recall that in order for something to be quantified, it has to be observed or measured.

If you manage to drop two different objects down a completely evacuated tube here on Earth and no additional or external forces to increase or decrease the rate of acceleration are applied to one object vs the other, and both air resistance and electromagnetism count as other forces that could and generally would alter the rate of acceleration, yet one object arrives at the bottom twice as fast as the other, then there's a Nobel Prize waiting for you with your name on it.

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#6 2020-04-08 16:41:26

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 23,099

Re: Physics Topics

Part of that proof is different materials but the same amount of mass which proved that gravity works on each in the same manner for the equation.
Everything starts at the atomic level for mass and energy.
With angular velocity built in to the atom as the electron shell circulars its core.

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#7 2020-04-08 19:23:08

louis
Member
From: UK
Registered: 2008-03-24
Posts: 6,865

Re: Physics Topics

You seem to be referencing motion which is a class of situations where forces interact. But how does acceleration come about? It doesn't just "happen". In most text book examples it comes about either through gravity or the electro-magnetic force. There is no such thing as a "force" of motion or acceleration or motion related to acceleration. When one object collides with another, there is an interaction in the electro-magnetic forces...IIRC it might be static electricity but that is what imparts energy from one object to another. It isn't some abstract "force" - it's a particular force with associated fundamental particles - electrons for example - and associated waves. 

How does acceleration fit in with magnetism? A magnet is not accelerating and I am not even sure mass is particularly  important to its force, but it exerts a "force", and can attract objects towards it. This is achieved at the level of fundamental particles.

Also I think this notion of "mass" is slippery. When does mass become an identifiable unit of mass?  If an asteroid 100 miles across hits the Earth at X speed there will be consequences. But if an interstellar gas cloud 100 billion miles across hits the Earth with ten times the mass of the asteroid, would there be worse consequences? Or would we not notice? You might object - well that's a fluid not a solid. But that just returns to my point, about it being a matter of how the surfaces interact at an electro-magnetic level when you have a collision.



kbd512 wrote:

Louis,

After 500 pages, no mention of the fact that force is defined as mass multiplied by acceleration?  I find that hard to believe.  If you're asking for a mathematical proof, I'm not sure there is one.  Without diving into quantum mechanics, "F = ma" is observationally true.  Meaning, the generated force is proportional to the quantity of mass and the acceleration vector quantity.  For all measured examples of a mass "m" accelerated at rate "a", force "F" is generated.  Recall that in order for something to be quantified, it has to be observed or measured.

If you manage to drop two different objects down a completely evacuated tube here on Earth and no additional or external forces to increase or decrease the rate of acceleration are applied to one object vs the other, and both air resistance and electromagnetism count as other forces that could and generally would alter the rate of acceleration, yet one object arrives at the bottom twice as fast as the other, then there's a Nobel Prize waiting for you with your name on it.


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#8 2020-04-08 19:44:38

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 23,099

Re: Physics Topics

Now you are confusing density with concentration and diffusion...

Magnets have a force of actraction and repulsion from domain alignments within its mass.

We say that there are just 4 natural forces but I think that there are more.

four-fundamental-forces-of-nature-300x173.jpg

To have mass (matter) we also have anti-mass (anti-matter) and that is why I consider there to be more forces.

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#9 2020-04-09 03:36:14

elderflower
Member
Registered: 2016-06-19
Posts: 1,262

Re: Physics Topics

Anti matter still has mass. The anti is anticharge. So a positron annihilates with an electron and all the mass of both appears as radiation energy according to e=mc2. (neglecting mass of Neutrinos, if they have any).

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#10 2020-04-09 08:25:29

kbd512
Administrator
Registered: 2015-01-02
Posts: 4,615

Re: Physics Topics

Louis,

Although I'm not a physicist and don't even play one on TV, I'm reasonably sure of the following:

All objects made of matter are in motion, so motion is an intrinsic characteristic of matter.  If you're asking what initially set all matter in motion, I don't know the answer to that question.  We have various theories with varying amounts of observational evidence supporting them, but no way to construct a conclusive test, so far as I know.  There are literally thousands of more knowledgeable people that I'd direct that particular question to if I were you.

If you're asking whether or not the observed phenomenon of magnets attracting or repelling each other or materials that can be magnetized is related to gravity, I only know what the physicists tell me and they describe it as a separate force.  Gravity could be a specific type of electromagnetic phenomenon, but I don't know of any way to prove that.  Despite that, the fact is, those magnets are made of matter, the matter has mass, and since that matter is in motion, force or kinetic energy is generated.

Here's how I broke down the logic behind your question:

1. both magnetic and non-magnetic objects are made from some specified quantity of atoms of matter
2. all atoms of matter have mass
3. if that mass of matter is moving, and it's always moving in some direction no matter how tiny those movements are, then some acceleration vector quantity has been imparted to it, mechanism of application unimportant
4. by measurement, the accelerated matter carries force / kinetic energy with it, which we observe / measure and label a "force" (the resultant product of moving matter)
5. if that matter strikes another object made of matter, magnetic or otherwise, then a kinetic energy transfer has occurred

As to precisely when a quanta of matter has mass, much like motion it's an intrinsic property.  Recall that all observed matter is in motion.  I suppose quantum physics would state that most of what creates mass in ordinary matter is the binding energy / gluon fields between the quarks that comprise protons and neutrons in the nucleus of atoms (which also happen to be in motion).

In your example of an asteroid vs a cloud of interstellar gas, both of those objects are made from ordinary matter and the quantity of kinetic energy they can transfer when they strike Earth remains proportionate to their mass and acceleration vector.  The matter comprising the asteroid is far denser than the gas cloud, even though the gas cloud may be many orders of magnitude larger and may carry many orders of magnitude more total kinetic energy with it.  The asteroid will likely transfer all of its energy from its atoms of moving matter to Earth in a matter of seconds.  Most of the gas cloud would miss Earth entirely and whatever did strike Earth would almost certainly be doing that over a substantially increased amount of time.  Energy transfer rate matters greatly.

Therefore, the kinetic energy transfer rate produced when that asteroid impacts Earth is vastly greater than that of the vastly more massive interstellar gas cloud.  The gas cloud may cause the upper atmosphere to glow, but won't produce any sort of extinction level event.  Earth has been flying through a never-ending stream of protons (ionized Hydrogen gas) thrown off by our Sun since Earth was formed.  The last time an asteroid 100 miles across hit the Earth, it was an extinction level event for most life present on Earth at that time.

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#11 2020-04-09 08:53:45

tahanson43206
Moderator
Registered: 2018-04-27
Posts: 7,326

Re: Physics Topics

For kbd512, elderflower and SpaceNut

Thank you for saving this new topic from the ignominious descent into psuedo-science that appeared to be happening.

For SpaceNut ... please use your powers to move any instances of psuedo-science that may occur into a suitable other topic.

I would very much like for this topic to be one that could safely be recommended by a responsible teacher or (conceivably even) professor.

Thus, the detailed and thoughtful answers provided by kbd512 are of a caliber that I think will stand the test of time.

(th)

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#12 2020-04-09 08:54:54

tahanson43206
Moderator
Registered: 2018-04-27
Posts: 7,326

Re: Physics Topics

For kbd512, elderflower and SpaceNut

Thank you for saving this new topic from the ignominious descent into psuedo-science that appeared to be happening.

For SpaceNut ... please use your powers to move any instances of psuedo-science that may occur into a suitable other topic.

I would very much like for this topic to be one that could safely be recommended by a responsible teacher or (conceivably even) professor.

Thus, the detailed and thoughtful answers provided by kbd512 are of a caliber that I think will stand the test of time.

(th)

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#13 2020-04-09 16:27:49

louis
Member
From: UK
Registered: 2008-03-24
Posts: 6,865

Re: Physics Topics

Some physicists do indeed propose more forces, but I think most tend to stick with the four as far as  what essentially determines the physical reality around us.

SpaceNut wrote:

Now you are confusing density with concentration and diffusion...

Magnets have a force of actraction and repulsion from domain alignments within its mass.

We say that there are just 4 natural forces but I think that there are more.

https://physicsabout.com/wp-content/upl … 00x173.jpg

To have mass (matter) we also have anti-mass (anti-matter) and that is why I consider there to be more forces.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#14 2020-04-09 17:05:46

louis
Member
From: UK
Registered: 2008-03-24
Posts: 6,865

Re: Physics Topics

My understanding is that most physicists think gravity is an entirely separate force from electro-magnetism and has its own fundamental particles - gravitons - which some experimenters claim to have identified.

kbd512 wrote:

Louis,

Although I'm not a physicist and don't even play one on TV, I'm reasonably sure of the following:

All objects made of matter are in motion, so motion is an intrinsic characteristic of matter.  If you're asking what initially set all matter in motion, I don't know the answer to that question.  We have various theories with varying amounts of observational evidence supporting them, but no way to construct a conclusive test, so far as I know.  There are literally thousands of more knowledgeable people that I'd direct that particular question to if I were you.

If you're asking whether or not the observed phenomenon of magnets attracting or repelling each other or materials that can be magnetized is related to gravity, I only know what the physicists tell me and they describe it as a separate force.  Gravity could be a specific type of electromagnetic phenomenon, but I don't know of any way to prove that.  Despite that, the fact is, those magnets are made of matter, the matter has mass, and since that matter is in motion, force or kinetic energy is generated.

Here's how I broke down the logic behind your question:

1. both magnetic and non-magnetic objects are made from some specified quantity of atoms of matter
2. all atoms of matter have mass
3. if that mass of matter is moving, and it's always moving in some direction no matter how tiny those movements are, then some acceleration vector quantity has been imparted to it, mechanism of application unimportant
4. by measurement, the accelerated matter carries force / kinetic energy with it, which we observe / measure and label a "force" (the resultant product of moving matter)
5. if that matter strikes another object made of matter, magnetic or otherwise, then a kinetic energy transfer has occurred

As to precisely when a quanta of matter has mass, much like motion it's an intrinsic property.  Recall that all observed matter is in motion.  I suppose quantum physics would state that most of what creates mass in ordinary matter is the binding energy / gluon fields between the quarks that comprise protons and neutrons in the nucleus of atoms (which also happen to be in motion).

In your example of an asteroid vs a cloud of interstellar gas, both of those objects are made from ordinary matter and the quantity of kinetic energy they can transfer when they strike Earth remains proportionate to their mass and acceleration vector.  The matter comprising the asteroid is far denser than the gas cloud, even though the gas cloud may be many orders of magnitude larger and may carry many orders of magnitude more total kinetic energy with it.  The asteroid will likely transfer all of its energy from its atoms of moving matter to Earth in a matter of seconds.  Most of the gas cloud would miss Earth entirely and whatever did strike Earth would almost certainly be doing that over a substantially increased amount of time.  Energy transfer rate matters greatly.

Therefore, the kinetic energy transfer rate produced when that asteroid impacts Earth is vastly greater than that of the vastly more massive interstellar gas cloud.  The gas cloud may cause the upper atmosphere to glow, but won't produce any sort of extinction level event.  Earth has been flying through a never-ending stream of protons (ionized Hydrogen gas) thrown off by our Sun since Earth was formed.  The last time an asteroid 100 miles across hit the Earth, it was an extinction level event for most life present on Earth at that time.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#15 2020-04-09 17:26:53

louis
Member
From: UK
Registered: 2008-03-24
Posts: 6,865

Re: Physics Topics

A silly comment.  I haven't challenged any central tenets of science. I fully accept the physical world is made up of the four fundamental forces.

tahanson43206 wrote:

For kbd512, elderflower and SpaceNut

Thank you for saving this new topic from the ignominious descent into psuedo-science that appeared to be happening.

For SpaceNut ... please use your powers to move any instances of psuedo-science that may occur into a suitable other topic.

I would very much like for this topic to be one that could safely be recommended by a responsible teacher or (conceivably even) professor.

Thus, the detailed and thoughtful answers provided by kbd512 are of a caliber that I think will stand the test of time.

(th)


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#16 2020-04-09 21:20:45

kbd512
Administrator
Registered: 2015-01-02
Posts: 4,615

Re: Physics Topics

Louis,

I was referring to gravito-electro-magnetism / GEM.  Frame dragging associated with GEM is a directly tested and measured effect associated with general relativity.  I'm pretty sure that gravity is NOT directly equivalent to magnetism.  There are predicted / tested / measured differences between the two and mathematical models that show how gravity and electromagnetism differ from each other.  However, there are also some inseparable effects associated with gravity and magnetism related to matter / energy / motion.  I can't claim to understand all of it, but it's definitely interesting and therefore I read about it.  This is the kind of stuff that a physics professor should be able to better explain.

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#17 2020-04-22 20:22:51

tahanson43206
Moderator
Registered: 2018-04-27
Posts: 7,326

Re: Physics Topics

In another topic, Void has introduced for discussion the idea of using steam to propel a vehicle to escape velocity from the surface of the Moon.

The idea did not arise directly from the article Void referenced, which was about development of steam powered thruster equipment suitable for use on asteroids.

The ISP quoted in a Wikipedia article on steam propulsion was 195 at the high end.

I am posting the idea here in the Physics topic in hopes that someone will be inspired to contribute a post which shows that Void's idea could work.

My expectation is that a vehicle designed to achieve escape velocity from the surface of the Moon would have to run along a horizontal track for some distance before it achieves sufficient velocity to rise every so slightly off the track, at which point any residual resistance of the wheels would be eliminated, and the entire thrust would be available to increase velocity.

It can be imagined that the energy to achieve steam thrust could be provided from an outside source such as the Sun, so the "propellant" mass would be that of the water to be consumed, and any fuel that might be needed to supplement solar energy for heating the water.

The balance of the mass to be launched would then consist of the structure of the vehicle, and any cargo it might carry.

As a reminder, escape velocity from the Moon is given as:

From its equator, the Moon has a radius of 1 738 km. It also has an estimated mass of 7.342 × 1022 kg. This means that the Moon's escape velocity is 2.38 km/s. That is much less than the 11.2 km/s it takes to get off the Earth.Jul 23, 2019

Escape Velocity | Let's Talk Science
letstalkscience.ca › educational-resources › stem-in-context

In another topic here in the forum, it was shown that an acceleration of 2.5 G's could provide a velocity of 2.38 km/s after a run of 71 miles.

The steam rocket would presumably take the same distance to achieve escape velocity if it can sustain 2.5 G's for the time required.

Edit#1: The velocity a vehicle would need to achieve to "lift off" the track described above would appear to be 1600 meters per second or so.

https://space.stackexchange.com/questio … turn-to-ea

The command/service module (CSM) would be in orbit around the moon traveling at a speed of about 1600 m/s. 

That would be 1.6 km/s, to match the 2.38 km/s escape velocity format.

I would expect the horizontal run at 2.5 G's would be significantly less than 71+ miles to achieve "lift off" from the track, but the steam engine would need to continue to provide that 2.5 G's for a period of time to advance from orbital velocity to escape velocity.

Edit #2: Physics 101 provides a starting set of figures:

Given a horizontal run at 2.5 G's, a vehicle would reach 1.6 km/s after an elapsed time of 65 seconds and change. 

The distance covered would be a bit over 32.5 miles (52,300 meters).

It should be possible to determine the amount of energy that would have to be input to achieve that performance, for various amounts of mass to be accelerated.

Something the system designer will need to work on is a way to continue to feed solar energy at the required rate after the vehicle leaves the track.

A "solution" is to simply continue the 2.5 G acceleration for the full 72 miles.  The time required is then 97 seconds.

The advantage to the designer is that the vehicle is no longer engaged with the track after 32.5 miles, but the solar power can be applied directly to the vehicle while it is in a "known" trajectory.

Can someone else help here?  I'm curious to know if a steam rocket could sustain 2.5 G's for 97 seconds, given solar power input.

SearchTerm:SteamRocketLunarLaunch
SearchTerm:LunarLaunchSteamRocket

Edit#3: Here is a paper that appears to come pretty close to Void's concept:

http://neofuel.com/moonicerocket/

The authors propose to use a nuclear rocket to throw water mined at the Lunar poles.

Edit #4: This paper is chock full of pertinent information.  It compares simple steam to water splitting, and shows significant benefits for the steam case.

The paper examines corrosion of vehicle materials exposed to high temperature steam. 

Conclusions

Quantitative estimates suggest a minimum performance nuclear-heated steam rocket (NSR) deployed with lunar water propellant achieving a power per mass of about 75 Megawatts per ton of rocket and developing a specific impulse of 195 seconds can deliver significant payloads to lunar escape.
The state-of-the-art in nuclear design and materials suggests that technology exists today to meet these nuclear reactor requirements for power per weight and specific impulse. The specific power requirement implies the NSR nuclear fuel must achieve 1.4 MW/liter. The specific impulse implies a mixed mean outlet temperature of 1100 K.

Calculations show the Baseline NSR infrastructure masses to be a factor between 150 and 720 lower than those of the cryo system.

A 2 ton Baseline NSR space tanker consisting of 1 ton engine and 1 ton tanks and structure could place about 20 tons per trip of arbitrary payload to a lunar escape orbit. Restart reliability of the reactor permits several trips per day.

The specific capacity of the Baseline NSR system is about 2400 tons of payload per year delivered to lunar escape from the lunar surface, per ton of infrastructure. The corresponding capacity for the cryofuel system lies between 3.3 and 16.

A steam rocket using water directly as propellant would consume about 2.5 times the propellant of its cryofuel counterpart for this payload delivery mission.

The cryofuel rocket compares poorly to the NSR because it requires an additional infrastructure to generate electricity in space, to split the water into hydrogen and oxygen using electrolysis, and to condense the gases into the premier cryogenic rocket fuels liquid oxygen (LOX) and liquid hydrogen (LH2).

A Baseline NSR could launch 14,400 tons per year using a single, 2 ton rocket supported by 4 tons of surface infrastructure. The Low Performance NSR would deliver 2880 tons (240 tons per month). The NSR principal processes require only thermal energy. The cryofuel system requires between 150 and 720 times more infrastructure mass to do the same job. This will translate into a proportionally lower mission cost.

Edit#5: Recovery of water used as propellant

Something not considered in the paper quoted above (or anywhere that I have seen) is the possibility of recovering water used as propellant for launch from the Moon. If the launch track is designed so that exhaust is captured, then it can be reused.  The momentum of the Moon itself would be changed by the sum of whatever water molecules are captured in flight, but for planning purposes that effect can be neglected.

An electromagnetic launch system, as discussed earlier in this forum (see topic created by kbd512) would provide a cleaner overall performance than any chemical system, but it would require a significant infrastructure investment in comparison.

(th)

Last edited by tahanson43206 (2020-04-23 07:44:46)

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#18 2020-04-23 13:04:34

tahanson43206
Moderator
Registered: 2018-04-27
Posts: 7,326

Re: Physics Topics

For today, I am following up on yesterday's exploration of Void's concept of using water as a propellant (in the form of steam) to accelerate a vehicle to escape velocity from the Moon.

It was established that a launch system operating at a thrust of 2.5 G, over a distance of 72 miles on the surface of the Moon, would deliver a vehicle and cargo to Escape velocity in 97 seconds.

In earlier posts and by following various links, it was shown that an ISP of (about) 200 would be practical for a steam rocket.

The use of a nuclear fission reactor to provide the energy for heating water to produce steam was explored in depth in a paper at one of the links.

Today, I'm adding a link to a NASA web site where rocket propulsion is discussed in some detail, at a level that looks (to me at least) as about right for high school students on a college preparation track.

https://www.grc.nasa.gov/www/k-12/airplane/specimp.html

What I have NOT yet found is an answer to my question: how much water is needed to achieve escape velocity from the Moon, using Void's scenario?

It is possible the nuclear fission reactor paper contains an answer to that precise question, but if it does, I missed it due to rapid scanning.

However, in case someone else feels inspired to toss a post into the topic, here are the givens:

1) Vehicle uses water in the form of steam to yield an ISP of about 200
2) Vehicle achieves a thrust level of 2.5 G's for 97 seconds
3) For simplicity, it is assumed solar power delivered to the moving vehicle is the source of energy to heat the vehicle.

Various sizes of vehicle are possible with these givens.

To provide a specific target for the (hypothetical) contributor, let the total mass of the vehicle, water(propellant) and cargo be 10 tons.

What part of the 10 tons would Void need to allocate to a supply of pure water, in order for the vehicle to reach Lunar Escape velocity?

Edit#1: The thrust achieved in item 2) above reflects the amount of mass being expelled at some rate.

The amount of water to load into the tanks is (presumably) equal to the amount of mass expelled in one second, for a total of 97 seconds.

Edit#2: Here is output of a web site that appears to be moving in the right direction:

https://www.calculatorsoup.com/calculat … /force.php

Choose a Calculation

F=ma
F =    (answer units)    F unit
m =   
10000 Kg
m unit
a =   
24.5 meters/seconds-squared
a units

Significant Figures
Answer:
F = 245000 Newtons

245000 Newtons is the force the steam propulsion system must deliver for 97 seconds

OK ... how to get from 245000 Newtons to the amount of water/steam flowing?

Rephrasing the question: What mass flow at 200 ISP provides 245000 Newtons of thrust?

I'm not sure that is even a valid question.  It may be.

Edit #3: From the NASA pdf:

The rocket weight will define the required value of thrust. Dividing the thrust required by the specific impulse will tell us how much weight flow of propellants our engine must produce. This information determines the physical size of the engine.

Following the text above, the thrust required is 245000 Newtons, and the ISP is 200, so the "weight flow of propellant" would be: 1225 something.

At this point, I have no idea what the "something" might be. 

(th)

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#19 2020-04-24 11:56:55

tahanson43206
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Re: Physics Topics

Continuing:

Following the text above, the thrust required is 245000 Newtons, and the ISP is 200, so the "weight flow of propellant" would be: 1225 something.

At this point, I have no idea what the "something" might be.

The document at the link below includes discussion of water molecules in the exhaust.

https://ocw.mit.edu/courses/aeronautics … cture2.pdf

A sentence in the document above caught my eye ... the author asserts that under certain circumstances, the specific impulse is equal to the exhaust velocity.

If we neglect pressure thrust, the specific impulse and exhaust velocity are the same and both can be computed through the use of Eq.(18). This is beneficial in understanding the fundamental differences between chemical and electric propulsion engines.

In the case of Void's (proposed) steam rocket to reach Escape velocity from the surface of the Moon, I am unsure if it is reasonable to "neglect" pressure thrust.

So far, I have not found (or more likely, have not recognized) text that would allow determination of the mass flow rate of steam in the hypothetical system.

What I am hoping to puzzle out (at some point) is the amount of water that must be loaded into the propellant tank for Void's steam rocket to achieve escape.

Edit#1: I ran a quick search looking for posts that contained the word "steam" and the word "rocket" and found way too many to deal with.

For that reason, I'll just tack this observation onto this subtopic of the Physics topic.

Void has imagined a launch vehicle able to reach escape velocity from the Moon, using water (from Lunar ice) as propellant.

A study reported earlier in this topic showed that if a nuclear fission reactor is employed to heat the water, such a rocket is feasible.

However, I've been wondering about how Solar Power might be employed to achieve the same result.

An idea that came to my attention while I was searching away on Google, is to use a laser beam to heat the water to be ejected from the vehicle.

In the scenario under discussion in this subtopic, the vehicle is accelerating along a straight track 72 miles in length on the Lunar surface.

This is (of course) different (by 90 degrees) from the "traditional" launch scenario considered for departure from the Moon.  This proposal is rooted in the work of Dr. Gerard O'Neill and his students, who imagined an electromagnetic launcher on the Moon, to send lunar material towards Earth for use in construction projects of various kinds.

The straight track concept lends itself to the use of a laser beam directed toward a port on the front of the launch vehicle, through which photons would be admitted to a chamber where water vapor would be delivered by a pump.  If the port is made of a material able to admit photons while keeping water molecules confined to the interior of the boiler, then the heating effect would be expressed in vigorous movement of the water molecules, who would be free to exit via the conveniently located exhaust port at the rear of the vehicle.

I bring this up because in another topic, kbd512 had launched a topic about use of electronmagnetic launchers based upon the proven US Navy carrier example, in use today for launching aircraft.

It is possible to imagine construction of an electromagnetic launcher on the surface of the Moon, but Void's steam rocket idea is a LOT simpler in principle.

While Void's steam rocket (as considered in this subtopic) would travel along a track for a considerable distance, that track could be ** much ** simpler and ** much ** less expensive than the same track fitted with magnets to drive a vehicle designed for electromagnetic acceleration.

I note that the common microwave heats water by filling a cavity with microwaves tuned to the frequency (per Google: 2,450 MHz).

Perhaps the idea above would work better if the energy supplied were at that frequency, but in that case, the transmitter would (logically) NOT be a laser.

Perhaps such a beam could be constructed with a maser:

A maser is a device that produces coherent electromagnetic waves through amplification by stimulated emission. The first maser was built by Charles H. Townes, James P. Gordon, and Herbert J. Zeiger at Columbia University in 1953. Wikipedia

Edit#2: This site looks promising: https://how2physics.com/2017/04/09/the- … ater-hose/

Void's idea for a steam rocket is not TOO different from a water hose.  The water on board has to last for only 97 seconds.

It has to produce 245000 Newtons for that period of time.

I'm thinking along the lines of allocating half the vehicle mass for propellant, or 5000 Kg.

Edit#3: My hope is that this gent's presentation for a garden hose can be adapted to the situation of Void's vision of a lunar launch steam rocket.

APRIL 9, 2017BORIS SAPOZHNIKOV
The force required to hold a water hose

The rate of flow of mass (ie, water molecules) would be 5000/97, or 51.5+ kilograms per second.

The force required is known, so I'm hoping the presentation can be adapted to yield the rate travel of the water molecules.

(th)

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#20 2020-04-24 17:18:13

louis
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From: UK
Registered: 2008-03-24
Posts: 6,865

Re: Physics Topics

I had the idea a while back - having watched the original Falcon 1 launches from Kwajelein and seen how large numbers of water droplets formed on the rocket - that (in tropical locations) you might be able to pump in air as a steam rocket rose and dehumidify it on board, meaning you didn't have to carry as large a water feedstock on the ground.

I further suggested you might use a rocket covered entirely in PV film, to generate power to (a) dehumidify the air and (b) heat the water to create the steam to propel the rocket. You wouldn't really have to pump in the air if the rocket was rising - you could feed it in through airlets as it rose I guess.

But I am not sure the numbers add up for any large rockets. Might work for smaller loads carrying mini satellites to orbit.

There has surely got to be some benefit from having a rocket that can draw its energy and the water feedstock from the surrounding environment. It might need large wings to boost the PV power supply. Or maybe you could have it attached to a cable itself attached to a floating aerial PV system - say up at 10,000 ft.

Another option might be to have ground hoses spray water on to the rocket as it rises. The water would pass through inlets and replenish the water tank. There are water features like Jet d'Eau that currently pump water 460 feet high. That height could like be bettered and you could possibly place your water "fountains" on a higher ground surrounding a rock launch pad.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#21 2020-04-24 19:51:39

tahanson43206
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Re: Physics Topics

For Louis re #20 ...

I'm hoping to be remembered as someone who encourages creative thinking.

You've provided numerous/many examples in the forum archive, and you're in fine form here.

The reason I like creative thinking by those who are blessed with the ability is that the ideas can stimulate a secondary order of creative thinking by those who are not as blessed at the high end, but who can take inspiring ideas and try to make something practical out of them.

You and Void are in the lead in the creativity department in recent times.

In the not too distant past kbd512 provided numerous examples.

RobertDyck is a consistent provider of well developed creative ideas, often encapsulated (like those of kbd512) in real-world wrappings.

Going even further back, JoshNH4H was(is?) in a class by himself, with visions based (as nearly as I could tell) on solid principles but extended to the breaking point, and occasionally beyond.

In Post #20, there were several word images that I enjoyed turning into mental images, such as the Kwajelein launches and the Jet d'Eau.

(th)

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#22 2020-04-24 21:16:40

tahanson43206
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Registered: 2018-04-27
Posts: 7,326

Re: Physics Topics

This is a continuation of Post #19

That post introduced an online reference which offers "The force required to hold a water hose" for study.

I am looking for characteristics of a rocket that might follow from Void's vision of a steam rocket to escape from the Moon.

The lesson from Boris Sapozhnikov opens with a simple equation:  l = v delta t (Length of tube of water = velocity times the time)

In this case, we know the length is just under 72 miles, so we'll round up to 72 for simplicity.

We know the mass of water we are going to throw is 5000 kg (an arbitrary division of vehicle 50% propellant and 50% rest)

2020/04/24 Trial run – use hose example to estimate Void's Launcher

115873 meters is 72 mile track length (rounded up to nearest mile)

Time interval is 97 seconds (2.5 G's acceleration to 2.38 km/s)

l = v delta t

115873 = v * 97

v = 1,194.567 meters/second

From notes in NewMars forum we have:
Following the text above, the thrust required is 245000 Newtons, and the ISP is 200, so the "weight flow of propellant" would be: 1225 something.

The similarity of those numbers may be coincidental.

In any case, it would appear that Void's steam plant needs to throw the 5000 kg of water out the exhaust port at a rate of about 1200 meters per second for 97 seconds, or 5000/97 or 51.5 and change kilograms per second.

As a side note ... an actual (normal) rocket will accelerate more rapidly as propellant is exhausted, if thrust remains the same.

However, in this (somewhat unique) situation, it is proposed that the G force on the vehicle remain a constant 2.5 G's for the comfort of the (presumed) passengers.  This requirement implies that the amount of thrust will decrease from the 245000 needed at the beginning of the run.

kbd512's electromagnetic launcher would not have this problem, because the mass of the vehicle would not change over the course of the run.

There is need for the work done in this series to be reviewed and corrected as appropriate.

(th)

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#23 2020-04-25 04:33:17

elderflower
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Registered: 2016-06-19
Posts: 1,262

Re: Physics Topics

Water is going to be a valuable and scarce resource on the Moon, and probably on most other bodies in the Inner Solar system. So you will likely need a device design that will enable capture of the majority of the water. A closeable large bore gun barrel, perhaps.
If using a gun barrel you don't need to put all the water on the projectile as steam could be injected at high pressure to accelerate it. Close the barrel as soon as the projectile passes and recover the steam. How about that?

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#24 2020-04-25 08:18:42

tahanson43206
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Registered: 2018-04-27
Posts: 7,326

Re: Physics Topics

For elderflower re #23

It is definitely encouraging to receive your suggestion.  Thanks!

This subset of the Physics topic was started after Void imagined a steam rocket could achieve escape velocity from the Moon.  Having been coached away from thinking of steam as a practical propulsion method (by a local engineer friend), I was skeptical the method would work.  However, as reported earlier in this series of posts, it turns out that ** very ** serious attention has been given to this question, and the answer is most definitely yes.

Today I'll add finishing touches to my exploration of the horizontal track steam rocket launch design.

However, ** this ** post is in response to the important suggestion that the water expelled in accelerating the vehicle along the track be collected and re-used.

The collection of propellant water is a distinct advantage of the horizontal launch concept, which itself is descended from an idea of kbd512 (in another topic) to use an electromagnetic accelerator to send a vehicle out over the Gulf of Mexico with sufficient velocity to traverse 400 miles of Earth atmosphere to reach a point above the atmosphere where further propulsion could be used to round the orbit.

Your suggestion of a large gun barrel fits nicely into the horizontal track concept.  My (slight) adjustment is to make the "barrel" into a half cylinder shelter.

The familiar Quonset Hut from World War II is a model for how I would like to adapt your idea.

From Google:

1941
Who invented the Quonset hut and when? The George Fuller construction company is credited with being the first company to manufacture Quonset huts on behalf of the US Navy in 1941. The exact inventor or designer isn't known, but the design is based on the Nissen Hut design that originated from World War I.

Quonset Hut History: From World War II & Beyond

I am pleased to add to my store of knowledge that the Quonset hut was based on the Nissen design from World War I.

The engineer friend mentioned earlier is a vast repository of stray facts like that, so the next time I see (or talk to) him, I'll see if he knows that history.

The reason I particularly like your suggestion is because it meets another need that was discovered in another topic, where kbd512's electronmagnetic launcher idea was being considered for the Moon.

On the Moon (it turns out) tiny particles kicked up by activity (such as rockets landing and taking off) can (and do) traverse vast distances at undiminished velocity until they finally impact upon human made artifacts, such as rovers, or in our case, a launch track.

Therefore, a 72 mile Quonset hut would protect the track from damage due to flying debris, AND it would provide a convenient surface for collecting frozen ice that has been emitted by the accelerating launch vehicle.

As a reminder, since I have no way of knowing if you read any of the series that preceded the one to which you replied, the 72 mile figure comes from the kbd512 discussion.  The current capability of the US Navy electromagnetic catapult launcher is (about) 2.5 G's.  That is a comfortable figure to work with for planning a system to launch humans from the surface of the Moon, so I've chosen to stick with it.

Others can (and have) chosen other G factors.

Your closing line suggests closing the barrel, but (in my understanding of the situation) there is little loss at the exit from a 72 mile Quonset hut.  Most of the exhaust will have been expelled towards the (already closed) end of the Quonset hut, and as the vehicle advances along the track, the water expelled will collect on the walls and on the (suitably prepared) floor of the Quonset.  Robot ice collectors could then periodically scrape ice from the walls and floor.

The mass efficiency of such a system should turn out to be reasonably good.  Naturally, it cannot match the 100% mass efficiency of kbd512's electromagnetic launcher, but what this simple steam concept loses in efficiency compared to the EML, it ** more ** than makes up for in simplicity of design, simplicity of construction, simplicity of maintenance and whatever else folks think of in Sols to come << grin >>.

(th)

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#25 2020-04-25 08:23:56

tahanson43206
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Registered: 2018-04-27
Posts: 7,326

Re: Physics Topics

This post is reserved for an extension of Post #22, which introduced a web based learning platform which offers a lesson on how to compute the force felt by a human holding a water hose.

Because of the remarkable simplicity of the presentation, I was (hopefully) able to break through the fog i'd managed to build up in thinking about Void's imagined steam rocket lunar launcher.

SearchTerm:Continue


Edit#2: Some calculations by Physics 101

Work done: Void's Steam Rocket

Force: 245000 Newtons
Distance: 115872 meters (72 miles)
Acceleration: 24.5 m/s^2 (2.5G's)

Result: 25832563082.823 NewtonMeters

Energy required:

Mass: 10000 kg
Velocity Final: 2380 m/s
Energy in Joules: 28,322,000,000
This energy would be imparted to a vehicle accelerated by an EML

The actual energy for a steam rocket would be less, because the propellant will be expelled during the run, and assuming the acceleration force is reduced to hold a constant 2.5 G's.

Edit#3: Asked Google about a Newton Meter (Nm)

What is newton meter (Nm)? definition and meaning ...www.businessdictionary.com › definition › newton-met...
newton meter (Nm) Metric (SI) unit of torque (tendency of a force to cause rotation), used also as a unit of energy. One newton meter is equal to about 0.74 pound-foot. USAGE EXAMPLES. The newly discovered energy source had many companies rushing out to buy newton meters, and, oddly, apples by the barrel.

I'll have to go online to find the F=ma calculator.  Physics 101 doesn't appear to have one, or if it does, I couldn't find it.

SearchTerm:Continue

(th)

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