Were the Lunar Rovers faked?

[anywho]

This is incorrect and gets to the heart of the confusion, in the army test they are certainly testing the weight/mass that can be pulled.

Yes, and I'll say it again: in a regime where the vehicle is already moving and has to pull its own weight up a hill. In a regime where the weight of the vehicle is actively opposing the forward motion.

So, just to be clear, you agree it is weight represented by the drawbar pull lb?

Earlier you said it was a force, there is a world of difference between a wheel being able to pull 30lbs in a test, and being able to exert a 30lb force.

[VQ]

This is incorrect and gets to the heart of the confusion, in the army test they are certainly testing the weight/mass that can be pulled.

Which one do you claim it is, weight or mass?

[Jason Thompson]

So, just to be clear, you agree it is weight represented by the drawbar pull lb?

Earlier you said it was a force, there is a world of difference between a wheel being able to pull 30lbs in a test, and being able to exert a 30lb force.

A weight is a force. It is the downward force exerted by a mass in a gravitational field. I know there is a difference between being able to pull 30 lb and being able to exert a force of 30 lb. The ability to pull a weight of 30 lb does not require a 30 lb force for one thing, unless you pull it upwards.

However (and this is the point you seem to be consistently missing), the amount of force needed to pull 30 lb increases depending on the angle you wish to pull it at relative to the horizontal/vertical. It varies according to the cosine of the angle to the perpendicular. If I want to hold a 30 lb weight up against gravity I have to exert a 30 lb force straight upwards. To lift it I have to exert a force in excess of 30 lb. To stop a 30 lb object rolling down a 45 degree slope I have to exert a force of 30 * cos 45 degrees, which is 30 * 0.707 = 21.2 lb. That's just to stop it rolling downhill. To lift it up the slope I have to exert a force greater than that, but it can be any little bit greater and I'll still be able to move it up the hill against its weight.

On the flat I don't have to exert any force to stop it moving because its weight won't make it move anyway. I can therefore apply any force to it and still move it, provided I can overcome the friction or rolling resistance. That resistance is related to its weight but is not equal to it. It is some fraction of its weight. If the force resisting its motion is 5 lb, I can move the object provided I can exert a force greater than 5 lb, no matter what the object weighs.

This is why I can move a car with its brakes off in neutral along a flat road, but I can't move it up an incline or when its brakes are on. On the flat I can exceed the resistance to its motion caused by rolling resistance and friction of the internal moving parts. When the brakes are on I cannot exceed the force provided by the friction of the wheels with the surface. On the incline I cannot exceed the portion of its weight making it accelerate down the hill, though I might be able to stop it rolling down the hill. On a steeper hill I would not be able to stop it rolling over me. The amount of force I can exert does not change. The conditions I have to overcome to make it move do.

[anywho]

This is incorrect and gets to the heart of the confusion, in the army test they are certainly testing the weight/mass that can be pulled.

Which one do you claim it is, weight or mass?

Good question, I'm really not too sure, the difference being the rolling resistance, for some reason i thought it was weight with the same rolling resistance as the wheel but I can't recall why.

If I get time later I will try to figure it out if I can, it will depend on how they set the carriage up which I don't think they elaborate enough about.

[Jason Thompson]

Um, guys? What's a pull coefficient? What is drawbar pull?

I read looked at the report being discussed and was totally lost. Just too many variables and unfamiliar terms to remember, and I couldn't get a picture in my head of how the test was being done.

Lil help? 

Sorry, missed this earlier.

The setup is a rover wheel in a dynamometer carriage. The wheel is resting on a bed of simulated lunar soil, and a load of 57 lb is applied to it to make it 'dig in' and get some traction. The wheel is running at a rate that would move the carriage forward at about 2.5 fps if it was allowed to roll freely. The wheel speed is kept constant.

The carriage, however, is moved independently, and it is moved along the test bed at speeds ranging from 0 to about 3 fps. At 0 fps the wheel is spinning without moving, a condition of 100% slip. At 2.5 fps the wheel is rolling along the surface with no slip. In between the amount of slip varies with the speed of the carriage.

As the wheel slips it digs into the soil and is 'trying' to move the carriage forward. It is exerting a pulling force that is measured by force gauges in the carriage. You can also, however, consider this force to be the force the carriage is applying to the wheel to make it slip: in other words the amount of force opposing the forward motion of the wheel that is required to induce that level of slip.

Applying that result to reality, as long as the wheel can overcome that force without slipping the rover can be driven. On a flat, that force is the rolling resistance of the surface. On a hill, the rover's weight acts against it, trying to pull it backwards down the hill. The portion of the weight acting opposite the rover's motion is the force it has to overcome without slipping. The 'pull coefficient' they are describing in the paper is that portion of weight.

Drawbar pull is the force left over when the vehicle has moved itself along a flat surface. An engine exerts a force. Some of that force goes to moving the vehicle itself. The leftover force is what can be used to pull an additional load. That additional load can be a trailer or it can be the extra effort required to move the vehicle up a hill due to the additional effect of the vehicle's own weight trying to make it roll back down the hill.

[VQ]

This is incorrect and gets to the heart of the confusion, in the army test they are certainly testing the weight/mass that can be pulled.

Which one do you claim it is, weight or mass?

Good question, I'm really not too sure, the difference being the rolling resistance, for some reason i thought it was weight with the same rolling resistance as the wheel but I can't recall why.

If I get time later I will try to figure it out if I can, it will depend on how they set the carriage up which I don't think they elaborate enough about.

It would probably be a good idea for you to understand the basic concepts prior drawing your conclusions (let alone accusing others of making mistakes).

[Sus_pilot]

To everyone who wants to use a locomotive, or a tugboat, or an airport tractor, please consider that none of these can get bogged, none of these are indented into the surface before they even try moving (apart from the tug lol). This, along with the low traction of a loose surface, is why a loose surface is such an easy one to get stuck on, as most of us have probably experienced.

I have towed a truck that was stuck on a loose surface with a 4wd, but that was with the truck using whatever traction it could muster as well, if the truck was in neutral forget it, it was also a very good surface which is why the driver though he could make it.

Do you really think a 4wd on a loose surface can act like a locomotive and pull many other 4wds?

A locomotive's wheels are indented into the surface.  The rail is deflected downward by the weight on the axle.   

[Sus_pilot]

and its doing so up a 1:40 incline with steel wheels on steel rails!!!!

I know steel wheels on steel rails give less traction than rubber tires on asphalt or concrete, as well as less rolling resistance. Anybody have some typical figures?

I often hear it claimed that trains are dramatically more energy efficient than trucks and cars because they run steel wheels on steel rails. I think that's incorrect; even on a highway, aerodynamic drag almost always exceeds rolling resistance, so further decreases in rolling resistance provide diminishing returns. I think trains are so energy-efficient because they're so long. Only the the locomotive (or lead car) has to push the air out of the way for all the other cars. It's like they're all drafting each other, something I've personally seen work surprisingly well on the highway (and extremely dangerously).

Of course, many trains (especially freight trains) run much slower than highway speeds, thus reducing aerodynamic drag considerably. But the differences are large even at comparable speeds.

I don't have the numbers, but it's mainly the fact that the overall rolling resistance of the cars (carriages to you outside North America) is far less than the equivalent tonnage of highway vehicles.  Aerodynamic drag is important, but nearly as much as the rolling resistance; locomotive engineers ("drivers" - sigh) can tell the difference in train handling when there are empty auto racks in the train, given the same overall length and tonnage.  Empty unit coal trains have higher aerodynamic drag coefficients than loaded ones because of the air swirling around in the cars. 

The biggest source of drag, though, are crosswinds, which push the wheel flanges up against the rail. That's huge, because, if your track structure is properly engineered and maintained, the flanges shouldn't touch the rail, at least ideally (the wheels are slightly conical - the flanges come more into play in turnouts, sharp curves at low speeds, or at very low speed on super-elevated curves, etc.).

Don't get me wrong, though.  For the amount of fuel we use, we are looking at aerodynamic drag - we always have.  One source of pain are double-stacked container trains.  Having, effectively, a flat plate sticking up behind the locomotives and moving it upwards 70 MPH is significant.  See this article for more:  http://www.marcgunther.com/the-power-of-one-union-pacific/

[LunarOrbit]

Better him than me. 

You can't discuss anything with someone who just goes "nope" all the time.

That's the first time I've ever seen anyone complain that Jay's responses are too short. Be careful what you wish for, anywho.

[geo7863]

The fact is that an object of 100 kg mass, weighs 100 kg on the Earth because it is in a 1G field

W = mg  ... 100 x 1 = 100

and in the lunar gravity its weight is 16.7 kg

W = mg ... 100 x 0.167 = 16.7 (disregarding mascons of course!!)

I think it's reasonable to say that a bathroom scale measures mass in kilograms as long as one remembers how it works and the limitations of that method: by measuring the force of gravity and converting that to kilograms, implicitly assuming an acceleration of 9.8... m/s^2. I.e., it's a mass-measuring device with a mechanism of operation that works fine on earth as long as the minor variations in gravity from place to place are below your accuracy requirements, as they usually are. Any precise mass-measuring instrument of this type would have to be calibrated for local gravity to give correct results.

Unless, of course, you're Anders Björkman.*



* For anyone not familiar with the reference, Anders (also known as Heiwa) famously claimed on JREF that a bathroom scale measured only weight, not force, and that if a person jumped on a scale from a height of 3.7 meters, it would read the same as if he just stepped upon it.

Funnily enough I have to spend a fair amount of time explaining to construction workers why their fall arrest anchorages must be 'strong' enough to withstand a dynamic shock loading of at least 1.2 Tonnes even if they weigh less than 100Kg. And on a 'rat run' lifeline the anchorage at each end must be even 'stronger' than this (cant remember the exact figure off the top of my head but 144% rings a bell).

[RAF]

This is incorrect and gets to the heart of the confusion, in the army test they are certainly testing the weight/mass that can be pulled.

Which one do you claim it is, weight or mass?

Good question, I'm really not too sure, the difference being the rolling resistance, for some reason i thought it was weight with the same rolling resistance as the wheel but I can't recall why.

We all know why. You got a preconceived notion that the rovers were faked, and now you're attempting to "fit" the evidence to "match" those preconceptions.

....and, hopefully, you are beginning to realize that just doesn't "work".

[JayUtah]

You can't discuss anything with someone who just goes "nope" all the time.

No, you don't get to play that card.  I wrote you several lengthy responses earlier in the thread and you ignored those too.  You're still fumbling with the concepts of weight versus mass, like a beginning high school physics student.  This, among other things, is leading you to misunderstand and misrepresent industry tests.

Let's be plain.  You're standing firmly in the realm of my professional activity and you don't know what you're talking about.  No, not even a little.

Many of your claims are based on allegations of fact you claim as premises to your physical model.  A number of these assertions are just plain wrong.  Those naked assertions are what's being called out and dismissed with "Nope."  When a professional engineer tells you you're wrong on a point of engineering, and minces no words in the telling, you ignore it at your peril.  And yes, that's what my staff engineers get.  If I red-line some assertion or assumption in their analysis and write the single word "Incorrect" and send it back to them for revision, they know what to fix.  You don't.

In your case you keep plodding along with the same misconceptions, unwilling to be corrected.  Hence why I don't waste much more time on your foolishness.  You have no interest in the right answer.  Fix the "Nopes" first, then you can be taken more seriously.

[JayUtah]

To everyone who wants to use a locomotive, or a tugboat, or an airport tractor, please consider...

No, all those putative objections have their proper analogues in the problem you're purporting to study and thus do not render the counter examples irrelevant.  You're trying to come up with feeble excuses for why your home-grown physics model doesn't predict the behavior of any other vehicle in the solar system but is somehow still accurate for the LRV.  But the answer is that your model is wrong, not that there's some nitpicky detail being left out.  As has already been belabored, you've omitted the important sanity-check of your purported analysis method.

You don't know how properly to interpret drawbar tension or tractive effort.  These counter-examples have been provided to you in the hope they will help you realize the qualitative nature of your error.  That hope is dwindling.  Over the past 30-odd pages you've advanced and abandoned half a dozen different ad hoc hypothesis and demonstrated a consistent inability to read and understand the technical literature.  You can't discuss any of these concepts competently.  What do you think the odds are that allegedly discrepant drawbar pull is really a smoking gun?  Or is it more likely that it's just the latest in a long line of arguments you don't understand and will eventually abandon as soon as you trump up something else?

[nomuse]

The fact is that an object of 100 kg mass, weighs 100 kg on the Earth because it is in a 1G field

W = mg  ... 100 x 1 = 100

and in the lunar gravity its weight is 16.7 kg

W = mg ... 100 x 0.167 = 16.7 (disregarding mascons of course!!)

I think it's reasonable to say that a bathroom scale measures mass in kilograms as long as one remembers how it works and the limitations of that method: by measuring the force of gravity and converting that to kilograms, implicitly assuming an acceleration of 9.8... m/s^2. I.e., it's a mass-measuring device with a mechanism of operation that works fine on earth as long as the minor variations in gravity from place to place are below your accuracy requirements, as they usually are. Any precise mass-measuring instrument of this type would have to be calibrated for local gravity to give correct results.

Unless, of course, you're Anders Björkman.*



* For anyone not familiar with the reference, Anders (also known as Heiwa) famously claimed on JREF that a bathroom scale measured only weight, not force, and that if a person jumped on a scale from a height of 3.7 meters, it would read the same as if he just stepped upon it.

Funnily enough I have to spend a fair amount of time explaining to construction workers why their fall arrest anchorages must be 'strong' enough to withstand a dynamic shock loading of at least 1.2 Tonnes even if they weigh less than 100Kg. And on a 'rat run' lifeline the anchorage at each end must be even 'stronger' than this (cant remember the exact figure off the top of my head but 144% rings a bell).

Probably why climbing gear is rated in kilo-newtons.  But practically, most only know if it is rated for a fall factor of 2 (worst-case lead fall).  Learning how to build an anchor that can hold that, though..!  And far too many still set the so-called "American Death Triangle"; a loop of webbing run between two anchors in such a way as to nearly double the load on them.

[geo7863]

The fact is that an object of 100 kg mass, weighs 100 kg on the Earth because it is in a 1G field

W = mg  ... 100 x 1 = 100

and in the lunar gravity its weight is 16.7 kg

W = mg ... 100 x 0.167 = 16.7 (disregarding mascons of course!!)

I think it's reasonable to say that a bathroom scale measures mass in kilograms as long as one remembers how it works and the limitations of that method: by measuring the force of gravity and converting that to kilograms, implicitly assuming an acceleration of 9.8... m/s^2. I.e., it's a mass-measuring device with a mechanism of operation that works fine on earth as long as the minor variations in gravity from place to place are below your accuracy requirements, as they usually are. Any precise mass-measuring instrument of this type would have to be calibrated for local gravity to give correct results.

Unless, of course, you're Anders Björkman.*



* For anyone not familiar with the reference, Anders (also known as Heiwa) famously claimed on JREF that a bathroom scale measured only weight, not force, and that if a person jumped on a scale from a height of 3.7 meters, it would read the same as if he just stepped upon it.

Funnily enough I have to spend a fair amount of time explaining to construction workers why their fall arrest anchorages must be 'strong' enough to withstand a dynamic shock loading of at least 1.2 Tonnes even if they weigh less than 100Kg. And on a 'rat run' lifeline the anchorage at each end must be even 'stronger' than this (cant remember the exact figure off the top of my head but 144% rings a bell).

Probably why climbing gear is rated in kilo-newtons.  But practically, most only know if it is rated for a fall factor of 2 (worst-case lead fall).  Learning how to build an anchor that can hold that, though..!  And far too many still set the so-called "American Death Triangle"; a loop of webbing run between two anchors in such a way as to nearly double the load on them.

As is so for Construction Fall Arrest Equipment (in fact some of the modern equipment is dual marked with EN Numbers applicable for both the workplace and the Rock face) At the Risk of sounding 'superior' I don't want to be explaining kN to East European bricklayers and carpenters...Keep It Simple Stupid.

To be fair they understand the principle of shock loading... they just cant quite understand why, if they weigh 100Kg, they need an anchor sling with a far higher capacity. I don't allow double anchors on my sites, but that usually falls in line with manufacturers specs so I just use that as my decision factor as far as the workforce are concerned.