Were the Lunar Rovers faked?

[Noldi400]

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.



 

[ka9q]

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.

[smartcooky]

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.

So I think it's simply wrong (as well as extremely confusing and error-prone) to say that an object that's 100 kg on earth becomes only 16.7 kg on the moon. It's still 100 kg on the moon, as it would be measured by a scale properly calibrated to the local gravity. It's just 6 times easier to pick up.

Suppose the bathroom-type scale had never been invented. Suppose we still measured things with the balance scale, matching the pull of gravity on our test mass with that on a set of calibrated masses. Then, without any changes, an object weighing 100 kg on earth would still weigh 100 kg on the moon, or anywhere that had a non-negligible gravity field. So would a bathroom-type scale with a built-in accelerometer to compensate for local gravity variations. I.e., the notion of the kilogram as a unit of gravity force depends not only on a specific local gravity field, but on the use of a specific type of device to measure it. That's silly.

SI carefully distinguishes between mass and force, something largely unknown to those who measure all forces and masses in pounds. It's a bit like the notion of grammatical gender in many non-English languages, only it actually makes very real sense. Just as English speakers can't impose their rules on other peoples' languages, they should not introduce their confusion between mass and force into other measurement systems.

Ok, strictly speaking you are right, but I have found that when I start talking to non-scientific minded people about the moon and Apollo, they understand when I tell them that a 60 kg object only weighs 10 kg on the moon because the moon's gravity is only 1/6th that of earth, and that is why the astronauts walked funny on the moon.

However, when I start launching into explanations about how 1G is actually 9.81 m/s², and I start using terms like "newtons" or "joules" which are not in everyday use, I find their eyes begin to glaze over, and at that point, I know I am on the way to losing them.

As I said, using kg for mass and weight might not be scientifically correct, but the lay public can understand them better because these are terms that are familiar to them.

[ka9q]

As I said, using kg for mass and weight might not be scientifically correct, but the lay public can understand them better because these are terms that are familiar to them.

Why not just say that 100 kg on the earth is still 100 kg on the moon, but it's 6 times easier to pick up because of the lower gravity?

These explanations usually go on to say how mass is still present in low gravity or even in weightlessness, which is why astronauts on the ISS or performing EVAs in orbit have to be careful when maneuvering themselves or heavy (massive) objects. So you can't avoid the distinction between mass and weight at some point.

[ka9q]

If I have a 100 kg mass I can accelerate it with any force whatsoever. If I apply 1 Newton force I can accelerate it at 1/100 m/s^2. If I apply 10 N of force I can accelerate it at 1/10 m/s^2. If I shove it with 1,000,000 Newtons it will accelerate at 10,000 m/s^2. There is NO 'minimum force' required to accelerate any given mass, ignoring all other factors such as friction, rolling resistance etc.

Does anyone happen to know the coefficient of rolling friction for the LRV's tires? It would be good to know before we dismiss it as small compared to the inertial forces and gravity slope forces.

On earth, at least, rolling resistance can be characterized by a dimensionless coefficient that relates the tractive force required to overcome it to the vehicle weight. E.g., if the RR coefficient is 0.1, then a vehicle with a weight (gravitational force) of 100 N would require a horizontal tractive force of 10 N to keep it moving (at any speed above zero) on a level surface. This energy goes into flexing and heating the tires, crunching up a soft roadbed, etc. This is why it takes a minimum tractive force to budge a car, train or airplane even on a level surface.

Because the tractive force needed to overcome rolling resistance is proportional to weight, it is also proportional to the local gravity field. All other things equal, a rover that takes, say, 100 N of tractive force to budge on earth would take only 16.7 N of tractive force on the moon.

Because the rolling resistance is a fixed force, and energy is force times distance, the power needed to overcome it increases linearly with velocity. This sets a maximum velocity that a rover could achieve on a level surface with a given amount of motor power. Can we work out what that is for the LRV?

[AtomicDog]

How about we just say that an object exerts a force due to gravity in kilos on the Moon that is one sixth of that force in kilos that it would exert on the Earth, and be done with it?

Yes, I know that force is measured in newtons, but the balances I used in physics class weighed objects in kilos.

[ka9q]

How about we just say that an object exerts a force due to gravity in kilos on the Moon that is one sixth of that force in kilos that it would exert on the Earth, and be done with it?

Not unless that's kiloNewtons. The kilogram is a unit of mass, not force.

Yes, I know that force is measured in newtons, but the balances I used in physics class weighed objects in kilos.

If they were classic balances with reference masses, either a set you manually added and removed or a set you slide along beams, they measured the objects' masses, and those are represented in kilograms. They used gravity to do it but they were not sensitive to the actual gravitational acceleration; they'd give the same answers on the moon.

[VQ]

Yes, I know that force is measured in newtons, but the balances I used in physics class weighed objects in kilos.

To my knowledge, the commercial world measures force in kgf and the USA/etc measures mass in lbm. But we are having an engineering discussion, where mass and force should be dealt with more rigorously.

Most scales measure force, while a balance measures mass. A laboratory electronic balance technically measures force, but since it is calibrated before use with reference masses, the readout is calibrated directly to mass and measured in kg. In the USA, some engineering disciplines (including mine) still use inconsistent systems of units in which lbm is the unit of mass, lbf is the unit of force, and a conversion factor of ~32.2 must be used for f to equal m*a. HVAC is more than a little behind the times in that regard (grains per lbm, anyone)?

[Not Myself]

"Bringing a knife to a gunfight" springs to mind.

Doesn't matter what weapons anyone brings; if you get to referee your own fight, who will the winner be?

[anywho]

Please note that when I am talking about pull coefficient or drawbar pull I am talking about the formula in the army test, so it is not a measurement of force, it is a measurement of the mass the wheel can pull before traction is useless.

No, it really is not. That is YOUR erroneous conclusion. I have already explained, twice, what those numbers actually represent and how that test applies to reality.

Drawbar pull, anywhere, is the force available to pull a load after the vehicle has moved itself. That's what it is in all cases, INLCUDING this test. You can keep on saying it means something else here, but the authors of that paper would not agree with you.

My bold:

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. I don't know why it differs from modern formulas but the most obvious reasons are that it is because of the specific nature of the soil/wheel interaction testing, or that it is simply how drawbar pull was measured in the 60's and 70's (how much extra weight the vehicle can tow).

They are not clear about the meaning of "drawbar pull" as they just list it as "lb", so this could be either a weight or a horizontal force. BUT, what they are very clear about is that to get the "pull coefficient" they divide "drawbar pull" by the "wheel load; weight, lb", this makes it very clear that the drawbar pull in the paper is also a weight and not a horizontal force.

If the drawbar pull was a force in the paper, they would have to divide it by the force needed to drive the wheel load to get the coefficient, not the wheel load itself.

I stand by the claim that the test show that the rover can only pull approx 50% more than its own weight before traction is too problematic.

Funny how Anywho has focused on Jason...

Better him than me. 

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

[anywho]

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?

[onebigmonkey]

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?

And your evidence that the lunar surface is loose and incapable of providing traction is...?

Hint: It's video from the lunar surface.

If you're insisting that no-one drove on the lunar surface, you have no evidence that it can't be driven on by the LRV.

[Jason Thompson]

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.

I don't know why it differs from modern formulas but the most obvious reasons are that it is because of the specific nature of the soil/wheel interaction testing, or that it is simply how drawbar pull was measured in the 60's and 70's (how much extra weight the vehicle can tow).

No, it's simply that you are consistently misunderstanding the test.

I stand by the claim that the test show that the rover can only pull approx 50% more than its own weight before traction is too problematic.

And the rest of the engineering world for over four decades disagrees with you. It does not have to 'pull' it's own weight on a level surface. It has to overcome the rolling resistance of the surface.

[Jason Thompson]

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).

It really doesn't matter. The point still stands that not one of these has a drawbar pull that is higher than the mass it can pull. You keep insisting that this one example is somehow different from every other counter-example, but that is only because it has to be if your argument is correct. Unfortunately your argument is very very wrong, as has been explained to you time and time again.

Drwabar pull is drawbar pull, and vehicles do not have to exert a force greater than the mass or weight of the load they pull in order to move that mass, here or on the moon.

[gillianren]

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

You could start by answering her questions.