Trains on Mars...

[edwin_m]

Not true.

Because Mars has 1/3rd gravity of earth, weight of train will be one third & adhesion will be one third. So no difference except one third of power will be needed on Mars to climb a gradient say of 1 in 50 as on earth. So greener!

But diesel won’t work as no oxygen in atmosphere to explode with the carbon/hydrogen diesel.

But tractive effort is train weight multiplied by coefficient of friction, and acceleration is tractive effort divided by train mass. So if the mass and coefficient are the same, the acceleration on level track will only be one third. Same applies to braking, but if you try to use an air brake it probably won't work at all.

 

[O L Leigh]

I don't have the skirt pressure data immediately to hand - but I do have the downwards pressure exerted, which will be broadly similar. The below data is of course for Earthly conditions with a 1 bar atmospheric pressure... but it should allow those with a mathematical bent to calculate an approximation for the Martian conditions - remembering also to allow for the differences in gravitational force.

Well I'm no mathematician, but I do have a conveniently close fag packet that I can scribble on the back of. I'm sure that if any of what follows is just "rose manure" then I'm sure that someone will come along later and correct me.

Lets set some assumptions. The gravity on Mars is 0.375 times that of gravity on Earth and the atmosphere more than 100 times less dense (source: NASA Science Mars Facts). For simplicity we'll just take 100 as a nice round figure rather than guesstimating too much. Lets also assume that the astronauts are packing light, which means that they won't have enough space for an SRN4, so they've brought a little Hov Pod instead.

To achieve lift, a hovercraft has to overcome the action of gravity on it's mass. Using the figures you've provided, on Earth the Hov Pod has to exert a force of not less than 102 kgf/m2 to achieve lift (I presume the figures that you quote do not include the occupant, if so I think you'd need to allow a greater mass on Mars due to the need to wear a little more than shorts, T-shirt and flip-flops). On Mars, with it's weaker gravity, this becomes 38.25 kgf/m2 (102 x 0.375 = 38.25). Nice!!

The issue is that it's trying to exert that force using an atmosphere 100 times less dense than on Earth. Although the pressure might only be slightly higher inside the skirt than outside, it still needs to exert a force of not less than 38.25 kgf/m2 in order to achieve lift. That surely means that it has to suck in 37.5 times more air to exert that force than would be the case on Earth (1 / 102 x 38.25 x 100 = 37.5).

What I'm unsure about is the effect of lower atmospheric pressure on Mars on a craft's ability to exert this force (6-7 millibars compared to 1013 millibars on Earth - source: Arizona State University), so the amount of air required may be less. It would certainly not require so much to inflate the skirt due to lower resistance, but I'm unsure how that helps it exert the force necessary to achieve lift.

So anyway, that's my science homework completed using the entirely non-scientific back-of-a-fag-packet-based-on-entirely-made-up-assumptions method. Hopefully someone with a more scientific bent will be along to correct where I've made an error.

 

[edwin_m]

Well I'm no mathematician, but I do have a conveniently close fag packet that I can scribble on the back of. I'm sure that if any of what follows is just "rose manure" then I'm sure that someone will come along later and correct me.

Lets set some assumptions. The gravity on Mars is 0.375 times that of gravity on Earth and the atmosphere more than 100 times less dense (source: NASA Science Mars Facts). For simplicity we'll just take 100 as a nice round figure rather than guesstimating too much. Lets also assume that the astronauts are packing light, which means that they won't have enough space for an SRN4, so they've brought a little Hov Pod instead.

To achieve lift, a hovercraft has to overcome the action of gravity on it's mass. Using the figures you've provided, on Earth the Hov Pod has to exert a force of not less than 102 kgf/m2 to achieve lift (I presume the figures that you quote do not include the occupant, if so I think you'd need to allow a greater mass on Mars due to the need to wear a little more than shorts, T-shirt and flip-flops). On Mars, with it's weaker gravity, this becomes 38.25 kgf/m2 (102 x 0.375 = 38.25). Nice!!

The issue is that it's trying to exert that force using an atmosphere 100 times less dense than on Earth. Although the pressure might only be slightly higher inside the skirt than outside, it still needs to exert a force of not less than 38.25 kgf/m2 in order to achieve lift. That surely means that it has to suck in 37.5 times more air to exert that force than would be the case on Earth (1 / 102 x 38.25 x 100 = 37.5).

What I'm unsure about is the effect of lower atmospheric pressure on Mars on a craft's ability to exert this force (6-7 millibars compared to 1013 millibars on Earth - source: Arizona State University), so the amount of air required may be less. It would certainly not require so much to inflate the skirt due to lower resistance, but I'm unsure how that helps it exert the force necessary to achieve lift.

So anyway, that's my science homework completed using the entirely non-scientific back-of-a-fag-packet-based-on-entirely-made-up-assumptions method. Hopefully someone with a more scientific bent will be along to correct where I've made an error.

The definition of kgf relies on Earth gravity and I can't get my head round what that would mean on Mars. I'm also unclear whether you measure "more air" in volume or in number of molecules. So this re-statement may or may not align with your thinking.

The pressure difference you need between the underneath of the hovercraft and the atmosphere is what lifts it up. So you'd need 0.375 times as much pressure difference.

If I remember rightly the amount of pressure per molecule is roughly constant so differences in air composition don't matter. The number of extra molecules needed is therefore 0.375 times as much as it would be on earth.

The problem is that the compressors on the hovercraft would have to gather these molecules from an atmosphere were the concentration of molecules is far less, so a lot more pumping would be needed. This energy-intensive process would have to be done in a way that doesn't rely on atmospheric oxygen, for example carrying both hydrogen and oxygen for a fuel cell. All this would make the hovercraft very much heavier, so invalidating the original assumption that it was the same.

 

[O L Leigh]

Um, yes and no.

The definition of kgf relies on Earth gravity and I can't get my head round what that would mean on Mars.

I did allow for that in my calculations by adjusting how much the Hov Pod would weigh on Mars due to the difference in gravity.

Weight, as I'm sure you're aware, is the gravitional force acting on an object, so changes in gravitational force alter an object's weight. This is different from mass which is defined as a property of an object and a measure of it's resistance to acceleration, which determines the strength of it's gravitational attraction to other bodies (from Wikipedia for convenience).

Therefore, if the maximum gross weight of the Hov Pod on Earth is 560kg and the gravity on Mars is 0.375 times that on Earth, the Hov Pod on Mars would now weigh 560 x 0.375 = 210kg irrespective of it's mass. The calculation given by @Peter Mugridge to calculate the pressure needed to achieve lift is maximum gross weight / surface area. Given that the Hov Pod has a surface area of 5.49m2, the pressure needed to achieve lift on Earth is 560 / 5.49 = 102kgf/m2 and on Mars is 210 / 5.49 = 38.25kgf/m2. This is because in order to achieve lift you need to exert a sufficiently large force to overcome the craft's weight; in other words, strong enough to overcome it's inertia due to gravity.

The pressure difference you need between the underneath of the hovercraft and the atmosphere is what lifts it up.

Um, sort of.

This goes back to Newton's First Law, which states that a body continues in a state of rest, or in uniform motion in a straight line, unless it is acted upon by a force. Therefore it is a force that lifts the body of a hovercraft, but that force is provided by the air pressure in the skirt.

Where it gets a little more complicated is due to the fact that there is already a force acting on the craft; that being the force of gravity. Where two or more forces are acting on an object at the same time the state of the object will depend on the sum of all the vectors of the forces. Our Hov Pod at rest is stationary because the downward force of gravity acting upon it is counteracted by an opposite and equal force acting upward from the ground supporting it.

It's not enough just to have a pressure difference underneath the craft. The difference has to be sufficiently large to generate enough force to counteract the force acting upon it by gravity; ergo, it's weight. If there is insufficient pressure difference you will not generate adequate force to gain lift. (This is also tangentially touched on in an article entitled Interplanetary Cessna.)

If I remember rightly the amount of pressure per molecule is roughly constant so differences in air composition don't matter. The number of extra molecules needed is therefore 0.375 times as much as it would be on earth.

Yes!! And this was my jumping-on point. You may only have to get 0.375 times more air molecules into the skirt than on Earth but, where the atmosphere is around 100 times less dense, finding that number of molecules is much harder; hence my assumption that the Hov Pod's compressor would have to work 37.5 times faster than it would on Earth to achieve the inflation required for lift.

This energy-intensive process would have to be done in a way that doesn't rely on atmospheric oxygen, for example carrying both hydrogen and oxygen for a fuel cell. All this would make the hovercraft very much heavier, so invalidating the original assumption that it was the same.

That's one solution. Another may be that the pressure required to generate lift is not dependent upon the atmosphere. In other words, have an onboard lifting engine of some sort with the exhaust contained within the skirt. A sufficiently energy-dense fuel would prevent it from being too badly weight-limited, as it wouldn't need to carry very much of it.

 

[61653 HTAFC]

At least with the thinner atmosphere, any Hyperloopery on Mars wouldn't need any implausible vacuum tubes.

Mars Emperor Elon wouldn't permit the use of technology that he couldn't directly profit from, so traditional railways wouldn't get a look-in even if the Martian atmosphere was identical to that of Earth!

 

[edwin_m]

I did allow for that in my calculations by adjusting how much the Hov Pod would weigh on Mars due to the difference in gravity.

Weight, as I'm sure you're aware, is the gravitional force acting on an object, so changes in gravitational force alter an object's weight. This is different from mass which is defined as a property of an object and a measure of it's resistance to acceleration, which determines the strength of it's gravitational attraction to other bodies (from Wikipedia for convenience).

Therefore, if the maximum gross weight of the Hov Pod on Earth is 560kg and the gravity on Mars is 0.375 times that on Earth, the Hov Pod on Mars would now weigh 560 x 0.375 = 210kg irrespective of it's mass. The calculation given by @Peter Mugridge to calculate the pressure needed to achieve lift is maximum gross weight / surface area. Given that the Hov Pod has a surface area of 5.49m2, the pressure needed to achieve lift on Earth is 560 / 5.49 = 102kgf/m2 and on Mars is 210 / 5.49 = 38.25kgf/m2. This is because in order to achieve lift you need to exert a sufficiently large force to overcome the craft's weight; in other words, strong enough to overcome it's inertia due to gravity.

You're probably right but my brain can't cope with the idea of a weight being expressed in kg or kgf if we're in a place where g isn't 9.81m/s2. Put it all in newtons and I may be able to agree with you.

 

[Vespa]

I watched the "Martian Chronicles" no sign of any trains, they did have land sail ships tho......

 

[Cowley]

Ok this is all good. But it’s about time we started talking about liveries…

 

[Irascible]

Given the near lack of atmosphere & low gravity, just shooting stuff around might work out better...

For a railway you'd have to use something that secured itself to the track ( or it'd jump ), so pick the simplest system. It'd all have to be shipped there too. What would you even be moving around?

 

[edwin_m]

Ok this is all good. But it’s about time we started talking about liveries…

We need to discuss seats first. Will the low gravity make ironing boards more acceptable?

 

[Cowley]

We need to discuss seats first. Will the low gravity make ironing boards more acceptable?

 

[Peter Mugridge]

What would you even be moving around?

Mars Bars....?

 

[Vespa]

Mars Bars....?

1st Class ticket meal

 

[Mcr Warrior]

As a space geek, I wonder how fast we'll have rails, trains, and all that stuff on Mars? I mean, is there even a possibility to have proper trains on this planet?

Not in our collective lifetimes. How are you even going to get the materials up there?

So I bet that by 2040 we will have the first train on the planet. What do you think?

I think no chance by 2040.