### Author Topic: Space Elevator for Mars  (Read 30616 times)

#### Hop_David

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##### Re: Space Elevator for Mars
« Reply #40 on: 07/17/2017 03:06 PM »
In any case, I don't think the folks suggesting some of these scenarios grasp what 1.1 e16 kilograms is. A civilization capable of moving this mass around is in the distant future.

Wikipedia says the Port of New Orleans handles 62 million short tons of cargo plus a million passengers. Ignoring passengers and barges they handle 5.6 x e10 kg.  Gravity on phobos is 5.81 e-4 earth.

You're wanting to move Phobos? It is Mars' gravity you want to look at, not Phobos gravity.

So the lift capacity of similar cranes would be around 9.6 e14.

And what distance do the cranes move the cargo? I would suppose From the deck of a ship to a dock. Not the same as moving Phobos thousands of kilometers.

At any rate, the cranes of New Orleans are quite massive infrastructure. Even if your model was sound, your scenario isn't plausible.

#### Hop_David

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##### Re: Space Elevator for Mars
« Reply #41 on: 07/17/2017 03:42 PM »
Solid rings are not stable. [...] There would be a constant expense to keep the ring from crashing into Mars.

As noted in Isaac Arthur's video, the advantage of orbital rings is that the ring's shell is stationary WRT to the surface, but vastly lower than geostationary (or areostationary) orbit (in theory even inside the atmosphere). That drastically lowers the strength requirements of the ground cables. And since you can use the ring for fast point-to-point ground transport, you end up with a lot of ground cables, stabilising the ring.

I admire Isaac Arthur. But I am skeptical that the rings he describes would be stable. Even with an interior counter rotating ring, both parts feel the same GM/r^2. If Mars center coincides with rings' center of rotation, a decrease in r also means a decrease in ω^2 r, regardless if the inner ring is retrograde. So dipping closer to Mars means stronger gravity and weaker centrifugal force. The instability remains.

The inner and outer component of a low Mars elevator would be moving at greater than orbital speed with regard to one another. That would be more than 3.4 km/s. How far apart are the inner and outer rings? Should they come in contact with one another, the failure mode would be spectacular.

And we're talking very massive infrastructure. A low Mars orbital ring would be 23,000 kilometers in circumference. What is the mass of this ring? Isaac Arthur has been talking about megastructures that might come to pass in the distant future.

My focus has been elevator to payload mass ratios. The less ambitious Deimos  and Phobos elevator scenarios described would take tonnes to tens of tonnes infrastructure. They could happen in the 21st century.

#### Paul451

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##### Re: Space Elevator for Mars
« Reply #42 on: 07/18/2017 12:12 AM »
and store in bags at the equivalent to Lagrange 4/5 points.

There's no [useful] equivalent around Mars, assuming you mean a Mars-moon Lagrange. Phobos and Deimos are too small.

(edit: pedantry)

The station could be placed in GSO of Mars or at a L station.  [...] and cargo and people could be sent directly to a fixed point on the Martian surface.

It sounds like you think that descending from GSO station to the point on the Martian surface directly under the GSO station's location is easier than descending to another point on the surface. It's not. The easiest spot would be approximately 180° away, but anywhere else along the orbital plane is pretty much the same. You do a small burn to lower your orbit a bit, drift around your orbit until you are roughly 180° from your landing site, and then do you re-orbit burn.

[It's not quite 180°, you'll hit the atmosphere first. But from GSO, with a minimum de-orbit burn, it's close enough.]
« Last Edit: 07/18/2017 12:12 AM by Paul451 »

#### Paul451

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##### Re: Space Elevator for Mars
« Reply #43 on: 07/18/2017 02:29 AM »
Solid rings are not stable. [...] There would be a constant expense to keep the ring from crashing into Mars.
As noted in Isaac Arthur's video, the advantage of orbital rings is that the ring's shell is stationary WRT to the surface, but vastly lower than geostationary (or areostationary) orbit (in theory even inside the atmosphere). That drastically lowers the strength requirements of the ground cables. And since you can use the ring for fast point-to-point ground transport, you end up with a lot of ground cables, stabilising the ring.
I am skeptical that the rings he describes would be stable.

Read my comment again. I wasn't arguing that they would be stable, I'm saying that they can be low enough to reach the Earth with many short cables, and those cables provide stability.

However, AIUI, you can also dynamically stabilise a ring that isn't secured against the ground, provided the inner-ring is made up of discrete objects, not a continuous ribbon of cable. You can vary the energy applied at each magnetic station, hence you can transfer momentum around the ring, from the high-side to the low-side, letting you actively re-centre the ring.

And we're talking very massive infrastructure. A low Mars orbital ring would be 23,000 kilometers in circumference.

Much less than the length of a Mars space elevator. Or an Earth or lunar space elevator. And with much lower material requirements. (Which is what it is being compared to.)

What is the mass of this ring?

It would be interesting to know the minimum mass possible. Assuming a continuous inner-ring, so it needs to be supported by ground-cables to Earth (because of instability), the ground-cables hang from the magnetic stations, and the cables must be large enough to pull against any instability. The mag-stations need to be powerful enough to suspend themselves and the mass of the ground-cables from the inner-ring. The inner-ring needs to have enough momentum in order to hold up the mass of the magnetic-stations and the ground-cables. Knowing the mass of the ring and the rate of ring-drift gives you the force of the potential instability that the ground cables need to be able to withstand, giving you the mass of the ground-cables...

It should be possible to work out the interconnected mass dependencies and then scale down until you reach the minimum possible sized component. But I don't know how to work out the size of the ring-instability, so I don't even know where to start.

The less ambitious Deimos  and Phobos elevator scenarios described would take tonnes to tens of tonnes infrastructure. They could happen in the 21st century.

I'm a fan of the Phobos elevator. And the Phobos/Deimos transit system. Obviously an orbital ring isn't going to compete against that.

But orbital rings are intended to be a better planetary space elevator. It allows direct connection to the ground, at multiple locations, at short lengths (a few hundred km instead of tens of thousands of km, hence travel times of hours vs multiple days) and within the material strengths of realistic substances.

Around Phobos a minimum space elevator is so short there's no point building an orbital ring. Mars/Phobos L1 is lower than the radius of Phobos. Plus if you were going to build a ring around Phobos, you might as well just build it on the surface, or tunnel a linear accelerator through the centre. When you have a giant momentum bank, your options increase.

However, by the time you've extended the Phobos tether to the edge of Mars' atmosphere, and you have the volume of traffic to justify it, you're are probably within the economic range of building an orbital ring.

#### Paul451

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##### Re: Space Elevator for Mars
« Reply #44 on: 07/18/2017 02:45 AM »
However, AIUI, you can also dynamically stabilise a ring that isn't secured against the ground, provided the inner-ring is made up of discrete objects, not a continuous ribbon of cable. You can vary the energy applied at each magnetic station, hence you can transfer momentum around the ring, from the high-side to the low-side, letting you actively re-centre the ring.

To use an example with just four stations at 90° intervals around the planet: Without the stations, the ring-particles would be in a natural circular orbit (V=Vc). Individually stable for long periods, although they would gradually drift relative to each other. If you placed the four magnetic stations in their non-orbits (V=~0), they would want to fall down (obviously). To provide them with their "statite" energy, you have each station steer ring-particles towards the next station at faster-than-orbital-velocity (V>Vc). The station bends the particle's trajectory down (& Mr. Newton lifts the station up in response) and steers it towards the next station at the same faster-that-orbital-velocity. Rinse/repeat.

That's the basic dynamic orbital ring concept, stripped to its essentials. Small variations in the trajectories of the ring-particles need to be dealt with by the capture mechanism on the stations, and hence more stations is better. (In theory, you could have just two stations exchanging a single ring-particle. But I think four and a stream of particles is easier to picture.)

But what do you do when the ring as a whole is becoming eccentric? If the particles are on a natural orbit between stations 1 & 3, they can provide no energy to station 2 and it falls, if they are on a faster-than-orbital-velocity trajectory they provide lift. So to raise or lower a station, you change the velocity of the ring-particles passing through. If you want to raise the station, tighten the bend, if you want to lower the station, soften the bend. And because the stations steer the ring-particles, the ring as a whole moves with the stations.

Hence by increasing the speed of the particles on one leg, reducing it on another, you transfer momentum around the ring, which lets you lift and lower stations (and hence the ring itself) asymmetrically; actively stabilising the ring. That varies the spacing of the ring-particles: the slower particles are on a wider, more circular orbit, hence a longer path than faster particles on a tighter, more direct trajectory; hence the spacing between particles on the slower leg is greater than on the faster leg. Which is why it won't work if you have a solid/continuous inner-ring.

#### Hop_David

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##### Re: Space Elevator for Mars
« Reply #45 on: 07/18/2017 01:57 PM »
Read my comment again. I wasn't arguing that they would be stable, I'm saying that they can be low enough to reach the Earth with many short cables, and those cables provide stability.

I believe that cables even to a low orbit would need to be tensile rather than compressive towers. So they would not prevent a ring from dipping and suffering regions where gravity exceeds centrifugal force.

However, AIUI, you can also dynamically stabilise a ring that isn't secured against the ground, provided the inner-ring is made up of discrete objects, not a continuous ribbon of cable. You can vary the energy applied at each magnetic station, hence you can transfer momentum around the ring, from the high-side to the low-side, letting you actively re-centre the ring.

Actively using energy to accelerate or decelerate discrete parts of an inner ring might be a way to keep the ring from spinning out of control.

And we're talking very massive infrastructure. A low Mars orbital ring would be 23,000 kilometers in circumference.

Much less than the length of a Mars space elevator. Or an Earth or lunar space elevator. And with much lower material requirements. (Which is what it is being compared to.)

In my opinion a Mars elevator would need to terminate with a counterweight below Deimos to avoid collision with that moon. Such an elevator would be about 20,000 kilometers.

And I've already said such an elevator is implausible. If made of Zylon with a safety factor of two, Zylon tether mass to payload ratio would be a little than a thousand.

I believe a lunar elevator is even less plausible than a Mars elevator. And I still haven't revised the lunar elevator scenario with a more sensible safety factor of three.  An Zylon earth elevator I don't even bother to look at.

The less ambitious Deimos  and Phobos elevator scenarios described would take tonnes to tens of tonnes infrastructure. They could happen in the 21st century.

I'm a fan of the Phobos elevator. And the Phobos/Deimos transit system. Obviously an orbital ring isn't going to compete against that.

But orbital rings are intended to be a better planetary space elevator. It allows direct connection to the ground, at multiple locations, at short lengths (a few hundred km instead of tens of thousands of km, hence travel times of hours vs multiple days) and within the material strengths of realistic substances.

I like the notion of an elevator linked to the ground. Also the notion of fast transportation from one point on the ground to another. I want it to be plausible. However I believe skepticism and some Devil's Advocacy can help solidify a sound idea. And if an idea isn't sound, the same Devil's Advocacy might prevent us from wasting time and energy on an implausible scheme

Around Phobos a minimum space elevator is so short there's no point building an orbital ring. Mars/Phobos L1 is lower than the radius of Phobos. Plus if you were going to build a ring around Phobos, you might as well just build it on the surface, or tunnel a linear accelerator through the centre. When you have a giant momentum bank, your options increase.

However, by the time you've extended the Phobos tether to the edge of Mars' atmosphere, and you have the volume of traffic to justify it, you're are probably within the economic range of building an orbital ring.

When you use a safety factor of three, a 5,800 km Zylon tether from Phobos to just above Mars atmosphere has a tether to payload mass ratio of 638. You'd need a 638 tonne elevator to accommodate a one tonne payload (which includes the elevator car as well as cargo & passengers). That makes it impractical, in my opinion. This was a painful conclusion for me as such an elevator had been one of my favorite day dreams for several years.

I still entertain the notion of a 1,400 kilometer elevator descending from Phobos. Zylon tether to payload ratio is .33. so a one tonne elevator could accommodate a three tonne payload.

A 1,400 km tether could drop payloads in a Mars orbit with an atmosphere grazing periapsis. Using repeated periapsis drag passes, aerobraking can bring this down to a low Mars orbit where the payload is moving 3.4 km/s. If the payload has propellent from Phobos, it could use that reaction mass to slow it further and Mars EDL would still be much simpler than the 6 km/s entry coming in from an earth to Mars Hohmann.

Both the 5,800 km and 1,400 km scenarios are examined in my Lower Phobos Elevator post.

#### stefan r

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##### Re: Space Elevator for Mars
« Reply #46 on: 07/18/2017 10:53 PM »
and store in bags at the equivalent to Lagrange 4/5 points.

There's no [useful] equivalent around Mars, assuming you mean a Mars-moon Lagrange. Phobos and Deimos are too small.

Exactly.  The gravity becomes so small it is not relevant.  If we move mass a distance along the orbital path we do not have to worry about it returning to Phobos.  Package tape or dental floss can easily resist the force of the solar wind on a cargo container.  Saying "equivalent of L4/L5" was probably a poor choice of words.  Preventing the mass on a Phobos orbital ring from reforming a moon does not require much force.  If you lose the material you will have dangerous debris flying around for a very long time.

In any case, I don't think the folks suggesting some of these scenarios grasp what 1.1 e16 kilograms is. A civilization capable of moving this mass around is in the distant future.

Wikipedia says the Port of New Orleans handles 62 million short tons of cargo plus a million passengers. Ignoring passengers and barges they handle 5.6 x e10 kg.  Gravity on phobos is 5.81 e-4 earth.

You're wanting to move Phobos? It is Mars' gravity you want to look at, not Phobos gravity.

So the lift capacity of similar cranes would be around 9.6 e14.

And what distance do the cranes move the cargo? I would suppose From the deck of a ship to a dock. Not the same as moving Phobos thousands of kilometers.

At any rate, the cranes of New Orleans are quite massive infrastructure. Even if your model was sound, your scenario isn't plausible.
Some of the containers get loaded in Shanghai and travel through the Panama canal before getting unloaded from container ships in New Orleans.  Some of the containers have a thousand kilometer trips before and after shipping.  Some of the components in the products already made ocean trips before assembly.  Of course some containers could be Huston to New Orleans.  I am not sure if they count unloading a container from a Hong Kong origin ship and reloading it onto ship heading to Boston as handling 2 containers.  On average 1000 km is probably the right order of magnitude.

We may be confused with the words "distant future".  A dwarf elephant is "bigger" than a huge ant.  If the goal is to sustain a small research outpost then an orbital ring will not pay off its investment.  If we are talking about "how to land a human" then this entire thread becomes suspect.  Elevators and orbital rings pay off as the total volume of cargo transport increases.  Depending on context the next century could be referred to as "near future".  "Within a child's lifetime" is probably better wording.

Elon Musk and SpaceX are talking about "a city of 1 million people" on Mars in the "next 40 to 100 years".  That requires moving a lot of mass.  It includes a lot of mass lifted out of earth's gravity well.  I guess I do not know how plausible that scenario is.  But they are throwing a lot of money and effort into it.

I do not see a good reason to move Phobos.  An orbital ring can pass through, go above, or go below Phobos.   We could do all three without too much trouble.  Phobos has a lot of inertia so attaching a ring to it has advantages that a free flying ring would not.  A Phobos ring is also a reasonable place to construct other rings.

The most basic orbital ring can be a thin tether like fishing line.  Tension does not need to add over thousands of kilometers. You can use spools spaced out around the ring.  Use several lines for redundancy and safety.  Pieces of the ring can be thicker chunks made out of low tensile strength material.  The problem with a single cheap thread is the launch capacity.  If most of the ring is a 50 newton fishing line then the prograde and retrograde launches have to balance within 50 newtons before the spools run out of line.

With a Phobos ring you can use a magnetic rail line to launch prograde and retrograde near phobos.  A few hundred kilometers mass driver track could catch/launch shuttles from the surface, launch shuttles to earth, and function as a landing strip for incoming spacecraft.  The momentum from a landing ship will not cause a huge change in Phobos's momentum.  So the far side of the ring do not need to spool out much fishing line to compensate.

An active support orbital ring is a bit bigger and more complicated but requires much lower technology than much of the Mars mission.

From an infrastructure standpoint orbital rings have a lot of advantages.  Mars does not have an ocean so none of Earth's shipping will be possible.  A rail line circling Mars is already a huge expense.  A rail or road only makes sense if it is part of a transportation network.  An active support orbital ring can bypass all of the long distance road and rail.

The entirety of "city on Mars" or "million colonists on Mars" may be implausible.  But if we start with that assumption then the orbital ring may be one of the easier paths.

#### Hop_David

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##### Re: Space Elevator for Mars
« Reply #47 on: 07/19/2017 12:23 AM »
Some of the containers get loaded in Shanghai and travel through the Panama canal before getting unloaded from container ships in New Orleans.  Some of the containers have a thousand kilometer trips before and after shipping.  Some of the components in the products already made ocean trips before assembly.  Of course some containers could be Huston to New Orleans.

So you're not just talking about the cranes at the New Orleans harbor. You are also talking about all the ships that move between New Orleans and points throughout the planet. You're also talking about the Panama Canal which is rather massive.

Also a ship traveling on the Atlantic has different fuel requirements than a tug moving stuff to different orbits. Delta V to move a mass from Phobos to Low Mars Orbit is about 1.2 km/s. By my arithmetic it would take about 3.1e15 kilograms of hydrogen/oxygen bipropellent. The world's annual production of oil was about 77,500,000 barrels per years as of 2014. That comes to about 4.4e12 kg of oil.

So the hydrogen/oxygen bipropellent to move Phobos would need to mass about 700 times as much as the world's annual oil production.

What is the mass of the infrastructure you imagine?

This is an important factor that people seem to want to ignore.

#### Paul451

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##### Re: Space Elevator for Mars
« Reply #48 on: 07/19/2017 12:30 AM »
Read my comment again. I wasn't arguing that they would be stable, I'm saying that they can be low enough to reach the Earth with many short cables, and those cables provide stability.
I believe that cables even to a low orbit would need to be tensile rather than compressive towers. So they would not prevent a ring from dipping and suffering regions where gravity exceeds centrifugal force.

For the ring to drift off-centre (so that one side brushes the planet), as one side lowers, the other side is lifted. If (tensile) ground-cables prevent that side from lifting, the ring itself prevents the other side from dipping.

However, AIUI, you can also dynamically stabilise a ring that isn't secured against the ground, provided the inner-ring is made up of discrete objects, not a continuous ribbon of cable. You can vary the energy applied at each magnetic station, hence you can transfer momentum around the ring, from the high-side to the low-side, letting you actively re-centre the ring.
Actively using energy to accelerate or decelerate discrete parts of an inner ring might be a way to keep the ring from spinning out of control.

The whole structure is active, that's the whole point. It only works because the mag-stations are guiding the inner-ring (even if you completely enclose the inner-ring(s), the mag system is still always on.) Actively changing the orientation of the ring would simply be a variation within that continuous process.

[BTW I might be wrong about my interpretation of how a non-grounded ring is stabilised. I'm going from fragments of what I've read, but its always pop.science stuff, I haven't tried to dig up any genuine scientific papers.]

#### stefan r

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##### Re: Space Elevator for Mars
« Reply #49 on: 07/21/2017 04:10 AM »
Some of the containers get loaded in Shanghai and travel through the Panama canal before getting unloaded from container ships in New Orleans.  Some of the containers have a thousand kilometer trips before and after shipping.  Some of the components in the products already made ocean trips before assembly.  Of course some containers could be Huston to New Orleans.

So you're not just talking about the cranes at the New Orleans harbor. You are also talking about all the ships that move between New Orleans and points throughout the planet. You're also talking about the Panama Canal which is rather massive.

Also a ship traveling on the Atlantic has different fuel requirements than a tug moving stuff to different orbits. Delta V to move a mass from Phobos to Low Mars Orbit is about 1.2 km/s. By my arithmetic it would take about 3.1e15 kilograms of hydrogen/oxygen bipropellent. The world's annual production of oil was about 77,500,000 barrels per years as of 2014. That comes to about 4.4e12 kg of oil.

So the hydrogen/oxygen bipropellent to move Phobos would need to mass about 700 times as much as the world's annual oil production.

What is the mass of the infrastructure you imagine?

This is an important factor that people seem to want to ignore.

I was expecting the converse.  Build the station at Phobos.  Take your numbers: 3.1e15 kg of oxy/hydrogen.  It is readily available as momentum.  Phobos may be much more valuable than the petro because we can use the momentum both ways.  Incoming craft add momentum out going craft drain it.

If we are dealing with 1 million colonists then a lot of numbers get large.  For instance SpaceX would be transporting 3 to 5 million meters of live human small intestine.  That is 5 to 8% of the length of an orbital ring.  A typical house in the USA has more than 60 meters of pipe.  Count hot, cold, drainage.  The Martians will also have some sort of farms.  Farms in the midwest have drainage lines every 3 meters.  Hydroponics would use more pipe.

I am not sure of the minimum mass needed.  The orbital ring is not excessive.
« Last Edit: 07/22/2017 01:18 AM by stefan r »

#### Hop_David

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##### Re: Space Elevator for Mars
« Reply #50 on: 08/07/2017 03:42 PM »
Read my comment again. I wasn't arguing that they would be stable, I'm saying that they can be low enough to reach the Earth with many short cables, and those cables provide stability.
I believe that cables even to a low orbit would need to be tensile rather than compressive towers. So they would not prevent a ring from dipping and suffering regions where gravity exceeds centrifugal force.

For the ring to drift off-centre (so that one side brushes the planet), as one side lowers, the other side is lifted. If (tensile) ground-cables prevent that side from lifting, the ring itself prevents the other side from dipping.

However, AIUI, you can also dynamically stabilise a ring that isn't secured against the ground, provided the inner-ring is made up of discrete objects, not a continuous ribbon of cable. You can vary the energy applied at each magnetic station, hence you can transfer momentum around the ring, from the high-side to the low-side, letting you actively re-centre the ring.
Actively using energy to accelerate or decelerate discrete parts of an inner ring might be a way to keep the ring from spinning out of control.

The whole structure is active, that's the whole point. It only works because the mag-stations are guiding the inner-ring (even if you completely enclose the inner-ring(s), the mag system is still always on.) Actively changing the orientation of the ring would simply be a variation within that continuous process.

[BTW I might be wrong about my interpretation of how a non-grounded ring is stabilised. I'm going from fragments of what I've read, but its always pop.science stuff, I haven't tried to dig up any genuine scientific papers.]

A low Mars orbital ring would be around 20,000 km in length. A low earth orbital ring about 40,000 km. They would not be as narrow as an elevator tether. So a large cross sectional area and thus larger chance of impact from a meteoroid or piece of orbital debris.

I attached a sketch of a piece of debris puncturing the walls and putting material in the path of the elements providing centrifugal lift. When the element strikes the debris in it's path it will itself turn into an expanding blob of debris further damaging the walls around the elements.

These guys are moving faster than something in a normal low earth or low Mars orbit. LEO orbital period is about an hour and a half. LMO is about two hours. In less than two hours after an impact all the rapidly moving elements will have reached the impact site.

The elevators to the ring as well as much of the ring structure would fall to the planet surface. There would also be an impressive debris cloud that would remain in orbit for some time.
« Last Edit: 08/07/2017 03:46 PM by Hop_David »

#### stefan r

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##### Re: Space Elevator for Mars
« Reply #51 on: 08/07/2017 05:05 PM »

These guys are moving faster than something in a normal low earth or low Mars orbit. LEO orbital period is about an hour and a half. LMO is about two hours. In less than two hours after an impact all the rapidly moving elements will have reached the impact site.

The elevators to the ring as well as much of the ring structure would fall to the planet surface. There would also be an impressive debris cloud that would remain in orbit for some time.

There are multiple models of orbital rings.  Would be nice if more specific terms could were common.  Your drawing looks like one of the active support structure.  And parts of your ring are co-rotating with the planet's surface.

Active support structures are not commonly available so it is hard to evaluate risks.  Suppose you have 10 or 100 conduit tubes and a matching set of wire/pellets.  Broken pieces of the co-rotating conduit would fall promptly or dangle.  Most of the 20,000 km of wire-pellets could switch to an alternate conduit.  Many km of wire would fly through the punctured section but they are moving at exit velocities.  (of course 100 wires means 2 orders of magnitude increase in minimum mass).

Meteor damage is not trivial.  Compare to a meteor puncturing a tank of chemical rocket fuel.  Most of the debris from an exploding storage depot will remain in mars orbit because it was at rest in low mars orbit before the explosion.  A supply line for 1 million Mars residents will contain a lot of dangerous energy and momentum.  Not simple to calculate which option is most dangerous.  Depends on a lot of variables.

An "orbital ring" can be actually orbital.  The active support exists only to stabilize the orbit and to cancel launch/landing momentum.  A meteor does not change anything except splash form the impact site.  Most of the ring could be fishing line.  Reels would be spread around the ring.  A Hoyt tether design would allow a comet to fly through without seriously effecting business as usual.  With an orbiting orbital ring you still need mass drivers, tethers, and/or rockets.  You could think of the (orbiting)orbital ring as a complex tether.

#### Paul451

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##### Re: Space Elevator for Mars
« Reply #52 on: 08/07/2017 10:47 PM »

The projectiles are guided by the magnetic rings, in your drawing they would be the sections connected to the ground-cables. Between, the projectiles are free-flying. The meteors achieve nothing. See below.

If you take out a mag-section, the projectiles in that section will continue on their super-orbital trajectory, missing the next mag-section and flying off into deep space. However, once the damage is detected, the previous mag-section would change its deflection angle and re-aim the following projectiles for the next intact section. The system as a whole wouldn't fail, although you'll need to re-inject new projectiles to make up for the loss.

Once you get to the point where the entire ring is enclosed (which means continuous mag-sections all the way round) then any strike would hit a mag-ring, but by the time you get to that point, you're talking a massive level of development and inevitably many redundancies and safety systems; at the very least, with so many mag-rings, each deflection would be so slight that losing a mag-ring or seven would still leave the projectiles within the target area of the remaining mag-rings. (Otherwise, how do you take rings off-line for maintenance.)

#### Asteroza

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##### Re: Space Elevator for Mars
« Reply #53 on: 08/08/2017 01:37 AM »
Aren't some active full ring systems predicated on very small projectiles (grain of sand or less) with a fairly generous vacuum tube area? Penetration events, if not very catastrophic, aren't going to make much of a tube debris cloud, and would basically sandblast any accident debris to dust while accelerating it down the tube right? Eventually debris dust would line the tube upper surface past the penetration site due to splatter because it can't be magnetically directed too right?

#### stefan r

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##### Re: Space Elevator for Mars
« Reply #54 on: 08/08/2017 08:41 PM »
Aren't some active full ring systems predicated on very small projectiles (grain of sand or less) with a fairly generous vacuum tube area? Penetration events, if not very catastrophic, aren't going to make much of a tube debris cloud, and would basically sandblast any accident debris to dust while accelerating it down the tube right? Eventually debris dust would line the tube upper surface past the penetration site due to splatter because it can't be magnetically directed too right?

The energy and momentum can be independent of size and shape.  106 spherical grains with millimeter diameter have the same momentum as 109 spherical grains with diameter 0.1 millimeter.  A single wire with 1.0mm diameter and 750 m length also has the same mass, momentum and energy.

My impression was that wire shaped particles are easier to work with and can overlap.  The particle model is used to counter concerns about stretching.

#### Dao Angkan

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##### Re: Space Elevator for Mars
« Reply #55 on: 08/12/2017 08:35 PM »
With regards to a Phobos tether;

Phobos L1 Operational Tether Experiment (PHLOTE)

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A sensor package that “floats” just above the surface of Phobos, suspended by a tether from a small spacecraft operating at the Mars/Phobos Lagrange 1 (L1) Point would offer exciting opportunities for science (SMD), for human exploration (HEOMD) and for advancements in space technology (STMD). Detailed information on the Martian moon Phobos is limited even though it is considered an important destination for near term human exploration. A PHLOTE spacecraft would perform fixed point station keeping at the Mars/Phobos L1 point to allow a tethered sensor package to “float” just above the moon’s surface and also park instruments on the surface for in situ science measurements. This can include ground penetrating radar for subsurface composition measurements to determine how thick the layer of fine grained regolith is for future landings. Other key instruments would be dosimeters for understanding the radiation environments for future human missions, cameras, and a spectrometer for surface mineral analysis. If deployed after a human landing, a PHLOTE spacecraft could provide a constant “eye in the sky” for ground controllers to monitor mission deployments and operational activities. The PHLOTE mission concept has only now become feasible due to recent technology advances, many of which have been supported by NASA’s STMD. Key technologies that make this mission concept feasible include: The Navigation Doppler Lidar (NDL) Sensor for the providing precise spacecraft position and rate knowledge relative to Phobos. This high precision is needed to maintain position at the L1 point; Carbon Nanotube (CNT) braided yarns for a structurally strong tether that doubles as a power and data conduit, Ultralightweight solar arrays, and highly efficient electrospray micro-propulsion thrusters for long term “hover mode” station keeping.

The Martian Moon Phobos offers a key waypoint toward enabling human surface landings on Mars. In particular Stickney Crater, which always faces Mars due to Phobos’ synchronous rotation, provides an excellent stepping stone destination as a precursor to a human Mars landing. There is very limited information on the composition and the environments at Stickney Crater on Phobos. Since Phobos has a composition similar to carbonaceous chondrite meteorites, it is believed that it could provide minerals that can be used for In Situ Resource Utilization (ISRU) to recover key elements such as Oxygen for use as return trip propellant. The mission concept below would answer many of these questions as well as provide TRL advancement in key technology areas for human exploration.

This mission concept is a synthesis of new technologies that would provide a unique platform for multiple sensors directed at Phobos as well as Mars. Since the Mars/Phobos L1 point is only ~3.1 km from the surface of Phobos, the PHLOTE tether length only needs to be a few kilometers long. A tether configuration with its Center of Gravity at the Mars/Phobos L1 point can place a sensor package on the moon’s surface or float it just above. Due to Phobos’ very low gravity, the tether will be under very low tensile loads.

Using a longer tether, this concept can be similarly used for other missions such as Mars/Deimos or at the Pluto/Charon L1 point where both bodies are tidally locked which means a PHLOTE spacecraft with a much longer tether could descend into Pluto’s tenuous atmosphere and sample its chemistry at all elevations unlike a traditional probe.
If selected, a feasibility study for the Phobos L1 Operational Tether Experiment (PHLOTE) mission would be performed that will define the PHLOTE mission, determine the technology needs and assess the technology readiness. The study would also model the system, identify risks, as well as explore new science opportunities that could be done with this unique sensor platform.

And a similar lunar tether;

NASA Studies Tethered CubeSat Mission to Study Lunar Swirls

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NASA’s Planetary Science Deep Space SmallSat Studies, or PSDS3, program recently selected a team at the Goddard Space Flight Center in Greenbelt, Maryland, to further develop a mission concept called the Bi-sat Observations of the Lunar Atmosphere above Swirls, or BOLAS. The study, led by Goddard Principal Investigator Timothy Stubbs, could lead to the first tethered planetary CubeSat mission, Stubbs said.

“This is an exciting concept,” said Michael Collier, a BOLAS co-investigator who has studied tether-based missions for gathering difficult-to-obtain lunar measurements since 2015. “Candidly, I think it’s groundbreaking. Tethered satellites are a very natural approach for targeting lunar science.”

As currently conceived, the mission would involve two 12-unit CubeSats, whose individual units would measure just four inches on a side. Once the pair reached a low-maintenance, quasi-stable orbit about 62 miles above the Moon’s surface, the two, connected by a 112-mile-long thin tether, would separate. The top satellite would climb 118 miles above the surface, while the lower, nearly identical twin would plunge to an altitude of about six miles above the surface.

“The tension in the tether keeps the CubeSats in vertical alignment as they orbit,” Stubbs said. “The configuration, with the center-of-mass in a quasi-stable orbit, should enable the lower CubeSat to fly for long durations at low altitudes.”

So, is it necessary to keep the tethers extended at all times putting them at risk of micrometeorite damage, or could we just reel the tethers in and out when needed from close to L1? This would also eliminate the need for specialised tether climbing and power equipment, it would all be powered from the L1 station. An extra thick tether could be used for the permanently exposed 3.1km Phobos - L1 tether, but this would not be used for transportation, simply tethering the L1 station. The "winched tethers" could be thinner, with spare spools kept on the L1 station in case of any damage to the existing tethers.

Any flaws with this?

(not to scale);

#### A_M_Swallow

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##### Re: Space Elevator for Mars
« Reply #56 on: 08/13/2017 01:10 PM »
That looks like the deployment method proposed for an Earth space elevator. For asteroids an alternative was to land the whole lot on the asteroid, dig in and fire the ribbon into space. Has this option been considered?

A 2 stage launch may be needed for the L1 equipment.

#### stefan r

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##### Re: Space Elevator for Mars
« Reply #57 on: 08/14/2017 09:10 PM »
That looks like the deployment method proposed for an Earth space elevator. For asteroids an alternative was to land the whole lot on the asteroid, dig in and fire the ribbon into space. Has this option been considered?

A 2 stage launch may be needed for the L1 equipment.

Ancient workers used clay bricks to make grain silos more than 10 meters high.  With gravity 1/2000 a structure made with equivalent material can be built to the Lagrange points.  Standard intermodal shipping containers can also be stacked over 10 meters on Earth's surface.

A challenge on Phobos will be to prevent things from flying around.  A typical cork in a champagne bottle could be popped into orbit around Mars.  A good table tennis (ping pong) match would also send balls out of Phobos's gravity well.  It becomes dangerous if corks or ping pong balls are accelerated by the solar wind and return through Phobos's orbit around Mars in an elliptical path.  Even if the cork remains in a circular orbit it could endanger a ship flying to or from Mars' surface and ships using a gravity assist.

#### Paul451

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##### Re: Space Elevator for Mars
« Reply #58 on: 08/14/2017 10:26 PM »
Pedantry, because it's one of those things that always bugs me:

For a tether, the part in circular orbit is not at the centre-of-mass. Because centripetal acceleration is linear to distance, but gravitational acceleration is to the square, the balance point of the forces on a tether (and hence its orbit) is below the centre-of-mass.

(Hence "COM orbit" (for a free flying object) would be an eccentric orbit (or even escape velocity) with periapsis at the COM's altitude.)

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