Author Topic: Space Elevator for Mars  (Read 17361 times)

Offline Hop_David

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Re: Space Elevator for Mars
« Reply #20 on: 05/30/2015 06:52 PM »
Not only are the lengths of Phobos/Deimos tethers less in total than an equivalent Mars-surface elevator, the strength requirements are a fraction as much. That means the taper (and thus mass) of any given tether-material is vastly less -- less than 1 percent for the full 9000km Phobos tether. Which means you can build a whole Phobos/Deimos tether network for a fraction of the mass, energy and effort as a Mars-surface elevator.

Better still, you can build out incrementally. Since Phobos L1/L2 is less than 4km, the minimum self-supporting tether from Phobos's surface is going to be dozen km or so. (As well as reducing the initial tether mass by several more orders of magnitude.) That's pretty close to tethers we've already deployed in experiments. In other words, the technology is already available, we just need to refine the design to make it safer and more reliable.

Once you have that anchor to Phobos, you no longer need a lander to move resources from Phobos into Mars orbit. You can slowly increase the length and usefulness of the Phobos tethers as your experience with the technology grows. Whereas a Mars-surface elevator requires the whole thing to be built before it's useful, so you have to already know how to build, maintain and control an ~40,000km tether before you can even start.

40,000km vs 12km. Hell of a learning curve difference.

Quite so.

The Phobos tether portrayed above has a taper ratio of about 8 if using Kevlar with 3,600 mega pascal tensile strength and 1.44 g/cm^3 density.

Using the same material, an elevator to Mars synchronous orbit and with a balancing length above synchronous would have a taper ratio of 45.

A payload at the foot of a Phobos tether would feel a gravity only slightly less than a foot at Mars surface. But that would be mitigated by centrifugal force. It'd feel a weight of about 3 newtons per kilogram.

A payload at the foot of an elevator to synchronous would feel a weight of about 3.7 newtons per kilogram.

An elevator's volume would be average cross section area times elevator length. I will take the elevator's average cross section area as (cross section at foot + cross section at max stress)/2. This is over estimating some but I believe it is in the right ball park.

A tether's mass is density times volume.

For a full blown Mars elevator using Kevlar I get a tether mass to payload mass ratio of about 2200. For the Phobos tether I portrayed above, tether to payload mass ratio is about 90.

Offline A_M_Swallow

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Re: Space Elevator for Mars
« Reply #21 on: 05/30/2015 08:30 PM »

The end of any tether has partial gravity (that is, after all, how tethers work.) So you can hang a simple platform and land income craft on that. You'll need to watch out for the cables/frame holding the platform, but the approach will still be much easier than any conventional docking.

To return to Mars, you just roll off the edge of the platform and fall.

Landing on that platform will probably be like landing on an aircraft carrier. A very tiny one because the platform mass comes off the tether's payload on a 1kg for 1kg basis.

Phobos has an elliptical orbit so the height above Mars can increase by about 200km.

There may be scope for a fun simulation here - landing on the Photos tether platform.

Offline Hop_David

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Re: Space Elevator for Mars
« Reply #22 on: 05/30/2015 09:27 PM »
Better still, you can build out incrementally.

Yes, the tether doesn't have to extend all the way to mars upper atmosphere to be useful.

One thing about tethers in the region of L1 or L2: An ion driven MTV could dock with it.

An ion driven MTV docking with Deimos is saved much of the spiral down Mars gravity well.

The Phobos and Deimos elevators share an ellipse. Going from/to Deimos tether to/from Phobos tether can take nearly zero delta V.

Dropping from a short distance beneath Phobos gives an atmosphere grazing ellipse with a periapsis speed of 3.8 km/s. Aerobraking can take this down to circular orbit moving 3.4 km/s. If Phobos is a source of propellent, much of that 3.4 could be removed. Mars EDL becomes much simpler.

Tether mass to payload ratios for attached diagram:
Deimos to Deimos drop off: 1/25
Phobos catch to Phobos: 1/18
Phobos to drop off for atmosphere grazing ellipse: 1/4

MTV could remain docked at Deimos.

Trip from Deimos tether to Phobos tether (or vice versa) takes about 8 hours so this transfer vehicle could be tiny.

The vehicle doing rendezvous/drop off with the Phobos tether foot would be a Mars ascent/descent vehicle.
« Last Edit: 05/30/2015 09:41 PM by Hop_David »

Offline A_M_Swallow

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Re: Space Elevator for Mars
« Reply #23 on: 05/31/2015 02:45 AM »
{snip}
Trip from Deimos tether to Phobos tether (or vice versa) takes about 8 hours so this transfer vehicle could be tiny.
{snip}

If the vehicle is tiny it can be the same machine as the tether climber.

Offline Hop_David

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Re: Space Elevator for Mars
« Reply #24 on: 06/17/2015 05:56 PM »
What if a carbonaceous asteroid (or comet? or kuiper object? saturn ring bits?) could be found and orbited around Mars, and a tether then gradually constructed from this body, and then slowly dangled down until it reached the surface?

Where would the key technical challenges be?

Importing an anchor mass/momentum bank for a tether to Mars orbit is an interesting idea. As mentioned in this thread, Deimos and Phobos are two existing masses already in Mars orbit.

A comet falling from the Kuiper Belt would have 10 km/s Vinf wrt to Mars. An ice ball falling from Saturn would have a nearly 8 km/s Vinf. It would take a great deal of energy and reaction mass to park these in Mars orbit.

But it may be possible to capture an asteroid to mars orbit. Similar to the Asteroid Redirect Mission but for Mars. But in Mars case you don't have a large moon that can shed up to 1 km/s Vinf. But there may be a good number of asteroids in nearly Mars like orbits.

Parking an anchor mass in a circular orbit deep in Mars gravity well would take a great deal of energy and reaction mass. Least challenging is to park it in an orbit as high as possible without the sun's influence destabilizing the rock.

If we set the orbital radius at half Hill Sphere radius, around 500,000 km, orbital period would be 125 days and speed would be about .3 km/s wrt to Mars. Synodic period wrt to Deimos would be about a day. So each day a payload could be dropped from the captured asteroid anchored tether to be caught by a Deimos tether. The drop from captured asteroid to Deimos would take nearly 50 days.

If the asteroid anchored tether were parked in a 100,000 km circular orbit, the drop to a Deimos tether would take about a week. Orbital velocity would be about .65 km/s and period would be about 12 days.

A tether relay (As I've already described between Deimos and Phobos) wouldn't require huge amounts of tether mass.

A full blown Mars elevator would need to have a tether mass 1000's of times greater than the payloads it moves (if we're using existing materials such as Kevlar).

 




Offline Hop_David

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Re: Space Elevator for Mars
« Reply #25 on: 06/17/2015 06:14 PM »
{snip}
Trip from Deimos tether to Phobos tether (or vice versa) takes about 8 hours so this transfer vehicle could be tiny.
{snip}

If the vehicle is tiny it can be the same machine as the tether climber.

I think so, yes.

Since Deimos and Phobos aren't exactly co-planar we would need some reaction mass for the rendezvous leading to a catch.

Half of the trip would be downhill (that is, in the same direction as dominant acceleration). For example the trip from Mars-Deimos L1 to Phobos drop would be downhill. If going the other direction, the trip from Mars-Phobos L2 to Deimos throw would be downhill.

Offline Hop_David

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Re: Space Elevator for Mars
« Reply #26 on: 06/19/2015 07:28 PM »
I've taken a closer look at the notion of a Phobos tether:

Phobos-Panama Canal of the Inner Solar System

Offline ilic78

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Re: Space Elevator for Mars
« Reply #27 on: 11/27/2016 02:25 PM »
Why not put a tether between Phobos and Deimos and use it to decrease the second's orbit toward geostationary and at the same increasing that of the first one?
Then you can think about a tether, anchored or not, between Phobos and Mars.
Sorry for my English but I'm Italian, thanks

Offline Hanelyp

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Re: Space Elevator for Mars
« Reply #28 on: 11/27/2016 11:07 PM »
Why not put a tether between Phobos and Deimos ...
I haven't run the numbers, but my gut says the forces on such a tether between those rocks would be immense.  If those moons are rubble piles with minimal self gravity they might not even hold together against such a tether.

Offline Paul451

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Re: Space Elevator for Mars
« Reply #29 on: 11/28/2016 11:29 AM »
Why not put a tether between Phobos and Deimos

How would you capture them?

Offline Hop_David

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Re: Space Elevator for Mars
« Reply #30 on: 12/16/2016 03:24 AM »
Why not put a tether between Phobos and Deimos and use it to decrease the second's orbit toward geostationary and at the same increasing that of the first one?
Then you can think about a tether, anchored or not, between Phobos and Mars.
Sorry for my English but I'm Italian, thanks

They are both very massive bodies whose speeds differ by .8 km/s. Not doable to tie them to together.

However a vertical tether could be placed at Mars synchronous orbit. There could be Zero Relative Velocity Transfer Orbit (ZRVTO) between the synchronous tether and a Deimos tether. Deimos material could be sent to the synchronous tether using very little reaction mass.

But catching material from Deimos would raise the orbit of the synchronous tether.

LIkewise there could also be ZRVTOs between the synchronous tether and a Phobos tether. But catching material from Phobos would lower the synchronous tether's orbit.

However if the synchronous receives material from Phobos and Deimos in the correct proportions, it can build up a respectable momentum bank while remaining at synchronous orbit altitude. There could be several such tethers. Mars synchronous tethers might be nice intermediate steps between a Deimos and a Phobos tether. There are obvious communication uses for infrastructure in Mars synchronous orbits.They might also be good spots from which to operate tele-robots on Mars surface.

I don't think a synchronous tether extending all the way to Mars' surface is practical. For a number of reasons. I hope to write an article on a full fledged Mars beanstalk soon.

Offline Hop_David

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Re: Space Elevator for Mars
« Reply #31 on: 06/15/2017 06:01 PM »
Could it one day be possible/practical to build a space elevator for Mars?

What if a carbonaceous asteroid (or comet? or kuiper object? saturn ring bits?) could be found and orbited around Mars, and a tether then gradually constructed from this body, and then slowly dangled down until it reached the surface?

here's a small thread from StackExchange:

http://physics.stackexchange.com/questions/33547/space-elevator-on-mars-with-todays-technology-possible

and another from Quora:

http://www.quora.com/Would-it-be-easier-building-a-space-elevator-on-Mars-or-the-Moon


What is the theoretical feasibility?
Where would the key technical challenges be?

I took a look at a Zylon Mars elevator using Wolfe's spreadsheet.

For examining scenarios I had been using a safety factor of one. This is pushing Zylon to the limits of its tensile strength. The slightest nick or scrape along an elevators length would cause it to break. I don't think a sensible player would risk valuable payloads on such an elevator. Much less risk human lives.

I've been trying to go back and redo the scenarios where I include the more sensible safety factor of three as well as a safety factor of one.

Given a safety factor of one and having the Mars elevator counterweight just below Deimos, it'd take about 200 tonnes of Zylon to lift a tonne from Mars surface. This tonne would include the elevator car, the elevator's power source and engine. So the mass of the actual cargo would be much less.

The sub Deimos counterweight would need to be 1200 times the mass lifted from Mars surface.

In my opinion, a Zylon Mars elevator wouldn't be worthwhile even given the very risky safety factor of one.

Given a sensible safety factor of three, tether to payload mass ratio would be around 53,000. The counterweight would need to be 180,000 times as massive as the mass lifted from Mars surface.

The Phobos anchored elevator is still my chief interest. An upper Phobos elevator capable of flinging payloads to the Main Belt and earth wouldn't have prohibitive tether to payload mass ratio. Even with a safety factor of three. However I no longer regard as plausible a Zylon Phobos tether descending to Mars upper atmosphere. See:
Phobos upper tether
Phobos lower tether
Deimos tether

I also look at some of the different scenarios in my Physics Stack Exchange answer.

Offline Hotblack Desiato

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Re: Space Elevator for Mars
« Reply #32 on: 06/30/2017 11:01 PM »
Rather than putting the elevator on Mars it may be better to attach it to the moon Phobos. Aim the ribbon at Mars and use a rocket powered aircraft for the last 100km.

Phobos is tidally locked, so at least you don't have to worry about your ribbon facing Mars. But it presents other problems.

Phobos is actually LOWER than geostationary orbit, so the ribbon would move around as phobos moves around in orbit. Might not be a problem going down, it's actually even better- just wait until you're at the spot you want at- but if you wanted to get up to it you'd need a launcher that can get you up to speed relative to Mar's rotation. What would that be, a few hundred km/h?

How would you dock with the end of the ribbon and keep it stable if it cannot be anchored to the ground? It moves around the planet with Phobos, and you need to reach it with a craft that's also moving very quickly; does this make a dangling ribbon too difficult?


*slightly off topic*
I've always been partial to orbital rings. Think about it- you could keep a ring of material in orbit if you made it magnetic and put it inside a tube. Then just have maglev engines all over it to accelerate the magnetic particles at a tangent to the surface. Their force against the outside of the ring would keep it in orbit. You could also precess the ring to reach other locations on the surface. The ring would only need to be a few hundred km up. You could easily have multiple rings  at different orbital planes and withdraw the rope when they need to pass over one another. Much simpler and more economic than a single very very long elevator. The rope for a orbital ring would not need to be any stronger than kevlar.

I think we should seriously consider this on Mars, as there is an easy source of material in the form of Phobos. We would use the carbon and silicon to produce the tube structure, the volatiles to get the energy to move the required 25 million tonnes of this material into lower orbit around Mars, and any metals we find as the magnetic material inside the tube. The only real issues are a) building a massive mining, manufacturing, and transportation operation 75 million miles away, and b) inventing and building huge maglev engines that can operate in space with extreme reliability.

Credit for this idea goes to Paul Birch. Though he assumed it would be built around Earth.
http://www.orionsarm.com/fm_store/OrbitalRings-III.pdf

How about turning the "lemon" phobos into lemonade?

Use phobos as the initial source for an orbital ring on the orbit of phobos (Birch Ring), then extend down and generate a second ring at ~100km height, which can be used to theter down to the surface.

It is challenging, and it would be actually a good idea to completely consume phobos in that process.

This way, the stationary Mars-orbit is in reach, and you can even throw things onto an escape trajectory.

Isaac Arthur did a piece on the topic orbital rings, although I'd call his ideas (interplanetary orbital rings, etc) as very advanced, even for this section of the board (yet no out of reach technologies required, no anti gravity, no warp drives, not even room temperature superconductors).


Offline Hotblack Desiato

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Re: Space Elevator for Mars
« Reply #33 on: 07/01/2017 07:12 AM »
Phobos' orbit is the problem here. See my post above about easier mass to obtain for the counterweight.

Low excentricity, low inclination, yet a bit high up. Maybe it would be better to just take the material from phobos and bring it down to a 200km orbit and construct the ring over there. That would also mean, that Phobos doesn't need to be consumed in total.

think about the initial ring as a wire rope, just like the ones used in modern elevators, cranes, ski-lifts etc, and then when the initial one is done, go for a few more, up to the thickness of for example the rope used in the golden gate bridge.

And then, set up non-orbiting platforms on that ring which levitate magnetically on that ring (at some point, get the plattforms all the way around). With that done, you then can extend kevlar-ropes down to the surface, leading to an orbital ring that is suspended to the surface of Mars. Climb up and down those ropes to transport goods from and to the martian surface.

It will be tedious to set up such a structure, but it is easier to do that than to do an elevator with phobos crossing the elevator track.

Offline Hop_David

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Re: Space Elevator for Mars
« Reply #34 on: 07/01/2017 02:50 PM »
Phobos' orbit is the problem here. See my post above about easier mass to obtain for the counterweight.

Low excentricity, low inclination, yet a bit high up. Maybe it would be better to just take the material from phobos and bring it down to a 200km orbit and construct the ring over there. That would also mean, that Phobos doesn't need to be consumed in total.

think about the initial ring as a wire rope, just like the ones used in modern elevators, cranes, ski-lifts etc, and then when the initial one is done, go for a few more, up to the thickness of for example the rope used in the golden gate bridge.

And then, set up non-orbiting platforms on that ring which levitate magnetically on that ring (at some point, get the plattforms all the way around). With that done, you then can extend kevlar-ropes down to the surface, leading to an orbital ring that is suspended to the surface of Mars. Climb up and down those ropes to transport goods from and to the martian surface.

It will be tedious to set up such a structure, but it is easier to do that than to do an elevator with phobos crossing the elevator track.

Solid rings are not stable. That's what led Niven to write sequels to his Ringworld story. He didn't want an obviously flawed sci fi device so he added stories where the ring used stationkeeping rocket engines. And so it would be with a solid ring about a planet. There would be a constant expense to keep the ring from crashing into Mars.

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.

Your notion of using Phobos material to make a lower orbit station has merit though.

I recently revised my Phobos tether posts using a more sensible safety factor. Early versions using a safety factor of 1, I had concluded that a Phobos tether extending to Mars' upper atmosphere could be made with Zylon. But a Zylon tether that long is impractical when using a safety factor of three. See Lower Phobos Tether.

While a 5800 kilometer lower Phobos tether (Phobos to Mars upper Mars atmosphere) would take an impractical amount of Zylon, A 1,400 km tether has a nice .33 tether to payload mass ratio. This means a ten tonne Zylon tether can drop thirty tonne payloads to a periapsis just above Mars surface.

A less than 1,400 km Phobos tether could toss a payload to a tether in low Mars orbit. Given two coplanar orbital tethers there exists a ZRVTO between the two tethers. ZRVTO is short for Zero Relative Velocity Transfer Orbit.

I am thinking of examining a scenario where a low Mars orbit tether tosses stuff up to a Phobos tether as well as dropping payloads into suborbital paths to Mars surface.
« Last Edit: 07/01/2017 02:55 PM by Hop_David »

Offline Paul451

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Re: Space Elevator for Mars
« Reply #35 on: 07/02/2017 10:06 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.

Offline rakaydos

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Re: Space Elevator for Mars
« Reply #36 on: 07/11/2017 01:00 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.
My question on the Orbital Ring is, what about the gyroscopic torque?
You've got an inner ring spinning at greater than orbital velocity. great, it's holding up a geostationary ring tethered to the ground.
Except the geosynchronus ring is rotating once per day, and unless it's precisely equatorial, that's going to cause some pretty severe torque on the ring as you basically force a constant inclination change on the inner ring through the geostationary one.

Offline Paul451

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Re: Space Elevator for Mars
« Reply #37 on: 07/11/2017 09:46 AM »
My question on the Orbital Ring is, what about the gyroscopic torque?
You've got an inner ring spinning at greater than orbital velocity. great, it's holding up a geostationary ring tethered to the ground.
Except the geosynchronus ring is rotating once per day, and unless it's precisely equatorial, that's going to cause some pretty severe torque on the ring as you basically force a constant inclination change on the inner ring through the geostationary one.

AIUI, for anything except the simplest structure (by necessity an equatorial ring), you need counter-rotating rings to cancel out the torque.


Offline stefan r

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Re: Space Elevator for Mars
« Reply #38 on: 07/17/2017 01:24 PM »
Phobos' orbit is the problem here. See my post above about easier mass to obtain for the counterweight.
like the ones used in modern elevators, cranes, ski-lifts etc, and then when the initial one is done, go for a few more, up to the thickness of for example the rope used in the golden gate bridge.
...
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.  So the lift capacity of similar cranes would be around 9.6 e14.  So just under 12 years.  Was that a complement to New Orleans workers or a claim that New Orleans is uncivilized?

Setting up the equivalent to the Port of New Orleans near mars is not a trivial project.  But if we were motivated then the whole Phobos destruction project could be done within today's children's life expectancy.   

Phobos would lose mass as you move pieces off.  So it could be done several years sooner [no need for that].  You could consume the mass from the center out.  Build most of the ring.  Then move fairly large pieces along the ring and store in bags at the equivalent to Lagrange 4/5 points. 

One of the hardest obstacles is preventing a swarm of debris.  You would need to build containers out of Phobos material or sinter dust/sand/cobbles into stackable blocks. 

Offline spacenut

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Re: Space Elevator for Mars
« Reply #39 on: 07/17/2017 01:40 PM »
Why not just build a giant space station for transfer of goods and people to Mars and then to the surface.   Large in space tugs or spacecraft wouldn't have to land, just transfer by docking with the station.  The station could be placed in GSO of Mars or at a L station.  That way it would stay put with thrusters and cargo and people could be sent directly to a fixed point on the Martian surface. 

I don't know how large this station should be, but a gateway station could be built at L2 lunar.  Then smaller rockets would ferry goods and people to and from earth to L2, then a deep space transfer ship could go between this station and the one at Mars where the space elevator may work.  An earth space elevator would, at this time, would not be practical due to distance and heavy atmosphere.  One at Mars may work. 

By the way what is the distance from the surface of Mars to GSO at Mars?

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