Author Topic: Space Elevator for Mars  (Read 27079 times)

Offline Nilof

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Re: Space Elevator for Mars
« Reply #100 on: 01/03/2018 03:15 AM »
Quote
Climber & Rappeller Utility

Yes, a climber would need something other than chemical energy.  Even HVDC power lines are inefficient and unsuitable over MSE distances.  It may be that areosync PV + a superconducting tether power line will be needed, to power an MSE climber system with performance superior to Mars launch.

A cargo descent rappeller is more efficient than a climber:  it uses 0 kW.  :)  And it does so while saving the tremendous energy that would otherwise be devoted to propellant manufacture on Mars, for the cargo ship's surface launch.

There's utility in that, yes?

Mars surface to mars escape is 5.2 km/s. Mars surface to the foot of a 1400 lower phobos tether is 4.1 km/s. Going from Mars surface to Phobos the traditional way takes 5 km/s.

You need the really ambitious versions of the lower Phobos tether to gain a substantial benefit from the rest of the system compared to just burning from LMO with an Oberth benefits. Otherwise, you just spent most of your delta-v alternative cost budget to get to Phobos, and you save a few hundred m/s of delta-b budget at best.

So it really boils down to Mars SSTO vs Mars suborbital vehicle + beamed power climber hybrid vs mass driver.

I think a more likely use for a Rapeller could be with a 1400 km tether to drop cargo to LMO in order to build infrastructure there.
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline LMT

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Re: Space Elevator for Mars
« Reply #101 on: 01/03/2018 04:43 AM »
I think a more likely use for a Rapeller could be with a 1400 km tether to drop cargo to LMO in order to build infrastructure there.

As regards the rappeller, it's used more effectively on the Mars Lift as given.  Dropping off cargo at Arestation and rappelling it to Mars obviates the need for suborbital transfer craft launch and landing, and the associated propellant manufacture on Mars.  It's not hard to make the business case for Omaha Trail cargo, vis-a-vis the upper/lower tethered system. 

--

And yes, the budget required to reach a Phobos tether from Mars, for transit in either direction, is considerable.  It does seem to consume the desired savings of the upper/lower tethered system.  Also some practical issues of Phobos cargo delivery were noted in Dr. Lades' talk.

Offline Hop_David

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Re: Space Elevator for Mars
« Reply #102 on: 01/03/2018 03:20 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.

Okay so we've eliminated the enclosing ring continuously accelerating the projectiles

Then the projectiles flying at faster than orbital velocity would not be flying a nice circular orbit from one platform to another. You would have to aim the ricochet to hit the next platform. See illustration below

Holding the platforms aloft by continuously ricocheting machine gun bullets poses some engineering challenges. How far apart are the platforms? What precision is needed in directing the ricochets?

I can imagine some spectacular failure modes with this scheme as well.
« Last Edit: 01/03/2018 03:22 PM by Hop_David »

Offline Paul451

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Re: Space Elevator for Mars
« Reply #103 on: 01/03/2018 06:46 PM »
Okay so we've eliminated the enclosing ring continuously accelerating the projectiles
Then the projectiles flying at faster than orbital velocity would not be flying a nice circular orbit from one platform to another.
You would have to aim the ricochet to hit the next platform.

You might want to read up on orbital rings, you seem to be thinking of it as a passive tube. The concept is that the particles are magnetically accelerated (not "ricocheted") faster than orbital velocity. It's like a launch-loop, but around the entire world. It wouldn't work if the particles were just in a natural circular orbit. The centripetal acceleration of the particles keeps the useful structure aloft.

My point was that if you damage one of the magnetic guides, the others would need to redirect to compensate. But they'll need to do that anyway whenever parts of the system is under maintenance. And I suspect they'll be constantly doing that based on the natural variability in the ring anyway.

But explaining how it works is off-topic for this thread, especially zombied from back in August. If you want to dig around, there might be an existing thread in Advanced Concepts that would be more appropriately re-animated.

Offline Hop_David

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Re: Space Elevator for Mars
« Reply #104 on: 01/03/2018 08:43 PM »
It wouldn't work if the particles were just in a natural circular orbit.

That was my point. You had altered my illustration of a continuous circular ring. You removed the ring  but left the projectiles still sailing nicely along circular paths from one platform to another.

But the paths would look more like straight lines.

So rather than constant acceleration along a circular ring, you'd have discreet, abrupt accelerations at the corners of a large polygon.

Below I drew the projectiles being magnetically redirected from one platform to another.

My objections remain.

It would take very precise aiming to send a projectile from one platform to another thousands of kilometers away. The projectile would have to hit the exact center of the platform or it would set the platform spinning instead of halting and reversing it's downward fall.

In the case of earth based platforms 60 degrees apart and approximating the hyperbolic paths as straight lines, the change in vertical velocity component would be about the same the projectile speed. How fast are the projectiles moving? 10 km/s? 16 km/s? That's a very drastic velocity change in a very short time. How many g's would the projectile endure? How many g's to the platform? The pusher plate in the boom boom Orion vehicle comes to mind.

But explaining how it works is off-topic for this thread, especially zombied from back in August. If you want to dig around, there might be an existing thread in Advanced Concepts that would be more appropriately re-animated.

Isaac Arthur's scheme was off topic back in August as well (in my opinion). But I address the August arguments because Nilof just responded to them within the past few days. If you like, you can open a new thread on Isaac Arthur's orbital rings.

Offline LMT

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Re: Space Elevator for Mars
« Reply #105 on: 01/05/2018 06:27 AM »
No Tankers

Notably, one of the benefits of the Omaha Trail proposal is that cargo flights can be launched without dedicated tanker ships. 

No tankers at all.  Just returning cargo ships.

This could be especially beneficial to the construction phase of SpaceX's Mars City, which could take decades.  The work might go 10x faster at Omaha Crater, but still, it's a benefit.



Cargo flight staging. Deimos propellant. Mars Lift space elevator in gold.

Depot

That slide didn't garner comment, but it shows on-orbit depot use, an ambition at SpaceX and elsewhere.

Quote from: Elon Musk
By establishing a propellant depot in the asteroid belt or one of the moons of Jupiter, you can make flights from Mars to Jupiter, no problem.

Quote from: Charles Miller
...produce propellant [from water in the lunar soil]. You’d put that in a propellant depot in lunar orbit to allow humans to go anywhere in the solar system.

Here the depot is at Deimos, with propellant shuttled to LEO.  Typical #s, allowing notional 55 t propellant reserves:

(1) Burn 397 t of 1008 t propellant after DRL, for transit and LEO circularization.

(2) Launch cargo to LEO.

(3) Transfer 540 t in LEO.

(4) Burn 16 t for EDL.

(5) Burn 540 t for transit and Mars Lift Arestation circularization.

Option:  To double payload, (1) is performed with 1066 t propellant load, twice, filling tanks in (3).  A second Earth launch transfers the second cargo.  With full tanks (5) delivers 300 t to Arestation.

All is done without an Earth tanker fleet, tanker booster fleet, and their propellant.  Cost savings would follow efficiency. 
« Last Edit: 01/08/2018 01:21 AM by LMT »

Online meberbs

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Re: Space Elevator for Mars
« Reply #106 on: 01/05/2018 02:49 PM »
That slide didn't garner comment, but it shows in-orbit depot use, an ambition at SpaceX and elsewhere.
False. My previous post was directly discussing that slide. Your claims in it are wrong, and the claimed numbers you just provided make this easy to show.

Here the depot is at Deimos, with propellant shuttled to LEO.  Typical #s, allowing notional 55 t propellant reserves:

(1) Burn 397 t of 1008 t propellant after DRL, for transit and LEO circularization.
Not sure why you are limiting yourself to 1008 t instead of the 1100 that constitutes a full load of the tanks.
Also, I get that to just do the 1.55 km/s burn to get on an Earth trajectory it would take 399 t with your claimed propellant load and the 50 t cargo needed for this architecture to be compared to SpaceX's.

This does not include losses in transit or LEO circularization.

(2) Launch cargo to LEO.

(3) Transfer 540 t in LEO.

This allows you 4.385 km/s delta-v in total. (Cargo is 150 tons for direct comparison of course) Typically LEO to MTO takes 4.3 km/s on its own, and that is not for fast transfers. If you want to get straight back to Deimos like you show, you would need to burn the same 0.94 km/s to circularize at Deimos that the rail launcher gave you on the way out, and that is assuming you accurately hit the aerocapature, getting into the exact desired capture orbit and have very accurate timing of the transfer as well.

(4) Burn 16 t for EDL.
I'm curious where you got this number, I don't think anyone outside SpaceX has a good idea how much fuel is needed forth this on BFR, there are too many unknowns.

Option:  To double payload, (1) is performed with 1066 t propellant load, twice, filling tanks in (3).  A second Earth launch transfers the second cargo.  With full tanks (5) delivers 300 t to Arestation.
Not going to bother with this since it  obviously just has larger problems than the first plan.

Note that the extra 92 tons I mentioned that you could start with would also cause an extra 25 t to be lost in the initial burn from Earth, and then with the losses in transit and LEO circularization, this will not come close to rescuing your plan.

All is done without an Earth tanker fleet, tanker booster fleet, and their propellant.  Cost savings would follow efficiency.
SpaceX is planning to fully fill up the ship's fuel before departing for Mars. They aren't doing this for no reason, so it should be obvious that your plan to only half fuel the ship will not work. None of the claimed benefits would emerge.
« Last Edit: 01/05/2018 02:50 PM by meberbs »

Offline LMT

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Re: Space Elevator for Mars
« Reply #107 on: 01/06/2018 09:01 PM »
Cargo Depot Redo

False. My previous post was directly discussing that slide.

Your architecture expects you to then fully fuel that other ship for it to transfer to Mars

compare with SpaceX's architecture  apples to apples, 50 tons return to Earth

No, you didn't mention the slide, and your text is not consistent with that flight plan.

There's no need to "fully fuel" cargo ships in LEO.  That's required only for crewed missions, where delta-v must be maximized.  Different flight plan.

Also there's no cargo on the return flight.  These cargo ships never land on Mars.  So unless you assume a booming market for Deimos cinder block at Home Depot, return cargo is naturally 0 t.

To discuss a particular flight plan, paste the image and reference the numbered steps; that'll keep your text clear.

Not sure why you are limiting yourself to 1008 t

Because that's the minimum needed.  We add more for options, such as doubled payload.

to just do the 1.55 km/s burn to get on an Earth trajectory it would take 399 t with your claimed propellant load and the 50 t cargo needed for this architecture to be compared to SpaceX's.

This does not include losses in transit or LEO circularization.

As we noted previously, perihelion requires ~1.55 km/s, aphelion requires ~0.8 km/s.  "Typical" #s are intermediate, not the extremes.  For example, 1.16 km/s.   

Then we tacked on 0.5 km/s for circularization.  (Your 0.94 km/s reasoning isn't right, and typical #s are smaller.)

As for losses, long-duration tankage scenarios expect some ZBOT method.  Any further loss would get some of the 92 t left at Deimos.

I'm curious where you got this number, [Burn 16 t for EDL.]

ITS drops 99%+ of KE aerodynamically, leaving only a landing burn.  We used a 40 s hover.

I don't think anyone outside SpaceX has a good idea how much fuel is needed forth this on BFR

"Fuel"?  "Propellant".  :)  Redo the cargo flight plan to see how the numbers improve.

Online meberbs

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Re: Space Elevator for Mars
« Reply #108 on: 01/06/2018 11:32 PM »
No, you didn't mention the slide, and your text is not consistent with that flight plan.
I shouldn't have to explicitly mention the slide, it should be obvious that is what I was discussing, and it follows your mission plan exactly, other than pointing out that you have entirely insufficient fuel. 

There's no need to "fully fuel" cargo ships in LEO.  That's required only for crewed missions, where delta-v must be maximized.  Different flight plan.
Wrong, please read the numbers in the post I just made again. You do not have anywhere near enough fuel for what you are proposing. I suggest you actually think about the fact that SpaceX is not planning to use fewer tankers for cargo vs. crew flights and why this is their plan.

Also there's no cargo on the return flight.
Wrong, look at SpaceX's architecture again.

These cargo ships never land on Mars.  So unless you assume a booming market for Deimos cinder block at Home Depot, return cargo is naturally 0 t.
Your architecture still involves transport capability from Mars surface, no reason cargo could only come from Deimos.

Not sure why you are limiting yourself to 1008 t

Because that's the minimum needed.  We add more for options, such as doubled payload.
Except as I showed it is nowhere near enough. Doubled payload would fail even with full tanks.

to just do the 1.55 km/s burn to get on an Earth trajectory it would take 399 t with your claimed propellant load and the 50 t cargo needed for this architecture to be compared to SpaceX's.

This does not include losses in transit or LEO circularization.

As we noted previously, perihelion requires ~1.55 km/s, aphelion requires ~0.8 km/s.  "Typical" #s are intermediate, not the extremes.  For example, 1.16 km/s.
Your architecture is basically useless unless it works every window, not just when you get an easy window. You need to plan for the worst case.

Then we tacked on 0.5 km/s for circularization.  (Your 0.94 km/s reasoning isn't right, and typical #s are smaller.)
The 0.5 km/s should be conservative for circularization at Earth after aerocapture, however as my calculation showed, you actually allowed less than 0 km/s for this.

The 0.94 number has nothing to do with the LEO circularization, and everything to do with the fact that you used 0.94 km/s to get from Deimos to an elliptical orbit with a periapsis close to Mars. In the best case scenario, that is also the orbit you would end up in after an aerocapture maneuver (actually you would need a lower periapsis to be deep enough in the atmosphere, but that just makes things worse for you). Circularizing that elliptical orbit to match Deimos takes exactly the same delta-v as entering that orbit from Deimos took in the other direction.

As for losses, long-duration tankage scenarios expect some ZBOT method.  Any further loss would get some of the 92 t left at Deimos.
Unfortunately for you, SpaceX is not using ZBOT, and their method of reducing boil off is to vent the outer tanks for greatly improved insulation. You can't do this, since you need the fuel that is in the outer tanks.

I'm curious where you got this number, [Burn 16 t for EDL.]

ITS drops 99%+ of KE aerodynamically, leaving only a landing burn.  We used a 40 s hover.
99% KE is for direct landing on Mars from MTO. Your use of it in this case is simply ignoring the deorbit burn entirely. It also isn't a number that is necessarily very exact. The 40s hover that you guessed doesn't mean much on its own. It depends on how many engines at what throttle.

Redo the cargo flight plan to see how the numbers improve.
No, you redo the numbers without using assumptions that contradict your architecture, and without assuming best case transfer windows, and without ignoring the fuel needed to circularize at Deimos.

Offline LMT

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Re: Space Elevator for Mars
« Reply #109 on: 01/10/2018 06:41 AM »
No, you didn't mention the slide, and your text is not consistent with that flight plan.
I shouldn't have to explicitly mention the slide, it should be obvious that is what I was discussing, and it follows your mission plan exactly, other than pointing out that you have entirely insufficient fuel.

Not at all.  The given, specific cargo flight plan has no requirements or assumptions for full LEO tanks or 50 t return cargo -- and of course in this flight plan there's no Mars landing for a 50-t cargo pickup anyway. 

So our mass and delta-v numbers work as given, with corresponding efficiency improvements over SpaceX baseline.  The numbers shouldn't be very surprising, especially given the familiarity of the route and spacecraft.

Then we tacked on 0.5 km/s for circularization.  (Your 0.94 km/s reasoning isn't right, and typical #s are smaller.)
The 0.94 number has nothing to do with the LEO circularization, and everything to do with the fact that you used 0.94 km/s to get from Deimos to an elliptical orbit with a periapsis close to Mars. In the best case scenario, that is also the orbit you would end up in after an aerocapture maneuver (actually you would need a lower periapsis to be deep enough in the atmosphere, but that just makes things worse for you). Circularizing that elliptical orbit to match Deimos takes exactly the same delta-v as entering that orbit from Deimos took in the other direction.

That reasoning is quite wrong.  The delta-v coming off the DRL by no means sets the delta-v for aerocapture circularization. 

For one thing, the DRL launch vector is certainly not the most efficient for lowering periapsis; so it's not the delta-v that one must "take".  That's easy to see with the example GMAT script: tweak the DRL VNB to reach the same periapsis using much less than 0.94 km/s delta-v.

Example vectors:

GMAT TOI.Element1 = 0.5;
GMAT TOI.Element2 = 0;
GMAT TOI.Element3 = 0.5; 

GMAT TOI.Element1 = 0.64;
GMAT TOI.Element2 = 0;
GMAT TOI.Element3 = 0;

Second, the mission literature shows you what's actually needed for circularization.  In one Lockheed Martin flight plan the spacecraft changes plane and circularizes to Deimos with a pair of burns totaling 607 m/s delta-v.



In an ITS cargo plan MOI is aerocapture, with aerodynamic plane change.  This removes propulsive plane change, lowering the burns for Arestation down toward 0.5 km/s.

But the rocket equation and orbit changes are, by themselves, OT.  Focus should be on possibilities for the Mars Space Elevator and related tether systems.

Online meberbs

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Re: Space Elevator for Mars
« Reply #110 on: 01/10/2018 08:41 AM »
Not at all.  The given, specific cargo flight plan has no requirements or assumptions for full LEO tanks or 50 t return cargo -- and of course in this flight plan there's no Mars landing for a 50-t cargo pickup anyway. 
50 tons return cargo is part of the SpaceX baseline you claim to be comparing to, you can't just throw out capability and claim your system is equivalent but more efficient.

Full LEO tanks isn't an assumption, it is a fact of what SpaceX has presented as their plan.

So our mass and delta-v numbers work as given, with corresponding efficiency improvements over SpaceX baseline.  The numbers shouldn't be very surprising, especially given the familiarity of the route and spacecraft.
No, your numbers don't work. You have simply ignored most of what I wrote, and have made no attempt to do things like update your numbers to account for worst case transfer windows.

Then we tacked on 0.5 km/s for circularization.  (Your 0.94 km/s reasoning isn't right, and typical #s are smaller.)
The 0.94 number has nothing to do with the LEO circularization, and everything to do with the fact that you used 0.94 km/s to get from Deimos to an elliptical orbit with a periapsis close to Mars. In the best case scenario, that is also the orbit you would end up in after an aerocapture maneuver (actually you would need a lower periapsis to be deep enough in the atmosphere, but that just makes things worse for you). Circularizing that elliptical orbit to match Deimos takes exactly the same delta-v as entering that orbit from Deimos took in the other direction.

That reasoning is quite wrong.  The delta-v coming off the DRL by no means sets the delta-v for aerocapture circularization. 

For one thing, the DRL launch vector is certainly not the most efficient for lowering periapsis; so it's not the delta-v that one must "take".  That's easy to see with the example GMAT script: tweak the DRL VNB to reach the same periapsis using much less than 0.94 km/s delta-v.

...

Second, the mission literature shows you what's actually needed for circularization.  In one Lockheed Martin flight plan the spacecraft changes plane and circularizes to Deimos with a pair of burns totaling 607 m/s delta-v.
I just used the number you provided for simplicity, assuming that you had bothered to orient the vector in close to optimal direction. I figured that a maneuver like in the picture you posted could reduce the delta-V some, but the order of magnitude is still the same. As I showed you barely have enough fuel transferred to get to MTO from LEO (using your number for transferred fuel rather than a corrected value.) You would have less than 100 m/s remaining in a typical case, so it doesn't matter whether you need 600 or 900 m/s for the last step, my point remains the same.

But the rocket equation and orbit changes are, by themselves, OT.  Focus should be on possibilities for the Mars Space Elevator and related tether systems.
You brought up your plan for supposedly improving on the BFR system using your tether architecture for Deimos-Mars transport. Pointing out that your numbers don't add up is on topic if your architecture is. (And if not your architecture can be split off into a separate topic.)

You have simply ignored multiple of the points I made about the flaws in your plan and analysis. I have stuck to ones provable with straightforward numbers, no point in bringing up the others when you can't recognize these.

Offline stefan r

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Re: Space Elevator for Mars
« Reply #111 on: 01/10/2018 08:41 PM »

(1) Burn 397 t of 1008 t propellant after DRL, for transit and LEO circularization.


Why LEO circularized?  The shuttle already has a tether attachment.  Seems like a waste of momentum. 

Offline LMT

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Re: Space Elevator for Mars
« Reply #112 on: 01/12/2018 04:25 AM »
I just used the [DRL] number you provided for simplicity, assuming that you had bothered to orient the vector in close to optimal direction.

?  The DRL vector isn't remotely "close to optimal direction", obviously.  You don't seem to understand VNB impulse.  You might "look it up," as you put it.  OT here.

The given, specific cargo flight plan has no requirements or assumptions for full LEO tanks or 50 t return cargo -- and of course in this flight plan there's no Mars landing for a 50-t cargo pickup anyway. 
50 tons return cargo is part of the SpaceX baseline you claim to be comparing to, you can't just throw out capability and claim your system is equivalent but more efficient.

Full LEO tanks isn't an assumption, it is a fact of what SpaceX has presented as their plan.

Oh, it's fair comparison because equivalent and properly calculated.  Same ship, same cargo, out and back, with and without Omaha Trail infrastructure.  "150 out / 0 back" is naturally the baseline ITS requirement.   

Each variation on cargo and crewed flights is calculated and compared separately.  We don't mix willy-nilly.

--

SpaceX hasn't proposed an Omaha Trail themselves.  But if tomorrow SpaceX proposed it under their own logo, I'm pretty sure no one would say, "But this isn't the plan!  They can't claim this is more efficient!"  More likely folks would brainstorm ways to deliver tunnel-boring machines and Tesla Roadsters by elevator.  ;)

Quote from: Rob Manning, Universe Today
"Mars is really begging for a space elevator," said Manning. "I think it has great potential. That would solve a lot of problems, and Mars would be an excellent platform to try it."

Online meberbs

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Re: Space Elevator for Mars
« Reply #113 on: 01/12/2018 06:45 AM »
I just used the [DRL] number you provided for simplicity, assuming that you had bothered to orient the vector in close to optimal direction.

?  The DRL vector isn't remotely "close to optimal direction", obviously.  You don't seem to understand VNB impulse.  You might "look it up," as you put it.  OT here.

I already said why I was wrong and why it doesn't matter, but let me rephrase since you seem to have not read my post.

I didn't bother looking at what vector you used in what frame, so the issue has nothing to do with my understanding of the frame. The 940 m/s seemed to be (and is, your next post showed that in a practical example, the answer is around 600 m/s) the correct order of magnitude, so I took the path of not doing more math, since my experience told me that the number was in the correct ballpark.

The real issue that you are failing to address is that you don't have the available delta V for any of these trajectories.

The given, specific cargo flight plan has no requirements or assumptions for full LEO tanks or 50 t return cargo -- and of course in this flight plan there's no Mars landing for a 50-t cargo pickup anyway. 
50 tons return cargo is part of the SpaceX baseline you claim to be comparing to, you can't just throw out capability and claim your system is equivalent but more efficient.

Full LEO tanks isn't an assumption, it is a fact of what SpaceX has presented as their plan.

Oh, it's fair comparison because equivalent and properly calculated.  Same ship, same cargo, out and back, with and without Omaha Trail infrastructure.  "150 out / 0 back" is naturally the baseline ITS requirement.   
Only if you ignore SpaceX's stated cargo return capability, so no that is not the baseline. When you are comparing to SpaceX the baseline is what SpaceX stated, 150 out and 50 back, not something you made up.

Maybe you didn't understand their presentation, they are using basically the same ship for cargo or crew the capability is the same either way.

SpaceX hasn't proposed an Omaha Trail themselves.  But if tomorrow SpaceX proposed it under their own logo, I'm pretty sure no one would say, "But this isn't the plan!  They can't claim this is more efficient!"  More likely folks would brainstorm ways to deliver tunnel-boring machines and Tesla Roadsters by elevator.
SpaceX wouldn't propose what you have themselves because the numbers don't work out. If they did people would add up the numbers and point out that fact. (This has already happened on this site, notably in L2 when people added up some numbers and determined the BFR rocket dimensions did not make sense. It turns out one of the numbers was out of date or miscommunicated, and their prediction came close to the actual dimension)

Quote from: Rob Manning, Universe Today
"Mars is really begging for a space elevator," said Manning. "I think it has great potential. That would solve a lot of problems, and Mars would be an excellent platform to try it."
Mars could be a great location for a space elevator, but your proposed use of it in combination with BFR simply does not work the way you claim it does.

Please stop ignoring all of the problems I have pointed out.

Offline alexterrell

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Re: Space Elevator for Mars
« Reply #114 on: 01/28/2018 07:16 PM »
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.


If you wanted to move Phobos to a higher orbit, you would build an elevator to near Mars, and lower a substantial fraction of Phobos' mass to a release point. I'm not sure what the Coriolis force would do dynamically to the cable though.

As it's lowered, Phobos would rise up. If it's released as small particles, they descend harmlessly to the surface.

Still a huge amount of mass to move - perhaps enough to build a O'Neil type cylinder inside Phobos. Or perhaps it';; be needed to prevent Phobos disintegration if a lot of material is being exported in other direction. (Either towards escape, or to build MSO colonies and mirrors) 

Offline LMT

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Re: Space Elevator for Mars
« Reply #115 on: 03/06/2018 03:27 AM »
The real issue that you are failing to address is that you don't have the available delta V

No, our very first thread presentation gave the novel propellant and delta-v numbers for the Omaha Trail.  Anyone could plug those numbers into the rocket equation, to compare against famous Earth/Mars baseline, and verify.  That much is easy.

You, Paul451 and Lar imagined otherwise, and ignored the presentation numbers. 

--

The Pendulum

If we want to drive the bus, we have to gas it up.  And if we want to justify a LEO propellant depot, we have to deliver propellant to LEO with great efficiency.  The most efficient system wins, all things being equal.

A DRL improves the efficiency of Deimos ISRU by increasing the delivered propellant load, but DRL benefit is limited by the Deimos / L2 tether launch geometry.  A DRL delta-v above 1 km/s intercepts Mars; course correction is needed, requiring propulsive maneuver at Mars, and reducing delivered load.  This caps net efficiency. 

An EM DRL delta-v of 1.9 km/s could, if vectored optimally, execute Earth-return without rocket propulsion.  Getting that vector is the first trick. 

GMAT modeling indicates that the 1.9 km/s DRL launch must be angled about 15 degrees off the Deimos / L2 line in order to pass above Mars and intercept Earth. 

How to get the 15-degree launch angle?

The DRL terminates at a counterweight.  Tapping the counterweight sets up an oscillation.  It's visually similar to a pendulum, but it differs in that varying gravitational and centripetal accelerations act on the counterweight and tether.  We can view the system as a "quasi-pendulum". 

Goal:

If this quasi-pendulum can be designed to give an oscillation period that's in resonance with Deimos' orbit, the launch window can be scheduled at the top of the pendular swing, where the tether's angle is at maximum.  The angle (swing amplitude) required for Earth-return is about 15 degrees.

Method:

A 2-D physical simulation of the Mars/Deimos system was used, with DRL tether and counterweight.  Properties of the tether, counterweight and perturbation force were systematically adjusted, to produce a table of quasi-pendulum oscillations.  The table was examined for useful swing amplitudes and swing/orbit resonances.

Result:

A DRL tether with length of 2500 km responds to a 60 m/s transverse perturbation of the counterweight by entering a quasi-pendulum oscillation having amplitude of 15 degrees.  This is the required Earth-return angle, and the 2500-km length is more than adequate for gentle DRL acceleration producing the required 1.9 km/s delta-v.



Oscillation period is about 20 hours.  This sets a 3:2 resonance with Deimos' 30.3-hour orbit.

In consequence, the DRL is aligned for Earth-launch every 2 Deimos days.  That is, the launch window opens every 60.6 hours.

Each pair of ITS tankers launching on the pendular DRL aerobrakes at Earth with tanks full.  This represents a nearly ideal efficiency of delivery to the LEO depot.

Online meberbs

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Re: Space Elevator for Mars
« Reply #116 on: 03/06/2018 02:20 PM »
The real issue that you are failing to address is that you don't have the available delta V

No, our very first thread presentation gave the novel propellant and delta-v numbers for the Omaha Trail.  Anyone could plug those numbers into the rocket equation, to compare against famous Earth/Mars baseline, and verify.  That much is easy.

You, Paul451 and Lar imagined otherwise, and ignored the presentation numbers. 
That presentation is obviously from before IAC 2017, so the numbers in it are inconsistent to begin with.

We have plugged the numbers in for you in this thread. They do not add up even ignoring things like boil off.

An EM DRL delta-v of 1.9 km/s could, if vectored optimally, execute Earth-return without rocket propulsion.  Getting that vector is the first trick. 
Just to be clear here, you are proposing a 2500km 370 km long rail launcher connected to a 2500km teather, which will have to support itself under variable gravity gradients. This could only support 1 launch every 2.5 days, so a maximum of about 25 launches assuming a 2 month window. (Feel free to show the calculations if you think you could squeeze more in than that per alignment, remembering that you need to calculate dV needed for worst case Mars-Earth alignments)

By the time there is resources at Mars for a project on that scale, we will want more than 30 BFS per year of combined cargo and crew.

There is still the problem that even if this does allow you to leave Mars with no spent propellant, you still haven't even taken a guess at how much boil off will occur.

Edit: calculated it and realized that you weren't talking about the launcher itself being 2500km long, it would "only" be 370 km for 0.5 g acceleration

Added: It is good that you are working on ideas to fix the problems that you had previously with your plan, but you would have more credibility doing so if you actually acknowledged the existence of the problems, rather than dismissing them with an irrelevant reference. As it is, your history of making best (or near-best) case assumptions makes me question if the 1.9 km/s would even work for all transfer windows, and what kind of transfer times it allows.
« Last Edit: 03/06/2018 04:48 PM by meberbs »

Offline LMT

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Re: Space Elevator for Mars
« Reply #117 on: 03/07/2018 04:22 AM »
This could only support 1 launch every 2.5 days, so a maximum of about 25 launches assuming a 2 month window...
By the time there is resources at Mars for a project on that scale, we will want more than 30 BFS per year of combined cargo and crew.

?  The DRL is always shown with paired launches, for balance; it's twice the launch rate you imagined. 

That's the rate for one pendular DRL -- and there's no cosmic limit of one.

i.e., roll out another.   ::)

--

Steam Engine

The envisioned pendular DRL delivers full tanks to Earth aerobrake.  However some propellant is used after aerobrake, for final orbit circularization to LEO depot. 

Q:  Can even that small burn be eliminated, to achieve perfect delivery efficiency?

Consider cargo flights: 



On the Omaha Trail, return flights (1) load methalox at Deimos Dock.  As cargo ships unload only at Arestation (5), they have no return cargo.

However one cargo can be loaded at Deimos Dock:  water. 

Water can act as a safe supplemental propellant.  A tiny water resistojet can be integrated into the water piping, to give a supplemental steam impulse.

Quantifying the challenge:  it's circularization from low aerobrake elliptical orbit down to LEO depot, using a "steam engine":

- Delta-v:  500 m/s

- ISP: 200 s

- Mass without water:  1185 t

Can an ITS cargo ship load enough water to do the job, and achieve perfect delivery efficiency?  We plug numbers into the rocket equation and find...


Online meberbs

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Re: Space Elevator for Mars
« Reply #118 on: 03/07/2018 06:00 AM »
This could only support 1 launch every 2.5 days, so a maximum of about 25 launches assuming a 2 month window...
By the time there is resources at Mars for a project on that scale, we will want more than 30 BFS per year of combined cargo and crew.

?  The DRL is always shown with paired launches, for balance; it's twice the launch rate you imagined. 
See slide 37 of the presentation you just linked. I can see now how you may have intended that to be 2 perspectives, with both being dual launch, but I never would have guessed that based on the BFR not being rotated in the images. I haven't seen you state with words that it is always dual launches before now. You can't expect people to magically get that knowledge from simplistic sketches.

That's the rate for one pendular DRL -- and there's no cosmic limit of one.

i.e., roll out another.   ::)
It is a major infrastructure project to just build 1. That part of the infrastructure needing to be duplicated for additional launch rates has a notable impact on your design feasibility.

The envisioned pendular DRL delivers full tanks to Earth aerobrake.
It doesn't matter what you envision, reality says that you will have lost significant propellant to boil off, and will lose quite a bit more before the propellant gets used. BFS does not have ZBOT, and it does not even have low boil off except when the outer tanks are empty.

As cargo ships unload only at Arestation (5), they have no return cargo.
As I said before, return cargo is part of the SpaceX architecture you are comparing to. If you want a fair comparison you don't get to eliminate that.

Offline LMT

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Re: Space Elevator for Mars
« Reply #119 on: 03/07/2018 06:36 PM »
I haven't seen you state with words that it is always dual launches before now.

The pairing is sensible and illustrated clearly in presentation, even with power and energy numbers to quantify launch of the tanker pair.  That's easy to see, and it's reiterated on this very page, viz:

Quote from: LMT
Each pair of ITS tankers launching on the pendular DRL aerobrakes at Earth...

--

We have plugged the numbers in for you in this thread. They do not add up

"We" have?  :)  No, you haven't demonstrated, because our numbers are ok.  Also you've struggled with spaceflight basics here, meberbs.  So don't repeat an untrue statement, but make corrections before moving forward.

--

Quote from: LMT
Can an ITS cargo ship load enough water to do the job, and achieve perfect delivery efficiency?  We plug numbers into the rocket equation and find...

344 t. 

And yes.

--

Hop_David, haven't heard from you.  You know tethers.  Got any rope tricks we might try, out on the Trail?



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