Balloon Landing System from Phobos to Mars

SpaceNut · 2020-05-15 18:31:06

The tether from Phobos will whip around it as it spins as its anchored to the moon and what we need is a slide ring for the attachment so that it will extend out straight and fall towards the planet instead as the moon spins so as to not whip the balloon around like a pail of water on a string....

tahanson43206 · 2020-05-15 19:55:54

For SpaceNut re #26

You have a talent for asking thought provoking questions. Your word picture of the behavior of the tether is certainly provocative.

Having ** just ** arrived at a resolution of the earlier challenge you and GW Johnson posed, I am going to take an overnight breather before tackling this latest one. All I know for sure is that the moon is tidally locked to Mars, so it definitely spins with respect to the Sun. I'm unsure of how to think about the moon's relationship to Mars itself, since it always points the same face toward the planet, just as our Moon always points the same face toward Earth (with a bit of wobble).

The revolution of the moon around Mars is given by our friends at Wikipedia as:

7 hours and 39 minutes
Phobos orbits 6,000 km (3,700 mi) from the Martian surface, closer to its primary body than any other known planetary moon. It is so close that it orbits Mars much faster than Mars rotates, and completes an orbit in just 7 hours and 39 minutes.
Phobos (moon) - Wikipedia

The question you have indirectly asked is: What will happen to a tether that is stretched out from Phobos?

I would expect the default condition would be to collapse back on Phobos, if the tether is shorter than the distance to the Mar/Phobos L1 point.

Thinking further, I would expect the tether to collapse back on Phobos if the tether is less than twice as long as the Mars/Phobos L1 point.

However, if the tether is ** longer ** than twice as long as the Mars/Phobos L1 point, then I would expect the gravity of Mars to exert supremacy over the gravity of Phobos, so that the tether would become tidally locked just as the moon is.

In that case, the tether would perform a stately 7 hour + rotation about Mars, always pointing directly at the point on Mars which lies on the axis between the centers of gravity of the planet and its moon.

That said, I need to let this interesting question simmer or bake or whatever questions like this do.

(th)

tahanson43206 · 2020-05-15 20:02:20

For SpaceNut re #27

This post is a continuation of Post #27, which was inspired by your interesting post #26

I decided to make this a separate post because I was surprised by what I found when I went looking for the Phobos/Mars l1 point.

About 37,300 results (0.83 seconds)
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Image result for where is the mars phobos l1 point
The Mars Phobos L1 is a little more than 3 kilometers above Phobos' surface.
How Many Martian Lagrange points are there? ...And are they ...
space.stackexchange.com › questions › how-many-mart...
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Phobos L1 Operational Tether Experiment (PHLOTE) | NASA
www.nasa.gov › niac › 2017_Phase_I_Phase_II › PHL...
Apr 6, 2017 - 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.

To begin with ... it would appear that a tether twice 3.1 kilometers long would "just" hang in the sky (of Phobos) pointing at Mars, with tension equally divided between the gravity of Mars and the gravity of Phobos.

A tether longer than 6.2 kilometers would (presumably) experience a greater pull from Mars, and therefore would reliably hang vertically in the sky (from the perspective of Phobos) pointing forever at the center of Mars. It would be a 6 kilometer long plumb bob, without the bob.

However, what is ** really ** interesting is to learn that NASA was thinking about a tether from Phobos in 2017.

SpaceNut ... this investigation is becoming more and more interesting as we (and I ** do ** mean we) go along.

Apparently NASA was thinking about making an instrument package the "bob" for their plumb bob.

Edit#1: What this means (NASA Plumb Bob) is that there is the nucleus of a collaboration lurking in that 2017 initiative.

I am proposing using a tether as a way to allow a balloon Mars lander to safely (and slowly) make its way toward Mars from Phobos, rather than accelerating as was considered in the earlier series of posts, which resulted in discovery that the velocity of the package would be in excess of 6 kilometers per second.

The slow descent of the package from Phobos towards Mars could be used to generate electricity (somehow).

I'm not thinking about a space elevator here (just in case someone zooms ahead to consider that). Instead, I am looking for a tether ** just long enough ** to let the balloon package loose at an altitude that its balloon envelope material can safely navigate. At this point, I have no idea what that elevation might be, because the temperature tolerance of the material is not known.

This inquiry has a way to run before all the unknowns have been identified, let alone values found for them.

Edit#2: However, I am feeling increasingly confident that the NewMars forum, having started from the vision of Void (Void's Conjecture) or [The Conjecture of Void], is meandering its way toward a safe, reliable, cost-effective means of transport of personnel and cargo between Phobos and Mars.

Edit#3: For a reader who is visiting the forum for the first time, and chanced upon this post, it is possible to find Void's Conjecture by entering into the search window of the forum main page the expression: VoidsConjecture

I recommend selecting Posts rather than Topics for a more precise search.

(th)

Last edited by tahanson43206 (2020-05-15 20:26:38)

tahanson43206 · 2020-05-15 20:57:14

For SpaceNut ...

Here's a long shot!

Dear Mr. Kempton:
Thank you for your description of the very interesting tether experiment proposed in 2017.
I am writing as a member of a discussion group considering using a tether to slowly drop payloads from Phobos to Mars.
We are corresponding at NewMars.com/forums, and I am participating as tahanson43206
The forum is an activity of the Mars Society, founded by Dr Robert Zubrin.
Dr. Zubrin (and the Society) fund the hardware and staff support for the forum, but otherwise it operates without direct interaction with the Society.
The NASA proposal to suspend an instrument package on the Mars side of the Phobos/Mars L1 point was discovered after one of the participants (SpaceNut) asked what would happen to a tether fixed to Phobos. SpaceNut rather imaginatively speculated the tether might "whip" about Phobos. I rather suspected the pull of Mars would help to prevent "whipping about", but was greatly relieved to find the 2017 NASA study proposal, which confirmed the stability which might be expected if a tether is deployed for more than 6 kilometers.
The purpose if ** this ** message is to invite your support of development of a proposal to safely, securely, reliably and cost effectively deliver personnel and freight to the surface of Mars, using Phobos as a staging point, a tether as a mechanism to drop a transport package toward Mars, and a hydrogen filled balloon as a major component of the delivery package.
The entire set of ideas arose when one of the more creative members of the forum (Void) speculated that a "bubble" could be used to deliver gas to the surface of Mars from orbit.
The need for hydrogen as a component of a fuel to be manufactured on Mars for safe return of a lander (see Zubrin/The Case for Mars) is the reason for focus on Hydrogen to be delivered. The selection of Hydrogen led to consideration of the buoyancy of Hydrogen with respect to the atmosphere of Mars, and through a series of steps the group arrived at a vision of a sturdy lander capable of carrying passengers and cargo, suspended under a hydrogen envelope whose fabric would be chosen to serve as the exterior of structures to be built on the surface of Mars.
A collaboration with NASA, for the purpose of developing a safe, reliable, cost-effective landing system for Mars would be beneficial to the Mars Society.
Perhaps the collaboration would also be of benefit to NASA, although that may be on the speculative side. NASA has many years and billions of dollars invested in traditional approaches.
With best regards,
tahanson43206
NewMars forum
Begin Quotation:
April 6, 2017
Phobos L1 Operational Tether Experiment (PHLOTE)
Kevin Kempton
NASA Langley Research Center
End Quotation.

(th)

tahanson43206 · 2020-05-15 21:01:17

SpaceNut ... for the Record:

Program Element Manager
Game Changing Development Program
Space Technology Mission Directorate
kevin.s.kempton@nasa.gov

BIOGRAPHY
BIOGRAPHY
PROJECTS
Kevin Kempton received his BSE from the University of Central Florida and, prior to joining NASA in 2001, led a large software test team at Warner Robins AFB. While working as a project element manager in the Game Changing Development Program Office, he has also recently proposed and led the Mars Ice Home habitat design study through the Center Innovation Fund as well as a NASA Advanced Innovative Concepts Study called PHLOTE. Prior to joining GCD, Mr. Kempton managed the Autonomous Landing and Hazard Avoidance Technology (ALHAT) Sensor Development at NASA’s Langley Research Center in Hampton, Va. While on ALHAT, Mr. Kempton planned, managed and executed the integration and flight test campaigns that led up to the successful flight test of the ALHAT system on the Morpheus vehicle at Kennedy Space Center. Prior to ALHAT, Mr. Kempton was the lead systems engineer for the CLARREO Climate Mission Development and was lead systems engineer and verification manager for the Ares I-X demonstration flight test. Mr. Kempton was awarded the NASA Exceptional Achievement Award as well as the Space Flight Awareness Award for leadership and innovations in systems engineering. Earlier Mr. Kempton was the project manager for the Reconfigurable Scaleable Computer Project as well as the Software Manager for the Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) Mission. Mr. Kempton has developed many software applications and tools for project management, systems engineering and test management.

(th)

tahanson43206 · 2020-05-16 07:09:05

For SpaceNut re this topic ...

At this point, I'd like to acknowledge the heavy lifting done by GW Johnson, to compute the DeltaV of 1.88 kilometers per second to dock with Phobos.

Void (of course) is the member who inspired this entire sequence of posts with his vision of a bubble delivering gas to Mars.

Your assistance has been helpful at numerous points, with your search skills and thought provoking questions.

What lies ahead is a ** lot ** of fill-in-the-gaps work by as many members as can spare the time.

It came to me overnight that the pull of gravity of Mars on the tether payload package can be harnessed if the tether is wrapped around a drum which is coupled to a suitably sturdy generator and energy storage facility.

The optimum length and strength of the tether remains to be determined.

The material to comprise the balloon envelope remains to be selected. It needs to be able to tolerate heating by the atmosphere of Mars after the balloon descent package is released, and subsequently to serve for many Mars years as the exterior covering for Quanset style structures on the surface.

The structure of the passenger and cargo compartments needs to be designed for application in construction on Mars.

The only piece that I am seeing right now as possibly being returned to Phobos is the electronics flight control package, which might be too specialized to find work on Mars itself. However, a general purpose computer might be adaptable for both controlling the landing, and subsequently, for controlling manufacturing processes or perhaps habitat systems.

Organization of human activity to support the funding, design, installation and operation of the landing system remains very much available as an activity.

(th)

Last edited by tahanson43206 (2020-05-16 07:09:27)

SpaceNut · 2020-05-16 08:52:32

Third time .. really starting to hate this new computer....

2012 topic https://forum.nasaspaceflight.com/index … ic=28279.0
Phobos surface access from Mars orbit (tether-based) is the use of L2 to drop a line to the moons surface

2009 topic https://forum.nasaspaceflight.com/index … ic=18812.0

Phobos tether using both locations about the moon for transfer http://clowder.net/hop/railroad/PhobosTether.php
post#6 has HOP's calculations

https://hopsblog-hop.blogspot.com/2016/ … ether.html
Upper%2BPhobos%2B7%2Bkilometer%2Btether.jpg

using both moons
Screen%2BShot%2B2017-07-31%2Bat%2B9.35.54%2BAM.png

https://www.nasa.gov/directorates/space … II/PHLOTE/
Phobos L1 Operational Tether Experiment (PHLOTE) was to suspend an equipment cluster on the end to perform science examination

https://www.nextbigfuture.com/2017/04/b … hobos.html

In support of the cable drum using carbon nano yarns to make the cable
https://www.nextbigfuture.com/2016/10/d … -five.html

Terraformer · 2020-05-16 09:03:09

Hop has written a lot about tethers on Phobos.

Extend it to the top of the atmosphere, and payloads can be dropped into the atmosphere at 600 m/s.

tahanson43206 · 2020-05-16 09:17:59

For SpaceNut re #32

Thanks for all those links. Please go back and edit #32 to add a brief summary of the value of each citation.
I have found SO many of your suggestions to be well worth my time. However, if you could provide a hint of what each citation offers, I could prioritize reading.

Terraformer re #33 ... This is ** excellent ** news! I am greatly encouraged by your report of Hop's work in this area. I'd like to see us move from theory to practice in a sound, deliberate manner. I'll follow the link you provided over the weekend.

It is contributions like these (and those of GW Johnson and others) which can put the Mars Society into the running for actual accomplishment.

The Planetary Society has demonstrated solar sailing after numerous failed attempts. The Mars Society can and should at least match that achievement.

Edit#1: I'd appreciate assistance with design of the tether reel power generator electricity storage system. A suitable candidate to assist in this specialty would be an engineer with training and experience in the field. It is understood that working in a vacuum with a mechanical system such as this is NOT in the CV of most engineers, so a learning curve is to be expected. Never-the-less, I think it is quite reasonable to look for a viable plan that can deliver a multi-ton balloon vehicle with passengers and cargo safely to the upper fringes of the atmosphere of Mars.

For Terraformer ... when I go to read Hop's work, I'll be looking to see how he dealt with the horizontal translation of the moon from which the tether is extended. As discussed in other posts recently, the orbital velocity of the moon is in excess of two kilometers per second. I would expect the design for the balloon vehicle to be able to deal with that. One approach is to include a thruster in the package design, to reduce the horizontal velocity so the combined vertical and horizontal vectors are within the capability of whatever material is chosen for the balloon envelope.

The thruster might be an ion engine, and power might be (somehow) derived from the pull of the gravity of Mars on the slowly descending landing package.

In the absence of Hop, I'm hoping there is a currently active member who can assist with the math needed to work out the design of the various components of the system.

This will be a success when all the trade-offs are optimized to yield a survivable package that can deliver multi-ton payloads to the surface.

(th)

Last edited by tahanson43206 (2020-05-16 09:31:09)

tahanson43206 · 2020-05-16 17:49:35

For SpaceNut re Post #32 ... thank you for the impressive enhancement of the post!

For Terraformer re #33 ... While SpaceNut has (most likely) included the link you provided, I followed the link you provided first.

I was happy to see that Hop reported another author working on the tether challenge.

From Hop's discussion, I picked up the observation that the velocity of Phobos in its orbit is substantially greater than the velocity of its footprint on the surface of Phobos. That explains how Hop was able to predict a much lower velocity of the bottom of the tether.

Hop predicts a 1;11 taper of the tether (if it is made of kevlar)

(th)

tahanson43206 · 2020-05-16 18:19:35

For SpaceNut and Terraformer ...

Following up on the link to Hop's work as shown by Terraformer, here is my attempt to arrive at Hop's prediction of .6 km/second of the footprint of the tether with respect to the surface of Mars. I came up with a slightly higher figure, but it is much better (lower) than the orbital velocity of the moon itself.

what is circumference of mars at the equator

about 13,300 miles
Mars' radius is, of course, half of planet's diameter. The circumference of Mars around the equator is about 13,300 miles (21,343 km), but from pole-to-pole Mars is only 13,200 miles (21,244 km) around. This shape is called an oblate spheroid.Aug 2, 2012
How Big is Mars? | Size of Planet Mars | Spacewww.space.com › 16871-how-big-is-mars
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what is orbital period of phobos around mars

Image result for what is orbital period of phobos around mars
Phobos orbits closer to Mars, with a semi-major axis of 9,377 km (5,827 mi) and an orbital period of 7.66 hours; Deimos's semi-major axis is 23,460 km (14,580 mi) with an orbital period of 30.35 hours.
Moons of Mars - Wikipedia

Given the Orbital period of Phobos of 7.66 hours...

Given the Circumference of Mars at the equator of 21,343 km ...

Assuming the track of Phobos is over the equator of Mars ...

Then the speed of the footprint of Phobos over the surface of Mars is:

21,243 / 7.66 or 3,034+ kilometers per hour, or 50.6 kilometers per minute.

That would be .84+ kilometers per second, or 840+ meters per second.

From Hop's blog we have:

http://hopsblog-hop.blogspot.com/2015/0 … solar.html

The tether foot pictured above moves about .6 km/s with regard to Mars surface. This is about 1/10 of the ~6 km/s the typical lander from earth needs to shed. Mars Entry Descent and Landing (EDL) would be vastly less difficult.

From this site: http://www.aerospaceweb.org/question/at … 0249.shtml

We are provided the speed of sound at the surface of Mars (taking temperature and the density of the atmosphere into account)

This methodology tells us that the standard "sea level" speed of sound on Mars is equal to
801.3 ft/s
244.2 m/s
546.4 mph
879.3 km/h
474.8 knots

Taking the 840+ meters per second figure above as the velocity of the bottom of the tether suspended from Phobos, then the velocity of the vehicle to be released into the atmosphere of Mars would be Mach 3.45+

The fabric of the envelope would need to be able to handle the temperature that would be generated after release, as the vehicle slows before landing.

Edit#1: I was puzzled by the difference between my first set of results, and those of Hop.

The moon travels from West to East (from the point of view of the surface of Mars).

The velocity of the equator of Mars is given as (about) 240 meters per second.

That would account for Hop's estimate of .6 kilometers per second, since Mars would be advancing under the descending tether.

Its surface is marred by debris that may have come from impacts on Mars. It travels around the planet three times a day, zipping across the Martian sky approximately once every four hours. The fast-flying moon appears to travel from west to east.Dec 8, 2017
Mars' Moons: Facts About Phobos & Deimos | Space

and ...

Image result for what is the spin of mars at the equator
Mars, like all of the planets except Venus, rotates in prograde(counter clockwise). The planet has a rotational speed of 868.22 km/h at the equator.Jun 6, 2008
Mars Rotation - Universe Today
This speed would be (about) 241 meters per second

(th)

Last edited by tahanson43206 (2020-05-16 18:36:19)

tahanson43206 · 2020-05-16 19:03:03

At this point in the topic, I've decided to investigate Hop's suggestion of Kevlar for the tether.

The site at the link below provides detailed comparisons of a variety of plastic ropes with the best available steel cable.

The application under study is cables used for winches.

The web site text takes up the advertised claim of "15 times better/stronger than steel" but more importantly, shows that for a given size of cable (3/8 inch) the plastic cable does indeed bear more load before breaking than the equal diameter steel cable.

https://blog.ronstanindustrial.com/is-s … teel-cable

However, the web site ** also ** points out the vulnerability of plastic rope to ultraviolet radiation, which can be anticipated in a space application such as the Phobos tether under discussion in this topic.

A consideration for selection of material for a tether, and design of the tether itself, is that the tether does NOT have to bear the full weight of the descending vehicle. The purpose of the tether (in this application) is to slow the acceleration of the vehicle which would otherwise occur due to the pull of gravity of Mars.

The goal (as I see it right now) is to find a combination of materials and procedures that allows a landing vehicle to enter the atmosphere of Mars at a velocity just below the limit of the tolerance of the envelope material for the conditions of encounter with the atmosphere at 600 meters per second or so of horizontal movement. The vertical component would be combined with the horizontal to produce the net velocity to be overcome by the balloon envelope.

Edit#1:

From the web site at the link above:

Next, if you were to apply gradually increasing straight-line tension to the two winch ropes, one of ⅜-inch 12-strand Dyneema® Ocean 5000, and the other a ⅜-inch Type 316 steel wire cable, the steel cable will be the one that snaps first. In fact, the Dyneema® will outlast it by several thousand pounds before it reaches the breaking point. The steel will tolerate 21,840 pounds by a point of partial or total rupture, whereas the Dyneema® Ocean 5000 has the ability to withstand 24,726 pounds before it breaks.
Dyneema® Rope vs. Steel Rope — Equal Weights
The claim that synthetic rope is 15 times stronger than steel cable actually originates in a comparison of the two materials by strength-to-weight ratio. Dyneema® has a dramatically higher tenacity (strength per weight) than steel and can tolerate up to 15 times the load that steel rope of identical length and weight can hold before failure (precise ratio pending other factors).
This is because synthetic fiber ropes are remarkably lighter than their steel counterparts in a similar size, length, and diameter.

For a tether application, with the gravity of Mars pulling on the cable as well as the payload, the strength-to-weight ratio advantage of Kevlar (and other synthetic rope) provides a distinct advantage.

In the blog post by Hop, he anticipates a 1:11 taper of the tether as it extends from Phobos toward Mars.

As an initial direction for investigation, I will try to find the weight of a Kevlar 3/8 inch cable 3700 miles in length. Whatever that weight is, it would be reduced to 38% of the Earth value when deployed at Mars. I expect to find the weight at Mars is greater than 24,726 pounds, else Hop would not have projected the 1;1 taper.

(th)

Last edited by tahanson43206 (2020-05-16 20:12:24)

tahanson43206 · 2020-05-16 21:53:08

For SpaceNut ... This post is about Kevlar ... Since Hop mentioned the material in connection with his vision of a tether from Phobos, I've started to collect information about it. What I'm looking for at the moment is the weight of a length of rope made of Kevlar, combined with the test strength for that particular kind of rope. My focus is to try to learn why Hop recommended (or suggested) a 1:11 taper for the tether.

The document at the link below is an early study of Kevlar by the US Military, not long after the material was developed/invented.

https://apps.dtic.mil/dtic/tr/fulltext/u2/a163255.pdf

1-1. INTRODUCTION. Kevlar is the registered trademark for a family of aromatic polyamide (aramid) fibers introduced by E. I. du Pont De Nemours and Company, Inc., in
1972.

Three different types of Kevlar aramid fibers are currently available from Du Pont:
Kevlar-formerly Fiber B, Kevlar 29-formerly PRD-49 IV, and Kevlar 49-formerly
PRD-49 III. Kevlar was developed primarily for tire reinforcement. Since its properties
are similar to those of Kevlar 29, Kevlar will not be discussed separately in this guide.
Both Kevlar 29 and Kevlar 49 have many industrial uses other than rope; however, this
discussion is limited to aramid ropes.

Edit#1: Even this early, there was understanding of the vulnerability of Kevlar to ultraviolet light, and the need for a protective sheath.

(th)

Last edited by tahanson43206 (2020-05-16 21:54:59)

tahanson43206 · 2020-05-17 07:36:03

Today I'm starting to look for a gravity gradient calculator. I'm interested in finding the force that will be exerted by the gravity of Mars on a tether as it is extended toward Mars from Phobos.

The force is toward Phobos during the first 3 kilometers (or so) to the L1 point, and toward Mars after the tether is extended beyond (about) 6 kilometers.

The goal is to deliver a balloon landing vehicle to an elevation above Mars at which it can be released so that it can withstand the heating that will occur as it enters the atmosphere of Mars. The vehicle will have a horizontal velocity due to movement of Phobos, and it will have a vertical component due to the gravity of Mars. The combination of these two vectors will determine the velocity the vehicle will have as it approaches the atmosphere.

A gravity gradient calculator would take as input the mass of the tether per unit of length, and the elevation of the given length above Mars, and deliver the force exerted upon that segment of the tether. The accumulated force would (presumably) approach and at some point exceed the snapping point of whatever material is considered.

It has been predicted by Hop (references were given in earlier posts by Terraformer and SpaceNut) that a tether made of Kevlar would need to be constructed or assembled with a taper ratio of 1:11.

I am looking forward to seeing how Hop arrived at that estimate.

(th)

Last edited by tahanson43206 (2020-05-17 11:26:20)

SpaceNut · 2020-05-17 08:04:27

https://en.wikipedia.org/wiki/Gravity_gradiometry
https://en.wikipedia.org/wiki/Gravity
https://en.wikipedia.org/wiki/Acceleration

The equation is the following:
F = G m 1 m 2 r 2 {\displaystyle F=G{\frac {m_{1}m_{2}}{r^{2}}}\ } F=G{\frac {m_{1}m_{2}}{r^{2}}}\
Where F is the force, m1 and m2 are the masses of the objects interacting, r is the distance between the centers of the masses and G is the gravitational constant.

Now for the r change it to the length and calculate the force at the new end of that cable. Select the ration of taper as new distance for the force to spread across with the attachment at phobos being 11 strength density for the cable for a distance, then 10 factor ect until the thin end of the cable is a factor of 1 to hold the desired payload before releasing.

tahanson43206 · 2020-05-17 11:25:53

For SpaceNut re #40

Thanks for the nice boost! In thinking ahead to implementing the suggestions in Post #40, it occurs to me that it does not matter what the actual mass of a given length of Kevlar (or any material) may be ... The equation just needs a mass, so a mass of 1 Kg will satisfy the requirement. The actual length of a segment of Kevlar (or other material) that has a mass of 1 Kg can be determined separately.

You've described the end game, with all 11 of Hop's planned taper elements in place, but I am far from the end game at this point. My first shot at this will be to try to understand the physics of a single strand of Kevlar. I'll be looking for the length of Kevlar that will ** just ** hold itself against the pull of Mars, aside from the presence of a payload.

(th)

tahanson43206 · 2020-05-17 12:56:10

For SpaceNut ...

While searching for an online calculator that might have been designed to compute the strength required for various tethers in various situations, I was surprised to find that the word "tether" has been appropriated by a company in the cryptocurrency business.

Accordingly, Google came back with numerous citations for calculators the compare the current value of various currencies to the US Dollar.

Search Results
Web results
Tether (cryptocurrency) - Wikipediaen.wikipedia.org › wiki › Tether_(cryptocurrency)
Tether is a controversial cryptocurrency with tokens issued by Tether Limited. It formerly claimed that each token was backed by one United States dollar, but on ...
History · Alleged price manipulation · Security and liquidity

There may be an opportunity to contribute a tether calculator (for astronomical tethers).

(th)

SpaceNut · 2020-05-17 17:11:22

https://blog.porterathletic.com/2016/03 … -vs-steel/

Table of diameters to length information
http://www.pelicanrope.com/12-strand-si … evlar.html

https://www.dupont.com/fabrics-fibers-a … ables.html
Lightweight, Flexible, and Strong Kevlar® Rope and Cables

https://www.thethreadexchange.com/miva/ … nformation
Kevlar Thread Guide

http://digipac.ca/chemical/mtom/content … tw_001.htm
Breaking Strength of a Cable

https://www.quora.com/What-tensile-stre … ator-cable

https://arxiv.org/pdf/cond-mat/0601668
On the strength of the space elevator cable

tahanson43206 · 2020-05-17 19:31:27

For SpaceNut re #43

Thanks for another (in my view) outstanding search session! The Pelican Rope page contained the direct strength/weight information I was looking for.

However, the other links were interesting in their respective ways. The Italian study of nanotechnology (carbon threads) for the space elevator was exhaustive and informative, and concluded that an Earth based space elevator cable would break if built with state-of-the-art-at-the-time carbon nanotubes, because defects would reduce the strength by 70%

I can't help thinking (in the absence of more information) that a technology that would fail in the Earth application might prove quite satisfactory at Mars.

The Pelican Rope page showed that a 1 inch diameter Kevlar cable would support 78,000 pounds, or 39 tons. I'll return to the question tomorrow, of how far that cable would stretch from Phobos before it has consumed all its strength by just holding itself together.

(th)

GW Johnson · 2020-05-17 21:13:45

Be aware that Kevlar, while very strong, and stiff, has very low elongation capability. On the order of 2% (and carbon epoxy is even worse!). That makes it fail in a brittle fashion: suddenly and without warning, under any significant shock loads at all. Been there and done that, with tow cables for towed aircraft decoys.

GW

Last edited by GW Johnson (2020-05-17 21:14:23)

tahanson43206 · 2020-05-18 05:50:59

For GW Johnson re #45 ...

Thank you for this cautionary note .... the 2% stretch figure was included in several of the articles at the links SpaceNut provided recently.

In particular, the marine web site described the failure mode of Kevlar as benign compared to the failure mode of steel cable, which tends to build up energy sufficient to kill bystanders when it lets go. The 2% stretch figure was cited as an advantage for most applications, where reliability of pull is important, and where the operator stays below the snap limit.

The suggestion of Hop (as revealed in the blog posts shown earlier in this topic by Terraformer and SpaceNut) is that a cable would be designed with a taper ratio of 11:1 for the Phobos application. An alternative to such a design is to assemble uniform diameter cable in bundles, with 11 at the top and one at the bottom, joined at attachment points along the route.

At this point, I am hoping to gain insight into the reason for Hop's prediction. Based on his work in general, I'm confident it will turn out there is a mathematical basis for the prediction.

From the standpoint of developing a business model to supply safe, reliable, cost-effective delivery service for people and cargo to the surface of Mars, it would seem undesirable to risk failure of a cable, so operation well within the limits of the material would seen advisable.

In addition, (and this will be more difficult to determine), since any material will be subject to gradual deterioration in space service, there needs to be found a way to (somehow) keep track of the condition of the material. Destructive influences I can think of right now include cosmic radiation, micro-meteors and the two Solar factors, ultraviolet radiation and simple radiant heating.

While Kevlar has superior performance with respect to heating, and that is good, it seems to me that it will be just as well that UV protection can also reflect sunlight at just the right rate to keep temperature of the cable within a healthy range. Since the cable will be in the shadow of Mars half the time, it needs to be assembled in such as way as to avoid temperature extremes. Accordingly, part of the mass of the system would include protective coverings, which may or (probably) may not contribute to the tensile strength of the package.

A concern I have is the means by which a descending vehicle would achieve a useful purchase on the cable without damaging it. I have not been a fan of the gripper idea imagined from the earliest days of the Space Elevator concept, and tested rigorously by annual ribbon climbing competition in the American West. To my way of thinking, rotating wheels slowly crawling up a ribbon thousands of kilometers long is considerably less than ideal.

However, more to the point here, the taper predicted by Hop means that the descending vehicle needs to (somehow) maintain a useful contact with the cable so as to slow itself as gravity pulls it down. An idea I've been considering is the venerable elevator cable concept, which (typically) involves use of a counterweight to (somewhat) balance the weight of the car, so that less tension is imposed upon the motor driven reel at the top of a building.

In the case of the Phobos tether, this might be accomplished by employing a series of loop cables which would be themselves supported by the primary cable. The loop cables would allow solid clamps to secure the vehicle to the cable, and then it could allow rapid travel to the bottom of the loop, at which point the vehicle would secure itself to the next loop cable and release the first.

A signal benefit of this system is that generators at the heads of the loop cables could generate electric power from the momentum of Phobos as the vehicle descends under the pull of Mars.

Periodically, as this topic proceeds, I would like to remind readers that the moon Phobos needs to receive acceleration from some source in order to remain in orbit over the long term, so planners (for the long term) can be expected to be thinking about how that can be done.

In his blog post, Hop mentioned this issue, but dismissed it as not relevant for millions of years. I'd prefer to see humans who enlist the beneficial presence of Phobos to further their projects take responsibility for adding to the momentum of Phobos (somehow) rather than making a habit of drawing from the reservoir without thought. A comparable phenomenon is the human drawdown of supplies of fresh water from underground lakes around the world. For hundreds of years, if not longer, humans have been happy to accept the generosity of Nature with no thought of replenishing the stored wealth. The time is at hand when failure to replenish what has been withdrawn is beginning to squeeze supplies and create anxiety in some members of the current generation.

I would like to see rebuilding the momentum of Phobos (or at least preventing further loss) a part of the human mind set as use of Phobos is considered.

(th)

tahanson43206 · 2020-05-18 06:13:56

This post is a continuation of Post #46, which is already long ...

To clarify ... the image of a building elevator was invoked in #46 to suggest an alternate way of thinking about transport along a Space Elevator. I've been skeptical of the wheeled gripper concept from the beginning (the "climber") and have been thinking about practical alternatives for some time.

An Earth-based building elevator (often if not usually) employs a counterweight to permit a balancing of load (to the extent possible) on the up/down sides of the motor driven winch at the top of the building/structure. I am NOT proposing such a system for the Phobos Landing Tether.

Instead, I ** am ** offering a vision of a continuous loop of cable with reels fitted with motor-generators at top and bottom reels.

These would be arranged somewhat like locks in a canal water transport system. In this case, the intent is to insure the maximum rate of safe descent between transfer nodes, and at the same time, to pull as much power from the descending vehicle as possible. The vehicle will be under acceleration by Mars, and the transfer cable will be resisting that pull by drawing power from the vehicle, which would otherwise fall to its destruction. That power could be fed back up to Phobos, for useful purposes, including accelerating the moon to prevent its further deterioration of orbit.

At each transfer node the vehicle would slow to a rate sufficient to permit the secure and safe connection to the next descent loop. That could include coming to a complete stop. The next descending loop would then invest some energy in accelerating the vehicle to resume a steady rapid but controlled descent to the next loop node.

At the bottom of the tether, the vehicle would release itself to begin a period of freefall before it enters the atmosphere of Mars, and there begins building up buoyancy to permit a zero velocity vertical arrival at the customer site.

Edit#1: An advanced system could accomplish a transfer at full descent speed between transfer nodes by clever computer supervised mechanical activities. These might include acceleration of the next descending loop to the velocity of the vehicle, and (very rapidly) engaging clamps on the lower loop while releasing them on the upper loop. The released upper clamps would then clamp onto the descending loop immediately after passing the upper head of the lower loop.

Edit#2: It was suggested that some of the power that might be collected from the descent of a vehicle would/could be used to help to accelerate Phobos in its orbit. Of course, whatever energy **is** invested in this manner would be a small fraction of the amount of momentum consumed in the descent. Most would be lost to heating of various components, but anything will help, and in any case, whatever energy can be collected would otherwise go completely to waste.

While I'm unsure of the practicality of the idea at this point, it ** is ** at least conceivable to deploy solar cells to collect energy that falls on Phobos, and to pull CO2 from the atmosphere of Mars (another possible application of the tether) to make CO and O2 for chemical propulsion.

(th)

Last edited by tahanson43206 (2020-05-18 06:23:53)

tahanson43206 · 2020-05-18 11:03:49

This is for SpaceNut ...

One of your recent posts contained a link to a web site that is set up to show available supplies of Kevlar thread.

The text is interesting (to me at least) because the author goes to some pains to try to dissuade customers from ordering Kevlar thread if it is not a good fit for the application. The author even goes so far as to characterize the situation as "almost cult-like behavior" (or words to that effect).

However, the site itself has been percolating or simmering or whatever sites do in the back of the mind, and this morning I am "looking" at mental images of the solution to the entire taper problem, in addition to the maintenance challenges of deterioration of sections of the tether under "real Universe" operating conditions.

The answer (as I am "seeing" it right now) is to make donuts/loops of thread. The problem I was considering is how to splice a tether made of sections of Kevlar. The answer (assuming it is based upon use of thread to make loops) is that the thread is manufactured in a continuous stream of material, which needs only ONE join after it is wrapped around the form to make a loop.

The loops themselves would be secured to each other with material whose tensile strength would be sufficient to the task, although I'll admit right now I don't know what that material might be. Thus, to replace a failing section (loop) the maintenance crew would pull the sections above and below the failing loop toward each other to free the existing loop, and to insert a custom made replacement.

Each loop in the tether would be custom wound to withstand the force at the assigned location in the chain.

I'll be looking to see if this idea can stand up to the intense scrutiny it deserves and (hopefully) will receive.

(th)

tahanson43206 · 2020-05-18 11:20:39

In an earlier post in this topic, I offered a vision of how the descent of a vehicle might be slowed to a reasonable speed without touching or otherwise disturbing the tether itself. The initial proposal was a mechanical system, consisting of a loop of material that would be gripped by the vehicle, and a set of motor/generators at the ends of each of a number of loops. Such a system would require transitions between loops, which would take time or require astonishing performance by gripper systems working in split seconds to transition from one loop to the next.

An alternative that is attractive (if feasible) is the inverse of kbd512's Electromagnetic Launcher.

This would be a continuous track along which the descending vehicle would move. The movement of the vehicle would induce EMF in coils of (something) comprising the track. I have seen demonstration of an object falling inside a magnetic block. The object took a long time to make the journey of a few inches.

I'm not sure (in pondering the memory) if the object was the magnet and the block simple iron, or the reverse.

Either way, the memory provides a vivid foundation for the confidence that magnet force can be effectively used to slow the progress of a moving object.

Electric current generated by the progress of the vehicle would be fed back to Phobos for storage and eventual use.

(th)

Last edited by tahanson43206 (2020-05-18 18:29:15)

tahanson43206 · 2020-05-18 18:39:03

This is a follow up to post #49 The demonstration I saw was of a "magnetic hysteresis" brake which depended upon eddy currents.

Electromagnetic brake
From Wikipedia, the free encyclopedia
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Find sources: "Electromagnetic brake" – news · newspapers · books · scholar · JSTOR (June 2011) (Learn how and when to remove this template message)
Electromagnetic brakes (also called electro-mechanical brakes or EM brakes) slow or stop motion using electromagnetic force to apply mechanical resistance (friction). The original name was "electro-mechanical brakes" but over the years the name changed to "electromagnetic brakes", referring to their actuation method. Since becoming popular in the mid-20th century especially in trains and trams, the variety of applications and brake designs has increased dramatically, but the basic operation remains the same.
Both electromagnetic brakes and eddy current brakes use electromagnetic force but electromagnetic brakes ultimately depend on friction and eddy current brakes use magnetic force directly.

The Wikipedia entry above adds 'eddy current brakes' after reporting the use of what I surmise were solenoids to actuate traditional mechanical calipers.

It is the 'eddy current brakes' that would seem most applicable to the Phobos Tether application.

Edit#1:

Search Results
Featured snippet from the web
The design of eddy-current magnet brakes. The eddy-current is created by the relative motion between a magnet and a metal (or alloy) conductor. The current induces the reverse magnetic field and results in the deceleration of motion. The proposed mechanism implements this phenomenon in developing a braking system.
(PDF) The design of eddy-current magnet brakes

https://www.researchgate.net/publicatio … net_brakes

Edit#2 ... the paper at the link above reports on research using several materials for the base plate. As I understand the work, permanent magnets were used to simulate magnets that might be carried in a train to be decelerated. An application of this idea in the Phobos Tether situation would allow for rapid movement of the descending vehicle while maintaining control of the rate of descent. The energy collected from the moving vehicle would be invested in the track as heat, and that heat would radiate rapidly to the cold of space. No energy would be recovered from the moving vehicle in this situation, but the simplicity of the method is a significant advantage.

speed is not equal to 1.0 m/sec. For clearer demonstration, the velocity profiles for different configurations are shown in Fig. 17.5. CONCLUSIONS The design of eddy current brakes is presented in the paper. Eddy current brakes provide non-contact means to produce braking forces required to decelerate the motion of a moving object. In this study, four systematic engineering design scenarios to design a braking system are presented: a constant magnetic field, an optimal magnetic field distribution, piecewise-constant magnetic fields and a section-wise guide rail with a constant magnetic field. Although the simulation results above show that the optimal magnetic field is better than the constant magnetic field, a deceleration peak of 9 git is not suitable for most people. Further, the sudden increase in current could cause wire overload. The piece wise constant magnetic field has the advantages of a preset terminal speed and predictable wire current but it produces a higher speed. Alternatively, it is much easier to keep the magnetic field constant and select the proper conductor materials. The advantages of these last two designs using different materials along the guide rail are tolerable deceleration; and easy manufacturing.
A nearly maintenance-frees system can be achieved if permanent magnet is utilized to establish the magnetic field.
It should be noted that the simulation results show in the paper are based on the assumption of using infinite conducting plate. For finite dimensional conducting plate, the required magnetic field has to be increased so that the same design results (velocity and deceleration) can be maintained. The amount of increase on the magnetic field depends on the physical dimension Fig. 16. The deceleration versus position with the carbon steel as middle section.Transactions of the Canadian Society for Mechanical Engineering, Vol. 35, No. 1, 2011

I have separated the key line for the Tether application. A maintenance free braking system would be the reward for investment in the mass needed to prepare the braking rail for use. That rail would be attached to the tether system, which as described in an earlier post, might consist of loops of kevlar thread, wound in sufficient numbers of loops to provide the strength needed at a particular point in the chain. The braking system would operate entirely without physical contact with the braking rail.

(th)

Last edited by tahanson43206 (2020-05-18 19:03:59)

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