Author Topic: Nose tethered BFS Spaceships for artificial gravity during the coastal phase.  (Read 15120 times)

Offline Paul451

  • Full Member
  • ****
  • Posts: 1289
  • Australia
  • Liked: 631
  • Likes Given: 535
https://www.reddit.com/r/spacex/comments/3ogkfa/artifical_gravity/cvxe4yu/?st=j77ls2q8&sh=1fabfcd4
Quote
We end up with a radius of only about 3500 meters.
and you want to simulate Mars gravity.

Less than one third of 1 RPM.

{sigh}

Why does talking about AG seem to turn people's brains to chicken soup?

Offline RonM

  • Senior Member
  • *****
  • Posts: 2142
  • Atlanta, Georgia USA
  • Liked: 1012
  • Likes Given: 788
I have changed my concept pic by reversing the spaceships so that radius is increased by length of the engine section. Note that this is opposite hanging direction as in ITS video so this may not be an improvement.

This won't work because AG direction will be 180 degrees from the landed configuration. The crew will be walking on the ceilings.

Offline GORDAP

  • Full Member
  • *
  • Posts: 160
  • St. Petersburg, FL
  • Liked: 56
  • Likes Given: 20
Fully fueled 12m ITS weights 2400 tons, unfueled ITS weights 450 tons. Artificial gravity makes sense during coasting so the strength of the cables depends also on how much fuel will remain in reserve after Mars transfer burn. As a lower bound, assuming there is no fuel left and we want 0.38g then cable(s) will have to be strong enough to support at least 450*2*0.38= 342 tons of weight. Anyone know how much kg per length this roughly could be?

I'm pretty sure this is off by a factor of 2.  I think the cable only has to be strong enough to hold up a single BFS (with appropriate safety factor, of course).

Online envy887

  • Senior Member
  • *****
  • Posts: 2764
  • Liked: 1276
  • Likes Given: 802
As for mass, how much does a 100m of cables weight? Lets say it must be sufficiently strong to not break under the weight of two partially fueled ITS spaceships at 0.38g.

Let's assume it is Zylon, density 1540 kg/m^3 and tensile strength 5.8 GPa. An ITS craft will mass up to 720 tonnes in cruise configuration, based on the fueled mass and delta-v given in the 2016 IAC presentation. At 0.38 g that is 2.68 million N of force on the cable.

Assuming a factor of safety of a very conservative 3x, that's 8.05 MN required strength, which Zylon can provide with only 13.9 cm^2 area of cable, which masses only 2.15 kg per meter. So a 100 m cable is only 215 kg.

But with a 100 m cable, the delta-v to spin up and down is 5250 kg per ship. The mass of the cable is pretty trivial compared to the mass of spin up/down propellant. Both propellant and cable mass increase with a longer cable, so the shortest possible cable is mass-optimal.

There doesn't appear to be any reason to spin at less than 3 RPM, which can be done with a 50 m cable and 2 ITS ships nose to nose, yielding .38 g on the lowest deck of the passenger cabin, and .27 g on the highest deck.

Offline Coastal Ron

  • Senior Member
  • *****
  • Posts: 3449
  • I live... along the coast
  • Liked: 2204
  • Likes Given: 2730
But with a 100 m cable, the delta-v to spin up and down is 5250 kg per ship. The mass of the cable is pretty trivial compared to the mass of spin up/down propellant. Both propellant and cable mass increase with a longer cable, so the shortest possible cable is mass-optimal.

Sure, the shortest possible cable is mass-optimal, but it may not provide the optimal conditions for humans.

Quote
There doesn't appear to be any reason to spin at less than 3 RPM, which can be done with a 50 m cable and 2 ITS ships nose to nose, yielding .38 g on the lowest deck of the passenger cabin, and .27 g on the highest deck.

If the weight of the cable is minimal, and if human comfort can be increased by increasing the radius of the rotation, then it only costs a small amount of cable weight to provide better human comfort. I'm assuming the amount of fuel required to spin up/down does not change given the radius as long as the resulting gravity is the same.
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline Coastal Ron

  • Senior Member
  • *****
  • Posts: 3449
  • I live... along the coast
  • Liked: 2204
  • Likes Given: 2730
For myself I'm OK with "worst case", [...] because if I can come up with solutions that work out OK with "worst case", then I should be OK with "reality".

No. This is why I keep harping on about people insisting on low RPM. It's pre-establishing that the trade will fail.

Radius increases with the inverse square of RPM. It's a huge effect. If you insist on 1g and 1 or 2RPM, then you've virtually ruled out AG in advance, because of course others are going to say "It adds too much mass/complexity", and any consideration of the idea dies. As I said, the difference between 1g/2RPM and 0.38g/4RPM is a tenfold difference in scale.

I understand your concern. Previously I didn't, but now I do, so thanks for explaining it. However there are some situations where your concerns should not be a problem, such as:

A. The example of two ITS joined by cable nose-to-nose, where the weight of the cable is negligible, so the radius of rotation can be lengthened without the inverse square problem.

B. If having a large vehicle/station is a benefit. A rotating space station design I'm looking at requires a large amount of habitable area to be useful, so having a longer radius with lower RPM can provide that.

Quote
It would be like designing rockets by insisting that any rocket technology must be capable of SSTO or you won't even look at it, because "if you can solve the worst case then you should be able to solve any other configuration". That's not reasonable, and it's not honest.

Again, I understand your point, but I would ask you to consider how SpaceX was able to create reusable rockets when everyone else assumed it could not be done. Sometimes you just need to ignore "the rules" to see if there are alternative solutions that still result in the desired outcome.
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline KelvinZero

  • Senior Member
  • *****
  • Posts: 3569
  • Liked: 483
  • Likes Given: 124
It just occurred to me, maybe a single cable can work. [etc]

This is where you make a system ridiculously complicated so you can keep the original idea, because the original idea was "simple".
No I don't think so, because the ships will (Im guessing) already have these flywheels and the software to deal with small corrections in orientation.

What was naive was the idea of the spaceship freely exchanging rotational energy with the cable, like a weight tied to the end of a rope, winding up, then spinning the other way.

Offline Paul451

  • Full Member
  • ****
  • Posts: 1289
  • Australia
  • Liked: 631
  • Likes Given: 535
I'm assuming the amount of fuel required to spin up/down does not change given the radius as long as the resulting gravity is the same.

Not even close. Angular momentum (and hence fuel requirement) increases with radius and velocity, both of which increase with reduced RPM for the same g-load.

Eg, at the same 0.38g, angular momentum per kg of ship mass at 4, 3, 2, 1 RPM is, approx. 190, 450, 1500 and 12,000 mē/s respectively. Hence, halve the RPM and you increase the angular momentum approximately 8-fold. Cubic increase.

Sometimes you just need to ignore "the rules" to see if there are alternative solutions that still result in the desired outcome.

I find it funny that you're saying this when you are the one fixating on SpinCalc's red/yellow/green indicators as if they are the word-of-god, and I'm trying to say "No, there's been 35 years of research that says otherwise. The old assumptions are wildly out of date."
« Last Edit: 09/05/2017 10:23 PM by Paul451 »

Offline Coastal Ron

  • Senior Member
  • *****
  • Posts: 3449
  • I live... along the coast
  • Liked: 2204
  • Likes Given: 2730
I'm assuming the amount of fuel required to spin up/down does not change given the radius as long as the resulting gravity is the same.

Not even close. Angular momentum (and hence fuel requirement) increases with radius and velocity, both of which increase with reduced RPM for the same g-load.

Eg, at the same 0.38g, angular momentum per kg of ship mass at 4, 3, 2, 1 RPM is, approx. 190, 450, 1500 and 12,000 mē/s respectively. Hence, halve the RPM and you increase the angular momentum approximately 8-fold. Cubic increase.

OK, it's good we're having this conversation, as I had the wrong assumption about the ice skater spinning with arms open and arms closed.

Quote
Sometimes you just need to ignore "the rules" to see if there are alternative solutions that still result in the desired outcome.

I find it funny that you're saying this when you are the one fixating on SpinCalc's red/yellow/green indicators as if they are the word-of-god, and I'm trying to say "No, there's been 35 years of research that says otherwise. The old assumptions are wildly out of date."

Well, 35 years of research here on Earth. What we need is a long-duration testing platform in space to validate everyone's assumptions - which I think I may have a valid concept for. Not ready to share yet, but it was inspired by a suggestion on a different topic thread. Specifically it would provide .38G at 3 RPM, while simultaneously providing levels of occupation that would be closer to the center of rotation, meaning less simulated gravity but more angular and tangential velocities (which may be detrimental at some upper limit, which needs to be determined thru actual testing).
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Online envy887

  • Senior Member
  • *****
  • Posts: 2764
  • Liked: 1276
  • Likes Given: 802
But with a 100 m cable, the delta-v to spin up and down is 5250 kg per ship. The mass of the cable is pretty trivial compared to the mass of spin up/down propellant. Both propellant and cable mass increase with a longer cable, so the shortest possible cable is mass-optimal.

Sure, the shortest possible cable is mass-optimal, but it may not provide the optimal conditions for humans.

Quote
There doesn't appear to be any reason to spin at less than 3 RPM, which can be done with a 50 m cable and 2 ITS ships nose to nose, yielding .38 g on the lowest deck of the passenger cabin, and .27 g on the highest deck.

If the weight of the cable is minimal, and if human comfort can be increased by increasing the radius of the rotation, then it only costs a small amount of cable weight to provide better human comfort. I'm assuming the amount of fuel required to spin up/down does not change given the radius as long as the resulting gravity is the same.

Propellant requirements are roughly proportional to tangential velocity, which increases with the square root of radius.

However, the difference between 2 and 3 RPM is only 6 m/s delta-v per ship, and 44 m (less than 100 kg) of cable. If the difference is worthwhile (which I doubt), then adding that capability is hardly difficult.

Offline biosehnsucht

  • Full Member
  • *
  • Posts: 187
  • Liked: 46
  • Likes Given: 61

There doesn't appear to be any reason to spin at less than 3 RPM, which can be done with a 50 m cable and 2 ITS ships nose to nose, yielding .38 g on the lowest deck of the passenger cabin, and .27 g on the highest deck.

Are you so sure about no reason to go less than 3 RPM? Perhaps a lower gravity target will help, but does 4 RPM for 0.38g come out to less Coriolis effect than 2 RPM at 1g?

Quote
The Coriolis effect gives an apparent force that acts on objects that move relative to a rotating reference frame. This apparent force acts at right angles to the motion and the rotation axis and tends to curve the motion in the opposite sense to the habitat's spin. If an astronaut inside a rotating artificial gravity environment moves towards or away from the axis of rotation, he or she will feel a force pushing him or her towards or away from the direction of spin. These forces act on the inner ear and can cause dizziness, nausea and disorientation. Lengthening the period of rotation (slower spin rate) reduces the Coriolis force and its effects. It is generally believed that at 2 rpm or less, no adverse effects from the Coriolis forces will occur, although humans have been shown to adapt to rates as high as 23 rpm.

Offline the_other_Doug

  • Senior Member
  • *****
  • Posts: 2502
  • Minneapolis, MN
  • Liked: 1532
  • Likes Given: 2717
I'd ask y'all to recall that the only experiments that I know of that have ever been done with spinning up two vehicles around a tether happened 51 years ago, on Geminis XI and XII.  They found that, on the mass scales and speed of movement they were testing, the recoil force from the tether itself caused the tether to have completely unacceptable dynamics, and even slight movements by the crew would take what appeared to be a stable tether dynamic and push it quickly into a series of disastrous loops.

The problem seems to be that you need the tether to be at full stress before you spin up, in order for it to act as if it is a solid, unbending link between the two end-point masses.  If you at any point stop thrusting the two masses away from each other, along the tether vector, the tether will recoil, start pulling the two masses back together, and start looping like crazy.  The spin-up maneuver, involving as it would need to the continuous thrust to apply pressure along the tether, but decreasing as the spin replaces the thrust to apply tension to the tether, is of such complexity that it would need to be demonstrated several times before you could consider using it operationally.

So -- those who believe that there needs to be an awful lot more real-world testing, in space, of the dynamics of such tethered systems before such a thing is designed into the cruise architecture of a Mars mission are the winners of this discussion at present, IMHO... :)
-Doug  (With my shield, not yet upon it)

Offline KelvinZero

  • Senior Member
  • *****
  • Posts: 3569
  • Liked: 483
  • Likes Given: 124
I'd ask y'all to recall that the only experiments that I know of that have ever been done with spinning up two vehicles around a tether happened 51 years ago, on Geminis XI and XII.  They found that, on the mass scales and speed of movement they were testing, the recoil force from the tether itself caused the tether to have completely unacceptable dynamics, and even slight movements by the crew would take what appeared to be a stable tether dynamic and push it quickly into a series of disastrous loops.

But since then we have things like the Colbert and the hoverslam, ie damping oscillations and control of laggy systems has advanced to an incredible art that is used for both trivial and extreme purposes.

I had assumed some sort of intelligent damping. Everything seems to have it now. Even the fact they actually physically did this experiment back then shows how much things have moved on. We are comparing biplanes to harriers

Offline the_other_Doug

  • Senior Member
  • *****
  • Posts: 2502
  • Minneapolis, MN
  • Liked: 1532
  • Likes Given: 2717
I'd ask y'all to recall that the only experiments that I know of that have ever been done with spinning up two vehicles around a tether happened 51 years ago, on Geminis XI and XII.  They found that, on the mass scales and speed of movement they were testing, the recoil force from the tether itself caused the tether to have completely unacceptable dynamics, and even slight movements by the crew would take what appeared to be a stable tether dynamic and push it quickly into a series of disastrous loops.

But since then we have things like the Colbert and the hoverslam, ie damping oscillations and control of laggy systems has advanced to an incredible art that is used for both trivial and extreme purposes.

I had assumed some sort of intelligent damping. Everything seems to have it now. Even the fact they actually physically did this experiment back then shows how much things have moved on. We are comparing biplanes to harriers

Invalid comparison.  Ever since Kitty Hawk, people have been working on VTOL systems.  The Harrier is not even the most sophisticated example -- but decades and decades of trial-and-error went into it, trying every damnfool thing you can think of, and others besides.  With the results that many different types of VTOL systems exist now - most of them some form of helicopter.

On the other hand, ISS does damping, when running microgravity experiments, by the expedient of telling the crew they can't exercise, and BTW, try not to bounce off any walls for the next couple of hours, OK?

Skylab's second and third crews were told not to run around the water tanks like Pete's crew did, because it was stuff like that that caused the CMGs to wear out way faster than anticipated.  And CMG technology is really no more advanced today than it was 45 years ago, I don't believe.  Especially enormous CMGs.

"Intelligent" damping can only accommodate so much moving and bumping around, and you can't keep a shipload of people still enough to bring the excursions down to anywhere close to the kinds of damping algorithms that are used in satellites and such.  And there has been no -- zero -- systems designed to keep two masses, with movements going on constantly within them, stable spinning around a flexible tether.  Because, unlike VTOL systems, there has never been a demand to develop such a thing.

So, once again -- as far as spinning two spacecraft around on a tether, we are still flying biplanes.  And will continue to do so until and unless someone actually addresses the extremely complex damping needed for such a dynamic and essentially chaotic system.  (How is your intelligent damping system gonna handle it when, out of pure chance, all of the people on the port side of one ship decide to flush their toilets within 15 seconds of each other?  That's gotta be a ton or more of flush-water moving around...)

I truly believe that this will end up being a lot harder than even the people who already think it will be hard really apprehend...
-Doug  (With my shield, not yet upon it)

Offline KelvinZero

  • Senior Member
  • *****
  • Posts: 3569
  • Liked: 483
  • Likes Given: 124
On the other hand, ISS does damping, when running microgravity experiments, by the expedient of telling the crew they can't exercise, and BTW, try not to bounce off any walls for the next couple of hours, OK?
The reason I brought up the COLBERT was this:
https://en.wikipedia.org/wiki/Treadmill_with_Vibration_Isolation_Stabilization

The rest is above my pay grade. Might make a great cubesat project, though I think that today you would basically know from simulations. I have seen simulations of how badly undamped cables can behave somewhere, probably on this site.

(edit)
Hey, has anyone mentioned that although this may prove unnecessary for mars, it could be a useful experiment in LEO? I don't think ITS is going to go haring off to mars as soon as it is built. I see all sorts of shakedown runs and other work, and maybe this could include a test with people and ECLSS for some months in mars gravity.

If mars gravity is not good enough for health, there are pretty good reasons to want to know this early.
« Last Edit: 09/12/2017 02:24 AM by KelvinZero »

Offline the_other_Doug

  • Senior Member
  • *****
  • Posts: 2502
  • Minneapolis, MN
  • Liked: 1532
  • Likes Given: 2717
On the other hand, ISS does damping, when running microgravity experiments, by the expedient of telling the crew they can't exercise, and BTW, try not to bounce off any walls for the next couple of hours, OK?
The reason I brought up the COLBERT was this:
https://en.wikipedia.org/wiki/Treadmill_with_Vibration_Isolation_Stabilization

The rest is above my pay grade. Might make a great cubesat project, though I think that today you would basically know from simulations. I have seen simulations of how badly undamped cables can behave somewhere, probably on this site.

(edit)
Hey, has anyone mentioned that although this may prove unnecessary for mars, it could be a useful experiment in LEO? I don't think ITS is going to go haring off to mars as soon as it is built. I see all sorts of shakedown runs and other work, and maybe this could include a test with people and ECLSS for some months in mars gravity.

If mars gravity is not good enough for health, there are pretty good reasons to want to know this early.

I hear you -- and even COLBERT has had some issues; it's a great improvement, but even its use is not allowed during some experiment runs.

And I agree wholeheartedly with your idea of running a significant series of tests of such systems nearer Earth than in truly deep space.  I wonder if it would be better to run them outside of LEO, though, maybe at a LaGrange point.  In LEO, you get such relatively strong gravity gradients that tethers much longer than 20 feet start curling, based on one end of the tether being in a different orbit than the other.  Spinning the pair just means this dynamic would change as the assembly moved around the center point; I'd be a bit concerned that this would inject a lot of variability and instability into the system that you wouldn't see in deep space.  No need to over-engineer the system to handle problems it won't see en route to Mars, after all.

Also, tethers run through Earth's magnetic field can build up a tremendous electrical charge -- witness the demise of the Shuttle's tethered satellite experiment.  I'm pretty certain that the Sun's magnetic field strength, at least between Earth and Mars, is quite a bit weaker than Earth's is while in LEO.  So, again, that would be a complicating factor you could dispense with by testing at a LaGrange point.

In all, testing stability and damping systems for such a rotation system during a nice, long test flight of your first two Mars-sized ITS'es at L2 or L4 might be the best plan.  Heck, sell seats for the tests at cut-rate prices, for a realistic test of the ECLSS and the damping systems, seeing as how they'll need to deal with lots of people moving around and doing, well, all the things that people do... :)

If the price is right, I'd sure buy a seat.  I'll be retiring in the next few years, and I can't think of anything better to do with my "golden years" than to act as a guinea pig for Elon.  :D
-Doug  (With my shield, not yet upon it)

Offline Aussie_Space_Nut

  • Full Member
  • *
  • Posts: 163
  • South Australia
  • Liked: 50
  • Likes Given: 158
I'd ask y'all to recall that the only experiments that I know of that have ever been done with spinning up two vehicles around a tether happened 51 years ago, on Geminis XI and XII.  They found that, on the mass scales and speed of movement they were testing, the recoil force from the tether itself caused the tether to have completely unacceptable dynamics, and even slight movements by the crew would take what appeared to be a stable tether dynamic and push it quickly into a series of disastrous loops.

The problem seems to be that you need the tether to be at full stress before you spin up, in order for it to act as if it is a solid, unbending link between the two end-point masses.  If you at any point stop thrusting the two masses away from each other, along the tether vector, the tether will recoil, start pulling the two masses back together, and start looping like crazy.  The spin-up maneuver, involving as it would need to the continuous thrust to apply pressure along the tether, but decreasing as the spin replaces the thrust to apply tension to the tether, is of such complexity that it would need to be demonstrated several times before you could consider using it operationally.

So -- those who believe that there needs to be an awful lot more real-world testing, in space, of the dynamics of such tethered systems before such a thing is designed into the cruise architecture of a Mars mission are the winners of this discussion at present, IMHO... :)

Please help me get my head around this.

1) Based on the Gemini Tether Experiment (GTE) it would appear that tethers don't work in the context of them being inherently stable.

2) So I went off to do some more reasearch and looked into it a bit more. According to wikipedia the GTE only got up to 0.00015g
https://en.wikipedia.org/wiki/Gemini_11#Scientific_experiments
"The passive stabilization experiment proved to be a bit troublesome. Conrad and Gordon separated the craft in a nose-down (i.e., Agena-down) position, but found that the tether would not be kept taut simply by the Earth's gravity gradient, as expected. However, they were able to generate a small amount of artificial gravity, about 0.00015 g, by firing their side thrusters to slowly rotate the combined craft like a slow-motion pair of bolas.[4]"

3) However even at this very low AG effect the NASA spiel states the following,
https://nssdc.gsfc.nasa.gov/nmc/masterCatalog.do?sc=1966-081A
"The hatch was closed at 9:57a.m. and shortly afterwards the spacecraft were undocked and Gemini 11
moved to the end of the 30 meter tether attaching the two spacecraft. At 11:55 a.m. Conrad initiated a slow rotation of the Gemini capsule about the GATV which kept the tether taut and the spacecraft a constant distance apart at the ends of the tether. Oscillations occurred initially, but damped out after about 20 minutes. The rotation rate was then increased, oscillations again occurred but damped out and the combination stabilized. The circular motion at the end of the tether imparted a slight artificial "gravitational acceleration" within Gemini 11, the first time such artificial gravity was demonstrated in space. After about three hours the tether was released and the spacecraft moved apart."

Saying that the GTE is therefore proof that tethers are unstable, given the extremely low AG effect attained, I think is unfair. (I admit this is my gut feel and is completely unsubstantiated.)

Is it wrong to say, that the greater the tension in the cable, the more stable the system will become?

Online Eric Hedman

  • Full Member
  • ****
  • Posts: 770
  • Liked: 186
  • Likes Given: 159
Is it wrong to say, that the greater the tension in the cable, the more stable the system will become?
It would at this point be unsubstantiated to say this.  It might be true, but at this point tests with larger spacecraft at greater tension and faster rotation need to be done before that assertion can be made.  In larger craft the mass of the human body will be moving around potentially inducing oscillations.  Determining the natural frequency of such a system would be an interesting problem.  It might become necessary to put some kind of dampening system in place.  Of course we won't know unless we try.  The first thing to do is to try to create a computer simulation.

Offline Aussie_Space_Nut

  • Full Member
  • *
  • Posts: 163
  • South Australia
  • Liked: 50
  • Likes Given: 158
I wonder if having a rigid pole attached to the nose of the spacecraft, with the tether attached to the end of the pole would help keep things stable?

Offline Peter.Colin

  • Full Member
  • *
  • Posts: 161
  • Belgium
  • Liked: 16
  • Likes Given: 36
I wonder if the use of the crane to lift the nose tethered BFS, onto the BFR.
And the oscillations caused by that action are indicative for what would happen in space?
 
« Last Edit: 09/17/2017 10:50 AM by Peter.Colin »

Tags: