Author Topic: Nose tethered BFS Spaceships for artificial gravity during the coastal phase.  (Read 15318 times)

Offline douglas100

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What "mass and added complexity"? At Mars gravity and 4-6 RPM, AG is almost free. The only reason not to do it would be out of spite.

There must be added mass and complexity to do this. How much is up for discussion.

As for a another reason not to do it, what about "it's unnecessary?" Spite has nothing to do with it.
Douglas Clark

Offline Paul451

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There must be added mass and complexity to do this.

Sounds like a religious tenet.

Online RonM

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There must be added mass and complexity to do this.

Sounds like a religious tenet.

If you look at the ITS design from the original presentation you'll see that there is no provision for docking. Two ITS spacecraft can't dock, let alone spin together for AG. Providing that capability would add extra mass.

Are you talking about spinning an ITS around its long axis? That would require a redesign of the deck layout to function properly in flight and when landed.

Offline alexterrell

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Obvious disadvantages: ....., communication with Earth and Mars, ....
Have a small satellite in "wifi range" - perhaps 2km away, with a big dish pointed at Earth. The "satellite" could also have a telescope on the BFR to check the outside.

Offline blasphemer

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I made a mock up picture of two ITS ships connected by a 200m long cable. Enough for 1g gravity at comfortable 2 RPM.


Offline Peter.Colin

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Very nice!

Now let's hope the cable is not to heavy or complex ..... lol
« Last Edit: 09/04/2017 06:33 PM by Peter.Colin »

Offline AncientU

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I made a mock up picture of two ITS ships connected by a 200m long cable. Enough for 1g gravity at comfortable 2 RPM.



I believe the 200-ish meters is a radius, not a 'length' -- the exact number is closer to 225m radius or 450m 'length'
"If we shared everything [we are working on] people would think we are insane!"
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Offline Paul451

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If you look at the ITS design from the original presentation you'll see that there is no provision for docking. Two ITS spacecraft can't dock, let alone spin together for AG. Providing that capability would add extra mass.

The refuellers attach side-by-side to the main ships, and on Earth the ships are lifted by the nose to be re-mounted on the booster. They have attachments up the wazzoo, and clearly the necessary prox-ops systems.

However, my objection to the original statement is that it solely assumes costs, but ignores any possibility of benefits or savings. That's pre-defining the issue to failure.

I made a mock up picture of two ITS ships connected by a 200m long cable. Enough for 1g gravity at comfortable 2 RPM.

A cable is a bad idea. Picture a 200m (or 400m, for 1g/2RPM) cable hanging in still air, with an ITS-like platform hanging off the bottom, with dozens to a hundred people moving around. It will twist and oscillate like crazy. In free-space it's actually worse, the oscillations dampen very slowly (bouncing back and forth along the cable). There's a similar problem with compressive structures like trusses. The suggested alternative is a tensegrity structure, a combination of tensile and compressive structures; eg, a truss held under compression by cables. A design suggested for long tethers is to have an inflated tube (or four) providing the compressive-resistive structure, which is held in compression by the tensile cables. In simulations, that apparently dampens all axes of oscillation, including twist, better than just cables, or just trusses.

IMO, 2RPM is ridiculously low. And 1g is too high, if you expect people to permanently colonise Mars. (If 0.38g isn't sufficient, we aren't colonising Mars, so the point is moot.) 4RPM and 0.38g gives you 40m total length. Including the length of the ITS cabins, it's barely worth the cable.

It would be nice to know for sure; since radius/diameter scales with the square of RPM, so doubling the RPM quarters the length. And linearly with g-load, so double the RPM at 38% gravity gives you a ten-fold reduction in length.

Offline Coastal Ron

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A cable is a bad idea. Picture a 200m (or 400m, for 1g/2RPM) cable hanging in still air, with an ITS-like platform hanging off the bottom, with dozens to a hundred people moving around. It will twist and oscillate like crazy.

I'm not sure what you envision all those people are doing, but I can't imagine anything they could be doing that would make an ITS twist and oscillate. And with the tension between the two ITS being so high, random movement of people on both ITS are likely to be very muted vibration-wise. At most all you'd need is a hydraulic damper located somewhere on the cable to smooth out minor vibrations.

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In free-space it's actually worse, the oscillations dampen very slowly (bouncing back and forth along the cable).

However because of the weight of the two ITS, and the rotation keeping the cable under tremendous compression tension* forces, it's going to be hard to get a cable vibrating in the first place, and the natural tendency would be to mute the vibrations out of the cable.

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The suggested alternative is a tensegrity structure, a combination of tensile and compressive structures; eg, a truss held under compression by cables.

Just stringing a cable between two ITS and getting them rotating without free body issues will be tricky by itself, but having to inflate things in between is going to be even harder.

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IMO, 2RPM is ridiculously low.

Low RPM's are much better than higher ones, not only for people but also for safety reasons.

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And 1g is too high, if you expect people to permanently colonise Mars. (If 0.38g isn't sufficient, we aren't colonising Mars, so the point is moot.)

I agree. Even if the ITS flotilla is returning to Earth it should be good enough to have Mars gravity for the trip.

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4RPM and 0.38g gives you 40m total length. Including the length of the ITS cabins, it's barely worth the cable.

It would be nice to know for sure; since radius/diameter scales with the square of RPM, so doubling the RPM quarters the length. And linearly with g-load, so double the RPM at 38% gravity gives you a ten-fold reduction in length.

Are you familiar with the website SpinCalc? It's where I do all of my "what if" simulations for spinning space station designing. Unfortunately though we lack enough real experience, so the variables and results in the calculator are only estimates based on research done here on Earth.

So if a 200m cable is used, and the required gravity is Mars normal (i.e. .38 Earth), then the RPM would be 1.8. According to SpinCalc that should provide artificial gravity with no known spin-related issues.

As for the cabling, there are Ultra-high-molecular-weight polyethylene (UHMWPE, UHMW) cables that could be used that are both very strong and lightweight.

* Corrected "compression" to "tension" as noted by Paul451 (thanks Paul451!)
« Last Edit: 09/04/2017 11:07 PM by Coastal Ron »
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 Paul451

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I'm not sure what you envision all those people are doing, but I can't imagine anything they could be doing that would make an ITS twist and oscillate.

You've obviously never seen a large mass on the end of a long cable.

and the rotation keeping the cable under tremendous compression forces

Que?

Just stringing a cable between two ITS and getting them rotating without free body issues will be tricky by itself, but having to inflate things in between is going to be even harder.

Quite the contrary, it makes everything easier, that's why it's suggested.

Are you familiar with the website SpinCalc?

Of course. (Although I use my own spreadsheet for quick'n'dirty RPM/radius/g-load calcs.) Ted is extremely conservative in his red/yellow/green warnings, which he has acknowledged in other presentations. He does link to the more recent Lackner and DiZio paper, which is part of modern AG research (including ultra-high RPM research like the "Space Cycle" exercise platform), but doesn't actually use it in the calc.

The old Apollo-era research just doesn't hold up. Even at the time, the result were wildly variable between experiments, which should tip you off that they stumbled onto a confounding factor rather than a real effect. Based on recent research, it looks like that uncontrolled variable was the amount of movement test subjects were allowed or encouraged to make, or actively discouraged from making, during the spin-up phase. The less movement, the worse the results. It's not surprising they did that; even today, you still get people saying "High RPMs might be possible if people limit their movements". The instinct is the opposite to how we actually need to adapt.

But it's funny when people say, "We can only go by Earth research," but then very pointedly ignore everything done in the last 35 years.

[IIRC, Space Cycle runs at around 40 RPM and up to 7g.]

Online KelvinZero

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If you look at the ITS design from the original presentation you'll see that there is no provision for docking. Two ITS spacecraft can't dock, let alone spin together for AG. Providing that capability would add extra mass.
There is a spacex animation showing an ITS being picked up by a cable to it's nose in order to place it on top of the 1st stage. That is probably what inspired all this.

My personal feeling is that we will find a way to avoid this, eg vr treadmills with elastic bands for gravity, but it is still fun to think about.

Despite that nose cable art, I would favour something wider, eg multiple widely spaced cables with cross connections specifically designed to damp any oscillation. Someone must have studied this extensively somewhere.

https://www.youtube.com/watch?v=0qo78R_yYFA?t=120

(for some reason, the t=120 did not work.. go to the 2:00 minute mark to see the ITS lifted by cable)
« Last Edit: 09/04/2017 11:07 PM by KelvinZero »

Offline Coastal Ron

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and the rotation keeping the cable under tremendous compression forces

Que?

Mea culpa. Should have said "tension" forces. Glad you pointed that out.

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Quite the contrary, it makes everything easier, that's why it's suggested.

If you say so. This obviously points to the need for actually DOING rotational gravity research in space instead of theorizing about it. Better to have facts than opinions.

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Of course. (Although I use my own spreadsheet for quick'n'dirty RPM/radius/g-load calcs.) Ted is extremely conservative in his red/yellow/green warnings, which he has acknowledged in other presentations. He does link to the more recent Lackner and DiZio paper, which is part of modern AG research (including ultra-high RPM research like the "Space Cycle" exercise platform), but doesn't actually use it in the calc.

OK good. For myself I'm OK with "worst case", which I'm hoping SpinCalc will be, because if I can come up with solutions that work out OK with "worst case", then I should be OK with "reality".
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 Paul451

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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.

Picture any other type of engineering where you just hand-waved an order of magnitude difference in the properties or scale.

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.

Online KelvinZero

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It just occurred to me, maybe a single cable can work.

I was imagining the ship twisting back and forward, with no way to damp the oscillation, sort of like a weight on the end of a string would tend to spin frictionlessly. But the cable would be able to turn it it's socket so there is no 'winding up', also each ship would have a flywheel that would keep each one rigidly oriented. I don't think there is any possibility of the flywheels becoming saturated since it is still a closed system.

That just leaves normal non-twisting oscillation. Even if it is tight like a violin string I think that sort of oscillation can be damped nicely at the endpoints.

Offline Paul451

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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".

Offline blasphemer

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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.

It may add mass, but complexity? I dont see why. In fact higher RPMs mean astronauts are more likely to be sick, experience Coriolis forces and any oscillations/instabilities are going to be higher frequency so if anything I would say that high RPM adds complexity. Unless you plan to make your connection rigid, you have to deal with oscillations no matter what.

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.
« Last Edit: 09/05/2017 12:25 PM by blasphemer »

Offline Paul451

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Offline blasphemer

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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?
« Last Edit: 09/05/2017 12:37 PM by blasphemer »

Offline blasphemer

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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.


Offline blasphemer

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https://www.reddit.com/r/spacex/comments/3ogkfa/artifical_gravity/cvxe4yu/?st=j77ls2q8&sh=1fabfcd4

I found an older reddit thread discussing this idea. Some relevant info:

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We end up with a radius of only about 3500 meters.

Assume MCT weighs 200 tonnes during cruise, and you want to simulate Mars gravity. Using three redundant Dyneema fibers, 50% engineering margin (NASA standard), 50% live load allowance, and 100% overhead for space environment protective coatings, I get a mass of 7.5 tonnes (or 3.8 tonnes per MCT). So it's totally doable.

The other thing is spin/despin fuel. That's another 8 tonnes per MCT.

These are some weird assumptions (3500m radius? 200 tons only?) but in the end cable mass is 7.5 tons and is actually less than half of spin/despin fuel mass. So maybe any AG solution will be dominated by fuel mass instead of mass of the mechanism itself?
« Last Edit: 09/05/2017 01:11 PM by blasphemer »

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