https://www.reddit.com/r/spacex/comments/3ogkfa/artifical_gravity/cvxe4yu/?st=j77ls2q8&sh=1fabfcd4QuoteWe end up with a radius of only about 3500 meters.and you want to simulate Mars gravity.
We end up with a radius of only about 3500 meters.and you want to simulate Mars gravity.
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.
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?
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.
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.
Quote from: Coastal Ron on 09/04/2017 11:08 pmFor 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.
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".
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.
Quote from: KelvinZero on 09/05/2017 11:32 amIt 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".
It just occurred to me, maybe a single cable can work. [etc]
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.
Sometimes you just need to ignore "the rules" to see if there are alternative solutions that still result in the desired outcome.
Quote from: Coastal Ron on 09/05/2017 09:14 pmI'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.
Quote from: Coastal Ron on 09/05/2017 09:24 pmSometimes 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."
Quote from: envy887 on 09/05/2017 06:57 pmBut 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.QuoteThere 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.
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.
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.
Quote from: the_other_Doug on 09/06/2017 06:45 pmI'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
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?
Quote from: the_other_Doug on 09/12/2017 01:35 amOn 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_StabilizationThe 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'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...
Is it wrong to say, that the greater the tension in the cable, the more stable the system will become?