Author Topic: Launching a Nanocraft to Alpha Centauri with a momentum tether?  (Read 3077 times)

Offline Journeyman

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Inspired by the Breakthrough Initiatives Starshot project, I had an idea for another way to accelerate a Nanocraft to Alpha Centauri.

Instead of blasting the Nanocraft with lasers, what if we instead used a momentum tether?

The concept is very simple, attach a 1 Gram Nanocraft at one end of the tether, a counter weight at the opposite end, then begin rotating the tether while extending the tether by a winch system until it reaches the desired length.

The rotation can either be done by eletrical motor at the center, or by Ion drive at the ends of the tether.

When the target velocity has been achieved the tether will carefully aim towards Alpha Centauri and release the Nanocraft at the precise point so it will head towards the destination.

As a thought experiment, will this work?

I'm no expert in the maths needed to calculate this. What are the physical limits? How fast can you accelerate the Nanocraft by this method?

What G loads will the craft experience? What material limits are there? The strongest material I  know is Carbon Nano Tubes.

So for this method to succeed, it needs to reach 20 percent of light speed or greater to be able to compete with the light sail concept.

So here are my questions for the experts here:

1. How long must the tether be to achieve at least 20 percent of light speed?
2. Will the Carbon Nano Tube tether have enough strength to withstand the centrifugal force at these speeds?
3. How much G forces will the craft experience at desired speed of 20 pecent light speed?

Usually there are issues with these ideas that makes it impossible when you do the math.
So if it's not possible to reach this speed, how fast can you send a 1 Gram Nanocraft with this concept using the best materials we know today?

For manned missions to Alpha Centauri, more likely Proxima Centauri if that Earthlike planet is confirmed. I assume the G forces and strain on the tether will be far greater than what it can tolerate if you want to reach any significant percent of light speed.

But if you instead send tiny Nanocrafts at 1, 10, 100 or even 1000 Grams. Maybe that will be a game changer as they will not create the extreme challenges as a giant manned spaceship will?

So my dear NSF members, lets hear your thoughts on this concept! :)


 

Offline Stan-1967

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I think if you ran the calculations for the tensile strength needed by a tether moving at relatavistic speeds, it would exceed by several orders of magnitude the strength of materials needed to build a space elevator, of which technology has not invented anything strong enough for even that.

In short, I think the idea will require unobtanium, not to mention from an energy standpoint, you are proposing accelerating a large mass tether to relatavistic speeds so that it can launch a smaller/lighter craft.   Your energy budget is therefore a show stopper
« Last Edit: 08/19/2016 03:53 AM by Stan-1967 »

Offline Star One

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Wrong star try Proxima Centauri instead.

Offline Stormbringer

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Wrong star try Proxima Centauri instead.

All of the Centauri stars referred to as Alpha Centauri A B and C (as Proxima Centauri is referred to by astronomers who believe that it is gravitationally bound) are fairly close to being in line with each other and you can basically stand on one and pee on the other two distance wise. If you send a probe to Proxima Centauri you might as well send one to the other two stars as well in the same project using the same infrastructure and ground crew.

It saves time. It probably saves development and other project costs. there would be very little delay between arrival of each probe set at each star. perhaps a year or two. It'd be analogous to voyager one and voyager two.
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Offline rdheld

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might not exist tether materials strong enough to do the job.   If youy are going to Alpha Centauri, you could have the ship split off payloads to visit Beta and Proxima.

Offline Journeyman

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Wrong star try Proxima Centauri instead.
I think if you ran the calculations for the tensile strength needed by a tether moving at relatavistic speeds, it would exceed by several orders of magnitude the strength of materials needed to build a space elevator, of which technology has not invented anything strong enough for even that.

In short, I think the idea will require unobtanium, not to mention from an energy standpoint, you are proposing accelerating a large mass tether to relatavistic speeds so that it can launch a smaller/lighter craft.   Your energy budget is therefore a show stopper

I would like someone knowledgable in tensile strength calculations to examine the limits of the strongest materials we know of today.

From my layman point of view the energy needed to rotate the tether should not be greater than what we could manage. As i see it, the tether dont need to spin that fast, the length of the tether is what will provide the speed. The question is how to balance the rotation speed with tether length?

The longer tether, the more mass and thus more strain. Or shorter tether but more rotational speed, but that probably means the same amount of strain on the tether?

It will likely as you say exceed by several orders of magnitude the strength of the tether eitherway.

But i'm still curious how fast we could send a tiny probe? What is the highest speed obtainable by current technology?

Can we reach1 percent of lightspeed? or not even faster than Voyager 1 at 17 km per second?

If we can get 10 x the speed of Voyager 1 then this might be worthwhile for exploring the outer solar system if nothing else.

Your input is greatly appreciated!


Online Robotbeat

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Specific strength is at most 4GPa/(g/cc).
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Offline Stan-1967

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I would like someone knowledgable in tensile strength calculations to examine the limits of the strongest materials we know of today.

From my layman point of view the energy needed to rotate the tether should not be greater than what we could manage. As i see it, the tether dont need to spin that fast, the length of the tether is what will provide the speed. The question is how to balance the rotation speed with tether length?

Your input is greatly appreciated!

How do you propose to rotate this tether?  To rotate it you will need to apply torque/moment to one end, so is it rigid?   If so it needs to be strong in compressive & shear modes as well. This may mean that a very long tether would take years to spin up to max velocity without bending the tether to failure or wrapping it around itself.

The speed at the tether tip scales linear to the length X angular velocity.   Reducing one will proportionaly increase the other.  Intuitively it shouldn't matter if is it short or long, the tangential velocity needs to be maximized to the ultimate tensile strength.   

Imagine the moon tied to a tether going around the earth at .2c, it would complete a revolution in about 7 seconds.  That is the type of velocity you are talking about.

Finally, at .2c, relatavistic effects on the tether mass will make the faster moving end heavier per unit length than slower moving segments.  No idea how to handle relatavistic length contraction.

All of these problems still pale in comparison to the energy you are going to need to spin up the tether.

Online Robotbeat

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The maximum achievable tip speed is related to the characteristic velocity. And the characteristic velocity "u" = sqrt(specific strength).

So for 4GPa/(g/cc), that's 4MPa/(kg/m^3) or 4e6 m^2/s^2. So square root of that is 2000m/s.

You can get a little better than that with tapering the tether and using a multiple stage tether, but that gets exponentially harder. Even with a collossal effort, you're not going to get more than 20km/s.

20km/s can be achieved with chemical or nuclear or solar-electric propulsion or some combination thereof.

An Oberth burn deep in the Sun's gravity well can get you 100km/s.

So no, there's no way to get near-relativistic speeds (30,000,000m/s for 10% c) with a tether the way you're imagining it. Even with carbon nanotubes.
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Offline KelvinZero

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Now if we could just find a way to grab two quarks and pull them apart.

My understanding is that the energy to do so just creates more quarks, so I am guessing this is an unbreakable elastic band that just stores more energy no matter how far you stretch it.

Sounds easy. It probably really, really isn't  :)

I don't suppose anyone has any math on that. Hypothetically can we calculate the mass/meter of such a thread? Does the question not even make sense for some reason I haven't grasped?

(edit)
https://www.quora.com/What-happens-when-you-pull-apart-a-pair-of-two-quarks

Rats. This says I was misunderstanding. They say the energy would just create new quarks for the separated quarks, but then you just have a pair of mesons. Quarks can't exist in isolation but there is no unbreakable thread.
« Last Edit: 08/26/2016 09:14 AM by KelvinZero »

Offline colbourne

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It is still possible to use this launch tether with a booster rocket on the probe.
Reaction can be against a large weight such as an asteroid , space station or another tether.


Offline ChrisWilson68

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It is still possible to use this launch tether with a booster rocket on the probe.
Reaction can be against a large weight such as an asteroid , space station or another tether.

It's not clear what you mean here, but anything I can think of that you might mean is wrong.

Robotbeat's calculations show that the tether would rip itself to pieces by the time the tip was going around 20 km/s at best, which is three orders of magnitude below the relativistic target of .1c.

If you mean to first use the tether, then release, then use a rocket, that's not going to be helpful because at the time you leave the tether you're still three orders of magnitude too slow -- anything that will get you from there to .1c can get you there nearly as easily without the tether.

If you mean use the rocket while still attached to the tether -- that won't work because the limit on the tether is that it will rip apart if the end goes any faster than that.  It doesn't matter what made it go that fast.

Offline colbourne

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The whole system would be in an eliptical orbit. Release is calculated to occur at the optimum position and speed.
The tether achieves an additional 20km/s.
The released probe is boosted by an attached rocket.
A laser could be used for the last acceleration phase.

Online Robotbeat

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The whole system would be in an eliptical orbit. Release is calculated to occur at the optimum position and speed.
The tether achieves an additional 20km/s.
The released probe is boosted by an attached rocket.
A laser could be used for the last acceleration phase.
20km/s is lost in a sea of the required 30000km/s for 10% c like spitting into the ocean.
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Offline Impaler

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A tapering tether might exploit some of the principles of a whip to achieve high tip speed, a rocket could be used at the base to impart a brief impulse to the tether which creates a ripple that will accelerate down the tapering tether and then impart most of it's energy into the probe.  Aiming may be rough so follow up laser light sail for fine tuning the trajectory may be needed.

Suppose that the tether base can receive an acceleration of 10 km/s which is achievable with chemical propellants, perhaps explosives even.  Then the whip tether tapers and achieves a 100 fold acceleration, standard whips do over 30 fold acceleration so this isn't inconceivable.  The tip speed is now 0.3 percent of the speed of light which would get you to alpha centauri in 1500 years.  I'd suspect that's about the best you could get out of a tether system as the tensile strength of the tether is certainly going to be an issue as the speed increases, presumably a longer tether lets the longer wave exist which puts less stress on the tether material.
« Last Edit: 08/26/2016 11:26 PM by Impaler »

Online Robotbeat

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10km/s can't be achieved with modern materials because this "characteristic velocity" of the tether (i.e. what can be done without tapering) is equal to the square root of the specific strength:
http://www.tethers.com/MXTethers.html

Only 2km/s can be achieved with current materials.

And due to the exponential nature of this problem, a 100 fold taper ratio is not feasible. The mass of a tether (divided by the mass at the tip, or the mass of the payload if you will) if you have a tip velocity 100 times that of the material's characteristic velocity is (according to this: http://www.nasaspaceflight.com/_docs/MXER%20TAG%202003%20Report.pdf ) :
m_tether/(m_payload)=sqrt(pi)*(v_tip/v_char)*erf(v_tip/v_char)*e^((v_tip/v_char)^2)

So if (v_tip/v_char) = 100:


m_tether/(m_payload)=sqrt(pi)*100*erf(100)*e^(100^2)= 1.56*10^4345
https://www.wolframalpha.com/input/?i=sqrt(pi)*100*erf(100)*exp(100*100)

In other words, even if your payload was just a single electron, your tether would have a mass far greater than the ENTIRE visible Universe.

(using a whip action doesn't avoid this, by the way...)
« Last Edit: 08/28/2016 02:47 AM by Robotbeat »
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To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0