Space Whip

[rogers_buck]

I like this idea so much, I think it deserves its own crackpot thread for discussion.<br /><br />The basics of the idea are as follows:<br />A long (perhaps several km) run of robust metal cable anchored at one end to a central hub. There is a counter balancing mass opposite the cable attached to the hub.<br /><br />This is an engineering detail, but let's pretend there is a solar generator in the hub and the electricity it produces is used to set the cable-hub-counter weight system in rotation by thrusting against the earths magnetic field with controlled currents in the cable. Whatever the means, the cable-hub-counter weidth system spins up to a high RPM that is just short of snapping the cable.<br /><br />Once the thing is spun up a mag-lev payload sled is released onto the leading edge of the cable (direction of rotation). The slippery magnetic field of the mass will allow the mass to slide down the cable, but the fact that the mass is not moving with the same angular motion as the cable will produce a sort of corriolis force between the cable and the payload that will convert the angular momentum of the cable into forward motion for the payload.<br /><br />This conversion of angular momentum of the cable to forward motion of the payload is integrated over the entire length of the cable as it encounters the payload and the mag-lev field. All of the angular momentum of the entire cable and some percentage of the counter weight's angular momentum will be converted to the forward momentum of the payload.<br /><br />This is the principle of a bull whip. The crack of a bull whip is the tip of the whip passing the sound barrier as it is propelled forward by the kink converting the angular momentum of the whip into forward movement of the kink. The payload is the kink. There is an enormous amount of energy available in this system to hurl payloads in free or orbital space.<br /><br />The physics here aren't that difficult to calculate for a few test cases, but I'd like to see

 

[j05h]

If you can build cables like that, why not just anchor both ends to the ground and run maglev launch-sleds up the thing? If you can manipulate multi-mile magnetic cabling, this kind of system would allow for a smoother passenger ride. The main loop would rise as a "St Louis Arch" several miles tall, supended by magnetism. Your sled accelerates up the Loop, releasing a payload at the top. <br /><br />Depending on available technology, the payload is either modules and rocket second stage or Marshall Savage style "Bifrost Bridge" laser array with ice reaction mass. Maybe it goes high enough to do a ballistic toss to a momentum exchange tether. <br /><br />The physics are daunting, never mind the engineering. 50 years from now with SPS and room-temp superconductors, perhaps it will be buildable.<br /><br />Josh <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[rogers_buck]

You are welcome to explore that in another thread, but i'd like to keep this one focused on the space whip because there isn't anything exotic about the tech required here. The cable isn't super strong and the rest of the tech is pretty basic.<br />

 

[nexium]

The system is similar to the bolo we have discussed recently at the forum at www.liftport.com with mag-lev added. We were thinking in vacuum and freefall. Were you thinking the axel is anchored to Earth's surface?<br />Unless you have two cables = double track or other attitude control, the mag sled will quickly twist to the trailing edge instead of the leading edge. Neil

 

[j05h]

> You are welcome to explore that in another thread, but i'd like to keep this one focused on the space whip because there isn't anything exotic about the tech required here. The cable isn't super strong and the rest of the tech is pretty basic. <br /><br />Sorry, shouldn't post that late at nite.<br /><br />I don't see how the cable strength is not super-strong in your bullwhip launcher. I would think it needs to carry the mass of the launcher plus the shearing force when the "whip tip" reverses. Might make more sense for launching from asteroids and other low-G objects.<br /><br />Josh<br /> <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[rogers_buck]

That's the crux of the matter. I think the major force on the cable is the centrifugal force caused by it's rotation. The translation/acceleration of the payload will create a torsion wave (which will look like a shear wave) but this will be 0 at the base and maximum at the top.<br /><br />The coriolis force is 2*m*omega*v, where omega is angular velocity of the cable, v is velocity of the payload and m is the mass of the payload. At the bottom of the cable, v is small so the shear force applied to the cable is the lowest where the tensile loading (from centrifugal force) is the greatest.<br /><br />The maximum power the cable can transmit to the payload without bursting from the shear is a product of the maximum shear strength of the cable and the linear speed of the payload (torsion wave velocity). Hence, the maximum power is extracted at the tip of the cable and not at its base in terms of shear.<br /><br />The maximum strain the cable can withstand goes up by the inverse square root of the cable's radius.<br /><br />Our problem is to balance the radius of the cable, the material of the cable (energy density), the length of the cable, and the rotation rate of the cable so that the torsion wave velocity (our payload's delta vee) achieves the desired value without snapping the cable.<br /><br />The total energy of the system is the integral of the centrifugal force taken over 0 to the legnth of the cable. The total stress on the cable is the instantaneous loading from the shear of the torsion wave and the centrifugal loading at each point. The total energy our payload must absorb from the cable is the integral of the coriolis force taken from 0 to the torsion wave velocity at the tip, which is our desired release velocity - this will be equal to the kinetic energy of the payload.<br /><br />The mass of the payload, and the release velocity we can stipulate. The tensile and shear strength for various cable materials we can look up. The problem then becomes a matter of

 

[scottb50]

Space Whip. Is that a dessert topping or a floor cleaner? <div class="Discussion_UserSignature"> </div>

 

[rogers_buck]

Kinda like a desert topping. It is what we apply to your naked body after throwing you out a airlock. By laying on the space whip we break the skin so that the juices can boil out more painfully.<br />

 

[rogers_buck]

Sorry, missed your post.<br /><br />No, this I was thinking would be inf ree space. The whip would be counter weighted and the whole system would be rotating about the axle. The spin-up would be electro-magnetic in the earth's magnetic field just for argument sake.<br /><br />I think the bolo is a simpler system to model than this, because the torsion wave mechanics are a bit complex. However, this system is capable of imparting far greater energies than a simple bola.<br /><br />

 

[j05h]

I see the payload being a craft made as a torus or tube split lengthwise. The two halves are held together via maglev, with the whip in between. Surounding and balancing the payloads presents fewer issues with the craft trying to twist around the whip. If anything goes wrong, the payload maglev can cut it's power and disengage from the whip. Otherwise, the maglev holds the cable in place, managing any "lashing" effects at the tip. The two ship halves could consist of a payload segment and a tug segment that returns to the Whip.<br /><br />I don't know enough about the physics involved, but it seems like a whip would make a rough ride. What scale are you thinking? 1 or many km length? Would save a lot on structures compared to big linear accelerator or larger "MXER" tether.<br /><br />Rogers_Buck - I never ever want to be on your bad side! Especially near an airlock! <img src="/images/icons/wink.gif" /><br /><br />Josh <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[rogers_buck]

I sketched out what I think are the physics in an earlier post. Everything seems to go in the right direction so as not to require super materials. To answer the length question, however, will require more than a sketch. The actual calculation for a few trial cases will take some effort. The equations for the stress wave will require some real world numbers, for example.<br /><br />However, if you ignore the stress wave component and simply look at the delta vee you need to achieve, you can easilly computer how long the cable would have to be for a specific gee load. I think in terms of rocks and donuts, but for chimpanzees you need to be gentle...<br /><br />Next, you can take choose a mass for your payload and then that will give you the total energy required for your spinning cable. Pick a density, steel for example, and that will give you a radius for your cable.<br /><br />Of course all that will be somewhat meaningless without plugging in the torsion wave velocity and feeding it back in, but it would be a good start for an order of magnitude sanity check.<br /><br />

 

[rogers_buck]

I've done some googling, and I can't find any space whip type systems in the proposaphere. There are some close concepts like "sky hook" rotating cable, but none of these propose surfing the torsion wave. So there are two possibilities:<br />1) It is a dumb idea that won't work,<br />2) It isn't turning up in my google searches.<br /><br />There could be various causes of 2) and we can't do anything about solving that question, but we can deal with 1). We will need to run some numbers...<br /><br />The first order calculation should be a sanity order of magnitude calculation. Find the total energy of the rotating cable. This is a product of the rotation and the integral of the moments of inertia over the volume of the cable. We will need the formula at this stage, nothing more.<br /><br />Next, we will need to decide on some parameters like release velocity and payload mass. We can then calculate the energy required to achieve the desired acceleration and plug this back into the energy equation to calculate cable length.<br /><br />We will need to play with the radius of the cable cross-section at this point as a paramter pickup to tweak the available energy.<br /><br />When we think we have reasonable length and cross sections then we can check the tensile strength of steel (I forgot to mention assuming a steel cable) and see how close we are to bursting the cable.<br /><br />That will give us a static order of magnitude sanity check for this system. The next part will be more difficult since the torsion wave will need to be analyzed in velocity, longitudinal shear force, etc.. Add to this that the effects are all second derrivative time effects as you move down the cable - we are accelerating...<br /><br />But for now, let's do the sanity.<br /><br />Anyone care to derrive the formula for the energy of the rotating cable in free space? I'll do it if I have to, but frankly, it hurts to sit here so I could use a friend...<br /><br />

 

[vogelbek]

Your concept is intriguing. Its a sort of cross between an electrodynamic space tether and a momentum exchange tether. Plus the clever sliding mass action. Cool.<br /><br />Heres some rain for your parade (I live in seattle, where all parades and festivals feature umbrella themes of some kind or other...)<br /><br />1) Consider that you will loose angular (orbital) momentum from your rotating hub, and consequently need to reboost after each throw. Alternatively, if you could guarentee a consistant supply of high-speed deep space objects to catch, you might beable to reboost from their high momentums (very tricky though)<br /><br />2) Its hard to deploy and control distributed space systems. Being light weight and very long and fiberous, it will be subject to gravitational gradients from the earth, tidal disturbances from the other planets and the moon, and possibly atmospheric drag. Because these effects are pretty complicated, you are likely to get stange oscilations in your whip that would be super hard to damp out. Not saying its imposible, just hard.

 

[rogers_buck]

Thanks for your coments. The physics of this thing are interesting.<br /><br />Concerning point 1), I think this system is a little different than you may be thinking. The orbital angular momentum of the system should not be altered. A rapidly rotating sphere has the same orbital angular momentum as a static sphere, however, the rapidly rotating sphere has large angular momentum compared to the static sphere's zed. If a counter to rotation thruster fires on the rapidly rotating sphere, it's orbit will not change as it slows rotation or even reverses.<br /><br />If you were thinking of a traditional skyhook, you would be correct. But in this concept, the angular momentum is not used as a lever against the orbital angular momentum to boost a payload. Instead, the roational angular momentum is converted to linear momentum of the payload and heat. The heat comes from the propagation of the torsion wave through the cable. <br /><br />Concerning 2), I am sure you are correct that there would be a plethora of second order perturbances interacting with this system. It is useless to discuss engineering details, but I imagine an active current looping system runing the length of the cable to spin the cable up to speed and stabilize it in earth's magnetic field. Fortunately, EM force is much stronger than second order effects so a good dynamic control system should tame the beast. After all, an F-117 flies doesn't it?<br /><br />

 

[vogelbek]

Point 1: If we draw a control boundary some distance away from the whip system as its firing, and watch some payload mass being expelled from one side of the control volume, we have to account for the fact that the entire whip system must gain an opposite ammount of momentum in the other way. Think of it like a delta-v from a single rocket pulse. If you dont counter it with a pulse in the opposite direction, you must change your orbit. <br /><br />I'm gussing that typical opperations would have you shooting stuff into higher orbits, so the whip would get knocked back into a lower orbital speed, thus lower orbit.<br /><br />Point 2: These second order effects are pretty important to understand how your very dynamic system opperates. Frankly, I cringe at the systems engineering implications of all the support opperations of making this thing happen, even if the whip is extremely well behaved. I'm just imagining unforseen vibrations shaking things apart, or making docking opperations impossible.<br /><br />For both points, I dont think either points I've brought up kill the idea, but I think they do need to be accounted for as you get into more detail.