tag:blogger.com,1999:blog-3596550435682943926.post8789480340474195652..comments2024-03-14T18:25:31.577-07:00Comments on Hop's Blog: Orbital Momentum as CommodityHop Davidhttp://www.blogger.com/profile/12923433894475072056noreply@blogger.comBlogger5125tag:blogger.com,1999:blog-3596550435682943926.post-51606431411040445212015-06-02T13:35:20.592-07:002015-06-02T13:35:20.592-07:00Hollister David, avoiding debris impacts is critic...Hollister David, avoiding debris impacts is critically important for tethers in LEO. Near the Moon you have less to worry about in terms of debris.<br /><br />It's not only the tether's angular velocity that can be altered by reeling but also the orbital eccentricity. A tether's orbit might become slightly eccentric after catching or releasing a payload. From the Lunavator paper:<br /><br />"When a tether is near the apoapsis of its orbit, the tidal forces on the tether are low. When it is near periapsis, the tidal forces on the tether are high. If it is desired to reduce the eccentricity of the tether’s orbit, then the tether can be reeled in when it is near apoapsis, under low tension, and then allowed to unreel under higher tension when it is at periapsis. Since the tidal forces that cause the tether tension are, to first order, proportional to the inverse radial distance cubed, more energy is dissipated as the tether is unreeled at periapsis than is restored to the tether’s orbit when it is reeled back in at apoapsis. Thus, energy is removed from the orbit. Conversely, energy can be added to the orbit by reeling in at periapsis and reeling out at apoapsis. Although energy is removed (or added) to the orbit by the reeling maneuvers, the orbital angular momentum of the orbit does not change. Thus the eccentricity of the orbit can be changed."<br /><br />In addition to Spectra, you might also look at Vectran as a possible tether material. This is the material used by NASA for the airbags on the Mars Exploration Rovers and for the tether on Curiosity's rocket crane. Vectran is known for high strength and modulus, low creep and stability at high temperatures. The tensile strength for this material can be as high as 3.2 GPa. http://www.vectranfiber.com/BrochureProductInformation/TensileProperties.aspxPeter McArthurhttp://www.eml2.spacenoreply@blogger.comtag:blogger.com,1999:blog-3596550435682943926.post-74762448184565707102015-06-02T09:25:18.966-07:002015-06-02T09:25:18.966-07:00Peter, vertical tethers with their foot in the upp...Peter, vertical tethers with their foot in the upper atmosphere could indeed drop payloads towards the earth at slow suborbital speeds.<br /><br />The reason I didn't look at these is they'd have a large cross sectional area presented to heavy debris flux. There's too much stuff in low earth orbit.<br /><br />As you mentioned on Facebook, a rotovator can have a shorter length and thus less vulnerable to debris impact. If the velocity of the tether tip is less than the material's critical velocity, we don't have a big exponent exploding the taper ratio. If memory serves, Spectra's Vcrit is 2.7 km/s. I haven't made a spreadsheet for Moravec's equations yet, but I'm guessing 1 or even 2 km/s tip speed could be done with a moderate taper ratio and that'd be more than enough to get an apogee above the heavy debris zone.<br /><br />A LEO rotovator tip moving 2 km/s could drop a payload at 5.7 rather than 7.7 km/s. That'd take care of nearly half the re-entry kinetic energy.<br /><br />Rereading it seems the rotovator's angular velocity can be altered by extending or pulling in tether tips. So it's more doable than I had thought to have tether tips at the right place and time for a launch window.<br /><br /><br />Hop Davidhttps://www.blogger.com/profile/12923433894475072056noreply@blogger.comtag:blogger.com,1999:blog-3596550435682943926.post-56511952154026380102015-06-02T03:04:26.522-07:002015-06-02T03:04:26.522-07:00The three-tether system is quite interesting. Woul...The three-tether system is quite interesting. Would the tethers orbit the Earth on the equatorial plane with transfers to and from the moon occuring twice a month as the moon crosses the equatorial plane?<br /><br />You make a lot of interesting arguments against rotovators. I'm still weighing the pros and cons with rotovators and vertical tethers. It's true that the stress is higher for rotovators. I think of a vertical tether as a subset of rotovators. They have a particular rate of one rotation per revolution. The additional mass from the higher taper ratio isn't necessarily bad. I would think a certain amount of tether mass would be desirable to prevent the tether's orbit being wrecked on catches and throws. You would need enough mass to keep the orbit stable but not so much that it would take decades for total payload throughput to equal structural mass. Perhaps it would be desirable to despin a rotovator on a catch and spin it up again for a throw. It has been proposed by Landis that reeling a tether in and out at various points in a gravitational gradient (assuming elliptical orbit) could adjust the tether's orbit.<br /><br />Your point about lowering reentry speeds to recover upper stages is very insightful. Although, if you plan to use momentum exchange to bring water to LEO to as a means to lower reentry velocity then I don't see why you can't just cut out the middle man. Just use momentum exchange to slow down the upper stages directly.<br /><br />One thing I remain convinced of is that the best near-term ISRU application is providing radiation shielding to platforms in lunar space. I doesn't matter to me if the shielding material comes from the moon or from asteroids or arrives by tether or by rocket. EML2 is where rock is needed.Peter McArthurhttp://www.eml2.spacenoreply@blogger.comtag:blogger.com,1999:blog-3596550435682943926.post-15972568393931029302015-06-01T09:42:38.989-07:002015-06-01T09:42:38.989-07:00LocalFluff,agree there are interesting manfacturin...LocalFluff,agree there are interesting manfacturing possibilities for space environments. Besides microgravity, A big variety of temperatures is available. Vacuum is a great insulator and with mirrors it's possible to pack a lot of thermal watts into a small volume. And with Multi Layer Insulation (MLI) shades, cryogenic temperatures are also doable.<br /><br />A dust free hard vacuum is nice for some industrial processes like vapor deposition. There might be a number of industrial uses of these exotic environments.<br /><br />To maintain a tether's orbit with the least station keeping mass, we would try to keep the imported mass equal to exported mass. Imported manufactured goods may indeed help with that goal.Hop Davidhttps://www.blogger.com/profile/12923433894475072056noreply@blogger.comtag:blogger.com,1999:blog-3596550435682943926.post-5351472898551440242015-05-31T11:56:56.324-07:002015-05-31T11:56:56.324-07:00What do you think about service production in deep...What do you think about service production in deep space, which could deliver their results via radio? I mean other than in Earth' orbit where there are many applications for this $122bn/year satellite service industry. Deep space environment itself is already used as a laboratory which returns services. Manufacturing in microgravity doesn't seem to have lived up to the high expectations and doesn't need more than LEO anyway. The Moon's far side and eternally shadowed polar craters are special places. One could also go near or far from the Sun or to an asteroid in order to find deep space environments which maybe could be useful for production of massless services, other than doing astronomy. I have no good ideas myself, nor has Mars One. But services would get rid of the return transportation problem.LocalFluffnoreply@blogger.com