Date: Mon, 11 Sep 1989 22:30-EDT From: space-tech-request@cs.cmu.edu To: "~/st/lists/stdigest" Subject: Space-tech Digest #32 Contents: John Hogg gravity gradients Kevin Ryan Re: gravity gradients Korac MacArthur Mars Missions Marc Ringuette Re: Mars Missions Korac MacArthur Re: Mars Missions Don Willits Re: Mars Missions John Roberts Re: Mars Missions John Roberts Mars mission, space elevators John Roberts Re: Mars Missions Marc Ringuette Re: Mars Missions ------------------------------------------------------------ From: John Hogg To: space-tech@cs.cmu.edu Subject: gravity gradients Cc: henry@utzoo.uucp Date: Mon, 11 Sep 89 09:09:43 EDT The Oracle at Ramsey-Wright says: >If you can haul considerable masses to LEO, there are other ways of dealing >with this -- you can cancel the gravity gradient with proper placement of >nearby masses. Could you explain how this is done? My intuition says that any additional mass will just set up a more tether-like situation, which is to say, it makes things worse. Masses at a greater orbital radius will merely ``pull from the other end''. (Maths on request.) Am I missing something? --- John Hogg hogg@csri.utoronto.ca Department of Computer Science, University of Toronto ------------------------------ Date: Mon, 11 Sep 89 09:55 EDT From: KEVIN@A.CFR.CMU.EDU Subject: Re: gravity gradients To: space-tech@CS.CMU.EDU > John Hogg (jh@db.toronto.edu?) asks: > - How can tital forces be cancelled with other masses? Won't they just > add to the problem? See _Dragons Egg_, by Bob Forward (one of the experts on gravity). The cancelling masses are put in a plane perpendicular to the radius from the planet (or neutron star, or whatever) to produce a cancelling tide. A tide producing a gradient of X to and from the gravity well also produces a gradient of -0.5*X perpendicular to the radius. If near a large gravity well you experience a gradient of 0.1 uG pulling your head from your feet you will experience a gradient of 0.05 uG squeezing your waist. A ring shaped countermass of the appropriate size and mass can cancel this out. Sorry I don't have the detailed math, but this gives the general idea. kwr kr0u+@andrew.cmu.edu ------------------------------ Date: Mon, 11 Sep 89 14:37 EDT From: Subject: Mars Missions To: space-tech@cs.cmu.edu I think that the tether idea would not be as good as a rotating cylinder. The cylinder would allow for cargo to be stored at the walls, serving as a rad hard shelter (especially if it was water and fuel tanks, metal parts,etc). Even better for control would be the two cylinder design (opposite rotation) so an outer framework would not spin, and allow good null-gee areas as well as a stable platform for course correction thrusters instead of stopping the rotation or figuring how to time the thrusters right. Admittedly, two cylinders and a framework would take longer to build, but if you can't do it right the first tim e... I'd hate to have a tether snap, then all the parts go their separate (off course) ways. At worse, a more solid hulled ship would still be on course if anyone could fix it by the time the injection burn had to be made. On propulsion: has anyone done a study on how much more efficient a linear acelerator type propulsion system would be with liqid nitrogen cooled super- conductor magnets? I would hope an order of magnitude or more. ****************************************************************************** Official pentagon definition of peace: "Permanent pre-hostility". Korac MacArthur K_Macart@unhh (bitnet) UNH Cosmic Ray Lab ****************************************************************************** ------------------------------ Date: Mon, 11 Sep 1989 15:36-EDT From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: space-tech@cs.cmu.edu Subject: Re: Mars Missions > I think that the tether idea would not be as good as a rotating cylinder.... Eek! You're comparing apples and oranges. A tether would add little weight to the system, and can be an add-on to most Mars mission scenarios. A cylinder would change everything, be extremely heavy, and make any mission many times more expensive. The gains are dubious also. Structural strength just isn't an important factor in a planetary mission; null-gee areas should be available regardless; and shielding would be very inefficiently placed at the rim of a rotating structure where it would require even greater structural strength. ===== The amount of thrust required to spin up a rotating structure is something to check; the velocity required of the capsules is the square root of the desired acceleration times the radius. For a 500m tether (250m radius), the capsules must be going 50 m/s relative to the center to produce 1 g at the outside. That's not too much compared to the thousands of m/s needed for the mission. ===== How about the shielding question, though? Should my Mars scenario include a well-shielded "box" for the astronauts to hide in during solar maxima? Maybe someone could check on that. ---------------------------------------------------------------------------- | Marc Ringuette | mnr@cs.cmu.edu | Never lick a gift | | Carnegie Mellon Comp. Sci. | 412-268-3728(w) | horse in the mouth. | | Pittsburgh, PA 15213 | 412-681-5408(h) | | ---------------------------------------------------------------------------- ------------------------------ Date: Mon, 11 Sep 89 18:16 EDT From: Subject: Mars mission To: space-tech@cs.cmu.edu Ok, a cylinder is the harder, more expensive way to go. But the mass required for a pair of rad shielded pods swinging around on tethers means the tethers need more tensile strength than probably exists, and also a really reliable method of making sure the things don't go . If it does, both pods go off course quite a bit, making it tough to carry on without excess propellant to change the mission into a no-gee one. Also: has anyone thought if the electrical inductance of long metal "wires" cutting across lines of Mag fields at planetary speeds is significant? I think even today's satellites in geosynchronous have to have charge dumps to keep static charges in the kilovolt range from frying the electronics on board. I know Mars has little field, but the Solar one might be strong enough for effects to be noticed. Hmmm... Korac MacArthur K_MACART@UNHH.BITNET ------------------------------ To: space-tech@cs.cmu.edu Subject: Mars mission Date: Mon, 11 Sep 89 16:47:22 PDT From: (Don Willits) * Ok, a cylinder is the harder, more expensive way to go. But the mass * required for a pair of rad shielded pods swinging around on tethers means * the tethers need more tensile strength than probably exists, and also a * really reliable method of making sure the things don't go . I'm not sure I can agree with that. Do you have some mathematics either way? i.e. Let's say two 10000 ton pods (let's be ambitious!) swinging on a cable far enough apart to provide between .1 and 1.0g. What would the length and tensile strength of the cable be? As a factor of rotational speed, probably. The recent who disagree with the "than probably exists" comment is that in Arthur C. Clarke's "ASCENT INTO ORBIT" he discusses the space elevator concept. In it he mentions laboratory composite's created which could conceivably be strong enough for an actual space elevator. If these composites could be manufactured on a large scale, then it would be possible to build a space elevator from the Earth's equator to a sufficiently large enough asteriod in geosynchronous orbit. However, this is second hand, and I don't have any mathematics to back up my earlier claims. Any "experts" out there who can comment either way? -Don ________________________________________________________________________ Don Willits - Head Consultant * Internet: willitd@mist.cs.orst.edu Computer Science Lab * UUCP: tektronix!orstcs!willitd Oregon State University * hplabs!hp-pcd!orstcs!willitd Corvallis, OR 97331 * Voice: (503) 737 - 3273 ------------------------------ Date: Mon, 11 Sep 89 19:56:37 EDT From: John Roberts Disclaimer: Opinions expressed are those of the sender and do not reflect NIST policy or agreement. To: space-tech@cs.cmu.edu Subject: Re: Mars missions >From mnr@daisy.learning.cs.cmu.edu Fri Sep 8 18:30:44 1989 >To: space-tech@cs.cmu.edu >To: questar.QUESTAR.MN.ORG!al@uunet.uu.net (Al Viall) >Subject: Mars Mission ship design ... >Certainly you should have two capsules on a cable 100m to 1000m in length, >spinning to provide between .1 and 1 G. My guess is, say, a 500m cable >providing 1G might be most practical. That's no sweat. Probably one capsule >will contain the people and life support, and the other one will contain >random stuff to balance the weight (although exact balance doesn't matter). >As much as possible can be left in at the center axis to reduce load on the >cable. The astronauts would probably have a small electrical cable-climbing >winch to transport one of them up the cable to the center, to get more >supplies or do maintenance. A system of two pods of equal mass connected by a cable and spinning in free-fall is very simple to describe, and I am willing to believe that it will perform as stated. When these conditions are altered, however, I am concerned that several problems might arise that would reduce the usefulness of the approach. Some of the possibilities: + Pods of unequal mass: presumably the two pods would move in circular paths at different distances from the center of mass, which would not be halfway along the line. The lighter pod would experience higher acceleration (not necessarily a problem). + Shifting masses: moving masses up and down the connecting cable would perturb the motion of the pods and the cable, for instance setting up oscillations which could last for hours or longer. + Center pod halfway along cable: I believe this would allow conditions in which the three pods were no longer in a straight line. Depending on the relative masses and the magnitude of perturbing forces, significant deviations could arise, including permanent imbalances and long-term oscillations. As a simple example, with unbalanced outer pods, the contents of the center pod would "slosh" around in small circles several times per minute. + Thrust applied while spinning: Any thrust applied while the system was spinning would have to be carefully calculated to avoid perturbing the internal motion of the system. I'm not saying that the system is impractical, but that the calculations to prove that it is practical are not trivial. (By the way, does anybody know of a computer program that can simulate the motion of a system such as this?) [ Re: the desired radius and coriolis forces... ] Like Henry, I am not convinced that problems with motion sickness in a rotating system would be as great in practice as has been predicted. I suspect that the main problem would be that your body's motion sensors would tell you that you were moving in a slightly different direction than your actual path. You might fall down occasionally until you learned to adjust, but might be less inclined to feel sick. (Problems with slow rotation might be more pronounced near 0G than at 1G.) A simple test for this case might be performed by finding a circular track 1600 feet in diameter and driving around it at 110 miles per hour, while test subjects move around in the vehicle. The vector of the force (~1.4G) would be at a 45 degree angle from the horizontal and there would be extraneous vibrations, but otherwise it should be good as a first approximation of the actual conditions in the pods. (In case they *do* get sick, the vehicle should belong to the test subjects :-) >How about the shielding question, though? Should my Mars scenario include >a well-shielded "box" for the astronauts to hide in during solar maxima? >Maybe someone could check on that. If the radiation occasionally reaches lethal levels at Earth's orbit, there should be times at which it is dangerous anywhere between there and the orbit of Mars. Cumulative effects must also be considered. John Roberts roberts@cmr.ncsl.nist.gov ------------------------------ Date: Mon, 11 Sep 89 20:42:15 EDT From: John Roberts Disclaimer: Opinions expressed are those of the sender and do not reflect NIST policy or agreement. To: space-tech@cs.cmu.edu Subject: Mars mission, space elevators >From: (Don Willits) >The recent who disagree with the "than probably exists" comment is that in >Arthur C. Clarke's "ASCENT INTO ORBIT" he discusses the space elevator >concept. In it he mentions laboratory composite's created which could >conceivably be strong enough for an actual space elevator. If these composites >could be manufactured on a large scale, then it would be possible to build >a space elevator from the Earth's equator to a sufficiently large enough >asteriod in geosynchronous orbit. Clarke also discusses the concept in detail in the novel "Fountains of Paradise". I am concerned about how the system would handle the transfer of orbital velocity. If the mass of material going up the elevator and the mass coming down did not balance, the elevator would tend to wrap around the earth, unless you had a rocket engine at the top to compensate. (Of course, sending spacecraft round trip, or people and spacecraft up and raw materials down might be worthwhile.) If you could use multiple tethers or tethers and compressive components to anchor the elevator rigidly to the earth, then material moving up or down the elevator would merely slow down or speed up the rotation of the earth. John Roberts roberts@cmr.ncsl.nist.gov ------------------------------ Date: Mon, 11 Sep 89 20:31:13 EDT From: John Roberts Disclaimer: Opinions expressed are those of the sender and do not reflect NIST policy or agreement. To: space-tech@cs.cmu.edu Subject: Re: Mars mission >From: >Subject: Mars mission > Ok, a cylinder is the harder, more expensive way to go. The cylinder would be thousands of times more expensive, especially if the entire surface has to be shielded. It might be suitable for moving thousands of people to Mars. >But the mass >required for a pair of rad shielded pods swinging around on tethers means >the tethers need more tensile strength than probably exists, and also a >really reliable method of making sure the things don't go . If it does, >both pods go off course quite a bit, making it tough to carry on without excess >propellant to change the mission into a no-gee one. The tether only needs to support half of its own weight plus the weight of one pod at one G. That doesn't seem too impractical. A steel cable should be able to handle the load easily. If there is a problem, a tapered cable (thicker in the middle) could be used. In any event, even if the cable snaps, remember that the pods are only moving apart ~50m/s. It should only take a tiny amount of fuel to get the pods back together, compared to the enormous quantity of fuel to move the huge cylinder toward Mars in the first place. I used to worry about the apparent lack of a method to rescue a shuttle astronaut if an MMU thruster should be stuck in the ON position. Then it occurred to me that the shuttle could easily use its thrusters to make up for any possible delta-v from the MMU, and intercept the astronaut directly. >Also: has anyone thought >if the electrical inductance of long metal "wires" cutting across lines of Mag >fields at planetary speeds is significant? I think even today's satellites in >geosynchronous have to have charge dumps to keep static charges in the kilovolt >range from frying the electronics on board. I know Mars has little field, but >the Solar one might be strong enough for effects to be noticed. Hmmm... I wouldn't think there would be too much of a problem. The tether is not really very long or moving very fast, there is no large (in area) closed loop, and the tether reverses direction several times per minute. If a system were built with a large closed loop that rotated *perpendicular* to the plane of the loop, it might induce currents that would tend to slow the rotation of the loop. On the other hand, much of this lost power could be recovered. > Korac MacArthur > K_MACART@UNHH.BITNET John Roberts roberts@cmr.ncsl.nist.gov ------------------------------ Date: Mon, 11 Sep 1989 21:38-EDT From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: space-tech@cs.cmu.edu Subject: Re: Mars missions On the weight issue: I got some sales literature from DuPont about Kevlar cables. Kevlar is roughly equivalent to steel cable of the same size, but has about 1/6 the weight. They quote 2.76E9 N/m/m tensile strength for Kevlar 29. A cable which can hold 100T at 1 gravity is 14 sq. cm. in cross section, assuming a 4x safety factor. At 1.44 g/cc, a 500m cable to hold 100T weighs 1T. I believe this is an actual practical figure, since they use this stuff to anchor oil rigs. That is, a 500m cable to hold up against 1 gravity can hold 100 times its own weight; since there are two capsules, that makes .5% of the mass of the system. The 500m length is arbitrary, but the rest are pretty solid. ========== John Roberts mentions some interesting questions... - shifting masses: if this is a problem, and to save energy, use a funicular setup with an hourglass-shaped crank near the axis. The crank would look like this: ------- ------- | ------- -------| | | | | ------ ------ | | | | | | | | ||||||||||| | | | | | | | | ------ ------| | | | | | | | | ------- | ------- | | | | ------- | ------- | | | weight | at | top | | | | | | | person at bottom Cute, huh? It preserves constant tension and costs no energy. - Oscillations: This is something to be dealt with at some point; part of the solution may be some small thrusters that do active damping of oscillations. - Thrust: good question! What's a good way to apply thrust to a rotating system like this? The axis stays mostly fixed, so it isn't possible to always thrust along the axis, unless you can change the axis as quickly as the thrust direction. But how do you apply thrust out of the axis without causing trouble? Perhaps you attach thrusters to every chunk of mass in the system and accelerate them equally... Anyone have an idea? ---------------------------------------------------------------------------- | Marc Ringuette | mnr@cs.cmu.edu | Never lick a gift | | Carnegie Mellon Comp. Sci. | 412-268-3728(w) | horse in the mouth. | | Pittsburgh, PA 15213 | 412-681-5408(h) | | ---------------------------------------------------------------------------- ------------------------------ [ How about other aspects of a Mars mission design? -- Marc ] ------------------------------ End of Space-tech Digest #32 *******************