Date: Wed, 21 Sep 1988 15:04-EDT From: space-tech-request@CS.CMU.EDU To: "~/st/lists/stdigest" Subject: Space-tech Digest #7 Contents: Bill Newman Re: design of a satellite Dale Amon Solar Sails Marc Ringuette Solar Sails (mini-discussion) ------------------------------ Date: Tue, 20 Sep 88 19:31:39 EDT From: newman@tcgould.tn.cornell.edu (Bill Newman) To: SPACE-TECH@cs.cmu.edu, SPACE@GWUVM.BITNET Subject: Re: design of a satellite I have no expertise in this area other than a fondness for AW&ST and some discussions with volunteers building a getaway special at Caltech, so you may safely discount my answer, but I will give it anyway: No, this group is not qualified to put together your lunar probe design, although a satellite might be possible. The biggest problem, as I see it, is that there is no proven propulsion technology which meets your cost requirements for getting from LEO to lunar polar orbit, and while a really well done project might be given a ride to LEO on a standard launch vehicle, I see no way that you are going to be given a ride to lunar orbit, since the costs of the trip would be so large. The costs are large enough that the ferryman could be expected to build his own probe; if he did not believe that he could build a probe which was lighter, higher performance, and better matched to the launch vehicle than anything part-time amateurs on a shoestring could manage, he would not be in the business of trying to ferry folks to the moon, I think. I have been fantasizing about a prospecting lunar rover for some time, and your polar survey satellite is of course appealing to me for the same reason. If you want to have a chance of making headway on an amateur basis, I suggest that you should: (1) If you haven't already done so, work out the details of orbital transfer, esp. using low thrust systems, from LEO to lunar polar orbit, and make sure that stability in the lunar polar orbit isn't a problem. BTW, if you get this kind of thing worked out so well you can do these calculations in your spare time, and you are ever looking for adulation, send me your programs, calculations, and a summary. (2) Work out all the tricks you can think of for getting the most out of a small satellite in lunar polar orbit and, if it appeals to you and you ever think it will be feasible, a surface rover too. But you really don't need worry too much about the details of the satellite, because the propulsion system is a bigger obstacle. Even if you never learn anything about clever remote sensing instruments, don't worry. If you work out a way to get from LEO to lunar polar orbit for cheap, and you demonstrate it with a prototype which flies with no nifty instruments at all, planetary scientists with experience in building instruments for satellites and space probes will come to you, tongues hanging out, begging you to carry their instruments on the next one. (3) So finally, what you've been waiting for: the amateur hardware project. My recommendation is simply that you work on a prototype high efficiency maneuvering vehicle, probably using solar sails, assuming that it is possible to use solar sails to get out of LEO, (does the solar wind beat atmospheric drag?) or else one of the weirder schemes like tethers (working off of either the tides in the Earth-Moon-Sun system or, when charged, the Earth's magnetic field) or ion drive (this isn't intrinsically weird, but the idea of getting a good enough power source into an amateur package the size of a getaway special seems a little bizarre). Good luck, whatever you decide to do.. Yours, Bill Newman newman@tcgould.tn.cornell.edu ------------------------------ Date: Wed, 21 Sep 1988 13:20-EDT From: Dale.Amon@H.GP.CS.CMU.EDU To: space-tech-request@CS.CMU.EDU Subject: Re: Solar Sails Keep in mind that you will have to get your sail into a pretty high orbit before deploying. In LEO the drag on the sail in the tenuous upper atmosphere is removes energy faster than it is added by the sail. I'm not sure where the break even altitutde is, but I belive you would want to deploy outside the outer Van Allen Belt, even if that altitude is lower. You really don't want to spend several weeks in the belts with inexpensive electronics... ------------------------------ Date: Wed, 21 Sep 1988 14:41-EDT From: Marc.Ringuette@DAISY.LEARNING.CS.CMU.EDU To: space-tech@cs.cmu.edu Subject: Solar Sails (mini-discussion) What follows is an email discussion between Steve Abrams and me about solar sails - I'm skeptical that they are feasible in the near future, and Steve is trying to convince me that they are. We had this discussion a few weeks ago, but it seems appropriate now. I give a comment at the end, relating this to the recent discussion on low-budget satellites. -- Marc ------------------------------ Mini-discussion on Solar Sails. Participants: sedspace@doc.cc.utexas.edu (Steve Abrams) mnr@cs.cmu.edu (Marc Ringuette) Summary: Steve: solar sail background + Steve's new angles Marc: Q - are they practical with current technology? Steve: A - yes!!! Marc: Q - what would a 3-year big-budget project look like, in detail? Steve: A - a 3-year solar sail project, with numbers (!!) ------------------------------ From: sedspace@doc.cc.utexas.edu (Steve Abrams) The concept behind solar sails - using solar radiation pressure for propulsion - was suggested by Tsiolkovskii and Tsander in the1920s. It is a well-considered aspect of satellite design. NASA spent nearly $4 million developing a sail-launched Halley comet *rendezvous* (not a fly-by), but funds were cut by the Carter administration. Solar radiation pressure is used today to damp out perturbations in satellite orbits. They are certainly "less bizarre" -- studied and developed longer -- than mass drivers, tethers/skyhooks, etc. and can be launched *today* with current engineering/manufacturing practices. The World Space Foundation has a prototype sail developed and is seeking funding to deploy it. It doesn't use unreasonably strong materials and requires no "breakthrough" technology. Louis Friedman (one of the co-founders of The Planetary Society) writes in his book, _Starsailing_, that only the "best people" in the world get interested in solar sails. Unfortunately, the influence of the American-captured WWII German Rocket Scientists and the Soviet-captured WWI German Rocket Scientists is felt even today because most aeorspace engineers are wedded to the notion of thinking only in terms of rockets for propulsion. It has worked, so why fix it? In my design of a Solar Sail Supply Ship, I take what's discussed in the literature, examine some of their simplifying assumptions and proceed to not use (or suitably modify) those assumptions to improve the desiribility of solar sails. The first assumption is that specular reflection is the only mechanism operative. In truth (as Robert Forward discusses in his paper, _Grey Solar Sails_), you must consider absorption & re- emission, diffuse and back reflection. Each one adds its own component of thrust -- admittedly lower than that of specular reflection, but it all adds up. The second assumption is that the absorptivity, transmissivity, and reflectivity (A,T, and R) is constant over time (although they, at least, admit that it isn't constant over wavelengths). However, all of these are constantly in a state of flux due to changes in temperature, pressure, stress/strain, electromagnetic fields, etc. What is more important is that *we* can induce changes to modulate the A, T, and R as we see fit. The easiest to control is thermal modulation as it has a scalar nature and all the other means have vector natures. It's simple...heat the sail film and you can *selectively* increase A, T, and R. This gives greater leeway in attainable trajectories. The third assumption is that the sail would only be used for propulsion. By vapor-depositing phase-reversal zone plates on the sail film itself (negligible extra mass) and thermal-electric generators, you can generate significant amounts of power - tens of kilowatts for reasonably- sized sails. In addition, the sail film (since it is conductive) can be used as an antenna, thereby allowing remote guidance control or re-programming (ala Voyager). The fourth assumption is that the sail film is metallic. Some recent studies indicate that conducting polymers might be just the ticket...they also would make great light-weight storage batteries for the power generation (this is currently an area of industry research). The polymer films would, of necessity, be thicker, but their densities are quite low (cis/trans-polyacetylene has a mass density of about .4 g/cm^3). This is due to the large amount of empty space between fibrils. This is what also allows the easy diffusion of dopant molecules in the improves the conductivity. The potential fields of the dopants, however, provide continuity to the reflectance at a molecular level. This also allows greater "room" (each molecule of the polymer fibril has a nearest neighbor only along the axis of the fibril) for the potential barrier to "spread out" under heating, thereby allowing for increased thermomodulatability. A fifth assumption is that the sail is one continuous, thin film. This isn't easy to manufacture or deploy but it does improve acceleration. By making the sail out of individually-controllable elements, manufacturing, deployment and navigation are much easier. Some other features I've considered are: framing the elements with a shape-memory alloy (e.g., nitinol) currently manufactured in a 5 thousandths-of-an-inch-diameter wire. By heating the nitinol frame, we heat the sail film, thereby modulating its A, T, and R. In addition, the restoring force of the nitinol keeps the film taut so that you don't have to spin the whole thing. (Spinning the sail greatly impedes its maneuverability - think of it as trying to change the orientation of a big gyroscope - but it does keep the film taut.) This is important because the effective thrust falls off rapidly when the sail film is wrinkled. Additonal nitinol struts could, under control, re-shape the sail into a parabolic reflector/antennae for communications, smelting, etc. Switching routines have been available since the early 60s that allow a sailcraft to "pump/de-pump" its orbit. The sailcraft can be placed in a moderately elliptic orbit (cheaper in fuel) by conventional rocketry. It can then boost itself to Earth's escape velocity in 6-8 months. This escape velocity can be directly with or against Earth's orbital velocity. If directed against the orbital velocity, the sail can be dropped close to the Sun where the high soalr constant will result in higher terminal velocities. Sails can be made to match trajectories with any planned manned exploration at various points of the exploration orbit to provide a continuous stream of supplies, food, fuel, etc. They would, of course have to be launched 2-5 years earlier which requires adequate long-range planning skills (that the US doesn't have). Travel outside the Belt could be facilitated by increasingly-hypothetical SDI lasers in Earth orbit or a "station- keeping" PRZP/FZP sail in Venus' orbit. In many cases, the total trip time is significantly less than would be attainable by conventional rocketry. And it could be much cheaper!!! Well, I've tried to give you an idea of why solar sails excite me. If we can ever get mission design out of the accountant's and budget manager's slimy paws and into the hands of scientists and engineers, we'll see more sail use over rocketry. ------------------------------ From: mnr@cs.cmu.edu (Marc Ringuette) Seems to me that solar sails are more practical than launch loops and ground-to-orbit skyhooks, but still a long way from reality. Don't you agree? If I gave you funding to build a solar sailer over the next 3 years, what materials & design would you use? What I'm getting at is, you mentioned a lot of visionary-style ideas, but I'm not clear on what today's technology would be. I presume it's something like mylar, but on what kind of frame? With what control (ouch)? ------------------------------ From: sedspace@doc.cc.utexas.edu (Steve Abrams) The "state of the art" for solar sails is at least as developed than the "state" for mass-drivers. There have been built prototypes for each concept. Of course, there's more to develop...remember, solar radiation pressure is and has been used for satellite orbit correction. We *know* that it works, but bureaucrats and big business are in love with rockets. Every idea I mentioned in reference to the sails has been widely developed in the literature...the effects are fairly well-known and the techniques are well-developed. The least developed idea is conducting polymers -- they've only been around for about 10 years and well-studied for the past 3-5 years. The best part about them is that you can "design" the polymers and dope them to produce conductors (with the necessary reflectance and, hence, modulatable reflectors. Their biggest problem currently is environmental stability. This is a major research topic in industrial applications - like batteries - and there is no reason to believe that the dopant molecules/atoms can't be "bound" to the polymer fibrils. Thermal modulation is well-known from modulation spectroscopy (it is used to study energy/band-gap structure of metals). Shape memory alloys are currently manufactured in appropriate ways. Orbital "switching" routines have been developed for 25 years to "pump/de- pump orbits" with low-thrust spacecraft. Gravity Maneuver Assists are quite well developed. I can't remember any other "visionary" techniques (oooops! phase-reversal zone plates and their construction were described in 1898) but, if you have any questions or want further explanation of any topic, just ask. I've tried to be very careful and consider technologies that have been around for at least a decade. Of course, they can always be improved... ------------------------------ From: mnr@cs.cmu.edu (Marc Ringuette) Don't you think there's a big difference between something you can tweak to work on paper or experimentally, and something you can produce a square mile of and control effectively? That's what I was getting at with the "3-year-project" question - I'm still interested in what design you would choose if you had 3 years and a big budget. In 3 years, could you construct, say, a box that you send up in an Ariane, which can put a solar sail in orbit around the moon capable of maneuvering a 100kg package into any lunar orbit? I know, orbiting the moon is pretty easy, but the time constraint is very tight - it means you can't develop new kinds of manufacturing facilities or anything fancy at all. Be as detailed as you can be without putting in a huge effort - what's the material, what's the area, what's the control mechanism, how long would it take to change from an equatorial to lunar polar orbit? I like the question, do you? Feel free to change the parameters, but I think it would be great to hear your answer to a specific question like this. ------------------------------ From: sedspace@doc.cc.utexas.edu (Steve Abrams) Of course there is a big difference between tweaking and producing. However, tweaking is the first step in the long road to production. I don't claim that all my ideas are workable, just that I've seen enough about them to justify (in my opinion) further study. OK, you asked for it...(don't hold me to all the numbers) I am "spiralling out" a sail from MEO to Earth escape velocity (the reason for this is that the final "loop", just before reaching this velocity, passes beyond lunar orbit at nearly zero radial velocity relative to the Earth and, hence, the Moon). Forgive me if I'm unclear anywhere as this should properly take a couple of months the do all the necessary calculations. I've been trying to get some generic simulation software to crank out numbers, but there are really too many degrees of freedom that need to be explored to discuss them all now... Within the 3-year time limit specified, I'm going to stretch it to the limit by launching three years from today. I assume that I have enough access to launch facilities and that political considerations are non-existant. The first thing I'd do is reserve commercial Soviet launches for the next two years. I'd also buy space on ESA's commercial launches beginning in 1989. I would then call Darryl Hodgsen at Raychem-Menlo Park and order 2000 nitinol (55 NiTi; resistivity = 80-100 micro-ohms per centimeter; restoring force at temperatures greater than 375K is around 10 micro-newtons) frames, hexagonally shaped, with diameters of 0.127 mm and sides of length 13.873 m. This yields a total sail area of 10^6 m^2. Darryl estimated that they could be delivered in 9-12 months. Additionally, 6 longer struts that, when heated form the sail into a parabolic reflector, thicker (uncertain as this is a newer idea) and 716.4m long (the longest extension of the sail in any one dimension) - they can be segmented - would be ordered. Then I would find some hotshot MIT-type lab and tell them that they have the funds to build a vapor deposition facility (including masks for phase reversal zone plates of aperture 1,200 zones) with delivery in 15 months - that would deposit an aluminum film (in space - at MIR) 20 nm thick on the nitinol frames when they finally got launched to Mir. The guidance and control systems would be the toughest (also because I'm less familiar with them) on this timescale. We'd need a radiation-resistant (at least as much as possible; I've heard that Tracor, Inc. is researching them) parallel processor, solar sensor/gyroscope combo used to continuously supply position and orientation data to the processor. It doesn't need to be fast due to the long timescales inherent in solar sail design. The processor then needs to be able to compare that data with the optimum course (algorithms optimizing solutions were presented at the 1988 Lunar Bases Symposium by a friend of mine working at Los Alamos National Laboratory) and determine which sail elements must be heated to what temperature to vary their reflectances around 20% maximum. Bulk aluminum can have its reflectance varied about10% in the visible portion of the =spectrum. In the infrared, the variance is 25-40% over the range of temperatures from 300 K to ~950 K (whatever the melting point of Al is). Additionally, some 55% of the Sun's power output comes in the relevant portion of the spectrum (690-10,600 nm)so it looks like aluminum may not be so bad after all. The processor could also be re-programmable via amplitude- modulated laser pulses focused onto the rather large sail (which, when parabolized - perhaps a default mode - focuses the laser onto a CCD array) from any visible light laser (or combination) in orbit yielding a total output on the order of 1 watt (it's overkill, but lasers diverge significantly over lunar distances). The first thing to be launched would just be a large (around 20m on a side), square, flat, thick piece of metal, oriented normal to the course of the Mir station, and controlled by conventional rocketry. This is used to produce a "ram" effect that creates a "higher" vacuum behind it. While this improves specular reflectances of the sail films, it primarily minimizes the film's exposure to atmospheric oxygen. The vapor deposition facility would then be launched, followed by the MBE devices. They would be followed by the first shipment of nitinol frames and a few kilos of pure aluminum. Vapor deposition on this scale would take around an hour per frame - to be safe - and the timetable allow for up to 4 hours of down time per hour of operation. Sail elements are then spot welded at vertices onto an "automatic deployment" structure such as those demonstrated at MIT in April '87 for communications antenna. For ease of calculation, I've only considered hexagonal sails. Use of a heliogyro design would improve deployment. Once manufactured, the MBE facility puts phase-reversal zone plates (looks like a two-dimensional bull's-eye with alternating rings varying in thickness between 7.5 nm and 15 nm) on the central frame elements; the 7.5 nm film will be coated a 7.5 nm film of solid sodium - index of refraction of 4.22 - to provide the necessary phase-reversal. A longitudinal boom, normal to the plane of the sail, holds a Brayton-cycle thermoelectric generator approximately 100 m behind the sail from the Sun with heat exchangers at the foci of the PRZPs. The number of PRZPs is determined by the power requirements - at least .3 kW per PRZP (limited primarily by the low efficiency of the thermoelectric generator, around 10%) can be obtained. To avoid arcing due to electrostatic charging, the fronts of each sail element are "shorted" to the backs of adjacent elements. The frame idea precludes the necessity of "ripstops." The self-deploying structure would have to be sealed ( in a large baggie) while being boosted to an orbit with perigee at MEO (7,500- 10,000 km radius orbit), where solar radiation force is greater than atmospheric drag, and an eccentricity in excess of 0.50 (this is cheaper in fuel and time than a circular orbit - an initally circular orbit would take *years* to reach escape velocity). The sail could then boost itself by pumping its orbit. This works by maximizing solar radiation force components along the sailcraft's instantaneous velocity vector as it approaches perigee and minimizing that component anti-parallel to the velocity vector as it climbs out of perigee to apogee. The boost time to lunar orbit is on the order of 7 months - it's the slowest part of the trip - depending upon the total mass. Once escape velocity is reached, it's less than 2.5 years to Jupiter, for example. Of course, with more time, the design can be improved, but that's what I'd try to do to launch in three years. I have no idea at what cost this would be. The above scenario is not realistic because the assumptions aren't realistic. I'm certain that I'm forgetting things, but I can't think what they are right now. One advantage of this type scenario is that, for the 6-12 months of orbit-pumping, your sailcraft is readily recoverable in the event of component failure. Changing from lunar equatorial orbit to lunar polar orbit would take quite a while to do - it's very energy intensive. Robert Forward recently published a paper describing a technique to extend the geosynchronous, equatorial Earth orbit - favored for satellites and getting crowded - into a "cylinder" (instead of ring) of orbits coaxial with the Earth's rotation axis by passive use of solar radiation pressure. Extrapolating from his results to the moon, I would estimate that we could start in Earth equatorial orbit and end up in lunar polar orbit in about 14 months. We could shorten this by accepting a "crash-landing" solution with impact velocites of less than 1 km/sec. ------------------------------ [ end of mini-discussion ] Thanks, Steve! I appreciate your willingness to take a crack at this. My final impression is that there are no fundamental flaws with this, but that it can't be done halfway. Deployment is such a problem that I'm willing to bet you can't send the sail up from earth, but instead you have to do a big-budget project with fabrication of the sail in space. If anybody disagrees, and has ideas on deployment which deal with the extreme fragility of the sail, please say so. Another possibility is a project which uses a thicker, more robust sail material which won't get practical accelerations, but might allow interesting experiments. -- Marc [ end of space-tech digest #7 ]