Subject: Space-tech Digest #53 Contents: Henry Spencer Re: Shuttle-C economies of scale? Henry Spencer Re: Space Exploration Faces Astronomical Costs Henry Spencer Re: CQ Moon Henry Spencer Re: CQ Moon Mike Butts Re: CQ Moon Mike Butts Re: CQ Moon Don Lindsay Radiation Damage to Fibers and Lasers Edward Wright LunaComm Requirements Lou Adornato Re: LunaComm Requirements Bill Davidsen Re: LunaComm Requirements Henry Spencer Re: LunaComm Requirements Rich Schroeppel Mirrors on the Moon John Sahr Re: Mirrors on the Moon George Herbert Re: Mirrors on the Moon ------------------------------------------------------------ From: henry@zoo.toronto.edu Date: Mon, 5 Mar 90 17:58:52 EST To: CS.CMU.EDU!space-tech@zoo.toronto.edu Subject: Re: Shuttle-C economies of scale? > Could one not launch a _number_ of payloads at the same time? Arianespace > does. Arianespace generally launches a maximum of two in one shot. (Barring secondary payloads that are willing to live with whatever constraints are imposed by the primaries.) Multi-payload launches run into all kinds of problems with insuring compatibility between the payloads, having them all ready at the same time, adding support structures to carry them, etc... not to mention the small question of whether they're all going to the same place. It can be done, but good opportunities are rare. > Also, somewhat related. I've been wondering why vehicles like Ulysses (sp) > have to wait so long to be launched. Couldn't it/they be launched any old > slot, parked in orbit and sent on their merry way when the window does open? Parking time in orbit is often severely limited. For example, the IUS that will boost Ulysses out of low orbit has limited battery life. Orbit is also a rather harsh thermal environment, and once it's in orbit you can't fix anything that goes wrong. If protected storage and repair facilities were available *in orbit*, it would make a lot of sense. Henry Spencer at U of Toronto Zoology uunet!attcan!utzoo!henry henry@zoo.toronto.edu ------------------------------ From: henry@zoo.toronto.edu Date: Mon, 5 Mar 90 19:06:52 EST To: space-tech@CS.CMU.EDU Subject: Re: Space Exploration Faces Astronomical Costs > [laser propulsion] > I thought it was still in theory and research state... It is, but there is good reason to believe that it could progress to working hardware development quickly if funded well. (Right now, it's not.) > >any savings, the U.S. would also have to build bigger, simpler rockets > >-- so-called big dumb boosters -- > > Whatever happened to Atlas? > about a year ago, someone(a professor) told me that they were still > being manufactured and that they were really cheap (like in the hundreds > of thousands, not millions, for each) which didn't sound quite right Atlas is still being built, but it is *not* cheap. General Dynamics will be happy to sell you an Atlas launch for a mere umpty-million dollars. Henry Spencer at U of Toronto Zoology uunet!attcan!utzoo!henry henry@zoo.toronto.edu ------------------------------ From: henry@zoo.toronto.edu Date: Mon, 5 Mar 90 17:05:45 EST To: space-tech@CS.CMU.EDU Subject: Re: CQ Moon > ...What's wrong with using a series of mirrors to provide the > equivalent of direct line-of-sight?... The mirrors will have to be very precisely aligned. The first mirror has to hit the last mirror, in its "virtual position" as seen reflected in the intervening mirrors, dead on. For a 1000km link with, say, 1m mirrors, that's pretty accurate aiming. I have doubts about how practical this is. I think even thermal expansion and contraction will kill you. Why bother? If you're planting mirrors every 30 km or so, you can just as easily plant microwave repeaters which don't have these problems. The reliability of the electronics is not likely to be much of an issue; certainly it's no more than a minor nuisance on Earth. Henry Spencer at U of Toronto Zoology uunet!attcan!utzoo!henry henry@zoo.toronto.edu ------------------------------ From: henry@zoo.toronto.edu Date: Mon, 5 Mar 90 19:12:44 EST To: space-tech@CS.CMU.EDU Subject: Re: CQ Moon > Question for the gallery: terrestrial comsats communicate only > with ground stations and not each other, correct? If not, is it simply a > matter of problems with pointing the narrowcast beams? Intersatellite communication is rare at present, unless you count use of the TDRS system to relay data from the shuttle and recent satellites. There may be a few military birds doing it. It is planned, for various purposes on both military and civilian satellites, in the not-distant future. It should not be much harder (or much easier!) than satellite- to-ground communications. Henry Spencer at U of Toronto Zoology uunet!attcan!utzoo!henry henry@zoo.toronto.edu ------------------------------ Date: Mon, 5 Mar 90 16:29:20 PST From: Mike Butts To: CS.CMU.EDU!space-tech%mntgfx@uunet.UU.NET Subject: Re: CQ Moon Why not use a chain of radio or laser relay stations on very tall towers and natural peaks, as we do on Earth? Unattended solar-powered electronics are probably no more difficult on the moon than Earth, maybe easier. The major impediment to tall towers on Earth is wind loading. I don't think that's much of a problem on the moon. 1/6 Gs reduces the vertical static load. It seems to me (admittedly without having run the numbers) that *very* tall masts could be cheaply erected on the moon with simple guy wire stabilizing supports. How tall a mast can be erected on the Moon with reasonable materials? On the subject of amateur earth-moon-earth communications, in the last few years there have been several quantitative technical articles on the subject in QST, the primary ham radio journal. Path loss and effective radiated power figures for various grades of stations are included. I could dig them up if anyone is seriously interested, or you could find them in most university libraries. While few hams run EME because of the $$$ and space required, the top EME stations, built by rich (retired) hams with lots of $$, land and time, are enormous. Arrays of >100 beam antennas, mounted on large structures which are rotated using truck chassis, have been built. The biggest stations have so much extra gain in their antennas they can run EME contacts with fairly ordinary stations at the other end. (FCC limits US hams to 1.5 KW transmitter power, but places no limit on the ERP resulting from antenna gain.) Amateur radio EME stations could probably contribute a greater communications ability to amateur space probe programs than one might think. --- Michael Butts, Research Engineer KC7IT 503-626-1302(fax:1282) Mentor Graphics Corporation, 8500 SW Creekside Place, Beaverton, Oregon 97005 !{ogicse,sequent,tessi,apollo}!mntgfx!mbutts mbutts@pdx.MENTOR.COM Any opinions are my own, and aren't necessarily shared by Mentor Graphics Corp. ------------------------------ Date: Mon, 5 Mar 90 16:38:24 PST From: Mike Butts To: CS.CMU.EDU!space-tech%mntgfx@uunet.UU.NET Subject: re: CQ Moon A further thought on my previous posting... > ordinary stations at the other end. (FCC limits US hams to 1.5 KW transmitter > power, but places no limit on the ERP resulting from antenna gain.) Amateur It is not uncommon for FCC to allow amateurs to conduct research operations beyond the regulations under special experimental permits. I'm guessing that if an amateur space probe were really to be launched, getting a permit for extra transmitter power at designated stations participating in the program would not be too hard. Other countries around the world also have EME stations and some probably have similar regs. --- Michael Butts, Research Engineer KC7IT 503-626-1302(fax:1282) Mentor Graphics Corporation, 8500 SW Creekside Place, Beaverton, Oregon 97005 !{ogicse,sequent,tessi,apollo}!mntgfx!mbutts mbutts@pdx.MENTOR.COM Any opinions are my own, and aren't necessarily shared by Mentor Graphics Corp. ------------------------------ Date: Mon, 5 Mar 1990 22:36-EST From: Donald.Lindsay@MATHOM.GANDALF.CS.CMU.EDU To: space-tech@CS.CMU.EDU Subject: Radiation Damage to Fibers and Lasers From: Christopher Neufeld > Question for the materials scientists: does anybody know the >vulnerability of fiber optics to particle radiation such as is present on >the moon's surface? I don't have the figures for the moon. But, I have equipment specs, damage vs. neutron fluence (n/cm^2), and also damage vs. Gy . [A "gray" == 1 Gy == J/kg == "absorbed dose". One rad is 10^-2 Gy.] InGaAsP LEDs keep their brightness to ~ 3x10^14 n/cm^2 or 10^7 Gy. You have to use the right photodetectors, because they are also ionizing radiation detectors. The best GaAs ones can ignore 10^6 Gy/s, but degrade by 10^6 Gy or 10^15 n/cm^2. Rad hard chips can take 10^4 - 10^6 Gy and 10^14 - 10^15 n/cm^2. Good fibers darken by 1 dB/km after 10^3 Gy or 10^13 n/cm^2. So, by the time something else is croaking, your fibers are darkened to (say) 10 dB/km, which is completely useless for long haul. Don ------------------------------ Date: Mon, 5 Mar 90 16:37:26 CST From: "Edward V. Wright" To: space-tech@CS.CMU.EDU Subject: LunaComm Requirements In the current discussion of lunar surface communications, suggestions have ranged all the way from low-data-rate Earth-bounce techniques and store-and-forward electronic mail satellites to high-data-rate fiber optic cables. But it seems to me that all this conversation puts the cart before the horse. The first step should be to estimate the communications requirements for a lunar base. This will depend greatly on the size and purpose of the base. A simple mining base, for example, would have far different requirements than a lunar astronomical observatory ala the current issue of Scientific American. It should be possible to estimate the communications requirements for a lunar scientific station by looking at the requirements of the Hubble Telescope, the Space Shuttle/Spacelab, space station Freedom, etc. What analogs exist for a lunar mining base? Offshore drilling platforms, maybe? Seismic exploration? What about teleoperated equipment? How much bandwidth does that require? Would anyone care to take a first stab at estimating the communications requirements of a lunar base? ------------------------------ Date: Tue, 6 Mar 90 11:04:19 CST From: Lou Adornato To: space-tech@CS.CMU.EDU Subject: LunaComm Requirements "Edward V. Wright" writes: >The first step should be to estimate the communications >requirements for a lunar base. I don't have time for a long response, but I have to disagree with this. We simply can't determine what the communications requirements are going to be. In fact, I wouldn't have too much confidence in someone who claimed to know what the communications system on Earth will need to handle in 20 years, and we have a lot more empirical data available for that. Any system that was locked into a set of specifications based on our current state (i.e. wishful thinking), would, at the least, have to be designed so that it wouldn't interfere with the _real_ communications system that would be installed later. IMHO, the better approach is to determine the real (mostly environmental) constraints, and try to design a system that will be expandable as new demands and technologies present themselves. Here are some of the contratins that I've identified: No atmosphere or weather to hinder optical line of sight High curvature limits line of sight distances Round trip propagation to stable "orbit" is 2.5 secs. Lower orbits highly unstable due to masscons and earth tidal effects. No RF emissions allowed (to keep from munging up Lunar radio astronomy) Unspecified surface radiation flux Unspecified probability of meteor impact damage High surface temperature variations (~300 deg. F?) High solar power density available Two week day/night cycle High equipment costs per pound High labor rates for installation and maintenance Need for high reliability (and graceful degradation modes?) Anyone care to add to this list (or fill in some missing numbers)? We can probably safely define three grades of communications service that will be needed: 1) Low bandwidth automatic data collection stations 2) Higher bandwidth (PCM?) human habitations 3) Very high bandwidth teleoperated robots (video+control channels) BTW: Does anyone have the firgure for orbital velocity of the Moon. How easy is it to generate orbital period from height (figuring a circular orbit)? A short tutorial on orbital mechanics would be appreciated. Also, I finally managed to dig up the diameter of the Moon, and it's 3475km. Lou Adornato | Statements herein do not represent the opinions or attitudes Cray Research | of Cray Research, Inc. or its subsidiaries. lfa@cray.com | (...yet) ------------------------------ Reply-To: davidsen@crdos1.crd.ge.com Date: Tue, 6 Mar 90 13:44:09 EST X-Mailer: Mail User's Shell (6.5 4/17/89) From: davidsen@crdos1.crd.ge.com To: space-tech@CS.CMU.EDU Subject: Re: LunaComm Requirements > Here are some of the contratins that I've identified: > No atmosphere or weather to hinder optical line of sight > High curvature limits line of sight distances As you imply, these somewhat contradict each other. Still, the low G makes tall towers very practical, and extends the line of sight quite a bit. > No RF emissions allowed (to keep from munging up Lunar radio astronomy) Sounds like a good argument for optical technology to me. > Unspecified probability of meteor impact damage > High surface temperature variations (~300 deg. F?) A good argument against strung cables, electrical or optical > High solar power density available > Two week day/night cycle Good arguments for/against solar power. > High labor rates for installation and maintenance I'm not sure I agree. The cost of transporting *stuff* would be about equal to cost of sending food, per pound. Certainly there's a tradeoff, but a long duration facility would recycle a lot and not treat everything as expendible. > Need for high reliability (and graceful degradation modes?) This is desirable, but the stations are unlikely to be able to interract quickly anyway, so a temporary fallback to an earth retransmit mode would be unpleasant but not cause desparate trouble. Makes a good case for Email rather than voice, with store and forward on the earth. Leaving the power source unspecified, I can say that just using a laser light beam between towers seems practical here. I can't see any gain from using fiber optics, and the best cable isn't nearly as good as a hard vacuum, nor as durable, light weight, or available on the moon in large quantity. I have no idea if using solar cells and batteries would be practical in a case like this. It is obviously possible, given cost is no object, but I have doubts that there is a low mass battery which would do the job. If we had better solar cells we could have the towers in the sunlight beam energy tower to tower with BIG lasers, but that doesn't scale at all well over many hops, and leads to all towers being single points of failure. I think each tower must be independent. -- bill davidsen (davidsen@crdos1.crd.GE.COM -or- uunet!crdgw1!crdos1!davidsen) "Stupidity, like virtue, is its own reward" -me ------------------------------ From: henry@zoo.toronto.edu Date: Tue, 6 Mar 90 14:52:48 EST To: space-tech@CS.CMU.EDU Subject: Re: LunaComm Requirements > Round trip propagation to stable "orbit" is 2.5 secs. Rather less than that to the Lagrange points or halo orbits around them, which are the real "stable orbits". They are not 100% stable, but close enough for this purpose. > Lower orbits highly unstable due to masscons and earth tidal effects. Dept of Nitpicking: Earth tidal effects are not significant on small satellites. Earth gravitational effects in general are the problem. > No RF emissions allowed (to keep from munging up Lunar radio astronomy) This is much more drastic than it needs to be, actually. Lunar radio astronomy probably will not be crippled by allocating one or two bands to Lunar surface communications. Agreed that optical communication is to be preferred when practical, but it will not always be possible. > Does anyone have the firgure for orbital velocity of the Moon. How easy > is it to generate orbital period from height (figuring a circular orbit)? > A short tutorial on orbital mechanics would be appreciated... Setting gravitational acceleration equal to centripetal acceleration, for a circular orbit, v = sqrt(G*M/r), where v is orbital velocity, r is orbital radius (altitude + Moon's radius), G is universal constant of gravitation, and M is mass of Moon. G is 6.67e-11, M is 7.3e22, Moon's radius is 1738e3 (all SI units), so a 200km orbit requires an orbital velocity of 1585m/s. Note that high orbits will be increasingly messed up due to Earth's gravitation, to the point where the orbital velocity for the Lagrange points is zero. Period is 2*pi*r/v, so our 200km orbit has a period of a little over two hours. Henry Spencer at U of Toronto Zoology uunet!attcan!utzoo!henry henry@zoo.toronto.edu ------------------------------ Date: Tue, 6 Mar 90 12:40:03 PST From: Richard Schroeppel To: space-tech@CS.CMU.EDU Subject: Mirrors on the Moon Discussing Lunar communications between bases, using lasers. (from Marc Ringuette) >> ...What's wrong with using a series of mirrors to provide the >> equivalent of direct line-of-sight?... (reply from Henry Spencer) > The mirrors will have to be very precisely aligned. The first mirror has to hit the last mirror, in its "virtual position" as seen reflected in the intervening mirrors, dead on. For a 1000km link with, say, 1m mirrors, that's pretty accurate aiming. I have doubts about how practical this is. I think even thermal expansion and contraction will kill you. Why bother? If you're planting mirrors every 30 km or so, you can just as easily plant microwave repeaters which don't have these problems. The reliability of the electronics is not likely to be much of an issue; certainly it's no more than a minor nuisance on Earth. Powering the repeaters might be a problem for the long term reliability, and it's one more thing to import from Earth. Even if repeaters are used, intermediate mirrors might be a good idea, to cut down on the number of repeaters. I suggest making the mirrors verrry slightly convex. Then an observer at mirror 16 will always be able to see an image of mirror 18 in mirror 17, although the image may move around slowly as the mirrors warm and cool. The system will require active pointing at the transmission end, with very fine adjustability, but slow reaction time. We can tolerate perhaps a 10^6 loss of transmitted power (send mW, receive nW). There might be problems with beam spread at these distances, even with flat mirrors; and diffraction losses from using "small" mirrors. Has anyone looked at the topography of the Moon? High mountains will really help on cutting down on the number of mirrors required. Since horizon distance scales as sqrt(radius), a given altitude on the Moon will get you half the horizon distance of Earth. A 5mi mountain will have a (lunar) horizon distance of 100 mi, and a mountain top observer could see the peak of another 5mi mountain up to 200 miles away. Rich Schroeppel rcs@la.tis.com ------------------------------ Date: Tue, 6 Mar 90 21:15:59 EST From: John Sahr To: space-tech@CS.CMU.EDU Subject: LunaComm Requirements >...I suggest making the mirrors verrry slightly convex.... Not all mirrors require precise pointing. There are nonlinear "phase conjugate" optical materials which will "prefer" to maintain an optical path even when there are imperfections in the pointing and the path. I believe that the nonlinear optical materials require fairly large fields (i.e. high power lasers) in order to achieve the requisite nonlinearity (could be wrong here). Actively distorting the mirror surface with piezo servos is also possible, but probably pointlessly complex. It is not necessary to make the mirrors big enough to see an image of the receiver or transmitter: the communication link is going to be interested in phase and amplitude information, not positional information. If it is possible to "see" the transmitter and the shack housing it from the receiver, then you have wasted effort on the mirror. Imperfections in a large mirror surface, perhaps distorting as it heats and cools, will give rise to speckle that will degrade the link performance: ideally every emitter should look like a point source. Although terrestrial microwave links are not immune to multipath fading, the absence of weather per se on Luna should alleviate many variable sources of multipath. Anyone know what typical power levels for terrestrial microwave repeaters are? ------------------------------ Date: Tue, 6 Mar 90 20:28:07 PST From: gwh%ocf.Berkeley.EDU@jade.berkeley.edu To: johns@VEGA.FAC.CS.CMU.EDU, space-tech@CS.CMU.EDU Subject: Re: LunaComm Requirements The phase conjugate mirror idea is great except that phase conjugate type mirrors need a large laser at the 'mirror' site. Once you have all that and have it alligned, and its power supply, etc. it ends up being easier to just put in another repeater. Back to square one. -george william herbert gwh@ocf.berkeley.edu ------------------------------ End of Space-tech Digest #53 *******************