From: vince@offshore.ai (Vincent Cate) Newsgroups: sci.space.tech Subject: Re: Lunar Sample Return via Tether References: <20031209194336.06226.00001965@mb-m19.aol.com> NNTP-Posting-Host: 207.42.133.230 Message-ID: <9186edb5.0312100949.3c86ca20@posting.google.com> skyefire@aol.com (SkyeFire) wrote in message news:<20031209194336.06226.00001965@mb-m19.aol.com>... > Stupid question time: assuming that the cargo schedule isn't > very heavy, why would we necessarily need incoming cargo or propulsion > to keep the tether's momentum up? If we had a momentum wheel powered > by a solar-electric motor mounted in the hub of the tether, couldn't > we win back rotation that way with minimum effort? There are 2 types of momentum you need to keep under control. One is the rotational momentum around your own center of mass and the other is orbital momentum around the moon. The rotational momentum of even 1 Kg at the end of a 100,000 meter long tether is so huge that no momentum wheel will have any impact on it. However, you can easily control a tether's rotational momentum, when near a gravitational body, by winching the tether in and out as it is going up or down relative to the body. If you want to rotate faster you let it out on the down side and winch in on the up side so it spends more time going down (and pulled faster) than going up (and pulled slower). So rotational momentum is an easily solved problem. The orbital momentum needs to be controlled by either leaving something on the surface of the moon of equal mass to what you are picking up, or using some kind of thruster. The thruster could be a conventional chemical rocket, an electric thruster, or a solar sail. All of these, including leaving something on the surface, can be looked at in terms of ISP or exhaust velocity. ISP Exhaust velocity Leaving mass on surface 163 1.6 km/sec Chemical Rocket 400 3.9 km/sec Hall Thruster 2,000 19.6 km/sec Ion Drive 10,000 98 km/sec Solar Sail infinite speed of light - solar photons The Hall Thrusters and Ion Drives come in different ISPs, these are just some sample values. Note that ISP times 9.8 equals the exhaust velocity in meters/sec. The ratio of lunar-pickup-mass/reaction-mass is the same as the reaction-mass-exhaust-velocity/lunar-orbital-speed so that momentum is conserved. The orbital speed you will be giving the regolith is about 1.6 km/sec. The higher the exhaust velocity the less reaction mass you need. In the solar sail case the reaction mass keeps coming to you from the sun, so it is sort of an infinite ISP. We are used to needing lots of rocket fuel to lift a small payload, since launching from Earth you might use 30 to 100 times as much reaction mass as you get payload to orbit. With a 10,000 second ISP ion drive and a tether, we could lift 61 Kg of lunar regolith for every 1 Kg of reaction mass (98/1.6=61). This is so amazingly good that it takes awhile to sink in. With a solar sail the only thing limiting how much you can lift is how long your system keeps working. -- Vince