Date: Mon, 28 Sep 92 05:02:39 From: Space Digest maintainer Reply-To: Space-request@isu.isunet.edu Subject: Space Digest V15 #256 To: Space Digest Readers Precedence: bulk Space Digest Mon, 28 Sep 92 Volume 15 : Issue 256 Today's Topics: Redefining failure, space camps, & other changes of subject Safety of flyby & aerobraking for large payloads at earth Space Platforms (political, not physical : -) Welcome to the Space Digest!! Please send your messages to "space@isu.isunet.edu", and (un)subscription requests of the form "Subscribe Space " to one of these addresses: listserv@uga (BITNET), rice::boyle (SPAN/NSInet), utadnx::utspan::rice::boyle (THENET), or space-REQUEST@isu.isunet.edu (Internet). ---------------------------------------------------------------------- Date: 27 Sep 92 23:08:17 GMT From: Nick Szabo Subject: Redefining failure, space camps, & other changes of subject Newsgroups: sci.space,talk.politics.space >> Paul Dietz writes, in response to Dennis Wingo: >>In other words, "the shuttle's not a failure, and it's not NASA's >>fault anyway!". Can you write two sentences without contradicting >>yourself? >> >>The purpose of the shuttle was to reduce the cost of getting into >>space. The shuttle has been a dismal failure in meeting this, >>its primary goal. > In article <27SEP199216051882@judy.uh.edu> wingo%cspara.decnet@Fedex.Msfc.Nasa.Gov writes: >Only half right. The primary reason for the Shuttle is REGULAR access to >space. The stated goal being a flight every week, the actual result being less than a flight per month. Of course, we're allowed to redefine that endlessly as well, just as we're allowed to decrease the functionality of the space station, increase its cost from the promised $8 billion to over $100 billion, and use accounting gimmickry and clever arguments to say, "hey, that's what we planned all along!" I was there when the promise was made, so let's not dredge up the Boeing accounting -- SSF was sold as an $8 billion project, in disregard of what Boeing engineers actually projected as the costs, information that was not made available to me at the time. We were shamefully involved in promoting the wrong numbers, because we were operating by wishful thinking; we didn't bother to think critically and ask the right questions. Similar NASA/NSS figures on astronaut projects in the future should be con sidered against this history of deceit. >["space camp"] We were talking about commerce here. The tiny revenue earned from space camps is lost in the NASA bottom line, and what NASA chooses to propagandize to children in space camps hardly constitutes what is good for industry. It's hard to remember, since Dennis keeps changing the subject, but this thread was about industrial competiveness. The camp's content doesn't reflect what children dream of, BTW. It reflects what NASA PR wanted them to dream of when they designed the camps -- big fat NASA contracts for all the traditional goals. In my high school physics class the Viking project was one of the teacher's motivational schticks; astronauts were nowhere in sight. Polls show that Americans support the astronaut and automated programs equally even when not shown the relative program costs. Furthermore, "keeping people interested in space" has no predictable correlation to space project funding. If anything there may be an inverse correlation; the height of interest during Apollo corresponded with the deepest cuts to NASA. The most visible programs are the most likely targets for budget cuts when Congress gets around to trying to convince us that they're fiscally responsible. -- szabo@techbook.COM Tuesday, November third ## Libertarian $$ vote Tuesday ^^ Libertarian -- change ** choice && November 3rd @@Libertarian ------------------------------ Date: Mon, 28 Sep 1992 06:15:17 GMT From: Nick Szabo Subject: Safety of flyby & aerobraking for large payloads at earth Newsgroups: sci.space What are the costs and benefits of using the earth for gravity assist and aerobraking of large payloads? What should be the policies concerning these flybies? Here's how a gravity assist works. The planet is moving, so there's our energy source. The slingshot can be computed with the patched-conics approximation. If we do a Hohmann ellipse to the planet in the inertial frame, the trajectory is a hyperbola in the frame of the planet. The energy of the vehicle is the same at symmetric points on opposite sides of the hyperbola in the planet frame. If we exit the rendezvous moving in the same direction as the planet, we gain velocity in that direction in the inertial frame. If we exit the rendezvous moving opposite the direction of the planet, we lose inertial velocity. Either gaining or losing velocity can be useful, depending on where we're going. Aerobraking is simpler to understand. In layman's terms, the air slows the spacecraft down, just like wind resistance slows down a bicycle. In orbital mechanics terms, the spacecraft exchanges momentum with the particles in the atmosphere. An interesting variant, called cometary aerobraking, vaporizes a piece of ice a split second before it intercepts the spacecraft at high velocity. The spacecraft uses the temporary cloud of gas to aerobrake, as if it were a planetary atmosphere. All these maneuvers allow us to tap into the energy already stored in the orbits of the planets and minor planets. They can greatly reduce the mass of propellant and tank needed for a mission; in the case of Galileo the Venus-Earth-Earth-Jupiter trajectory saved it from being cancelled when it had to substitute a smaller upper stage for the powerful Centaur. Very large payloads can benefit from these trajectories for the same reason, especially at earth, which raises the important question of safety. We cannot tolerate bringing a dinosaur-killer sized asteroid anywhere near earth, or coming towards earth near an intercept trajectory. Reentry of c. 100 ton Shuttles is safely performed and tolerated towards inhabited areas, and natural fireballs and meteorites massing several tons each hit the earth harmlessly every year. Somewhere between these two extremes, we need to figure out the margins of safety and enforce them. There are several techniques for using earth to change the orbital trajectory of objects: * fast aerobraking (eg Shuttle, Apollo) * slow aerobraking (eg Hiten) * gravity assist (eg Galileo) All three of these can play important roles in reducing the costs of capturing space materials from comet fragments and asteroids into various earth orbits. The delta-v savings are roughly up to an order of magnitude for gravity flyby, and up to two orders of magnitude for aerobraking. For gravity assist, the following need to be considered: * what is the margin of error due to fringe atmospheric density, gravity anomaly and trajectory measurement error? * how quickly and precisely can the on-board engines compensate for trajectory errors directly before and during the flyby? * can the operation be timed so that a worst-case error will cause reentry over an uninhabited area (eg the ocean)? * how much material on board is strongly toxic or radioactive? * what is the worst-case scenario wrt the mass, composition, and worst-case error trajectory of the payload? For slow aerobraking, we must also consider the above points, paying close attention to the fringe atmospheric density, since that is what we are using to change the trajectory. For fast aerobraking, we need to pay very close attention to upper atmospheric density at all levels. The error margins are much less. Unless the worst-case scenario is trivial or the spacecraft is well-controlled aerodynamically, fast aerobraking is much more dangerous than slow aerobraking or gravity flyby. Given these data points, we must then determine whether the project is ethically and politically tolerable, and whether it can be insured. For the sake of discussion, I make the following initial proposed rules of thumb: gravity assist & slow aerobrake * <.0001% chance of trajectory error sufficiently large for reentry * if reentry, >98% chance it will occur over & towards uninhabited area * payload mass limits: solid metallic materials: 5,000 tons (no piece > 1 ton) stony materials: 10,000 tons (no piece > 5 tons) carbonaceous materials or loose regolith: 30,000 tons volatile ices w/pores: 50,000 tons strongly toxic or radioactive material: (varies by material; 1 ton typical) The shape and attitude of the container also play a major role. For example a long, thin cylinder will be dispersed more widely than a sphere if it hits the atmosphere sideways instead of headlong. fast aerobrake: * aerobrake must contain control surfaces sufficient to give <.001% chance of reentry due to error * if reentry, >98% chance it will occur over & towards uninhabited area * no strongly toxic or nuclear materials on board * mass limits: solid metallic materials: 200 tons all other materials: 400 tons (note: ceramic heat shield will be significant % of mass for any payload) Significant amounts of simulation, study of real-life artificial and natural reentries, and benefit/risk analysis need to go into determining the actual safety margins. Calculations like those done by Zdenek Sekanina, to predict the ability of comet material to penetrate the earth's atmosphere, need to be perfected. Privately financed insurance with unlimited liability should be required for all such payloads. If no insurance company is willing to underwrite the risk it is a good sign for the public that the maneuver is too risky and should be not be allowed. On the other hand if the insurance industry volunteers to take on the risk, this is a good sign that the risks, financial and physical, are minimal, and that the payoff directly to the companies, and indirectly to mankind as a whole, are well worth it. -- szabo@techbook.COM Tuesday, November third ## Libertarian $$ vote Tuesday ^^ Libertarian -- change ** choice && November 3rd @@Libertarian ------------------------------ Date: 28 Sep 92 07:43:29 GMT From: Tim Starr Subject: Space Platforms (political, not physical : -) Newsgroups: sci.space,talk.politics.space,alt.politics.marrou,alt.politics.libertarian In article exukjb@exu.ericsson.se (ken bell) writes: } }Does the libertarian party support US withdrawl from the UN? }////////////////////////////////////// }/* Kenny * Welcome to Mind Wars! */ }////////////////////////////////////// Yep. Tim Starr - Renaissance Now! - Think Universally, Act Selfishly starr@genie.slhs.udel.edu "True greatness consists in the use of a powerful understanding to enlighten oneself and others." - Voltaire ------------------------------ End of Space Digest Volume 15 : Issue 256 ------------------------------