Return-path: X-Andrew-Authenticated-as: 7997;andrew.cmu.edu;Ted Anderson Received: from beak.andrew.cmu.edu via trymail for +dist+/afs/andrew.cmu.edu/usr11/tm2b/space/space.dl@andrew.cmu.edu (->+dist+/afs/andrew.cmu.edu/usr11/tm2b/space/space.dl) (->ota+space.digests) ID ; Fri, 9 Mar 90 02:09:55 -0500 (EST) Message-ID: Reply-To: space+@Andrew.CMU.EDU From: space-request+@Andrew.CMU.EDU To: space+@Andrew.CMU.EDU Date: Fri, 9 Mar 90 02:09:27 -0500 (EST) Subject: SPACE Digest V11 #129 SPACE Digest Volume 11 : Issue 129 Today's Topics: Re: hubble telescope power Re: Spacecraft drives and fuel efficiency Re: Americanisms (was: Ulysses Update - 03/06/90 {sic}) Re: Any news on a new comet? specific impulse Re: specific impulse ---------------------------------------------------------------------- Date: 8 Mar 90 16:45:05 GMT From: wuarchive!cs.utexas.edu!jarvis.csri.toronto.edu!utgpu!utzoo!henry@decwrl.dec.com (Henry Spencer) Subject: Re: hubble telescope power In article <3914@nmtsun.nmt.edu> dbriggs@nrao.edu (Daniel Briggs) writes: >...no idea what the current price tag is on Hubble (Henry?), but it is in the >many billions class. I don't have numbers on hand, but not even NASA could afford a many-billion telescope. One or two billion is the right sort of number, I think. -- MSDOS, abbrev: Maybe SomeDay | Henry Spencer at U of Toronto Zoology an Operating System. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu ------------------------------ Date: 7 Mar 90 23:30:33 GMT From: kr0u+@andrew.cmu.edu (Kevin William Ryan) Subject: Re: Spacecraft drives and fuel efficiency In response to various debates on antimatter propulsion launch vehicle safety, I thought I should mention something: The energy content of an antimatter launcher (per comparative kilo to orbit) will be _lower_ than a chemical fuel launcher. The chemical launcher has a considerably lower efficiency, and must carry fuel to move the fuel to launch the payload. Mass ratios go up exponentially as exhaust velocity decreases. Also, as the fuel requirement goes up, so does the tankage, which raises the fuel requirement some more. Therefore, in terms of pure energy release in the explosions of comparable payload launchers, the antimatter launcher is safer. As I doubt that anyone will be standing within a kilometer of either launcher when it takes off (I'm guessing that the fraction of gamma radiation penetrating one kilometer of air is neligible - feel free to correct me on this) I don't think that the radiation is a threat either. kwr "Jest so ya know..." ------------------------------ Date: 8 Mar 90 16:34:29 GMT From: unmvax!nmtsun!john@ucbvax.Berkeley.EDU (John Shipman) Subject: Re: Americanisms (was: Ulysses Update - 03/06/90 {sic}) Chris Thompson (cet1@cl.cam.ac.uk) writes: +-- | 10/05/90 - Launch from Space Shuttle... | It can be quite confusing for us to read dates in your | weird local format :-) I am not suggesting you should | use *our* weird local format, though. How about using | ISO format dates? (e.g. 1990-03-06, though the ISO standard | does allow some other seperators than '-' to be used, I think) +-- Bravo! The YYYY-MM-DD ordering is also an ANSI (American National Standards Institute) standard for date recording in data processing. I feel that it is the only logical format for several reasons: (a) If you see a date like 3/6/90, you don't know whether it is European (June 3) or American (March 6). The presence of an obvious year number at the beginning of a YYYY-MM-DD date signals that it is neither. (b) In eleven years, we will have dates like 01/02/01, giving the opportunity for a THREEFOLD ambiguity! The sooner we switch to a four-digit date, the less painful the millennium will be. (c) It is a pure positional notation, like decimal numbers or days/hours/minutes/seconds, in which the units get smaller from left to right. (d) When sorting dates electronically, a date in YYYY-MM-DD format can be handled as a single sort key. A date in either DD/MM/YY or MM/DD/YY format must be handled as multiple key fields. -- John Shipman/Zoological Data Processing/Socorro, New Mexico USENET: ucbvax!unmvax!nmtsun!john CSNET: john@jupiter.nmt.edu ``Let's go outside and commiserate with nature.'' --Dave Farber ------------------------------ Date: 5 Mar 90 02:35:30 GMT From: usc!samsung!munnari.oz.au!cluster!metro!ipso!stcns3!dave@ucsd.edu (Dave Horsfall) Subject: Re: Any news on a new comet? As it happens, I received a submission for our local Amateur radio broadcast, from an astronomy enthusiast. Here it is: A rather bright comet should soon make an appearance in our sky. Comet Austin, 1989 C1, according to observations and computer calculations may become as bright as zero magnitude in April, although caution should be exercised as such new comets often tend to be "fizzers." Perhaps second magnitude may be more realistic, however will will have to wait and see. The comet was discovered by Rod Austin of Blenheim, New Zealand on 6th December 1989. Perihelion or closest approach to the sun is on 9th April at 0.349 Astronomical Units, that is 34.9% of the mean Earth-Sun distance, about the distance of the planet Mercury from the sun. It is now in the evening sky, but after perihelion it becomes a morning object and finally a night-time object. The Orbital Elements etc come from the International Astronomical Union Circulars originating at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, USA. -- Dave Horsfall (VK2KFU) Alcatel STC Australia dave@stcns3.stc.oz.AU dave%stcns3.stc.oz.AU@uunet.UU.NET ...munnari!stcns3.stc.oz.AU!dave ------------------------------ Date: 8 Mar 90 20:32:43 GMT From: psuvm!mrw104@psuvax1.cs.psu.edu Subject: specific impulse >>- One pound of hydrogen and oxygen burning for one second exerts 456 >>pounds of force. This gives it a 'specific impulse' of 456 seconds. >>"Hydrogen-Oxygen is the highest specific impulse of any chemical >>combination of fuels in use today". >> >Specific impulse is simply a measure of the rocket's exhaust velocity, >not the thrust. A specific impulse of 456 s corresponds to an exhaust >velocity of 4469 m/s. How much thrust is produced depends on the >mass flow rate also. Nope-- specific impulse is defined as Thrust I = -------------- SP Mass flow rate The equivalent exhaust velocity is the specific impulse of the fuel times the acceleration due to gravity at sea level (this assumes conventional rocket engines, ie one that throw mass out the back end for thrust.) Mike Williams mrw104@psuvm.edu ------------------------------ Date: 8 Mar 90 23:40:11 GMT From: cs.utexas.edu!uwm.edu!ux1.cso.uiuc.edu!uxh.cso.uiuc.edu!jep@tut.cis.ohio-state.edu (John E. Prussing) Subject: Re: specific impulse >>Specific impulse is simply a measure of the rocket's exhaust velocity, >>not the thrust. A specific impulse of 456 s corresponds to an exhaust >>velocity of 4469 m/s. How much thrust is produced depends on the >>mass flow rate also. > >Nope-- specific impulse is defined as > > Thrust > I = -------------- > SP Mass flow rate > >The equivalent exhaust velocity is the specific impulse of the fuel times >the acceleration due to gravity at sea level (this assumes conventional >rocket engines, ie one that throw mass out the back end for thrust.) > >Mike Williams >mrw104@psuvm.edu =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= We seem to be going around in circles here. All of the above is correct. No matter how you say it, the thrust is the product of the mass flow rate times the effective exhaust velocity (or, EQUIVALENTLY, the specific impulse times g_zero). At psu, you can ask Prof. Bob Melton in Aerospace Engineering. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- John E. Prussing Internet: jep@uxh.cso.uiuc.edu Aeronautical & Astronautical Engineering Bitnet: jep@uiucuxh University of Illinois at Urbana-Champaign Tel: (217) 333-8231 ------------------------------ End of SPACE Digest V11 #129 *******************