>Date: Wed, 20 Mar 1996 02:19:07 -0700 (MST) >From: Steven Martindale >To: Chuck & Kathy Bennett >cc: gurpsnet-l@io.com >Subject: Re: Realistic Interplanetary Travel >Sender: owner-gurpsnet-l@io.com >Reply-To: Steven Martindale >Content-Return: prohibited >X-URL: http://www.io.com/sjgames/gurps >X-URL: http://www.io.com/~ftp/GURPSnet >X-URL: http://www.io.com/user/epopt/gnetfaq.html > >On Tue, 19 Mar 1996, Chuck & Kathy Bennett wrote: > >> I'm aware that the most fuel-efficient travel is by Hohmann transfers, in > > Hmmm, I need to dig through my recent copies of Discover >Magazine, as they had an article recently about another efficient >transfer for which the short and sweet is that it's mroe fuel efficient, >but not faster (if I'm remembering correctly). > > Originally developed by a guy trying to figure out how to get a >ship/probe/whatever from Earth to the Moon using electric propulsion (is >this the 'official' name for ion engines?) which are good for long >duration burns, but don't provide as much 'kick' as a good old chemical >rocket. When he tried modelling such a vehicle using a Hohmann transfer >it would turn on the drive, spend a few weeks spiralling outwards from >Earth picking up speed, make its turnaround, and whiz by the Moon unable >to slow down in time to enter orbit around the moon... > > His solution was to use finess, and unfortunately I can't >describe it worth anything without the source material on hand. (And >it's been used at least once, so we know it works) > > >Steve, Date: Wed, 20 Mar 1996 02:19:07 -0700 (MST) From: Steven Martindale To: Chuck & Kathy Bennett cc: gurpsnet-l@io.com Subject: Re: Realistic Interplanetary Travel On Tue, 19 Mar 1996, Chuck & Kathy Bennett wrote: > I'm aware that the most fuel-efficient travel is by Hohmann transfers, in Hmmm, I need to dig through my recent copies of Discover Magazine, as they had an article recently about another efficient transfer for which the short and sweet is that it's mroe fuel efficient, but not faster (if I'm remembering correctly). Originally developed by a guy trying to figure out how to get a ship/probe/whatever from Earth to the Moon using electric propulsion (is this the 'official' name for ion engines?) which are good for long duration burns, but don't provide as much 'kick' as a good old chemical rocket. When he tried modelling such a vehicle using a Hohmann transfer it would turn on the drive, spend a few weeks spiralling outwards from Earth picking up speed, make its turnaround, and whiz by the Moon unable to slow down in time to enter orbit around the moon... His solution was to use finess, and unfortunately I can't describe it worth anything without the source material on hand. (And it's been used at least once, so we know it works) Steve, Date: Wed, 20 Mar 1996 07:22:58 -0800 From: sands@netcom.com (Sue and Sean) To: kendrice@indirect.com CC: bennett@rollanet.org, gurpsnet-l@io.com Subject: Re: Realistic Interplanetary Travel Sender: owner-gurpsnet-l@io.com Reply-To: sands@netcom.com (Sue and Sean) Content-Return: prohibited X-URL: http://www.io.com/sjgames/gurps X-URL: http://www.io.com/~ftp/GURPSnet X-URL: http://www.io.com/user/epopt/gnetfaq.html > Originally developed by a guy trying to figure out how to get a >ship/probe/whatever from Earth to the Moon using electric propulsion (is >this the 'official' name for ion engines?) which are good for long >duration burns, but don't provide as much 'kick' as a good old chemical >rocket. When he tried modelling such a vehicle using a Hohmann transfer >it would turn on the drive, spend a few weeks spiralling outwards from >Earth picking up speed, make its turnaround, and whiz by the Moon unable >to slow down in time to enter orbit around the moon... A Hohmann orbit requires one short eccentricity-changing burn at the apo/periapsis and another at the peri/apoapsis. It is not intended for the constant low thrust of an electric drive, so I'm sure that a different course (not "orbit," since the spacecraft would not be in free-fall) would be used. Susan and Sean (order optional) Politics: poli-, many; -tics, S & S Enterprises blood sucking parasites. sands@netcom.com Date: 20 Mar 96 18:34:25 EST From: "S.D. Anderson" <102250.1425@compuserve.com> To: GURPSnet Subject: Re: Realistic Interplanetary Travel Sender: owner-gurpsnet-l@io.com Reply-To: "S.D. Anderson" <102250.1425@compuserve.com> Content-Return: prohibited X-URL: http://www.io.com/sjgames/gurps X-URL: http://www.io.com/~ftp/GURPSnet X-URL: http://www.io.com/user/epopt/gnetfaq.html About 20 years ago in his column "A Step Further Out", Jerry Pournelle crunched the numbers for the "Torch Ship" theorized for belter prospectors in countless space operas of yesteryear. (I don't remember what issue. If anyone on the list knows or can find out, please let me know. Very useful for any Terradyne or Terradyne-ish setting.) His conclusion: An ship with the delta v to travel the belt in a practical manner would not be seriously hindered by Earth's gravity well. Being more centrally located, Earth or Luna would make a much better location for trade and administration. Date: 21 Mar 96 01:59:25 EST From: "Chuck & Kathy Bennett" To: GURPSnet Subject: Re: Realistic Interplanetary Travel At 06:34 PM 3/20/96 EST, S.D. Anderson wrote: >About 20 years ago in his column "A Step Further Out", Jerry Pournelle crunched >the numbers for the "Torch Ship" theorized for belter prospectors in countless >space operas of yesteryear. > (I don't remember what issue. If anyone on the list knows or can find out, >please let me know. Very useful for any Terradyne or Terradyne-ish setting.) > His conclusion: An ship with the delta v to travel the belt in a practical >manner would not be seriously hindered by Earth's gravity well. Being more >centrally located, Earth or Luna would make a much better location for trade and >administration. > I remember reading that. A bunch of those articles are collected in a book, naturally titled "A Step Farther Out", 1979, from Ace SF. A lot of the info that I've put together on Isp and delta-v for different types of drives comes from that book or extrapolations from it. Chuck Bennett bennett@rollanet.org Date: Thu, 21 Mar 96 17:23 EST From: Aerodesic Research <0001998364@mcimail.com> To: Chuck Bennett Subject: interplanetary travel Chuck, For futuristic, and possable, crAft Bussard's right ( as told by Ligon letter ) the trips will tend to be straight lines. The veloc ities artre 10s to 100s of times those of Holmann transfers. What little deviations can be corrected with side thrusters of the same magnitude. However, from your postings in sflit, I gatther you are interested in smaller Delta Vees, 100-200Kps. These are much more then the 30 Kps or less Holmann delta vees but still need to consider the local gravity wells as a major factor. They're called hyperballic orbits and you can figure then to a first approximation as quadratics. The rubberbook ( CRC) only has the tables for our own solar system and doesn't list there equations. Arther C Clarke has the equations in the promise of space, but again figures the trips from Earth. P_Powells Technical Books in Oregon has used copies of some of these and they will ship. Its a toll free call... For that quadratic figuse v=( v2^2-v1^2)^1/2, and a= -delta Vee/t with the constand as the distance D. You solve for t, timre.V1 is the escape velocity of planet A( which you can figure from its mass and radius or 2^1/2 times the orbiting velocity. v is excess velocity at infinity, and v2 is v1 pluss what extra delta vee you can add for the trip. This way you don't have to wafe through a perponderance of partial differentials and tripple integral:s for a change under 5% in energy or time as in the Fundamentals of Astro@hics. Clarke's bod and the quadratic are as good as cirxcular orbits for figuring delta vees and trip times. me, I'd go for Bussard's figures as we will have them anyway as soon as we get any real industry into LEO Jim From: Chuck Bennett To: Aerodesic Research <0001998364@mcimail.com> Subject: interplanetary travel At 05:23 PM 3/21/96 EST, you wrote: >Chuck, > For futuristic, and possable, crAft Bussard's right ( as told by >Ligon letter ) the trips will tend to be straight lines. The veloc >ities artre 10s to 100s of times those of Holmann transfers. What >little deviations can be corrected with side thrusters of the same >magnitude. > However, from your postings in sflit, I gatther you are interested >in smaller Delta Vees, 100-200Kps. These are much more then the 30 >Kps or less Holmann delta vees but still need to consider the local >gravity wells as a major factor. They're called hyperballic orbits >and you can figure then to a first approximation as quadratics. Thank you, you are one of the few to acknowledge what I'm actually looking for - not ultra-tech drives w/ huge delta-v, but something where it still takes some time and planning to get around the solar system, so it isn't a routine trip. I have a lot of info on estimates of Isp for different drives, and I know how to use Isp and mass-ratio to figure delta-v. What I need is what you're talking about - some equations to =approximate= trip times and required delta-v (therby fuel/mass usage). I don't need to actually be able to send a probe there and hit a particular orbit, like those fellows at NASA and JPL - I just want some approximations for a =game=. Approximation by a quadratic equation I can handle - but the next part here is where I start to get lost... > The rubberbook ( CRC) only has the tables for our own solar system >and doesn't list there equations. Arther C Clarke has the equations in >the promise of space, but again figures the trips from Earth. P_Powells >Technical Books in Oregon has used copies of some of these and they >will ship. Its a toll free call... OK, so what are the equations? Or do you mean that I should get Clarke's book and that it has the simple quadratics that you were talking about to get approximate figures? > > For that quadratic figuse v=( v2^2-v1^2)^1/2, and a= -delta Vee/t >with the constand as the distance D. You solve for t, timre.V1 is the >escape velocity of planet A( which you can figure from its mass and >radius or 2^1/2 times the orbiting velocity. v is excess velocity at >infinity, and v2 is v1 pluss what extra delta vee you can add for the >trip. Let me make sure I'm reading your notation correctly: x^y means x to the y power? So v2^2 means v2 squared? And ^1/2 means to the one-half power, or square root? I see you're giving figures for v, a, and D, and I solve for t. OK. What equation do I plug these into to relate v, a, D, and t? Am I missing something? Do these go into an equation from Clarke's book? Could you give an example w/ some numbers plugged into the variables? > This way you don't have to wade through a perponderance of partial >differentials and tripple integral:s for a change under 5% in energy >or time as in the Fundamentals of Astro@hics. Clarke's bod and the >quadratic are as good as cirxcular orbits for figuring delta vees and >trip times. Thanks, approximation is fine. (If only I could understand what you are telling me as to how to =do= the approximation!) > me, I'd go for Bussard's figures as we will have them anyway as soon >as we get any real industry into LEO Sigh. It may come to that. But I'd really like to have the flavor of Red Mars/Green Mars/Tinker et al, where even the inner planets are really a =long= way apart, and folks have to think carefully about how they use their fuel and reaction mass, rather than just jaunting around in their space-yachts for a day's outing. >Jim > Thanks for your help. I like the direction you're heading with the approximations, I just got lost behind you somewhere along the way. If you can get me back on the path, I'd appreciate it. I'm planning to head over to the UMR library this weekend to see if I can find any of the various books that have been suggested to me here and in the CI$ forums, to see if there are some with math on a level I can handle for the kind of approximation we are talking about. Chuck Bennett bennett@rollanet.org Date: Fri, 22 Mar 1996 11:53:43 -0600 (CST) X-Sender: bennett@rollanet.org To: tomvall@ix.netcom.com (Thomas A. Vallejos ) From: Charles Bennett Subject: Re: Realistic Interplanetary Travel Cc: gurpsnet-L@io.com Sender: owner-gurpsnet-l@io.com Reply-To: Charles Bennett Content-Return: prohibited X-URL: http://www.io.com/sjgames/gurps X-URL: http://www.io.com/~ftp/GURPSnet X-URL: http://www.io.com/user/epopt/gnetfaq.html At 11:08 PM 3/21/96 -0800, you wrote: [snip repeat of my original message] >I suggest you check out "A Step Farther Out", by Jerry Pournelle. [snip] Thanks. I have this book, and have gotten a lot of info about realistic space-drives and Hohmann orbits from it. Unfortunately, most of Pournelle's calculations are done "off-screen", with tables of results for a few examples, but no equations to figure out your own trajectories or figure non-Hohmann trajectories. >Also check out Arthur C. Clarke's >"The Promise of Space." It may help you with STL stuff. If the articles >themselves aren't that useful, look in the Bibliography section, they >may point to some articles and books that could help you more. > Thanks. I'll see if I can locate a copy. >Good Luck. > >Tom Vallejos > Chuck Bennett bennett@rollanet.org Date: Fri, 22 Mar 1996 13:37:45 -0600 (CST) X-Sender: rwright@pop3.umr.edu To: bennett@rollanet.org From: rwright@umr.edu (Robert Wright) Subject: GURPS and Space Dynamics I read your message on the GURPS List and did not notice that you were local until you mentioned going to the UMR Library. As a Aerospace student I may be able to help. One book to try is _Introduction to Space Flight_ by Francis Hale. The library should have it as I used it for a course last semester. If you would like I could meet you at the library and I may be of some help. Rob Wright ----------------------------------------------------------------------- From the College that Lazurus Long's Pop went to and Alexander Hergensheimer failed out of -- IT'S Robert G. Wright RolePlayer/SpaceFanatic/NeophyteFan/ComputerEnthusiast/AndInLoveAgain Date: Fri, 22 Mar 1996 22:30:18 -0600 (CST) X-Sender: bennett@rollanet.org To: rwright@umr.edu (Robert Wright) From: Chuck & Kathy Bennett Subject: Re: GURPS and Space Dynamics Cc: gurpsnet-L@io.com Sender: owner-gurpsnet-l@io.com Reply-To: Chuck & Kathy Bennett Content-Return: prohibited X-URL: http://www.io.com/sjgames/gurps X-URL: http://www.io.com/~ftp/GURPSnet X-URL: http://www.io.com/user/epopt/gnetfaq.html At 01:37 PM 3/22/96 -0600, you wrote: >I read your message on the GURPS List and did not notice that you were local >until you mentioned going to the UMR Library. > >As a Aerospace student I may be able to help. One book to try is >_Introduction to Space Flight_ by Francis Hale. The library should have it >as I used it for a course last semester. If you would like I could meet you >at the library and I may be of some help. > >Rob Wright [cool sig snipped] Wow, this is like finding the Grail! Not only have I got a lot of good references to track down at the library, but now an AE student right here in my hometown offering to help! I must be livin' right. Thanks, Rob. My wife has informed me that we are cleaning the house tomorrow, since her folks are coming to visit next week, so I don't think I'm going to make it to UMR tomorrow. Is the library open on Sunday afternoon? I'll let you know whenever I manage to head in that direction. Thank you very much for your kind offer of help. Chuck Bennett bennett@rollanet.org Date: Fri, 22 Mar 1996 15:07:59 -0800 X-Sender: henryrap@ix.netcom.com To: Chuck & Kathy Bennett From: "Paul D. Henrichsen" Subject: Re: interplanetary travel Cc: gurpsnet-l@io.com At 10:52 PM 3/21/96 -0600, Chuck & Kathy Bennett wrote: >At 05:23 PM 3/21/96 EST, you wrote: >>Chuck, [snip] >OK, so what are the equations? Or do you mean that I should get Clarke's >book and that it has the simple quadratics that you were talking about to >get approximate figures? I believe that the equations he refers to are .5at^2 + vt + d(initial) = d(final) (where d represents position) v(final)=v(initial) + at >> For that quadratic figuse v=( v2^2-v1^2)^1/2, and a= -delta Vee/t >>with the constand as the distance D. You solve for t, timre.V1 is the >>escape velocity of planet A( which you can figure from its mass and >>radius or 2^1/2 times the orbiting velocity. v is excess velocity at >>infinity, and v2 is v1 pluss what extra delta vee you can add for the >>trip. >Let me make sure I'm reading your notation correctly: x^y means x to the y >power? So v2^2 means v2 squared? And ^1/2 means to the one-half power, or >square root? yes, that's standard notation >I see you're giving figures for v, a, and D, and I solve for t. OK. What >equation do I plug these into to relate v, a, D, and t? Am I missing >something? Do these go into an equation from Clarke's book? Could you give >an example w/ some numbers plugged into the variables? > check the formulas listed above. [snip] I hope this is useful to you. If you have web access, you might see what you can find at the Lunar Institute of Technology. Their URL is http://sunsite.unc.edu/lunar/index.html. Enjoy! --Paul ------------------------------------------------------------ Paul D. Henrichsen woefulhc@io.com henryrap@ix.netcom.com homepage http://www.io.com/~woefulhc/ pauldh@aol.com Date: Sat, 23 Mar 1996 00:22:33 -0800 From: tomvall@ix.netcom.com (Thomas A. Vallejos ) Subject: Re: Realistic Interplanetary Travel To: Charles Bennett The way I do STL travel is to use classic Newtonian physics. My ships don't go anywhere near the light barrier. That is point ship, fire engines calculate distance = 1/2 (at^2) in a straight line between two points. Uses lots of fuel and or reaction mass (depends on TL). Maybe it's a bit simplistic, but it works. If a ship is accelerating fast enough gravity won't be factor unless it gets too close. How close? Varies from planet to planet. I remember another source. Physics Tday from 1968 (I think). Discusses Freeman Dyson's idea for a Nuclear Pulse Rocket. Don't get behind this baby... Dr. Robert L. Forward discusses other STL drives in Future Magic. Tom Date: Sat, 23 Mar 1996 08:44:24 -0600 (CST) X-Sender: bennett@rollanet.org To: tomvall@ix.netcom.com (Thomas A. Vallejos ) From: Chuck & Kathy Bennett Subject: Re: Realistic Interplanetary Travel Cc: gurpsnet-l@io.com Sender: owner-gurpsnet-l@io.com Reply-To: Chuck & Kathy Bennett Content-Return: prohibited X-URL: http://www.io.com/sjgames/gurps X-URL: http://www.io.com/~ftp/GURPSnet X-URL: http://www.io.com/user/epopt/gnetfaq.html At 12:22 AM 3/23/96 -0800, you wrote: [re-quotes snipped] >> >The way I do STL travel is to use classic Newtonian physics. My ships >don't go anywhere near the light barrier. That is point ship, fire >engines calculate distance = 1/2 (at^2) in a straight line between two >points. Uses lots of fuel and or reaction mass (depends on TL). Maybe >it's a bit simplistic, but it works. If a ship is accelerating fast >enough gravity won't be factor unless it gets too close. How close? >Varies from planet to planet. > This is how I have done interplanetary travel up to now, due to not knowing how to figure delta-v requirements and travel times for slower trajectories, Hohmann transfers and variations thereof. What I am looking for here is a slightly lower tech setting, where travel between even the inner planets still takes a =long= time, and travellers have to think carefully about their use of fuel and reaction mass - sort of like in Robinson's Red Mars/Green Mars and Pournelle's Tinker (and others). >I remember another source. Physics Tday from 1968 (I think). Discusses >Freeman Dyson's idea for a Nuclear Pulse Rocket. Don't get behind this >baby... > >Dr. Robert L. Forward discusses other STL drives in Future Magic. > >Tom Thanks, I'll look for Future Magic. Chuck Bennett bennett@rollanet.org Date: Sat, 23 Mar 1996 12:46:01 -0500 X-Sender: caroth@iu.net To: bennett@rollanet.org From: Craig Alan Roth Subject: Realistic Interplanetary Travel Sir, For a truly GOOD book for ideas on near-future concepts, read a book by Robert L. Forward called "Indistinguishable From Magic". The first section, ANTIMATTER, gives a good look at how anitmatter can be used in a reaction drive. Essentially, for any length trip, the reaction mass remains the same (four times the mass of the combined total of the rest of the craft), only the amount of antimatter is changed. If what Forward, a noted engineer, says is true about the creation and storage of antimatter, GURPS may have to relocate the tech-level down to 8 or 9. I hope you find the book helpful, Craig Roth Date: Sat, 23 Mar 1996 19:41:48 -0800 From: lpurple@netcom.com (Lance Purple) To: bennett@rollanet.org Subject: Re: Realistic Space Travel (update) Newsgroups: rec.games.frp.gurps Organization: NETCOM On-line Communication Services (408 261-4700 guest) In article <4iqrah$ggo@gamma.rollanet.org> you write: >Last night (3/19) at about 10pm I posted a request for help in finding >materials to simulate interplanetary travel for a game/story setting >using realistic space transport - no reactionless drives. You might want to check out the "Magsail" WWW page at http://152.2.22.81/lunar/magsail.html This describes Robert Zubrin's "magnetic sail" propulsion system. It is simply a 30-km diameter loop of superconducting wire, which is charged with electrical current to produce a 10^-5 Tesla field. This field repels protons from the solar wind, giving a constant acceleration of 0.02 m/s^2 to the 100 kg. sail *and* 100 tons of payload. No moving parts; no reaction mass; and it even helps to protect your crew from solar flares. It is almost feasible with current technology. The only obstacle is that the wire needs a critical current exceeding 10^6 amps/cm^2. The new ceramic superconductors can't carry this much current yet. Niobium wire can, but has to be refrigerated down to 10 K, which is a real pain. (The sail would constantly be boiling off liquid helium, which kinda defeats the advantage of not needing reaction mass...) Anyway, hope this is interesting and useful to you. ------------------------------ Lance Purple (lpurple@netcom.com) Date: Sun, 24 Mar 96 02:39 EST From: Aerodesic Research <0001998364@mcimail.com> To: Chuck & Kathy Bennett Subject: approximations & fuel Dear Chuck, It occured to me that I had assumed a working knowledge of rocket propulsion and astronomy in my last letter. There are several equations you have to keep in mind to use the quadratic effectivly in planing fuel usage. Since I don't play, or even like, roll playing games and videogames I don't know -how- detailed you want it. Using the right figures for a trip to Saturn, and not the off-the cuff ones I said earlier: From LEO delta vee for a holmann is 10.3 KPS Taking into consideration both the Earth's and the Sun's gravity wells ( they vary with Distance D as does the period or "year") g(sun, or planet) varies as g'=g(0)/(delta d^2) and delta y^2=delta d^3 The trip takes about 6.34 years ( standard) Adding 5,15 kps at LEO cuts that time to 3.1 years, but it also adds a velocity at infinity of 11.48 kps that you have to account for in the fuel use for a total delta vee of 16.63. alowing for course corrections and matching orbital planes you need 17 to 18 KPS extra delta vee and not just the 5.15 you started out with. FMR=e^(total delta Vee/ Vex), where Vex==Isp*9.807 and total delta vee. You can use up 100K_s easily in one trip this way from A to B in one sysytem and then from b to A in another Jim From: lpurple@netcom.com (Lance Purple) Subject: Re: Realistic Space Travel (update) To: bennett@rollanet.org (Chuck & Kathy Bennett) Date: Sun, 24 Mar 1996 09:39:15 -0800 (PST) You wrote: > Thanks, Lance. I'd never heard of this before. Kind of a different breed > of solar sail, instead of the usual large, very thin mirror. > > Yours is actually the only response I've gotten from the GURPS newsgroup so > far, but I've had a lot of help from the GURPSnet-L@io.com mailing list. > I'm putting a cc of this reply to the list so others there can see it. Glad to help. Another resource you might want to see is Dani Eder's "Canonical List of Space Transportation Methods", which attempts to catalogue all known proposals for space transportation systems. http://www.ifi.unizh.ch/groups/ailab/people/schaad/space/ Canonical_Space_transport_list [size 85K - 4 Mar 94] You might also ask around in the newsgroups "sci.space.tech" and "rec.arts.sf.science". Lance Purple (lpurple@netcom.com) Date: Mon, 25 Mar 96 00:16 NZST X-Sender: banj0@deepsouth.co.nz (Unverified) To: Chuck & Kathy Bennett From: banj0@deepsouth.co.nz (banj0 øß|iViøn) Subject: Re: Realistic Interplanetary Travel Jerry Pournelle's email address = jerryp@bix.com Date: Tue, 26 Mar 1996 17:36:34 -0600 (CST) X-Sender: bennett@rollanet.org To: me From: Aerodesic Research <0001998364@mcimail.com> (by way of Charles Bennett ) Subject: Re: interplanetary travel Chuck, You got that right, I'm just a lousy typest. I skipped some but you set all of the polynomial as functions of D and T. Like so: 1/2*a*t^2 +v*t -D=0 you set the v by finding out the escape v of the starting planet ( either from tables or from 1st year physics from the given planet) if you know the orbiting velocity just multiply it by 2^1/2.. You get the effective a by iterating the quadratic with differing values for v2, basically the starting out vee lowers over time so its a -negative -accileration., its vary small. but you can also get this by using the 1/2*a*t^2 over the trip length. All unites should be in MKS to get consistant seconds. I usually figure t as yje half distance and find round trip times. Using Saturn v1==1.12*10^4 m/s, D==1.65*10^12, ,a==10^-4( at least at first> Total delta Vee==v2+a*t. per trip if you get close to any well, the hyperbalic distorts due to the steep hgravity gradient even a close orbit or brush with a small moon could divert the course by 60 degrees. CVClarke(s book has examples in the chapter called Trillion Mile whirlpool if I misremember correctly. I lost my copy... Jim Date: Sat, 4 May 1996 10:54:08 -0700 From: tomvall@ix.netcom.com (Thomas A. Vallejos) Subject: Realistic Space Travel To: bennett@rollanet.org Hi Chuck: Just wondering if you saw my post on where you can find info on Hohmann transfer orbits? Just in case here it is again: http://www.planetary.caltech.edu/~eww/physics/node9 Tom Subject: Travel time, Re: Ship and Starship ... To: gurpsnet-l@io.com Date: Mon, 6 May 1996 09:17:03 +0200 (MET DST) X-Aliased: From "Onno Meyer" From: "Onno Meyer" Sender: owner-gurpsnet-l@io.com Reply-To: "Onno Meyer" Content-Return: prohibited X-URL: http://www.io.com/sjgames/gurps X-URL: http://www.io.com/~ftp/GURPSnet X-URL: http://www.io.com/user/epopt/gnetfaq.html > > And does anyone know a good equation for figuring out how much time it > takes to go a given distance at a set acceleration? I have sworn off > reaction drives in favor of reactionless, and I need some idea how long > it takes to get places. > > Gilmer Brendan Young If a is the constant acceleration, t the time and s the distance (units in meter and second), you have the equation s = 0.5*a*t^2 or sqrt ( (2*s)/a ) = t But that means you accelerate all the time towards the destination and zip past at a high velocity. Thus you have to travel half the distance two times, once accelerating and once decelerating. t = 2*sqrt(s/a) Note: This assumes the pull of stars and planets are neglible and the speed is well below lightspeed. On insystem operations with a decent reactionless drive, it is a safe bet. BTW, Space contains a table with acceleration in Gs and distance in AUs, probably more comfortable units for you. Onno Meyer Date: Mon, 6 May 1996 07:34:05 -0500 (CDT) From: MA Lloyd To: Onno Meyer cc: gurpsnet-l@io.com Subject: Re: Travel time, Re: Ship and Starship ... Sender: owner-gurpsnet-l@io.com Reply-To: MA Lloyd Content-Return: prohibited X-URL: http://www.io.com/sjgames/gurps X-URL: http://www.io.com/~ftp/GURPSnet X-URL: http://www.io.com/user/epopt/gnetfaq.html On Mon, 6 May 1996, Onno Meyer wrote: > But that means you accelerate all the time towards the destination > and zip past at a high velocity. Thus you have to travel half the > distance two times, once accelerating and once decelerating. > > t = 2*sqrt(s/a) > > Note: This assumes the pull of stars and planets are neglible and > the speed is well below lightspeed. On insystem operations with a > decent reactionless drive, it is a safe bet. Anything under 1200 G-hrs keeps relativistic distortions under 1%, safely ignorable. > BTW, Space contains a table with acceleration in Gs and distance > in AUs, probably more comfortable units for you. Or you can convert the units and still use the formula t(hours) = 68.7 *sqrt(s(AU)/a(G)) There are two realistic cases where interplanetary trajectories can be solved analytically without ignoring gravity: One is the Hohmann transfer orbit, the minimum energy course resulting from a short (compared to trip length) high acceleration starting phase. Trip time is .25 x P (1+R2/R1)^1.5, where P is the period of the starting orbit (1 year for a vehicle leaving Earth) and R1 and R2 are the distances from the sun of the starting and ending orbits. The other is the constant low acceleration transfer, which goes to the limit t = 6.2832 (R1/P*a) * (1-R1/R2)^0.5, where a is the acceleration approaching zero (valid approximation where a << R2/P^2). Careful, units matter here. -- MA Lloyd (malloy00@io.com) Date: Tue, 9 Jul 1996 16:36:45 -0500 (CDT) From: MA Lloyd To: traveler@io.com cc: gurpsnet-l@io.com Subject: Re: Space Campaign Sender: owner-gurpsnet-l@io.com Reply-To: MA Lloyd > Magnetic sails are also nice for interplanetary travel, and not too slow > > near a planet, but lousy for interstellar trips. > > Why? From what I've read about them, they'd provide decent thrust > longer than a lightsail would on the outgoing trip, and would work > beautifully as a braking system when you get to the new system. They do make good brakes, but on the outbound leg your velocity is limited to the speed of the solar wind (which is pretty slow on an interstellar scale), and once you cross the heliopause you lose the solar wind, and will have a hard time doing any sort of midcourse correction. -- MA Lloyd (malloy00@io.com) Comments: Authenticated sender is From: "Dataweaver" To: traveler@io.com, MA Lloyd Date: Tue, 9 Jul 1996 22:07:05 +0007 Subject: Re: Space Campaign CC: gurpsnet-l@io.com Priority: normal Sender: owner-gurpsnet-l@io.com Reply-To: "Dataweaver" Content-Return: prohibited X-URL: http://www.io.com/sjgames/gurps X-URL: http://www.io.com/~ftp/GURPSnet X-URL: http://www.io.com/user/epopt/gnetfaq.html > Date: Tue, 9 Jul 1996 16:36:45 -0500 (CDT) > From: MA Lloyd > To: traveler@io.com > Cc: gurpsnet-l@io.com > Subject: Re: Space Campaign > Reply-to: MA Lloyd > On Sat, 6 Jul 1996, Dataweaver wrote: > > > > Magnetic sails are also nice for interplanetary travel, and not too slow > > > near a planet, but lousy for interstellar trips. > > > > Why? From what I've read about them, they'd provide > > longer than a lightsail would on the outgoing trip, and would work > > beautifully as a braking system when you get to the new system. > > They do make good brakes, but on the outbound leg your velocity is limited > to the speed of the solar wind (which is pretty slow on an interstellar > scale), and once you cross the heliopause you lose the solar wind, and > will have a hard time doing any sort of midcourse correction. > > -- MA Lloyd (malloy00@io.com) Agreed; as the _sole_ means of propulsion, a magsail would have some problems in terms of interstellar travel; but why not use magsails for the initial outward stage of the journey - until the speed of the solar wind is achieved - then switch over to another psystem for the second and third stages (further acceleration and course correction, respectively), and finally switch back to the magsail for the fourth stage (braking)? Overall, this would require a smaller system and less fuel than the backup propulsion system alone, because you wouldn't need fuel for part of your acceleration or for any of your braking. BTW, I read the _Roleplayer_ article about magsails; I was wondering, though, how much efficiency is gained by using multiple coils? -- Jonathan "Dataweaver" Lang mailto:traveler@io.com http://www.io.com/~traveler/ Date: Wed, 10 Jul 1996 08:03:36 -0400 (EDT) X-Sender: tratclif@brutus.bright.net To: gurpsnet-l@io.com From: Tracy Ratcliff Subject: Re: Space Campaign Sender: owner-gurpsnet-l@io.com Reply-To: Tracy Ratcliff Content-Return: prohibited X-URL: http://www.io.com/sjgames/gurps X-URL: http://www.io.com/~ftp/GURPSnet X-URL: http://www.io.com/user/epopt/gnetfaq.html At 04:36 PM 7/9/96 -0500, MA Lloyd wrote: >On Sat, 6 Jul 1996, Dataweaver wrote: > >> From what I've read about [magnetic sails], they'd provide decent thrust >> longer than a lightsail would on the outgoing trip, and would work >> beautifully as a braking system when you get to the new system. > >They do make good brakes, but on the outbound leg your velocity is limited >to the speed of the solar wind (which is pretty slow on an interstellar >scale), and once you cross the heliopause you lose the solar wind, and >will have a hard time doing any sort of midcourse correction. OTOH, the combination of magsail + particle cannon is by one estimate 6 times The October '95 Discover has an article that sketches out Bob Forward's laser sail and the Andrews/Zubrin particle cannon magsail.the magsail is that a charged particle beam falls apart rapidly in a vacuum, so the sail has to accelerate a 1000 g's initially. Tracy Date: Thu, 11 Jul 1996 14:23:14 +0100 (BST) From: Alun Thomas To: gurpsnet-l@lists.io.com Subject: Re: Space Campaign X-Sun-Charset: US-ASCII Sender: owner-gurpsnet-l@io.com Reply-To: Alun Thomas Content-Return: prohibited X-URL: http://www.io.com/sjgames/gurps X-URL: http://www.io.com/~ftp/GURPSnet X-URL: http://www.io.com/user/epopt/gnetfaq.html brandor@juno.com wrote: > It would seem that one could 'tack' the solar wind, much as a yachtsman > tacks the earthly wind. Adjusting sails to provide forward thrust in a > different direction than that of the wind itself. > Granted, in space, there is no 'friction' of water (or land) to help keep > the nose of the craft in the proper direction. You can use flywheels or small thrusters to control the orientation of the craft. > tacking into the wind would be nigh impossible, without some sort of > additional propulsion. This is where the analogy with Earth-bound sailing breaks down a bit. You *can* use a solar sail to fly directly towards the Sun. The trick is to remember that you start in orbit around the Sun, and its only your speed that stops you falling into it - if you use the sail to push against your current movement, you'll drop towards the Sun. Alun. ~~~~