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Regarding acceleration and travel times

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  • Regarding acceleration and travel times

    Ships using Epstein drives are not able to maintain full acceleration for weeks at a time (fuel considerations) so the longer jaunts on the travel table while technically accurate arenít realistic

    For instance from the books the Roci can only pull .25gís for 3-4 weeks

    This is also an important point for thematic reasons, without this limitation you can tool around the solar system really fast, reach relativistic speeds etc

    We also donít have any stats on fuel for ships just thrust

    Any chance of fleshing this out some ?

    ó-

    Note for some reason I canít seem to reply to this thread but let me clarify

    - Epstein drives do in fact require reaction mass to produce thrust (usually water )
    - This is different from the power to actually run the fusion reactor (which in cannon does appear to be a nonissue)
    - there are many examples in the book of ships taking weeks or months to teach a destination. This would simply never happen with unlimited constant acceleration
    - there are areas of the solar system which are still relatively undeveloped because of distance, again this would never happen if you could pop over to Neptune in two weeks
    - star travel is a generation ship thing in canon. With unlimited constant acceleration of even 1g you donít need generation ships to get to the nearest star
    Last edited by Unholyguy; 12th March 2019, 10:15 PM.

  • #2
    Re: Regarding acceleration and travel times

    As a more concrete example, take the Earth->Pluto route

    Now we know that the outer planets (Uranus, Neptune etc ) are considered helluvah far away and on the outskirts of civilization. Doubly so for Pluto

    However according to the travel table a 1g burn will get me there in 17 days . No problem for an Earth crew at least, letís go to Pluto

    Except ships in the expanse cannot do a 1g burn for 17 days they donít have the fuel

    Instead Iím probably looking at 0.25g for about 10 days, 5 days to accelerate then i coast then 5 days to deaccelerate (need fuel for the return trip)

    This leaves me with a travel time to Pluto of around 90 days. And a round trip of 180 days

    Now Pluto feels far. Heck better stop at the belt and refuel get there faster

    Comment


    • #3
      Re: Regarding acceleration and travel times

      Hello,

      to my understanding:

      - the Epstein Drive as a modified fusion drive does not require fuel (pg 113 CRB); āonlyĎ thruster do so, but as far as I understand it thrustersĎ use is limited to maneuvering, which does not raise fuel issue on long-distance travel
      - the tables 3A through 3D (pg 118 CRB) are meant to be understood as mere guidelines considering the acceleration values of 0.3g, 1g, 7g, and 12g as average values (pg 117 CRB) for the whole trip, which means that the values change during the course of travelling. Also the values are based on average distances between the stellar objects and tjus do not display the closest possible to maximum distances between the objects. Therefore the values may change a lot. I wonder: since I do not understand the maths behind all of this, is anyone out there able to provide tables like 3A through 3D, but displaying distances and travelling-times for the shortest and maximum possible distances between the different planets?

      Best wishes!
      Cpt. Cthulhu
      The stars are right. Always. If you disagree, Cpt. Cthulhu swallows your soul.

      Comment


      • #4
        Re: Regarding acceleration and travel times

        My understanding from the fiction is that Epstein drives are so incredibly efficient that the bigger concern on long trips is water for both potables and as a thruster propellent. Running low on fuel never comes up and the Roci even manages a honking great six month round trip to [spoilertown] in Cibola Burn without a refuel.

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        • #5
          Re: Regarding acceleration and travel times

          Originally posted by Hyperspacey View Post
          My understanding from the fiction is that Epstein drives are so incredibly efficient that the bigger concern on long trips is water for both potables and as a thruster propellent. Running low on fuel never comes up and the Roci even manages a honking great six month round trip to [spoilertown] in Cibola Burn without a refuel.
          In general you would never take a six month round trip to anywhere if this was true. The farthest destination in the solar system would be 3 weeks away

          Several times in the cannon itís mentioned that fuel is a concern, once Alex says specifically that the Roci can only maintain 3-4 weeks at 0.3g

          Similarity it took 9 months to get to a certain object at the edge of the solar system (spoiler) which was roughly the same distance away and Pluto

          For more info

          http://www.projectrho.com/public_htm...st.php#epstein

          Comment


          • #6
            Re: Regarding acceleration and travel times

            There would be no six months trips anywhere in the solar system with a constant acceleration of 1g or even .3g. Your talking a couple weeks to get to Pluto

            Thatís the point, travel times as presented in the series are significantly longer then would be delivered. The explanation for that is fuel and the associated constraints on delta-v

            Assume a coupe weeks of .3g (as the book confirms at one point) more or less makes the series work

            For more information, copious analysis here

            http://www.projectrho.com/public_htm...st.php#epstein

            Comment


            • #7
              Re: Regarding acceleration and travel times

              Thatís the point though, without fuel constraints there are no six month trips. The farthest trip will be a couple weeks. Which doesnít fit the canon at all

              Also constraints are mentioned several times in the book, not so much in fuel for the drive itself but reaction mass to propel the ship (probably water).

              Lots of math and details here

              http://www.projectrho.com/public_htm...st.php#epstein

              Comment


              • #8
                Re: Regarding acceleration and travel times

                - Epstein drives do in fact require reaction mass to produce thrust (usually water )
                - This is different from the power to actually run the fusion reactor (which in cannon does appear to be a nonissue)
                - there are many examples in the book of ships taking weeks or months to teach a destination. This would simply never happen with unlimited constant acceleration
                - there are areas of the solar system which are still relatively undeveloped because of distance, again this would never happen if you could pop over to Neptune in two weeks
                - star travel is a generation ship thing in canon. With unlimited constant acceleration of even 1g you donít need generation ships to get to the nearest star

                Comment


                • #9
                  Re: Regarding acceleration and travel times

                  Originally posted by The Expanse RPG;page 117
                  This information is included as a useful reference and guideline, not to put bounds on the storytelling in your Expanse
                  series! As noted previously, any number of factors can affect communication and travel times, so feel free to tinker with
                  these basic numbers as best suits your particular story or adventure. Often the narrative taking place during a transit or
                  involving intra-system communication involves an interlude during or between adventures (see Interludes in Chapter 5
                  for details).
                  If you demand absolute precision, feel free to go ahead and tinker with the times presented. The numbers presented in the book are approximations, as we had no wish to require higher math for people to play the game. Most ships are going to travel at the lowest presented speed, .3g, unless they are doing a hard burn for a seriously time critical mission.
                  ALL HAIL THE SPAM-O-MATIC!

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                  • #10
                    Re: Regarding acceleration and travel times

                    Originally posted by Fildrigar View Post
                    If you demand absolute precision, feel free to go ahead and tinker with the times presented. The numbers presented in the book are approximations, as we had no wish to require higher math for people to play the game. Most ships are going to travel at the lowest presented speed, .3g, unless they are doing a hard burn for a seriously time critical mission.
                    Itís not really absolute precision though itís thematically broken, even if you assume .3g which there isnít really any reason to, considering the Martian military at least train under 1g

                    I can put together a table of alternate travel times that assume a limited delta-v, the math isnít that bad and there are online calculators to help there. Agree full precision would be a pain though since each ship would have a different mass ratio

                    Would also make chases and races more fun ...

                    Comment


                    • #11
                      Re: Regarding acceleration and travel times

                      There's an estimate of what's involved in an Epstein drive over on the Atomic Rockets website. That gives 6.9g with a .09kg/s flow rate, for a rocket with a wet mass of 15 tonnes and a dry mass of 3.75 tonnes. This breaks down to about 36 hours of burn at 6.9g, or around 250 hours at 1g (which is probably what you'd use to get around).

                      From this, you can expect to get wherever you need to go out to about Saturn within 10 days, without running out of fuel. If you're going to something farther away, you'll probably stop to refuel, breaking it up into two trips. Essentially, use the 1G travel times table unless the hours are more than 250, in which case a normal ship needs to refuel or go slowly. If you need to do it without stopping, at .3g, you can get between any two systems, but you're looking at travel times of a month to get to Pluto. You'd be a bit faster at 1g, since you can accelerate for 5 days, maybe cruise for 15 more, then decelerate for 5 days at the end (I'm just guessing on the cruise time).
                      Last edited by ubik2; 14th March 2019, 06:59 PM.

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                      • #12
                        Re: Regarding acceleration and travel times

                        Yeah I was looking at the AR site as well, but they were also doing their estimate for the Roci which is a war ship and doesnít really carry much other then a small crew, reaction mass and weapons. I also think their swag for the efficiency seemed pretty high and not well supported , higher then seemed to be justified by the examples of travel in the book for instance

                        I was thinking about tuning down their efficiency assumptions to about half. That would explain why places like Saturn and farther are hard to get to and why the belt matters so much , and would still be a damn efficient torch drive

                        For more common civilian ships that are devoting a fair amount of space to cargo I think reaction mass reserves are considerably smaller. I think you probably need another stat for the ship to capture actual % of ship devoted to reaction mass

                        It also leads to more intersting ship vs ship interactions, if itís not just raw acceleration but also staying power that factor in races and chases

                        Comment


                        • #13
                          Re: Regarding acceleration and travel times

                          This thread has been bugging me.. scared as many other things regarding a series that I love that even the smallest wrong detail can ruin my relationship with the rpg.

                          I'm not finished with the manual yet, nor with the books, and mainly I don't have time to search for data to confirm this, but I think the problem with the traveling time issue is human survivability to Hi G, not the fuel.
                          Think of when the Roci pursues Eros, it's not the motor the limit in the pursue, it's the crew capacity to sustain Hi Gs even under Juice..
                          The manual says that common drugs lasts for 2d6 minutes, doesn't stats how much the Juice last actually. But in the end it's clear to me that it's not very dangerous for the crew to speed up to 1G but if you sail a manned vessel to pluto at 7 or 12G it will arrive quickly, full of dead bodies.

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                          • #14
                            Gone
                            Last edited by Harper; 4th September 2019, 09:17 AM.

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                            • #15
                              Re: Regarding acceleration and travel times

                              Traveling between outer system planets, the relative positions of planets matter a lot but if either end point is in the inner system, doesnít matter much

                              So like going from Jupiter to Saturn the relative positions matter but not if you are going from Mars to Saturn

                              The six month trip to Uranus was exactly my point, the math presented in the rule book absolutely does not support anything like that even with the least favorable planetary positions imaginable

                              Your talking 11 days according to the rules

                              Now if you assume a limited reaction mass itís easy to explain
                              Last edited by Unholyguy; 6th April 2019, 01:49 PM.

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