Steve Jackson Games Forums - Pyramid #3/79: Space Atlas

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Re: Pyramid #3/79: Space Atlas

Quote:

Originally Posted by Steven Marsh (Post 1909341)

So, folks... are there any comments on the issue? This was the first non-spaceship, non-Transhuman Space science-fiction issue we've done in a couple of years, and the first included spreadsheet in... wow, a lot of months. Was this issue any good? Too crunchy? Too fluffy? (I honestly like to try to give y'all what you want, if I can.) :-)

When I saw the title “Space Atlas” for this issue, I was sort of expecting at least one space map for an interstellar neighborhood (or at least a star system). The closest we got was the Diagram of Bi-Elliptic Transfers on page 27. <sigh> Of the articles, Halfway to Anywhere was my favorite. I’ve always been a bit of a math geek, so any time I get a list of equations I’m very happy. I was a bit surprised that that you included the equations for Bi-Elliptic Orbits (btw. these do “suffer launch window restrictions” unless you plan to loiter at transfer orbit apoapsis by splitting the second burn to achieve a temporary circular orbit) but not those for One Tangent Burns that can be used for Free Return trajectories. While minimizing ΔV is important for unmanned missions, minimizing TOF (Time of Flight) may be more important for manned ones. BTW, the spreadsheet needs a bit of work; you've got the article title wrong on each sheet. ;)

Dalton “who wouldn't mind a GURPS Equations sourcebook of game math” Spence

Re: Pyramid #3/79: Space Atlas

Quote:

Originally Posted by DaltonS (Post 1915480)

BTW, the spreadsheet needs a bit of work; you've got the article title wrong on each sheet. ;)

Thanks for pointing that out! I've patched it up.

Re: Pyramid #3/79: Space Atlas

Specific bi-elliptic orbits suffer launch window reatrictions, but you can change that window by altering the length of the semi-major axes of the transfer orbits [a(c) iirc; idhmbwm]. The article does sacrifice detail - e.g., which a(c) will let you launch now with minimum time of flight in exchange for not tackling some nasty calculus. But its safe to say that by manipulating a(c), a navigator can manipulate the necessary launch window for a bi elliptic transfer without circularizing the orbit and loitering.

Re: Pyramid #3/79: Space Atlas

Quote:

Originally Posted by Steven Marsh (Post 1915578)

Quote:

Originally Posted by DaltonS (Post 1915480)

BTW, the spreadsheet needs a bit of work; you've got the article title wrong on each sheet. ;)

Thanks for pointing that out! I've patched it up.

Great, where can I get a copy?

To continue the errata, in the “Formulae by the Numbers” sidebar on p.29,

the first “Achieving Orbit” equation has a few problems;
1. the last part should be [1 + S/(A-S)] (actually [A/(A-S)] would be simpler).
2. no allowance is made for atmospheric drag here.
3. Should "winged lift" reduce gravitational drag?
the first two “Oberth Effect” equations use V(per) instead of V(esc) as in the text.
the second term of the Brachistochrone Transfers ΔV equation does not match the ones in the preceding main text or the “Index of Formulae” sidebar. Which one is correct?

Also, on page 27 you state “the total delta-V of fuel needed equals the absolute value of ΔV1 plus the absolute value of ΔV2.” While technically accurate, in a Hohmann Transfer both burns are either prograde or retrograde and will thus have the same sign, making “the absolute value of (ΔV1 + ΔV2)” equally true.

Dalton “who is a simple soul who likes simplified equations” Spence

PS. I'm confused about this paragraph:

Quote:

A ship breaking orbit can use the equations above by replacing initial and escape velocity with the ship’s velocity at periapsis (Vper):

Both? Really? How does that work? Also, just to clarify terms: when breaking a planetary orbit to enter a solar transfer one, is the final velocity relative to the planet the transfer ΔV relative to the star?

Re: Pyramid #3/79: Space Atlas

A suggestion for the “Halfway to Anywhere” spreadsheet: see my thread FWIW: A Significant Formula for GURPS Spreadsheets for a formula you can use to round the raw value of calculations to 2 significant digits.

Dalton “who prefers 3 digits himself, but I'm not the Secret Masters” Spence

PS. Where can I can the updated version of the spreadsheet? Do I have to D/L the Pyramid issue again?

Re: Pyramid #3/79: Space Atlas

Quote:

Originally Posted by DaltonS (Post 1916004)

Great, where can I get a copy?

Just re-download the issue.

Re: Pyramid #3/79: Space Atlas

Quote:

Originally Posted by PK (Post 1918625)

Just re-download the issue.

Okay. Is this change to the enclosed spreadsheet enough to generate an issue update notification? Have the errata I've mentioned above been addressed?

Dalton “Expiring minds want to k....” Spence

Re: Pyramid #3/79: Space Atlas

In the latest “Halfway to Anywhere” spreadsheet, the formula for Transfer Duration: of “Brachistochrone Transfer” is:

Code:

=2*SQRT((F13+F14)/F11)+IF((F14-F13)<0;-1;1)*SQRT(F11*ABS(F14-F13)/(0.0000000000667384*F12))

In order to match the equation on page 29, it should be:

Code:

=2*SQRT((F13+F14)/F11)+IF((F14-F13)<0;-1;1)*F11*SQRT(ABS(F14-F13)/(0.0000000000667384*F12))

Note the position of the second F11. This causes a significant difference in the result at very low accelerations. Which one is in error?

Dalton “who needs to know how long a VASMIR powered 'Mars Express' would take” Spence

Re: Pyramid #3/79: Space Atlas

Thanks for your efforts!

When the issue and/or spreadsheet are significantly updated, I'll let folks know. (I didn't send an email update when was the wrong title on the spreadsheet, because that was a minor issue.)

Re: Pyramid #3/79: Space Atlas

I have just moved, so it will take me a few days to adress these properly. I will look into them, though.