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When Space Toolkit came out (with the Tachyon Squadron kickstarter), I was glad it included what looked like a way to abstract space travel while maintaining Newtonian behaviour of spaceships. But now I'm taking a closer look at those either missed something, or have trouble understanding the game-mechanical trade-offs between engines (among other things).

I'm looking specifically at engine rules of the Mass Drivers setting, on page 146 of the Space Toolkit:

a ship can travel from one space to another on the space map in a few days, which counts as one exchange. Modify this travel time per exchange by steps equal to the difference between the ship’s Mass and Thrust, decreasing travel time if its Thrust is greater than its Mass, and vice versa (Fate Core, page 197). Thus, a ship with Fantastic (+6) Mass and Good (+3) Thrust would shift a travel time from a few days up to a few weeks or half a month.

Modify the distance that the ship is able to travel in one exchange by the difference between its Mass and its Impulse, so that a ship with Impulse greater than Mass is able to move one more space on the map per exchange for each step of difference. This reflects how longer periods of acceleration allow for the accumulation of velocity.

And looking at the above formula, and I don't see a game-mechanical reason to pick the low-thrust-high-impulse drive over the high-thrust-low-impulse drive.

In real life, the former has the benefit of generally having a higher ΔV, which adds up to a higher average and peak velocity over the course of the whole trip even despite low acceleration. When I look at the mechanics, a reduction of exchange duration seems to always result in a faster trip to the destination than an increase in the number of nodes crossed per exchange. (Specifically, as per Core 197, a step of time reduction offers at least a doubling of speed, and on average a bit more than that.)

Another way of looking at the increased fuel efficiency would be a drastic reduction in the rate of depleting the fuel stress track (minutes vs. weeks or more until empty), but I haven't found a mechanical support for that either. Maybe I overlooked it somewhere.

What are the game-mechanical benefit of impulse-oriented drives that represent the things they're cherished for, assuming literal reading of the rules?

It's pretty clear that by Rules as Intended, the impulse-oriented drive should provide faster movement on interplanetary scales (but not on small scales like launches, of course), however that's not what I'm asking about. Rather, I'd like to know if there are game-mechanical benefits of such drives in the literal reading of the rules that I may have missed - whether directly mentioned ones, or emergent due to any rules interactions I didn't think of. (If the rules are indeed faulty as they seem to be, applying changes/fixes will be the next phase of tackling this issue.)

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They do different things, and each makes you faster in a different way.

a ship can travel from one space to another on the space map in a few days, which counts as one exchange. Modify this travel time per exchange by steps equal to the difference between the ship’s Mass and Thrust

...

Modify the distance that the ship is able to travel in one exchange by the difference between its Mass and its Impulse

Having extra Thrust makes each exchange take less time. Having extra Impulse lets you travel farther in each exchange. Depending on what you want to achieve (travel a moderate distance in a short time, cover a vast distance in an average time, etc.) and how the exchange duration chart scales (from the example shown, I assume it's non-linear), different combinations of Thrust and Impulse may be preferred.

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    \$\begingroup\$ The exchange duration scales in accordance to the rules on Core 197, which is the reason why, according to my observations, the there doesn't seem to be a benefit of picking Impulse over Thrust (since the latter seems always better). \$\endgroup\$ – vicky_molokh Mar 13 at 11:08
  • \$\begingroup\$ The rest of the world might not be waiting for you to travel to your destination. Having a longer exchange might allow BBEG to get prepared for your arrival. \$\endgroup\$ – aragaer Mar 13 at 19:25
  • \$\begingroup\$ @aragaer But the length of BBEG's exchange would depend on BBEG's engine's Thrust. \$\endgroup\$ – vicky_molokh Mar 13 at 19:42
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I haven't played this system, but I'll try to answer based on the quoted rules and what I know about Fate generally, and a charitable interpretation of the rule:

Modify the distance that the ship is able to travel in one exchange [...] so that a ship with Impulse greater than Mass is able to move one more space on the map per exchange for each step of difference

I would parse this as follows: Similar to how velocity is written out as "meters per second" and acceleration is written out (SI Units) as "meters per second per second", you can think of "the ship is able to travel in one exchange [...] one more space" as having units of "spaces per exchange" already. Then "the ship is able to travel in one exchange [...] one more space per exchange" would have units of "spaces per exchange per exchange" which is acceleration (accumulation of velocity over time).

Mechanically, this is identical to "Each exchange, permanently add the difference between Impulse and Mass to the number of nodes you can move (this effect can compound with itself over the course of multiple exchanges)." and this is how I would describe it to players to prevent it from being ambiguous (as the original text is) if you decide to interpret it this way.

The alternative is that the bonus does not stack: you only get +X nodes travelled in the current exchange, but that does not become your new "base speed" for future exchanges. In this interpretation, the second "per exchange" is redundant, which is also a reasonable way to parse the sentence if it stood alone. If you read it this way, it does seem clearly inferior.

I can't argue for the first interpretation over the second based purely on the grammar of that singular sentence, but I think that the following sentence makes the intent clear:

This reflects how longer periods of acceleration allow for the accumulation of velocity.

You get faster and faster each exchange. Since an exchange is a fixed time period, "accumulation of velocity" over longer periods can only be referring to accumulating velocity over multiple exchanges

Example: If a ship with +3 Mass and +3 Impulse can move X spaces on the map in an exchange, then a ship with +3 Mass and +5 Impulse (difference of 2 between Impulse and Mass) can move:

  • X + 2 spaces in exchange 1
  • (X + 2) + 2 == X + 4 spaces in exchange 2
  • ((X + 2) + 2) + 2 == X + 6 spaces in exchange 3

Over a long enough trip, this will result in a higher average velocity and peak velocity than a High Thrust / Low Impulse engine.

If your fuel is depleted at a constant rate per exchange, then this will eventually result in arbitrarily high fuel efficiency.

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  • \$\begingroup\$ Welcome to RPGSE. I don't want to discourage you, and am thankful for you trying to tackle the issue, but I'm not convinced that the charitable interpretation of what the mechanics as written are (even though I see it as a reasonable variant of what they should be based on what they're meant to represent). Could you elaborate please why you parse the text as saying that, given that it seems to not explicitly mention mechanical carry-over effects (that I can see, at least)? \$\endgroup\$ – vicky_molokh Mar 13 at 11:38
  • \$\begingroup\$ I tried to do that by quoting This reflects how longer periods of acceleration allow for the accumulation of velocity. You accumulate velocity over longer periods. Since an exchange is a fixed time period, this can only be referring to accumulating velocity over multiple exchanges. I will edit that into my answer, but does it make sense? \$\endgroup\$ – Steven Jackson Mar 13 at 13:45
  • \$\begingroup\$ My impression is that 'this reflects' actually serves against your case, as it seems to be synonymous with 'this is what the mechanic represents'. So I'm alas not convinced that would make a difference in terms of RAW. I do think it's a good change if I end up using this mechanic with some houserules, since then the mechanics would become closer to the thing they represent. \$\endgroup\$ – vicky_molokh Mar 13 at 13:56
  • \$\begingroup\$ I cleaned up the formatting, and also clarified how I parsed the sentence more clearly: Similar to how velocity is written out as "meters per second" and acceleration is written out (SI Units) as "meters per second per second", you can think of "the ship is able to travel in one exchange [...] one more space" as having units of "nodes per exchange" already. Then "the ship is able to travel in one exchange [...] one more space per exchange" would have units of "nodes per exchange per exchange" which is acceleration (accumulation of velocity over time). Let me know if that is clearer. \$\endgroup\$ – Steven Jackson Mar 13 at 13:58
  • \$\begingroup\$ I also re-arranged it so that the actual "parse the sentence like this" piece comes first, and the "mechanically this is equivalent to" piece comes second. I originally skipped the "parse the sentence like this" part and re-wrote the rule completely differently. I can see why that made it impossible to follow my thought process. Thanks for the requesting clarification, i think the answer is better now! \$\endgroup\$ – Steven Jackson Mar 13 at 14:11

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