Tag: worldbuilding

[Battletech] Realistic gravity-based Jump Point Distances

I’ve got back into Battletech recently and I’ve been trying to reverse-engineer how the Proximity Limit is calculated (because I like to know how these things work). First, here’s a little background if you’re unaware of how jump works in Battletech. According to the fluff, Jumpships use a “Kearny-Fuchida drive” to travel over interstellar distances almost instantaneously. Traditionally they arrive at the Zenith/Nadir points – above the north and south poles of a star respectively – at the Proximity Limit, which is (supposedly) determined by the gravitational field of the star. These Jumpships usually carry Dropships, which then detach from the Jumpship with their cargo of mechs and head towards their target planet using normal fusion drives. Meanwhile, the Jumpship (which has little more than a few manoeuvering thrusters) generally sits at the arrival point and unfurls a large solar sail about a kilometre in diameter that it uses to recharge its drive in readiness for the next jump, which usually takes around a week (150-200 hrs).

After looking at the latest version of the stellar data table on pg. 100 of Campaign Ops, and I made an interesting observation: the Proximity Limit is not based on gravity. Given all the fluff says that this distance is determined by gravity (e.g. the notes on pg. 122 of Strategic Ops, summarised as “hyperspace fields don’t like gravity”) this is somewhat surprising. It turns out I’m not actually the first to notice this either – a thread on the battletech forums from over a decade ago discusses this too, but no solution to this is presented there. So I wondered – what would it look like if the Proximity Limit was calculated using gravity?

If you just want to skip to the point – here’s the gravity-based Safe Jump Distance table that you can use in your Battletech games instead of the published ones (this is just intended as a variant to use if you want more realistic values):

Table showing recalculated gravity-based Safe Jump Distance values.

If you want to know the technical details behind how I calculated this then read on…
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[2300AD] Pushing the Stutterwarp Drive


While I was looking at the side branches of the French Arm (I’m still working on it!), I found a group of systems that were 7.769ly from the nearest star on one of the side branches. Which got me to wonder – what happens if a system in the 2300AD setting is slightly over 7.7 ly away – between 7.7 to 7.8ly? Can people still reach it without tugs or multiple drives/tuning? Do they have to “push the drive” even for such a small increment?

Let’s have a look at how “pushing the drive” really works (using the MGT rules). The rules say: “Pushing a stutterwarp drive past its discharge limit: 1–6 hours, Very Difficult, range increased by Effect x10%.” So when do you start “pushing the drive”? I’d guess no later than 6 hours before it gets to 7.7 ly (otherwise you wouldn’t be able to finish before hitting the limit).

Getting into the nitty-gritty of the game engine, I guess theoretically an engineer could spend a lot of time on it (reduce by two timesteps to 10-60 hours – so break out the coffee and stims!) to knock the DM penalty from DM-4 to DM-2. I’d imagine most engineers would have skill level 2-3 (DM +2 or +3), and Int or Edu of at least 9-11 (DM+1) – let’s just say that a competent engineer should be able to get a DM+4 between their skill and characteristic. So if they spend 10-60 hrs on it they’d be rolling 2d6 against difficulty 8 with a net DM of +2, which would give them a 72% chance of success. Though granted, it’s unlikely they’d be able to do the task for 10-60 hours (on their own at least) – at worst I think they’d just do it at Very Difficult with 1-6 hours and have all the DMs cancel out for a 42% chance of success. It seems to me that a competent engineer could reasonably be willing to spend at least 6-24 hours to push the drive to get a 58% chance of success with no possibility of destroying the ship (though there’s still a 42% chance that he’ll destroy the drive, so it’s probably not something that would be tried regularly).

Exceptional Failure only occurs if the Effect is -6 or less (I think that’s what the rules mean – not less than -6, since -6 isn’t on the table otherwise), so they’d only have a 2.8% chance of exploding the ship if they didn’t take more time to do it (Very difficult, no DMs, 8+ required) – otherwise Exceptional Failures can’t happen if there are net positive DMs. Though Average or Marginal Failures still at least disable the drive (I’d imagine the drive would just explode but not necessarily destroy the ship with it, and the the ship would suddenly drop out of stutterwarp).

If the roll succeeds, then there’s a decent chance that the range is increased by 10-30% (the chance is lower for the 40-60% increase, since that would require bigger effect – though spending longer on the roll would make those more likely) – so theoretically a drive has a reasonable chance of being pushed to 10.01 lightyears (maybe up to 12.32 ly if the engineer is very good and very lucky)!

So the question really is whether all this is even necessary to go slightly over 7.700 ly. If it is, then I guess it’s not going to be very likely that anyone would want to be regularly heading out to systems that are even 7.71 – 7.80 lightyears away since there’s a significant chance the drive would be destroyed/rendered inoperative (even though it’s at most about 1-2% further in range). Maybe risk-taking explorers would do it, but it wouldn’t be a regular route. Or maybe a 1-2% range increase is OK, but the drive MUST be checked over/recalibrated/retuned at the destination (after discharging) before it can be reused? I’d be more inclined to go with the latter option – it adds possibilities and doesn’t seem unreasonable.

Extending the drive range

There are at least three options (possibly more) to increase the range of the drive:
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