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):
If you want to know the technical details behind how I calculated this then read on…