Posts in Category: Interstellar Travel (FTL)

Jump Drive

Jump drives are pretty common in SF – most recently they appeared in the remake of Battlestar Galactica.  BSG ships could jump every 33 minutes without problems, but I wanted something with more restrictions than that.  I envision something closer to the jump drive in the Traveller RPG, or in C.J. Cherryh’s “Alliance-Union” setting.  Other than jump drive, I wanted everything to be very “hard” SF – meaning keeping the more futuristic elements to a strict minimum. Basically, I settled on this:

Jump Requirements

  • Jump drives need to somehow “recharge” (at least 4-6 hours before the next jump)
  • Jump drives should be small
  • Jump drives shouldn’t need much power to activate (but still need to “recharge” somehow)
  • Jump drives should work for ships larger than a few hundred tons, but no larger than a million tons (may be adjusted if I decide to expand/restrict that)
  • Jump drives shouldn’t be effective for distances longer than about 7-8 light years
  • Military ships can jump farther
  • There are consequences if the jump is not perfect (or at least “good enough”)
  • Jumps are instantaneous
  • Jumps should not be possible next to a planet, too close to a star or gas giant, attached to a station, or in an atmosphere
  • I shouldn’t have to explain how they work, just the limitations
  • Jump drives should be difficult and expensive to manufacture, so that they aren’t ubiquitous

My notes below have the details, but basically, you have to be far enough away from a mass for gravity to be below a certain force in order to start a jump, and the jump goes in a straight line until you hit another sufficient gravity well that pulls you out of jump space.  I get around explaining how they work by making them alien technology – ship crashes on Earth, crew all dead, carrying multiple jump drives onboard (basically a freighter with a small crew) – and fortunately if you take the drive apart and manufacture the components exactly, you can build something that will work once it’s activated (passed through a jump).  Also fortunately, if you scale the drive up, it will work on bigger ships, up to a point.  Details are below.

These are my raw notes for how Jump Drive would work in the “Achilles” universe:

For accurate jump:

  • Need to know mass of ship and mass distribution – specifically the location of CoM in relation to the jump governor
  • Need to know position and velocity in relation to the system primary or system barycenter (multiple star systems are less accurate)
  • Need to know exact bearing to the jump target

All objects create “bumps” or indentations in the sheet of spacetime – gravity peaks (the reverse concept of gravity well). Jump is possible if the gravity gradient is sufficiently low – 21,870.1337N or less. The actual distance out from a star, gas giant, or other large body is dependent on the masses in question. The larger the ship, the greater the distance. Close in, the mass of the primary (or primaries) will dominate, but for outer planets, the driving factor will be the planet, moon, or asteroid the ship is near. Jumps are unrestricted in open space when sufficiently far from large bodies.

Jumps are effectively instantaneous, and a ship travels along a jump vector governed by the jump axis. The jump vector must intersect a gravity peak before the jump field becomes unstable – if the vector does not encounter a gravity peak, the ship reappears in “normal” spacetime when the jump field collapses. (a mis-jump) Since it’s not an orderly exit caused by intersection with a gravity gradient, the field collapse will not be uniform and the exit position and velocity may vary significantly from expected values. Typically this will be 9-15LY from the point of origin depending on the mass of the ship and several other factors, but a small percentage of the time, it can be significantly further. For an on-target jump, the maximum safe jump distance is 7.454 LY – and this is regarded as the “normal” maximum range. Military vessels have more crew and better support systems maintaining the stability of the jump governor, and their maximum range is slightly more than 7.6LY. In practice, military ships and the desperate rarely attempt jumps longer than the maximum safe distance of 7.454 LY.

Misjumps, in addition to missing the target and having a randomized exit point and exit vector, will typically damage or destroy the jump governor and any “spare” jump governors kept onboard. In the rare event that the jump governor is not damaged, a jump can be again attempted once the jump drive has cooled.

The jump axis runs through the center of the jump governor. This controls the creation of the entry field which transitions the ship into jump space. Ideally, the jump axis should pass through the center of mass – when it does not, the jump vector will precess (“wobble”) around the axis of mass, creating a less accurate path through jumpspace. The jump governor is typically mounted on 3-axis gyro wheels to allow the governor to be rotated so that the jump axis can be centered on the mass axis. This reduces jump error to the precision of the gyro wheels in pointing the jump axis, and typically is 1 arcsecond or less.

Jump itself doesn’t use a lot of energy – maybe a few kilowatts per thousand tons to initiate the jump. At the completion of the jump, there is a lot of thermal energy in the jump governor, which prevents another jump until the governor cools. Cooling generally cannot be accelerated without causing thermal shock that cracks components and destroys the governor. Cooldown is typically on the order of several hours. Large jump governors for large ships can have some external cooling, which allows cooldown times to remain fairly constant regardless of size. If a second jump is attempted before cooldown, the ship may fail to jump entirely, or might succeed and be lost on entry when the governor fractures, or might complete the jump successfully and merely destroy the jump governor.

Jump drive origin: An unknown vessel crashes on Earth. Onboard are hundreds of jump governors, along with other technologies, most of which cannot be used, but many of which lead to breakthroughs in power storage, thermal management, metallurgy, physics, and various engineering disciplines. The vessel is recovered by the Canadian government, and quickly a consortium develops between Canada, the US, Russia, China, the UK, and Norway (all of which had fragments of the ship after impact) and several large aerospace and tech companies (Boeing the most notable). The knowledge of advanced alien life spurs an alliance which eventually develops into United Earth, and the corporate alliance led by Boeing eventually develops into the defense/tech megaconglomerate of Federal Dynamics – a public/private partnership with the UE government. Within 20 years, the secrets of jump are unlocked, UE/FD discover how to make more of them, and the first exploration ships are outfitted and begin their survey work. Over the next century, routes are opened to dozens of star systems.

Size limits: There is a practical size to how small the jump governor can be – this directly determines the lower size limit of a starship. On the larger end, there are several conditions which interact to create a maximum jump governor size. A given jump governor can jump a ship up to 1/4 – 2x the governor rating. The “sweet spot” is 0.0093165 of ship mass. This means that ships with large mass fractions may be unable to jump when most of their reaction mass has been spent, unless the ship has a second (smaller) jump drive.

Jump range of effect: Jump affects the entire ship and anything contained within, attached to, or in direct contact with the ship at the time of jump. Items which are within inches of, but not touching the ship, will be left behind.

Jump drive manufacture: The savant Bartholomew Matthias initially discovered how to disassemble and catalog the components of the jump governor. He successfully created several devices, none of which worked. Jelenn McKitry performed initial remote testing of the devices in space, and discovered how to initiate them.  After several remote tests, the decision was made to build a manned vessel for additional testing.  The UES Asimov (XS-1) carried 17 crew, including Captain Alistair Flint, Pilot Jelenn McKitry, and Dr. Irene Bhattacharya, an experimental physicist. Bhattacharya had obtained one of Bartholomew Matthias’ jump governors, and used it to determine the best points for monitoring equipment. She ran extensive tests before and after the jump with the onboard equipment, and discovered variations in the results of Matthias’ governor before and after the jump. The initial manned test was a run to Jupiter from just outside the solar jump radius. Previous tests of the Matthias governors had not been as detailed, so the differences had not been detected. After the jump, the results of the Matthias governor matched the installed governor (of identical size) in every respect – the jump had somehow “activated” the Matthias jump governor. While many of the Matthias governors had been lost over the intervening years, his meticulous documentation of their creation was intact, and Federal Dynamics was able to manufacture additional units and “activate” them on subsequent tests. Experimentation showed that they could be made larger or smaller, to a point.

Jump distances: Jump distances are limited based on pointing accuracy. If aiming at a star 7.5 light years away, if you can point the ship (or the drive if the axis is misaligned) to an accuracy of 1 arcsecond, then the circular error of probability (CEP) is between 2 and 3 astronomical units (AU). Most ships jumping to a sun-sized star would fall out of jump within 1-5AU of the star. As long as the CEP is less than the jump distance limit, there is no chance for the ship to “miss”. If the CEP is larger, then there is a chance to miss – so there is a practical limit to the distance that can be jumped, which depends on being able to properly target your destination and accurately point the ship (or the drive). Additionally, since the ship’s center of mass and axis of mass can cause inaccuracies, it means that the ship can’t be doing things which change those parameters at the time of jump – things like crew moving around, engines firing and burning fuel or causing water or fuel to slosh in the tanks, moving cargo, firing ordinance, or anything else that would muck up the jump.

end of raw notes

Final Notes

This is a work in progress, and additional info and commentary will be added over time.