Reynard said:
The bubble isn't a physical object per se as a soap bubble. It's a three dimensional field centered inside the vessel and extending out moving relative to the vessel and holding the hydrogen in place. It can be hard to describe but this field 'draws' the vessel into a hyper universe without effecting the speed, inertia and vector though the field itself is manipulated to vector the ship to the destination as programmed into astrogation. When you pop out, you will still have the speed and direction from the launch point. Even if you shut your engine down at jump you still retain the velocity and direction at that moment.
One big reason to slow down or even stop before jumping you don't know what's in the region of the destination you enter. Space is big but there's still stuff out there. You won't have much reaction time if you come screaming out of the tunnel blind plus you might not be aiming toward your final destination.
The field is, yes. Though I'll address some questions on how that field is formed below. However, according to what we have in the rules as written, that field must be filled with hydrogen, thus its also a physical "bubble" of hydrogen (I'm assuming in gaseous form by reason that when released to the larger volume surrounding the ship it would go from liquid to gaseous states). That bubble of hydrogen, not being physically constrained, would continue on its original vector and velocity if the ship were to change velocity or vector while prior to jump. That's the part I was getting at and why I'm suggesting a ship must either stop completely or at minimum must not change vector or velocity prior to jumping.
That would then seem to raise the question, how long does it take to form and fill this bubble with hydrogen since that amount of time becomes important. It becomes the amount of time your ship cannot maneuver and I would also think would be unable to return fire if fired upon. Given the frequency with which some Traveller's get themselves into... let's call it "interesting situations"... that could become an important point in play.
Now as far as how the field is formed. I've wondered about that too. There's nothing much about it specifically in Traveller that I'm aware of (if there is I'm sure one of you will point it out). And to be honest, its a level of detail I'm personally willing to "hand-wave" away and ignore because I don't think its significant to actually playing. That is it probably won't have any bearing on ship operations, tactics, escaping those patrol ships while you smuggle the daughter of a wealthy noble offworld for a tryst with her lover... or other such "interesting" events. That said...
If the field is formed by creating a micro black hole, I don't think you could create that within the ship (and then expand the field outward). The intense gravity effects would rip your ship apart. There is mention of using exotic particles (which is sci-fi speak for "we made up some mysterious material that pretty much magically does this" and I'm fine with that for this purpose). So perhaps the particles are ejected in front of the ship, and then some form of energy... intense laser, tachyon particle beams, or some other "gadget" is used to excite the exotic particles and form a micro black hole which then forms the bubble while ripping a hole in the fabric of space. Not exactly hard sci-fi, but I'm cool with that, it works for Traveller and its playable. But that still begs the question, the "field" formed must be filled with hydrogen to form the jump bubble. How long does that take and what happens if the ship maneuvers during this process? What happens if the ship and/or the bubble are fired upon (in other words how hard is that bubble to disrupt)?
Those last questions are the essential questions I'm pondering because they do affect play directly and could affect it in some big ways. It could make escaping combat by jumping (a tactic I've seen used a lot over the years) virtually impossible. A ship preparing to jump would... for a few seconds? minutes? tens of minutes? be more vulnerable, making it a tense (and possibly dramatic) moment during play if the ship is in an unsafe area (like unexplored space, a hostile system, being searched for by patrols, chased by pirates, etc. etc.).