Today’s question for Dr. Science is, “Why do lighthuggers have to stay so far out? Can’t they use the same highports as normal starships?”
While it would certainly be more convenient to avoid the lengthy shuttle trip to meet a lighthugger, the risks attached to the amount of energy needed to propel a ship between the stars at near-light speed make them something best kept away from population centers.
The smallest lighthugger in production, close to the practical lower size limit, is the Evelantar-class staryacht, whose unfueled mass is 5,451 tons. It is propelled by a Nucleodyne Thrust Applications antimatter pion drive, with fusion supplementation for lower velocities, giving it a maximum cruising speed of 0.9 c.
The mass ratio, including operational safety margin, of the NTA pion drive – the ratio between its fueled mass and its unfueled mass – is 25; but since a lighthugger in many cases cannot guarantee that it can refuel at its destination, the Evelantar is equipped to carry fuel for a two-way trip.
Thus, such a staryacht can carry up to 136,275 tons of fuel one-way, or 272,550 tons fully fueled, of which just under half is antimatter in the form of metastable metallic antideuterium. And, of course, when fueled for a two-way trip, over half of its fuel – because of the additional fuel carried as a safety margin – remains in its cryocels when it arrives at its destination.
Such an amount of matter/antimatter fuel would, if detonated, produce an explosion of approximately 2.6 teratons. In orbit of a garden world, this would be sufficient to create massive earthquakes and volcanism, megatsunamis, global wildfires, major atmospheric damage, and a high-probability extinction-level event, in addition to the radiation effects. These radiation effects and indirect impulsive shock would also be lethal to any habitats or drifts within tens of thousands of miles of the explosion. And this is the fuel mass of the smallest production lighthugger.
While the probability of a cryocel containment-safety systems failure is infinitesimal, the magnitude of these consequences – along with the possibility of deliberate sabotage or the use of lighthugger fuel as a terror weapon – is sufficient for virtually all civilized systems to restrict lighthuggers to far outer-system ports of call.
– from Children’s Science Corner magazine
I think I have to point out that if you have a mass ratio of 25 for a one-way trip, you need a mass ratio of 25^2 = 625 for a two-way trip, unless you want to skimp on the delta-V and have a much slower return trip.
(Kinda wish I’d caught that particular numerical oops before the book version went to press, but I guess one can’t have everything… 🙂 )
The stated mass ratio of 50 does indeed give you just enough delta-V to accelerate to or from 90% of lightspeed (from the ship’s perspective, I don’t feel up to wrestling Einstein enough to figure out how fast the rest of the galaxy would think you were going) four times between refuelings, provided you’re spitting out those pions at more than 92% of the speed of light.
My question is, how the heck do you make a mass ratio of 50 work from a structural standpoint?
Putting the engine in front, exotic materials (and the use of non-material forces to reinforce them), exquisitely clever engineering, and a snifter of space magic.
(This is, admittedly, a little thin on the details, but there’s a limit to how detailed I can get regarding a technology roughly five millennia ahead of ours. 🙂 )