A thought about FTL drive

[Graham Kennedy]

So a lot of the time in sci fi, the writer wants to have it so that you have to fly at sublight speed for a while before you can use your FTL drive. Trek messes with this a couple of times, claiming that you can't engage warp within the solar system. Heinlein's done it, Weber's Honorverse does it, Niven's Known Space stories does it, probably too many others to list. Usually, the rationale given is that your FTL drive needs to be away from gravitational influences to kick in.

So I was thinking on a variation of this that might be an interesting wrinkle. What about an FTL drive system that can only be engaged or disengaged from deep within a gravitational well, rather than far outside one?

What I had in mind was that you have to be in a gravity field of, say, 20g before you can engage your warp drive. And once in warp, you have to reach another such gravity well before you can safely emerge from warp again. Trying to go into or out of warp drive anywhere else converts your ship into its component molecules.

Now there's only one place in our entire solar system with that much gravity - you'd have to be within 820,000 km of the centre of the sun. Since the sun is about 700,000 km in radius... this would be problematic.

Wrinkles that occur to me with this idea :

1) For most SF universes, the only real difference between a sublight ship and an interstellar one is the presence of an FTL drive, along with maybe minor things like life support endurance. In this system, all FTL ships would need to be designed with massive capability to withstand heat / light. There would be a major distinction between those and purely in-system craft. Much as there is between inshore craft and open ocean ships today. Though that capability would perhaps be common with warships that would need armour or shields anyway.

2) It would make travelling in FTL a lot less convenient. Problem with your warp drive in Trek? Just drop to sublight and work on fixing it. Problem with your FTL in this system, you have to make it to the nearest exit point.

3) It would radically alter solar system defence. Under the usual system, the idea of defending a solar system from attack is absurd unless you have FTL sensors with ranges in the tens of AU, near light speed travel, weapons with multi-AU range, or can fly around your system at FTL. We've seen Trek talk about absurdities like "mining the edge of the solar system", apparently without any real idea of what that would entail. Even something like the "Mars defence perimeter" is a bit silly, given that it would have to cover about 10^25 square km. Under my suggestion, you don't need to defend the outer system at all - all you need to defend is the area immediately around the star, because that's the only vector any attack can come from. Of course, that's still a very large volume of space by our standards.

4) On those lines, inner planets would be important! Mercury would be the most vital strategic planet in the solar system. Such would be true in any system.

5) Red giant stars are lower mass than our sun, but much larger volume. So the warp limit would actually be inside the star, which would make for a very interesting entry/exit. In fairness, something very similar to this can happen with the Alderson jump drive in the Niven/Pournelle book "The Mote in God's Eye", so that's unfortunately not an original idea. Ah well.

6) Likewise, many stars may be too light to even have a useable FTL perimeter. Such systems would be total mysteries, forever unreachable by FTL or STL travel.

That's all the implications I can think off, offhand. Any thoughts or suggestions?

[McAvoy]

Interesting premise. Maybe a some sort of slingshot effect or some drive that absorbs gravity?

[IanKennedy]

Another example of this is the B5 gate system. Small ships typically can't enter FTL travel on their own and have to enter and leave via a gate, using whatever sub light speed they can to get to and from the gate. Larger, more advanced ships are able to have the gate technology built in.

[IanKennedy]

A high gravity requirement suggests a wormhole type thing.

[Graham Kennedy]

Another example of this is the B5 gate system. Small ships typically can't enter FTL travel on their own and have to enter and leave via a gate, using whatever sub light speed they can to get to and from the gate. Larger, more advanced ships are able to have the gate technology built in.

Yep (just finished rewatching much of B5!), but as far as we know you can place jumpgates anywhere and open jump points anywhere. I'm suggesting a system where you could only do that very close to a massive body like a star.

Ooo, there's a thought - you'd also be able to do it very close to a black hole or neutron star.

[Captain Seafort]

6) Likewise, many stars may be too light to even have a useable FTL perimeter. Such systems would be total mysteries, forever unreachable by FTL or STL travel.

Not necessarily - you'd just have to take your time getting there from the nearest jump-reachable system.

[Graham Kennedy]

Well it depends on how good your STL is. Trek could reach another star at impulse, if they were willing to spend a few decades going each way. But it's unlikely a normal Trek ship could run that long without outside support, given how often they seem to need outside supply.

But if you were limited to a few gee and few days acceleration they'd be effectively out of reach.

[Mikey]

Interstellar war would be extinct, considering how easy it would be to defend a particular system.

[Coalition]

Interstellar war would be extinct, considering how easy it would be to defend a particular system.

Not really. Two systems would have more resources than the one defender, and could attack. The defenses would know where the attackers will emerge from (somewhere inside X volume), but wouldn't know when or where. So attacking forces could emerge at one location and concentrate their full weaponry on specific parts of the defense, especially if the defenses are not at general quarters.

Automate weaponry would help since all they need is someone at a base (any base) to designate the inbound ships as hostile. From there, the automated platforms start firing until they or the targets within range are destroyed. This can range from orbiting short-burn missiles (aka minefields), expendable weapon platforms (bomb-pumped X-Ray satellites), capacitor fed energy satellites (only fire one shot per hour, but you don't have to replace them when used), all the way to automated bases using their better computers to coordinate on-board weaponry, plus thicker shields/armor.

The other fun the defenders have is trying to spot an attacking vessel against the background giant ball of fusion. They will need excellent communications so bases/sensor platforms off to the side can spot enemy vessels without having the sun as a background emitter.

The attackers have an advantage where their targets are all backlit by the cold of interstellar space. The defenses will be much easier to spot and target, though the defenses don't have to include a large FTL drive on board. Even better, the attacker only has to deal with solar emissions for a few hours, while the defenders have to deal with solar emissions 24/7. The attackers can be stored within a shielded/cooled base (similar to the Nkllon shield ships in Star wars), dropped off within FTL jump range, and as soon as they exit FTL they only have to deal with the local stars heat/energy for a few hours. Since the defenders are often in fixed orbits (or higher fuel requirements for changing orbits), that means attackers can jump in, look for the older infrared images of the defender units, and fire missiles/weapons at the estimated positions of the defenders. Missiles would be useful as they could maneuver after launch, while energy weapons would be near or at the speed of light.

Defenders could have one useful defense/decoy - mirrors. If an unknown ship pops in, the mirror decoys would rotate themselves so the sun's light is reflected at the target. This will cause a series of false targets to pop up for any attacker to have to ignore. If the mirrors are placed near the actual defending units, that means attackers firing at them will have to deal with a lot of distractions.

Just some thoughts.

[Graham Kennedy]

A lot of how the attacking/defending works would come down to the tech level employed. They have FTL, obviously, but otherwise? I lean towards a pretty low tech setting... no shields, no "impulse drive" that zips around at half the speed of light, no phasers.

So say ships are limited to the acceleration that a person can take - five gees or so. Which means it's going to take you on the order of five or six hours to get out from the jump zone to as far as Mercury's orbit, putting you past the inner defences and out into the system proper.

As Coalition says, on the one hand the attackers will have an excellent view of the defenders, presuming their ships/defence platforms are orbiting somewhat above the jump zone, whilst the defenders are trying to pick attackers out against bad interference - an extreme case of "coming in out of the sun" there. But you could alleviate that with a network of sensors that block the sun and scan around it, much as we see the area around the sun clearly during an eclipse.

The biggest advantage the defenders would have would be that there is plenty of energy on tap for them. Stick a layer of solar cells 10km on a side in orbit, say 10 million km out. Cluster your defensive lasers behind it in the shade and you have about 30 TW on hand to power them with. And the attackers can't do the same because you can't reasonably build something to fly around in FTL that's as big as an orbital platform. They'd have to generate all that energy from their power system, which would likely be a fusion one at the levels we're talking about.

Course this presumes that the defending system has the money to build such a defence system, which it may well not.

[Coalition]

A lot of how the attacking/defending works would come down to the tech level employed. They have FTL, obviously, but otherwise? I lean towards a pretty low tech setting... no shields, no "impulse drive" that zips around at half the speed of light, no phasers.

So say ships are limited to the acceleration that a person can take - five gees or so. Which means it's going to take you on the order of five or six hours to get out from the jump zone to as far as Mercury's orbit, putting you past the inner defences and out into the system proper.

As Coalition says, on the one hand the attackers will have an excellent view of the defenders, presuming their ships/defence platforms are orbiting somewhat above the jump zone, whilst the defenders are trying to pick attackers out against bad interference - an extreme case of "coming in out of the sun" there. But you could alleviate that with a network of sensors that block the sun and scan around it, much as we see the area around the sun clearly during an eclipse.

The biggest advantage the defenders would have would be that there is plenty of energy on tap for them. Stick a layer of solar cells 10km on a side in orbit, say 10 million km out. Cluster your defensive lasers behind it in the shade and you have about 30 TW on hand to power them with. And the attackers can't do the same because you can't reasonably build something to fly around in FTL that's as big as an orbital platform. They'd have to generate all that energy from their power system, which would likely be a fusion one at the levels we're talking about.

Course this presumes that the defending system has the money to build such a defence system, which it may well not.

We can use the sun shade style to protect the sensors, the problem is that the attackers will be seeing the defenders directly, while the defenders will have to reply upon a sensor station to the side, which has to receive the light-speed information about the attacker arriving, then transmit that to the defender. The attacker only has to see the defending station, so the attacker will have the initiative.

For the solar cells, that also means they are fragile. Any base that uses lots of solar cells will have to detach from them before combat, or decent dodge maneuvers will rip the solar panels off. It will reduce maintenance needs outside of combat due to lower fuel needs, but in combat the station will still need internal power supplies to avoid being a stationary target (compare the math needed to aim the Voyager probe at Neptune vs trying to dock ships in Earth orbit).

Now the local force having the funds to build a defense system is tricky, as you are trying to protect enemy ships from getting past the surface of a sphere 820,000 km in radius, or ~8.4 trillion square kilometers. The local defender can try to protect that entire surface, or use a local defense fleet (based near xeno-Mercury?) to set up roving patrols and intercept attackers.