Regarding high acceleration (or lack of), the late Robert L. Forward wrote about several interesting ideas both as a scientist/engineer and as a sf writer.
Within the solar system, it is wasteful to accelerate and de-accelerate a cargo, when in the end you just balance the momentum. Think of how a space elevator is different from a rocket, especially if counterweighted by incoming cargo.
Take a long pole and spin it in space, rapidly. The hub where it pivots is an easy place to dock, and then tractor rails pull it to one end where it is released, thrown out to the destination. Similarly, the arm can catch an incoming pod and carry it to the hub to be released.
The cargo pod will feel a large g force while it is being held to the wheel. The energy and spin-up of the tether can be balanced between incoming and outgoing, so it doesn't take new energy input to get from here to there.
For manned pods, a way of tolerating the acceleration would enable that use.
For black holes etc. A "slingshot" involves gravity and the ship does not feel any acceleration. When New Horizons passed behind Jupiter it gained angular momentum at the expense of Jupiter losing some, slowing its orbit around the sun. The ship gained 4 km/s, which isn't much on the scale of the solar system but did save 3 years, or in other uses can save fuel and expense.
If you collapsed Jupiter to a black hole using Clarke's monolith or somesuch, then you could pass much closer to the mass and get more attraction. But you are only closer for a brief time, so you have diminishing returns and it doesn't give as much as you would wish. In this case, the close encounter would give tidal forces and a ship would feel stress and the occupants high-g, as in Nivin's short story Neutron Star.
A chain of Saturn-mass black holes is absurd. Like normal planets they need to be spaced apart by billions of miles, and they only are useful when lined up just right.
Now back to Forward: imagine a super-dense material (not a black hole, but dense enough so gravity is useful) shaped like a torus. It's spinning around, such that a point on its surface is seen to go through the hole and circle around the limb (think of the motion of rolling down a sock while you're wearing it.
This would cause a gravito-magnetic effect and an object flying through the hole would be accelerated. Again, this acceleration is not felt by the ship since it affects every part of it. But, un-even-ness would be noted as g-forces.
If you had a set of rings so the ship passed through one after another it could build up acceleration. What do you make it out of, how do you keep it from collapsing into a sphere, how does it turn inside out like a smoke ring, and how do you replenish the spin after use? If you can build that, keeping biological bodies intact is not going to be an issue. The two topics should not meet, unless it's a found artifact or something like that.
Now consider a "railgun" of any technology. Not gravity but perhaps electric, or even pneumatic: whatever. Assume you can get a continuous acceleration, not just spots of high acceleration with gaps from one to the next. At 100g, how long would the barrel be in order to boost it up to ultra-relativistic speeds?
See this page for the math. Here is some GEL if someone who knows more how to use it wants to generate some graphs:
c = 1; # units used: c is 1 lyr/yr
g = 1.03; # 1g is 1.03 lyr/yr^2
function f_t (a,T) = (c/a) * sinh(a*T/c)
function f_d (a,T) = (c^2/a) * (cosh(a*T/c)-1)
function f_v (a,T) = c * tanh(a*T/c)
function f_T (a,t) = (c/a) * asinh(a*t/c);
day = 1/365.25
t = day
a = 100*g
T = f_T(a, t) # proper time
d = f_d(a,T); # distance traveled
v = f_v(a,T); # velocity
display ("distance in miles", d*5.87849981e12)
display ("final velocity", v)
So, if your railgun could give a continuous 100g acceleration for one day, the projectile would have a final velocity of a mere 27% c, and the device would be 2¼ billion miles long.
After two days, you are up to 49% c and the barrel needs to be 8½ billion miles long.
What was that someone was saying about ultra-relativistic speeds, that a slingshot (or small number of them) could get up to 0.99c? Let's amp it up: 400g of continuous acceleration, applied for 8 days. And a railgun over 83 billion miles long.
The orbit of Sedna is not quite half of that. In this diagram, note the the purple orbit is Pluto.
why have high end-point acceleration if continuous 1g acceleration is available?
Someone earlier was thinking that high endpoint-only acceleration would give shorter transit time than 1g continuous acceleration. My own intuition is that any external mechanism (railgun) that is suitably compact will operate briefly, before the ship leaves the mechanism. Continuous acceleration builds up over time and you have the entire voyage to use it. So, there is no way that a gun will get a ship to its destination (or to the half way point, where both craft use the same on-board engine to show down) sooner than the 1g engine.
In terms of on-ship proper time, there is not the same speed limit. From the outside world, two ships traveling at near the speed of light will take the same time to transit. But on board, the one with higher dilation will experience less time during flight. So more is still better, from the passengers' point of view.
The advantage of something like a slingshot or external flinger of any kind is that you leave the engine behind and don't have to carry all that weight and fuel, and you can use conservation of round-trip counter momentum to reduce the actual energy needed. So even if you could build 1g craft, that would be the luxury passenger liner, while Walmart cargo would use the rotating tether for raw materials in one direction and finished goods in the other.
