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Set in the late 22nd century AD, spaceship designs are becoming more streamlined and have longer bodies measuring from the tip at the front of ship to the thruster located at the back of the ship. The longitudinal strength of the body of the space vessel may experience different degrees of acceleration which contribute to stress inside the structure.

I am thinking of applying a pendulum similar to the one in tall buildings, especially in earthquake prone zones on Earth, the swinging of the weight to counteract the vibration caused by the swaying of the structure. But I don't think a pendulum would work in a spaceship.

How do I mitigate or negate the stress built up while moving in space? No FTL and adamantium/vibranium.

I am not sure why you expect to have stress on a space ship. During travel in space the only forces acting on a ship would be gravitation attraction from some attractor in the surrounding and the vectorial thrust of the rockets, if active.

You would have no drag nor lift forces inducing torques on the structure. If your thrust vector is not passing through the center of mass, your ship will also rotate as a result. This is why anything we have sent to space had a much simpler structure than the Space Shuttle, that was supposed to dive into the atmosphere.

Just add some damping elements if you want to prevent resonating frequencies to damage your structure.

Ignoring the question of why they're making ships more streamlined for an environment without air resistance one answer may be to include thrusters at multiple points along the hull.

These multiple thrusters would work together to accelerate different parts of the ship at the same rate (rather than slowing the ship at one end and forcing the compression to accelerate the other end).

You would still experience some compression/stress in the areas between thrusters but if they were positioned correctly this could be minimised to a manageable amount.

One reason to have very long and seemingly streamlined ships might be they use a mass driver as their propulsion system. This offers some advantages in that the ship can theoretically use almost anything as reaction mass, and if the situation demands, the ship has a built in weapons system.

The excellent Atomic Rockets site contains this set of stats for a mass driver, but the exhaust velocity of 30 km/s isn't quite up to scratch for an interstellar spacecraft, even if you simply strap your craft to a convenient small asteroid to use for fuel. If we were to multiply the length of the drive by ten, according to the equation v2f=v2i+2aΔdvf2=vi2+2aΔd (relativistic effects are negligible at these velocities, so we can ignore them), for a mass driver with the same acceleration, but ten times longer, we get an exhaust velocity of a bit under 95km/s, which is quite a bit nicer, even though we had to increase the mass of the drive and its power source by a factor of ten.

So you can envision the ship being essentially a long "boom" representing the mass driver sticking out the tail of the ship. With large radiators it might even resemble a dragonfly.

Elegant spacecraft design

However, if the ship is undergoing hard maneuvering in space (perhaps it is a warship), then the best design might be similar to a paper airplane. The long "fins" are the radiators, but they work like trusses to keep the mass driver rigid and stable. While paper airplanes usually have 3 fins, a spacecraft is likely to be symmetric and have 4 equally spaced fins to brace the mass driver.

Classic paper airplane. Visualize the fins as the radiators bracing the mass driver in the centre

Ok, so the forces you're describing are generally only going to come into play when your ship is experiencing force that's NOT being applied along its longitudinal axis. Generally that's only going to happen if something hits you, or anytime you need to change the direction your ship is pointing. In either of those cases, you're going to have forces trying to 'bend' your ship out of the straight line.

In the case of maneuvering adjustments, the best way to manage this is by spreading the forces that are rotating the ship along its entire length. While you get the most mechanical advantage by putting your maneuvering thrusters as far from your ship's center of mass as possible (because leverage), this also creates the most stress. Spreading your thrusters out along the entire length of the ship also spreads those stresses out so that no particular area is likely to accumulate failure.

This is ALSO a good way to try and manage the forces associated with a collision, assuming that the a: you see it coming and b: it's not enough force to shear your ship in half outright. An impact against the side of your long skinny ship is effectively going to apply force to the the part of the ship it impacts which may cause that location to experience acceleration the rest of your ship can't keep up with, at which point structural failure ensures.

If you know something is going to slam into the side of your ship, you would want to have all your maneuvering thrusters pushing the entire ship along the same vector in which the collision is occurring, and the further from the point of impact it is, the more thrust you want.