In starbound I have a path down to the core, but it is very difficult to use my second jump to alleviate fall damage. Can I use water or a certain block to eliminate this, such as slime blocks in Minecraft?
Use Gravity Bubble tech/ability to instantly remove all velocity from a safe distance from the ground. You can then start falling again, with the fall distance reset to zero, which means no damage when you touch the ground.
Beware of trying to use Bouncy tech because if you don't have enough space around you for the bouncy orb to appear, you will fall and receive damage. Only use it in wide drops, and even then consider Gravity Bubble for more safety.
A good way I would suggest you try is make climbing ropes, you might need a few of them, but they should help you descend downwards as well as come back to the surface.
Trap some water (or other fluid) at the bottom of your hellevator. The pool only needs to be a few blocks high to stop your character's fall and negate falling damage completely.
A good suggestion would be the Grappling Hook. Not only can you use it to descend safely, but it comes in handy much more often than you'd think; it gives you an advantage in battle, it lets you get to tricky spots that you couldn't normally reach, and with a little planning you can sneak through anything with it.
Water works but you need a deep pool (at least 100 blocks or so) to negate all your velocity from a fall from the surface.