Space will be colonized by human race, 3d printing will help but chemical rockets won't get us far. Here are two propulsion methods that you will be able to make and demonstrate here on Earth with some help of your 3d printer. Ideal for educational purposes.
3D Printable DIY Ion Thruster
Ion thrusters are proven space propulsion method and you can learn more about it on Wikipedia:
With some spare parts and 3d printed structure you can make a working model:
Learn more about the project and get more detailed construction instructions:
3D Printable EM drive or RF resonant cavity thruster
EM-drive is much more exotic method of electromagnetic space travel which was considered a hoax when first presented but turned out to be potentially revolutionary since some tests show it works thus enabling future spacecrafts to fly without fuel mass.
Learn more here:
Now you can also make a model and test it with few metal parts and a 3d printed support.