Our partners at Science NetLinks have a lesson called "Rocket Launch" that might be worth taking a look at. It lists under the grades 9 - 12 category, but I think it would be effective with many 8th graders as well. It makes use of some NASA resources and takes a good comprehensive look at the factors that affect the success of a rocket flight. Another interactive resource is the RocketModeler III at the NASA website.
Hopefully that will get you started and maybe we'll hear from some others who have ideas as well!
This interactive challenges students to virtually launch a rocket into space to see how the force of gravity can pull an object toward the earth and moon. Students can also experiment with different thrust and angle settings to learn how these settings change the motion of the rocket.
This activity uses the same interactive listed above but it’s targeted at afterschool or parents.
Lastly, Science NetLinks created the space science resource page Celebrating Space Exploration for NASA’s 50th Anniversary. Here you’ll find many space related resources there including more interactives and podcasts.