The middle school years are a crucial time for cultivating students' interest in and preparedness for future STEM careers. However, not all middle school children are provided opportunities to engage, learn and achieve in STEM subject areas. Engineering, in particular, is neglected in these grades because it usually is not part of science or mathematics curricula. This study investigates the effectiveness of an engineering-integrated STEM curriculum designed for use in an after-school environment. The inquiry-based activities comprising the unit, "Think Like an Astronaut", were intended to introduce students to STEM careers--specifically engineering and aerospace engineering--and enhance their skills and knowledge applicable related to typical middle school science objectives. Results of a field test with a diverse population of 5th grade students in nine schools revealed that "Think Like an Astronaut" lessons are appropriate for an after-school environment, and may potentially help increase students' STEM-related content knowledge and skills.