Earth-to-orbit propulsion for the 21st century

This paper presents an overview of NASA's liquid and solid rocket and air-breathing propulsion research programs along with some recent accomplishments. Attention is focused on two contrasting, but complementary, space transportation system elements. One is directed toward the unmanned delivery of massive and/or bulky 'space cargo' to low earth orbit. The other is tailored to the transportation of humans and light priority cargo in smaller manned piloted vehicles.