When funding for the program came into question, there were concerns that the project might be cancelled. This led to an effort to interest the US Air Force in using the shuttle for their missions as well. The Air Force was mildly interested, but demanded a much larger vehicle, far larger than the original concepts. To lower the development costs of the resulting designs, boosters were added, a throw-away fuel tank was adopted, and many other changes made that greatly lowered the reusability and greatly added to vehicle and operational costs. With the Air Force's assistance, the system emerged in its operational form.