German Aerospace Center studied reusable VTHL Liquid Fly-back Boosters from 1999. Design was intended to replace Ariane 5 solid rocket boosters.[13] The U.S. government-funded, US$250,000,000, Reusable Booster System program, initiated by the USAF in 2010,[14] had specified a high-level requirement that the design be VTHL,[15] but the funding was discontinued after 2012.[16]