¶Goodbye, Delta. The last of the long-lived Delta family of rockets is scheduled to launch tomorrow. The final Delva IV Heavy will take off from Cape Canaveral Space Force Station NET 1:40 p.m. EDT (video stream; launch is pending weather that’s only 30% favorable for an on-time liftoff). The mission will deliver a classified NRO payload to (an also classified) orbit. The Delta family was first launched almost 65 years ago in 1960, converting Douglas Aircraft’s (later McDonnell Douglas, then Boeing, and now ULA) hurriedly-developed and failure-prone Thor intermediate-range ballistic missile into the first stage of an orbital-class launch vehicle. The first Thor DM-19 Delta variant could place just 45 kg into GTO—but almost innumerable iterations over the intervening 64 years have eventually yielded a final variant with little relation to the original, capable of carrying 13,810 kg into GTO. Delta IV Heavy uses three side-by-side Common Booster Cores (CBCs) shared with its Medium configurations, the last of which flew in 2019. With tomorrow’s launch, 77 CBCs will have been launched across the three variants since the booster’s first static fire in 2001. The Heavy variant’s unique three liquid-fueled booster configuration clearly influenced SpaceX’s development of Falcon Heavy and was perhaps itself influenced by Proton-M’s six-booster cluster configuration. Delta IV has always been a particularly expensive rocket, though, relying on the hydrolox RS-68A engine (initially developed with the intention of being a “cheaper” version of the RS-25 Space Shuttle Main Engine). Due to a cost of $165M+, or up to $440M for a single Heavy launch, the US government has paid for every Delta IV launch except its maiden launch of Eutelsat W5. Tomorrow’s final Delta IV Heavy launch should be a typically fiery affair. The rocket flows liquid hydrogen through the engines to purge oxidizer prior to launch, creating a cloud of H2 that the rocket ignites as it lifts off, enveloping it in an iconic fireball—a fittingly fiery departure in Viking fashion for a rocket initially named after the Norse god of thunder. 🔥🚀 | 
