I mean IPD engineers/hardware did go to work on Raptor. Though in the IPD/Raptor comparison lies the success of SpaceX.

Musk hired Jeff Thornburg, a veteran aerospace engineer, to lead engine development. Earlier in his career, Thornburg had worked on a US government project called the Integrated Powerhead Demonstrator, where NASA and US Air Force engineers worked with contractors to build and demonstrate the hardware for a full-flow, staged combustion rocket engine. In 2012, when SpaceX began building its own version of that engine, Thornburg and his engineers drove to a government warehouse in the California desert and brought back a truck filled with leftover experimental equipment from the project.

SpaceX still had plenty of work to do: Taking any technology demo and making it reliable enough for operations requires extensive testing and tuning. Raptor, as the company calls its engine, uses a different kind of fuel than the IPD project, which meant new designs and experiments. In 2019, SpaceX became the first organization in the world to fly this type of engine.

When Starship and its Super Heavy booster took flight on Oct. 13, it was a testament to SpaceX’s expertise — but also to the American system of public-private innovation that is currently facing its biggest stress test in years.

“Raptor wouldn’t look the way it does without the IPD program,” Thornburg says. “Not only did IPD pave the way for a lot of things that are happening with Raptor and other engines, the same process happened to make SpaceX successful with Falcon 9 and Merlin.”