With a clear ground test and development programme.
There are three core building blocks to the SABRE engine, the precooler, the engine core and the thrust chamber. Each of these systems can be developed and validated using ground based demonstrations, which saves cost and time relative to flight test, a design feature that benefited the development of the propeller and jet engine.
We are demonstrating each of these independently. The high temperature test of the precooler took place in 2019.