For the first time, Boeing released two different aircraft carrier-compatible missile concepts: [Figure 1] A sub-twin combustion chamber scramjet engine powered by a sub/super-twin combustion chamber scramjet engine for the HyFly2

For the first time, Boeing has released two different aircraft carrier-compatible missile concepts: [Figure 1] A sub-/super-twin-chamber scramjet engine powered by a HyFly 2 scramjet engine and the "Advanced Ramjet Engine Supporting Supersonic Propulsion" (SPEAR) program

Figures 2 and 3: Another type powered by a hypersonic ramjet engine, based on the X-51A, may also be used in the SPEAR program

The U.S. Navy's SPEAR program of technology demonstrators will debut in late 2022 to help the Navy determine future air-launched missile needs, said Boeing's SPEAR program manager, "Spear Flight Demonstrator will provide significant improvements in the F/A-18 Super Hornet and carrier strike group in terms of range and survivability against advanced threat defense systems." ”

HyFly's predecessor, HyFly, uses sub/super-twin combustion chamber technology: the rocket-boosted missile is first accelerated to Mach 3.3 or so and then the sub-(sonic) combustion (burning) ramjet engine is activated, and the sub-combustion engine is used as a subsonic gas generator, which transports the superheated gas discharged by the acceleration downstream to the super(sonic) combustion (burning) ramjet engine. The second start of the scram engine then propelled the aircraft to Mach 6.

This design does not require the missile to be accelerated to near hypersonic speeds, so a smaller solid booster can be used to launch the missile, while the Boeing X-51A "Waverider" powered by a scramjet engine needs to accelerate to more than Mach 4 to start the engine.