Short notes: pulsejet and hybrid engines

(a) Pulsejet engine

A pulsejet burns fuel intermittently in a tube. Air enters through spring-loaded flapper valves at the front.

 valves     combustor        tail pipe
 |>|=====+===========+=================>  exhaust

Cycle: (1) air and fuel enter when pressure inside is low; (2) a spark ignites the mixture and the pressure rise closes the valves; (3) hot gas leaves through the tail pipe, producing thrust, and inertia of the gas column lowers the pressure; (4) the valves reopen and the cycle repeats at 40 to 250 cycles per second. Later cycles self-ignite from the hot walls. Its cycle is close to the Humphrey (constant-volume) cycle.

• Advantages: simple, cheap, thrust at rest. Disadvantages: loud, strong vibration, low efficiency, poor valve life, speed limited to low subsonic. The German V-1 flying bomb used one.

(b) Hybrid rocket engine

It combines a solid fuel grain (e.g. rubber/HTPB or paraffin) in the combustion chamber with a liquid or gaseous oxidiser (e.g. liquid oxygen, nitrous oxide) injected into the port.

• Can be throttled, shut down and restarted by controlling the oxidiser valve.
• Safer to handle than solid motors (fuel is inert); simpler than liquid engines (one liquid only).
• Disadvantages: lower regression rate (lower thrust density), mixture ratio shift during burn, lower combustion efficiency.

(c) Combined-cycle (turbo-ramjet) engines

At low speed a ramjet gives no thrust, while a turbojet is limited at high Mach number by compressor temperature. A turbo-ramjet has a turbojet core inside a ramjet duct: the turbojet works from take-off to about $Ma$ 2.5, then it is shut off, bypassed, and the air passes to an afterburner/ramjet combustor up to $Ma$ 3 to 4. Related concepts are the air-turbo-rocket and the rocket-based combined cycle. They give a single engine over a wide speed range, at the cost of complexity and weight.