Scramjet engines and how to start them?

Ramjet engines are used in missiles for several years. But placing those engines in aircraft is the new thing. Sometimes is expected that the legendary "Aurora system" was simple the ramjet-driven concept aircraft. The difference between ramjet and scramjet engines is that ramjet is meant for supersonic speed. And scramjet is meant for hypersonic flight.  

 Ramjet and scramjet engines are the systems that need certain airspeed for ignition. The super- and hypersonic engines have no moving parts. They are tubes that have a certain form. That form causes friction what will ignite the fuel- and oxygen mixture. The acceleration to the necessary speed can happen by using certain engine types in stages. At the first. The turbojet will accelerate the aircraft to the speed where the ramjet can start. 

Then ramjets will accelerate the aircraft to speed where scramjet can begin its operation. That kind of staged engine solution gives the aircraft also capacity to operate at a slow speed. Another way is to combinate rocket and ramjet or scramjet engines. The iris or flap can close the air inlet and then the engine system can use internal oxygen. When the speed of the aircraft is high enough. That system can turn to take oxygen from the atmosphere. 

But there is another way to start the ramjet or scramjet. 

The system must only make a vacuum behind the nozzle. In that case, the vacuum will just pull the gas through the scramjet or ramjet engine. And that thing makes those systems ignite the fuel. Those high-speed engines use some other fuel than kerosene. 

Acetone, propane, methane, acetylene, or hydrogen are the most promising things. The liquid hydrogen is a useful thing. In that case, the liquid hydrogen can use for cooling the core of the super- or hypersonic aircraft. If the scramjet engine can start on the airfield. 

That thing makes the aircraft lighter. The rocket ramjets and scramjets require internal oxygen. The thing why those hypersonic aircraft are interesting. Is that they can fly to ballistic trajectory outside the atmosphere. 

So the idea is simple. There is a hypersonic wind tunnel in the base. Then the hypersonic aircraft will position in the wind tunnel and the vacuum will ignite the engines. Then the hatches will open. And aircraft can fly away. Those hypersonic aircraft can return by gliding to the ground. 

Another thing is that there is the possibility to connect ramjets or scramjets. With conventional aircraft bodies. The aircraft can launch that supersonic or hypersonic engine by forming a vacuum behind the aircraft just like the wind tunnel makes. The idea is just to increase the airflow in those very simple-looking but otherways complicated engines. 

https://en.wikipedia.org/wiki/Aurora_(aircraft)

https://en.wikipedia.org/wiki/Hypersonic_speed

https://en.wikipedia.org/wiki/Ramjet

https://en.wikipedia.org/wiki/Scramjet

https://en.wikipedia.org/wiki/Supersonic_speed

Image:) https://en.wikipedia.org/wiki/Lockheed_Martin_SR-72

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