Breaking the elusive barrier of Scram-jets
What are Scram-jets?
Well, to make it clear to non-aerospace readers, there is no mechanical fan in the engine, unlike the conventional ones you see on commercial plane engines.
To give a rough breakdown of the engine technologies that are available right now and the associated speed they can achieve, I present to you the table below (approximations, don’t cite it for your undergraduate course-works!)
Hence, air-breathing Scram-jet engine is one of the fundamental keys to hyper-sonic flight (Mach > 5). While there are many concepts to explain in order to lead up to the development of a Scram-jet, this article would provide an appreciative breakdown of why general Scram-jet technologies are still somewhat lacking for practical usage.
First of all, it is vital to note that Scram-jets operate with supersonic combustion. The word ‘supersonic’ might sound cool to you, but it can bring some severe drawbacks, like the materialization of a ‘shock’. These shocks would present losses in stagnation pressure and decrease the overall efficiency of the engine! Let’s go straight into the key points I would like to highlight.
1. Thrust Availability
With an engine that travels at hyper-sonic speeds, drag is enormous if the conventional operating altitudes are applied.
Therefore, it is necessary to fly the Scram-jet equipped aircraft at higher altitudes to reduce drag, which invites a new problem of insufficient momentum/mass flux. The density of air is too low at high altitudes to generate sufficient thrust and the general thrust formulation is shown below.
To resolve this problem, the engine inlets have to be enormous in order to allow sufficient mass flux to be obtained, which is why Scram-jets are essentially the entire air-frame itself. This might be impractical and is a food for thought.
2. Flame-holder and Mixing Problem
In most continuous combustion jet engines, flame-holders are utilized to generate a low speed region with eddies that allow better mixing and combustion, while preventing the flame from blowing out.
The transit time of the flow within the combustion chamber is very short-lived due to the supersonic speed. This means the heat addition to the flow by the combustion might not be fully complete as the fuel has not fully distributed itself into the flow. With the flame-holders, this problem is only partially addressed.
Another way to aid this problem is the lengthening of the combustion chamber to allow a longer time for combustion to take place. However, this is impractical as the engine could be as long as the fuselage!
Uniquely to supersonic combustion, adding a conventional flame holder in the middle of the flow would generate a Bow Shock. Bow shock is defined as a detached shock that occurs when supersonic flow meets a bluff body, in which the required shock angle of the flow exceeds the maximum achievable shock angle for an attached oblique shock. Remember that I mentioned earlier, shocks are extremely bad for efficiency!
A potential solution is to introduce near-wall fuel injectors instead to prevent the existence of bluff bodies in the middle of the flow. This sounds rather attractive as a subsonic boundary layer would exist adjacent to the wall that allows better mixing for combustion. However, the boundary layer thickness is relatively thin as compared to the combustion chamber, which means that it is still not effective enough.
Or, we could make use of the near-wall fuel injectors to generate an imposed designed series of reflecting weak oblique shocks or expansion waves to augment the mixing process for combustion. Shocks, though tagged with a negative connotation, is able to provide compression that speeds up the combustion process, allowing more kinetic energy to be derived.
3. Fuel Issues
Fundamentally, fuel has to be as highly energetic (high energy density) as possible to allow fast combustion reaction due to the previously mentioned point on low transit time. And we do have that, in the form of cryogenic Liquid Hydrogen, but there are some serious storage problems to that. In order to keep it at its liquid state, it has to be pressurized in a full steel pressure chamber, which is going to be a dead-weight and introduce inefficiencies.
4. Engine Self-Start
The absence of a fan to draw in air and its inherent design to work at extreme hyper-sonic speeds make Scram-jets unable to self-start on ground. It only works at high altitudes and high speeds. One possible way is to utilize rocket-fired Scram-jets to give it the initial boost, but I’m pretty sure you and I would not want to experience that extreme g-load. Another alternative is carry a turbine engine for self-start, but it would eventually become a dead-weight when the propulsion system is switched to the Scram-jet. Not to mention the huge drag generated from the windmilling of the fans at high speed. It is possible to introduce some jettisoning device to recover the turbine engine, but I have yet to come across one.
5. Materials Limitation
This is as fundamental as it gets. Materials need to be able to withstand extreme high temperatures that conventional ones cannot. External compression that occurs at the inlet of the Scram-jet would increase temperature rapidly at hyper-sonic speeds; coupled with high heat addition from the fuel. In regard to the near-wall fuel injectors, it would also concentrate the burning process adjacent to the wall, which all indicates to the need for materials that can withstand high temperature rise.
There are many other reasons that might make Scram-jets impractical for our current reality and also several improvements that are currently being researched on which could potentially put Scram-jets into practical usage. The fundamental ideas are that there will definitely be shock losses and the requirement for an enormous inlet would generate high levels of drag itself.
This is simply a fundamental and low-level analysis of the impracticality of Scram-jet engines as of now and I hope this benefits someone!
(This article was written with the aid of the Advanced Propulsion Course taught by Dr David M. Birch from University of Surrey)
— Greg