The basis for all the rocketry and wind tunnel posts discussed previously are primarily for the development of a micro-ramjet engine. For now I have gone as far as I can on the supersonic wind tunnel and the design has more or less been finalised, all the tunnel wall contours for the test sections are complete and plans have been drawn. However I am a little pushed for both work space and money after completing my PhD and not having access to a dedicated workshop means building things has been put to one side for now. However that hasn’t stopped me building micro gas turbines in the past, even going as far as building my own lathe once so I really do need to pull my socks up and get on with things now summer is approaching.
I am keen to see how small of a practical ramjet engine I can build, one that can not only be demonstrated to be functional but can actually be used as an air breathing stage on a small rocket. I have an idea that this will be somewhere in the range of around 20mm in diameter, assuming a circular cross section and low enough total rocket drag. This puts it into the range of using small F and G sized solid fuel rocket motors as a first stage to get it up to speed quickly. I happen to have such motor casings lying around, as one does. Also it means I don’t have to contend with high altitude flights and all the issues that brings in trying to recover the rocket from several kilometres altitude. Onboard avionics can record accelerations and barometric pressure so once the solid rocket motor burns out the data should show how well a ramjet engine stage performs, hopefully maintaining speed, however a slow decay in the flight speed would also be a great result. So I just have to plug away at the maths and run a few simulations to see if that hunch is correct. If I can make a smaller, beefed up composite version of something akin to an Estes model rocket, running relatively oversized first stage solid motor and small ramjet sustainer stage might make things more interesting…… I have seen people fly cheap Estes kits, reinforced with some careful application of fibreglass to ridiculous speeds on small, single use composite Aerotech rocket motors. But Mach 2+ is difficult enough with larger, high power rocketry in general and I rarely had anything break Mach 1.8 when I used to fly rockets over a decade ago. But we shall see.
Some careful consideration as to the integration of a rocket and ramjet is needed, an engine simply mounted to the side of the fuselage would produce a lot of drag and an offset thrust line, so either a control system to correct for flight attitude (difficult and complex) or two engines 180 apart to cancel these forces. However two engines, twice the weight and twice the drag. One could feasibly create an engine within the rocket fuselage, and house the ancillary and recover equipment within the centre body but I am less drawn to this idea. A hybrid of the two approaches could be used through use of two intakes, feeding a combustor in the aft of the fuselage. I shall worry about this later, I still have much to learn about the design and combustion processes at these small scales before I can even think about integrating an engine on a rocket.
As the project will be based on the design of a supersonic ramjet I will be only considering designs and applications for this flight regime. They are interesting engines in their own right and I am aware that subsonic ramjets are very popular on the internet, being easy to make and many examples can be found produced by interested people online . However there was also a period when there was some serious military and commercial interest in subsonic ramjets, particularly in the 1950s when research on almost any form of jet propulsion was being carried out. One of the most famous of these were the Hiller Hornet YH-32 helicopters equipped with ramjets at the tips of each rotor blade, shown below, removing the need for a tail rotor and main engine.
The main drawback is the pressure recovery and performance of subsonic ramjets is already poor even at high subsonic speeds, let alone those below compressibility effects which start to occur at around Mach 0.3. Generating airflows of sufficient flow rates above these speeds is both costly and somewhat dangerous, which has lead to some interesting concepts by those experimenting with subsonic ramjets. The most commonly used method I have seen from amateur scientists has been to use a leaf blower to produce a stream of air at a modest speed. Similar methods have been used where the exhaust stream from a pulsejet has been used to increase the speed of the air entering a ramjet engine, however this method requires the ramjet ingesting hot exhaust products. Although this hot gas will still contain enough free oxygen for combustion, the high temperatures would have a serious effect on the already poor efficiencies of a small, subsonic ramjet engine. I did consider the use of a small turbojet engine to provide a high speed flow of air at almost sonic speeds, but the heat of the exhaust lead me to abandon the idea. Another idea I mooted at the time was the use of a static single blade rotor rig, similar to a helicopter but fixed to a stand with an engine mounted at the tip of the blade, in order to create a rotary test bench. But the forces involved and the difficulty in gathering meaningful data meant I quickly abandoned this idea also. But the use of a small wind tunnel to investigate combustion stability and the interaction of shockwaves within the intake seems to be the best route.
Introduction to Ramjet Engine Components
The ramjet engine produces propulsive power similarly to all air breathing jet engines, in that the momentum of the working fluid is increased through some form of heat release such that the momentum of the exiting jet is higher than the momentum of the air stream entering the engine. However the ramjet differs from other air breathing engines in that the working cycle is accomplished without the assistance of mechanical compression of the working fluid. Pulsejet engines also accomplish this but they differ in that combustion occurs intermittently, while the combustion process in ramjets is a constant process, consequently the ramjet can be considered the simplest of air breathing jet engines.
Fundamentally the ramjet can be described as a simple shaped tube, this simplicity belies the complexity that governs jet engine design. A simple schematic of a representative ramjet engine is shown here.
High speed air is inducted into the engine via the intake, shockwaves are then used to reduce the speed of the flow to subsonic speeds and in turn raise the pressure of the flow, the air speed is then reduced again in the diffuser further increasing the pressure. Engines that employ the use of gaseous or liquid fuels to add energy to the compressed air, this fuel will be injected via fuel orifices or injectors upstream of the combustor often within the diffuser itself, some engines even inject fuel into the external supersonic flow. The required location of the fuel injectors will be a function of the degree of atomisation and mixing required in order to produce stable combustion. One can imagine that smaller engines will need to atomise, mix and combust fuels in a much shorter physical length than large scale engines. This has lead to some interesting concepts such as the use of solid fuels to help negate issues such as these and even more bizarre was a conceptual design for the use of a nuclear powered ramjet, utilising a heat exchanger to heat the flow .
The use of hydrogen gas as a fuel source might be worthy of investigation also, as it readily burns and is often the fuel of choice for high speed scramjets. It has a much higher calorific value, with a Lower Heating Value (LHV) of almost 120 MJ/Kg in comparison to 43 MJ/Kg for kerosene. It also has a much wider range of air-fuel ratios over which it will combust compared to most other commonly used fuels, however it isn’t dense and would require either high pressure storage, or carry a larger volume at lower pressures. Both having the same result of increased take off mass for a small rocket. Propane can be stored quite easily, handled and small pressure vessels refilled safely, pressurised gas also negates the need for a pump and other ancillary equipment.
The design and image shown here  is for a miniature Mach 4 ramjet of around the
same scale as the one being considered for this project. Due to the difficulties in providing adequate mixing before combustion, the fuel here is injected from the very tip of the intake cone and the centre body supporting struts (both from the rear and side faces of each strut) some of these injection orifices are highlighted in the image. Although the effects of the injected fuel on the airflow around the intake when in operation at supersonic speeds is unknown at this point. Some modification to the shockwave is to be expected, potentially unstarting the intake and leading to ‘intake buzz’.
One idea considered and still needing research has been the use of a fuel rich, solid fuel and oxidiser pellet which should produce both heat and a fuel rich vapour, however mixing the vapour with the incoming air would still be an issue. A similar idea involved the use of a slow burning solid fuel pellet, similar to Jetex systems and simply using the reaction gases to heat the air without the need for further combustion. Due to the small scales involved this on the surface at least, it appears feasible and reduces the overall complexity still further. Note, this concept is not to be confused with ‘ram-rockets’ and similar hybrid engines, the solid fuel here is used purely to heat the air, and not generate thrust on it’s own. An electrical element could also be used, like a hair drier, if one could provide the required electrical power on-board a small rocket without having to carry several kilograms of lithium batteries. At least something on the order of several kilowatts of power would need to be consumed constantly at this scale.
Somewhere aft of the diffuser will be a flame holder, a device used to generate localised highly turbulent regions which promote areas of intense combustion, protecting the combustion flame front from being extinguished and providing a constant source of ignition. Ideally the combustion process occurs entirely within the combustion region, which has a constant area. However as alluded to previously, small engines may well suffer from having combustion still occurring outside of this region, even extending beyond the engine due to the small length scales involved in this project. The combustion length available might only extend two diameters in length, therefore complete combustion to occur within 60 mm for a 30 mm diameter engine is going to be a struggle for certain fuels, particularity liquids, and some combustor designs.
Flame holders in ramjets can range from simple wire fences, through V-shaped gutters and complex combustion cans, all three are shown in the image here. Wire fences (a) have the advantage of being easy to manufacture and replace, they also have the benefit of glowing red hot when in operation, providing a continuous ignition source furthermore the turbulence they produce immediately aft of the twisted wire fence will produce very high levels of turbulence intensity, within a small volume immediately aft of the wire, giving low overall drag. Annular rings of V or U shaped gutters (b) are commonly employed for use within afterburners, and operate similarly to wire fences but on a much larger scale. Aagain relying on the bluff body aerodynamics of such designs to create regions of strong turbulence and therefore promote intense regions of combustion. Combustion cans as shown in (c), here used in the Bristol Thor ramjet engine for use on the Bloodhound surface to air missile, show just how complex some designs can be and are similar to the combustors used in gas turbine engines. However at small scales the large pressure drop across the combustor stage, aerodynamic complexity, blockage effects and subsequent impact on performance mean these combustors are of no practical use. I have a paper on the design and testing of a small afterburner fitted to a micro gas turbine engine which covers some of the issues with regards to the stability of combustion, blockage effects, and thermodynamics which may throw up some unusual issues at smaller scales .
I fully intended to go through the design process from scratch in upcoming posts, developing a set of tools in the form of numerical simulations along the way which can then be used to fine tune design ideas and explore possibilities. However being a simple thermodynamic models they will not predict problems which arise, such as unsteady aerodynamic effects which can lead to flame outs, combustion instabilities, and vibration. Problems like these will only become apparent through experimental testing.
 Ferguson, Kevin M. , ‘Design and cold flow evaluation of a miniature Mach 4 Ramjet‘ https://calhoun.nps.edu/handle/10945/986
 Cooper, Jonathan., ‘Engineering an afterburner for a miniature gas turbine engine‘, https://www.emeraldinsight.com/doi/abs/10.1108/00022660510585929