ABOUT RAMJETS
- A ramjet is a type of engine that utilizes forward motion for compression instead of an axial compressor or centrifugal compressor
- Capable of operating between Mach 2.5 and Mach 6
- Has been used in several aircraft but has primarily been used in missiles
Ramjets have been incorporated into artillery shells for improving the range compared to the non ramjet powered counterparts
Nammo has been developing a ramjet powered artillery shell capable of reaching Mach 3 and a range of 150 kilometers much farther than the 20 kilometers artillery is currently capable of
THE Theory
Isentropic Inlet - Utilizes an infinite series of weak shocks to maintain a constant stagnation pressure. Weak Leading shocks focus on the cowling lip to reduce drag.
Ramjets are a supersonic propulsion method that is used for flights ranging from Mach 2.5 to Mach 6
Lower limit of Mach 2.5 is limited by the compression that can be achieved by flying through air
At Mach 2.5 assuming an isentropic process the compression ratio between inlet and combustor pressures could be up to 17
The upper range of Mach 6 is determined by the temperature differential between the compressor and the heat added by the combustor
𝑇𝑠𝑡𝑎𝑡𝑖𝑐/𝑇𝑠𝑡𝑎𝑔𝑛𝑎𝑡𝑖𝑜𝑛 =(1+𝑀∞2 ((𝛄−1))/2)(−1)
For example Mach 6 with a static air temp of -40℉ the combustion chamber could reach 3000℉ before heat is added by the fuel
Non-isentropic inlets are found on ramjet projectiles
Non-isentropic inlets are used because they still give acceptable pressure recovery with more robustness when compared to the isentropic designs
Using oblique shock calculation, the pressure ratio can be found
𝑃𝑜1/𝑃𝑜2 ∙𝑃𝑜2/𝑃𝑜3 ∙𝑃𝑜3/𝑃𝑜4 =𝑃𝑜1/𝑃𝑜4
- Critical case is the Ideal operation the mass flow rate is maximum with a low loss in stagnation pressure
- Supercritical case has maximum mass flow rate but more stagnation pressure loss
- Subcritical cases will have similar stagnation pressure loss as the critical case, but the mass flow rate will be lower
Buzz phenomenon is something that occurs on supersonic inlets and can be fatal to the flight vehicle since it can upstart the engine and cause a large drag increase. It can occur by changing the entrance or exit conditions of the inlet.