Furhter updates the propulsion documentation

docs/documentation/sizing/propulsion_design/index.md

 # Introduction {#mainpage}
-The tool _propulsion_design_ is one of the core design tools in UNICADO. The overall goal is the design the propulsion system based on... 
+The tool _propulsion_design_ is one of the core design tools in UNICADO. The overall goal is the design the propulsion system based on...
+
 - the architecture (e.g. 2 turbofan at rear fuselage, 4 fuel cell prop engine over the front wing) set by the user and,
 - the total required thrust and system off-takes calculated within the aircraft design loop.
-This tool is exciting!🔥 because the propulsion is one of the critical components in the aircraft design loop. It provides the thrust or power, enabling powered flight of the aircraft letting it move through the skies.🌍
+The propulsion is one of the critical components in the aircraft design loop. It provides the thrust or power, enabling powered flight of the aircraft letting it move through the skies.
 
 There are different propulsion architectures for the aircraft conceptual design process. To give you a general taste, here are a few illustrations of possible concepts.
 ![](figures/different_engines.svg)
@@ -11,7 +12,7 @@ The [getting started](getting_started.md) gives you a first insight in how to ex
 
 Prior to that, let's summarize what the tool can currently do and what is planned (terms like _method_ or _strategy_ will be explained in the sections):
 
-| Engine type                  | Methods*                                             | Status     |
+| Engine type                  | Methods (engine design/ nacelle design/ pylon design/ integrator/ mass analysis )  | Status     |
 |------------------------------|------------------------------------------------------|------------|
 |kerosene-powered turbofan     |Rubber(V2527-A5)/ Default/ Default/ Default/ Default  |running     |
 |hydrogen-powered turbofan     |Rubber(V2527-H2)/ Default/ Default/ Default/ Default  |to be tested|

docs/documentation/sizing/propulsion_design/overview.md

+# Overview {#mainpage}
+The propulsion_design tool provides the engine data and the engine integration on the aircraft. The steps are shown in the following figure:
+
+![](figures/propulsion_design_flow.png)
+
+Required inputs for propulsion design are therefore:
+
+    - The thrust to weight ratio (First from initial_sizing sizing then from constraint_analysis).
+    - The MTOW of the aircraft.
+    - The type of propulsors and the according thrust share.
+
+With this the engines are designed one by one with the following approach:
+
+![](figures/engine_sizing.png){html: width=600}
+
+The outputs are the engine xml file as well as the different deck values as csv files in projects as well as the following xml snippet for each engine:
+
+![](figures/xml_output.PNG){html: width=1200}
+
+Readout of the engine data can and should only be done using the engine library!
+
+The csv files contain engine data that depends on the operating point. The operating point is defined as
+
+    - Flight Mach number
+    - Flight altitude 
+    - Low pressure spool speed / power setting
+
+An example is shown in the following figure.
+
+![](figures/deck_example.PNG)
+
+The data is readout by the engine library which has an efficient parser for the deck values using a linear interpolation between two existing deck values. Penalties, like shaft power offtake or bleed air offtake, are also calculated using the engine library. The description of the engine library can be found here as well.
+The description of the engine library can be found [here](../../../libraries/engine/).
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