A proposal on Gas Turbine Engine Mechanics.

[Alias72]

Hello and welcome to my proposal for a new mechanic encompassing jet engines!

The current system for Gas Turbine or "jet" engines is to place an intake, which generates intake air, and a nozzle that represents the entire engine. This system has a few problems.

1: the placement of the engine mass at the nozzle shifts the center of mass for the entire aircraft backwards. this requires lifting surfaces to be placed rearward and causes stability issues and unintuitive design.

2: By scaling engine ISP instead of thrust engines remain incredibly powerful outside of their range of usefulness. additionally the limited customization of engine configuration lends itself to a more limited set of designs for engine use and placement.

My proposal:

building a working jet engine would require three parts. an Inlet, a Nozzle and an engine. the inlet would determine intake air based on speed, the engine determines ISP and Thrust, and the Nozzle determines net thrust at altitude.

Inlets determine the amount of intake air, the efficiency of the inlet at altitude, and provide a limited set of placement options. generally speaking there would be three types of inlets. low speed, high speed, and variable speed inlets. Additionally each inlet will have an inline and a radial version for a total of at least 6 inlets.

-A low speed inlet will be most efficient operating between mach 0.3 and 0.8. the inlet will not work with supersonic flows (the inlet will not produce intake air.)

-A High speed inlet will be most efficient operating above mach 1.2. This inlet will work below that number but will be less efficient.

-A variable speed inlet sacrifices weight for performance. this inlet has a variable geometry and will be sufficiently efficient at all flight regimes.

Engines determine how air and jet fuel is converted into thrust. There is in this category, a wide range of potential options. High thrust low ISP, Low thrust High ISP, compromise engines, and dual mode engines. basically all the engine does is take intake air and fuel and applies the rocket equation. Engine ISP remains fixed.

-If their is less intake air than the rated maximum the engine will produce proportionally less thrust until it reaches a flameout condition. (an example would be 20% flameout, so if there is 20% of the rated intake air then the engine shuts off.)

-Dual mode engines perform like the SABRE. they have the option of burning liquid fuel and oxidizer instead of air and kerosene.

Nozzles determine the amount of thrust ACTUALLY produced. The provide an efficiency scale based on altitude. there would be low altitude nozzles, high altitude nozzles, and multi-altitude nozzles. In addition some nozzles would provide thrust vectoring.

-Low altitude nozzles provide excellent efficiency at low altitudes(95%) but significantly less as altitude increases.

-High altitude Nozzles are most efficient at high altitudes, though will function at lower altitudes.

-Variable altitude nozzles such as the aerospike are heavy but remain very efficient at different altitudes.

Consequences on Design:

Though this proposal may sound complicated it should, in practice, be both simple and intuitive. To build an engine you pick an inlet.

-The inlet you choose depends on whether you wish to go fast or slow and if you have place in line or radially.

then you pick an engine.

-The engine you pick is determined by how efficient you want your plane to be vs how much thrust it must produce.

then you pick a nozzle

-The nozzle you pick is determined by how high you wish to fly, and whether or not you want thrust vectoring.

I believe this system is also intuitive, as it allows you to actually choose where your plane will perform. It should provide results that conform better to expectations. Also by moving engine mass further forward it should help with aircrfat balancing.

[camlost]

First, Problem 1 can be solved be adding a CoMOffset to the part.

Problems with the main topic:

1.The current system is built around "IntakeAir" as a resource. You did not make clear of whether it is to be remained. If so, limiting and categorizing inlets will not stop people from spamming it but encourage it further. And as long as they are spammed, the thrust will be the same at altitude. Auto-throttle-down when IntakeAir is insufficient was implemented in v0.21 but it does not solve the problem.

Besides, the number seems arbitrary and lacks realistic reason, and it could be rather confusing to players.

2.Speed performance and fuel efficiency are intertwined not separated. For example, high bypass turbofans have high Isp, low TWR and works at low speed only. Afterburning turbojets have low Isp, high TWR and works at high speed. It is hard to imagine an engine that has high Isp, low TWR but works at high speed. That is against thermodynamics.

The reason why rocket engine have choices like that is, they choose different cycles (closed cycle vs. gas generator etc.). Jet engines are not that case.

3.The nozzles are not like rockets where you worry about expansions. Jet engine nozzles are variable at the throat, in order to cater to different throttle and speed. Having them perform differently solely based on altitude is incorrect (that it rocket thinking into jet realm again XD)

4.The whole system contains too many configurations (3x3x3=27) that are categorized arbitrarily, thus difficult to balance.

Edited January 17, 2015 by camlost

[Alias72]

First, Problem 1 can be solved be adding a CoMOffset to the part.

4.The whole system contains too many configurations (3x3x3=27) that are categorized arbitrarily, thus difficult to balance.

the categories were meant to be arbitrary, there would not be 27 definite categories, but different parts that are better suited towards different regimes of flight. You are right about this not fixing intake spam. That said the need for differing intakes for different flight regimes may mitigate ti somewhat. And I have nothing to say about the other 3 comments because you are probably right.

Edited January 17, 2015 by Alias72