Better Thrust control on jet engines?

[icemonkey]

Just a thought I had in the shower this morning, and decided I would ask someone about it.

We all love making VTOL jet planes in KSP, but the only problem is the amount of time for a jet engine to spool up/down. Surely it would be easier to just change the amount of air that enters the engine? As in, at lower thrust the intake would just let in less air into the jet, and just keep the spinning at a constant RPM?

This might be a stupid idea, but I thought I might as well ask someone if it existed/was theoretically possible.

[shynung]

Shouldn't this be in the Suggestions section?

[icemonkey]

It's not a mod suggestion, its a genuine question about real life jet engines. I just used the KSP engines as an example

[shynung]

Ah, sorry, misinterpreted your post.

Jet engine intakes IRL can't be closed completely, or even throttled down. The intake is fixed by design (at least, commercial airliners' are). They throttle the engine by varying fuel input. The spool up/down lag is the turbine's rotational inertia.

The only jet engines I know of that has closeable intakes are REL's SABRE engine, though it is a jet-rocket hybrid.

[SargeRho]

The Harrier has flaps on its intakes that open during VTOL to increase the ammount of air the engines take in. I guess you can count the lift fan on the F-35B as "Closeable" too

[Streetwind]

There's all sorts of problems associated with that approach.

- If you have a jet turbine spinning at a supposedly fixed RPM with a fixed fuel flow, and you cut the amount of air that enters it in half, then the engine RPM will change. The air density acts as a source of drag and as a medium for the turbine blades to push against, and if it gets reduced then the turbine has less drag and less to push against so it speeds up. By definition you cannot have a fixed RPM turbine controlled by air inlets.

- Fuel combustion requires a certain fuel/air mixture ratio in order to work. The jet turbine controls its own mixture ratio by virtue of rotation speed and fuel input being linked - if you throw in more fuel, the engine speeds up, which in turn pulls in more air so the mixture is kept within operational ranges. If you deprive the turbine of its ability to do this by limiting its access to air, you could literally shut down your engine by accident, especially at high throttle settings.

- Whatever acts as a shutter on the inlet will be a major source of aerodynamic drag for the plane as a whole, and will need to be fairly sturdy as the turbine inside will create a sharp pressure differential.

- Turbonfan engines generate most of their thrust from their bypass flow - that is, only very little air goes through the turbine, the largest amount is simply blown out back, bypassing the turbine. The turbofan is essentially a large high-performance ducted fan driven by a turbine. Most commercial and many military large aircraft use these engines. And in this design, it is literally impossible to close an intake on the turbine without also closing it on the bypass, which would instantly cut its thrust to near-nothing.

I'm sure there's more...

[Red Iron Crown]

Variable geometry jet intakes are a very real thing, the F-14, F-15 and SR-71 all used them, probably others, too. Though I think it was more for optimizing airflow at different speeds than varying thrust.

[Streetwind]

Yeah, those are mostly for letting the engines breathe normally while flying at supersonic speeds (the SR-71 wikipedia page has an excellent description on how this worked).

[Soda Popinski]

If you limit the air, you'll have excess fuel in the mix (which is wasted fuel). Makes more sense to me to throttle the fuel. I assume that's what's normally done.

[shynung]

If you limit the air, you'll have excess fuel in the mix (which is wasted fuel). Makes more sense to me to throttle the fuel. I assume that's what's normally done.

Actually, yeah, they do throttle the fuel input. As far as I know, at least.

Edited May 15, 2014 by shynung

[PakledHostage]

We all love making VTOL jet planes in KSP, but the only problem is the amount of time for a jet engine to spool up/down. Surely it would be easier to just change the amount of air that enters the engine? As in, at lower thrust the intake would just let in less air into the jet, and just keep the spinning at a constant RPM?

As others have pointed out already, the throttle on a jet engine controls the fuel flow. On the turbofan engines used on commercial airliners, the piece of hardware that controls the fuel flow is typically called a hydro-mechanical unit (HMU) or Fuel Control Unit (FCU). Depending on how modern the engine is, the throttle will be connected directly to the HMU/FCU or to a computer which is in turn connected to the HMU/FCU. The most sophisticated engine control computers are called FADEC systems. FADEC stands for "Fully Automatic Digital Engine Control". The FADEC computer manages fuel flow in response from throttle inputs and to keep engine parameters within operating limits. Throttle response on turbofan engines is slow due to the engine's spool up time.

Turboprop powered aircraft typically have variable pitch props and their throttle response is quicker, despite the fact that the core of the engine is really no different than that of a turbofan. Propellers on turboprop aircraft like Dash 8s are constant speed propellers. They are governed to turn at a fixed RPM, regardless of throttle setting. You don't have to wait for them to spool up and spool down.

As an interesting side note, you can actually hear the governors on turboprop airliners like Dash 8s working during takeoff if you pay attention. During initial climb, you will often hear an annoying "wow-wow-wow" sound that fades to a uniform droning of the propellers once you're climbing comfortably. The annoying sound is called beating. It is caused by the difference in RPM between the two props. The governors are synchronized to each other during normal flight to eliminate the beating sound, but the system is normally turned off during takeoff because you don't want the slave engine's RPM to follow the master's down if the master quits.