To me the only difference between a jet engine and a rocket engine is that the jet is the most complicated one way valve immaginable so that the explosion is directed out the back but one of the two components for combustion can still get in the front. Maybe I am looking at it wrong as I know less about aviation than I do about electronics :). So with my view in mind how is an electric motor going to replace some/all of the jet engine or is my understanding wrong.
Yes.
No. SoS is related to the average molecular mass and the square root of the temperature in degrees Kelvin - nothing else. This is because temperature is based on average molecular kinetic energy of collision, which is m*v^2 - and SoS is directly related to v.
Air expanding with a pressure ratio of about 1:0.58 reaches sonic velocity (this ratio varies slightly with the gamma of the gas, which is related to the moment of angular inertia of the molecules). Above this pressure ratio it's possible to use a divergent nozzle to further accelerate the flow to supersonic.
However, a supersonic exhaust is a waste of energy unless you're travelling supersonic - you can use the energy better by exhausting more gas at a lower speed (higher momentum transfer, less wasted kinetic energy). Hence airliners use high-bypass fans.
That's why the shuttle solid rocket booster burns a kind of rubber to produce a high molecular mass, lower velocity exhaust, during the early parts of the flight, and relies on LH2/LO2 fuel for a lower MM, higher velocity exhaust when it's travelling faster.
Basic rocket science :-).
It depens on number of poles.(for asynchronous motors). Double pole->3000 rpm (at 50 Hz) ->3600 rpm (at 60 Hz) 4 pole->1500 rpm or
1800 rpm. A DC motor, either series, shunt or compound excitation runs at about the same speed.Or slower, of course.-- Dimitris Tzortzakakis,Iraklion Crete,Greece Analogue technology rules-digital sucks
necessarily
Are you joking?GEARED?Steam turbine?They are on a single-cast shaft.THAT shaft is expensive, thus it connects the turbine and generator.Imagine a gear for 2,500,000 hp (usual power of a nuclear plant generator).The generator and turbine are designed to run at the same speed.Even train locomotives use diesel-electric transmission, and the traction motors are directly coupled on the wheels.So must be happening at the ships, too.
-- Dimitris Tzortzakakis,Irakli>
No.The thrust of a jet engine is not created solebly by the high rpm, but also from the high speed of the exhaust gases, coming from the back of the engine.The jet engine intakes air, compresses it, the fuel is ignited in the combustion chamber, and the gases simultaneously rotate the turbine and propel the plane.
-- Dimitris Tzortzakakis,Iraklion Crete,Greece Analogue technology rules-digital sucks
GEARED down?I have seen a WWII airplane engine, and the crankshaft is directly coupled to the propeller.The jet engines are ~2500 hp, so it's impossible to gear.The pilot controls only the fuel supply.
-- Dimitris Tzortzakakis,Iraklion Crete,Greece Analogue technology rules-digital sucks
25000RPM isn't unusual for an unloaded router. ...some a little higher.
You're quite right of course. I shouldn't do my inventing while tired and drunk :)
Tim
DC, or universal, motors can run much faster (add a zero). As I posted earlier, it's not uncommon for a router (woodworking tool) to have a no-load speed of 25,000RPM or more.
example:
If you read _Surely You're Joking Mr Feynman_, you will find most of a chapter devoted to how he sold his patent for a nuclear- powered airplane for the sum of $1.
Tim.
On Tue, 8 Jun 2004 02:01:50 -0700, Rene Tschaggelar wrote (in article ):
Thanks, I'll look at that.
Universal motors from washing machines (that's european style washing machines) initially look like a good bet if you're after a mains motor. However, with no load and without their servo control, they can get to speeds where they fly to pieces.
On Mon, 7 Jun 2004 22:50:51 -0700, Don Pearce wrote (in article ):
Your post answers several of the questions -- many, unexpressed -- that I've been after.
So, basically, turning a fan in a tube (spinning a turbojet engine without fuel) doesn't gain you much efficiency. If electrics are to power an aircraft, it seems that an efficient propeller is the best that you can do.
On Tue, 8 Jun 2004 06:40:33 -0700, DaveC wrote (in article ):
And then hi rpms isn't important any more. Indeed, since torque doesn't increase with speed (I *do* have that fact right, don't I?), gearing isn't necessary and propellers have a relatively low maximum speed requirement.
AFAIK the required speed of the propellor can be put anywhere controlled by the pitch and shape of the blade. It gets more difficult when the blade tips start going faster than sound and these make a distinctive sound like the hughes 500 helicopter, the noise it makes is different to most helicopters because the blade tips go supersonic. Thinking about it I reckon that propellor blade design is like antenna design, there is a lot of "magic" in it.
Dave, I think you might want to look at the ducted fan engine. Also, there were test made where one of the jet engines on a DC9 was replaced by a turbine engine driving an open pusher propeller with what looked like maybe 10 - 16 blades. BTW, I am 99.99% sure that the propeller on turboprop engines is geared down. You don't want the speed at the tip of the propeller to exceed the sped of sound. When automobile V8 engines have been installed in light aircraft, they have been geared down to allow an engine speed of ~4500 rpm.
Tam
You would indeed do far better with just the propeller.
d Pearce Consulting
On Tue, 8 Jun 2004 07:19:34 -0700, Tam/WB2TT wrote (in article ):
I'll move the discussion of fans & propellers, etc. to a more appropriate group.
Any more comments on the use of high-speed electric motors, I'd be grateful to hear.
Thanks,
Have a look at the spindle motors on the lpkf PCB prototyping machines. The fastest of these go to 100,000RPM, and are speed controlled brushless DC motors. Brush designs, are generally rare beyond perhaps 25,000RPM, but brushless designs are remarkably common at these speeds. Also look at:
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