Electric jet engine max speed

cells · Jul 6, 2015

I was just thinking, what is the max speed and acceleration an electric jet could reach if it didn't have to carry any batteries. Imagine a jet flying just a few feet off the ground pulling power from a live and earth strip it is just touching lightly via some conductive pole or even perhaps via an electric arch if that is possible

You would still need a massive length to get upto any usable speed but at least the track just becomes two conductive plates

The highest thrust jet engine seems to be 500KN for a 7.5T engine, and the max speed a jet has reached seems to be just shy of 1km/s so an electric engine not carrying fuel should surely best this but by how much?

Two ideas

1: If very fast speeds can be achieved could it help get things into orbit. I think this would be a no and difficult as you would need to get towards 10km/s

2: Could air planes of the future use such a system and just use their momentum as "fuel" and maybe a small battery pack for small backup etc.

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would an electric jet going through the sound barrier create much of a sonic boom?

MsElectric · Jul 6, 2015

Wouldn't the problem be at higher altitudes you hardly have much air at all to generate sufficient thrust?

cells · Jul 6, 2015

Im not sure what you mean

The blackbird aircraft holds the record of speed at close to 1km/s and at a high altitude. That is done by a normal jet engine carrying fuel and breathing air

But anyway this idea is to get a lot of speed when very close to the ground, in the order of 1km/s - 10km/s

Of course the main problem is that the runway needs to be in the order of 10-20km or even longer but at least its a simple runway which is just two conductors

mmccord · Jul 6, 2015

It seems to me that travelling at those kinds of speed at low altitude would present other problems. (Remember that air at low altitudes is significantly denser than above)

This is why we can't rail-gun stuff into space.

cells · Jul 6, 2015

Just being silly but lets say an electric jet motor could produce twice as much thrust at half the weight of a conventional jet engine.

Well they can do 500KN at a mass of 8 tons so lets just pretend an electric version can do 1,000 KN at a mass of 4 tons

Lets pretend we have two such engines and the total craft is 20 tons (8 ton engines 12 ton payload)

20 ton craft with a force of 1,000kN would accelerate at a crazy 50m/s/s or ~5g

a 20km "runway" would get you to ~1.4km/s

Going to need a very long runway.....

How far could such an aircraft or glider cruise oncce at that speed

jaguar36 · Jul 6, 2015

You can't fly at anywhere near 1km/s near the ground, the air is way to dense. In addition if you are talking about going anything close to Mach 1 its going to take a fairly long time to get there even at a high rate of acceleration. Such that your rails would need to be on the order of 10s of miles long. The track would also need to be very very straight, and very very flat.

A electric plane would make just as big a sonic boom as a gas powered one, the boom is caused by the compression of the air, not by the engines.

cells · Jul 6, 2015

I would have thought during the acceleration phase being lower in thicker air would help as thats the stuff you are pushing against?

jaguar36 · Jul 6, 2015

The force of drag (which is the force the force the engines need to exert just to maintain speed let alone accelerate) is directly proportional to the air density. Therefore if it takes the SR-71 64,000lbs of force to go 2,200mph at 80,000ft where air density is .86 10[SUP]-4[/SUP] slugs/ft[SUP]3[/SUP] it would take almost 30 times that thrust to do it at sea level. That's almost 2 million lbf or about the same as one of the massive F1 engines the Saturn V used.

deonb · Jul 6, 2015

cells said:
I would have thought during the acceleration phase being lower in thicker air would help as thats the stuff you are pushing against?

Just... No.

Jet engines don't push against anything. A jet engine will work fine in outer space, if you bring your own air for combustion instead of getting it from the air intakes.

The only slight advantage you get from a lower altitude is that you can make the air intakes smaller, but that in no way defeats the effect of drag on the airplane wings that has to be overcome at low altitude.

Having said that, an electrical airplane engine as you've seen them so far is not a jet engine... It's a ducted fan engine. It would in theory be possible to create a plasma engine, but they don't exist yet. Either way, doesn't change how the physics work.

JRod0802 · Jul 6, 2015

For those that are interested in reading about high speed vehicles at low altitude, here's a (non-electric) "car" that traveled faster than mach 1:

ThrustSSC - Wikipedia, the free encyclopedia
Thrust SSC - Run Details

Chris TX · Jul 6, 2015

If by "Electric Jet Engine" you mean electric turbofan, you're still going to run into relatively thick air at ground level. The thrust needed to maintain speed at higher altitude is much less, and that's why jets go 20-50k feet up. If you're following along, the removal of all that "thick" air at ground level is what Hyperloop does. It does that by sucking out the air from the tubes.

RDoc · Jul 6, 2015

What do you mean by "electric jet engine"?

Most modern jet engines are a kind of hybrid between a rocket and a propeller. Electric propeller engines are well known. Electric rockets (apart from vacuum only ion engines) are AFAIK not, although I suppose they would be possible if you used electricity to drive a compressor and heat the air inside the engine. I'm very doubtful that would be at all practical or efficient.

cells · Jul 7, 2015

RDoc said:
What do you mean by "electric jet engine"?

Most modern jet engines are a kind of hybrid between a rocket and a propeller. Electric propeller engines are well known. Electric rockets (apart from vacuum only ion engines) are AFAIK not, although I suppose they would be possible if you used electricity to drive a compressor and heat the air inside the engine. I'm very doubtful that would be at all practical or efficient.

by electric jet engine i meant an electric engine on a jet so no specific design.

I remember watching a video about jet engines and it said that at high speeds (>2x the speed of sound) one of the bigger problems is the need to slow down the air in the compression stage and then speed it up again so if you could combust it without having to slow it down first that would be better. With an electric engine you dont need to do the slowing down as you dont need to compress air to combust it

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Ok another crazy idea. Not suggesting its possible but these things are sometimes interesting to talk about

Lets imagine two drones in the sky 10 km high connected by a say 20km line to each other both orbiting and spinning.

There is some tension in the line but a lot of the spinning is from the flying/wings of the drones.

Another line connects to the middle of this setup which sends up power to the drones to speed them up

What sort of speed could they attain?

deonb · Jul 7, 2015

cells said:
Lets imagine two drones in the sky 10 km high connected by a say 20km line to each other...

What sort of speed could they attain?

About 9.8 m/s/s ...

cells · Jul 7, 2015

deonb said:
About 9.8 m/s/s ...

is that a joke to say that they will fall and not fly?

google have some project called project makani where they seem to be flying a drone on a teather

jaguar36 · Jul 7, 2015

cells said:
by electric jet engine i meant an electric engine on a jet so no specific design.

I remember watching a video about jet engines and it said that at high speeds (>2x the speed of sound) one of the bigger problems is the need to slow down the air in the compression stage and then speed it up again so if you could combust it without having to slow it down first that would be better. With an electric engine you dont need to do the slowing down as you dont need to compress air to combust it

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Ok another crazy idea. Not suggesting its possible but these things are sometimes interesting to talk about

Lets imagine two drones in the sky 10 km high connected by a say 20km line to each other both orbiting and spinning.

There is some tension in the line but a lot of the spinning is from the flying/wings of the drones.

Another line connects to the middle of this setup which sends up power to the drones to speed them up

What sort of speed could they attain?

Not a lot before centripetal force turned the pilots into goo.

cells · Jul 7, 2015

the longer the line the lower the acceleration force for a given velocity

at 10km radius and 100g, assuming a drone could do 100g velocity should be ~3.1km/s

being crazy you would need to get toward 300g and 20km radius to get towards 8km/s and into orbit.
detach a payload and let it fly and reuse everything else

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