Jet propulsion in space?

  • #1
Here's a really complex & difficult question...

Satellites orbit Earth in the upper thermosphere. Could a jet engine (not a rocket) produce enough thrust to move a small craft? The reason I'm asking is because it most certainly would produce "some" thrust in low orbit. The atmosphere extends to roughly 70 miles above the surface. Why bother with rockets at all & just use slower / safer jets?
 

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  • #2
Drakkith
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At 70 miles above the surface the density of the atmosphere is a mere fraction of that nearer to the surface where jet engines are normally used. There simply isn't enough air to support combustion, even with a compressor, so a jet engine cannot function at all.

Also, I've changed the thread prefix from A to B. The prefixes indicate at what level you want the explanations. An A would indicate you want a graduate-level explanation, full of complicated equations and such if appropriate.
 
  • #3
I apologize for the error I'm a bit fresh here : )

The engine doesn't need to be powered by combustion, so it could still work. An electrical (or mechanically powered) engine would serve our purposes here just fine.
 
  • #4
Drakkith
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The engine doesn't need to be powered by combustion, so it could still work. An electrical (or mechanically powered) engine would serve our purposes here just fine.
Alright, but then it isn't a jet engine. How will it provide thrust to a spacecraft?
 
  • #5
DaveC426913
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The engine doesn't need to be powered by combustion, so it could still work. An electrical (or mechanically powered) engine would serve our purposes here just fine.
What electrically-powered or mechanically-powered engine would work in the upper atmosphere?

I can think of one type of engine that works very well there: a rocket engine.
 
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  • #6
What electrically-powered or mechanically-powered engine would work in the upper atmosphere?

I can think of one type of engine that works very well there: a rocket engine.
A jet works by sucking in air & propelling it out the back. In low orbit, there is still *some* atmosphere present. My thoughts were that maybe a jet could still produce a useful amount of thrust even in such sparse atmosphere.
 
  • #7
davenn
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A jet works by sucking in air & propelling it out the back. In low orbit, there is still *some* atmosphere present.
nowhere near enough for that ..... even low earth orbit satellites are well above the significant atmosphere

from wiki

A low Earth orbit (LEO) is an orbit around Earth with an altitude between 160 kilometers (99 mi) (orbital period of about 88 minutes), and 2,000 kilometers (1,200 mi) (about 127 minutes). Objects below approximately 160 kilometers (99 mi) will experience very rapid orbital decay and altitude loss.[1][2] The orbital velocity needed to maintain a stable low Earth orbit is about 7.8 km/s, but reduces with increased orbital altitude

The atmosphere has a mass of about 5.15×1018 kg,[2] three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The Kármán line, at 100 km (62 mi), or 1.57% of Earth's radius, is often used as the border between the atmosphere and outer space. Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km (75 mi).

In summary, the mass of Earth's atmosphere is distributed approximately as follows:[26]

  • 50% is below 5.6 km (18,000 ft).
  • 90% is below 16 km (52,000 ft).
  • 99.99997% is below 100 km (62 mi; 330,000 ft), the Kármán line. By international convention, this marks the beginning of space where human travelers are considered astronauts.


cheers
Dave
 
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  • #8
nowhere near enough for that ..... even low earth orbit satellites are well above the significant atmosphere

from wiki





cheers
Dave
Aye. This is good. I understand that 1/100,000th of normal air pressure might not be enough to produce enough thrust to move an object.
 
  • #9
DaveC426913
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Aye. This is good. I understand that 1/100,000th of normal air pressure might not be enough to produce enough thrust to move an object.
I'm not sure it's enough to sustain combustion at all.
 
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  • #10
Janus
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Aye. This is good. I understand that 1/100,000th of normal air pressure might not be enough to produce enough thrust to move an object.
It's much worse than that. At an altitude of 200 km, the density has dropped to ~2.79e-10 kg/m3. The density at sea level is 1.225 kg/m3
or better than 4,000,000,000 times denser.
 
  • #11
D H
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Orbiting is a matter of "falling with style": thrust is rarely needed. At 70 miles (110 km), an object is not "falling with style." It is instead just falling. Assuming a spherical satellite, the satellite needs to be orbiting at more than 140 to 160 km (depending on density) to have a chance of making even one orbit about the Earth. Anything less than that and the object is not "orbiting". It is just falling.

I suspect that thanks to boatloads of bad science in Star Trek, Star Wars, and a plethora of other TV shows and movies, the opening poster may be confused with regard to what "orbiting" truly means. A spacecraft in orbit does not need to be constantly firing its thrusters to stay in orbit. Thrusters are used rather infrequently to counteract the tiny effects drag, perturbing effects from the non-spherical nature of the Earth's gravitational field, perturbing effects from the Moon and the Sun (and to a much lesser extent, other bodies), and to keep the spacecraft pointed in the desired direction.
 
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  • #12
Nidum
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just use slower / safer jets?
Generally with air breathing engines the higher the altitude the faster you need to fly . Put crudely you need to fly faster at higher altitudes so that engines can scoop up enough air /second to function properly and to produce required thrust .
 
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  • #13
DaveC426913
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I suspect that thanks to boatloads of bad science in Star Trek ...... A spacecraft in orbit does not need to be constantly firing its thrusters to stay in orbit.
Are you trying to tell me that, the moment the Enterprise shuts off its engines, its orbit doesn't decay in a matter of mere hours?

cuz them's fightin' words.

:-p
 
  • #14
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Although some oxygen exists at very high altitude, gathering the amount required to combust jet fuel would need a gigantic air intake, around a kilometer in diameter, and an enormously powerful compressor stage.
Such an engine would consequently be very large and heavy, way too heavy to be supported by lift generated from a wing it would be attached to.
 
  • #15
I feel like my question has a been answered in a way that I can chew it : ) Thank you all for your contributions!!!
I also want to add that I was not referring to a conventional combustion powered air-breathing jet. Rather a jet with an electrical engine, or mechanical one leaving combustion moot. I'm only interested in producing a meaningful amount of thrust in low orbit without rockets... (Impossible?)
 
  • #16
davenn
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Rather a jet with an electrical engine, or mechanical one leaving combustion moot. I'm only interested in producing a meaningful amount of thrust in low orbit without rockets... (Impossible?)
you are still using the words jet engine
a jet engine requires something to be sucked in one end, burnt and thrust out the other end

your idea of a electrical or mechanical powered jet engine is therefore not workable ... impossible ... most likely ...YES

so ask yourself
"what is your electrical powered motor going to thrust out its "tail pipe" since there is nothing coming in the other end ? "


Dave
 
  • #17
Chronos
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An ion engine requires no oxygen or combustion, they work just fine in space using EM force to eject charged particles at high velocity. In fact, they are pretty much useless in the atmosphere.
 
  • #18
Drakkith
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An ion engine requires no oxygen or combustion, they work just fine in space using EM force to eject charged particles at high velocity. In fact, they are pretty much useless in the atmosphere.
True, but you still have to carry your fuel on-board, correct?
 
  • #19
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Following the OP idea, no it's just not feasible in the usual concept of a 'jet engine'.
However most jet engines used in modern airliners are actually fan-jets where the fan part is doing most of the work.
If you could make something like that without needing combustion to power it, the idea is credible.
No presently known method can do this that I know of though, certainly not electrical - batteries are heavy..
 
  • #20
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I remember reading (in the late 50s or mid-60s) about a proposal of an 'orbital jet'. It should work like a ramjet, its 'fuel' being the thermal energy supplied by the compression-induced recombination of ionized gases in the outer layers of the atmosphere, 70km above the surface (or something like that)...
 
  • #22
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Following the OP idea, no it's just not feasible in the usual concept of a 'jet engine'.
However most jet engines used in modern airliners are actually fan-jets where the fan part is doing most of the work.
If you could make something like that without needing combustion to power it, the idea is credible.
No presently known method can do this that I know of though, certainly not electrical - batteries are heavy..
The OP has changed from jet engine to electrical engine (fan). It seems that if there is enough atmosphere to bring a thing out of orbit, then a solar powered fan should produce enough thrust (downward) to keep the object afloat, assuming the fan is big enough.
 
  • #23
Janus
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The OP has changed from jet engine to electrical engine (fan). It seems that if there is enough atmosphere to bring a thing out of orbit, then a solar powered fan should produce enough thrust (downward) to keep the object afloat, assuming the fan is big enough.
You don't want to apply the thrust downward no matter how it is provided. Downward thrust would change the eccentricity of the orbit, causing the perigee to swing closer to the Earth. And even if you arranged the downward thrust so that it kept the satellite at the same altitude, this would not prevent it from slowing in its orbit. Since it is its orbital speed that allows it to maintain orbit, you would have to keep increasing the thrust in order keep it at altitude.

Any thrust designed to keep the satellite in orbit needs to be applied so that it pushes the craft i the same direction as its orbital motion.

As far as using a fan. Remember that at an altitude of 200 km the satellite is moving at over 7.8 km/sec relative to the local "atmosphere". For a fan to provide thrust, it would have to be spinning fast enough to take in that atmosphere and add speed to it. We are looking at 100's of thousands to maybe millions of rpm.
 
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  • #24
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IIRC, some 'low flying' satellites have used an ion-engine to offset atmospheric drag and compensate for geoid variations. Sadly, such atmosphere is too thin to use as propellant, so that must be carried after all...

And, yes, there's a proposal to use a conductive tether in Earth's magnetic field to either trade velocity for power, or feed with eg solar power to gain velocity.

Um, I'm no Treckie but, given the frontal area of a starship, it would face a lot more drag in LEO than our much smaller space-station which must still be boosted at regular intervals. Don't forget Skylab, whose orbit decayed early due 'puffing' of atmosphere by a SolarMax...

Another Treckie possibility is the starship holds a very low, power-maintained orbit so their baulky transporters stay within optimum range...
 
  • #25
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Sadly, jet engines would DEFINITELY not work up there. The air is too thin, but there is another reason why.
Oxygen is rather heavy compared to other elements found in the atmosphere, and up there, most of the "air" would be composed of hydrogen, which is not suitable for oxygen-combustion.
I remember reading (in the late 50s or mid-60s) about a proposal of an 'orbital jet'. It should work like a ramjet, its 'fuel' being the thermal energy supplied by the compression-induced recombination of ionized gases in the outer layers of the atmosphere, 70km above the surface (or something like that)...
I think you are talking about the Buzzard Ramjet. It works like a charm, but it requires a lot of electricity, and it is inappropriate for long range travels which the energy is harnessed by the sun.
 

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