Ionizing Air Going Through A turbojet engine... could it increase thrust?

[Spacescifi]

2 hours ago, Terwin said:

This would do nothing useful for the rocket propulsion wise.

For dumping raw energy/heat into the exhaust to be useful, it needs to be before the throat of the engine, which in turn is before the nozzle. 

But you can only do this up to the point where the throat of your engine starts to melt, and this is a point that we already reach quite easily with just chemical combustion.

Shooting a laser into the exhaust of your rocket is functionally equivalent to mounting a small, off-center photon engine next to your chemical engine: lots of complexity for no practical gain.

If you are adding magical uber-power sources without associated magical uber-materials, and also constrain those uber-power sources with a destructively-high minimum functional output, then you have basically re-skinned the nuclear fission bomb.

The only use-case where pusher-plates make sense is when you are limited to fission bombs or a close analog, with no materials or propulsion technologies significantly in advance of 50 years ago. 

 

Just remember: using an Orion engine in an atmosphere will cause significant damage to both the vehicle and the planet, so they are only of use once you are already in-orbit, where things like ion or plasma propulsion can get similar ISP with far less engineering and damage, even if the thrust is much lower.

 

 

I meant shooting the laser into the combustion chamber so the exhaust is pushed out both by the uber laser pulse and combustion at the SAME time.

47 minutes ago, farmerben said:

Are you sure that is true? Superheating the exhaust could have a positive thrust.

 

Unless the laser hits the exhaust while still in contact with the nozzle it won't do anything.

 

The faster the exhaust is shot out, the greater the propulsive force on the rocket.

 

So you are right so long the exhaust is touching the nozzle that the laser zaps.... but direction matters, since the laser needs to push the exhaust straight on out, not radially.

Edited September 15, 2022 by Spacescifi

[Terwin]

50 minutes ago, farmerben said:

Are you sure that is true? Superheating the exhaust could have a positive thrust.

I might be misremembering the mechanics, but I am pretty sure that the throat turns the heat into pressure, and the nozzle turns the pressure into thrust.

Heating up the exhaust after it leaves the nozzle is effectively heating up a near-by nebula that happens to be behind you.  There might be a little something from the photon pressure, but nothing significant.

[sevenperforce]

20 minutes ago, Terwin said:

I might be misremembering the mechanics, but I am pretty sure that the throat turns the heat into pressure, and the nozzle turns the pressure into thrust.

That is the ordinary way it works, yes.

A converging-diverging throat creates a shock transition where the flow upstream is subsonic and the flow downstream is supersonic. Since pressure waves cannot propagate backwards in supersonic flow, this directly converts pressure into velocity downstream of the throat.

For in-atmosphere operation, you don't want to make the downstream nozzle too large, or the pressure at the nozzle exit will drop lower than the atmospheric pressure, allowing atmospheric air to push its way inside the nozzle as the exhaust rushes out. In a near-vacuum you can make the nozzle as large as you want.   

20 minutes ago, Terwin said:

Heating up the exhaust after it leaves the nozzle is effectively heating up a near-by nebula that happens to be behind you.  There might be a little something from the photon pressure, but nothing significant.

Heating up the exhaust after it completely leaves the nozzle is useless.

Heating up the exhaust while it is still inside the nozzle will increase thrust (also known as afterburning or reheat) but it will not do so as efficiently as if you added that heat inside the combustion chamber, before the shock at the choke point in the throat.

Lasers are generally not considered to be a good way to reheat exhaust.

[Spacescifi]

12 minutes ago, sevenperforce said:

That is the ordinary way it works, yes.

A converging-diverging throat creates a shock transition where the flow upstream is subsonic and the flow downstream is supersonic. Since pressure waves cannot propagate backwards in supersonic flow, this directly converts pressure into velocity downstream of the throat.

For in-atmosphere operation, you don't want to make the downstream nozzle too large, or the pressure at the nozzle exit will drop lower than the atmospheric pressure, allowing atmospheric air to push its way inside the nozzle as the exhaust rushes out. In a near-vacuum you can make the nozzle as large as you want.   

Heating up the exhaust after it completely leaves the nozzle is useless.

Heating up the exhaust while it is still inside the nozzle will increase thrust (also known as afterburning or reheat) but it will not do so as efficiently as if you added that heat inside the combustion chamber, before the shock at the choke point in the throat.

Lasers are generally not considered to be a good way to reheat exhaust.

Lasers could be a good way if one had a compact and uber sources of power.

Imagine if you will an uber laser that is linked to the propellant so that when it fires, it dumps it's waste heat into the propellant which is shot out while simutaneously zapping the propellant inside the combustion/detonation chamber.

 

I really am hard pressed to think of other options that can deliver high performance for rocket SSTOs.

 

Great power is needed.

 

If one tried to put playing marble size pellet of metallic hydrogen in the combustion chamber it would probably blow it up since the blast would not be directed.

A laser by comparison is directed. So a laser beam of sufficient powsr could do the job.

 

In scifi if you have guys toting visible beam laser pistols that can melt, cut, or blast, then having more power for propuslion would make sense.

[sevenperforce]

37 minutes ago, Spacescifi said:

If one tried to put playing marble size pellet of metallic hydrogen in the combustion chamber it would probably blow it up since the blast would not be directed.

A laser by comparison is directed. So a laser beam of sufficient powsr could do the job.

No, that is not correct.

The process of converting heat and pressure into velocity through a converging-diverging nozzle is a function of the total heat and pressure in the supercritical fluid in the chamber. Directionality has nothing to do with it. The nature of choke point at the throat is such that whatever is happening inside the combustion chamber is immaterial once the fluid passes the throat.

Second, lasers are not magic. Even though the photons in the laser beam are parallel and coherent, that coherence and directionality is lost once their energy is absorbed by the fluid in the chamber. 

If you have something like metastable metallic hydrogen that you can decompose at will, then just put in smaller amounts continuously. Way easier than trying to do something with a laser. Getting a fluid to absorb photons from a laser will require that the exhaust be opaque to the laser wavelengths, which also may not be the case automatically.

1 hour ago, Spacescifi said:

Lasers could be a good way if one had a compact and uber sources of power.

Imagine if you will an uber laser that is linked to the propellant so that when it fires, it dumps it's waste heat into the propellant which is shot out while simutaneously zapping the propellant inside the combustion/detonation chamber.

If you have a power source that has high specific energy, high energy density, and high specific power, then there is absolutely no reason to worry about pumping a laser gain medium to transfer that energy into the propellant. Just pump that energy directly into the propellant itself. 

_{(There is a sense in which the throat choke of a rocket engine is the sonic equivalent of the coherence gain process in a pumped laser gain medium but that is beyond the scope of this discussion.)}

[sevenperforce]

2 hours ago, Spacescifi said:

In scifi if you have guys toting visible beam laser pistols that can melt, cut, or blast, then having more power for propuslion would make sense.

Which is precisely why you would use those power packs with a propellant tank.

You can simply handwave power density and specific energy, say you have this "uber" energy source, and move on from there.

[Spacescifi]

4 hours ago, sevenperforce said:

No, that is not correct.

The process of converting heat and pressure into velocity through a converging-diverging nozzle is a function of the total heat and pressure in the supercritical fluid in the chamber. Directionality has nothing to do with it. The nature of choke point at the throat is such that whatever is happening inside the combustion chamber is immaterial once the fluid passes the throat.

Second, lasers are not magic. Even though the photons in the laser beam are parallel and coherent, that coherence and directionality is lost once their energy is absorbed by the fluid in the chamber. 

If you have something like metastable metallic hydrogen that you can decompose at will, then just put in smaller amounts continuously. Way easier than trying to do something with a laser. Getting a fluid to absorb photons from a laser will require that the exhaust be opaque to the laser wavelengths, which also may not be the case automatically.

If you have a power source that has high specific energy, high energy density, and high specific power, then there is absolutely no reason to worry about pumping a laser gain medium to transfer that energy into the propellant. Just pump that energy directly into the propellant itself. 

_{(There is a sense in which the throat choke of a rocket engine is the sonic equivalent of the coherence gain process in a pumped laser gain medium but that is beyond the scope of this discussion.)}

 

3 hours ago, sevenperforce said:

Which is precisely why you would use those power packs with a propellant tank.

You can simply handwave power density and specific energy, say you have this "uber" energy source, and move on from there.

 

I mostly agree but a rocket SSTO with uber power is limited by how hot the combustion chamber gets, which is also the reason why the amount of thrust you can get from metallic hydrogen pellets in the chamber is limited.

The easy solution is to just add more engines/nozzles... since making an engine that can tolerate more heat than normal has hard limits.

As for high energy power packs etc... those too have limits.

For example the reason nuclear reactor rockets are lower thrust compared to chemical rockets is because the reactor cannot survive the heat to get hotter that chemical combustion entails. Not only that, but erosion is also an issue... which means the power source is gradually being eaten away by the propellant which is like acid.

It would be cool if you could just put a power pack in the combustion chamber and run propellant over it, but that is also a way to destroy the expensive power pack.

So you would need an intermediary heat exchanger, which would lower performance still.

 

So from my view, I think metallic hydrogen pellets and uber laser pulses are the highest performance engines a rocket SSTO could have.

Lasers also have the advantage of making engines safer. Also making a propellant opaque to lasers is hardly impossible. But that would involve some processing... maybe mixing powder into it? Better yet is that ANY liquid propellant would work given how uber the laser is, so ISRU is really easy as you can gather almost any liquid propellant and get by since the laser is doing most of the work to propel it and bot the chemicals. No need for refueling at fuel depots with specially manufactured metallic hydrogen pellet enriched propellant. All you have to do is mix powder into the propellant to make it opaque to the laser blast.

Use asteroid dust I dunno... plenty of it.

You could really pull off the lone explorer Star Trek ship using the laser method.

Also with metallic hydrogen pellets in the fuel mix, if something goes wrong... BOOM!

I realize the power pack of the uber laser has the same issue, but likely it would be smaller and take up less volume, making it less likely to be hit in any given scenario... compared to a high volume fuel tank.

Edited September 16, 2022 by Spacescifi

[kerbiloid]

The metastable metallic hydrogen would probably reflect the laser light. It's shiny.

[18Watt]

Easiest way to increase the thrust on a turbojet (or any gas turbine) is to add dihydrogen monoxide ahead of the combustion chamber.  That's right, plain old water.

Some of the equipment I've used injected a mixture of water and methanol into the intake air.  The methanol was only there to keep the water from freezing.

Most of the thrust gain is due to the water absorbing heat from the air as it goes from liquid to gas, thus the temperature of the air is cooler, and you can combust more fuel without exceeding the max engine temperatures.  I suppose you are also increasing the mass of the gas going through the engine, but the mass of the injected water is trivial compared to the mass of the air that was already going through the engine.  So your thrust gains are mainly due to the cooler intake air after you spray water in the intake.

Ionizing gas coming into a turbine would yield zero benefits.  The gas would now be potentially corrosive to the engine components.  The energy required to ionize even a small fraction of the incoming air would be colossal, where would that energy come from?  

[Spacescifi]

8 minutes ago, 18Watt said:

Easiest way to increase the thrust on a turbojet (or any gas turbine) is to add dihydrogen monoxide ahead of the combustion chamber.  That's right, plain old water.

Some of the equipment I've used injected a mixture of water and methanol into the intake air.  The methanol was only there to keep the water from freezing.

Most of the thrust gain is due to the water absorbing heat from the air as it goes from liquid to gas, thus the temperature of the air is cooler, and you can combust more fuel without exceeding the max engine temperatures.  I suppose you are also increasing the mass of the gas going through the engine, but the mass of the injected water is trivial compared to the mass of the air that was already going through the engine.  So your thrust gains are mainly due to the cooler intake air after you spray water in the intake.

Ionizing gas coming into a turbine would yield zero benefits.  The gas would now be potentially corrosive to the engine components.  The energy required to ionize even a small fraction of the incoming air would be colossal, where would that energy come from?  

I agree... this was pointed out earlier though.

[farmerben]

Running an electric current through the exhaust plume is different than simply adding heat to it.  It's possible to get electric thrust downstream of the throat, if you can somehow push against the ionized plume.  

[magnemoe]

11 hours ago, 18Watt said:

Easiest way to increase the thrust on a turbojet (or any gas turbine) is to add dihydrogen monoxide ahead of the combustion chamber.  That's right, plain old water.

Some of the equipment I've used injected a mixture of water and methanol into the intake air.  The methanol was only there to keep the water from freezing.

Most of the thrust gain is due to the water absorbing heat from the air as it goes from liquid to gas, thus the temperature of the air is cooler, and you can combust more fuel without exceeding the max engine temperatures.  I suppose you are also increasing the mass of the gas going through the engine, but the mass of the injected water is trivial compared to the mass of the air that was already going through the engine.  So your thrust gains are mainly due to the cooler intake air after you spray water in the intake.

Ionizing gas coming into a turbine would yield zero benefits.  The gas would now be potentially corrosive to the engine components.  The energy required to ionize even a small fraction of the incoming air would be colossal, where would that energy come from?  

This was done on combat jet engines before the afterburner was invented. Today I say its more relevant for rocket engines as you can use more reaction mass but reduced velocity to increase trust at the cost of lower ISP, vasmir was designed with this option, adding oxygen to an nerva engine is another way who also add energy to the flow but require an separate tank with oxygen.