Could Thermal Turbojets work with precontained air?

[Coga19000]

I have been searching for ways to construct not just an SSTO, but an SSTO that consumes no fuel at all. Being a huge KSP Interstellar fan, I obviously steered towards the Thermal Turbojet, but decided not to include a Quantum Vacuum engine, for its feasibility is anything but solidly proved.

For those who don't know, a Thermal Turbojet is an engine which works by running the atmospheric air from the Intakes over the walls of a working reactor, which heats it to high velocities before it gets expelled from the nozzle. This allows an aircraft to fly (in theory) without consuming any fuel other than the reactor fuels, as long as it stays in the atmosphere.

However, I realised that this kind of engine could work anywhere in deep space, as long as there is a gas flowing through the assembly. Obviously, this gas cannot come from the environment of the ship, but could be derived from gas stored on high pressure in internal tanks. This would consume the internally stored air, of course, but the propellant would still be an absurdly abundant resource that it could be refueled with on terrestrial (or other planet's with an atmosphere) soil for as long as humans have air to breathe.

So, which are your opinions? Would such an assembly actually work? Would an element I did not know of prevent this? Would I need impracticably huge tanks to accomplish this? Am I missing something else? Am I ready for the funny farm after proposing such a concept? Whatever it is, let me know!

[Armchair Rocket Scientist]

What you're proposing is basically a combined-cycle nuclear-thermal engine that can use either internal propellant (which should be stored as a liquid because high-pressure gas tanks have abysmal mass ratios and density) or external air. I'm not sure how feasible it would be to make one that worked well because your cooling channels need to be either compatible with both liquid and gas or you need multiple sets of cooling channels.

[windows_x_seven]

So... basically a RAPIER thermal turbojet?

It would require a very large amount of air stored.

[Coga19000]

What you're proposing is basically a combined-cycle nuclear-thermal engine that can use either internal propellant (which should be stored as a liquid because high-pressure gas tanks have abysmal mass ratios and density) or external air. I'm not sure how feasible it would be to make one that worked well because your cooling channels need to be either compatible with both liquid and gas or you need multiple sets of cooling channels.

Hmmm, I see your concern. First of all, yes, I hadn't taken the mass ratio of air in account. Well, that would complicate the refueling process slightly, but we can liquify air can't we? (Well, at this point the handling difficulties and propellant inefficiency might be so large we might as well turn to specialised fuels as hudrogen, which beats the purpose a little -since the engine is now literally a NERVA in all ways possible. I'm not too sure, though )

And well, the whole point is that the Thermal energy is transferred to the propellant which is then expelled, simplifying the cooling system. Now, how different cooling systems are for liquids and gasses, I have no idea, so someone will have to indulge me on that. And a possible workaround for this would be liquidating the air from the Intakes on the fly -but at this point, again I think this is too similar to a proposed engine concept. I would think SABRE without being sure , but if that's the case, there is still the matter of the reactors presence to differentiate them.

So... basically a RAPIER thermal turbojet?

It would require a very large amount of air stored.

Actually... Yes, that's what I'm proposing And yes, I do agree on the air amount problem. So, would this work better with liquids? I can only assume it would, if I judge from the above post.

[windows_x_seven]

Actually... Yes, that's what I'm proposing And yes, I do agree on the air amount problem. So, would this work better with liquids? I can only assume it would, if I judge from the above post.

I'd guess so. It wouldn't be as efficient as a "closed-cycle only" NERVA engine, but I think it'd work.

[SomeGuy12]

It wouldn't work very well. The problem's very simple - thermodynamics means that at a given temperature, a gas with heavier molecules has a slower average velocity.

So for a nuclear thermal rocket, the temperature of the rocket jet has to be low for 2 important reasons :

1. If it's too high, the reactor parts or the engine bell will melt

2. You're getting rid of reactor waste heat by the gas flowing through the reactor

If you use pure hydrogen as propellant, you get impressive ISPs of about 1000 or more, but if you use air - oxygen and nitrogen in a 40/60 mix, say - you get worse performance. https://en.wikipedia.org/wiki/Root-mean-square_speed

If the ISP with hydrogen is 1000, a gas that has a mass of 1, oxygen has a relative mass of 16 (we don't care what units they are to use this mathematical relationship). If you look at the equation, you are dividing by the mass and taking the square root. Anyways, it means that you divide by the square root of 16, or 4, so you get an ISP of 250 using oxygen.

That's awful. So now your nuclear scramjet has to have this gigantic tanks of liquid oxygen, since it's going to need impractical amounts of it, and that adds drag, and so it can't get as high a velocity in the upper atmosphere, and basically we aren't going to space today.

Oh, several more show-stoppers :

1. The neutrons and gamma rays coming off a respectable nuclear scramjet engine will fry any astronauts in the same vehicle, giving them a lethal dose in seconds. You would need impractically heavy amounts of shielding - 50 tons or more of it.

2. If you ever do make it to space, you better deploy some radiator fins fast, because your nuclear thermal rocket is emitting many gigawatts of waste heat. Going to be hard to get rid of that - droplet radiators are a lightweight way to do it, but you can't deploy those until you're in real vacuum.

3. If you use tanks of liquid hydrogen, the only practical way, then your reactor core has to survive both exposure to hydrogen (reduction) and high temperature oxidation. It is very difficult to find a material that resists both at high temperature. Not sure if it's chemically possible.

I did this exercise as well. I concluded that you want a rocket with several stages that each boost the next one up, and you propulsively land the stages back on the planet. You refuel the stages with ISRU - on an earth-like planet, this would be really easy since there's huge amounts of water, C02, and oxygen in the air. You basically give up the ISRU plant, and the lower stages of your rocket permanently - but you can reuse em over and over if you had the technology to maintain them in the field.

Nuclear is just a bad idea. It's ok if you're going to stay in a vacuum and you don't need as much thrust and you can have the reactor really far from the crew on a long thin boom.

But for leaving a planet, in a way, a rocket is a vast pile of chemical energy that you've set up in just the right way to release all that energy very quickly. You could use a nuclear reactor to supply the energy to run the ISRU plant to make the fuel, but you don't want to carry that reactor on your lift vehicle. For one thing, the weight of the reactor is dead weight, it represents potential fuel you could have carried instead.

Edited July 7, 2015 by SomeGuy12

[prophet_01]

The US airforce tried nuclear planes in order to design the ultimate strategic bomber. But the Reactors and especually the radiation shielding were too heavy to make their designs practical.

They also designed and tested a missile with a nuclear ramjet. That thing didn't need much shielding and therefor achieved very impressive (and rly rly scary) results. It was called project pluto and ultimately was canceled, because the US goverment was afraid to trigger another arms race.

Many of the technologies developed for this project are used in modern missiles.

[Rune]

Yeah... nuclear engines running on fission reactors have a lot of special handling required. But I will direct you all to a (I'll admit, very speculative) paper on a little known fusion engine concept, the QED, specifically the ARC version (Quiet Elestrostatic Discharge, All-Regeneratively Cooled).

http://www.askmar.com/Fusion_files/The%20QED%20Engine.pdf

Basically, an electric engine running attached to a Polywell fusion reactor. And no, the Polywell hasn't been built yet, and it hasn't even been proven to work conclusively. But if we believe the numbers there, you get systematic TWRs of between 2 and 6, with isps of between 1000s and... 5500s, by heating the propellant with high-voltage electron guns running directly from the reactor current (it's a direct conversion reactor, too). That would make SSTOs with reasonable payload ratios for Earth! And then forget about combined/closed cycle, you fill up your tanks on the ground and that's it, and your engine only runs optimized for one configuration. It probably allows the payload fraction to put a pump and electrolyzer to refill the tanks if that is what you need for storytelling purposes.

Yup, it uses superconducting magnets to do, among other things, magnetic nozzles and direct conversion of the fusion products into a high voltage current driving the relativistic electron beams that heat the propellant. Probably a very complicated piece of kit. But if it can be built, this is the propulsion system in all those cheesy sci-fi shows! Propellant-agnostic (methane or water can be used, by the time it's finished coupling with the electron beams it's a plasma anyway), TWR over one, runs on a mostly-anaeutronic reaction that is probably only significantly dangerous during operation.... what more could you ask of it? You can even tap it for power! And loads of it, since you can get all your jet power as high-voltage current. Though probably the conversion equipment to use it will be heavy.

Rune. So, not going to be built tomorrow. But plausible.

[KerikBalm]

Air? well, in the "Up-Goer 5" schematic, hydrogen was described as cold, wet air.

So... sure, if by precontained air, you mean H2.

But normal air, as in basically 20% O2 and 80% N2.... nope....

Youd get better results with steam (MW 18 vs that 32 and 28 mw mixture).... but your reactor won't trech the temps of the chemical reaction of LH2+O2, and Isp will be worse than a chemical rocket still.

BTW... polywells have been built, and they do work for producing fusion... just like Dense plasma foci have been built.

They haven't been built big enough to break even, nor have they done the P-B11 reaction yet.

[sgt_flyer]

Yeah... nuclear engines running on fission reactors have a lot of special handling required. But I will direct you all to a (I'll admit, very speculative) paper on a little known fusion engine concept, the QED, specifically the ARC version (Quiet Elestrostatic Discharge, All-Regeneratively Cooled).

http://www.askmar.com/Fusion_files/The%20QED%20Engine.pdf

Basically, an electric engine running attached to a Polywell fusion reactor. And no, the Polywell hasn't been built yet, and it hasn't even been proven to work conclusively. But if we believe the numbers there, you get systematic TWRs of between 2 and 6, with isps of between 1000s and... 5500s, by heating the propellant with high-voltage electron guns running directly from the reactor current (it's a direct conversion reactor, too). That would make SSTOs with reasonable payload ratios for Earth! And then forget about combined/closed cycle, you fill up your tanks on the ground and that's it, and your engine only runs optimized for one configuration. It probably allows the payload fraction to put a pump and electrolyzer to refill the tanks if that is what you need for storytelling purposes.

Yup, it uses superconducting magnets to do, among other things, magnetic nozzles and direct conversion of the fusion products into a high voltage current driving the relativistic electron beams that heat the propellant. Probably a very complicated piece of kit. But if it can be built, this is the propulsion system in all those cheesy sci-fi shows! Propellant-agnostic (methane or water can be used, by the time it's finished coupling with the electron beams it's a plasma anyway), TWR over one, runs on a mostly-anaeutronic reaction that is probably only significantly dangerous during operation.... what more could you ask of it? You can even tap it for power! And loads of it, since you can get all your jet power as high-voltage current. Though probably the conversion equipment to use it will be heavy.

Rune. So, not going to be built tomorrow. But plausible.

if you can get a polywell to work, why not directly use it as a fusion drive instead of doing an electric conversion ? If they can shape the confinment fields to create fusion, they should be able to control were the fusion products escape - that would give you tremendeously more ISP than QED (though, it would obviously be much better suited to in space operations ^^) (from QEDs, that's basically the DFP concept of QEDs )

Edited July 7, 2015 by sgt_flyer

[Rune]

if you can get a polywell to work, why not directly use it as a fusion drive instead of doing an electric conversion ? If they can shape the confinment fields to create fusion, they should be able to control were the fusion products escape - that would give you tremendeously more ISP than QED (though, it would obviously be much better suited to in space operations ^^) (from QEDs, that's basically the DFP concept of QEDs )

I think you answered yourself there (and mentioned the big if, too): a 20,000s isp would give you the times less TWR than a 2,000s one, in ideal conditions. Besides, cooling when you can't dump the extra heat into the fuel is tricky, the designs in the paper I showed are thermal-limited, hence the ARC acronym. This way, you get a compact high TWR engine that still has awesome ISP, without having to lug a crapton of radiators around. But that application has also been envisioned for Polywell engines, I just can't find the paper outside my computer. I think it got lost to the internet god of "too old and unknown".

Also, the fusion cycle lends itself to it: the p-B11 reaction produces 80% of the energy as charged alphas, which are "ridiculously easy" to convert into electric power, and only 20% as thermal "waste" in other fusion products (He and neutrons, basically).

Rune. A shame, I have yet to see any half-serious concept that promises so much for so little.

[KerikBalm]

I'm also quite optimistic about dense plasma focus reactors... those readily turn into a very high Isp propulsion device, if you don't slow down the reaction products against a magnetic field to generate electricity.

[sgt_flyer]

I think you answered yourself there (and mentioned the big if, too): a 20,000s isp would give you the times less TWR than a 2,000s one, in ideal conditions. Besides, cooling when you can't dump the extra heat into the fuel is tricky, the designs in the paper I showed are thermal-limited, hence the ARC acronym. This way, you get a compact high TWR engine that still has awesome ISP, without having to lug a crapton of radiators around. But that application has also been envisioned for Polywell engines, I just can't find the paper outside my computer. I think it got lost to the internet god of "too old and unknown".

Also, the fusion cycle lends itself to it: the p-B11 reaction produces 80% of the energy as charged alphas, which are "ridiculously easy" to convert into electric power, and only 20% as thermal "waste" in other fusion products (He and neutrons, basically).

Rune. A shame, I have yet to see any half-serious concept that promises so much for so little.

Well, for vacuum engine or electric generators, there's the dusty plasma concept variant of the fission fragment reactor (also direct energy conversion, but fission based instead of fusion)

The efficiency of those direct energy conversion is anyway miles ahead of current tech thermal energy conversion) - and the more efficiently you can extract this energy, the less the waste heat you need to manage

Edited July 7, 2015 by sgt_flyer

[PB666]

It wouldn't work very well. The problem's very simple - thermodynamics means that at a given temperature, a gas with heavier molecules has a slower average velocity

1. The neutrons and gamma rays coming off a respectable nuclear scramjet engine will fry any astronauts in the same vehicle, giving them a lethal dose in seconds. You would need impractically heavy amounts of shielding - 50 tons or more of it.

Once in space simply expand the distance. What about the folks on the ground, the astronaut is in front of the intake, the Kerbal citizens are in the path of the ejecta.

2. If you ever do make it to space, you better deploy some radiator fins fast, because your nuclear thermal rocket is emitting many gigawatts of waste heat. Going to be hard to get rid of that - droplet radiators are a lightweight way to do it, but you can't deploy those until you're in real vacuum.

Actually I don't think this is possible, the waste heat is too much, you have to separate the fuel and slow down the reaction or slide the fuel into graphite cylinders. The problem is that once initiated the fissile products remain radioactive and produce heat for years.

3. If you use tanks of liquid hydrogen, the only practical way, then your reactor core has to survive both exposure to hydrogen (reduction) and high temperature oxidation. It is very difficult to find a material that resists both at high temperature. Not sure if it's chemically possible.

Its plasma anyway at 2500-4000 degrees, you have corrosive protons and electrons running around at high temperature, your fuel is going to ablate. Ultimately the solution is RF MR direction of the plasma away from the fixed parts of the ship, such as the VASMIR engine does. But that generates RF heat and tramsformer waste.

At such temperatures there is no energy gain of oxygen reacting with metals, since the stabilization energies are the addition of electons at those temperatures the labile outer shell electrons are gone. Heating metals in this way is the way that oxides are driven off of metals. Redox based chemistry is irrelevant because there are no stable states. Most every thing will be fluxing into a plasma state. Metals and gases all will lose some outer shell electrons (or they will be in high energy outer shell orbitals).

Some of the radioactive engines have the hydrogen going through the fuel sand and becoming hot plasma with an operating temperature around 4000 degrees at the center, the ejected hydrogen carries most of the heat out of the system, there is no way to separate the U once injected into neutron stabilizer bed and loss of some U is expected with friction and erosion.

I have my doubts about the space practicality of NERVA like engines.

[Rune]

Well, for vacuum engine or electric generators, there's the dusty plasma concept variant of the fission fragment reactor (also direct energy conversion, but fission based instead of fusion)

The efficiency of those direct energy conversion is anyway miles ahead of current tech thermal energy conversion) - and the more efficiently you can extract this energy, the less the waste heat you need to manage

You don't even need those huge isps. The solar system ain't that big, a 5,000s spaceship would already be able to single stage most places here. And on the other end of the scale, even 50,000s, or 500,000, are not enough to do relativistic interstellar travel. So really, once you have ion-engine-like isp, you are golden to do in-system travel, and you should start worrying about TWR, because only probes can wait months to complete a maneuver, and lifting from the ground is where we have the big issues now.

Not that they aren't nifty thought exercises, mind you. But the really cool thing for a proper Space Age would be an engine that you can use all the way from the ground to the Kuiper Belt, with manned payloads. I thought it was impossible in the realm of real world science, and then I found this. It's even by the guy that dreamt up the (flawed, I know), Bussard Ramjet! You can imagine why it's my favourite engine concept.

Some of the radioactive engines have the hydrogen going through the fuel sand and becoming hot plasma with an operating temperature around 4000 degrees at the center, the ejected hydrogen carries most of the heat out of the system, there is no way to separate the U once injected into neutron stabilizer bed and loss of some U is expected with friction and erosion.

I have my doubts about the space practicality of NERVA like engines.

Yet the ones seriously studied for development run cooler than some chemical engines, just to prevent that short of thing. NERVA or, even better, the RD-0410 (the russian version, which was both simpler and better performing than NERVA in many aspects, and was actually built and tested in flight configuration), would have been very practical 900s in-space engines.

Rune. Wow I really went off topic there.

Edited July 8, 2015 by Rune