Non nuclear non fuel jet engines

You'll want coolant flowing through everything to keep temperatures uniform. If you have superbatteries that don't put off much heat, use air, but you want a working fluid of some sort. Just bleed it off the engine.

There's an expansion nozzle before that. The turbine doesn't take all the energy out of the air, since that would result in an engine with no thrust. (and therefore probably used for power generation on the ground) The air traveling through the engine still has a lot of energy in it, and by the thrust equation, you can utilize more of it if you expand that air out to ambient pressure. Therefore, there's an expansion nozzle, which is where you actually generate thrust. The pressures here are what move the whole thing forward. Now, after this expansion nozzle you can put on TV paddles. (or, more commonly, the tail end of the expansion nozzle /is/ the TV paddle setup)

In your application, you don't have the turbine sucking a ton of energy out of the flow (to power the compressor) Assuming your after-compressor energy is exactly the same as the after-combustor energy in a normal engine, you need more nozzle to expand back out to ambient. Get shitloads of thrust, though.

This is a valid point, though most of the temperature is taken care of by using wall cooling and superalloys, rather than thick conventional materials.




I read V10's post, and edited in a response above, but I'll reproduce it here anyways:
Having reviewed his post, I don't think that's the case. I think you can get a lot of thrust out of this, especially since thrust in real-world applications is temperature limited. (the first stage of the turbine starts melting above a certain point) I'm trying to figure what you can get, but so long as you can take the pressure, and can manage a large enough nozzle, you can go right on up until your compression heats the air to your materials' melting points. This means lots more compressor stages though. LOTS.

Point. I'd be interested to see how you'd get that fronal area.

Indeed. But it's much simpler than running the coolant through a pump or something and than using radiators in the engine. I have a few ideas for air cooling already. And I could probably exploit the existing air flow over the wing in some way.

Thanks for that insight.

Considering I have no combustion and thus no extra heat. Would my after-compressor energy not be equal to the after-compressor energy of a normal engine? As in, the before-combustor energy as opposed to the after-combustor energy?

As far as nozzle sizes go, I don't think I want a larger nozzle per engine than modern jet fighters have. After all, a huge nozzle means a huge surface area and wider engine no? I feel that the issue I would most likely run into first is that of my peak pressure being limited not by the materials of the engine but by the amount of air my nozzle can suck in before it gets too unwieldy for my aircraft to use.

Well there are several ideas for now. One was to simply mount the engines under a flat wing XB-70 style. That way I can get them to line up with the air flow nicely and use a very large surface area unobstructed by the fuselage.

Although I have also been thinking of having the fuselage taper rearward (like on that image I posted on the 1st page) to direct the air flow toward the engine. But at the same time provide a more aerodynamic front surface for the air to hit. With the ideal outcome being that the air hits the aerodynamic front of the fuselage and is directed across it surface and into the engine nozzles in the rear.

But this is all still in a very, very alpha stage.

Modern engines are limited by the temperature of the compressed air *after* you burn jet fuel in it. The temperature limitation in all jet engines is the first stage of the turbine: if your after-combustor temp (called the "turbine inlet temperature") is too hot, your turbine blades start to relax, impact the outer wall of the engine, and your engine disassembles itself.

You don't have a combustor, so you can compress your air until it hits that temperature. Your Compressor Pressure Ratio can be absolutely ungodly since you're not burning anything.

Your max CPR can be found thusly:
CPR=(T2/T1)^3.5, where T2 is the temp where things start melting, and T1 is your max design inlet temp.

Take 330K as the max design, and 1600K where things start melting (for instance), and you get a possible CPR of 250. (most jet engines do.... 30s.) This is obviously a bad idea, as 250 atmospheres of absolute pressure would blow out any pressure vessel you can think of, and the engine would be MASSIVE, but it tells you that you're really not temp limited anymore. Note that this uses temp values used today...

you're mostly replacing turbine with nozzle. You do expand the nozzle some too, but mostly you just replace the turbine with more nozzle.

While I certainly see (and have seen) the theoretical possibility, I also see this as impractical beyond any normal use.

Thrust is limited by pressure, velocity and temperature not due to melting, but due to strength loss at high temperature. The blades are kept subsonic to avoid shocks, the heat at certain point would cause the materials to no longer be strong enough for the loads.

Increasing the pressure produces rapidly diminishing returns. I have some figures for expansion with either a given nozzle or an optimal nozzle at different pressures. For instance, the same idealized (successful combustion is assumed) JP-5+air engine will produce, with an optimal nozzle for each case, with different exhaust pressures:

15 psi - 0 thrust
30 psi - 70 units of thrust
75 psi - 104 units
150 psi - 123 units
300 psi - 140 units
600 psi - 152 units
1200 psi - 163 units
3000 psi - 176 units
30000 psi - 196 units

Now, you probably don't need any comments, but I struggle to think of them anyway.
30,000 psi is a firearm, 3,000 psi is SSME, 150-600 jet engines, 30-75 ramjets and such.
3,000 psi is about the proposed 1:250 OPR.

Different temperatures would change things more than pressure alone would, but at that point you're essentially back to his original proposal, which is simpler and better than a very-many-stages jet engine. At least it's better once you add an initial electric compressor.

I'll review those numbers whenever I get a chance, all my stuff is currently in FedEx. It does square broadly with what I know, but you run into structural issues before you flatline IIRC. Also, I'll need to check the nozzle stuff.

Additionally, I'm not suggesting 250, I'm just stating that this is what's required to push it to the temp limit. (also, the math is somewhat sketchy because you need compressibility in there, but ~100 would be achievable without too much trouble)

Relaxation of the first stage turbine is the primary failure mode you're worried about IIRC.

These figures are for an idealized engine, they are solid there. The units used are seconds, of course. Gas properties are accounted for. The nozzle is assumed to produce full expansion to atmospheric pressure, for higher pressure engines it's going to be larger.

So with these lots of stages and extreme pressures you get an engine that's way longer and easily 10 times heavier than a simple one, maybe more - and all you get to show for it is an extra 25% of thrust.

But... I thought that the turbine is the bit that is in the back of the engine. As in, the engine being a tube with the compressor, combustion chamber and turbine being lined up one after the other. How would I replace it with a nozzle?

That's what I am worried about as well. The thing should not be less efficient than a normal jet engine in terms of thrust per unit of mass/volume. Or else what's the point?

Well, long heavy engines present less frontal area for drag, so can give more top speed. But would you rather have a Mach 3 fighter with TWR of 0.4 or a Mach 1.5 one with TWR of 1.5?

Just use a set of short engines. Except you need further more area, like 3x that. Unless you need stealth, use just 2 engines, but each taller and wider than two current ones combined. If you need radar stealth, your TWR and speed will suffer.
Replace delta wings (not that great anyway) with diamond or trapezoid and make the wing pretty much end where the engines begin. To keep with the Whitcomb rule, you don't want anything else where the engines are.

Add a thick tail boom extending past the engines, place an extra sensor set there. Have an actual tail for maneuverability. The arrangement will be a little more like airliners, with the engines behind the wings and the tail further behind. The plane will spend a lot of its time at transonic speeds, only Mach 1.5, 1.7, maybe Mach 2 top speed depending on tech level in areas other than batteries, but able to spend whole missions there.

Ideally I am looking at the modern fighters range. So something like the F-22/F-35/T-50. And due to the nature of the conflicts this will be fighting in VTOL or at least STOL is preferable. (I intend to use this as a fighter dropped with an invading force to give air support and superiority for a planetary invasion.)


EDIT: What you said in the last bit pretty much hits it.
The plane will spend a lot of its time at transonic speeds, only Mach 1.5, 1.7 as you said. I would love to try and squeeze Mach 3 out of it. But definitively not as its operational speed.

Stealth is a pretty big factor with fighters. I was thinking of using 5 short engines in the back (you can see that image on page 1). With each having the same diameter inlet than modern jet engines.

But if the engines are short, as in 1/3 to 1/2 of the length of a normal engine. Won't that and what you just said result in really, really short wings? I am not sure I get it.

I thought that tailless + trust vectoring was good for stealth. Also, my battery time is fixed at 24h at cruising conditions. What ever that ends up meaning.

There are tradeoffs to everything. The setting seems to be otherwise low PMT (else you wouldn't care about stealth), so with this technology you can't. Except at the cost of ruining what good you get from it.
Mach 1.7 is going to be about the top speed. Think of it this way, it's the supercruise speed of F-22, it's fine to have the same supercruise speed. And you don't have afterburners, so supercruise speed is top speed.

Engines behind the wings, not under the wings. You'll keep a smooth top over them and under them, but no wings (per se) where the engines are. No control surfaces there either.
Your leading edge sweep is too high anyway, reduce it, get a lower wing chord.

Well, if maneuverability isn't very important. 2D TVC is good for stealth, not 3D.

F-22 has a V-tail, and trying to get a lower RCS than it has while remaining a viable fighter is not too productive. But if you want. Still, you want some kind of a tail, tailless is for UAV and bombers.

Stealth will newer go away, regardless how far we go into FT. The reason for this is simple. The only way to be less detectable than stealth is to get Star Trek like cloaking devices. And as a rule, these have to work by absorbing all emissions that come from the craft they are cloaking. Now that's fine for a starship or a tank. But fighter jets tend to rely massively on active sensors. And active sensors don't work well with having their emissions reabsorbed.

I understand that much. But considering the decrease in mass per engine here, could I not simply mount more engines like I intended (3 or maybe 4 instead of 2) and thus get more thrust and thus more speed? Assuming I can figure out a good way of mounting them that is. After all, fuel economy is a non issue.

So you mean like have the engines be in the middle of a "wing that isn't a wing". Sort of blended wing body style? Or do you mean literally to take a secondary pair of rear "wings" of some sort and mount them above and bellow the engine mounts whilst these are behind the main wing? I am not quite sure I understand.

I think I will go with the F-22 route than. Although I guess at this point I will have to draw up a more complex list of requirements to fallow. Basically what I am looking at is:

• Supermaneuverable
• Supercruising
• Stealth
• Capable of VTOL or at least STOL
• Good for both air superiority/control and ground strikes but with an emphasis on air to air fighting more than anything

Also, in terms of the engines. It seems that there are two conflicting approaches here. One is to go with a lighter engine that will have much less weight and length than a normal one but with about 20-25% less thrust. And the other seems to be to make one engine that would be as heavy/large (maybe heavier/larger? would it be heavier/larger? It sounds like it would from what you are saying.) than modern engines but have the same thrust. (All thrust being dry of course.) Did I get that right?

Not all sensors can be "stealthified" against. Despite the setbacks here and there, the history of warfare has been progressing towards easier detection, not harder. Sure, a modern F-22 is stealthier than a Sopwith... but it's still detected at a far longer range. There are no magical absorbers coming that will defeat everything.

The improvements in stealth are logarithmic, you find some aspect that has been neglected, patch it up, and brickwall there. The improvements in sensor effectiveness are radical, i.e. square, cubic, quadric root. On infinity, radical wins over logarithmic.

You start just fighting the drag of your own engines. Your drag at high speed mostly comes from them as it is.
And as speed increases, thrust decreases.

This isn't a way to make a super-fast fighter. For super-fast, NE's way is better. But this is a way to make a super-maneuverable one with massive TWR. For that you have to keep a low weight however.

Basically, you just place your engines right behind the wing. But keep them in square housings, so the tops lie flush with the wing. And are actually connected to its top surface, its extension.

The key word being on infinite. As it goes I don't think the point where sensors > stealth has been reached in the setting yet. I think at least. I shall check with the GM. In the meanwhile, I do believe that just like in defending against anything else, every bit counts.

I have to ask what TWR stands for. I have been googling it and searching for an answer but to no avail. My guess would be Thrust to Weight ratio or something along those lines. But is that it?

Oh... I get it now.

I just want to bump this.

Also, I would like to return to my original idea for a moment and further explain and expand upon it. What I am thinking is the fallowing. Take a pure electric engine like the one we talked about until now. Now, you know how engines are basically contained within a tube/box. Well, my idea is to install a massively powerful yet comparatively lightweight electric heaters into these walls. With insulation ensuring that the heat goes where it is supposed to. Alternatively the heater could be mounted in the expansion nozzle. Either way, the heaters would draw power from the main batteries and heat up the passing air to the kind of temperatures found in modern jet engines. Maybe even higher right up to the limits of the materials I am using. I certainly do have access to heaters that can achieve such temperatures and that are lightweight enough not to seriously weight down the engine. As in it won't be heavier than a compatible diameter jet engine for sure. And it would certainly be lighter and more mass efficient than a complicated coolant flow system. I know this would be very energy inefficient. But that is of no concern to me due to the initial assumption. And I can go a bit over with the mass since I just realized I might be saving as much as 90% of the fuel mass due to the light batteries.

So? Opinions?