Extremely high power to weight ratio: results.

[breadbasketbomb]

During the Second World War, aircraft had large powerful engines that to our standards, are actually week, inefficient and bulky. And it will be like that for engines of now, in the future. Today, we can modify modern engines, or make engines of very high output for there size compared to WW2. Today, an engine used to power aircraft that is fast to some extent in WW2 can be used in some street legal cars, or at least possible.

http://www.nelsonracingengines.com/cars-and-projects.html

Let’s go insane on this. 705 ci twin turbocharged V8 with 3000 horsepower. Multiply it by 4 because let’s say we are using it to replace a twin wasp which is 4 times the displacement, and we get around 12000 horsepower. Now it’s a wankel engine with he same displacement, and we get 24,000 horsepower. We have an aircraft that has a power to weight ratio of 1.4. Also the blades are swept.

If such an aircraft were to exist, and there are no mechanical issues. There are two I am considering actually, a conventional front mounted propeller blade, and a less conventional rear mounted version. How would those two fly?

And now I am going to take this to the extreme. Burt Rutan flies in with his flying Kevlar composite carpet and cast a spell upon the airplane and most of it’s structure is now carbon fiber and magnesium lithium alloys of the lightest type.

what would happen then?
all I know is that it would be really loud and fast, hell maybe even supersonic.

[Chevan]

@breadbasketbomb

And now I am going to take this to the extreme. Burt Rutan flies in with his flying Kevlar composite carpet and cast a spell upon the airplane and most of it’s structure is now carbon fiber and magnesium lithium alloys of the lightest type.

Hi breadbasketbomb.Its very interesting topic.
Do you mean D_ick Rutan’s Voayger?How exactly are you going to make this else to extreme?Any flying projects planns available?

[Nickdfresh]

My very limited knowledge tells me that “extreme” power often comes with a huge price in reliability, service life, ease-of-maintenance, and a whole hose of other issues–especially when we’re talking about lives of pilots whom aren’t easy to train and develop…,.

[breadbasketbomb]

@Nickdfresh

My very limited knowledge tells me that “extreme” power often comes with a huge price in reliability, service life, ease-of-maintenance, and a whole hose of other issues–especially when we’re talking about lives of pilots whom aren’t easy to train and develop…,.

Yes, but it’s a wankel engine so it will blow up. SO in the entirety of the time that we can fly this thing, about 10 minutes in the air what could we do.

[pdf27]

Lots of issues here:

1. Engine cooling. As engines get bigger, they get harder to cool. Higher power densities compound this process. I know for certain that Rolls-Royce bench tested engines of pretty similar power densities to this (the Rolls-Royce Crecy, a two-stroke producing equivalent to 5,000 bhp from 1536 cubic inches in single-cylinder tests). The majority of problems were to do with extreme overheating, even using sleeve valves (less problematic than poppet valves).
2. Absorbing the power. It is next to impossible to drive a propeller aircraft through the sound barrier - all sorts of nasty things happen at the blade tips, essentially turning most of your power to noise and heat and unless you have very short, stubby propeller blades probably ripping them off too. It should be noted that even with enormously powerful turboprop engines available today (11,000 BHP per engine on the A400M for instance) the fastest propeller aircraft around are still WW2 warbirds racing at Reno. More power in this case does NOT equal more speed.
3. Jet engines exist, and have since the early 1940s. They are massively more reliable than piston engines, much more pleasant to fly with (next to no vibration), and much more fuel efficient at high speed.
4. Reliability - big powerful piston engines running at high specific powers have very low reliability. Not the end of the world in a car race - you’ll accept losing the odd race in return for winning a lot more. In an aircraft, the same problem may well kill you.
5. Handling - single engined piston fighters, particularly late war ones with big engines, had vicious handling characteristics on takeoff. Add in the long nose (to house the big engine) and they would regularly kill inexperienced or tired pilots. Jet engines don’t have these problems, and adapt well to tricycle undercarriages.
6. Power to weight - if you a good power to weight ratio, you’re better off building a light aircraft with a light, reliable engine. There are lots of aircraft out there by the likes of Pitts, Yak or Extra with a power to weight ratio in excess of unity. You also get all sorts of handling and manoeuvrability benefits.

[breadbasketbomb]

@pdf27

Lots of issues here:

1. Engine cooling. As engines get bigger, they get harder to cool. Higher power densities compound this process. I know for certain that Rolls-Royce bench tested engines of pretty similar power densities to this (the Rolls-Royce Crecy, a two-stroke producing equivalent to 5,000 bhp from 1536 cubic inches in single-cylinder tests). The majority of problems were to do with extreme overheating, even using sleeve valves (less problematic than poppet valves).
2. Absorbing the power. It is next to impossible to drive a propeller aircraft through the sound barrier - all sorts of nasty things happen at the blade tips, essentially turning most of your power to noise and heat and unless you have very short, stubby propeller blades probably ripping them off too. It should be noted that even with enormously powerful turboprop engines available today (11,000 BHP per engine on the A400M for instance) the fastest propeller aircraft around are still WW2 warbirds racing at Reno. More power in this case does NOT equal more speed.
3. Jet engines exist, and have since the early 1940s. They are massively more reliable than piston engines, much more pleasant to fly with (next to no vibration), and much more fuel efficient at high speed.
4. Reliability - big powerful piston engines running at high specific powers have very low reliability. Not the end of the world in a car race - you’ll accept losing the odd race in return for winning a lot more. In an aircraft, the same problem may well kill you.
5. Handling - single engined piston fighters, particularly late war ones with big engines, had vicious handling characteristics on takeoff. Add in the long nose (to house the big engine) and they would regularly kill inexperienced or tired pilots. Jet engines don’t have these problems, and adapt well to tricycle undercarriage
6. Power to weight - if you a good power to weight ratio, you’re better off building a light aircraft with a light, reliable engine. There are lots of aircraft out there by the likes of Pitts, Yak or Extra with a power to weight ratio in excess of unity. You also get all sorts of handling and manoeuvrability benefits.

Thanks for all the information, that’s what I need to know. Except about the reliability, I know the aircraft would be a total wreck, except for the heating part. The engine is not bigger, it’s just as efficient as modern engines, but is still inefficient compared to a modern car engine. Though I do think the heating is going to be a major problem because we are talking about a 50 liter engine that has a power density of a modern wankel engine.

If I had to commit suicide, I would do it by flying this aircraft fast and hard, not by crashing, but by catastrophic failure in mid air.

I just had a new idea this mourning BTW.

Use the same engine I made up that produces too much horsepower, and use it as a heater in a boiler, with it’s drive shaft attached to a steam turbine. You get the image I’m seeing. This is how a hybrid engine should work dammit!

[pdf27]

Been done:
http://en.wikipedia.org/wiki/Napier_Nomad

Doing it with a steam cycle is only really practical in very large ships and more commonly power stations as the steam plant is EXTREMELY heavy compared to the rest of the engine. Most commonly done with gas turbines as diesel engine exhaust can be made cool enough (with turbocharging) that the efficiency gain from an additional steam cycle is minimal.

[breadbasketbomb]

@pdf27

Been done:
http://en.wikipedia.org/wiki/Napier_Nomad

Doing it with a steam cycle is only really practical in very large ships and more commonly power stations as the steam plant is EXTREMELY heavy compared to the rest of the engine. Most commonly done with gas turbines as diesel engine exhaust can be made cool enough (with turbocharging) that the efficiency gain from an additional steam cycle is minimal.

Well I have only one idea left for improving the performance of a propeller driven aircraft, two actually.

The first is a Ultra lean burn engine. I heard they went 40 miles to the gallon. If I had a B-36 peacemaker with lean burn engines of equivalent power to weight ratio, will that help the range by allot? It has too. I heard the only disadvantage was the complex catalytic converter to keep emissions low. Why do we need a even more complex (but gradually simplifying, so might as well not care) hybrid car. I don’t hate hybrid cars, it’s just… why the cant we just use lean burn engines with complex catalytic converters? Anyhow, back to the subject. Really how would a lean burn engine do.

The second is in regards to a non ww2 aircraft. The SR-71. What happens if we take the same engine pod with the spike, and instead of using a J58, what happens if we just used titanium counter rotating propellers making it a ducted fan, will the intake spike allows them to run for how fast and high?

[pdf27]

OK, you’ve got some very mixed up ideas here@

1. Lean burn engines. They weren’t deigned to increase fuel economy (that was a small side effect - can’t actually remember if it was actually any more efficient, I have a feeling it wasn’t), they were designed to have a low flame temperature inside the engine, achieved by using a leaner fuel mixture so the flame was diluted with air and burned cooler. This reduces NOx formation inside the engine, and gave you low emissions without the need for a catalytic converter. Also, they don’t have the same power to weight ratio - burning lean means you need a bigger combustion chamber, and hence a bigger/heavier engine to burn the same amount of fuel and generate the same amount of power.
   Hence, with a heavier engine cutting into your payload and roughly the same fuel efficiency, your range will be reduced.
2. SR-71: the engine inside the spike essentially runs as a ramjet at high speed. That more or less means that once you get to ~Mach 1.5 or so, you could take out everything but the afterburner section of that engine and it would still fly just fine. A ducted fan will probably get you up to this speed. You still run into the original problem though - the lightest and most efficient way of running this ducted fan is to attach it to the front end of a gas turbine engine! At that point, you’ve just reinvented the J58

[breadbasketbomb]

@pdf27

OK, you’ve got some very mixed up ideas here@

1. Lean burn engines. They weren’t deigned to increase fuel economy (that was a small side effect - can’t actually remember if it was actually any more efficient, I have a feeling it wasn’t), they were designed to have a low flame temperature inside the engine, achieved by using a leaner fuel mixture so the flame was diluted with air and burned cooler. This reduces NOx formation inside the engine, and gave you low emissions without the need for a catalytic converter. Also, they don’t have the same power to weight ratio - burning lean means you need a bigger combustion chamber, and hence a bigger/heavier engine to burn the same amount of fuel and generate the same amount of power.
   Hence, with a heavier engine cutting into your payload and roughly the same fuel efficiency, your range will be reduced.
2. SR-71: the engine inside the spike essentially runs as a ramjet at high speed. That more or less means that once you get to ~Mach 1.5 or so, you could take out everything but the afterburner section of that engine and it would still fly just fine. A ducted fan will probably get you up to this speed. You still run into the original problem though - the lightest and most efficient way of running this ducted fan is to attach it to the front end of a gas turbine engine! At that point, you’ve just reinvented the J58

Well I would have known that if someone made a simplified summary of it, which you have thankfully did and now I know.

And for the last part do you see what I am getting at? I’m trying to make a high altitude supersonic airplane that gives of minimal heat signatures, it’s a science fiction thing I have going on that I am aware, is not suitable for this sight. And are you sure it is the lightest and most efficient way of getting a ducted fan to get me up to mach 1.5? because car engines can end up becoming insanely powerful these days compared to the last of the piston engines built for speed and power, and relatively cheap because I can make it out of cast aluminum, not machined superalloys. Remember, Nelson Racing Engines?. Just to tell you, I don’t read books so that’s why I’m clueless.

[pdf27]

You’re being a little harsh on yourself - I have a Masters degree in this stuff, I haven’t just read a few books

For Mach 1.5, aerodynamic heating will start to become significant. I can’t remember what exact temperature they reach and my notes aren’t to hand right now (might check tonight if I remember), but I do remember that the cruising speed for Concorde was set by aerodynamic heating limits at Mach 2. For what I remember of Aerospace aluminium grades, that equates to around 100 deg C. The US wanted to go to Mach 3 with their competing aircraft, which required a Titanium structure.

Supercruising is possible (not just for the F-22, the English Electric Lightning in the early 1950s was the first), and that means you don’t need the very hot reheat. It also means exhaust gas temperatures aren’t really any hotter than for a standard low-bypass turbofan at subsonic speeds. With a bit of care (designing it to fly at high altitude and having the engine exhausts on the top of the wing) and your wing leading edge becomes the hottest thing going.

The other issue is how much heat each engine will put out. Assuming they are set up for the same cruising speed, the exhaust gas speed and mass flow rate will be about the same (optimum propulsive efficiency). That means the compression ratio in reality - the higher the compression ratio, the cooler the exhaust and hence the higher the efficiency. Racing car engines have a problem here - one of the ways they get high specific powers is to use turbo or supercharging, which let them get high power to weight ratios. Unfortunately, they also limit the compression ratio inside the engine (and petrol engines are inherently limited anyway as petrol will self-ignite before top dead centre if the compression ratio is too high). Gas turbine engines don’t have this problem - they use a heavier fuel and burn continuously around the injectors so could live with it anyway. Compression is instead limited by weight and diminishing returns for efficiency - if you add 1% to the weight of the aircraft to gain 1% in engine efficiency, that’s a net loss.

[breadbasketbomb]

@pdf27

You’re being a little harsh on yourself - I have a Masters degree in this stuff, I haven’t just read a few books

For Mach 1.5, aerodynamic heating will start to become significant. I can’t remember what exact temperature they reach and my notes aren’t to hand right now (might check tonight if I remember), but I do remember that the cruising speed for Concorde was set by aerodynamic heating limits at Mach 2. For what I remember of Aerospace aluminium grades, that equates to around 100 deg C. The US wanted to go to Mach 3 with their competing aircraft, which required a Titanium structure.

Supercruising is possible (not just for the F-22, the English Electric Lightning in the early 1950s was the first), and that means you don’t need the very hot reheat. It also means exhaust gas temperatures aren’t really any hotter than for a standard low-bypass turbofan at subsonic speeds. With a bit of care (designing it to fly at high altitude and having the engine exhausts on the top of the wing) and your wing leading edge becomes the hottest thing going.

The other issue is how much heat each engine will put out. Assuming they are set up for the same cruising speed, the exhaust gas speed and mass flow rate will be about the same (optimum propulsive efficiency). That means the compression ratio in reality - the higher the compression ratio, the cooler the exhaust and hence the higher the efficiency. Racing car engines have a problem here - one of the ways they get high specific powers is to use turbo or supercharging, which let them get high power to weight ratios. Unfortunately, they also limit the compression ratio inside the engine (and petrol engines are inherently limited anyway as petrol will self-ignite before top dead centre if the compression ratio is too high). Gas turbine engines don’t have this problem - they use a heavier fuel and burn continuously around the injectors so could live with it anyway. Compression is instead limited by weight and diminishing returns for efficiency - if you add 1% to the weight of the aircraft to gain 1% in engine efficiency, that’s a net loss.

Well, here is what I am trying to do for the sake of science fiction and bad literature that litters the internet (my immortal… anyone?). I will leave out the plot and only give out some setting. A given civilization is staring at 1980’s America, and said that they want an SR-71. At the time all they got is piston driven aircraft and a giant radio station to talk to humans. How will they get an SR-71 equivalent with only a propeller driven aircraft. I’m sure that all problems stated with the motor can be fixed with engineering (I heard the me 262 can run on diesel), and ignoring efficiency not by adding weight, just making the engine more modern. OK I’m changing the priority.

I dont care about heat, or efficiency anymore (unless it ends up being less efficient than a J58).

Is there a way to make a SR-71 competitor in a car factory is all I want in it’s most basic form. The point is that gas turbines are much more complex than piston engines or wankel engines (which I want to use). Is it possible to use aviation fuel in any of the engines. I believe that just because we had a given technology in the past does not mean we can do it then. And for the temperature aah, just leave that there, fix the thing later. The only solution I have created is using diesel, and with engineering it can at high speeds and so and so. All I have is one question now that I scrapped all other wanted benefits that wont come: How do I make it go mach 1.5 or up. The original concern I had was with the ducted fan.

And about the last part, what do you mean by specific power? like power to weight ratio, or fuel efficiency. I just want the fan to spin fast enough now.

[pdf27]

@BreadBasketBomb

Well, here is what I am trying to do for the sake of science fiction and bad literature that litters the internet (my immortal… anyone?). I will leave out the plot and only give out some setting. A given civilization is staring at 1980’s America, and said that they want an SR-71. At the time all they got is piston driven aircraft and a giant radio station to talk to humans. How will they get an SR-71 equivalent with only a propeller driven aircraft. I’m sure that all problems stated with the motor can be fixed with engineering (I heard the me 262 can run on diesel), and ignoring efficiency not by adding weight, just making the engine more modern. OK I’m changing the priority.

I dont care about heat, or efficiency anymore (unless it ends up being less efficient than a J58).

Is there a way to make a SR-71 competitor in a car factory is all I want in it’s most basic form. The point is that gas turbines are much more complex than piston engines or wankel engines (which I want to use). Is it possible to use aviation fuel in any of the engines. I believe that just because we had a given technology in the past does not mean we can do it then. And for the temperature aah, just leave that there, fix the thing later. The only solution I have created is using diesel, and with engineering it can at high speeds and so and so. All I have is one question now that I scrapped all other wanted benefits that wont come: How do I make it go mach 1.5 or up. The original concern I had was with the ducted fan.

And about the last part, what do you mean by specific power? like power to weight ratio, or fuel efficiency. I just want the fan to spin fast enough now.

Hmmm…. Best bet I would say would be a composite aircraft, the mother aircraft having piston engines to get off the ground and the daughter aircraft only a ramjet for speed. Ramjets need a minimum speed to start (quite high, but achievable for WW2-era aircraft – exact speed depends on the design) and then if designed right can reach up to Mach 2-3. Conceptually they’re pretty simple (the first patents were about 5 years after the Wright brothers). This would have the mother aircraft climbing to altitude and reasonable speed, with the daughter aircraft then being dropped to dive steeply and start the engine. It would then run the mission on ramjet only, and glide into land (much like the Me-163 in WW2 in some ways). That’s actually a pretty simple aircraft, probably shaped something like a Bell X-1 or Miles Messenger (fuselage shaped like a bullet, which is known to be stable supersonically, and very thin straight wings which all the experience with piston engined aircraft suggested would cause the fewest problems).
Trying to replicate a jet engine with ducted fans and piston engines can be done, but the results are pretty awful – stick “Coanda engine” into google for details. It’s a nice concept, but the actual practicalities are horrible – you end up essentially having to reinvent the jet engine to get it to work.
Specific power is the power output of an engine per unit mass – it was only when I googled it to work out why I was unclear that I realised there are other uses for the term.

[breadbasketbomb]

@pdf27

Hmmm…. Best bet I would say would be a composite aircraft, the mother aircraft having piston engines to get off the ground and the daughter aircraft only a ramjet for speed. Ramjets need a minimum speed to start (quite high, but achievable for WW2-era aircraft – exact speed depends on the design) and then if designed right can reach up to Mach 2-3. Conceptually they’re pretty simple (the first patents were about 5 years after the Wright brothers). This would have the mother aircraft climbing to altitude and reasonable speed, with the daughter aircraft then being dropped to dive steeply and start the engine. It would then run the mission on ramjet only, and glide into land (much like the Me-163 in WW2 in some ways). That’s actually a pretty simple aircraft, probably shaped something like a Bell X-1 or Miles Messenger (fuselage shaped like a bullet, which is known to be stable supersonically, and very thin straight wings which all the experience with piston engined aircraft suggested would cause the fewest problems).
Trying to replicate a jet engine with ducted fans and piston engines can be done, but the results are pretty awful – stick “Coanda engine” into google for details. It’s a nice concept, but the actual practicalities are horrible – you end up essentially having to reinvent the jet engine to get it to work.
Specific power is the power output of an engine per unit mass – it was only when I googled it to work out why I was unclear that I realised there are other uses for the term.

I do not know why I should compare the design to the the Coanda 1910. The Coanda is a jet powered aircraft, albeit a bad one. The aircraft I am using looks like a normal single engine jet (most comparable a mig 21). The design is best comparable to the Stipa-Caproni flying beer can, and at this point I hope you got the image. What I am about to state is a second problem other than just engine design (which I believe I may have fixed by using a twin turbocharged -and supercharged- diesel wankel engine producing some 10 thousand horsepower or far more.); and that issue is drag. The venturi tube of both designs should increase engine efficiency, but the Stipa caproni had an issue with aerodynamic drag. I do not know the source of the drag, is it the fact that the airplane is extremely stubby, or is it something else, like the mass flow rate of the vanturi tube being to small to fill an a vacuum, or something else. To states again, it’s like a Mig 21 with the engine replaced with multiple supersonic propellers inside a longer, sleeker venturi tube.

[pdf27]

OK, I’ve got the idea of what you’re thinking of now. As always, there’s good news and bad news.

Good news is that you can forget about having to invent a supersonic propeller - using the duct design you can fit a conventional shock cone or similar to slow the airflow down to subsonic speeds before it reaches the propeller (all current jet aircraft do this too - they require subsonic airflow at the engine inlet). Further good news is that doing this with a converging-diverging nozzle will give you supersonic airflow at the exhaust and potentially very high speeds.

Bad news is that you’re limited to two bad options aerodynamically. The drag problem that Caproni had is most likely due to the very high wetted area of the aircraft - it had a lot more skin than a similarly sized conventional aircraft, and hence a lot more skin friction. At high speeds this gets worse - wave drag (from shockwaves) is a function of how fat the body you’re pushing through the air and how fast the cross-sectional area changes. Anything with a large cross-sectional area (required unless you’re going to be getting enormous thrust per unit mass of air) will have so much drag you won’t be able to break the sound barrier without additional thrust (rockets).
The second option is to accept that you’re going to be getting low propulsive efficiencies (i.e. the air leaving the engine will be at very high speed indeed to get the thrust needed). This enables you to run your shaft compressor at high pressure ratios (cutting down on your air mass flow needed and hence cross sectional area of the front of the aircraft). The high pressures can then be converted to extremely high exhaust velocities, and hence relatively large amounts of thrust per unit area.

For working out your thrust, this calculator should prove helpful - http://www.grc.nasa.gov/WWW/k-12/airplane/ienzl.html . If you look for an aircraft of similar size and performance to what you’re looking for (so if it’s a MiG-21 use that), that will give you required thrust for the performance you want. For a MiG-21 that’s 70kN on a 1.1m diameter. That gives 70kg/sec of air at a compression ratio of 4.8:1 - conveniently with a much smaller diameter. Stealing somebody else’s spreadsheet (because it’s too early this morning to work it out by hand and the cat is lying on me!) gives a power requirement of ~22,000 hp. In reality you will probably need more like 15,000 hp or so as for engines this big the exhaust gas provides a very significant amount of thrust.
The NASA calculator doesn’t seem to benefit from increasing the temperature of the gas flowing through the nozzle, which seems odd to me - I would have expected a significant thrust benefit from burning fuel in the compressed air before it hits the nozzle. I’ll have to think about why that might be if I get around to it today.

[breadbasketbomb]

@pdf27

OK, I’ve got the idea of what you’re thinking of now. As always, there’s good news and bad news.

Good news is that you can forget about having to invent a supersonic propeller - using the duct design you can fit a conventional shock cone or similar to slow the airflow down to subsonic speeds before it reaches the propeller (all current jet aircraft do this too - they require subsonic airflow at the engine inlet). Further good news is that doing this with a converging-diverging nozzle will give you supersonic airflow at the exhaust and potentially very high speeds.

Bad news is that you’re limited to two bad options aerodynamically. The drag problem that Caproni had is most likely due to the very high wetted area of the aircraft - it had a lot more skin than a similarly sized conventional aircraft, and hence a lot more skin friction. At high speeds this gets worse - wave drag (from shockwaves) is a function of how fat the body you’re pushing through the air and how fast the cross-sectional area changes. Anything with a large cross-sectional area (required unless you’re going to be getting enormous thrust per unit mass of air) will have so much drag you won’t be able to break the sound barrier without additional thrust (rockets).
The second option is to accept that you’re going to be getting low propulsive efficiencies (i.e. the air leaving the engine will be at very high speed indeed to get the thrust needed). This enables you to run your shaft compressor at high pressure ratios (cutting down on your air mass flow needed and hence cross sectional area of the front of the aircraft). The high pressures can then be converted to extremely high exhaust velocities, and hence relatively large amounts of thrust per unit area.

For working out your thrust, this calculator should prove helpful - http://www.grc.nasa.gov/WWW/k-12/airplane/ienzl.html . If you look for an aircraft of similar size and performance to what you’re looking for (so if it’s a MiG-21 use that), that will give you required thrust for the performance you want. For a MiG-21 that’s 70kN on a 1.1m diameter. That gives 70kg/sec of air at a compression ratio of 4.8:1 - conveniently with a much smaller diameter. Stealing somebody else’s spreadsheet (because it’s too early this morning to work it out by hand and the cat is lying on me!) gives a power requirement of ~22,000 hp. In reality you will probably need more like 15,000 hp or so as for engines this big the exhaust gas provides a very significant amount of thrust.
The NASA calculator doesn’t seem to benefit from increasing the temperature of the gas flowing through the nozzle, which seems odd to me - I would have expected a significant thrust benefit from burning fuel in the compressed air before it hits the nozzle. I’ll have to think about why that might be if I get around to it today.

OK so here is the design in my head:

A tube shaped, delta winged, relatively small sized fighter when compared to an F-15. It has a 46 liter, 24,000 horsepower diesel wankel engine to power 4 counter rotating propellers inside the long venturi tube, each becoming more oriented towards the front from the first to the last. And at high enough speeds, the wankel engine exhaust is producing enough thrust to go to mach 1.5 or up. Well, is it less efficient than a cold war turbojet? We got the speed, aerodynamics is fixed (because the airplane I am speaking of is not a flying beer can), is it efficient enough?

[pdf27]

@breadbasketbomb

OK so here is the design in my head:A tube shaped, delta winged, relatively small sized fighter when compared to an F-15. It has a 46 liter, 24,000 horsepower diesel wankel engine to power 4 counter rotating propellers inside the long venturi tube, each becoming more oriented towards the front from the first to the last. And at high enough speeds, the wankel engine exhaust is producing enough thrust to go to mach 1.5 or up. Well, is it less efficient than a cold war turbojet? We got the speed, aerodynamics is fixed (because the airplane I am speaking of is not a flying beer can), is it efficient enough?

OK, so a fairly simple UAV engine using current technology can do ~250 bhp/litre (Wankel-type engine). You’re looking at twice that per litre – really pushing it and would probably be more credible if you dropped it back a bit, but it certainly doesn’t need the intervention of Skippy the Alien Space Bat.You’ve got the wrong end of the stick with the propellers. I’ll try to explain - but it’s pretty complicated so bear with me.There is a concept in aerodynamics known as choking or choked flow. This is the concept that the maximum mass flow rate past a point will be at exactly the speed of sound. To get the air to go faster, you need to suck rather than blow, and this can practically only be done by using a converging – diverging nozzle. For the thrust and frontal area you’re after, that means the pressure upstream needs to be about 4.5 times the pressure downstream. A centrifugal compressor could to this in a single stage, you’re looking at 8 or 9 for an early axial compressor.One of the oddities of this is that it really doesn’t matter what speed the upstream air goes through the compressor at – you just change the converging part of the nozzle slightly. This makes life a LOT easier, since you don’t need to worry about transonic flow in the compressor and all the losses that will give you, but instead means you need to pay attention to the air inlets (that’s what all the weird and wonderful air inlet spikes do that you see in early supersonic jets).For that power and size, you’re looking at something with similar performance to a MiG-21, maybe a bit better but with shorter range/less payload. Fuel efficiency will be worse and empty weight will be a lot higher, but it should reach the speeds you need. Personally I think your biggest problem will be engine reliability and cooling – the rest of it will probably not be a big deal.

[breadbasketbomb]

@pdf27

OK, so a fairly simple UAV engine using current technology can do ~250 bhp/litre (Wankel-type engine). You’re looking at twice that per litre – really pushing it and would probably be more credible if you dropped it back a bit, but it certainly doesn’t need the intervention of Skippy the Alien Space Bat.You’ve got the wrong end of the stick with the propellers. I’ll try to explain - but it’s pretty complicated so bear with me.There is a concept in aerodynamics known as choking or choked flow. This is the concept that the maximum mass flow rate past a point will be at exactly the speed of sound. To get the air to go faster, you need to suck rather than blow, and this can practically only be done by using a converging – diverging nozzle. For the thrust and frontal area you’re after, that means the pressure upstream needs to be about 4.5 times the pressure downstream. A centrifugal compressor could to this in a single stage, you’re looking at 8 or 9 for an early axial compressor.One of the oddities of this is that it really doesn’t matter what speed the upstream air goes through the compressor at – you just change the converging part of the nozzle slightly. This makes life a LOT easier, since you don’t need to worry about transonic flow in the compressor and all the losses that will give you, but instead means you need to pay attention to the air inlets (that’s what all the weird and wonderful air inlet spikes do that you see in early supersonic jets).For that power and size, you’re looking at something with similar performance to a MiG-21, maybe a bit better but with shorter range/less payload. Fuel efficiency will be worse and empty weight will be a lot higher, but it should reach the speeds you need. Personally I think your biggest problem will be engine reliability and cooling – the rest of it will probably not be a big deal.

We are almost at the finish line. With creativity, our alien friends (I did not create them, someone in Japan did) which I will not reveal may be able to fix the engine problem, and that is only up to me or someone else who finds an application in a wankel engine that works reliably 60,000 feet in the air and at mach 1.5. As for the efficiency how less efficient is it when compared to the MiG-21, which had a range of about 1000 miles. Does this airplane have 700 miles? 500 miles?
300 miles! DON’T TELL ME 100 MILES DON’T TELL ME IT’S A PLANE THAT GOES A HUNDRED MILES ON THAT MUCH FUEL… nah you tell me how much range it has and we should be done. Unless you give me another idea…

[pdf27]

Working at altitude isn’t a problem - provided it’s mainly spending it’s time at that height then you just fit a higher pressure supercharger on the engine (or even just use bleed air from the big compressor you’re using to provide propulsion, which would be my favoured option.
Fuel efficiency is a bit of a grey area - it mostly depends on how light you can make everything else, the problem is weight not how much you can cram in. If you go for really lightweight engines (I saw a UAV engine the other day that does 1 hp/lb - look up cubewano) and don’t require it to be able to pull lots of g-force, then you could probably reach the same range. Certainly it should be able to do 500 miles without major problems.

[breadbasketbomb]

@pdf27

Working at altitude isn’t a problem - provided it’s mainly spending it’s time at that height then you just fit a higher pressure supercharger on the engine (or even just use bleed air from the big compressor you’re using to provide propulsion, which would be my favoured option.
Fuel efficiency is a bit of a grey area - it mostly depends on how light you can make everything else, the problem is weight not how much you can cram in. If you go for really lightweight engines (I saw a UAV engine the other day that does 1 hp/lb - look up cubewano) and don’t require it to be able to pull lots of g-force, then you could probably reach the same range. Certainly it should be able to do 500 miles without major problems.

OK so the guys working in the other planet has an interim solution to getting a jet engine. 10 years later, they get a functioning jet engine. But for working out the kinks and some ingenuity (which will remain a mystery), they extend it’s service life to 30 years. Hooray!

Actually I forgot, there is still one question left. Cost.

At the time, these guys are pre ww2 level tech. I think this airplane will be a cost effective solution to not having a viable jet engine YET.