Is there speed/height limit for air-breathing engines?

[raxo2222]

KSP AJE mod allows engines to run at up to 5 mach and 25 kilometers,

KSP Interstellar mod allow higher speeds and altitudes for air breathing mode.

What are limits for real world good old propeller / turbofan / turbojet / ramjet / scramjet / future (KSPI Interstellar) engines?

Edited November 5, 2016 by raxo2222

[Shpaget]

Each engine has its limit, but it's not a strict line. As you go higher you start losing performance and need to go faster, but you can only go so fast before you lose performance due to supersonic/hypersonic air flow.

There is optimal regime and suboptimal and engineers need to find a balance for each aircraft/engine/payload/flight plan combination.

If you want to know specs of a specific engine, wikipedia usually is enough, but your question is too broad for a simple reply.

[raxo2222]

1 hour ago, Shpaget said:

Each engine has its limit, but it's not a strict line. As you go higher you start losing performance and need to go faster, but you can only go so fast before you lose performance due to supersonic/hypersonic air flow.

There is optimal regime and suboptimal and engineers need to find a balance for each aircraft/engine/payload/flight plan combination.

If you want to know specs of a specific engine, wikipedia usually is enough, but your question is too broad for a simple reply.

Well my question is: How fast aircraft can travel with these engines before they fail due to high dynamic pressure/low static pressure/high temperature.

[SargeRho]

As far as I know, High-Bypass turbofans, that's the ones used by passenger aircraft, stop working at the speed of sound, Jet Fighters use low-bypass turbofans, or turbojets, and have specially shaped intakes, and intake flaps to slow down the incoming air. Jet Fighters still can't keep their top speed up for long without overheating the engines, though. Ramjets and Scramjets need to be moving at high speed, and supersonic speed respectively, to work.

[Green Baron]

Very rough numbers, depending on circumstances (aircraft, atmosphere), don't nail me to the cross with these numbers:
Piston engine uncharged 15.000ft, turbocharged 25.000ft, turboprop 30.000ft, turbofan 35.000ft. Can be more, can be less, but not much ... none of these aircrafts work at the speed of sound. You want a number ? Piston engines (WWII) mach 0.5, Turboprop mach 0.7, turbofans could principally work low supersonic, as said not the airliner ones.

A typical Cessna/Piper/Soccata 2-4 seater goes at 80-120knots, a small turboprop 250-350knots, airliners 450, 480 if in a hurry. Legal speed limits apply ;-)

No idea about jets ...

Aircraft failure at high speeds might not be due to the engine but to the structure ... you know, structural integrity and so on ... :-)

If you want specific numbers on a special aircraft (service ceiling, cruising speed), i'm sure you can find them ... each aircraft has a Vne, never exceed speed, a red dial on the airpseed indicator. It's construction is so that above that filght stability and structural integrity can not be guaranteed, like flaps flying off for example or wings saying good-bye to the rest due to high load. Bad things can happen :-)

 

Edited November 5, 2016 by Green Baron

[Steel]

Broadly speaking:

Turbofan: will stop working efficiently somewhere in the transonic region (0.8<M<1.2) due to shock wave build up on the turbine blades.

Turbojets: somewhere above Mach 1 they'll start becoming inefficient due again to the fact they rely mainly on blades for compression.

Ramjet: dependent almost entirely on intake geometry. Will stop working efficiently once intake can no longer slow the air to subsonic speeds, or compresses the air so much by doing so that it becomes too hot to use efficiently.

Scramjet: again, dependent almost entirely on intake geometry. A scram jet designed to go at Mach 5 will no longer be efficient once it goes a reasonable amount faster or slower than that.

 

Morale of the story: Max speed is dependent on individual engine design. Actual engine failure is highly unlikely, rather the engine will become so inefficient and so produce so little thrust that it will just slow back down to a more efficient speed.

[raxo2222]

What about exotic designs like VASMIR or ARCJET just like in @FreeThinker Interstellar mod?

Can these work at even higher speeds and altitude than SCRAMJET? (they would be inefficient at low speed/altitude)

Can magnetic fields protect engine insides against very hot air flow and make engine work?

 

[Guest]

27 minutes ago, raxo2222 said:

What about exotic designs like VASMIR or ARCJET just like in @FreeThinker Interstellar mod?

Can these work at even higher speeds and altitude than SCRAMJET? (they would be inefficient at low speed/altitude)

Can magnetic fields protect engine insides against very hot air flow and make engine work?

 

VASIMR don't work in atmosphere at all. All that you will get is blown fuses and a lots of bad smell.

ARCJET ... may be. With poor ISP and thrust.

Both of them are intended to be used in vacuum only, thus velocity is irrelevant.

About magnetic field protection ... don't know, probably no-one tried it. Probably will not help at all.

[raxo2222]

39 minutes ago, WildLynx said:

VASIMR don't work in atmosphere at all. All that you will get is blown fuses and a lots of bad smell.

ARCJET ... may be. With poor ISP and thrust.

Both of them are intended to be used in vacuum only, thus velocity is irrelevant.

About magnetic field protection ... don't know, probably no-one tried it. Probably will not help at all.

Well for VASMIR I was thinking about static pressure in range of 1 - 1000 pa. Also powered by fusion power plant either directly or trough microwave network.

[magnemoe]

2 hours ago, Green Baron said:

Very rough numbers, depending on circumstances (aircraft, atmosphere), don't nail me to the cross with these numbers:
Piston engine uncharged 15.000ft, turbocharged 25.000ft, turboprop 30.000ft, turbofan 35.000ft. Can be more, can be less, but not much ... none of these aircrafts work at the speed of sound. You want a number ? Piston engines (WWII) mach 0.5, Turboprop mach 0.7, turbofans could principally work low supersonic, as said not the airliner ones.

A typical Cessna/Piper/Soccata 2-4 seater goes at 80-120knots, a small turboprop 250-350knots, airliners 450, 480 if in a hurry. Legal speed limits apply ;-)

No idea about jets ...

Aircraft failure at high speeds might not be due to the engine but to the structure ... you know, structural integrity and so on ... :-)

If you want specific numbers on a special aircraft (service ceiling, cruising speed), i'm sure you can find them ... each aircraft has a Vne, never exceed speed, a red dial on the airpseed indicator. It's construction is so that above that filght stability and structural integrity can not be guaranteed, like flaps flying off for example or wings saying good-bye to the rest due to high load. Bad things can happen :-)

 

Note that fuel efficiency and cost is critical here, small private planes are slow as they are cheap as airplanes goes, going faster and you need an far larger and more expensive engine so you need an larger plane who is also more expensive. 
And yes planes has an maximum speed because of air-frame, was in an private plane who went into an steep dive after passing over an cliff, I and the other passanger was afraid we would crash, the pilot was afraid of passing the Vne as then the plane he had rented had to do an major overhaul and he would probably not be allowed to rent a plane from the club again. 

[Shpaget]

1 hour ago, raxo2222 said:

Also powered by fusion power plant either directly or trough microwave network.

Neither exists. Also, what does it matter how is an electric engine powered?

The maximum speed of an aircraft is usually limited by airframe, not the engine. Either the airframe is not designed for excessive speeds or it's not economical to fly faster then they do. The engines that propel that aircraft could go faster if they were mounted on a different body.

Turbofans are not good for supersonic, but jet fighter engines (turbojets) could operate at higher speeds than the aircraft, if they were mounted on a more streamlined body.

VASIMR (not VASMIR) is strictly a vacuum engine and as such will not operate in atmosphere. In vacuum it doesn't have a max speed.

Same goes for Arcjet.

Why do you keep on mixing speed, thrust, air breathing and rocket engines? What do you want with this topic?

Edited November 5, 2016 by Shpaget

[raxo2222]

7 hours ago, Shpaget said:

Neither exists. Also, what does it matter how is an electric engine powered?

The maximum speed of an aircraft is usually limited by airframe, not the engine. Either the airframe is not designed for excessive speeds or it's not economical to fly faster then they do. The engines that propel that aircraft could go faster if they were mounted on a different body.

Turbofans are not good for supersonic, but jet fighter engines (turbojets) could operate at higher speeds than the aircraft, if they were mounted on a more streamlined body.

VASIMR (not VASMIR) is strictly a vacuum engine and as such will not operate in atmosphere. In vacuum it doesn't have a max speed.

Same goes for Arcjet.

Why do you keep on mixing speed, thrust, air breathing and rocket engines? What do you want with this topic?

Well ARCJET and VASMIR can use atmosphere in Interstellar mod.

and I just wanted to know what is speed barrier for aircraft/spaceplane - when they have to switch to rocket engines (those fastest only), if they want reach space.

Electrical engines are more like "what if" inspired by Interstellar mod.

[Nibb31]

The record for a jet aircraft is 123 500 ft on a modified Mig-25. An F-104 reached 120 000 ft. I wouldn't call that "flight" really, because reaching those altitudes is basically just flinging the aircraft as high as possible. The U-2 could do steady flight at 70000 ft and the SR-71 flew up to 85000 ft at 3500km/h, which is pretty much a record for unassisted level flight.

The scramjet-powered X-43 reached Mach 10 at 98000 ft, but needed a Pegasus rocket to reach Mach 7.

 

 

[Shpaget]

There's an Air Traffic Joke (may be a real thing, who knows) involving an SR-71 pilot and an inattentive ATC

The pilot requests FL600.
ATC: Buddy, if you can get up there, you can have it.
Pilot: Roger that, descending to FL600.

A little explanation for those who are not into aviation:
For any change in altitude (among other things) inside controlled airspace, and to enter the controlled airspace, a pilot must be cleared by Air Traffic Control and be given permission before the change. Controlled airspace extends from about (may vary between countries) 1000 feet to usually about 60 000 feet (also called Flight Level 600, or FL600).
Since the SR-71 was above FL600 he was not in controlled airspace and was not in contact with ATC beforehand.

No one flies at FL600. Even the highest flying airliners top out at around FL420. SR-71 being a notable exception, though quite a rare sight.

Of course, a proper request for FL600, like in this scenario, would necessarily include much more info, such as position and altitude. If no flight plan was filed, then also aircraft type and other data. Suffice to say, ATC would understand what is going on, so I classify this as a joke and not an anecdote.

Also, military aircraft are controlled by military ATC (who may be sitting right next to civil ones, as is the case here where I live), who would surely know of any military activity.

Edited November 6, 2016 by Shpaget

[cantab]

Is there any theoretical speed limit for a scramjet, apart from those imposed by the properties of the materials it could be made of?

[p1t1o]

19 hours ago, cantab said:

Is there any theoretical speed limit for a scramjet, apart from those imposed by the properties of the materials it could be made of?

Its hard to answer quantitatively questions that have magical premises such as "apart from those imposed by the properties of the materials it could be made of" as we are now operating with a massless engine capable of surviving many many thousands of degrees with no change in structural properties. With this material, you could make some really fancy stuff...

But considering an air-breather for the moment:

Thermodynamics starts to set a hard ceiling. Essentially, the faster your regime, the hotter the air entering the combustion process - simply by being compressed by the passage of the craft, apart from any required compression within the engine. The hotter the air incoming, the more energy you have to dump into it in order to get a decent expansion and extract work, and as you get faster, chemical combustion stops being able to provide enough energy to raise the temperature of incoming air very much. At this point we are in or near the regime where we are talking about temperatures that will ionise atoms and split molecules apart, so even getting combustion to *work* gets harder.

There are also issues involved with the fact that your aircraft will now be travelling around as fast as molecules in a reaction, which makes sustaining combustion even harder, possibly can be tackled with fancy engineering, though anything that adds even a hair of drag is going to be dragging that ceiling down.

This is where SABRE engines generate a significant part of their benefit - even though they (nor any other engine) do not operate anywhere near these limiting conditions - by cooling incoming air you are increasing the amount of work that can be done by the engine for a given core temperature. (SABRE also gets benefit from not having to operate at limit-conditions, so lighter metals can be used in construction, significantly benefiting its TWR.)

So even if you had a perfectly heat-resistant scramjet (which also needs to be lightweight and assuming you have solved the supersonic combustion problem itself) you will still hit a limit set by the amount of energy that can be stored in chemical bonds. You might find some new exotic fuel that gives you a few more joules per oxygen molecule burned, but it will only be incremental benefits. 

Edited November 7, 2016 by p1t1o

[wumpus]

On 11/5/2016 at 7:50 AM, SargeRho said:

As far as I know, High-Bypass turbofans, that's the ones used by passenger aircraft, stop working at the speed of sound, Jet Fighters use low-bypass turbofans, or turbojets, and have specially shaped intakes, and intake flaps to slow down the incoming air. Jet Fighters still can't keep their top speed up for long without overheating the engines, though. Ramjets and Scramjets need to be moving at high speed, and supersonic speed respectively, to work.

As far as I know, the Concorde's engines were pretty much standard turbojets* (or whatever was state of the art in the 1970s), only with a restricted intake and larger fan areas to keep the air moving through the engine at less than mach 1.  This apparently works fine up to the mach 2 or so that the Concorde flew.  The SR-71 goes to much greater extremes to get to mach 3, and at that point is largely acting as a ramjet engine.  I can't be sure if the engine or the body of the plane is the limit for the top speed (the plane is pretty much all titanium and/or unobtanium, there's little chance that anything could be designed with much extra headroom).  Also the SR-71 [nearly] always ran on afterburners, with the exceptions involving refueling and then requiring a shot of [limited] igniting fuel to get the JP-7 burning again.  Presumably the cost of JP-7 is still classified (to keep hordes of angry taxpayers from storming the Pentagon), but has been said to be similar to a "good scotch".  Don't expect to see the like of either craft soon.

On 11/5/2016 at 5:29 PM, raxo2222 said:

Well for VASMIR I was thinking about static pressure in range of 1 - 1000 pa. Also powered by fusion power plant either directly or trough microwave network.

VASMIR won't work in an atmosphere (as already mentioned).  Microwave power would have issues of attenuation of power as the plane got further from the microwave source, not any altitude effects.  Atmospheric dynamics had a project based on microwave power that has since shut down, but perhaps the dream is only dormant  while the US Navy works on "killer laser beams" that could provide the needed power (warning: I suspect the Navy is willing to maximize power by producing it in milliseconds.  This prevents incoming rockets to survive merely by spinning.  To fly into space you are going  to need minutes of outrageous amounts of power, meaning the Navy's work might not help at all).

On 11/6/2016 at 2:19 AM, Nibb31 said:

The record for a jet aircraft is 123 500 ft on a modified Mig-25. An F-104 reached 120 000 ft. I wouldn't call that "flight" really, because reaching those altitudes is basically just flinging the aircraft as high as possible. The U-2 could do steady flight at 70000 ft and the SR-71 flew up to 85000 ft at 3500km/h, which is pretty much a record for unassisted level flight.

The scramjet-powered X-43 reached Mach 10 at 98000 ft, but needed a Pegasus rocket to reach Mach 7.

The scramjet hit 9.6, and I suspect the Pegasus to "mach 7" was an earlier flight.  Looking at a NASA paper ( https://hapb-www.larc.nasa.gov/Public/Documents/AIAA-2006-1-317.pdf ), the X-43 barely had positive acceleration at any point in the flight (the graph isn't labeled, but any acceleration would be a tiny fraction of the expected acceleration).  While this certainly shows that mach ~10 is within the limits for airbreathing vehicles, it is equally clear that more research is needed before NASA (or anyone else) designs one.  But yes, 4 figure ISPs to a significant fraction of orbital velocity could change a lot of aerospace economics.

On 11/5/2016 at 5:10 AM, raxo2222 said:

What are limits for real world good old propeller / turbofan [the rest has mostly been covered in this thread]

The big catch for propeller design is that allowing the propeller tips to hit supersonic speeds is typically harmful to to the propellers and always harmful to anyone in earshot (which is a *long* way considering how loud this can be).  Supposedly there is a plane made in the old USSR that has [intentionally] super sonic propellers and it is the noisiest plane on the planet.  From the sound of it turboprops should be an effective means of moving an aircraft, but are sufficiently expensive that people simply go all the way to jets (well, high-bypass turbofans).

Turbofans are typically what is on a modern "jet".  Expect the same issues in going supersonic (although the wiki insists that they only go up to mach ~1.4, so presumably would make a concorde 2.0 slower than the original).

* I originally thought they were high-bypass turbofans.  Wiki insists that they had to be turbojets.  They did have (and use) afterburners, both for takeoff and for getting through the sound barrier.

[cantab]

4 minutes ago, wumpus said:

Supposedly there is a plane made in the old USSR that has [intentionally] super sonic propellers and it is the noisiest plane on the planet.

The Tupolev Tu-95 "Bear", it's still operated by the Russian air force. In recent years they have been regularly flying close to NATO airspace. And yes, they are LOUD.

[Nibb31]

22 hours ago, cantab said:

Is there any theoretical speed limit for a scramjet, apart from those imposed by the properties of the materials it could be made of?

The physical properties of materials are what determine a theoretical speed limit. 

As I said above, the fastest scramjet ever built was the X-43.

Edited November 7, 2016 by Nibb31

[magnemoe]

56 minutes ago, cantab said:

The Tupolev Tu-95 "Bear", it's still operated by the Russian air force. In recent years they have been regularly flying close to NATO airspace. And yes, they are LOUD.

Know about them, plenty of them outside Norway but did not know of the supersonic propellers. 
A bit weird they are not replaced, had been plenty of talk about rebuilding an transport plane to bomber to replace the B52, for the US this is mostly an cost issue as the B52 is an fuel hog and expensive to maintain. 
For Russia it would also increase performance, however its a lot like the B52, it works well enough and replacing it cost money. 

[p1t1o]

@magnemoe @cantab @wumpus

Relevant: https://en.wikipedia.org/wiki/Republic_XF-84H "Thunderscreech"

"The XF-84H was quite possibly the loudest aircraft ever built (rivaled only by the Russian Tupolev Tu-95 "Bear" bomber[16]), earning the nickname "Thunderscreech" as well as the "Mighty Ear Banger".[17] On the ground "run ups", the prototypes could reportedly be heard 25 miles (40 km) away.[18] Unlike standard propellers that turn at subsonic speeds, the outer 24–30 inches (61–76 cm) of the blades on the XF-84H's propeller traveled faster than the speed of sound even at idle thrust, producing a continuous visible sonic boom that radiated laterally from the propellers for hundreds of yards. The shock wave was actually powerful enough to knock a man down; an unfortunate crew chief who was inside a nearby C-47 was severely incapacitated during a 30-minute ground run.[18] Coupled with the already considerable noise from the subsonic aspect of the propeller and the dual turbines, the aircraft was notorious for inducing severe nausea and headaches among ground crews.[11] In one report, a Republic engineer suffered a seizure after close range exposure to the shock waves emanating from a powered-up XF-84H."

[Rath]

14 hours ago, cantab said:

The Tupolev Tu-95 "Bear", it's still operated by the Russian air force. In recent years they have been regularly flying close to NATO airspace. And yes, they are LOUD.

They put a extra electronic countermeasures pod on it, but the radar crews in Alaska would hear it farther away than the aircraft without the pod would be detected.

[Racescort666]

@Shpaget this story is also worth mentioning: http://oppositelock.kinja.com/favorite-sr-71-story-1079127041

[p1t1o]

39 minutes ago, Racescort666 said:

@Shpaget this story is also worth mentioning: http://oppositelock.kinja.com/favorite-sr-71-story-1079127041

I heard a similar anecdote about an SR-71 pilot requesting "Clearance to Flight Level 60" (60,000feet) and the reply came back something along the lines of "Sure, but I dont think you'll make it that high!", the response: "Sorry Control, we are requesting clearance down to FL 60"