Hypersonic Transport

[KJ_Lesnick]

I'm inspired obviously by the LAPCAT design, at least the idea of a Mach 5 ultra-long ranged hypersonic commercial jet.  The fact that the plane can fly sub-sonically in a manner that is quite efficient too is also quite awesome.

Honestly though, as I'm thinking about it, the technology to do this has been around for such a long time.  I'm not sure how much of it was not classified and only for government use and research, and military applications, but during the 1980's so much effort was going into making a National Aerospace Plane, and McDonnell Douglas wanted to make a hypersonic airliner that a lot was probably available even then. 

The propulsion technology existed, metallurgy and heat-shield technology existed, avionics obviously existed.  The only factor that didn't exist for commercial applications was money.


KJ Lesnick
BTW:  Anybody have any knowledge on how the Scimitar works.  I have an idea, but I'm not completely certain (assuming it's not classified or anything). 

[Mossie]

I've got to admit, I've always been a fan of hypersonic aircraft.  I was astounded by Blackbird when I was a kid & I assumed we'd just keep on going faster.  I also assumed we'd be in space regularly & I might get a chance at it.  Maybe the fledgling space tourism industry will finally kick start it, we'll just have to see, eh? 

Unfortunately I don't think the technology is just there to take off the self given enough money.  The cancellation of the X-33 & therefore the VentureStar program due to the technology to build a composite hydrogen tank not being ready showed it's going to take a lot of work to get a hypersonic aircraft or spaceplane into the air.

Archibald described to me the Sabre engine that is proposed to power the Skylon spaceplane in a thread I created some years back.  The basic principal is pretty much the same as the Scimitar.  There's some more info on the Reaction Engines site as well.

http://www.whatifmodelers.com/index.php/topic,12605.0/highlight,skylon.html
http://www.reactionengines.co.uk/sabre.html
http://www.reactionengines.co.uk/lapcat_scim.html

Kendra, I assume you have British Secret Projects: Hypersonics, Ramjets & Missiles & Secret Projects: Military Space Tecnology?  Both are well worth a read for this kind of stuff.

[KJ_Lesnick]

Mossie,

Looking at the Scimitar diagram, it looks like a turbojet with a gigantic inter-cooler in the front, I'm not sure if that's an intercooler wrapped around the outer part of the compressor and combustion chamber area or that's part of the combustion chamber.  Either way it has a rather unusual combustion chamber in at least two respects, it has a pre-burner which seems to imply multiple stage combustion as I understand it, and that that the exhaust from the combustion-chamber appears to route through a normal-turbine and through an outer-turbine which wraps around the core-nozzle and is located in the outer duct which drives an aft-fan.  There seems to be a duct-burner behind the fan according to the diagram.  

I have a number of questions to ask regarding the Scimitar engine.
- While I know the structure in front of the engine is an inter-cooler, the structure wrapped around the turbo-compressor part of the engine, is that another intercooler or heat-exchanger?
- I don't know how cold the intercooler has to cool the air down to, but since there's water vapor in the air, couldn't it ice-over?  How'd they keep it from icing and still keep it cool?
- Why do you need a pre-burner?  Why not just use one combustion chamber?
- Does the combustion chamber always route through the main-turbine and aft-fan turbine, or can it be routed through just the main core, and through the core and aft-fan turbine?

Also, I have a few questions pertaining to the SABRE engine
- In the engine, helium or liquid helium was used.  What purpose was this for?
- For achieving orbit, would you say this device was more efficient than a ramjet/scramjet duct with a strut-jet RBCC or PDWE (to get the ramjet/scramjet up to speed, and for the last push into orbit, both can be cycled into rockets)?


KJ Lesnick

[Mossie]

I'm in the dark myself here (& fully open to holes being poked in my understanding!), but I would imagine the structure around the compressor is a heat exchanger (HX3 suggests there are two before it, one being the precooler).

Part of the reason that the water vapour doesn't freeze due to the airflow.  Water can supercool to very low temperatures as long as it has nothing for the ice crystals to form around.  If the water hasn't got time to form on the walls of the pre-cooler, it can't freeze.  I don't think this is the full explanation though, as the precooler takes the air down to -150^oC.  The video at the bottom mentions 'frost control', but I can't find any info so I get the feeling they don't 100% know how they're going to deel with this yet!

In the Sabre engine, the helium is there to provide cooling.  From what I gather, it's in an enclosed loop and doesn't physically come into contact with the fuel components.

Have you seen this video from the Lapcat entry?  It may help with some aspects, it describes the Sabre engine but the air breathing function is applicable to Scimitar:
Sabre engine video

[KJ_Lesnick]

Mossie,

According to that video regarding the SABRE engine, it had a pressure-ratio of 150-to-1.  How the hell did they manage that?

In the video there was one part that I didn't understand was the need for the bypass tubes, something about the efficiency of the engine breaking down at transonic speed and the need for a bypass-system.  I didn't really understand it due to the way it was explained.

[KJ_Lesnick]

While I'm definitely fascinated by the SABRE and SCIMITAR engine designs, I'm not entirely fixated on those designs.  How efficient is a hydrogen-expander engine over a traditional gas-turbine engine at sea-level and at high-mach?

It strikes me as being more efficient in that the hydrogen vaporizes readily in contact with air without the need to burn it, they don't use a traditional turbine (usually you use a small, high RPM turbine which through a gear-box drives a multi-stage fan/compressor) which isn't riding in the hot exhaust gases, probably steadily cooled to an extent with the LH2 flowing through it, and can go to higher mach numbers allowing either a higher pressure-ratio for the same mach number (unless the airflow runs the risk of ionizing or something) or a higher mach-number for the same pressure-ratio.  I would assume combustion could be more efficient as you'd be burning a completely pre-vaporized gas rather than just atomized droplets.

Also, does LH2 combustion also pose the risk of forming NoX compounds?  It rarely is mentioned, but I remember seeing a video about the LAPCAT/Scimitar in which it *was* mentioned.  So I don't know.


KJ Lesnick

[KJ_Lesnick]

I'm also wondering, how much cooling could be derived by using LH2 for active cooling by circulating it around the compressor (and any fixed guide-vanes) of a jet-engine through small passages to maximize surface-area at Mach 5?

For example, how much of a drop in temperature could you produce, and or how much higher a pressure ratio could you achieve for the same temperature? 

Any guesses, thoughts, ideas?


KJ Lesnick

[Mossie]

I think that's how the LACE engines work that are mentioned in the Sabre article?
http://en.wikipedia.org/wiki/Liquid_air_cycle_engine

Going on NO_x compounds, I don't think their should be to much to worry about.  Hydrogen burns at a much lower temperature than nitrogen.  As long as their is enough hydrogen available (double that of the available oxygen), this should tie up all the oxygen.  If the engine is starved of hydrogen for any reason (so there is unreacted oxygen) then there could be problems.

The problems of introducing nitrogen oxide in significant amounts would be two fold.  Firstly, there would be a change in the energy of the reaction, which could cause a change of fuel flow through the engine.  The second is production of nitrogen oxides that are harmful to health & the environment.

If this was a particular risk, I guess you could introduce a compound that would react with nitrogen in favour of the oxygen (unusual) that would be stable in the exhaust.  I'm sure it's doable, but I would think that it'd be more desireable to introduce an engineering soloution to restrict the amount of air coming in if their was ever any.

Any body who is more sure of there chemistry & engineering please feel free to chip in!

[KJ_Lesnick]

Mossie,

I think that's how the LACE engines work that are mentioned in the Sabre article?

I don't want to liquefy the air.  I just want to use the LH2 for active cooling.  Use the LH2 to cool off the airflow into the compressor than burn the LH2 up

Going on NO_x compounds, I don't think their should be to much to worry about.  Hydrogen burns at a much lower temperature than nitrogen.  As long as their is enough hydrogen available (double that of the available oxygen), this should tie up all the oxygen.  If the engine is starved of hydrogen for any reason (so there is unreacted oxygen) then there could be problems.

The problems of introducing nitrogen oxide in significant amounts would be two fold.  Firstly, there would be a change in the energy of the reaction, which could cause a change of fuel flow through the engine.  The second is production of nitrogen oxides that are harmful to health & the environment.

Huh?  Nonononono...  What I'm worried about is NOx formation during combustion.  When you burn fuel and air under certain conditions you get NOx.  I'm just wondering if LH2 combustion would produce less or the same NOx levels as hydrocarbon combustion. 


KJ Lesnick

[Mossie]

I don't want to liquefy the air.  I just want to use the LH2 for active cooling.  Use the LH2 to cool off the airflow into the compressor than burn the LH2 up

If I've understood Archies comments in the first link, there are a number of problems with this.  The hydrogen to the point where it's no longer liquid, your using hydrogen which you need for fuel & the liquid hydrogen tends to freeze everything it's meets.

Huh?  Nonononono...  What I'm worried about is NOx formation during combustion.  When you burn fuel and air under certain conditions you get NOx.  I'm just wondering if LH2 combustion would produce less or the same NOx levels as hydrocarbon combustion. 


KJ Lesnick

The flashpoint of hydrogen is about -253^oC, so it should start to react long before the nitrogen.  The temperature will rise quickly so you could get some NO_x forming before all the hydrogen & oxygen reacted.  If you get the amount of hydrogen right, my feeling is that it all should react before the nitrogen gets a chance.  I might be wrong though.

[coops213]

Ben Rich briefly mentions a hypersonic plane in his book "Skunk Works". Pretty much he saw the whole idea as infeasible, even if there is an engine capable of giving the require thrust. According to him, the main sticking points were finding a material from which to build the aircraft that would be capable of withstanding the tremendous heat on the plane's surface. He also writes "And by the way, our crews wore space suits and we still worried about boiling them alive if our air conditioning system failed. And you are proposing to fly at Mach 12 where the surface heat on the fuselage would be 2500 degrees and still have a passenger cabin filled with women, children, and businessmen sitting around in their shirtsleeves!"

Chris

[KJ_Lesnick]

Mossie,

If I've understood Archies comments in the first link, there are a number of problems with this.  The hydrogen to the point where it's no longer liquid, your using hydrogen which you need for fuel & the liquid hydrogen tends to freeze everything it's meets.

But you can use the heat to vaporize the hydrogen which can be burned.  Gaseous H2 can be burned way easier than LH2.

I don't know about freezing everything it meets, I would think it would depend on how much LH2 was used for how much airflow, and the heat of the airflow?

The flashpoint of hydrogen is about -253^oC, so it should start to react long before the nitrogen.  The temperature will rise quickly so you could get some NO_x forming before all the hydrogen & oxygen reacted.  If you get the amount of hydrogen right, my feeling is that it all should react before the nitrogen gets a chance.  I might be wrong though.

But the NOx would be lower than regular hydrocarbon combustion?


Coops 213,

Ben Rich briefly mentions a hypersonic plane in his book "Skunk Works". Pretty much he saw the whole idea as infeasible, even if there is an engine capable of giving the require thrust.

I don't think the thrust would be that big an issue especially once up at speed.  Ram compression would seriously augment the thrust of almost any engine.  The issue is generating enough thrust to takeoff and accelerate up to hypersonic speed.  I'm not planning a hypothetical Mach 12, but a hypothetical Mach 5 or Mach 6 so it isn't as difficult to achieve.

The J-91, J-58 and J-93 while officially designed for the Mach 3 realm, were in fact, all designed for Mach 4 performance.  There is at least one book I have about the XB-70 which states the X279E which is the YJ-93 was rated for Mach 4.  The J-91, and J-58 were both designed for the same speeds.  While the XB-70 was officially designed for Mach 3 performance, the design is actually designed for Mach 4 flight.  Walt Spivak actually stated that the inlets were designed to withstand the temperatures of Mach 4 flight, and looking at a diagram of the inlet, which shows the shockwave formation, it depicts approximately 14 to 16 shockwaves, and large portions of those inlets had porous ramps as well.  For Mach 3 you do not need that many shockwaves for even a very highly efficient inlet.  From what I remember the L-2000, which was a supersonic transport, capable of protracted Mach 3 flight and had inlets that possessed a high pressure-recovery and their shock-count was only about 4 or 5 (and porous ramps).

The J-91 had a pressure-ratio of 7 to 1, the J-58 had a pressure-ratio a little under 9 to 1, but used a variable guide-vane to lower pressures at higher mach to keep the turbine temps in check, the J-93 had a pressure ratio of 8.8 to 1.  None of these engines were turbo-ramjets (I'm talking about the original J-58 prior to it being re-designed for the A-12 program).

You could probably get hypersonic performance out of a bleed-bypass J-58/J-93, and I wouldn't be surprised if the SR-71 could achieve such performance.  

According to him, the main sticking points were finding a material from which to build the aircraft that would be capable of withstanding the tremendous heat on the plane's surface.

Not really.  According to Paul Csysz, I believe, and I've heard other sources state similarly that the A-12 was made out of an unusual titanium called Beta-Titanium.  From what was said, it had useable temperatures up to 1,200 C.  That seems more than suitable for Mach 5 or Mach 6 flight.  

Additionally, also according to Paul Csysz, McDonnell did a lot of work in the 1960's with the Flight-Dynamics Laboratory at Wright-Patterson, and developed a variety of light-weight, high-strength thermal protection systems, or heat-shields which could maintain room-temperature even at speeds of at least Mach 12, if not up to orbital velocity without the need for using the fuel as a heat-sink.  They were said to be sturdier, lighter, and had a much longer service life than the Space-Shuttle's tiles.  I wouldn't be surprised if they were heavier than a typical aircraft skin, but if you were going for Mach 5 or Mach 6, you probably wouldn't need as much insulation as you would for Mach 12, Mach 22, or Mach 25.

He also writes "And by the way, our crews wore space suits and we still worried about boiling them alive if our air conditioning system failed.

Technically, they had two air-conditioning systems, one for the suit (which I gather was quite insulated) and one for the cockpit itself.  Both were capable of keeping the pilot cool.  There was at least one case where one of the air conditioning systems in the RSO's cockpit of the SR-71 failed, the air-conditioning in the pressure-suit remained operational and the pilot lived.

And you are proposing to fly at Mach 12 where the surface heat on the fuselage would be 2500 degrees and still have a passenger cabin filled with women, children, and businessmen sitting around in their shirtsleeves!"

Well, for the purposes of an airliner I think the heat-shield approach would be a more practical solution.  It would work by simply reflecting almost all the heat away from the plane rather than relying on a complex cooling system.  Cooling systems could fail.  I suppose structures can fail too, but structures are tested so that doesn't happen and a plane wouldn't be allowed to enter service that was sufficiently reliable.


KJ Lesnick

[kitnut617]

I have a number of questions to ask regarding the Scimitar engine.
- While I know the structure in front of the engine is an inter-cooler, the structure wrapped around the turbo-compressor part of the engine, is that another intercooler or heat-exchanger?
- I don't know how cold the intercooler has to cool the air down to, but since there's water vapor in the air, couldn't it ice-over?  How'd they keep it from icing and still keep it cool?
- Why do you need a pre-burner?  Why not just use one combustion chamber?

Kendra, do a search using 'turborocket' this is what the Brits originally called these types of engines.  A turborocket is part turbine and part rocket as we traditionally know them.  The pre-burner is used to power the turbine part until it reaches an altitude (and speed) where the rocket works full time.  BSP-Hypersonics, Ramjets & Missiles goes into a lot of discussion about these engines which can be powered two ways, liquid oxygen and air breathing. The book says that the intake temperature at Mach 5 can be as much as 1000C (1832F) just from ram air compression alone and the exchanger cools this air down, from what I understand from the book this is how the pressure is made be 150 psi, at least that's how I understand a ram jet works.

[KJ_Lesnick]

Kitnut,

So the Pre-burner is the combustion chamber for the gas-turbine?

[kitnut617]

Kitnut,

So the Pre-burner is the combustion chamber for the gas-turbine?

Yes, that's how I understand it.  The idea for these engines was to get around the huge size that 'combination' engines were progressing too, see Andy's (General Melchet) E.E. P.42 build to see what I mean.