Hypersonic Transport Part 2

[KJ_Lesnick]

From a purely technical standpoint, I'm wondering how difficult it would have been to have developed a liquid-gas powered hypersonic transport with technology available in the mid 1970's through 1990.  

Instinctively, the idea sounds absolutely insane, however a number of technological innovations were developed prior to those time periods and would have allowed such an aircraft to be developed.  During the 1960's there were a number of aircraft designs conceived by the Flight-Dynamics Laboratory at Wright-Patterson, as well as McDonnell, and Convair, whose designs incorporated high temperature, light-weight thermal protection systems (heat-shields) which often took the shape of multi-layered shingles, eliminated the need for using fuel tanks as heat-sinks, as well as elaborate air-conditioning systems, and allowed for sharp noses to be incorporated into the designs (though piped coolant was often required for the nose).  These TPS were often substantially lighter and sturdier than the ceramic and carbon tiles utilized on the Space-Shuttle design, and possessed a longer service life, and additionally, required far less maintenance, though they weighed substantially more than a typical aircraft's aluminum or titanium skin.

Advances in jet-engine technology, starting in the 1960's and proceeding to present, has allowed higher mach numbers to be achieved with increasingly high pressure ratios due to the advances in air-cooling, metallurgy, ceramics and fabrication techniques, compressor design has also improved immensely, allowing greater pressure ratios to be achieved with fewer stages.  Higher pressure ratios per given stage would allow a more compact engine for the same thrust, and higher temperature metals could allow higher pressure ratios to be achieved for the same mach-number.

Additional, numerous engine designs were conceived which could be used at high mach-numbers which include bleed-bypass turbojets such as that used on the SR-71 which bleed increasingly large amounts of air off the compressor and feed it into the afterburner at higher mach-numbers which keep the turbine temperatures in line and increase afterburner thrust; Liquid-gas expander-engines which utilize a cryogenic/liquefied gas and external heat (as well as engine heat) to drive a small turbine, which through a reduction gear spins a multi-stage compressor/fan (since the turbine is driven by a relatively cool expanding gas, the typical turbine temperatures are lower for the same pressure-ratio/mach-number; the exhaust-gasses do not travel through the turbine and go straight out the back producing higher exhaust velocities); various other variable-cycle proposals.

Hypersonic aircraft produce rather steep shock-wave angles off their nose and tails that generally do not reach the ground from the high altitudes in which they fly (~100,000 feet and higher), eliminating the sonic boom problem once the aircraft is at speed and altitude.


There are of course a number of technical issues that would get in the way of things, and this would include the following.

• While these relatively lightweight heat-shields were created, they were highly, highly classified, and very few people appeared to be in the know.  I don't know at what point in history they became declassified, let alone became public knowledge, and who in the know had the power to order this technology to be de-classified (I don't even know if some presidents were in the know about this).
  
• While the "light-weight" thermal protection systems mentioned earlier are quite light compared to the shuttles ceramic and carbon tiles, I have been told that they were generally substantially heavier than an aluminum or titanium skin.  Interestingly, the thermal protection systems mentioned were designed for speeds of up to Mach 20 at least, if not higher, so one designed for Mach 5.0 to 6.5 would probably be somewhat lighter and might not possess the same problems.
  
• Many airplanes optimized for hypersonic speeds, generally have very high takeoff and landing-velocities, which affect takeoff and landing distance.  Landing distance would be a greater obstacle than takeoff distance as supersonic/hypersonic aircraft generally are more streamlined and have more powerful engines than a subsonic design, which yield superior acceleration -- landing distance on the other hand, is largely a function of breaking
  
• Noise levels would likely be very high as high-speed aircraft generally require a fast exhaust velocity to operate at high-speeds, though it depends on the particulars of the engine used.  A sound suppression system would probably be needed of a potentially significant complexity.
  
• I don't know, even from a technical standpoint, how difficult it would be to set-up an infrastructure for providing LH2 or LCH4 in sufficient quantities for a fleet of airliners in the 1990 to 2010 time-frame.  
  
• The aircraft would probably possess an unusual fuselage configuration consistent with many BWB designs, very wide, and flat requiring non-tubular designs (either extreme ovals, or rounded rectangles) and large amounts of composites to avoid metal fatigue problems.  Windows would be difficult to integrate into such a design without weight penalties, so you'd have to work around that.
  
• The aircraft would heat up quite a lot in flight, and would probably take a considerable amount of time to cool down, though I'm honestly not 100% sure about that.  Since the TPS do dispel heat very effectively, it might not be a gigantic problem, though I'm not 100% sure about that.  I do remember seeing a YF-12 during testing with NASA that actually used a water spray for some reason, though I don't know why.

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The best practical design from a high-speed standpoint would be some form of wave-rider design, as they tend to produce some of the highest L/D ratios, with spatular-noses, or an ogival-wing configuration (considering the designs would be around the ability to produce a design from the technical standpoint utilizing technology from the 1970's, 1980's, and 1990's, an inward-turning Busemann-type inlet probably isn't applicable) performing the best of them at the time.  Despite the fact that these designs perform the best, they are certainly not the only feasible designs.

For Reference Purposes:  A spatular-nosed wave-rider would be epitomized by this design, conceived in 1966 called the Kuchemann Tau




For Reference Purposes:  An ogival-wing wave-rider configuration would be well illustrated in this Northrop-Grumman hypersonic-bomber proposal




What do you guys think?

[Cobra]

Very Cool Stuff the Hypersonic Bomber design made me wait to hear,"Thunderbirds are Go!"! Hope to see more.btw, IIRC, i seem to remember hearing about a Hypersonic Transport/Bomber Concept that was Rejected for looking Too Much Like Concorde! Stay Cool. :cheers:Dan

[KJ_Lesnick]

I have a good question.  I'm surprised I never thought of this up to now. 

An airliner is going to need a pressurized cabin.  Even if the plane is heat-shielded, you're still going to need to take in air for the environmental system, and you're going to have to cool it down and such.  I know you could use the fuel as a heat-sink in one way or another.  I assume cooling just the air for the environmental system would require less hassle than having to cool the whole airframe down (if the plane wasn't heat-shielded), though I am not sure.

[Hobbes]

The cabin air is traditionally drawn in from the engines (from somewhere along the compressor) and IIRC even on Concorde it needs to be heated up before entering the cabin.

[KJ_Lesnick]

Hobbes,

The cabin air is traditionally drawn in from the engines (from somewhere along the compressor) and IIRC even on Concorde it needs to be heated up before entering the cabin.

You're correct about tapping the air off the compressor, but to the best of my knowledge, the air needs to be cooled before it can go into the cabin, and the fuel is used as a heat-sink for this purpose on Concorde

[KJ_Lesnick]

An important issue is the type of fuel and engine to use.  My favored fuel types include Liquid Hydrogen and Liquid Methane as their extreme cold could be used to actually do work, either by cooling and densifying incoming air, driving the engine, or a combination of both cooling/densifying incoming air, and driving the engine.  They also seem to be able to produce a lot of energy for their weight, and Liquid Hydrogen at the very least seems to produce very low NOx emissions (At least Liquid Hydrogen does).

Secondly, I was thinking about the engine types, which revolve on three basic concepts

• The liquid-gas expander, which uses the low boiling point of cryogenic fuels relative to the outside air-temperature to vaporize the fuel, which then drives the engine, with the now vaporized fuel burned in the combustion chamber
  
• The pre-cooler concept, which is similar to Reaction-Engine's SABRE and Scimitar designs, and utilizes the low temperature of the cryogenic-fuel to drive a pre-cooler, which cools down and densifies the incoming air; which is then compressed further and mixed with the now vaporized gas and burned.
  
• A pre-cooler/hydrogen-expander concept similar to the Japanese ATREX engine design which uses cryogenic fuels to drive both the engine and run a pre-cooler

..
I'm wondering which design most members here would find preferable?


K.J. Lesnick

[KJ_Lesnick]

When did the composite-fabrication technology reach a point, military or civilian, where large aircraft portions could be built out of composites, particularly pressure hulls?

KJ_Lesnick
BTW:  I think I remember reading about a proposed supersonic transport design which was proposed in the 1970's and it had some composites proposed

[Hobbes]

Wasn't the Rutan Starship the first with a composite pressure hull?

[KJ_Lesnick]

Hobbes,

I'm not sure, but the Starship first flew in 1986