Jet engine with LENR realistic?

  • I started to wonder NASA:s sldes about using LENR in JET and space engines. My concern is heat transfer from reactor to air.
    Jet planes have pretty powerful engines, and they operate purely by heating incoming cold (+40...-50) air to hot air (800C?) which expands causing needed thrust.

    My assumptions:
    -Kerosine 77000kg Airbus 300
    -Flight time 14,5h
    -Energy 42.8 MJ/kg (Kerosene)
    => Flight average power 63.13 MW! (In ideal engine like ramjet, but in real is somewhat less 50MW?)

    If convection would be conservative 200W/m2/K and temp diff would be 500C (1300C-800C) that would require roughly 500m2 convection surface
    (when assuming that high speed air would really be able to capture heat with that 200w/m2/K from reactor surface,

    Taking heat directly from burning kerosine gets all energy to heating and that minus compressor etc. produces thrust power, but hard to believe that LENR engine efficiency would be equal nor better than Kerosene powered engine.

    Anyone having better calculations?

  • Argon, ( almost bedtime, don't need a critique on my math)
    quick swag, assume big high bypass turbofan , 100,000 lb thrust
    at 400 mph, 1 lb thrust about 1 hp
    for 100k lb thrust you will need another 200,000 hp for the compressor,
    guessing that's about 220 MW total
    to get efficiency required, need to get the delta T to 700-750 C for the working fluid (air)
    hard part would be a heat exchanger with low weight, size, and internal drag for airborne apps

    if certain parties ever deliver so called "hot tube" technology, however, ground based gas turbines using only air for a working fluid would be "YUGE", and you could use the waste heat for other industrial processes

  • Thanks Mike, i didn't have turbofan part in my calculations so it was closer to Ramjet than modern jet engine. And I also used total average fuel consumption, but during takeoff it would need much more, so I think your number is much better (it depends on plane size anyway).

    That would make design even harder. Challenge of (>1000C) heat distribution to surface of that size (2000m2) needs some new innovation.

    BTW I have sometimes calculated on ballbark that even pressurized air becomes hotter than intake, it is gets compensated with more efficient convection under pressure, so no need to take into calculations as long as there is enough temp diff between surface and compressed air. But not completely sure about this.

    • Official Post

    You have got to step back a little here. About 100 years. A closed-cycle steam-turbine powering a fan (as in 'turbofan') would be much easier to produce, and also close to silent. Plenty of cooling surface (the wings) to re-condense the steam back to water and plenty of cold air available. Flash-steam boilers are not big or heavy either. Since most large civilian aircraft already use turbofans the airframe knowledge already exists, the tricky engineering problem is those pesky intercoolers in the wings.

  • Interesting idea. Condensing capacity needs to be designed on takeoff conditions, but as you said lots of wing surface available. Thrust available without expanding (heating) the air is another concern.

    Intersting to see if these will ever replace kerosine engines.

    • Official Post

    A supercritical flash boiler running at 300-400C is more or less a coil of heated tubing with a turbo-pump at the inlet end. No real need to go all the way back to the liquid phase - cool steam (>110C) from the low-pressure end of the turbine would probably be ok -maybe better than feeding it water in terms of boiler life. As for heating the blown air - why not put the primary condenser circuits into the tailpipe wall? Extreme high-pressure steam turbines are very compact btw - the HP end of a 7,500hp steam turbine is often no more than 250mm across the blade tips.

    Any weight penalty due to the need for proper containment of the high-pressure steam is offset by the reduced need for tons of fuel. Biggest problem for an airframe designer night eventually be that take-off and landing weights are the same - they normally like to land without too much fuel on board.

  • A supercritical flash boiler running at 300-400C is more or less a coil of heated tubing with a turbo-pump at the inlet end. No real need to go all the way back to the liquid phase - cool steam (>110C) from the low-pressure end of the turbine would…

    Alan, I was also concernerd on size of needed heat transfer surface. Normal jet-engine works in principle that inflowing air is heated 600-700C which makes it to expand quite a lot and thus increasing pressure and increasing air velocity in tail pipe. Even modern engines have turbofans in front compressing 4/5 of intake to bypass actual heat chamber, but still air expansion is needed by heating. (bypass airflow is partly helping on thrust, but also acting as noice control).

    Or are you saying that passenger jet speeds (500mph) would be achievable with fan only? (I have some difficulties to get the idea)

  • Alan and Argon,

    I am pleased to see this discussed. Applications may well be sooner than most think.

    I think the closed cycle turbine driving a "ducted fan" is implied here. The working fluid in the closed cycle turbine can be steam or other suitable fluid (argon, ammonia, propane, freon, hydrogen, sodium or whatever the temperature regime dictates) with or without phase change.

    • Official Post

    @ Argon. Think of an engine the size of an RB211 (Airbus etc) Both the inside and the outside skins of the engine nacelle could be used as exhaust condensers. There is no need for the blown air from the big fan at the front to be superhot- though it could pass over a streamined turbine/boiler casing and be warmed as it went. The limit on speed for a ducted fan engine (which is what this is) is approached when the blade tips go 'supersonic.' Based on piston-engine fighters this is probably around 400mph.

  • OK now I got it, thanks to both. You mean pure 'ducted fan' engine approach. Makes sense just to make mechanical energy to drive the fan with lower speed ranges.
    I was too stuck on my old experience on fighter jets with afterburners and all where main thrust comes from heating the air.

    What I had in mind was few years ago I saw concept pictures of (NASA?) supersonic long distance passenger jet with Ramjet engines. From pictures I could see 4-5 big pipes on top of plane. With quick calculation 5 pipes with 1m diameter each having 50 m (heat transfer) length, would give 785m2 heat transfer surface.
    So that construction could also be plausible if you could distribute heat to whole length (small Quark-X:s if they are real).
    In Ramjet you don't need fans since speed of the engine (aircraft) causes air compression in intake. So only thing needed would be heating air in pipes, and no moving parts! Well in practice takeoff part need additional design, since this works well only in high speeds.

    I can't say is this long pipes construction realistic though, just throwing ideas and curious to hear from people who know what LENR could make possible.

    BTW NASA has lot of supersonic studies. Here is sample page…ctSheets/FS-040-DFRC.html

  • Agreed that take off conditions will be the limiting factor. A way to cool the condenser whether in the wings or nacelle is to have ground based fans that cool them. After the engines have reved up, withdraw the ground based fans. Presumably the fans would be behind the wings with rectangular ducts temporarily covering the wings on top/bottom and rear sides. It might also be possible to have a ground based "launcher" that cuts into the plane cooling circuit and goes partway down the runway with it for a while, then disconnects before lift-off. Then there is the JATO possibility as well.

  • Argon,
    the less you beat up the air, the more efficient the propulsion, hence high bypass turbofans

    supersonic may seem glamorous, but then you drastically limit the number of available airports

    lastly, interesting tidbit, the ultimate speed of the SR 71 was limited by the impact air temp on the
    engine's fan face

  • Yes I agree, it would be practical, and most beneficial, in long distance flights and first response patrol fighters, where flight time would be limited by pilots toilet breaks instead of fuel capacity.
    Also passing sound barrier is problem when flying over populated areas. I noticed that NASA is studying this sound barrier problem more closely maybe trying to minimize disadvantages in future planes.
    I would be really happy on planes humming, with more laminar flow, over my head with very large, or many ducted fans even speed would be limited. Pollution and sound is big problem in current planes.

    BTW Mike I think NASA concept was planned to achieve actual travel speed only in much higher altitudes to limit surfaces overheating you mention.

  • You are correct that the ceiling for the Blackbird was "classified" and the generally allowed guess was 80,000 feet (Janes Fighter Aircraft). There may have been ways to push it somewhat higher, likely dependent on special fuels, which effectively contain oxidizers in situ and could push considerably higher, but my guess, having paid some attention over the years, is surely not over 110,000 feet (~34 km). The designation of borane (such as triethylborane) fuels for this aircraft is conveniently and deceptively (IMHO) attributed to takeoff requirements. To this enthusiast, the mix of internally oxidizing "fuels" could easily have included nitrated organics such as nitromethane and/or nitrobenzene that may have provided not only tremendous over-rating for the Pratt & Whitney JP58 cores, but effectively taken the ram configuration somewhat into "rocket" mode. Although such altitude boosting would still likely be classified.

    A blackbird when first on display at an Air Museum outside McMinnville OR some years back (currently the Evergreen Aviation and Space Museum).... the original pile of SR 71 components had a sign that humorously indicated "some assembly required". Later viewing showed a "complete" appearance and quite impressive at that, considering the design was conceived in the early 60s. One key advance for the the SR 71 was the ability of the core engine to remain subsonic while allowing a variable compression from the position variable "spike" (inverse Coanda cone) to provide varied bypass ram jet functionality for out-pacing missile defenses or other fighters of the day. One can appreciate that form of ducting looking at the "bird" today in repose.

  • Yes heck of the engineering indeed in SR-71. I was first thinking that Ramjet/Scramjet could be starting point on design with effective heating surface (15-30 km altitudes Mach 2-5). But after your comments and some googling it seems not to be quick path to design. I was thinking many smaller air intake nozzles that leads to bigger heating chamber (air velocity decreases), but yes overall heating of surfaces is still the problematic one. It would quiclkly lead space shuttle design with ceramic tiles and everything, not suitable for frequent commercial flights.

    Even Mach 3 seems to be challenging in practice: