back to article X-51 hypersonic scramjet test bird ready for second flight

The X-51A hypersonic scramjet project is back on track, according to its chiefs, with problems that occurred on the inaugural test flight now ironed out and a second flight pencilled in for 22 March. The first X-51A and its booster rocket mounted ready for release from B-52 mothership. Credit: USAF All right, very nice, but …


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  1. norman


    I wonder what happens in a Mach 22 bird strike?

    /it can't be good.

    1. BatCat


      Thankfully prolly don't get that many birds at the altitude where it's possible to do Mach 22... :o)

      1. TwistUrCapBack

        march != mach

        where has mach 22 come from ???

        here perhaps ?? "second flight pencilled in for 22 March"

        date != speed

  2. Matt_V

    whereas they would be un-bothered by a hypersonic

    until someone puts a nuke on the end of one...

  3. Anonymous Coward

    SCRAM jet?!

    SCRAM? Oh, noes, more nucular reactors, more PACTS WITH THE ATOMIC DEVIL!!

    You, LEWIS, may be 'CELEBRATING', but this project is only 'back on the track' that rises straight from the Bottomless Pit of Hell That is Fukushima.

    Lewis and that font of truth Barack Hussein Osama tell us the Protective Pentagram Vehicle is nearing completion. WOE to ALL of the world's TRUE CITIZENS wishing to get on with their lives without contemplating the Chernoblian Spektor of DOOM

    1. ian 22

      Liked that did you

      You're welcome. You may quote me.

      I did try to work in a reference to Cthulhu, however it appears he is standing for president on the Republican ticket (

      1. Anonymous Coward

        Like yours, my incoherent rant got downvoted.

        However, unlike you, I remembered to use the Joke Alert icon.

        AC because I don't want AManFromMars to be coming after me for trademark infringement.

        1. ian 22

          Downvotes? I sneer at downvotes!

          Impossible to say what sets off a downvoter (or an upvoter for that matter). IMHO amassing either is A Good Thing- an indication of a quality post. And if a post becomes the root of an entire thread, its brilliant.

          As I may be made redundant shortly, I've wondered how to apply to be a scribe/wordsmith at El Reg. Is there financial compensation involved (from El Reg to me, in case I was unclear)? I've demonstrated my command of clichés, I think.

    2. Msan

      Not sure about thumbs down

      Reads to me like irony. It is irony isn't it? Please say it is irony.

      Does anyone know what the black/deep violet really fast plane was used in the footage for the opening sequence of the movie 'Performance'? Best bit of the movie I thought.

  4. Anonymous Coward

    More pointless tech

    Why oh why are we still developing technology that requires combustable fuel. It makes not sense what so ever. I for one will not be buying one although I am sure you might be able to buy one in Iran in 10 years which will be pointing at the US and UK with a warhead on the end of it !

    1. AlexH

      not buying one?

      Oh, well, if AC isn't buying one then they might as well not bother!

      I currently have hilarious visions of AC sitting in front of the telly watching science documentaries and screaming 'What the bloody hell do I need an LHC for!? If they ring me up trying to sell me one I'll tell them where to stick it!'

    2. ravenviz

      Re: More pointless tech...

      What do you suggest then, tethered geese?

    3. Dr. Mouse Silver badge


      "Why oh why are we still developing technology that requires combustable fuel"

      Maybe because it's the only technology we have which is currently usable in this situation?

      I would LOVE to see someone try to power a hypersonic aircraft with batteries. At this point in time, it is just not possible.

      Cars, I can see the point. We are nearly at the stage where an electric car could replace one with an ICE for more than short distance use. It's already there with hybrid tech (although I don't see why they are all petrol, diesel seems a better fit for most).

      But aircraft still have a LONG way to go, and spacecraft will be in the "distant" future. For now, we still need combustion-based technology for these applications.

    4. longbeast

      Fuel is not a problem if you don't need it to be ultra-cheap.

      Hydrocarbons as an energy store will be around for a very, very long time. Whether it's biofuel derivatives or something synthesised directly from co2 and water, fuels can be manufactured.

      Even if every oil well on the planet unexpectedly ran dry tomorrow, we'd still be able to produce plenty at a higher cost for neat applications like this scramjet.

      Well... we'd be able to, once the economic chaos settled down a bit.

  5. rav
    Thumb Up

    Hey you forgot Burt and Sir Richard!

    Hey for a runway to space plane you forgot Burt Rutan and Spaceship 1 and Sir Richards Virgin Galactic Space Program.

  6. JDX Gold badge


    At Mach-22 you are in air too thin for birds, I think.

    1. TwistUrCapBack

      mach what ??

      "pencilled in for MARCH 22"


      its a date on the calander - not a measure of speed

  7. Anonymous Coward

    wishful military analysts

    "The idea here is that launching a conventionally-armed ICBM would cause other nations to panic and assume that a nuclear war was underway, whereas they would be un-bothered by a hypersonic."

    In a period of hightened tension between two (or more) nuclear armed militaries, they're just as likely to go hot at the mach 5 missile heading their way, as they are with anything else that could deliver a nuke or CBW warhead.


    Note the author didn't say whether they thought it was a good or bad idea

  8. lIsRT

    abc and/or 123

    "The idea here is that launching a conventionally-armed ICBM would cause other nations to panic and assume that a nuclear war was underway, whereas they would be un-bothered by a hypersonic."


    Is there some reason a nuke can't go in a hypersonic cruise missile?

  9. Gene Cash Silver badge
    Thumb Up

    Well done, Lewis!

    I'm American and read a half-dozen aerospace sites a day, yet the first mention I see of this is an El Reg article... with a nifty pic too.

  10. Anonymous Coward
    Anonymous Coward

    Each time I read about scramjets..

    .. I get the feeling they arn't really flying, they're just falling very very fast

  11. Anonymous Coward
    Anonymous Coward

    Un bothered ?

    As in permanently?

  12. John Smith 19 Gold badge

    Potentially *slightly* more useful than a Hydrogen fuelled scramjet.

    But only just.

    While making it run on JP7 makes it a *lot* more operable than either liquid or compressed Hydrogen. the ethylene is not really an issue. The SR71 needed a start tank of a very fierce organo-metal to fire up. It was only a few litres in capacity but good IIRC for 10s of starts. Ethylene is *benign* by comparison your still left with 2 problems.

    Fixed internal geometry ramjets have *limited* speed ranges. If its designed for M2 it won't stay in ram mode much below and won't take much above. Widening that speed range *without* geometry change is something which AFAIK has *never* been attempted. The temperatures of the internal flow paths are Earth reentry level *continuously*, as are substantial parts of the outer skin.

    So the fuel is really the *only* place to dump the heat if you want to run it continuously.

    Problem 2 is you still need to get it *up* to ignition speed in the first place, preferably a *long* way from the ground.

    And when you have made it *work* you've basically saved yourself the cost of a lox tank.

    LOX is roughly 4600x the oxygen density at *ground* level (and at least 4 times better at its likely *minimum* operating altitude).

    Its also the *cheapest* oxidizer (High concentration Hydrogen Peroxide is fairly rare and Nitrogen Tetroxide *very* hazardous), literally cents per lb.

    This is really a weapons technology. It's *useless* for commercial functions until the problem of getting it up to ignition speed. No problem for missiles. Not much use for commercial traffic.

    1. blah111

      Isp and Payload/Mass/Fuel Fraction

      So the density of LOX is 4600x that at sea level. So *what*? Carrying that LOX *onboard* reduces the useful payload. The best hydrocarbon rockets have a specific impulse well short of *400sec*. A ramjet has a specific impulse more than *double* that and a turbojet (jet not fan) can easily get close to *2000sec*. Since the exhaust velocity will be very high for a scramjet I expect its Isp will be closer to that of the turbojet than the ramjet. That means you get a *much* higher Isp and your payload mass fraction goes *up*. By not carrying *most* of your *reaction mass* and instead borrowing it from the atmosphere you can deliver more mass to orbit. Of course if the engines themselves are extremely heavy then that would be *bad*. A 350sec Isp rocket can deliver about 6% of it's starting fueled mass to orbit. An 800sec Isp (sc)ramjet operated for the entire burn to LEO would deliver *70%* of it's fully fueled ground mass to orbit. Breaking it up into a turbojet/ramjet/scramjet/rocket profile is left as an exercise to the reader.

      Ramjet's are great for high speed applications and relatively efficient in the supersonic regime. Their only upper limitation is that combustion must occur subsonically (hence the missing SC) and when they get to the hypersonic range they have to slow down their intake air so much that they melt. SCramjets get around that by magically only slowing the intake air speed by a fraction and so have less heat load to contend with. They substitute the problem of keeping the flame lit which apparently P&W are solving. You are also right that they need to do most of their acceleration in the atmosphere and so skin temperatures, really leading edge temps, are a serious concern. And yes, you still have to get it up to at least a mid supersonic regime to transition through a ramjet and then scramjet mode, but no one outlawed variable inlet geometries. Without a doubt scramjets *are* potentially *very* useful for commercial space applications.

      1. David Dawson


        Nice info.

        If I could suggest though, *less* of the starring.

        I know this means bolding or emphasis; but I, for one, struggle to read when every sentence has an associated star cluster.

        For fun, I went over the posts to pick out constellations.

        I reckon I can see pleiades.

      2. John Smith 19 Gold badge


        "Breaking it up into a turbojet/ramjet/scramjet/rocket profile is left as an exercise to the reader"

        Do not feed.

  13. Anonymous Coward
    Thumb Up



    high speed tech and funky toys, incomprehensible to those even as young as the baby boomers. More please El Reg!


  14. Galeed
    IT Angle

    bootnote high flyer?

    Good article - BootNote - 40000 ft??? Airliners do that!!!

    1. Captain TickTock



  15. Captain DaFt
    Paris Hilton

    So who needs warheads?

    Mach 22 = 7.48638 kilometers / second according to Google.

    Quote: "The basic metric of kinetic weapons is that anything hitting you at 3 km/s - a rock, a throw pillow, whatever - delivers 4.5 megajoules of kinetic energy per kg of mass. For comparison, TNT delivers about 4.2 megajoules" - from

    So an incoming hypersonic missile would deliver over twice the explosive force of an equal weight of TNT just by virtue of its speed and mass alone, Kind of makes the whole warhead thing obsolete.

    Paris Hilton icon because.... Well why not?

    1. Anonymous Coward
      Anonymous Coward

      Whilst I agree with the maths...

      unless the scramjet hits something solid it may merely pass through an object such as a ship.

      1. TeeCee Gold badge

        Passing through a ship.

        It may well. However, I'd have thought that the amount of kinetic energy it would give up on the way through would make the experience pretty much indistinguishable from being blown up by a conventional warhead for the ship concerned.

        1. Anonymous Coward
          Anonymous Coward


          If it doesn't hit anything with significant mass it will just punch through.

          Most naval vessels are just thin tin boxes. It'd just be like a rifle bullet and tissue paper.

          1. This post has been deleted by its author

          2. laird cummings


            Even presuming the scram-missile doesn't hit anything crucually important, they're still going to know they've been kissed. Secondary projectiles from bits knocked off, incandescent metalic vapor from friction and impact, crumbly bits of missile as it starts to come apart, and a nasty bit of overpressure / underpressure from shockwaves will make a right proper mess of things. Expect a warship so rudely treated to be out of action immediately following, and likely for some time subsequent.

            BIG grenade, 'cause that's what it'll look like.

            1. Anonymous Coward
              Anonymous Coward


              I can't believe that - evidence of this sort of impact on this sort of target would be good.

  16. John Smith 19 Gold badge

    A note on what kinetic energy can do.

    There is a picture in the book "Birth of the missile" which shows a V2 test site. The guidance system during development was so unreliable the test team felt the safest place to be was the *target*.

    That day the V2 was spot on.

    IIRC correctly the V2 (*without* warhead) made a 30m crater.

    At these speeds just *hitting* something will do some pretty serious damage to most things. The only sort of "warhead" you needs is something sharp, strong and *highly* temperature resistant to drive into the target.

    Something like the Tungsten or depleted Uranium sabots in artillery shells for example.

    It remains a *very* complicated way to do something which *can* be done in a *lot* simpler way.

    Like a fashion model with a PhD in Comp.Sci. Interesting certainly but not actually *essential* to the job.

  17. Anonymous Coward
    Anonymous Coward

    Looks like a shark

    Can it behave like one and attack from below - ie take out satellites?

  18. John Smith 19 Gold badge

    Some notes for air breathing fanbois

    To learn some history

    NASA web site|1&N=0 search for "Facing the heat barrier" to get a history of hypersonics in the US.


    Page 118 in the PDF is the most recent history along with the background to how it got started.

    For the math (and why people call it the "Airbreathers burden")

    BTW If you think the author is wrong, check *your* reasoning first.

    For more math around LACE, Ramjets and Scramjets

    Rocket engineering *is* his day job.

    Reaction engines have worked on the problems of making air breathing to orbit for the last 30 years. Rocket engineering is their day job too. Their download page is.

    The pdf listed as "A Comparison of Propulsion Concepts for SSTO Reuseable Launchers"

    explains the alternatives in non mathematical language. "The SKYLON Spaceplane - Progress to Realisation" describes their progress as of 2008. They are currently commencing their "D1" design update which should reflect lessons learned and issues closed since their design freeze in the early 90's.

    Note that when Skylon was first designed (c1989) a *lot* of it's tech (or the equipment to build it) had simply *never* been built. Since then they worked with various partners to design (the fairly easy part), *build* and *test* key elements of the hardware. A useful lesson *any* developers looking to get into the space business with a limited budget should keep in mind.

  19. rbrtwjohnson

    Expelling-mass propulsion has limitations.

    Hypersonic scramjet is a kind of expelling-mass propulsion. Expelling-mass propulsion will ever limit space exploration to our solar system. To go to other stars it is needed new means of propulsion such as phase-shifted electrodynamic propulsion powered by nuclear fusion.

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