back to article X-51 hypersonic scramjet test: Flameout at Mach 5?

US government aerospace agencies have achieved the world's first hypersonic scramjet flight using hydrocarbon fuel. The test did not go perfectly, but further flights will follow; organisers said they were "ecstatic" with progress thus far. Concept graphic of the Waverider in flight, during rocket boost. Credit: AFNS …

COMMENTS

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  1. Philip Lord

    Meh - Barnes Wallis

    Didn't Barnes Wallis forsee this sort of thing years ago - IIRC a flight from London to Oz in about two hours? Remember seeing a programme about his work on the telly in the 70s or early 80s and showed his design for an 'aeroplane' to undertake such a flight. If IIRC also the 'plane had very slender wings with the engines mounted towards the tips of the wings.

  2. Yesnomaybe
    Joke

    What's in a name?

    "tried to get funding for such a super-Blackbird - dubbed "Blackswift""

    ...Because Honda already owns the name "Super-Blackbird"???

  3. Anonymous Coward
    Boffin

    Minor Correction:

    It burned for 140s the Powered flight time was 200s that includes 60s Rocket boost burn to get up to mach 4.5

    1. dpg21
      Paris Hilton

      Ramjet

      The original design of Concorde was for two of its four engines to be ramjets, which would have given it incredible range and fuel economy. That plan was scrapper in part due to a new Labour government at the time. Make's you want to weep, eh?

      (Paris, cos I bet she'd never scrap a ramjet)

    2. Arclight

      Hard to....

      Swallow was the name. Swing-wing airliner, another great still born British idea like the TSR-2

    3. Ian Michael Gumby Silver badge
      Thumb Up

      LOL... A design on paper isn't the real deal...

      If you listen to the You Tube video, the X15 project had started this effort. However the materials required to handle the heat and stress didn't exist.

      Knowing how something might work and having all of the requisite technologies working and available is another.

      Thumbs up because this is really cool. If they can work out the kinks, it would mean cheaper access to space.

      1. Anonymous Coward
        Anonymous Coward

        Unstart

        Don't ramjets occasionally "unstart"? And isn't that a potentially nasty situation? Ask a Blackbird pilot. I for one would never have flown on Concorde if it was fitted with ramjets.

        Oh and as for the Labour government, you must remember it was none other than that shuperior shocialist Tony Ben who kept Concorde going whenever it looked like the project was going to die. He it was who made sure both sides agreed to a no pullout clause in the contract. Even on silly little arguments like the letter E (the french wanted it, the brits didn't) it was TB who came up with a compromise.

      2. John Smith 19 Gold badge
        Happy

        @Ian Michale Gumby

        "If you listen to the You Tube video, the X15 project had started this effort. However the materials required to handle the heat and stress didn't exist."

        It would seem you are unaware that the X-15 flew 199 times. It hit M6.15 (on pure rocket power). It was *designed* to study the *problems* of prolonged hypersonic flight. To do so it used a thicker (but not *much* thicker) skin than conventional aircraft in a high temperature Nickel alloy (rather than aluminium) to act as a heat sink in a "hot structure" design concept.

        Getting to M6 or 8 has *never* been a problem. Doing it with oxygen from the *atmosphere* has taken 6 decades to get to a flight vehicle.

        And it *still* can't do so from a standing start.

        1. fatchap
          WTF?

          Never?

          I think that your definition of never may need revising! Surely the Victorians had some issues with it.

    4. Graham Dawson Silver badge
      FAIL

      Wrong!

      Knowing where to put the sensors and knowing what the sensors will say are two very different things. They will have created computer models of the craft before building the real thing, which will have given them an idea of where potential problem sites and useful areas to monitor will be; what it won't tell them is what actually happens there. The sensors will tell them how close their model us to reality. With the new data they can improve the model and use that to inform the next round of physical tests, thus improving the actual machine.

      The first time someone looked at Saturn they had no idea what they would see, but they still knew where to look. This is the same principle.

      1. Graham Dawson Silver badge
        Coat

        Er...

        Obviously meant as a reply further down. Oops.

    5. Annihilator
      Boffin

      @Philip Lord

      Yeah, and Da Vinci invented the helicopter.

      I myself have invented the time machine. The finer details have yet to be worked out but I'm not rushing it, the future me will come back and fill me in at some point.

  4. TeeCee Gold badge

    Spaceplane.

    The obvious move here would be to take off on rockets, briefly boost to scramjet speeds, then cutover to scramjets for the trip to the edge of space and finally go back to rockets for orbital insertion.

    That way you'd only need two sets of engines, the rockets and the scramjets and you could do away with all the problems associated with the transition between the performance envelopes of turbo/ram/scramjets. An external droptank containing the fuel/oxidiser for the first rocket burn should solve any fuel capacity problems associated with the takeoff side of things.

    1. Poor Coco

      You totally missed the point

      An external oxidizer droptank would be huge, and the whole point is to get a spacecraft into space WITHOUT dropping large parts of airframe.

      Jets kick rockets into the dust for lifting capacity because the oxidizer is ambient and does not require acceleration or carriage on the craft. This is an inherent limitation. You can carry a lot of jet engines and fuel for the volume and mass of an oxidizer tank.

      1. John Smith 19 Gold badge
        Happy

        @Poor Coco

        "An external oxidizer droptank would be huge, and the whole point is to get a spacecraft into space WITHOUT dropping large parts of airframe."

        I think you'll find he was proposing a *pair* of drop tanks, oxidiser and fuel, to bring the vehicle up to ramjet (or rather scramjet) ignition speed.

        "Jets kick rockets into the dust for lifting capacity because the oxidizer is ambient and does not require acceleration or carriage on the craft. "

        No. Modern good turbojets (something like those on the JSF or Eurofighter) achieve T/W of 10:1.*Poor* rocket engines hit T/W of 40:1. *good* hydrocarbon fueled ones hit 100:1.

        A *true* bill needs to take into account *wings*,inlets and landing gear. Thrust on aircraft is *typically* 1/3 of takeoff weight. On the Virgin jet powered round the world aircraft it was 1/9. With *anything* but a rocket wings (and *all* their mass) are *essential* rockets don't need them.

        BTW 1 cubic metre of Liquid Oxygen is *roughly* equal to ingesting 700 cubic metres of air.

        Now you were saying what exactly?

    2. Zolkó
      Pint

      a scramjet would be useless for going to space

      To get into orbit, you need to be at 400km and flying at 28000km. Therefore, flying at 20km and 6000 doesn't help you much. In fact, the scramjet engine would be dead weight.

      This is entirely a military stuff. Or pure curiosity.

      Sorry to spoil the dreams. I'm going to get a beer.

      1. Tim Bates

        Wrong units?

        I think some units there are wrong and/or missing.

  5. The First Dave Silver badge
    Boffin

    Failure

    Sure, you can learn a lot from failure, _if_ you have something to examine, but the plan all along was to destroy the vehicle entirely at the end of the flight, so if their sensors were in the wrong place then they will learn nothing. And if they knew where to put the sensors in the first place, then they probably didn't need the test flight.

    1. Mr Grumblefish

      I have a feeling

      you don't quite understand the concept of an 'experiment'.

      1. Doug Glass
        Go

        Uh Huh

        You got that right.

  6. Tom 7 Silver badge

    Santa Claus the thing!

    Launch the thing from a very long chimney/gun so that it comes out the top of a high mountain ready to go straight into scramjet mode.

    They use catapults to launch planes from carriers - even that might work.

    Wouldn't want to live next door mind!

    1. Anonymous Coward
      Boffin

      could be done

      The Iraqi Super Gun "could" have been used for putting satellites into a low orbit,

      and that's old technology.

  7. ReaderOfTheRegister
    Boffin

    "Concept graphic" indeed

    But even an artist should realize that something that flies at 50k-70k ft would be *above* the clouds, no?

  8. Anonymous Coward
    Stop

    U2-based Launcher ??

    The whole oxygen argument could also be made with a launcher that is lifted by a U2-type aircraft (extremely long winspan/high altitude capability) to about 20000 meters and then the "conventional" rocket would be ignited to shoot the thing into orbit.

    Above 20000 meters (60000 feet) there is not much oxygen left....

    Anyway, it will be difficult to challenge the tried and tested, cheap Russian throw-away launchers.

    What happens when aerospace guys do economics arguments can be seen with the space shuttle - much more expensive than the conventional way of doing things....

    1. Doug Glass
      Go

      Words

      Does the word "depiction" have meaning for you? You know, something like Gordon Brown with white teeth.

    2. Doug Glass
      Go

      Well ....

      ... we sort of wanted to re-use the crews. Didn't always work out that way but that was the plan.

      1. Ian Stephenson Silver badge
        Coat

        RE: Words

        Beyond the realms of imagination you mean?

    3. Anonymous Coward
      Anonymous Coward

      The shuttle is in fact an example...

      ...of what happens when a budget is slashed and when an airforce insists on a cross range capability. NASA had to agree to the Air Force request to get some money out of them.

      The original design prior to the cuts was way different to the finished product. It was supposed to haver smaller wings and sit on top of a booster, not strapped to a tank as if it was humping it...

    4. Andrew Norris

      Artistic

      >>But even an artist should realize that something that flies at 50k-70k ft would be *above* the >>clouds, no?

      Ah but it still has the booster attached in the picture so it is just at launch altitude.

  9. Ned Leprosy

    Intestines

    Looks quite a bit like something I remember seeing around 25 years back - wasn't the name Hotol, or something along those lines? I forget now. Last I heard was some government type supposedly staking a claim on the design and then sitting on it, in that time-honoured fashion.

    1. Mike Richards

      You're thinking right

      It was the HOrizontal Take-Off and Landing - HOTOL which brought a bit of Thunderbirds flash to the 1980s before vanishing in a big puff of bureaucracy.

      Its designer, Alan Bond now runs Reaction Engines:

      http://www.reactionengines.co.uk/

  10. GeorgeTuk
    Thumb Up

    I Spy With My Little Eye...

    ...something beginning with T.

    Trident Replacement

  11. Ross 7

    Blackbird v2

    I can't see turbojet -> scramjet being a particularly pleasant (or survivable) experience for a squidgy human. Then there's the whole issue of materials science - throw away experiments (as impressive as this one is) are one thing, having a boat that can fly mach 5+, be refuelled and turn around for another sortie without bits (like engines, wings etc) falling off is another.

    The friction on that thing must be crazy. I know it's not quite in the region of atmospheric re-entry, but it ain't *that* far off.

    I don't imagine ejection would be that healthy an option either!

    Hope they (as in America) don't think that using it as a cruise/exocet type missile is a good idea tho. You can imagine the news story already - "an American missile today destroyed a terrorist training camp killing 80 combatants. On its path to its target its shockwave destroyed 14 schools and 3 market towns killing 1500 civilians. Officials said they were 'looking into the matter'".

    1. Anonymous Coward
      Anonymous Coward

      However

      As a nuclear delivery vehicle it could rock. Think SRAM/ALCM. Try picking out this MF with air defences. Making it stealthy is going to be a bitch though.

  12. Dodgy Geezer Silver badge
    Paris Hilton

    What happened...?

    "...Something then occurred that caused the vehicle to lose acceleration....."

    My guess is either a very (very, very...) high-flying bird strike, or the hypersonic vehicle cabin crew decided on a go-slow.....

    Paris, because I think she'll be a great contender for the 'Mile-High AND Mile-A-Second' club....

  13. Anonymous Coward
    Stop

    Birds....

    " either a very (very, very...) high-flying bird strike"

    Now please point me to the wiki entry of the 20000 meter-high flying bird. I remember reading that 12000 meters is the record for birds.

    More likely a piece of shit falling down from ISS. Or maybe just a malfunction of the experiment. Which is OK, if I compare it with the contraptions I daily create. They normally don't work at all on the first run :-)

    1. Paul A. Walker
      Coat

      obvious what caused the failure...

      ...bloody volcanic ash AGAIN!

  14. Anonymous Coward
    Thumb Down

    Manned Spaceflight

    "we sort of wanted to re-use the crews. Didn't always work out that way but that was the plan."

    I haven't researched it, but my feeling is that the Russian throw-away booster tech has a better record of "reusing" humanoids traveling in spacecraft than the Shuttle. Maybe that is because everything is just so much simpler than the shuttle ?

    The only main usefulness of the Shuttle is the ability to steal a Russian sat for the USAF. Go figure the politcial implications of that....

    1. Captain TickTock
      Paris Hilton

      Irony detector failure?

      "...the Russian throw-away booster tech has a better record of "reusing" humanoids..."

      Possibly, just possibly, that might have been Doug's point...no?

      What's that? He said just the opposite?

      1. Steven Knox
        Boffin

        Humanoid Reuse

        According to Wikipedia (http://en.wikipedia.org/wiki/Space_accidents_and_incidents):

        "About two percent of the manned launch/reentry attempts have killed their crew, with Soyuz and the Shuttle having almost the same death percentage rates....

        In total, Shuttle accidents have claimed the lives of fourteen.

        Soyuz accidents have claimed the lives of four. No deaths have occurred on Soyuz missions since 1971, and none with the current design of the Soyuz. Including the early Soyuz design, the average deaths per launched crew member on Soyuz are currently under two percent. However, there have also been several serious injuries, and some other incidents in which crews nearly died."

        The apparent discrepancy in death rates is explained by the fact that the Shuttle flights generally carry more people, so the same number of fatal flights (2) results in more deaths.

        I'd say neither system has a statistically significant edge in safety, partly due to the fact that we're only talking about ~240 flights (~130 for the Shuttle, ~110 for Soyuz) over the past 40 years.

  15. Dodgy Geezer Silver badge
    WTF?

    There is a lack here....

    ...a lack of a 'humorous exaggeration ' icon.

    Perhaps it would help if I said 'very, very, very, very, very, very, very, very, very, very high'?

    I was imagining something like a grouse tied to a weather balloon.........

    icon is the best I can do...

  16. Graham Bartlett

    @Ross 7

    When it comes to putting a human in it, it doesn't matter how fast it ends up going - the important bit is how much acceleration you experience.

    Let's say you're dropped from a parent aircraft at 36,000ft at 150m/s (335mph). According to Wikipedia, Mach 1 is 295m/s at 36,000ft. If we assume most humans can tolerate 4G with reasonable discomfort, then we want to be accelerating upwards at 3G. If we want to end up at 20G (2950m/s), we need an extra 2800m/s. 1G=9.81m/s/s, so with continuous at 3G acceleration, that only takes 95s. Which is perfectly acceptable. Dial this back to 3G on the people inside, 2G aircraft acceleration, and you're talking 142s.

    As for ejection, they figured that out ages back for the F111. You don't eject the pilots individually in their seats and faff about with canopies and stuff like that. No, you just blow the whole front of the plane off, because it's already a nice strong armoured unit designed to protect the pilots. Then the whole thing floats down on a chute like a space capsule.

  17. This post has been deleted by its author

  18. Stevie Silver badge

    Bah!

    Yet another initiative that gets us back into bed with Big Oil.

    When may we expect a hybrid version of this technology?

  19. Anonymous Coward
    Joke

    I 'll wait for the Dyson vortex air intake edition

    Dyson could do better :).

    On a serious note - why use fuel when they could just do all the computational processing on a group of nvidia GPU's and and run the heatsinks into the expansion chamber to use those to heat thinks up. Would be one solution to passive cooling GPU's :D.

    What next - sensors that detect the thermal boundiaries and ride between those.

    Another thought - how small can these be scaled down - but guess would end up using the expansion chamber as the main flight winds on that scale.

    ANON for the bad comedy

  20. John Smith 19 Gold badge
    Coat

    Some notes on Hypersonics

    "I can't see turbojet -> scramjet being a particularly pleasant (or survivable) experience for a squidgy human. "

    Acceleration is *not* velocity. Provided the environment system is up to the job (and that's a pretty *big* if) the crew won't have a problem.

    Then there's the whole issue of materials science - throw away experiments (as impressive as this one is) are one thing, having a boat that can fly mach 5+, be refuelled and turn around for another sortie without bits (like engines, wings etc) falling off is another.

    Correct.

    The friction on that thing must be crazy. I know it's not quite in the region of atmospheric re-entry, but it ain't *that* far off.

    Actually it's *very* close to re-entry conditions in terms of heat input per unit area, *especially* in the intake area. Proposed solutions included ones proposes to cool the Shuttle wings on reentry.

    I don't imagine ejection would be that healthy an option either!

    As pointed out a self contained crew capsule would be the proffered option. BTW the F111 capsule had some "issues," concerning misfiring of the guillotine severing the control cables prior to separation. A number were lost over Vietnam before this little fault got found out.

    Hope they (as in America) don't think that using it as a cruise/exocet type missile is a good idea tho. You can imagine the news story already - "an American missile today destroyed a terrorist training camp killing 80 combatants. On its path to its target its shockwave destroyed 14 schools and 3 market towns killing 1500 civilians. Officials said they were 'looking into the matter'"

    Ballistic flight path. Likely to remain *very* high as long as possible. Inlet pressure is a multiple of surrounding atmospheric pressure. IIRC at M3 it is 37x surrounding air pressure, but at 60-80 000 (roughly SR 71 ceiling) it's roughly 1/16 sea level. Running full tilt at ground level would rupture it unless *ridiculously* strong.

    Hypersonic cruise has *some* potential uses. Boosting to orbit (especially single stage) is *highly* speculative.

    Mine would have a PMP loaded with "Facing The Heat Barrier" including the history of NASP and its, er, "creative" promoter Anthony DuPont.

    1. Il Midga di Macaroni
      Headmaster

      F-111s lost over Vietnam?

      Six was the figure. Yes technically it *is* a number but your statement was misleading.

      BTW there's a hyphen in F-111. It's an American concept, foreign to the British forces. But then so is the concept of a supersonic swing wing bomber with more range than any other save the B-1.

      Farewell F-111s. You will be missed.

  21. Anonymous Coward
    Anonymous Coward

    Title, required, one, government issue.

    Ever notice how for speedy progress we need to be in a war? You know, getting from tri- and biplane to monoplane, getting the engines up to serious capability, jet engines, long range rocketry, getting into space even if that war was mostly fought by posturing and proxy, and so on and so forth. Right now the US is actually slacking because they already have a large fat technical edge over their enemy du jour, and they change enemies too often, so the big advances are in subtly or not so subtly destroying privacy in the name of finding new citizen-enemies to pursue. And in pork barreling for the beltway bandits, of course, but that's a given since we're all well and truly caught in the military industrial complex, currently expanding into security scareware. It's big business.

    The thing is, beyond a certain point the earth gets too small to have a jolly old war big enough to advance the current technology in the traditional way.

  22. fred #257
    Terminator

    R.I.P. X-51A

    "Something then occurred that caused the vehicle to lose acceleration. At that point, the X-51A was terminated as planned."

    Sounds terribly... final. Was this a CIA project then?

    1. Charles 9 Silver badge

      No. It simply means...

      ...that the craft ran to the end of its useful life and, having served its purpose, was allowed to complete its willy-nilly course into the deep blue sea. Meanwhile, back at base, the eggheads are going, "OK looks like something came up about 200 seconds in. Let's take the data back home and see what we stumbled upon." IOW, not preferred but not unexpected, either. Initial tests going awry is actually par for the course (which is why they have a few more remaining). As others have said, simulations can only take you so far; eventually you have to just get out there and, well, blow stuff up.

  23. BossHog
    Thumb Up

    Worth it just for this...

    "modified B-52 bomber test mothership"

    Get in!

  24. Dodgy Geezer Silver badge
    Grenade

    Not a problem...

    "...beyond a certain point the Earth gets too small to have a jolly old way big enough to advance..." AC

    Just do what the Brits used to do.

    Pick a war with a small nation, preferably one with mountains around it to keep the war localised. Pick a nation which is small, but has a world-renowned fighting reputation. Examples might be the Irish, Zulus, or Afghans. At a pinch, the Scots. But not France or Italy. Nepal, certainly, but not Tibet, who would just gaze at you uncuriously if you drove a tank up their main roads and offer you flowers...

    A small nation means that Treasury will only allow you a small budget, so it won't cost much. But fighting a warrior nation means you will have to work hard and apply maximum ingenuity to stay alive. Together, these provide the perfect recipe for maintaining a well-honed but low-cost military and a thriving weapons research industry.

    The Israelis have been doing this ever since they were invented...

  25. Richard Jukes

    Yup

    As some other chap said, Tridant Replacement. Or indeed they could just use conventional missile war heads. They could be launched from anywhere in the world and would bring an entirely new slant on artillery strikes...

    1. Bitsucker
      Black Helicopters

      Warheads? Why warheads?

      The Waverider weighs about 4000 Kg (dunno how much of that is fuel though). If it's reached Mach 5 so far, that equates to a collision impact energy of up to just over a ton of TNT. That's about as much as a standard Tomahawk cruise missile carries, and 30% more than the Tomahawk warhead used against hardened targets.

      If a militarised Waverider could be aimed accurately then just hitting something would probably destroy it, and (as the Youtube video of a F4 being driven into a concrete wall shows) nothing identifiable/incriminating would be left afterwards.

  26. Chris 242
    Coat

    @Ross

    Looking into it.

    Reminds me of the old joke.

    A huge hole has appeared on the M25, the police are looking into it !

    On the same pathetic humour theme:

    Burgulars have stolen the toilets from paddington police station.

    Authorities say, they have nothing to go on !!

    Shiite jokes, coat on - walking down the wet/windy street of social procrastination.

  27. Chris 242
    Grenade

    Replacement for Trident ?!

    How fast is the response ICBM versus a land/sea based M5 scramjet minimal warhead?

    Although I" love the bomb" and all the reasons why they are deterents.

    I find it hard to believe that delivering death at mach 5 is any different than carpet bombing.

    This brings us back 50 years to the Blitz.

    Comments Please.

  28. John Smith 19 Gold badge
    Happy

    History repeats itself, badly

    Prior to the 1st generation ICBMs the US was developing the Navajo 2 stage ramjet powered cruise missile.

    It was *huge*, expensive (and like *all* defense programmes getting more so as the number of unknown unknowns kept increasing) *until* the point it was realised that as the bomb was getting smaller and it gave *incremental* (M2 fighters were in the field and it was expected their top speed would keep rising till eventually one *could* chase down an M5 missile)

    So why not go the whole hog, use rockets and make the payload go *really* fast. The orbiting of Sputnik1 also helped convince planners that it *could* be done (the Soviets had done it).

    BTW I'm not sure if it's built into the physics of the combustion process but Scramjets seem to have a narrow operating Mach range. While a regular ramjet works well at just above M1 this one seems to have *needed* M4. A number of modern military planes could manage a M1+ wing or bay launch, eliminating the booster entirely. If you bought up some F105s (1 to fly, the rest for spare parts) you *could* get release of at least 14000lb at M1.98 at 29000 at least (NACA wind tunnel tests. You should get more with low fuel loads and stripping the airframe of most of that 1950's Cold War avionics kit).

    *Might* get you SSTO, probably won't. The *massive* Isp *always* look good on paper. It does not last up to full orbital velocity.

  29. Chris in NZ

    We feel a lot safer now...

    >The technologies in the Waverider, however, might see the appearance of hypersonic missiles in the near future, able to travel huge distances quickly and close in on their targets so fast they would be almost impossible to defend against.

    Fantastic news indeed! Not.

    1. BorkedAgain

      Well quite.

      I'm never too comfy at the thought of anyone actually winning an arms race. Can't see too many realistic peaceful uses for this; all that "cheaper access to space" stuff sounds too much like a sop...

  30. John Smith 19 Gold badge
    Coat

    @BorkedAgain

    "Can't see too many realistic peaceful uses for this; all that "cheaper access to space" stuff sounds too much like a sop..."

    You're right to be suspicious.

    Hypersonic flight is a bit like fusion power. Good for generating a nearly inexhaustible supply of PhDs with *limited* use outside their (highly) specialized areas.

    As an *idea* its advocates break down into 2 groups. Those who would like to field a hypersonic (M5+) vehicle to *prove* you don't need rockets to go this fast, *ideally* leading to a hypersonic passenger carrier (say 3x the speed of Concorde). the 2nd group reckon the way to lower launch costs is to get more "aircraft" like operations. In this mindset this is a stage in ramping up the aircraft speed until *ultimately* it gets to launch velocity.

    Note that the missions are *qualitatively* different. Cruise in a missile or passenger is fine. It can run rough at startup and getting up to speed because that will be transient. A launcher wants to be *continually* accelerating. If it's not its crashing. This implies a launcher engine is *continually* altering geometry to compensate for its acceleration. And if (big if) it *can* get to M8, that still leaves getting to orbital velocity at M23. It's now above 95-99% of the atmosphere (so any wings are pretty useless).

    There is a *reason* NASA chose to pursue RBCC (Rocket Based Combined Cycle). A launcher will need rocket drive. Treating it as a *systems* integration problem and designing it in up front helps spread the weight. IN a cruise vehicle (passenger carrying or not) it would eliminate the dropping of booster rockets at the expense of a smallish oxidizer tank.

    What *really* lowers weight is junking the wings, landing gear and huge high temperature (huge x high pressure x high temp capable = V. heavy ) air handling ducts running end to end and replacing them with a well designed rocket engine.

    Honestly figuring the weights of these things at the concept design stage is tricky enough that *any* group of supporters can concoct a set of assumptions that will *prove* their preferred development plan will solve the problem. You might note that the USAF (whose top brass *love* the idea of crewed *very* fast aircraft and would bend over backwards to justify one) have still *never* managed to find a mission it can do better *enough* (than a crewed aircraft pushing at most M2) to justify the huge development costs.

    Human beings have built at least a *dozen* orbit capable rocket systems in the last 54 years. No one has flown a *single* hypersonic system (1 or more stage) to orbit.

    Air is *bulky*. Liquid oxygen is not (by about 700:1) and pretty cheap. Storage tank design is well understood and can be surprisingly compact as long as you avoid a propellant that is colder than LOX (other heat leaks accumulate pools of LOX. Add spark for "interesting" effects) or insanely dangerous to handle (OF2 for example. It can burn through concrete).

    Mine will still have a copy of "Surviving the Heat Barrier" in it.

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