New reports suggest that Virgin Galactic’s SpaceShipTwo will be incapable of flying to an altitude of 100 kilometers.


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A new report suggests that Virgin Galactic’s SpaceShipTwo will be incapable of flying to an altitude of 100 kilometers.

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8 comments

  • Pzatchok

    Isn’t Helium non combustible?

    If so wouldn’t adding it to the engine mix cause the engine to lose power?

    Making the engine less powerful could be how they plan on keeping the vibrations down to a manageable level.

    But that would also mean a lower top speed than what they have already reached and thus a lower altitude.
    Unless they think they will be able to keep the engine running for a longer time thus making up the difference.

  • Dick Eagleson

    Anyone who has ever inhaled helium from a balloon knows it causes sonic changes in the gas column it is mixed into – hence the high “Donald Duck” voice of the helium huffer. Blowing pressurized helium into the exhaust stream should raise the pitch – i.e., increase the frequency – of the exhaust plume just like it does to the human voice when inhaled. Presumably, this has a beneficial effect on the engine vibration problems. I can see how this might be so. For a given total amount of energy driving a vibration, raising the frequency also lowers the amplitude. That seems likely to be the object of this exercise.

    As to helium’s non-combustability, that is certainly true, but helium also has a very low molecular weight. In theory, it could improve the engine’s specific impulse to lower the average molecular weight of exhaust molecules, but the helium would also absorb heat from the combustible fuel and oxidizer to get it moving and this would have the effect of lowering the average velocity of other exhaust molecules. Not sure what the net effect is. Might be close to a wash. If the change to engine vibration/resonance modes was sufficient to allow an increase in oxidizer mass flow rate, though, then more fuel would get burned faster, thrust would increase and perhaps do so with adequate margins to make the complete, minute-long burn VG needs to make to get SS2 anywhere near space.

  • Edward

    Human organs have a natural frequency around 20 Hz, and with strong shaking or sounds, they will vibrate against each other until they have turned to mush. Avoid that frequency range!

    It is my understanding that, since the speed of the reaction mass passing through the throat of the nozzle is always at mach 1, heavier molecular weight results in greater thrust. It is a reason that solid rocket fuels sometimes/often contain such things as aluminum and iron oxide. They burn to produce the energy needed, plus the resulting heavy molecular weight produces a greater mass flow and thrust.

    This, and (as you noted) the energy lost heating it, may be why they can’t make it to 100 km with the helium injection.

  • Pzatchok

    Good catch Mr. Eagleson.
    I never thought of that.
    It would require quite a bit of He though unless they only intend to ‘change the pitch’ of the vibrations a very little bit.

    The longer this Virgin space race goes on though the more and more its starting to look like a very stubborn man refusing to change his idea. It looks like Branson is willing to reduce seats just to stay with his original engine fuel choices.
    By reducing seats he could be taking the whole profit margin right out of his flights.But then again I don’t think this is about profit anymore. Its all about advertising for his brand. He no longer has any real goal of making space or making a flight company.

    This He trick could be the cheapest way they could think of of making the ship safe enough to fly.

  • Dick Eagleson

    Rocket exhausts are not limited to Mach 1. That’s why so-called “Mach diamonds” form in the exhaust plumes of rockets fired in atmosphere; the plume is doing its best, against atmospheric pressure, to be as supersonic as the energy of its chemistry can make it. Specific impulse is closely tied to exhaust plume velocity. This velocity tends to maximize for highly energetic reactions involving low-molecular-weight reagents.

    The highest Isp fuel-oxidizer combo in common use, for example, is LOX-LH2. The result of this combustion is all water, molecular weight 18. Methane (LNG) burns, using LOX oxidizer, to two water molecules (combined mol. wt. 36) and a molecule of carbon dioxide (mol. wt. 44). The average exhaust molecule in a LOX-methane exhaust plume has a molecular weight of 36+44/3 = 26.67. The Isp of LOX-methane is lower than that of LOX-LH2. Kerosene has, proportionally, much more carbon than methane. Methane has four hydrogens for every carbon, hence the two water molecules when it burns completely. Kerosene has, essentially, two hydrogens for every carbon, thus only one water per carbon dioxide when it burns. The average exhaust plume molecule from a LOX-kerosene engine, therefore, has a molecular weight of 18+44/2 = 31. Sure enough, the Isp of LOX-kerosene is still less than that of LOX-methane.

    The inclusion of metal powders in solid fuel formulations is not done because heavier exhaust molecules are better – they’re not – but because light metals are the lightest fuel molecules – that’s right, the metal powder is the fuel – that can be combined with the usual oxidizer and binder chemicals to make a slurry which is castable and still forms a stable solid at terrestrial surface temperatures. Fuel metal in solid motors tends to be mainly aluminum. There are lighter metals: magnesium, sodium, etc., but these are generally too-easily touched off by accident to be safe in most weapons-related solid fuel applications. Needless to say, one doesn’t want touchy fuel in a man-rated vehicle either. Iron oxide is sometimes used in solid motors, but not as a fuel or oxidizer. It functions as an inert limiter of the fuel-oxidizer reaction rate.

  • Dick Eagleson

    You got it. Based on what the article had to say about the helium tankage displacing two whole seats in a six-seat vehicle, I think it’s very fair to say that it takes quite a bit of helium to do its thing. I share your mystification as to why Branson hasn’t long since given up on the rubber-nitrous hybrid engine as an unfixable bad job and moved on to something else. Even switching to nylon-nitrous several years ago would, in retrospect, seem to have been a call he could have made and maybe gotten aloft a lot sooner. Of course it remains to be seen if nylon-nitrous is a real solution or just a less-marginal alternate problem. Personally, I’m no longer expecting any more powered test flights soon, nor to see revenue service commence by year’s end. I’d love to be wrong on both scores, but VG has long since used up any slack I’d otherwise be inclined to cut them. It’s looking more and more like the good old plodding tortoise, XCOR, may well beat the flashy rabbit, VG, into both space – at least the 50-mile-high kind – and regular service. Swashbuckling flash is only a viable marketing strategy if you can back your brag.

  • Pzatchok

    We used to mix powdered magnesium with an oxidizer to make ‘fire works’.

    Quite an energetic little mix I must say.

  • Edward

    I reviewed the equations, and you are correct: lower exhaust-gas molecular weight produces higher thrust. The problem of relying on an increasingly failing memory.

    “Rocket exhausts are not limited to Mach 1.”

    That is the beauty of the bell portion of the engine. Because the exhaust is at mach 1 at the throat (that is the limit), instead of slowing when the stream widens, as would happen with an incompressible fluid, it accelerates and produces even more thrust by the time it exits the bell. What magic!

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