31 thoughts on “Hypersonic Hype”

  1. Whatta drag. SABRE is a wonderful technology solving all the wrong problems.

    Starship P2P is going to own that business, esp. if they can minimize the 0g experience most travelers in that mode don’t want.

      1. If the fuel is liquid hydrogen, the plane will rapidly disintegrate due to hydrogen embrittlement. The people I knew who worked on NASP said that was the ultimate reason the program was killed; the titanium aluminide skin material was never able to survive hydrogen embrittlement.

        Similarly, the X-33 aerospike was to have undergone a subscale flight test mounted on the back of one of NASA’s SR-71s (the LASRE project). Steve Ishmael, an Armstrong (then Dryden) SR-71 pilot, told me that, despite having a miniscule amount of LH2 aboard, the engineers were so concerned about “casual hydrogen” embrittling the SR’s titanium skin that the project was cancelled.

        1. Titanium reacts exothermically with hydrogen. TiH2 is used as a hydrogen storage compound and neutron shielding material.

  3. Easy way to avoid aero drag – fly where there is no aero.
    Starship or more likely a derivative modified version thereof will own that business as David Spain says. Great for priority cargo and I’m not sure that an hour or less of zero gee, looking at the Earth from space might not be a huge attraction.
    SABRE isn’t even a good first stage. As Rand has pointed out, liquid oxygen is cheap.

  4. Let me know when they have even a prototype, I should live so long.

    If you look at the URL, it’s on a thing called Luxury Launches which means it’s a press release written to read like a news story. The real give away is that the grammar and syntax are far too good to be produced by the hacks that would work for a “real paper” at the wages they pay now days. The actual information content is about the same.

    The tell that there is nothing there and never will be is when they talk about cooling the intake with liquid He. Are they planning on just letting it boil off? If not, they not only have to build an engine that I find entirely implausible and an air frame that somehow manages low hypersonic drag while providing commercially viable volume, but He liquefaction in flight as well. I’d love to hear how they intend to get rid of that heat.

    Bad Star Trek fanfic is more plausible than this.

    1. The helium is just a heat transfer fluid between the engine intake and the liquid hydrogen tank. They could run liquid hydrogen directly through the intake heat exchanger. But if they did, and there’s a leak, bad things would happen. I haven’t looked at their heat-exchanger materials to know if H2 would also cause embrittlement issues with it.

      I assume the design will just have lots of little valves to close off any parts of the heat exchanger that start leaking, otherwise they might run out of helium mid-flight. I don’t see that as catastrophic, since it would just mean they couldn’t run the engine in hypersonic mode, but could still run it through the slower parts of the flight envelope.

      As for the liquid hydrogen, I ran some calculations to compare it to using LCH4. It came out about the same on heat capacity versus the relative surface area of the fuel tanks. The trouble is, unless seriously super-chilled, liquid methane isn’t cold enough to liquify inert heat-transfer fluids like helium, nitrogen, or argon.

      1. What happens to the heat? That’s a lot of He to load if it’s one and done, I’m not going to waste time trying to figure out how much.

        It doesn’t even make sense to use liquid He, it’s the worst possible heat transfer fluid unless you need to hold something near 0°K. The operative word is still transfer; to where? How are you going to build a heat exchanger to dump it the the air when just exposing it to the air steam heats it red hot.

        The whole thing is a collection of random buzz words to impress investors with more money than brains.

        1. Working “fluid” for the HTGR reactors — a situation where you need a heat transfer fluid that will not react to anything else. Why would anyone use it in any non-nuclear engine…?

          1. He is a very good heat transfer fluid, non-reactive, very good capacity for a gas, only H is better. Low pumping losses.

            This is supposed to be liquid He. Where it is used as a liquid you have to be very careful it NEVER gets above critical temp. Once it does, it takes a large plant to refrigerate it back to a liquid. When a superconducting magnet is quenched, you have to vent it and replace it, very expensive.

            They have to be trolling for really stupid investors. If it is going to use liquid He, it must be really good.

            In my mind, it’s gone from wildly speculative to pure scam.

  5. The Sabre engine uses gaseous, not liquid helium, modestly compressed by a blower driven by the main turbo assembly (possibly with its own turbine, I’m not sure). It liquefies the inlet air, then transfers that heat to the high pressure LH2, vaporizing it and chilling the GHe down to around 30-40K, cold enough to liquefy the air. Round and round she goes…

    GHe is a decent heat exchange fluid, we cooled the aluminum nozzle of the 5K18 LOX-kero engine with ~100K GHe at around 600 psi, with modest pressure drop through the nozzle coolant passages, keeping the aluminum happy against a stagnation temperature over 3600K. I did the heat transfer calculations, predicted that it would work, then tried to look nonchalant the first time we ran it. Sweating bullets.

    But I agree that the Sabre engine is a solution in search of a problem. Starship is going to eat pretty much everyone’s lunch.

    Riding Starship on a P-to-P flight would be real freaking sporty, and that 90-degree pitchup and multi-gee landing at the end will be an E ticket ride. Sign me up.

    2. It’s going to take a lot more LH2 to cool the entire air stream than the O2 in it will burn. And the “article” said liquid He.

    3. I suspect the Moneyball business traveler isn’t really interested in an E-ticket ride every other day. SpaceX is gonna find that a nice comfortable ride is going to drive ticket sales, not the Space Mountain experience.

  6. Does anyone know the cross-range for Starship alone? Would the single stage be good enough for suborbital P2P throughout CONUS in 45 minutes? Then, for international travel, you bolt on the Super Heavy to convert Starship into an ICBT.

      1. Now that is cool, but for it to be effective SpaceX is going to have to seek environmental relief against sonic boom ordinances. Good luck with that…

  7. The heat exchanger tubing material in SABRE is just stainless hypodermic tubing- 301L, typically. It has no hydrogen embrittlement issues.

    The real secret sauce has to do with how they avoid icing on the heat exchanger tubes. After all, the engine will be taking in air containing water until it gets above around 15 km. I’ve seen how they do it, but I think I’m still under NDA. It’s pretty clever, and they’ve demonstrated it completely.

    Still, Reaction Engines is still developing an engine. Aircraft development requires airframe/engine integration from the get-go, and the higher speed the aircraft, the more important that early integration is. If you look at the X-51A, you’ll see how tightly integrated the airframe and propulsion are.

    They may have a fine overall approach to developing an airbreathing hypersonic propulsion system, but before it flies, it will have to be started from scratch in the context of a hypersonic aircraft design process.

    By the way, all vehicle engines, of any kind whatsoever, produce a momentum exchange with the vehicle surroundings. Thus all vehicle engines are “reaction engines.”

    1. By the way, all vehicle engines, of any kind whatsoever, produce a momentum exchange with the vehicle surroundings. Thus all vehicle engines are “reaction engines.”

      Oh contraire Michael. Here is my design for a fully reaction-less vehicle design that depends on propulsion simply on a space-time anomaly known as a moving wormhole. Totally reaction-less. I’m thinking of applying for a patent. Still working on making it determinate in the temporal dimension however….

      http://media.liveauctiongroup.net/i/5927/8746687_1.jpg?v=8CC43F73963EB30

    2. The real secret sauce has to do with how they avoid icing on the heat exchanger tubes.

      Hm. That’s a shame. SABRE would be a hell of a way for making a lot of ice quickly for those flash mob patio parties….

      2. Now seriously, folks, I could use some of that Secret Sauce to prevent my eyeglasses from fogging talking to you with this mask on.

        I’ll be here all week — try the veal.

  8. The Sabre anti-icing secret sauce is methanol wetting the OD of the HX tubes (I saw a public article about it a year or two ago) and the diluted ME is regenerated by a thermal process IIRC. Like the Napier Nomad turbo compound diesel, it is an engine designed to be shot down before first overhaul.

  9. Correction to my comment above, the air is deeply cooled but not liquefied. The methanol-weeping method was disclosed quite a while back, a summary here:

    https://en.wikipedia.org/wiki/SABRE_%28rocket_engine%29

    The system diagram shows *four* turbopump spools, two driven by helium, two by hydrogen, and the helium loop has shaft seals against both LH2 and LOX, so contamination of the GHe can have, er, exciting results. Three heat exchangers and a staged-combustion preburner add extra thrills.

    I love the caption “Simplified Sabre Cycle”. Riiiight.
    https://upload.wikimedia.org/wikipedia/en/d/dc/Sabre_cycle_m.jpg

    1. I don’t think I’d have done it that way. In fact, it would probably have been simpler and more reliable to make a pure turbojet with cooling via the liquid helium loop, and then a separate rocket engine that doesn’t use helium at all.

      As it is, the heat exchanger for the inlet air is always in the helium loop, and the helium loop also directly runs the turbines for the LOX and LH2 pump. The LH2 pump is required even in airbreathing mode.

      If there’s a leak in the heat exchanger for the incoming air, the entire helium loop is going to get vented and the engine, in whatever mode its in, is going to shut down. The helium loop is acting like a cars uni-belt. If that leak occurs during a critical flight phase, which would be the phase between takeoff and normal landing, the vehicle becomes a glider.

      Yet the only thing the helium loop is directly required for is chilling the incoming air above Mach 3, but before the engine mode switches to rocket power entirely.

  10. I don’t think the Starship re-entry and landing will be that uncomfortable. In airline type seat, head towards nose of Starship, your back towards heatshield side. Eyeballs in, 3 g is no big deal.
    Then during the skydive part you are on your back at 1 g. Engines start for flip which feels like the seat tilted forward some degrees (maybe up to 45 degrees) and you got a little heavier. During the flip the seat apparently tilts forward the rest of the way to upright. Not too different from lying in your favorite lounge chair and flipping it to upright quickly. You are on the ground stopped before you’ve had time to think about it.
    Alternatively you tilt the seat backwards during the flip at the same rate as the flip, so the ship rotates around it which is the position that needs to be available for launch anyway. Once landed, seat tilts forward so you are sitting upright. To prevent optical confusion wear VR googles with stabilised outside view of horizon. The adventurous can observe inside of cabin and view out of windows, if any.

  11. I am guessing you could not launch a fully fueled starship {the reusable second stage} from runway on Earth. Though maybe from
    from Mars.
    So I mean having some kind horizontally launched mothership carrying the starship.
    Or in terms of it’s structural strength, it must be launched vertically?

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