15 votes

SpaceX's Starship prototype aces first untethered hop test

2 comments

  1. [2]
    zlsa
    (edited )
    Link
    This is the first flight of SpaceX's Starship prototype, known as Starhopper. Starhopper is powered by a single SpaceX Raptor engine. The Raptor engine is a powerful full-flow staged combustion...

    This is the first flight of SpaceX's Starship prototype, known as Starhopper. Starhopper is powered by a single SpaceX Raptor engine. The Raptor engine is a powerful full-flow staged combustion engine, which improves efficiency and performance at the cost of complexity.


    Drone view of the flight: https://twitter.com/elonmusk/status/1154674872041103360
    Engine view of the flight: https://twitter.com/elonmusk/status/1154629726914220032

    Why this is important

    Essentially, rocket engines need pumps to pump fuel into the combustion chamber itself. Most engines today [1] are powered by turbopumps, where the pumps (one each for the fuel and oxidizer) are powered by a gas turbine, running off the same two fuels. The resulting exhaust from the gas turbine is dumped overboard (very prominently visible in this photo of a SpaceX Merlin 1D engine test.)

    However, dumping the exhaust overboard reduces efficiency. The gas turbine is typically run fuel-rich to keep temperatures down (basically using some extra unburnt fuel to carry the heat away.) This means a fraction of the fuel is essentially wasted and does not contribute to the thrust of the engine.

    The Space Shuttle Main Engines (aka SSME, aka RS-25) were staged combustion cycle engines. In a staged combustion cycle engine, the two propellants are combined in a preburner, and the resulting high-pressure fuel-rich exhaust is routed through the turbine and then to the main combustion chamber. The remaining fuel in the preburner exhaust is burnt in the combustion chamber, which increases efficiency (and performance) compared to gas-generator engines.

    Full-flow staged combustion is even more complex. Rather than the one shared turbopump shaft of gas-generator or staged combustion cycle engines, FFSC engines have two turbines, each one running its own pump (one for fuel, one for oxidizer.) There are two preburners, one running fuel-rich and the other running oxidizer-rich. Unlike a normal staged combustion engine, which must still feed fuel and oxidizer directly into the combustion chamber alongside the preburner exhaust, FFSC engines run all of the fuel and oxidizer through the turbines. This improves performance in a number of ways:

    1. Because the preburners have such an extreme ratio of fuel to oxidizer (and therefore much more mass flow than other engine types), the remaining unburnt propellant helps keep the turbine much cooler, improving engine lifetime.
    2. Because the propellant all goes through the preburner (and is heated up by the combustion within the preburner), much of the propellant is gaseous, not liquid. This improves combustion efficiency in the combustion chamber. (This gasification of propellants to improve combustion efficiency is exactly what fuel injectors do for the engine in your car[2].)
    3. Unlike gas generator or normal staged combustion engines, FFSC engines don't need to have a single shaft for the turbine (very very hot), the oxygen (very very cold, making metals extremely brittle), and the fuel (which cannot come into contact with the oxidizer at the risk of engine-rich combustion [3].) This keeps the disparate fluids separate, simplifying the materials chosen for the turbine shaft, housing, and any seals.

    The primary downside of FFSC engines are the inordinate engineering complexity compared to gas generator or even staged combustion engines[4]. There's also the careful management of two turbines: now, you have two turbines and pumps to keep track of, and the fuel to oxidizer ratio must be carefully maintained by controlling the ratio of propellant sent to the preburners.


    Now, back to Starhopper. It's basically a flying water tank (no seriously, the thing was built by Caldwell Water Tanks.) Starhopper is a shortened prototype of the future Starship[5], which will have longer fuel tanks and an area at the nose for cargo and crew.

    If you've ever seen close-up photos of Starhopper, you'll have noticed that there's not much flattering to say about its construction. But smooth surfaces and sealed pipes aren't necessary if you're just flying a a couple of kilometers and staying well below Mach 1. Starhopper's primary purpose is to test the Raptor engine in-flight, with all the moving and shaking that entails. It also gives SpaceX a great test article for trying out new ground-side equipment, procedures, and other aspects not strictly related to the vehicle itself.

    At the same time, SpaceX is building the structure for the first full-scale prototypes in Florida and Texas. Elon's (wildly optimistic) timeline for these is first flights in 2-3 months, then reaching Earth orbit in another 2-3 months (presumably with just the single stage, which would also be a new first if they pull it off.) That said, don't expect it to happen that quickly. I'd be surprised if either of them hits 10 kilometers altitude by the end of this year.


    [1]: Rocket Lab's [Rutherford](https://en.wikipedia.org/wiki/Rutherford_(rocket_engine)) engine is a notable exception with its battery-powered fuel pump.

    [2]: Yes, you drive an EV. I'm not talking about your car.

    [3]: When engine parts begin to enter the turbine. This is not a good thing, and will typically result in complete destruction of your rocket engine.

    [4]: Yes, the Raptor engine in the photo probably has a lot of development sensors on it.

    [5]: Source: https://twitter.com/elonmusk/status/1122829331401601029

    15 votes
    1. sqew
      Link Parent
      Also notable is the fact that last night's hop was the first ever free flight of a full-flow staged combustion rocket engine. Neither the Soviet attempt of the 1960s, the RD-270, nor the US's one...

      Also notable is the fact that last night's hop was the first ever free flight of a full-flow staged combustion rocket engine. Neither the Soviet attempt of the 1960s, the RD-270, nor the US's one attempt, the IPD, ever made it into the air.

      It's absolutely impossible to understate just how impressive it is that SpaceX managed to get a FFSC rocket engine off the ground. They had some hiccoughs along the way ("liberated" turbine stator, etc.), but they pulled it off.

      5 votes