17 thoughts on “Safe, Simple, Soon”

  1. I’m not an engineer, so I don’;t know whether I’m correct or not, but I’ve always said that when it comes to rockets, one should design the rocket to suit the engines, which means getting the engines largely done first. For example, the Merlin 1C had been used in flight (Falcon F1) before they started building the Falcon 9.

    The RS-25’s for SLS are especially problematic for SLS design because its payload capacity is predicated on a significant performance increase vs. the reusable RS-25s. If it turns out that they can’t get that kind of performance and reliability out of the new RS-25s, they have a major problem.

    1. A performance increase? Over the RS-25? I think we’ll see engines blow up on the pad. I thought it already was operating over the design spec to begin with.

      1. Yup, the original RS-25’s were operating over original design spec, that’s why you had the somewhat jarring (to me) call-outs in Shuttle launches of “throttles to 109%” once they’d upgraded to the RS-25D for the SSME.

        But, taking that 109% as a baseline, the new expendable RS-25’s (Sometimes called RS-25E, though I’m unsure if that’s official) are supposed to have improved thrust-to-weight, improved ISP, and higher thrust.

        The original (late model) RS-25 (RS-25D) had (at 109% of rated power) a vac ISP of 452, and sea level of 366. Thrust to weight was 73:1

        The new RS-25s are supposed to improve on that, though I can’t find the numbers at the moment, plus they kept changing. Last I saw was a design target of 535 vac ISP, which I thought rather, um, ambitious (and quite possibly impossible). That’s a VERY significant ISP boost, especially when coupled with a mass decrease.

        Whether they can achieve that or not is debatable. What isn’t debatable is that the SLS performance specs rely on it happening. If they don’t get it (or close), then SLS won’t have the capabilities claimed.

        1. I can’t remember a LOX/LH2 engine with more than 480 vac ISP. Those engines typically use the expander cycle. Which is not the case of the RS-25.

          I can see them improving the thrust-to-weight ratio by reducing the thickness of the pipes, etc, since it does not need to be reusable anymore. They could also use a single-shaft turbine like the Russian LOX/LH2 engine in the Energia.

          But that’s basically making a new engine. I’m not sure if you can call it an SSME anymore.

          1. SSME is staged combustion, which in theory is even higher Isp. I’ve no idea what they’ve done to make it expendable. I think, as with all involved with SLS, the primary goal is to spend money in the right zip codes.

  2. It’s a good thing they’re reusable because that allows them to test it many times till they get it right. Then they can fly it and throw it away.

  3. Was the “significant anomaly” anything like how the first test flight of Ariane 5 “did not meet all test objectives”? (For those who came in late, that’s how Arianespace (or the ESA?) described the fact that it blew up.)

    1. Well, reusing the avionics software for Ariane 4 on the Ariane 5 without testing it first was not that good an idea, some variable overflowed and the guidance systems decided that up was down.

      So, the engines worked, and things like that. But the guidance software needs to be modified and tested properly.

  4. SLS project management better hope that puppy is repairable. There are exactly enough usable engines for four SLS missions. Lose even one in testing and that drops to three.

    1. What is the back story on how the power head on that engine started shaking towards the end? Some manner of hydraulic hammer (pressure pulses from flow interruptions) from the shutdown? A turbopump throwing a blade?

      I am amazed that something handling that much mechanical, fluid and combustion gas power can trip out like that without doing a “Russian shutdown” (a reference to the N1 booster).

      1. I no longer recall in which of his works it was, but Robert A. Heinlein once made reference to rocket engineers and “the fitful devils that live in their engines.” As with so much else, RAH neatly nailed that one.

        1. Jetmen were the most carefree of the lot, and the meanest. Compared with them the masters, the radarmen, and the astrogators (there were no supers nor stewards in those days) were gentle vegetarians. Jetmen knew too much. The others trusted the skill of the captain to get them down safely; jetmen knew that skill was useless against the blind and fitful devils chained inside their rocket motors.

          The Green Hills of Earth

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