Supersonic Man

May 30, 2016

the dimensionality of torque

Filed under: Hobbyism and Nerdry,thoughtful handwaving — Supersonic Man @ 10:33 am

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1 Comment »

  1. Here’s another dimensional abuse I want to complain about: the way they use the term “specific impulse” in rocketry. What it actually means is the amount of velocity change that a given mass of fuel can give to an equal mass of material being pushed. In short, it’s simply the exhaust velocity of the flame coming out of the bottom of the engine. But rocketeers don’t like to express it as a velocity; they express it as a span of time. “SpaceX’s Merlin 1D engine, burning RP-1 kerosene and LOX, has an ISP of 281 seconds in sea level air, or 311 seconds in vacuum.” They do this by conflating mass with weight, meaning you have to multiply the time by Earth’s gravity to get the true impulse.

    One might suppose that their excuse is that they’re always thinking about lifting stuff up against gravity. But it’s been claimed that the real reason has something to do with finding common ground with how airplane engine makers talk about specific impulse. (In those engines it doesn’t equate to exhaust velocity — it’s more complicated.) That does’t make a ton of sense to me. Either way, let’s use velocity, dammit, because that’s what it actually is.

    Comment by Supersonic Man — April 23, 2017 @ 4:01 pm | Reply

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