stuff that’s hard to do (Atlas 5 this morning)


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  1. Yeah, that’s pretty impressive, especially the way that the smoke changes colors. Or that orchid shaped bloom on the end.

    I know, curvature of the earth and all that, but these launches always look like they’re coming right back down again. I’m not sure I understand the whole contrails thing, (shouldn’t the plume look smaller the further away the rocket is?) so feel free to do some rocket-splainin at me if you want.

    Comment by Drew458 — August 9, 2019 @ 1:35 pm

  2. ^ Yeah, the photo’s spectacular because the launch was around dawn, so the lower-altitude smoke is in a cloud’s shadow, then it’s high enough for the sunrise-color lighting, then at high-altitude, it’s in daylight.

    Think of the ascent path as needing to go over the horizon eventually. Orbital velocity is basically horizontal speed, not vertical speed (but you need to get high enough to get out of the atmosphere). After liftoff, the rocket starts to pitch over in an efficient path that goes from straight up to near-horizontal, then it proceeds on over the horizon. It’s like a fast sunset (the Sun goes down, right?).

    Not really a contrail (condensation trail from water vapor produced by combustion in an airplane jet or piston engine when altitude/temp/humidity allow it. See: foggy breath in the Winter).
    When a rocket is going through Mach-1 or so, it may also produce a ring of condensed water vapor (a cloud), usually at a conical adaptor and/or nose (wherever the shock wave begins), as the air is compressed/quickly-decompressed by the shock-wave onset.

    A rocket’s exhaust smoke is narrow near the ground, where engine exit pressure nearly matches the surrounding atmospheric pressure (1-atmosphere, 14.7-psi). As it gains altitude, the surrounding atmospheric pressure drops quickly below 1-atm, but the engine exit pressure is still near 1-atm, so the exhaust begins to bloom out sideways. Up near space’s vacuum (0-atm, 0-psi), the plume gets very wide. When the sunlight catches it, you see a pretty residue of tiny ice particles.

    When you see the exhaust spreading, that’s a reduction in efficiency, since not all of the exhaust mass is traveling directly aft — its sideways action-reaction contribution is wasted.
    Stage-1 engine bells are designed to produce an exit pressure near 1-atm, because max thrust and efficiency are needed for the heaviest case at liftoff near sea level.
    Upper-stages have vacuum engines with a longer, wider engine bell to drop exhaust pressure down closer* to 0-atm for best efficiency in the vacuum of space.
    * See: engineering limitations


    Comment by DougM (speak three names) — August 9, 2019 @ 3:50 pm

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