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I've seen countless photos of rocket launches that show the path taken by a rocket, and they always seem to arc rather than go in a straight line. Like the following:

photo of gentle launch arc

The adage "the shortest distance between two points is a straight line" comes to mind when I see this, so I wonder: am I overlooking something because reality is in three dimensions or is there another reason to launch a rocket in an arc?

Nathan Tuggy
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Jon
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1 Answers1

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Drag is highest at low altitudes, and drag can cause problems with heating, pressure strain on various parts, and of course slowing acceleration. So the rocket starts by climbing above most of that, then converting most of its upward velocity into sideways and accelerating a whole lot more to get up to orbital speed (nearly 8 km/s, or about an order of magnitude faster than most bullets; slower, higher orbits require even more energy). This acceleration takes the vast majority of the rocket's fuel, at least 90%.

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Nathan Tuggy
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    You might expand this to make it clear that orbit requires extremely high horizontal speed, not just altitude; this seems to be what most people are missing when they ask this question. – Russell Borogove Sep 02 '15 at 19:30
  • @RussellBorogove: Added a link to an answer about delta-V breakdown I found while checking for dupes earlier. – Nathan Tuggy Sep 02 '15 at 19:36
  • @NathanTuggy I think a bit more detail about the required horizontal speed should probably be dropped directly into this answer, too. Also, a thought: Would it be possible (and maybe fuel efficient) for a spacecraft to achieve GEO directly above its launch site by launching straight up, and would that launch still appear curved to the observer? For anything other than GEO, the curve makes (relatively) obvious sense. But it's a bit less intuitive for a near-site GEO launch. (Not that that happens a lot.) – Iszi Sep 02 '15 at 21:21
  • @Iszi: Maintaining the same angular velocity as you move outward requires a large increase in momentum. In principle this acceleration could be spread across the climb proportionally, so that the rocket would be thrusting at various angle while rising (relatively) slowly straight up. In practice that's much less efficient, essential to the point of impossibility, because of drag, the range of thrusts needed at different altitudes, the necessity of sustained powered hovering at low altitudes where orbital speeds are impossible, and so forth. – Nathan Tuggy Sep 02 '15 at 21:43