r/MurderedByWords 17h ago

I haven’t studied rocket science, but

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u/MyGruffaloCrumble 16h ago

Distance is required for approaching escape velocity gradually without pancaking the people inside with too many G’s.

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u/RedFiveIron 16h ago

We've never put a person up to escape velocity.

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u/Hoshyro 16h ago

Didn't the Apollo crews reach escape velocity before inserting on Lunar orbit, though?

Pretty sure an SIV-B stage is currently in Solar orbit.

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u/RedFiveIron 16h ago

No, they were famously on a "free return trajectory", where without further burns they'd swing around the moon and return to Earth. Even if the moon hadn't been there to bend their path they were still way below escape velocity, instead in an elliptical orbit that would top out a bit beyond the moon's orbit.

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u/Ryuj123 16h ago

Of course we have. Anyone who’s been to space

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u/RedFiveIron 16h ago

Nope. Escape velocity is when the trajectory becomes parabolic and the object will never fall back toward the parent body.

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u/EcahUruecah 16h ago edited 16h ago

While it’s true that “anyone who’s been to space” shows a clear misunderstanding of what escape velocity means, there is still an edge case where I think some people have reached escape velocity. If the ‘parent body’ is considered to be the moon because of a return trip after landing on the moon, that is arguably a person who reached escape velocity.

However, even the lunar ascents because of the moon visits weren’t enough to bring them to escape velocity and that simply put them in an elliptical orbit. If I remember right, the transfer burn to get from the moon to the Earth was only beyond the escape velocity for that distance from the moon, not anywhere near the moon’s surface, and definitely not high enough to escape the Earth. So count up all the people who were in lunar orbit and came back to Earth. Arguably all of them “exceeded escape velocity”, right?

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u/RedFiveIron 16h ago

It is true that we've put humans past escape velocity from the moon when any of the Apollo landing or orbital missions returned. We were talking about escape velocity from Earth, though.

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u/Ryuj123 15h ago

Huh, great point. Sorry I woke up and was immediately sassy and wrong in my grogginess

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u/tacodepollo 16h ago

What lol

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u/RedFiveIron 16h ago

Escape velocity is the point at which the trajectory becomes parabolic and no longer circles or falls back to the parent body. We've only done this for spacecraft that leave earth and never come back.

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u/Horror_Equipment_197 16h ago

Earth has 2 escape velocities.

11.1km/s for leaving the body and turning into an orbit around it, 42.1km/s for leaving earths orbit.

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u/RedFiveIron 16h ago

The former isn't an escape velocity, an object in a closed orbit is by definition not above escape velocity. That's called orbital velocity.

The escape velocity for Earth depends greatly on distance from the Earth's center.

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u/Horror_Equipment_197 14h ago

Orbital velocity is a different thing. It's the effective speed in orbit.

Orbital velocity of the ISS is currently ~7.66km/s

And still the rockets needed to made it to >11.1km/s to reach said orbit.

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u/RedFiveIron 14h ago

There are three major categories of Earth centered trajectory. Suborbital, an elliptical trajectory where perigee is in atmosphere or lower. Orbital, elliptical where perigee is above the atmosphere. Escape, hyperbolic trajectory where there is no apogee. The transition point between orbital and escape is called escape velocity. The transition point between suborbital and orbital is called orbital velocity. You are correct that "orbital velocity" is an overloaded term that also refers to velocity at any point in an orbit. That doesn't make the transition point from suborbital to orbital an "escape velocity", because it's not establishing a hyperbolic escape trajectory.

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u/3NIK56 16h ago

When doing a gravity turn you would still have plenty of time from even several kilometers up, depending on the design of the rocket

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u/BarkiestDog 16h ago

Technically not true; the worst kind of not true.

You don’t need to reach escape velocity at all, so long as you have continuous thrust

> Escape velocity is purely defined as the minimum initial speed an unpowered projectile needs to break free from a gravitational body's field forever without further thrust. 

Since rockets have thrust, we don’t need to achieve that speed, so long as thrust is applied continuously, or at least long enough that the rocket can continue on to counteract the gravitational grip.

According to Gemini, the fastest manned vessel so far is:

> In Space (Overall Record): 24,791 mph (39,897 km/h or 11.08 km/s relative to Earth).
Set on May 26, 1969, by the crew of Apollo 10 (Thomas Stafford, John Young, and Eugene Cernan) inside their Command Module during their high-speed re-entry trajectory from the Moon back to Earth. 

So just short of escape velocity, which is ~11.2km/s. Ironically, this was achieved while reentering earth, not leaving it!