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Comment by enjeyw | Hacker News Reader

Ok so doing super-approximate maths:

A fully fueled and loaded Saturn V weighs about 3,000,000 Kg, and has a payload of about 40,000 Kg to the moon.

So that means once you've put all the bits of the rocket together that collectively give you sufficient thrust to get you to the moon, you've only got about 1.3% left for actual payload.

S if earth's gravity had been ~1.3% stronger, then the weight of the "getting us there" bits of the rocket would have left no room for payload. Any more gravity, and we wouldn't even be getting to the moon.

Equally, if the earth's gravity had been 1.3% weaker, we could have doubled our payload to the moon (or done things way easier).

Contrast that with a Boeing 747 which has 50% of total mass as payload, and so is relatively insensitive to changes in gravity.

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That's not really how it works. If Earth's gravity were 1.3% stronger, you'd need about 0.6% (√1.013) more delta-v to escape Earth, or about an extra 70m/s. That's not hard to achieve. Increasing the total mass to dry mass ratio by about 0.6% would do it (and the Saturn 5 was about 85% fuel) since ln(x*1.006) = ln(x) + ln(1.006) ≈ ln(x) + 0.006.

Someone else posted somewhere that if the Earth was about 3x heavier, we wouldn't be able to use chemical rockets to escape.

If the Earth were 3x heavier, birds wouldn't be able to fly, at least not the way they are now. Sparrows can barely get airborne when they're full, let alone at triple their weight.

Air density would be three times higher though. So they might still be able to fly.

Ooo, that's a good point!

> at least not the way they are now.

Yeah, but they wouldn't be, because the extra mass would have influenced their evolution.

Extra weight. If gravity is the only thing changed, mass would remain constant.

Since the discussion is about "earth's gravity", I'm assuming the change would be in its mass, not in the universal gravitational constant, which is not Earth-specific.

Did that take into account the possibility of an air breathing first stage?

Probably not. But, while an air-breather would be more useful on a larger planet, it would still be the case that almost all of the space launch system's work must be done outside the sensible atmosphere. Air breathing would only add the tiniest margin to "chemically escapable planets."

It's not. That thing is called the rocket equation and the law is logarithmic. https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

So basically in your example, if the Earth gravity was 1.3% stronger, payload would be ~37.8t instead of 40.

Or we'd have to use different lift technologies, e.g., https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion .

I recently realized that if we got in Jupiter's gravity well we could probably never escape it again. Someone correct me if I'm wrong. The thought made me feel really uneasy.

If that were true, it would be impossible to leave the solar system, since everything is currently in the Sun's gravity well, and the Sun is larger than Jupiter. But we have sent probes outside the solar system (did they fully leave the Sun's gravity well yet though? Not sure)

But we didn't send the probes from the Sun.

Plus I don't think it got its velocity from Earth, but by using gravity assists to steal momentum from other planets (which, if you don't know how that works, do yourself a favor and read the wikipedia page, it's pretty great).

Sure, we could orbit Jupiter and get back out. But if we ever descended to the "surface" we couldn't climb back out.

Voyager 1/2 and New Horizons are at escape velocity and leaving the solar system.

You have to have high thrust to get out of the dragging atmosphere.

The premise is about current chemical propellants and structural components. If you build a rocket out of nanotubes with nuclear propulsion you could go wherever you want from anywhere else. Except black holes.

Remember that we’re already orbiting the sun though. A lot of the hard work is already done.

Launching from the (nonexistent) surface of Jupiter would be the comparison. From a low orbit of Jupiter you need a lot of delta v to escape, but you don't need extremely high thrust as would be needed to get in to orbit in the first place from the surface of a body. You could use a very efficient thruster over a long period of time which is a lot easier to design for.

So along this same line the PSP Parker Solar Probe is exploring our gravity well in the other direction, and in some ways will be more gone forever than voyagers.

I think the Jovian Moon Io is very low, look up an eclipse photo from Juno; it would be key to exploring low Jovian orbit, just like Venus kicks PSP and Jupiter kicked Voyagers.

Some (funny) sources list Saturn-V payload as 140 tons on LEO :) so according to this logic to get to orbit one has plenty of margin. And as soon as the payload is in orbit it uses full value of accumulated speed, so it's used to get further and getting to the Moon is no harder than with Earth...

Why can't you aerodynamically fly up to high altitudes and then start rocketing?

You can, but it introduces its own set of complexities. The plane needs to carry a massive payload, and doesn't really get you much delta v.

The problem is not how do we get up high, but rather how do we get going really, really, really fast.

There is in fact a remarkably useful set of well developed air launch rockets that help provide a nice “benchmark” for comparison with air launching satellites/etc using rockets. Anti-SATellite (ASAT) weapons are making similar engineering trade offs but aiming at a different set of capabilities, optimising for smaller sizes, being “rack ready” in the event war breaks out, and so on.

Unlike the more modern ship launched SM3 that is part of the Ageis system, the older ASM-135 https://en.wikipedia.org/wiki/ASM-135_ASAT was designed to be air launched in a supersonic zoom climb to get maximum speed and altitude from the aircraft in order to maximise its capabilities.

For an even better comparison, you can compare air launches ASAT missiles with sounding rockets, they have a greater similarity in their trajectories/launch profiles, so it’s easier to see what the air launch “gains” over the ground launch.

There's always also e.g. Orbital Pegasus.

Rockets tend to throttle down at lower altitudes so they're not accelerating through thick atmosphere though? So Delta V isnt much of a loss.

When not burning sideways, you have the issue of gravity drag. Specifically, all the force spent firing downwards is wasted on fighting gravity rather than gaining speed. The reason rockets go up is to get above the atmosphere. Without an atmosphere, you'd launch rockets sideways.

It doesn't help much. Most of the work in achieving orbit is in gaining speed "sideways". Your average orbital launch achieves high airplane altitudes in the first 15-20 seconds out of a 500 second flight. It's easier to build a slightly bigger rocket than to add a giant airplane to the system.

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