trubble76 wrote:

Joe09 wrote:v2 = 2GM/r

Hmm, you might be marked down for not showing your working.

1/2mv2 = (-)GMm/r

mv2 = 2GMm/r

v2 = 2GM/r

Oh, but it is rocket science.

As opposed to relativity and quantum mechanics.

There's another factor that leads to better results from burning the fuel low in the ascent - the fuel that is consumed doesn't need to be lifted, so less overall work needs to be done (rocket fuel is quite heavy and the factor is significant).

So consensus is that for the space shuttle it's to get rapidly to orbital velocity (matching the speed of the ISS for instance), but for stuff leaving earth's orbit it's just more fuel efficient to twat it like mad* lower down in the atmosphere (and gravity well).

*I'm an engineer, that's a technical term.

IBP wrote:So consensus is that for the space shuttle it's to get rapidly to orbital velocity (matching the speed of the ISS for instance), but for stuff leaving earth's orbit it's just more fuel efficient to twat it like mad* lower down in the atmosphere (and gravity well).

*I'm an engineer, that's a technical term.

The most efficient way to do it would be to blast the space shuttle out of a big gun. No astronauts have signed up for this project yet .

klazmon wrote:

IBP wrote:So consensus is that for the space shuttle it's to get rapidly to orbital velocity (matching the speed of the ISS for instance), but for stuff leaving earth's orbit it's just more fuel efficient to twat it like mad* lower down in the atmosphere (and gravity well).

*I'm an engineer, that's a technical term.

The most efficient way to do it would be to blast the space shuttle out of a big gun. No astronauts have signed up for this project yet .

What's the maximum force people could stand? From 0 to ~6mi/s over such a short distance of a gun (though a rather large one) seems like it would squash you flat.

IBP wrote:I could escape earth's gravity at walking pace, if I could find a ladder long enough...

If we restrict ourselves to an analysis in classical gravitation, climbing a tall enough ladder, eventually you would arrive at the boundary where the classical forces from Earth's gravitation become weaker than those of another celestial body. Incidentally, this is (part of) how the Apollo astronauts managed to arrive in the vicinity of the Moon.

However, pursuing the analysis a bit, we can see that the Moon has not yet escaped earth's gravity, though it is getting farther and farther away. Cue "As Time Goes By". Time and tide. Weight for no man.

pcCoder wrote:What's the maximum force people could stand? From 0 to ~6mi/s over such a short distance of a gun (though a rather large one) seems like it would squash you flat.

People have survived accelerations in excess of 100g for very short amounts of time during car crashes, etc. However, for a sustained acceleration of several seconds, 17-25g is the practical limit.

Consequently, a hypothetical space gun capable of launching people into orbit would need a barrel about 100-160km in length.

Matt_B wrote:

pcCoder wrote:What's the maximum force people could stand? From 0 to ~6mi/s over such a short distance of a gun (though a rather large one) seems like it would squash you flat.

People have survived accelerations in excess of 100g for very short amounts of time during car crashes, etc. However, for a sustained acceleration of several seconds, 17-25g is the practical limit.

Consequently, a hypothetical space gun capable of launching people into orbit would need a barrel about 100-160km in length.

It doesn't seem impossible, especially on the moon, as a rail, perhaps with linear motors to push it evenly at constant acceleration. Atmospheric effects near the ground probably make it impractical on Earth.

Cito di Pense wrote:

IBP wrote:I could escape earth's gravity at walking pace, if I could find a ladder long enough...

If we restrict ourselves to an analysis in classical gravitation, climbing a tall enough ladder, eventually you would arrive at the boundary where the classical forces from Earth's gravitation become weaker than those of another celestial body. Incidentally, this is (part of) how the Apollo astronauts managed to arrive in the vicinity of the Moon.

However, pursuing the analysis a bit, we can see that the Moon has not yet escaped earth's gravity, though it is getting farther and farther away. Cue "As Time Goes By". Time and tide. Weight for no man.

If the ladder were firmly mounted at the equator the climber would reach a stable orbital velocity long before other celestial bodies became significant. That would be when stepping off the ladder would mean a fall to a stable orbit above the atmosphere. With a long enough ladder escape velocity could be achieved.

Space elevators are theoretically possible, given tethers that are strong and light enough.

GrahamH wrote:
If the ladder were firmly mounted at the equator the climber would reach a stable orbital velocity long before other celestial bodies became significant.

That is of course true, but does not answer the question.

If one has a rope to slither up, one will always be in geosynchronous orbit, no matter what the orbital radius. That is, until you let go of the rope. This is a brain teaser, not rocket science.

Escape velocity is with respect to zero, on the pad. With a long enough rope, VE can be 4 km/hr.

How high would you have to climb to reach standard stable LEO velocity? I'm going to guess somewhere about 250 km above the surface of the planet. Duh. Somewhere not far below that, you start encountering annoying atmospheric resistance.

v2 = 2GM/r