Space Elevator

In most cases, 100km is less than the distance between sizeable metropolitan areas. It's a day long bike ride. Air runs out less than a bus ride across town. A 15k jog/hike would put you in the stratosphere. Those jet aircraft that seem so high are closer than that. Closer than your friends house or the local stadium probably.

Look at a map or globe with that in mind and everything feels so thin!

Going up is the comparatively easy part, it's not exactly rocket science. Going fast enough sideways so you stay up there is the tricky bit.

nah, thats the simple part. getting up there efficiently is the difficulty. once we're up, its just a matter of force over time to create a nice orbit.

The faster you go, the more friction you face, and the more heat and vibration your equipment must endure.

Going slower reduce friction and stress but use more energy just negating gravity. Slow rocket is inefficient rocket.

So we wanna leave the atmosphere as soon as possible, but not so fast that the rocket melts or engines collapse. Prefferably just below the sound barrier.

once we're up, its pretty chill... until you wanna go down again. Slow rocket is alive rocket.

Now, I have no idea how practical it is to build one (Angela Collier has a video saying it's kinda ridiculous), but it's a cool idea.

https://www.youtube.com/watch?v=Z5aHMB4Tje4

Also since rockets have moved away from hydrolox, it would be nice to have a greener launching system.

It depends what you mean by "up there". ChatGpt tells me you'd free fall from 1000 km to 100km in about 8 minutes. It also did the math that you'd need 1.65G of sideways thrust to reach orbital speed. That's quite a bit of force for spacecraft sized objects.

If you have an actual space elevator, sure, you can go to close to geosynchronous altitude and by that time you'd have enormous sideways velocity just by being dragged sideways by space elevator and indeed it would be easy to propel yourself to orbit (above a certain altitude my intuition tells me you could let go of the rope and while you'd end up on an eliptise you'd still be in orbit)

There are other concepts like space fountains, orbital rings and sky hooks that seem more doable -- especially the sky hook seems close to do-able, especially on the Moon.

You trusted an LLM to do the maths when it is just s = 5t^2?

Even more, your delta v required is still huge. I can't be bothered to run the numbers right now but most of the delta v is in the orbital velocity, not in the altitude.

Energy to reach LEO velocity ~ (1/2 * 1kg * (8km/s)^2) ~ 32MJ

Eh. Going up is easy. A Frenchman, a sheep, duck, and rooster solved the whole ‘up’ thing over two centuries ago.

But going DOWN? That’s far more difficult. What wonders may lie beneath our feet: vast caverns, ore, underground oceans… hard to get to though.

Space elevators only (theoretically) work because the entire structure is in tension. And the only material we currently know of that can handle the tensile forces is carbon fiber.

/s

Either way, you need the same total velocity delta.

They’re both acceleration. At high thrust, virtually equivalent.

If earth were a grapefruit, our atmosphere would be ~1mm thick!

The issue, I think, is more about balancing drag and air intake at appropriate atmospheric densities for different speeds. An SR-71 Blackbird could fly at 85,000 feet continuously, and a MiG-25 set what I believe is still the air-breathing record max altitude by pulling a "zoom climb" (accelerating in higher-density air that the engines could use effectively, then pulling the stick back and coasting up through rarefied air too thin for the engines) to 38km or 123,000 feet.

Most experimental hypersonic aircraft use rockets because that's what works.

So either fire a rocket in space to circularize the orbit or reach more than Earth’s escape velocity 25,020 mph (11.186 km/s, 40,270 km/h) ~ Mach 32.6, due to some drag in air to thin for any kind of air breathing engine to work.

X-30 was aiming far lower ~Mach 20. Nuclear could make it more realistic than any form of chemical combustion. It might be physically possible using Hydrogen but you’re talking generating extreme thrust at vastly more extreme conditions than the space shuttle’s retry.

Estimate for a standard classroom globe at 13" in diameter (I'm seeing a rnage of 12-14 inches as typical). I'm reporting in inches because that is what came up first and most of the globes are for sale in the US. Mixing units here, but, it works out.

But, in meters, the diameter of the Earth is 12,742,000 m on average. if we use the 'Karman line' as defining the edge of what the atmosphere is, that is 100,000 meters. Solving for X ... (13" / 12742000 m)=(X / 100,000 m). gives us an atmosphere thickness of approximately 0.1". -----

Paper glued to the globe would have a thickness of maybe, 0.004" (thin paper) to 0.012" (like a card stock paper).... so that analogy is off by an order of magnitude or more.

Even if you use the mesosphere as the definition for the top of the atmosphere, that is still 85,000 meters and thus similar.

People can check the numbers I used.

* Perhaps the analogy should go more like: the thickness of the cardboard sphere the globe is made out of is about the thickness of the atmosphere. Because, having completely destroyed a globe once in my youth, I remember the cardboard shell being approximately a tenth of an inch thick. But, that's maybe not a great reference for the analogy because not everyone has cut apart a classroom globe....

Another comment talks about atmosphere being a 1 mm layer on a grapefruit... so definition of atmosphere extents might be different in these two anecdotes.

(edit: I submitted this comment two minutes after another comment did the math on the globe/paper layer version...)

Once you're in space, force over distance until your fuel runs out.

90% of the atmosphere is below 16 km.

16 km * (12" / Earth diameter) :: https://www.wolframalpha.com/input?i=16+km+*+%2812%22+%2F+Ea...

0.015 inches, 0.38 mm

... and tossing sheets of paper into that ( https://www.wolframalpha.com/input?i=thickness+of+paper ) ...

16 km * (12" / Earth diameter) / thickness of paper :: https://www.wolframalpha.com/input?i=16+km+*+%2812%22+%2F+Ea...

4

Note that that's copy paper rather than card stock...

Adjusting this to 5.6km (the 50% atmosphere amount) ...

5.6 km * (12" / Earth diameter) / thickness of paper :: https://www.wolframalpha.com/input?i=5.6+km+*+%2812%22+%2F+E...

1

So it's a matter of selecting the proper globe, proper paper, and proper threshold for the atmosphere.

This is essentially what Scaled Composites and Virgin Galactic were trying to do with their cargo plane system, only you don’t have to worry about the ignition timing because you’re not in free fall.

I don't know of any launch sites significantly above sea-level, the marginal performance increase wouldn't be worth the logistical nightmare. It's easier to fly up a 747 than build a launch facility on top of a mountain.

I think this thread would also be loved by the nerdy folks at https://Reddit.com/r/theydidthemath

This was part of the Plot of Halo: ODST, where fragments of the space elevator collapsed onto New Mombasa.

The highest jet record is 37km in MiG-25. The scramjet record is 33km. I found source that says the limit is 40km at Mach 15.

... and the source document for the numbers was based on a paper that is fairly easy to find given the proper keywords in google search... https://pubmed.ncbi.nlm.nih.gov/1648028/ (and I learned that methane more rare in flatus than not).

> There is an art, it says, or rather, a knack to flying. The knack lies in learning how to throw yourself at the ground and miss.

You're not going sideways - you're actively falling continuously, but somehow missing the ground for the entire length of your orbit.

https://dahosek.substack.com/p/one-million-stories

The other French method included two dogs, a bunch of chicken, and a very large cannon, which had quite a bit more showmanship.

There's no theoretical limitation on how fast an air breathing jet can move. You just have to redesign everything every few mach numbers, and deal with the atmospheric drag.

The NASA X-43A hit Mach 9.6 as an air breathing engine (test used a rocket at lower speeds for cost reasons) which in theory should be capable of ~65 km assuming it could survive a similar maneuver. Actual limits are heavily influenced by how much thrust you can generate while slowing down etc not just max velocity.

So yea no actually built air breathing aircraft can hit space, but it’s within the realm of possibility.

Twice.

Because the only known material strong enough at the time was diamond, and diamond doesn’t ablate much when falling through atmosphere.

One of the early space elevator research companies specifically designed a cable that was made by stacking successive layers of material both to slowly increase carrying capacity by building the tether in iterative layers, and hoping it would ablate or at least reach a quick terminal velocity in the case of catastrophic failure.

It is undoubtedly the case that the senior staff on that project were familiar with the Mars trilogy.

What’s the downrange safety cone look like for space launch sites around the world? A little S curve in your insertion orbit would certainly waste a bit of delta V. But not all orbits are equatorial anyway.

This all changes in outer space, where 0.01g is a valid propulsion mechanism for long duration missions.

On the planetary scale, humans are tiny. We're more or less equivalent to bacteria.

Our entire civilisation is a skin rash.

Basically, I would have thought that any momentum that can be imparted to the rocket before it has to rely on its self propulsion would be a huge help. Not talking about eliminating self propulsion, just an assist so the rocket could carry a larger payload or be smaller or whatever.

IE like a variation on Jules Verne's big gun for throwing the payload up there but engineered to be plausible and having the rocket still be self propelled. And safe.

But we don't seem to do this. So why?

Edit: First part of video [0]. Apparently it's not completely dumb. Just stupid-hard/impossible to do practically at the size required for big rockets and payloads. But small ones might work. Maybe.

[0] https://www.youtube.com/watch?v=lWYn5hl4QWg

This is why Starship is aiming for a catch rather than legs. Legs that work at 1g added too much weight.

Chuckers are the optimal large scale solution for airless bodies, but they're horizontal. You spread the acceleration out over a very long distance so you don't need a super beefy spacecraft and your humans won't turn to goo. Basically, a maglev train except it has track above as well as below and it doesn't have a maximum speed. Wrap one around the lunar equator and it can eject anywhere from sundiver to interstellar escape with human-tolerable acceleration.

But it was the optimum solution because that engine had a long burn to take it to L2. Hauling less engine to L2 was worth more than the loss of the engine not being powerful enough to fight gravity.

Build towers around the equator. Build a ring around the equator on those towers. Build a ring inside the ring, maglev supports. Evacuate the ring, spin the inner ring above orbital velocity. The objective is to generate as much outward force as the weight of the entire assembly including the towers.

Do it again, on top of the first one. Keep doing it until you reach synchronous orbit. If you want to go higher the inner ring is not moving, exerting downward force countering the outward force of the rest of it being above orbital velocity.

Forces:

1) Compression on the towers. Note that this goes to zero as tower height goes to zero.

2) The outward force on the ring exists across the whole ring, but the downward force of supporting the towers only exists where there is a tower. Your ring needs to be stiff enough to counter this. But, again, note that this force goes to zero as the space between towers goes to zero.

3) Maintaining a very hard vacuum in the structure.

We know how to do #3, the others must have answers. Thus it can be built.

Nothing else is feasible with current technology, the taper of the cables is highly dependent on the strength of the material (in tension, an elevator is based on always pulling out as the upper end isn't hooked to anything.) You need much better than anything we can currently do before the taper blows up so badly you can't build it.

Orbital rings--only if you have elevators. Remember, the Ringworld is unstable. So is every other planetary ring.

I do not recall numbers on hooks so I will not address them.

The Moon has a whole different set of problems. There is no synchronous orbit, elevators must go above synchronous orbit, so the normal version can't exist. Nor can anything stand up to be yanked around by the Earth.

But there are two cases that avoid the yanking problem: pointed towards and pointed away from Earth. Current cables are good enough for a useful Earth-pointing cable. The free end dips below synchronous orbit, but it's moving very slowly. You do what people think rockets do--go up. It takes a lot less energy to catch the cable than it does to even reach orbit.

How to have such a cable in an environment with geosynchronous satellites is another matter...

There's also another interesting cable situation. Cable on Mars? Iffy--and those two moons would be a major problem. But flip the problem over--put the cables on the moons. The low end dips into the atmosphere at aircraft-type speeds. The cables can toss to each other. The high end can capture/eject to Earth or the asteroid belt.

If you inflate a 18cm diameter head to the size of our planet, a 75um hair would be about 5km wide - which is about the average depth of our oceans.

It's one hair, not a whole head of hair!

Some would argue is quite literally rocket science, even if suborbital rockets are much simpler, like they can just burn solid fuel.

I made a pikachu face when they returned the first image.

After the rocket is clear, activate compressors inside of the blimp and return it to base for re-use.

The base would need to be enormous and I'm sure the power draw would be insane, but being able to break the elevator cable into smaller lines and take the weight off of any individual strand might make it a touch more plausible without currently non-existent technologies.