Space Elevator

> it's just hard to go up

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.

> 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.

Which is another part of why a space elevator is nifty - by definition it extends out to a distance where you are going fast-enough-sideways.

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.

> once we're up, its just a matter of force over time to create a nice orbit.

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)

> Angela Collier has a video saying it's kinda ridiculous.

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.

>ChatGpt tells me you'd free fall from 1000 km to 100km in about 8 minutes

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

It's not that simple though. The rocket equation still applies so it's almost as hard to do (you just get rid of atmospheric drag), and failed launches are also extra catastrophic.

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 for 1kg to reach LEO (800km * 1kg * 9.8m/s2) ~ 8MJ

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

It's NOT rocket science?

You can reach space using air breathing jets. You can’t stay in space using air breathing jets.

The rocket fuel needed to produce that 40 MJ weighs close to 1 kg, especially when you include the oxidiser. So the energy needed to accelerate 1kg of payload to LEO velocity is much more.

I don't think there are any physics reasons why it'd be impossible, but certainly we can't do it with existing technology. You'd need an air breathing jet that could get a vehicle to go about five or six times faster than any current such engine has ever achieved (i.e. around mach 20-30), which is perhaps ridiculous, but I don't think it's necessarily impossible, just something we don't know how to do. There have been some (failed) efforts to get there, like the X-30.

Delta v feels reductive, since your fighting negative acceleration to go up and a fraction of that to go laterally, no?

What if we just made a huge mountain? Space ramp? Is that anything?

Can an air breathing jet actually attain those velocities? I thought most supersonic aircraft use rockets after a certain point

IIRC there is no material we're aware of that has anywhere near enough compressive strength to build that high, regardless of how wide the base is.

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.

They’re the same thing. Either you go sideways really fast, or go straight up really far (geo-synchronous distance).

Either way, you need the same total velocity delta.

> since your fighting negative acceleration to go up and a fraction of that to go laterally

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

You can build a decent sounding rocket with just trial and error.

"Most" supersonic aircraft are fighter jets and other military aircraft that use jet engines, not rockets. They may have afterburners that are much like a rocket that just injects jet fuel in the exhaust stream, but that's still using atmospheric oxygen.

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.

Basically when you cut thrust you must pass through that altitude again or escape orbit.

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.

But the energy needed is not an indicator of what is difficult or dangerous. Leaving the atmosphere intact is the most difficult part of launching a rocket going by failure rate. Of those that reach space, those that still fail often took damage from the launch.

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

Well you can't reach a high orbit using air breathing engines because your impulse must be given within the atmosphere, and then your trajectory inevitably re-intercepts the atmosphere (unless you achieve an escape trajectory) and would decay quickly. You can get around this by packing a small rocket engine and circularizing on apogee!

There isn't enough air at high altitudes for jets to reach space even if you count 100km as space.

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.

This is where Douglas Adams was right, of course:

> 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.

> Can an air breathing jet actually attain those velocities?

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 MiG didn’t reach 37km in level flight, which is probably where Mach 15 @40km comes from. Instead the MiG was doing a nearly parabolic trajectory starting with Mach ~3 worth of kinetic energy.

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.

The best way I’ve seen it described is that the first 9.8 m/s² of thrust only makes you hover in place. So the more g forces you generate the higher the efficiency of the engine - as long as thrust per kg of fuel doesn’t dip too far.

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

Missing involves going sideways.

Didn’t Chuck Yeager burn his face off doing that?

It could just involve falling long enough the the ground gets out of your way.

Failures rate is even less a suitable indicator. Going up 100km is achievable by a simple single stage solid fuel rocket. Going to orbit requires way way more complexity, including a giant first stage that can fail in atmosphere.

That whole "tyranny of the rocket equation" thing is why I am surprised the actual first stage for launching a rocket is NOT a ground based reusable "up-chucker".

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

The structural support needed to keep the rocket from crumpling under the throw is extra weight to carry the rest of the way.

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

Actually, not quite. You tolerate a bit of extra fire in exchange for reduced gravity loss. Going straight up until you clear atmosphere is not the minimum energy trajectory.

Building your rocket to survive the upchucker costs more than the savings from being upchucked.

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.

Yup. Look at the launch trajectory of the Webb telescope. The upper stage engine was too weak for the orbital insertion and the booster had to waste energy putting it higher than need be so the upper stage engine had altitude to trade for time to keep the telescope from hitting the atmosphere.

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.

Or go high enough to let the moon alter your orbit into one that doesn't hit the atmosphere.

Nobody has been able to build one, but I'm not aware of any proof that it's impossible. You need some way to build an engine that doesn't appreciably slow the air that's passing through it.

I've worked out one that "can" be built (it's beyond ridiculous, though.)

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.

Fountain--while it could be done in a perfect world I do not believe it would be feasible and the whole thing is so vulnerable to disruption. Dragon's Egg got it very wrong, if one of your fountain bits misbehaves it misbehaves very badly. They're moving way above orbital velocity if they hit something that energy is promptly liberated. The Cheela couldn't catch those rings, they had to be moving at high relativistic speeds and would have hit with something akin to antimatter level force.

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.

There's an XKCD for that: https://what-if.xkcd.com/58/

> Going up is the comparatively easy part, it's not exactly rocket science.

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

Nearly so, yeah. But a stock starfighter couldn't do that, the one Yeager was flying was outfitted with a rocket engine.

Yea thus ‘Basically’ you can also escape earth’s orbit slightly more easily using the sun. However, none of this really helps much you’re still looking at more than escape velocity in atmosphere with a purely air breathing engine due to drag.

God bless the Webb team but if a manager came to me to pitch a software project with as many sequential events that had to go perfect I would have told him to fuck right off. After I stopped laughing so hard I couldn’t breathe.

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

Except that you save maybe 30% of the cost to just launch from Earth. Once youre off planet you're over half way to anywhere, and you don't need to land on the Moon to go further

Not quite; g drops materially between 100 km and 1000 km. 8 minutes I believe is quite close, whereas 5t^2 (or even 4.9t^2) will lead to underestimating time.

The radius of the earth is around 6300km. The difference in g at the start (between 6400km and 7400km) is 25%. But gets less as you fall. So it might make somewhere around a 10-15% difference overall? So s=5t^2 is fine unless you need a super accurate figure, in which case you need to do some calculus. I would trust an LLM with calculus even less.

I've also wondered about a balloon launch. Strap the rocket to an enormous blimp, it handles the initial 1-2m/s acceleration, saving an enormous first volley of rocket fuel needed to break objects at rest out of their state of rest.

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