6 ms·

> 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 tric

by messe 1y ago

> 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.

aDyslecticCrow 1y ago
> 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.
- raducu 1y ago
  > 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)
  - hermitcrab 1y ago
    >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?
    - kaonwarb 1y ago
      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.
      - hermitcrab 1y ago
        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.
- literalAardvark 1y ago
  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.
  - BobaFloutist 1y ago
    Delta v feels reductive, since your fighting negative acceleration to go up and a fraction of that to go laterally, no?
    - lazide 1y ago
      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.
    - JumpCrisscross 1y ago
      > 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.
- advisedwang 1y ago
  Energy for 1kg to reach LEO (800km * 1kg * 9.8m/s2) ~ 8MJ Energy to reach LEO velocity ~ (1/2 * 1kg * (8km/s)^2) ~ 32MJ
  - nroets 1y ago
    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.
    - _carbyau_ 1y ago
      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 https://www.youtube.com/watch?v=lWYn5hl4QWg
      - btilly 1y ago
        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.
      - LorenPechtel 1y ago
        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.
- hinkley 1y ago
  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.
  - LorenPechtel 1y ago
    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.
    - hinkley 1y ago
      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.
      - seg_lol 1y ago
        It is unbelievable really. That team should do some things on earth, like fix healthcare or implement a national rail system.
        rkomorn 1y ago
        It's probably a lot simpler to get something to a Lagrange point, unfortunately. Way fewer variables.
- LorenPechtel 1y ago
  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.
Pxtl 1y ago
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 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.
- raducu 1y ago
  > 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.
  - BobaFloutist 1y ago
    What if we just made a huge mountain? Space ramp? Is that anything?
    - estimator7292 1y ago
      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.
  - LorenPechtel 1y ago
    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.
block_dagger 1y ago
It's NOT rocket science?
- Retric 1y ago
  You can reach space using air breathing jets. You can’t stay in space using air breathing jets.
  - CobrastanJorji 1y ago
    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.
    - estimator7292 1y ago
      Can an air breathing jet actually attain those velocities? I thought most supersonic aircraft use rockets after a certain point
      - LeifCarrotson 1y ago
        "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.
sebastiennight 1y ago
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.
- hinkley 1y ago
  Missing involves going sideways.
  - themaninthedark 1y ago
    It could just involve falling long enough the the ground gets out of your way.
zwog 1y ago
There's an XKCD for that: https://what-if.xkcd.com/58/ https://what-if.xkcd.com/58/
DoktorL 1y ago
> 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.

by messe 1y ago

> 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.

aDyslecticCrow 1y ago

> 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.

raducu 1y ago
> 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)
- hermitcrab 1y ago
  >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?
  - kaonwarb 1y ago
    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.
    - hermitcrab 1y ago
      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.
literalAardvark 1y ago
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.
- BobaFloutist 1y ago
  Delta v feels reductive, since your fighting negative acceleration to go up and a fraction of that to go laterally, no?
  - lazide 1y ago
    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.
  - JumpCrisscross 1y ago
    > 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.
advisedwang 1y ago
Energy for 1kg to reach LEO (800km * 1kg * 9.8m/s2) ~ 8MJ Energy to reach LEO velocity ~ (1/2 * 1kg * (8km/s)^2) ~ 32MJ
- nroets 1y ago
  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.
  - _carbyau_ 1y ago
    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 https://www.youtube.com/watch?v=lWYn5hl4QWg
    - btilly 1y ago
      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.
    - LorenPechtel 1y ago
      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.
hinkley 1y ago
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.
- LorenPechtel 1y ago
  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.
  - hinkley 1y ago
    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.
    - seg_lol 1y ago
      It is unbelievable really. That team should do some things on earth, like fix healthcare or implement a national rail system.
      - rkomorn 1y ago
        It's probably a lot simpler to get something to a Lagrange point, unfortunately. Way fewer variables.
LorenPechtel 1y ago
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.

Pxtl 1y ago

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 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.

raducu 1y ago
> 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.
- BobaFloutist 1y ago
  What if we just made a huge mountain? Space ramp? Is that anything?
  - estimator7292 1y ago
    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.
- LorenPechtel 1y ago
  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.

block_dagger 1y ago

It's NOT rocket science?

Retric 1y ago
You can reach space using air breathing jets. You can’t stay in space using air breathing jets.
- CobrastanJorji 1y ago
  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.
  - estimator7292 1y ago
    Can an air breathing jet actually attain those velocities? I thought most supersonic aircraft use rockets after a certain point
    - LeifCarrotson 1y ago
      "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.

sebastiennight 1y ago

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.

hinkley 1y ago
Missing involves going sideways.
- themaninthedark 1y ago
  It could just involve falling long enough the the ground gets out of your way.

zwog 1y ago

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

DoktorL 1y ago

> 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.