5 ms·

> 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,

> 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.

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.