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There is a cost in mass. Rockets today are essentially a giant fuel tank with a few engines connected to one side and cargo connected to the other side. Every

There is a cost in mass. Rockets today are essentially a giant fuel tank with a few engines connected to one side and cargo connected to the other side. Every kg of fuel is a kg of cargo you can’t launch. That’s the cost.

pfdietz 4y ago
None of that justifies claiming the cost is due to gravity. The problem of getting to space is obviously affected by gravity, but the cost is dictated by other considerations. It's not like "oh well, gravity, we might as well give up because it's not possible to do much better."
- FPGAhacker 4y ago
  > None of that justifies claiming the cost is due to gravity. Where do you suppose the need for so much energy comes from?
  - FeepingCreature 4y ago
    It's more that the cost in money doesn't come from the cost in energy.
  - DarmokJalad1701 4y ago
    Congressional mandates forcing NASA to use Space Shuttle components for the SLS that are de facto non-competitive, single-source requirements assuring contracts to existing Shuttle suppliers is contributing significantly to the costs. For example, the NASA contract to Aerojet Rocketdyne to manufacture RS-25 engines is costing $146 million per engine. Each SLS launch will toss four of these engines in the ocean for a total cost $580 million. And that's just the engines of the core stage. For the cost of a single engine, they could have instead purchased an entire Falcon Heavy launch, which has two-thirds of the SLS lift capacity (and has to obey the same laws of physics as SLS). SLS is the price NASA had to pay to get Congress off its back.
    - matheusmoreira 4y ago
      Why would the US congress mandate stuff like this?
      - ambicapter 4y ago
        Because US congress is made up of elected representatives, and each representative is incentivized to do so by the desire for re-election.
      - pfdietz 4y ago
        Because it benefits them (in kickbacks from contractors, for example) and because they can get away with it. Having a well functioning space program? Doesn't benefit them much or at all. Ultimately, it's because voters don't call them on their BS. If you've been a NASA fan, encouraging NASA funding regardless of what it's for, you're part of this problem.
        matheusmoreira 4y ago
        I'm not an US citizen. I can't call out US politicians on anything because I don't have a vote.
      - zardo 4y ago
        The more dollars and jobs you can claim you brought to your district during the next election, the better. This isn't just a NASA thing, a NASA contract is probably pretty small compared to getting a new military base, or keeping one open that the military wants to close.
  - pfdietz 4y ago
    Why do you think the cost of launch is due to the cost of energy?
- mr_toad 4y ago
  > None of that justifies claiming the cost is due to gravity. It scales a lot with gravity. Compare the cost of a vehicle capable of reaching lunar orbit (e.g. the Eagle), with the cost of getting humans into Earth orbit (e.g. the Mercury-Atlas). And the Eagle carried two people.
  - pfdietz 4y ago
    Sure, but once you're in space things can become easier, with high Isp engines and in situ propellant production. It's not necessarily all chemical rockets with fuel lifted from Earth.
humaniania 4y ago
Why do they launch from sea level instead of a high elevation?
- sophacles 4y ago
  The goal of the rocket is to get the payload high enough to be in space AND fast enough to be in orbit. It's the later that takes most of the fuel, IIUC.
- pfdietz 4y ago
  Mostly because it's nice to have an uninhabited area downrange of the pad. The benefit of launching from altitude would not be the potential energy, but the lower air pressure and density. The first would enable rocket engines to operate at higher expansion ratio, and the second would reduce aerodynamic forces. The benefit is apparently not large enough to justify the difficulty of operating on a mountain.
- krisoft 4y ago
  Why do you think that would help? I can imagine two different answers to that and depending on which one you tend to think more there are two different answers to your question. If you are thinking: space is high up therefore the higher we start the easier we will get there. The problem with this is that getting into outer space is not that hard, staying there is the hard bit. The international space station orbits about 408km away from the surface of the earth. That is not a long distance. If you would have a car which can travel the same speed a car can usually travel but straight up you could reach that altitude in about 4 hour easy driving. What is important is what would happen after you reach that elevation and turn off the engine of your car. If you have seen astronauts serenly drifting in space, you might expect that your imaginary car would do the same. But that is not what would happen. You would see that your car starts falling and rapidly! In fact there is a word for such a flight path. It is called a suborbital space flight. Space flight because it went to space, but “sub” orbital because it lacked something to stay in orbit. What is that something it lacked? Why do the astronauts float gracefully while your car plumets? The trick is that the astronauts plumet too! They are constantly falling back towards earth, they just go so fast sideways that the earth rolls out from under them. In fact that is what an orbit is. You are falling around the earth with a very high sideways speed. In essence staying in space equals being in orbit which further equals going very fast. So now you can see that being high is not the hard bit of staying in space. Flying as fast as a bullet is the hard bit, and of course starting from high won’t help with that. But! You might think a different thing why launching from a high elevation might help. Maybe you already know that staying in space == going fast. So you are thinking: what hinders us from going fast? The drag of our atmosphere! If we would launch from high there would be less atmosphere around us, thus there would be less drag, thus we wouldn’t waste so much energy to fight it. And you would be right! If you could launch from a high elevation you could spare some energy because there the atmosphere is thinner. But when you run the numbers you see that this is a very small percentage of your total energy expenditure. Mostly because our rockets fly through the dense part of our atmosphere relatively quickly wasting only a little energy to fight drag. Launching from a high elevation would come with it’s own set of challenges of course and wouldn’t help that much relatively. So depending on which thing you were thinking about you have an answer. Hope it explained the problem better. If you have any more questions about any of the details let me know and I will try my best to explain. Also sorry for answering this long a seemingly simple question. Would love to answer shorter, but you know it is “rocket science”. ;)

pfdietz 4y ago

None of that justifies claiming the cost is due to gravity. The problem of getting to space is obviously affected by gravity, but the cost is dictated by other considerations. It's not like "oh well, gravity, we might as well give up because it's not possible to do much better."

FPGAhacker 4y ago
> None of that justifies claiming the cost is due to gravity. Where do you suppose the need for so much energy comes from?
- FeepingCreature 4y ago
  It's more that the cost in money doesn't come from the cost in energy.
- DarmokJalad1701 4y ago
  Congressional mandates forcing NASA to use Space Shuttle components for the SLS that are de facto non-competitive, single-source requirements assuring contracts to existing Shuttle suppliers is contributing significantly to the costs. For example, the NASA contract to Aerojet Rocketdyne to manufacture RS-25 engines is costing $146 million per engine. Each SLS launch will toss four of these engines in the ocean for a total cost $580 million. And that's just the engines of the core stage. For the cost of a single engine, they could have instead purchased an entire Falcon Heavy launch, which has two-thirds of the SLS lift capacity (and has to obey the same laws of physics as SLS). SLS is the price NASA had to pay to get Congress off its back.
  - matheusmoreira 4y ago
    Why would the US congress mandate stuff like this?
    - ambicapter 4y ago
      Because US congress is made up of elected representatives, and each representative is incentivized to do so by the desire for re-election.
    - pfdietz 4y ago
      Because it benefits them (in kickbacks from contractors, for example) and because they can get away with it. Having a well functioning space program? Doesn't benefit them much or at all. Ultimately, it's because voters don't call them on their BS. If you've been a NASA fan, encouraging NASA funding regardless of what it's for, you're part of this problem.
      - matheusmoreira 4y ago
        I'm not an US citizen. I can't call out US politicians on anything because I don't have a vote.
    - zardo 4y ago
      The more dollars and jobs you can claim you brought to your district during the next election, the better. This isn't just a NASA thing, a NASA contract is probably pretty small compared to getting a new military base, or keeping one open that the military wants to close.
- pfdietz 4y ago
  Why do you think the cost of launch is due to the cost of energy?
mr_toad 4y ago
> None of that justifies claiming the cost is due to gravity. It scales a lot with gravity. Compare the cost of a vehicle capable of reaching lunar orbit (e.g. the Eagle), with the cost of getting humans into Earth orbit (e.g. the Mercury-Atlas). And the Eagle carried two people.
- pfdietz 4y ago
  Sure, but once you're in space things can become easier, with high Isp engines and in situ propellant production. It's not necessarily all chemical rockets with fuel lifted from Earth.

humaniania 4y ago

Why do they launch from sea level instead of a high elevation?

sophacles 4y ago
The goal of the rocket is to get the payload high enough to be in space AND fast enough to be in orbit. It's the later that takes most of the fuel, IIUC.
pfdietz 4y ago
Mostly because it's nice to have an uninhabited area downrange of the pad. The benefit of launching from altitude would not be the potential energy, but the lower air pressure and density. The first would enable rocket engines to operate at higher expansion ratio, and the second would reduce aerodynamic forces. The benefit is apparently not large enough to justify the difficulty of operating on a mountain.
krisoft 4y ago
Why do you think that would help? I can imagine two different answers to that and depending on which one you tend to think more there are two different answers to your question. If you are thinking: space is high up therefore the higher we start the easier we will get there. The problem with this is that getting into outer space is not that hard, staying there is the hard bit. The international space station orbits about 408km away from the surface of the earth. That is not a long distance. If you would have a car which can travel the same speed a car can usually travel but straight up you could reach that altitude in about 4 hour easy driving. What is important is what would happen after you reach that elevation and turn off the engine of your car. If you have seen astronauts serenly drifting in space, you might expect that your imaginary car would do the same. But that is not what would happen. You would see that your car starts falling and rapidly! In fact there is a word for such a flight path. It is called a suborbital space flight. Space flight because it went to space, but “sub” orbital because it lacked something to stay in orbit. What is that something it lacked? Why do the astronauts float gracefully while your car plumets? The trick is that the astronauts plumet too! They are constantly falling back towards earth, they just go so fast sideways that the earth rolls out from under them. In fact that is what an orbit is. You are falling around the earth with a very high sideways speed. In essence staying in space equals being in orbit which further equals going very fast. So now you can see that being high is not the hard bit of staying in space. Flying as fast as a bullet is the hard bit, and of course starting from high won’t help with that. But! You might think a different thing why launching from a high elevation might help. Maybe you already know that staying in space == going fast. So you are thinking: what hinders us from going fast? The drag of our atmosphere! If we would launch from high there would be less atmosphere around us, thus there would be less drag, thus we wouldn’t waste so much energy to fight it. And you would be right! If you could launch from a high elevation you could spare some energy because there the atmosphere is thinner. But when you run the numbers you see that this is a very small percentage of your total energy expenditure. Mostly because our rockets fly through the dense part of our atmosphere relatively quickly wasting only a little energy to fight drag. Launching from a high elevation would come with it’s own set of challenges of course and wouldn’t help that much relatively. So depending on which thing you were thinking about you have an answer. Hope it explained the problem better. If you have any more questions about any of the details let me know and I will try my best to explain. Also sorry for answering this long a seemingly simple question. Would love to answer shorter, but you know it is “rocket science”. ;)