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To go higher with tons of weight requires even more fuel. This is a _very_ common rocket science problem and is the reason why almost all space vehicles have s

by virtue3 12y ago

To go higher with tons of weight requires even more fuel. This is a _very_ common rocket science problem and is the reason why almost all space vehicles have stages that drop off. To go high, you need more fuel, to get more fuel up you need more fuel... etc etc. "we just go higher with less air resistance" is a bullshit marketing thing and not really showing the math. Engines designed to be supersonic are also _very_ shitty at subsonic speeds. Sure, cooling 1000 degree air for use in a turbo fan is great. You're not gonna get _any_ benefit doing it at ground level.

m_mueller 12y ago
You don't need the fuel to go high.[1] You need it to go fast. Orbital velocity is Mach 25, not the puny Mach 5 that they talk about for transporting passengers here. [1] See Virgin Galactic's 'spacecraft'. You see lots of fuel tanks? No? That's because they only do a little 'hop' to 100km or so, not accelerating sideways.
- idlewords 12y ago
  The fuel required to accelerate from Mach 5 in near vaccum to Mach 25 is a fraction of what you need to accelerate the vehicle + fuel combo to Mach 5 in air. Remember how the space shuttle had those massive rockets on the side, that fell off way before it was close to horizontal? You need the fuel to get high enough to go fast.
  - XorNot 12y ago
    This is actually incorrect. The space shuttle had massive launch engines because it needed the thrust to get off the ground and into space. But from the words coming out of SpaceX, about 80% of a rocket's fuel is actually expended accelerating to orbital velocity once you're in space. But, once you're no longer needing to fight gravity so much, it's a lot more efficient to use lower thrust engines for the job, provided you've got the time.
    - m_mueller 12y ago
      In defense of parent (although I'm speaking against my own initial argument right now): I think there are two ways to look at it. One is to look at the final energy state in orbit, where kinetic energy is much higher than potential energy. One is to look at how much fuel energy is spent for each delta-T for potential and kinetic energy, and integrating that over time. Intuitively I expect that to give a different result, e.g. most fuel spent for increasing potential energy. The difference[1] comes from the potential energy that gets stored in the remaining fuel that will be converted into kinetic energy later at a lower cost. So, there is an argument to be made for both sides, however one has to be careful in how to apply this knowledge to a new technology like air breathing rocket motors - it has a very different economy. [1] edit: There is a second factor that will also change that result compared to the first: air drag. I.e. some fuel energy will be lost in heating up the surrounding air.
  - m_mueller 12y ago
    The basics of your argument are true, but the air breathing capabilities change the magnitude of the effect quite a bit. Saving the oxygen weight in the 'first stage' helps a lot. One oxygen atom has a molar mass 15 times larger than one hydrogen atom. Burning O and H to H2O thus means you have 2/17 of the mass laying in the hydrogen, 15/17 in the oxygen. It's an order of magnitude savings. Again, look at the Virgin Galactic Spacecraft as an example. The reason why they can go so high without much of anything, is that their first stage is an air breather. Furthermore, the fuel that you need to bring to go from Mach 5 to Mach 25 is what's responsible for a large size increase of your first stage - to have more fuel to burn at a high altitude / high velocity requires even more fuel at the ground. Overall, you have to look at an airbreather quite differently than conventional rockets and you have to look at suborbital flights differently than orbital - you can't really approximate it from the space shuttle without going into the details.

by virtue3 12y ago

m_mueller 12y ago

You don't need the fuel to go high.[1] You need it to go fast. Orbital velocity is Mach 25, not the puny Mach 5 that they talk about for transporting passengers here. [1] See Virgin Galactic's 'spacecraft'. You see lots of fuel tanks? No? That's because they only do a little 'hop' to 100km or so, not accelerating sideways.

idlewords 12y ago
The fuel required to accelerate from Mach 5 in near vaccum to Mach 25 is a fraction of what you need to accelerate the vehicle + fuel combo to Mach 5 in air. Remember how the space shuttle had those massive rockets on the side, that fell off way before it was close to horizontal? You need the fuel to get high enough to go fast.
- XorNot 12y ago
  This is actually incorrect. The space shuttle had massive launch engines because it needed the thrust to get off the ground and into space. But from the words coming out of SpaceX, about 80% of a rocket's fuel is actually expended accelerating to orbital velocity once you're in space. But, once you're no longer needing to fight gravity so much, it's a lot more efficient to use lower thrust engines for the job, provided you've got the time.
  - m_mueller 12y ago
    In defense of parent (although I'm speaking against my own initial argument right now): I think there are two ways to look at it. One is to look at the final energy state in orbit, where kinetic energy is much higher than potential energy. One is to look at how much fuel energy is spent for each delta-T for potential and kinetic energy, and integrating that over time. Intuitively I expect that to give a different result, e.g. most fuel spent for increasing potential energy. The difference[1] comes from the potential energy that gets stored in the remaining fuel that will be converted into kinetic energy later at a lower cost. So, there is an argument to be made for both sides, however one has to be careful in how to apply this knowledge to a new technology like air breathing rocket motors - it has a very different economy. [1] edit: There is a second factor that will also change that result compared to the first: air drag. I.e. some fuel energy will be lost in heating up the surrounding air.
- m_mueller 12y ago
  The basics of your argument are true, but the air breathing capabilities change the magnitude of the effect quite a bit. Saving the oxygen weight in the 'first stage' helps a lot. One oxygen atom has a molar mass 15 times larger than one hydrogen atom. Burning O and H to H2O thus means you have 2/17 of the mass laying in the hydrogen, 15/17 in the oxygen. It's an order of magnitude savings. Again, look at the Virgin Galactic Spacecraft as an example. The reason why they can go so high without much of anything, is that their first stage is an air breather. Furthermore, the fuel that you need to bring to go from Mach 5 to Mach 25 is what's responsible for a large size increase of your first stage - to have more fuel to burn at a high altitude / high velocity requires even more fuel at the ground. Overall, you have to look at an airbreather quite differently than conventional rockets and you have to look at suborbital flights differently than orbital - you can't really approximate it from the space shuttle without going into the details.