4 ms·

It has nothing to do with sheer weight and everything to do with the thrust-to-weight ratio. Saturn V was slow to liftoff because the launch configuration had

It has nothing to do with sheer weight and everything to do with the thrust-to-weight ratio. Saturn V was slow to liftoff because the launch configuration had a very low thrust-to-weight ratio, IIRC about 1.1

MS90 7y ago
And you don't think that has anything to do with the scale of it? The size of the rocket necessitating a lower T-W ratio due to the impracticality of making a rocket engine large enough to increase it? And yeah, sheer weight definitely has something to do with the thrust to weight ratio. It's half the equation.
- singingboyo 7y ago
  Honestly, maybe partially scale, but not really. It's more a factor of what it was meant to do. Going to the moon means extra hardware on top, which means more fuel, which means lower TWR at launch. If you're just going to drop a 6000kg bomb on earth somewhere, you need a lot less rocket on top of that first stage, and it's going to take off a lot faster.
  - MS90 7y ago
    I think we're just going in circles now. The scale was a result of what it was meant to do. The extra hardware on top and extra fuel all add to the scale of it. It was a massive rocket because it needed to do a massive job. Anyway, I think we've gone off track from the original point, which was that full size launch vehicles taking off slow isn't because of gimbals vs. fins, it's because they're big and heavy. We can be pedantic and say it's specifically because of a low T-W ratio, but the T-W is low because the weight is high and it's not practical to build a massive rocket engine to increase it.
- andbberger 7y ago
  um, sure.
taneq 7y ago
Yep, and it's all down to the squared/cubed law. The thrust that a rocket engine makes at a given operating pressure is proportional to the area of the bell, and so scales with the square of its linear size. The mass scales with the cube of the linear size, and so for a given shape of rocket, doubling the size will multiply the thrust/weight by a factor of 2^2 / 2^3 = 1/2.

MS90 7y ago

And you don't think that has anything to do with the scale of it? The size of the rocket necessitating a lower T-W ratio due to the impracticality of making a rocket engine large enough to increase it? And yeah, sheer weight definitely has something to do with the thrust to weight ratio. It's half the equation.

singingboyo 7y ago
Honestly, maybe partially scale, but not really. It's more a factor of what it was meant to do. Going to the moon means extra hardware on top, which means more fuel, which means lower TWR at launch. If you're just going to drop a 6000kg bomb on earth somewhere, you need a lot less rocket on top of that first stage, and it's going to take off a lot faster.
- MS90 7y ago
  I think we're just going in circles now. The scale was a result of what it was meant to do. The extra hardware on top and extra fuel all add to the scale of it. It was a massive rocket because it needed to do a massive job. Anyway, I think we've gone off track from the original point, which was that full size launch vehicles taking off slow isn't because of gimbals vs. fins, it's because they're big and heavy. We can be pedantic and say it's specifically because of a low T-W ratio, but the T-W is low because the weight is high and it's not practical to build a massive rocket engine to increase it.
andbberger 7y ago
um, sure.

taneq 7y ago

Yep, and it's all down to the squared/cubed law. The thrust that a rocket engine makes at a given operating pressure is proportional to the area of the bell, and so scales with the square of its linear size. The mass scales with the cube of the linear size, and so for a given shape of rocket, doubling the size will multiply the thrust/weight by a factor of 2^2 / 2^3 = 1/2.