Comment by MS90 | Hacker News Reader

Comment by MS90 | Hacker News Reader

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I think you raise a good point about scale. I don't think full sized rockets lifting off has as much to do with gimbals vs. fins as it does with sheer weight. The Saturn V had fins on it and it lifted off slow, and I'm willing to bet that it had a lot to do with the fact that a Saturn V on the launch pad weighed somewhere around 6.2 million pounds.

Scale it down a ways to something like a RIM-116 and you can see the scale come into play. This thing goes from launch to almost disappearing over the horizon in under three seconds and then hits the target.

https://www.youtube.com/watch?v=pVk9VnUkvaU

Granted, that RIM-116 does have fins. So let's take the fins away and scale up to something like a Polaris SLBM

https://www.youtube.com/watch?v=sUlXty69-Y8

Still unbelievably fast, same with the Tridents. These get popped up out of the water before they fire their main engines. It's basically free standing in the air when it fires the main and it still shoots off super fast.

https://www.youtube.com/watch?v=h5KejRbD5s0

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

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.

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.

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.

um, sure.

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.

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