[N_A_T_E]

Wow, so this thing needs to be pointed directly at the target 8,000 miles away and will miss the target by the amount of error in aim.

"To target a Minuteman I missile, the missile had to be physically rotated in the silo to be aligned with the target, an angle called the launch azimuth. This angle had to be extremely precise, since even a tiny angle error will be greatly magnified over the missile's journey. " ... "The guidance platform was completely redesigned for Minuteman II and III, eliminating the time-consuming alignment that Minuteman I required. The new platform had an alignment block with rotating mirrors. Instead of rotating the missile, the autocollimator remained fixed in the East position and the mirror (and thus the stable platform) was rotated to the desired launch azimuth. "

[kens]

There are two factors. First, any missile with inertial guidance needs to have a precise angle reference as a basis for the guidance system. If the guidance system starts off slightly wrong about which way is North, it's going to miss the target. Second, the guidance system in Minuteman I could only turn about 10 degrees from its initial angle before the wires would get tangled up. The solution in Minuteman I was to use the launch azimuth as the reference angle, so it was precisely lined up against this angle. Most of the alignment was physically rotating the missile, but the last bit of alignment was by constantly rotating the stable platform for alignment with the light beam from the autocollimator.

[ianbicking]

I noticed that too. That seemed odd at first read... after all, it has a guidance system, it's not relying on exact aim. I'm assuming it's more that its guidance system can only has so much fuel at its disposal and ability to correct errors, and if it's aimed incorrectly it would exhaust its fuel before it corrects its trajectory.

[ethbr1]

Sometimes it's less work to engineer a hard problem into an easy one, than to solve the hard problem.

Most of the tech for the Minuteman I was developed in the mid-1950s.

With that level of processing, would you rather solve a 2d problem by precisely orienting the missile before launch? Or a 3d one by requiring it to orient during flight?

Keep in mind: any equipment to self-orient in-flight also needs to be carried on the missile itself, while being tolerant of launch, acceleration, and reentry forces.

Any precision machinery at the launch site has no such requirements.

[aeonik]

This doesn't make sense to me. I would assume the engines starting by themselves would introduce enough error to throw the entire system off. Let alone natural seismic events in the ground, plus wind.

I would guess you must solve the 3D problem at least to some degree.

[ethbr1]

I'm not a rocket scientist, but I think thrust is pretty constant at that scale. That's why they start, spool up, then release from cradle.

Vs something like a Polaris SLBM that has a much more variable guidance problem

It'd be curious to see how early ICBM and SLBM guidance systems differ.

[kens]

I haven't looked at submarine systems in detail, but my understanding is that the big problem is that an ICBM knows where it's starting, but the submarine travels. So submarines have super-accurate inertial navigation systems on board to determine their position.

[ethbr1]

I was thinking more for the sequence where it broaches the surface then lights its engine.

https://m.youtube.com/watch?v=h5KejRbD5s0&t=34s

That's a lot more dynamic of a launch orientation. Which way is it rotated? Is it inclined off vertical?

[ecshafer]

Shooting a projectile, accounting for the earths rotation and wind, is essentially a solved problem (with computers). So I don't think this is that outlandish and I imagine it gets pretty accurate. Creating an analytical solution by hand is a junior level physics problem.