How ICBMs get to their targets

[yungman]

TL;DR Summary

I am NOT knowledgeable on space science, just a simple question I want to verify after listening to someone described how ICBM hit the target.

Hi

Someone told me that to hit the target, they just shoot the ICBM up the space in the right latitude, then wait for the earth to rotate until the target is right below the ICBM, then just drop down to hit the target.

That sounded strange, but seems doable. But I would think it's a lot faster to reach the target if it actually speed towards the target then drop. Also give less time for enemy to react. Instead of waiting half a day if the target is on the opposite side of the earth before dropping, the ICBM can reach the target in minutes if it actually flies towards the target. You don't want to give the enemy time to react.

Like I said, I don't know the first thing about space science, it's not a homework, it seems very obvious to speed towards the target, but what do I know.

Thanks

Alan

 

[Drakkith]

Someone told me that to hit the target, they just shoot the ICBM up the space in the right latitude, then wait for the earth to rotate until the target is right below the ICBM, then just drop down to hit the target.

No, that's about 90% wrong. While the rotation of the Earth has to be taken into account, ICBM's have to travel at nearly orbital speed to get all the way across to the other side of the planet. That means they need to travel about about 7 km/s (15,600 mph) at maximum velocity, while the Earth's surface moves, at best, at around 0.46 km/s (1,000 mph).

The thing with ICBM's is that they are ballistic, meaning that the majority of their flight time is spent in free fall. Other than minor corrections for the warheads prior to firing from the final stage, the missiles quickly accelerate up to some velocity, often near orbital speeds, and then simply shutoff their engines and coast until the warheads are fired. The warheads then enter the atmosphere and guide themselves to their targets.

This coasting might seem wasteful, but there are two reasons for it:

1. They have to go AROUND the Earth. You can't launch them into space and then immediately point them straight towards their target, as their target would be blocked by the ground or the ocean.
2. Carrying enough fuel to continuously burn until the warheads launch, while definitely faster, is monstrously more difficult. The launch vehicle would be MUCH more massive. So large that it wouldn't be financially or technically feasible to build such a weapon system.

Also give less time for enemy to react. Instead of waiting half a day if the target is on the opposite side of the earth before dropping, the ICBM can reach the target in minutes if it actually flies towards the target. You don't want to give the enemy time to react.

The entire process, between launch of the vehicle and impact of the warheads, lasts around 30 minutes at most. Your enemy already doesn't have much time to react, and the vehicle is traveling so fast and so high that almost nothing can destroy it.

 

[Vanadium 50]

No, that's about 90% wrong.

I'm looking for the other 10%.

The descriptions has ICBMs starting and stopping and changing direction willy-nilly. Stopping a rocket takes as much energy as it did to get it going in the fisrt place (and then you need more fuel to lift that fuel, and more fuel still to lift that fuel, and so on and so on.) Once launched, apart from course corrections, the destination is pretty much fixed.

Once rockets go up, who cares where they come down? "That's not my department", says Werner von Braun.

 

[Baluncore]

I came across this book a couple of months ago. It is a great read for anyone interested in the engineering technology and history of inertial guidance.
Title: Inventing Accuracy : A Historical Sociology of Nuclear Missile Guidance.
Donald MacKenzie, 1993, MIT Press.

 

[sophiecentaur]

TL;DR Summary: I am NOT knowledgeable on space science, just a simple question I want to verify after listening to someone described how ICBM hit the target.

it seems very obvious to speed towards the target, but what do I know.

If the purpose were to get a very small payload to the target then that could be a good system (with a bit of 'aiming ahead' to compensate for a target rotating around the centre of the Earth) But we want to get a big payload to the target so we have a limited amount of fuel (energy). The most efficient system uses a ballistic path - as with a Moon shot- and most of the journey is in free fall. To detect and deal with an ICBM requires fast and accurate detection and tracking in order to get to it in time.
Note the date on the link from @Baluncore in the above post. It's been a long term study, since the fifties and earlier. We still have the same old problem of gravity and distance.

There is limited gain in using orbiting missiles because they can' t be relied on to be in the right place at the right time to beat the surface to surface approach. And you would need a sky full of them!

 

[berkeman]

TL;DR Summary: I am NOT knowledgeable on space science, just a simple question I want to verify after listening to someone described how ICBM hit the target.

Hi

Someone told me that to hit the target, they just shoot the ICBM up the space in the right latitude, then wait for the earth to rotate until the target is right below the ICBM, then just drop down to hit the target.

That sounded strange, but seems doable. But I would think it's a lot faster to reach the target if it actually speed towards the target then drop. Also give less time for enemy to react. Instead of waiting half a day if the target is on the opposite side of the earth before dropping, the ICBM can reach the target in minutes if it actually flies towards the target. You don't want to give the enemy time to react.

Like I said, I don't know the first thing about space science, it's not a homework, it seems very obvious to speed towards the target, but what do I know.

Thanks

Alan

It sounds like you can learn a lot about ballistic trajectories from American football passes. Modulo the spin coupling to the atmosphere, of course...

https://www.mercurynews.com/2023/11...chances-at-no-1-seed-wilks-big-sideline-move/

 

[Baluncore]

Note the date on the link from @Baluncore in the above post.

I suspect the date, 1993, coincides with the advent of the game-changing GPS, and the solid-state Fibre-Optic Gyroscope.

 

[Filip Larsen]

Someone told me that to hit the target, they just shoot the ICBM up the space in the right latitude, then wait for the earth to rotate until the target is right below the ICBM, then just drop down to hit the target.

This idea is probably fueled by the common misconception that getting into space is only about altitude, when in fact speed is the primary thing you need to obtain. On top of that, only thrusting vertically up (directly against gravity) is extremely wasteful due to what is called gravity loss, so any rocket, whether its payload is designed to be delivered halfway around the globe or to a another planet, invariably uses a gravity turn launch profile to get into orbit.

 

[Orodruin]

Once rockets go up, who cares where they come down? "That's not my department", says Werner von Braun.

In German or English, I knov hov to count dovn
… und I’m learning Chinese

 

[sophiecentaur]

I'll just mention the term "Coriolis effect", which throws everything out on a rotating Earth.

 

[Vanadium 50]

This is an issue even for naval gunfire, and it's been a solved problem for more than a century. You just have to aim properly.

 

[Filip Larsen]

I'll just mention the term "Coriolis effect", which throws everything out on a rotating Earth.

Or just use a more inertial system, like any of the Earth-centered inertial systems, and forget about Coriolis.

 

[.Scott]

I think the arithmetic would work out something like this:

If the missile had no acceleration time and the Earth had no atmosphere and was not rotating, then you would simply launch for a minimal suborbital flight that would follow a great circle path. If only the target was rotating with the Earth, you would need to adjust your trajectory's destination based on the time-to-target. Since the launch pad is also rotating with the Earth, you will use that to your advantage (maybe even switching a westerly launch to the east) - adding to that starting velocity to get to your suborbital time-to-target-adjusted trajectory.

Then, since your rocket will burn through fuel at it accelerates and both climbs and descends through an atmosphere of changing density, you will need to plan out two profiles - an ascent profile and a descent profile - which you can paste into your main trajectory.

Of course, every time you add in another factor, it will create secondary effects when combined with those previous factors.

So, rather than trying to solve the entire ascent/suborbit/descent trajectory as a closed math problem, I would suggest you model the whole thing on a computer, start with an estimated trajectory, work out the "miss" distance and the correction, and retry the model with the estimated correction. Then repeat until the correction is significantly smaller than other error factors in your missile system.

If it's just a one-off problem, I would probably use a spreadsheet program like Excel with each iteration using one row.

 

[sophiecentaur]

Or just use a more inertial system, like any of the Earth-centered inertial systems, and forget about Coriolis.

Whatever you call it (force or imaginary force) going from point A to point B on the Earth requires you to include the effect in your navigation or balistics plan.