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In a previous post here, I explained how Minuteman Intercontinental Ballistic Missiles are ballistic. At the end of stage 1, 2, and 3 firings (about 3 minutes) what’s left of the missile after the stages have fallen away is called the post boost vehicle (PBV). So by definition, the PBV is always on a ballistic trajectory, much like a basketball leaving the players’ hands.
If you get the right speed and direction and let go at the right instant, you will swish through the hoop (like John Stockton) or place your warhead on target. So here I talked about direction, and here I talked about letting go at the right instant (or for a missile, cutting off the thrust).
But the astute reader will recall that a single Minuteman III can be configured to deliver 3 warheads to 3 different targets. So the whole ballistics thing doesn’t make sense anymore. How can 3 basketballs go to 3 different hoops off of one hand? Even John couldn’t do that. No matter how good he is. It is just physics.
It all comes down to using a non-engine engine to finesse a non-ballistic ballistic trajectory using a post-boost boost vehicle.
The PBV is a post boost booster because it includes not only the warhead and guidance system, but also something called a PSRE, a Post Boost Rocket Engine. With this engine (really, a collection of engines), the guidance system can tweak the path after the main boost has occurred. It can adjust or maintain the direction the post boost vehicle is pointing via its attitude control engines (the flames really are blue as in the photo) and it can also give a little more boost with its axial engine (in the center of the PSRE in the photo). So… it can boost, post-boost. A post-boost booster.
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Minuteman III Propulsion System Rocket Engine (PSRE), built by Bell Aerospace Corporation (now Aerojet Rocketdyne), provides the Minuteman III missile with postboost propulsion (both down-range and/or cross-range extension) and precise attitude and velocity adjustment. — http://www.rocket.com/minuteman-iii-psre
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But that’s not all. It is also a non-engine engine. Some dictionaries will tell you an engine is “…a machine with moving parts that converts power into motion”. But the engines in the PSRE are just chambers where the fuel and oxidizer mix. The shape and flow create the resulting thrust, not any moving parts. The fuel and oxidizer are hypergolic. That is, when they touch, they ignite.
In a discussion in this blog post here, I mentioned the hypergolic on-orbit attitude control system used by the Space Shuttle. It used a fuel called monomethylhydrazine (CH3(NH)NH2) and an oxidizer called dinitrogen tetroxide (N2O4). (This is nasty stuff as you might imagine. Don’t try this at home.)
The PSRE has two propellant storage assemblies (the two big silver cylinders in the photo) containing monomethylhydrazine and nitrogen tetroxide. It also has a gimbaled axial engine and 10 attitude-control engines for roll, pitch, and yaw.
Can they really be engines without any moving parts? I suppose there are the pyrotechnics that open the fuel and oxidizer lines just before they are needed. And there are metering valves that allow fuel and oxidizer to flow or shut off so the guidance computer can tweak the flight path. So if you include that, these engines move a little bit. And, to be completely fair, there are also little electric motors that move the axial engine slightly in two dimensions, pitch and yaw.
But I still claim it all comes down to using a non-engine engine to finesse a non-ballistic ballistic trajectory using a post-boost boost vehicle.
I know, you are still trying to imagine John Stockton tossing 3 balls at three hoops all at once from one hand. What really would happen is that he would carry the balls and move and toss. This is what Minuteman does, with the help of a PSRE that can finesse just as good as an NBA high scorer.
More on that in a later post.
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