SR-71 Refueling at Groom Lake

SR-71 Refueling at Groom Lake

Thursday, December 22, 2016

Minuteman Thrust Termination

From January 1985 Publicly Released Document
"Effectiveness of  The Minuteman II Stage III Refurbishment Program"

In a previous post here, I explained that throwing a warhead to the other side of the world is essentially the same thing as throwing a basketball through a hoop. They are both ballistic. You just need to let go at the right speed. All those pesky nuances like atmospheric drag and Earth rotation are just minor details compared to the ballistic math. 

In a previous post here, I explained how, in addition to speed, you need to throw the basketball or warhead in the right direction as well. Then I explained how the missile knows it is going in the right direction. 

All of that is necessary. But the missile must also "let go" of the warhead, just like you let go when you've thrown the basketball. As a minimum, the missile must stop thrusting.

This is easy to do if you have a liquid fuel rocket. You just close the valves. Or rather, the on-board computer closes the valves when the right speed is reached. In fact, you could close just the fuel or just the oxidizer valves and the "fire goes out". But the Minuteman ICBM is powered by solid rocket fuel. 

Solid rocket motors cannot be metered. Solid rocket motors contain both fuel and oxidizer in one big mix. If you visit a solid rocket motor factory you will go away thinking it is a lot like mixing up a cake. A very, very large cake that can explode at any minute. But mixed up in a big bowl with a big paddle attached to a big motor. 

Anyway, once you ignite solid rockets, they go until the fuel and oxidizer are depleted. 

Chemical-powered rockets, liquid or solid, are not much different from any chemical fire. A forest fire has fuel in the form of the entire forest and oxidizer in the form of oxygen in the air. Block the fuel with a trench or the oxygen with fire retardant and the fire struggles to stay lit. Fires generally need an ignition source, such as an untended campfire.  

The Space Shuttle on-orbit attitude control (orientation of the vehicle, where it points) used a fuel called monomethylhydrazine (CH3(NH)NH2) and an oxidizer called dinitrogen tetroxide (N2O4). These chemicals are used on Minuteman III, but not in the main solid fuel stages, stages 1, 2, and 3. More on that in a later post. This hypergolic system is popular because no ignition source is necessary. When they touch, they ignite. 

Space Shuttle reaction control system for on orbit attitude control 
The fuel and oxidizer create resultant chemicals as their original molecular structure is destroyed to release the energy. That is, monomethylhydrazine combined with the right proportions of dinitrogen tetroxide create water, carbon dioxide and nitrogen oxide.

Specifically, adding 2 parts monomethylhydrazine to 7 parts dinitrogen tetroxide results in 6 parts water, 2 parts carbon dioxide, and 18 parts nitrogen oxide. An understanding of chemistry allows us to "balance the equation" to know what proportions of fuel and oxidizer to use. 

7 N2O4 + 2 CH3(NH)NH2 = 6 H2O + 2 CO2 + 18 NO

Solid fuels, such as those used in Minuteman III can be endlessly fascinating in the various amounts and types that chemical engineers and other rocket scientists try. Typical oxidizers include both ammonium perchlorate and also various metal oxides to create high temperatures. High energy fuels include aluminum, magnesium, and zinc. Some fuels are there primarily for their ability to bind everything together: hydroxyl-terminated polybutadiene,  carboxyl-terminated polybutadiene, and polybutadiene acrylonitrile, for instance. One of the key ingredients for this chemical reaction is sufficient pressure in the combustion chamber of the rocket. 

So if we can't dampen the fuel or cut off the oxidizer, how do we stop the rocket motor from thrusting? 

First, we make it easy on ourselves by making sure the rocket is always powered by stage 3 when we want to stop thrust. Just figure out the shortest route to the nearest target and the farthest route to the farthest target and design stages 1, 2, and 3 so that stage 3 is thrusting in all cases. This is actually pretty easy energy management.

So how does Minuteman III stop stage 3 from thrusting? 

The diagram below shows a side view of a Minuteman II Stage 3. The secret is in the item highlighted on the top center with a dashed line circle: a thrust termination port. 



The next diagram is a "blow up" that port, which is what the port is for, blowing up. 

Actually, it just blows out, on command from the flight computer. When the right speed has been reached, the port blows open and the thrust chamber pressure that was present goes from full normal pressure to the vacuum of space. 

Ports are evenly spaced around the rocket's circumference to provide an even cut off. Detonation cords leading to each thrust termination port are of equal length to help ensure a simultaneous port plug blow. 

Look for the thrust termination just before the minute and a half point on this video

Yeah. Actually rocket science is hard, but kinda fun. 


2 comments:

  1. Thanks! I worked shuttle OMS/RCS in Mission Control for the first flights, but don't know solids from spaghetti. In retirement I've begun wondering if some weird 'sky spirals; seen around the rim of Russia [and commonly considered UFOs or alien wormholes, etc] were caused by ICBM tests with dump port activation with combustion continued but neutralized. Can I show you some of my reports on this theory? www.jamesoberg.com, email jamesEoberg@comcast.net

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  2. This is some very interesting reading.
    My father invented and designed the thrust termination ports on the minuteman third stage when he worked at Hercules a long, long time ago.
    he told me the story many times about it and testing and how much a pound of weight savings was worth back then.
    The same basic design was used on the minuteman I and II and submarine based missiles.

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