How a Minuteman III ICBM works

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An Air Force Global Strike Command unarmed Minuteman III intercontinental ballistic missile launches during an operational test Oct. 29, 2020, at Vandenberg Air Force Base, Calif. ICBM test launches demonstrate the U.S. nuclear enterprise is safe, secure, effective and ready to defend the United States and its allies. ICBMs provide the U.S. and its allies the necessary deterrent capability to maintain freedom to operate and navigate globally in accordance with international laws and norms. (Air Force photograph by Michael Peterson)
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by Charlie Vono, special to Aerotech News
The new Ground Based Strategic Deterrent intercontinental ballistic missile is now in its engineering and manufacturing development phase.

So now is perhaps an excellent time to describe how the old missile, the one that GBSD will replace, works. That way, as we learn of the new missile’s design and its support structure, we should be able to see how it solves many expensive problems of the current incumbent, the Minuteman III.

Despite being fielded in the 1970s, that old missile works pretty darn good at deterring attack. This is not terribly surprising since Boeing not only built the Minuteman ICBM series, but also the B-52 and KC-135 at about the same time — the mid-20th century. And they are all still in service.

My theory is that these examples of aerospace excellence, because they were designed using slide rules and other approximation techniques, inspired cautious engineers to add significant margins to the specifications and ensure back-ups to main systems. I experienced for myself how these wide design margins saved us when various subsystem degradation issues became apparent, not only as an ICBM sustainment engineer, but also as a KC-135 pilot.

Minuteman, as a very complex system, also benefited from TRW. TRW Inc. was created by Doctors Simon Ramo and Dean Woolridge. They provided systems engineering and technical assistance to the U.S. Air Force to help ensure hundreds of contractors were all working to the same plan. Some people like to say the “successful aerospace vehicle is a collection of thousands of parts and assemblies all flying in close formation.” On April 29, 1957, these two appeared on the cover of Time Magazine. It was around this time you started hearing the phrase “It doesn’t take a TRW rocket scientist to …” TRW was acquired by Northrop Grumman in 2001.

Minuteman III had, and has, hundreds of contractors, prime, subs, suppliers, vendors, and more. Plus, it had the expertise of amazing Air Force officers, enlisted members and civil servants. A few more will be mentioned below.

Multiple flight tests of the Minuteman III each year out of Vandenberg Air Force Base, Calif., give us confidence and our adversaries pause. In other, more specialized tests, the Air Force demonstrates how the Minuteman III can consistently deliver nuclear destruction reliably and accurately despite enemy attacks. Many of those tests occur at the very specialized environmental test facility north west of Hill AFB, Utah, called Little Mountain. All of that, and much more, is Minuteman III and the team meeting the deterrence mission, which is:

“Make our adversaries re-think any consideration of direct attack on our homeland.”

Like most well-designed and well-maintained missiles, the Minuteman III has the pointy end on top and the fire comes out the bottom. It is composed of three solid rocket motors, a guidance system, and a warhead. It also has, in a way, “4th stage” between the 3rd stage and the guidance system with enough liquid fuel on board to maneuver up to 3 warheads onto 3 targets, if assigned.

Around 60 years ago, brand-new Thiokol engineers in their 20s were given the task of designing a Minuteman I stage 1 that could blast itself and the upper parts of the rocket out of a well-protected and buried silo. In 2010, I had the privilege to meet and discuss with these 3 seasoned men this breakthrough technology. Two examples:

“Back then, we were in a big hurry. The static tests were scheduled to happen one after another supporting a rapid development schedule. Failures were common. The engineers had to keep up and come up with design tweaks to make the next test more successful. I feel like those first few months I spent all my time looking for nozzles in the Utah desert.” (Static tests are tests where rocket stages lay on their sides to be fired.)

“When we were hired, our boss put us in the room and said: ‘We need a cylinder that can withstand the pressure of our new solid rocket fuel and propel itself out of a buried silo intact.’ ‘How do we do that?’ I asked. He said: ‘That’s why I hired you, figure it out.’”

They figured it out. The ability to launch a Minuteman from a silo was demonstrated in September 1959 at Leuhman Ridge at Edwards AFB. Calif.

Thiokol was (and still is) located in the desert north of Ogden. While many of us in Utah still call it “Thiokol”, it has gone through a few hands such as Morton Salt, ATK, Orbital-ATK, and now is a part of Northrop Grumman.

One member of that original three-person team is still around and available to come to your location to speak: Allan McDonald at “ethicskeynotespeaker.com.” The name should be familiar.

Another familiar name from the early days of Intercontinental Ballistic Missiles is Charles Stark Draper, the MIT professor that convinced our government to guide ICBMs via internal, un-spoof-able, inertial guidance systems. Draper Labs in Cambridge Massachusetts is his legacy and is still a valued advisor on every US government inertial navigation system and much more. Draper showed us how to take somewhat crude spinning wheels used to cut-off fuel in German V-2 rockets and create ultra-precise guidance systems. Lookup Dr. Fritz Mueller’s Mechanical Integrating Accelerometer for more.

Figure 1: Basketball is Ballistic. (Courtesy graphic)

Placing a bomb at a target half-way around the world is an exercise in ballistics.

Getting a basketball into a hoop is the same basic problem. Stripped of minor, second-order effects like wind and weather, if you can place the basketball in the air at the right place with the right velocity vector (direction and speed), it will pass through the hoop every time. That’s what an ICBM does. And here is how a Minuteman III Draper-inspired Autonetics-built guidance system does it:

A gimbaled platform is placed towards the nose of the missile and is kept stable, in one position, by using gyroscopes. By keeping the platform stable, the on-board computer knows where the pendulous integrating gyroscopic (PIG) accelerometers are pointed. These spinning gyroscopes are deliberately placed out of balance such that the pendulous mass senses acceleration. Physics tells us that counting the precession of the gyro gives us a number proportional to the speed of the missile. Combining direction and speed tells us when to cut off the motor and let loose of the “basketball”. I am pretty sure this is when people started being confused about the fact that PIGs do fly.

A flying pig perched outside my home office. (Courtesy photograph)

Those with a bit of background in solid rocket propellant would have a question right now. How does the third stage stop firing when the correct location and velocity is achieved? Shutting off a solid rocket motor is pretty much impossible, right? The computer sends a signal to the third stage that blows open ports in the side of the motor, effectively driving the pressure in the combustion chamber to zero. Loss of thrust is instantaneous.

At this point, the Propulsion System Rocket Engine, designed and built by Bell Aerospace, takes over and, using small amounts of hypergolic fuel, maneuvers the guidance system and re-entry system (which contains the warhead) to an even more precise position and velocity in space as required by the target. Hypergolic fuel is widely used in military and civilian space vehicles to provide very reliable tweaks. For instance, the same kind of system was used on the Space Shuttle. In hypergolic engines, a fuel (usually some version of hydrazine) and an oxidizer (usually something like dinitrogen tetroxide) are pushed into a combustion chamber where they explosively combine producing carbon monoxide, water, various nitrogen molecules, and other stuff …. and a lot of force.

So, just like a basketball players’ hand, at the precise point in space with the precise speed and direction, the re-entry system gingerly releases the warhead and the PSRE gently backs away sending the re-entry vehicle upwards along its ballistic trajectory.

The re-entry vehicle (Lockheed-Martin) protects the warhead through intense atmospheric re-entry. This re-entry capability is very hard to achieve and is one of the major barriers to nations creating an ICBM force. So, let’s not give out any more details on that design here.

A few examples will illustrate the headaches involved in keeping this extremely complex bit of machinery called Minuteman III reliable and available at a moment’s notice.

First, recall that these missiles are buried underground under a 100-ton steel and concrete door that blasts open when the missile is commanded to launch. Any maintenance, routine or emergency, or any upgrades, must be delivered to hundreds of these silos buried all across the northern tier, sometimes in a deep snowfield. The Minuteman III was therefore built to be extremely reliable and not require many visits. But upgrades happen and must be precisely planned to not disrupt the mission. Routine maintenance of the guidance system is the biggest logistics nightmare. In the 1970s there was no substitute for spinning wheels. But spinning wheels wear out and must be dealt with via depot repair.

And you can’t simply open a drawer on the side to get to these wheels. The warhead must be removed to get to the guidance system. This requires significant numbers of Air Force personnel, both maintenance and security.

Let’s hope the GBSD minimizes the use of spinning wheels. Perhaps the new guidance system will be easier to access? I expect its reliability will be phenomenal.

Solid rocket fuel is extremely reliable. But it must be monitored. And when it starts to age, it must be replaced. Perhaps besides more durable and longer-lived fuel and container vessels we will have a more efficient monitoring and replacement process?

The thousands of parts and pieces that make up the rocket and all its associated equipment has to be kept procured and in stock. Obsolescence must be anticipated, and new sources found. Environmentally questionable materials must be recognized and phased out. I anticipate a very modern supply system for the new missile.

Complex pieces, such as rocket nozzles or electronics assemblies, can get low in stock and have to be re-manufactured by understanding the old specifications and building the new assemblies using modern capabilities while preserving their hardness against nuclear attack. I hear rumors of GBSD using the very latest systems engineering techniques to ensure documentation is available and useful.

And all of this, and much more, must be carried out in affordable ways as funding for ICBMs becomes more and more scarce each year. I expect annual costs of sustaining the new missile will be much less than the current one.

As we learn of the new solutions being engineered and manufactured for the new rocket, we can think about what this article discussed, read other descriptions of Minuteman III, and get a good feel for how well it is going.

Through the course of this article, I have mentioned a few people and a few companies, and the amazing people of the Air Force. Sadly, this means that many hundreds of major and significant designers, managers, vendors, suppliers, repairers, engineers, logisticians, systems engineers — even the list of names of the fields of expertise — is too long to state here, and have remained unnamed.

There are two books I can recommend providing a bit more history: “Fiery Peace in a Cold War” by Neil Sheehan which recounts the early history of ICBMs; and “Minuteman III” by my friend, David Stumpf, which should be published very soon. He has a book out already called “Titan II.” But that’s a whole other story.

Editor’s note: Charles Vono is a retired U.S. Air Force colonel and retired defense contractor with experience as a pilot, engineer and manager. This included Boeing KC-135 aircraft commander, Inertial Upper Stage software manager, and an ICBM Guidance Systems manager for a defense contractor. In 2014, he retired from his ICBM defense contractor job after 25 years as part of a vast nation-wide team keeping Minuteman IIIs meeting the deterrence mission. In that job, one of his additional duties was teaching the Minuteman III general familiarization class to contractor and government personnel.
 
 
 

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