Solid Propellant Primer

Solid rocket motors are both old and young; it is true that the Chinese were tinkering with them long before the word “rocket” itself was a thing, but it’s also true that, until Jack Parsons decided to mix asphalt and Potassium Perchlorate back at Caltech in 1943, they were limited to small, low-impulse and unreliable applications. And yet, even after Parsons came up with the big breakthrough, it took a lot of time before solid rocket motors reached any sort of maturity.

This is because, speaking politely, solid propellant motor design is f*cking hellish. In a liquid rocket engine, you generally have just two tanks of chemicals which, if a spark is provided, will burn with each other (and sometimes you don’t even need a spark), with all those “little” things like mass flow, chamber pressure and the like being easily acted upon in real-time. Solids are not like that.

In a solid rocket motor, you must build a propellant tank which is also a thrust chamber, where you must arrange the propellant in such a way that they will give you the thrust profile you want, while keeping in mind that the changing chamber pressures and temperatures during the burn will also change the rate at which the propellant burns; you must find a way to make the propellant solidly (pun not intended) adhere to the tank walls, so it won’t be sh*t out mid-flight with the ensuing embarrassment; you must take care that the propellant will keep being a propellant while it is stored and not turn into an explosive while you are busy; you must design a nozzle and chamber that can keep themselves together at the hellish temperatures of a burning rocket without having the opportunity to rely on transpiration or regenerative cooling; you must make, all in all, something that will passively carry out all the self-regulation that, in a liquid rocket engine, can be done actively and dynamically. And that’s before you even try to make a rocket that’s got a competitive specific impulse.

No wonder that Parsons himself, the archetypal solid rocketeer, ended up being obsessed with occultism - if there is a single piece of aerospace hardware that is likely to be easier to design when your office is haunted by a supernatural entity, it’s the solid rocket motor.

But what’s inside a solid rocket motor?

The insides of your average Solid Rocket Motor

Contrary to common sense, which sees solid propellant motors as metal cans full of undifferentiated boom juice, they’re actually rather complex pieces of engineering - being simpler than liquid rocket motors is not a hard bar to clear. Generally speaking, you have a structural pressure vessel, built out of either metal or composites, then a liner, thermally insulating the pressure vessel from the interior of the rocket and giving the propellant a surface where it can adhere well, then the propellant itself and, in the end, a pyrotechnic train to ensure ignition. After that, you have an ablative liner ensuring that the rocket’s throat won’t be burned off during flight by the hot combustion gases and a nozzle with, optionally, thrust vectoring mechanisms.

As far as the propellant itself is concerned, the precise formulation you will go with depends heavily on the mission the solid rocket booster is designed to carry out. Some formulations give you high specific impulse, some allow extraordinarily high thrusts, some have hard-to-spot exhaust plumes, some are just cheap and comparatively easy to work with.

But, before we introduce the various broad families of solid rocket propellants and their use cases, we must first introduce the general use case of a solid propellant motor over a liquid propellant one.

First of all, it is important to note that solid rocket motors are almost never the performance option - the highest specific impulse ones have specific impulses broadly on par with the lowest specific impulse storable liquids in common use, and those that are not ruthlessly optimized in that direction have specific impulses broadly comparable with those of the highest specific impulse monopropellants.

However, specific impulse is not everything: many other design drivers exist, and solids are generally pretty damn good at satisfying them.

What solids can offer you are high thrusts to weight ratios (because they don’t have to lug around heavy turbomachinery) with fast reaction times (because the engine can light the very moment you start the pyro train, without care for propellant settling, fueling, or anything of the sort) in a storable, extremely compact package (because solid rocket propellant mixtures are as a rule much denser than liquid ones) with no user maintainable parts inside (because you don’t have to lug around complex turbomachinery).

All of this, combined with the lack of toxic or corrosive or cryogenic spills if damaged, often makes us overlook their temperamental, tsundere attitude, their design complexity, their expense, and the fact that they can’t be easily throttled.

Today, you can find solid rocket motors serving as engines in military rockets, going from small anti-aircraft and anti-tank missiles to heavy intercontinental strategic systems, but also in space lift vehicles (which are absolutely not related to the previously mentioned intercontinental strategic systems, mostly because some of the countries employing them are not even supposed to possess intercontinental strategic systems of that sort), emergency crew escape towers, takeoff-assist units for aircraft, boosters for ramjet systems and so on and so forth.

Whenever and wherever convenience, compactness, and ease of use trump sheer specific impulse, you can bet solids will be a competitive option.

But then again, what gives solids their boom?

The insides of your average Solid Rocket Motor, this time for real

Broadly speaking, modern solid rocket motor propellant compositions fall into two wide categories: Double Bases and Composites. In a double base solid rocket propellant grain, you have an energetic solid monopropellant such as, for example, Nitroglycerine, awash in a gelling material, which can be another solid monopropellant too, for added bang - a classic example being Nitrocellulose.

Double bases don’t have a particularly ravishing specific impulse and they can age badly - for example, in the classic Nitroglycerine/Nitrocellulose combination, Nitroglycerine tends to sweat out of the Nitrocellulose, which is bound to lead to exciting fireworks of the unexpected kind when the rocket is fired, as the mixture stops burning and starts exploding. However, not all is bad with double bases - in fact, most are good: they are cheaper than dirt, comparatively easy to design and build and their exhaust trails are pretty hard to spot too.

Composite propellants, on the other hand, are made by mixing very intimately a solid fuel and a solid oxidizer - a classic combination being Hydroxyl-terminated Polybutadiene (HTPB), which I am assured is basically a fancy way of saying tire rubber, with Ammonium Perchlorate (AP).

Compared to double bases, composites age better (though they do tend to develop dangerous cracks if they experience too many temperature swing cycles) and have higher performance, but nothing comes for free in this world, and they are much harder to design and make, more expensive, and some of the best combinations (such as the aforementioned HTPB/AP) tend to produce billowing clouds of smoke (ever seen a tire fire?), which is bad for a lot of applications. All in all, the application is key - a ballistic missile, for example, doesn’t care much about producing smoke on the ascent, while an anti-tank or anti-aircraft missile should do its best not to make the launcher’s position immediately obvious.

Additionally, once we have put together our basic elements, the true fun can begin: sprinkling a bit of Aluminium powder in a double base or composite propellant gives you higher specific impulse and higher density, at the cost of producing even more smoke, while adding some rubbery binder will give your fuel grain better mechanical properties and less of a propensity to develop cracks (bonus points if your rubbery binder is also a good fuel, as in the case with HTPB), adding HMX, FOX-7 or other energetic nitramines will boost your specific impulse, and so on and so forth. It is kind of a messy field, this is what I am trying to explain.

Credit:

• To David Black for writing the article
• To Tshhmon for minor edits