Blog-a-Day #4 -- Screamer

I’m not sure about this picture. I guess I get the sense that the wind is up and maybe really roaring as the squall approaches. Also, it’s the wind, the rushing air that is driving the boat. Let’s see what analogies we can draw between this vehicle on the cusp of a storm and our next spacecraft design, which I call the Screamer.

As we discussed last time, Single Stage to Orbit vehicles, or SSTOs, have been the Holy Grail of launch vehicles for decades. The prospect of much lower costs to deliver payloads to Low Earth Orbit (LEO) holds the promise of greatly expanded space activities including exploration, tourism, and even colonization. A successful SSTO design has never come to fruition, however, and it’s likely none ever will. But, I contend that we don’t need a vehicle that goes directly from the surface of the Earth into orbit without shedding any dead weight along the way.

Using a staged launch system, as the Soviets demonstrated in the 1950s, addresses the dreaded Rocket Equation problem. It turns out to be much better to burn some of your fuel then jettison the empty tanks and maybe even the engines, at which point you can light up a whole new rocket with its own fuel and engines. Getting rid of the dead weight of empty fuel tanks and their associated plumbing makes sense, but why throw away all of that hardware? Chucking expensive gear, especially highly-tuned, extreme precision machines like rocket engines is not very cost-effective. Enter the notion of reusability.

Some space launch systems now include components that can be reused on later flights, but they can easily be lost or damaged and even under perfect conditions need extensive refurbishment before they can fly again. Why can’t spaceships be like jetliners, taking off and landing multiple times per day? There are three reasons why: air, air, and air. First of all, a jet engines uses the ambient air as an oxidizer for its fuel, whereas rockets have to carry their own oxidizer, either as part of the solid fuel, or as separate liquid oxygen. The second reason is that while relatively dense air (below 90,000 feet) keeps a jet aloft as it flows over the wings, air at even much higher altitudes creates drag which counteracts the thrust of the engines. And finally, slamming into even thin air super-heats the vehicle’s skin through compressive heating and friction.

These factors all act to limit aircraft speeds to about Mach 3, or only about 2200 mph. The SR-71B, otherwise known as the Blackbird, actually used this compressive heating in its ramjet engines, but it also meant that the panels of its titanium skin expanded so much from the heat that it leaked jet fuel like a sieve until it got up to operating speed. And that was at twice the altitude, 85,000 feet, of most commercial airliners.

Which, at long last, brings us to my Screamer design in which I try to take advantage of all of these perceived disadvantages by turning them upside down. First, the Screamer (so named because of the noise I imagine it making in flight) is a two-stage launch system. Its first stage is the size of a giant cargo jet consisting mainly of jet fuel tanks, fairly traditional turbofan jet engines, and really big wings. The fuel, engines, and wings serve one purpose: getting the Screamer orbital stage, which is strapped to its back, very high. The whole contraption takes off horizontally and climbs to a maximum height of perhaps 80,000 feet, but well under Mach 1.

At the first stage’s maximum altitude, the Screamer stage pops off and ignites its hybrid engines. What, pray tell, are hybrid engines? They are the latest, by which I mean still mostly theoretical, in rocket engine design. They are “hybrid” because they use the ambient air as an oxidizer for as long as possible, only switching to onboard liquid oxygen when the air gets too thin.

The Screamer design takes maximum advantage of that big impediment, the air, in multiple ways. First is the shape of the craft itself, which approximates a shape known as a “lifting body”. A lifting body aircraft, when dropped, will spontaneously orient itself onto a smoothly descending glide path. This will help a lot on re-entry, but it also helps during ascent by keeping the Screamer within the upper reaches of the atmosphere. Yes, that’s right I said keeping it in the atmosphere helps. Remember the Screamer turns traditional rocketry on its head.

We want the Screamer to stay within the atmosphere for as long as possible because the more air its engines breathe, the less oxidizer it needs to carry. But what about that nasty compressive heating that limits the Blackbird to Mach 3? Well, that’s at an altitude of 85,000 feet. At four or five times that height, which is where the Screamer would be optimized to operate, the air is much, much thinner. With the ever increasing speed of ascent, the thinnest air can still be dense enough to burn the fuel. In effect, the Screamer must rise through the atmosphere as its speed increases, keeping a balance between the air creating too much drag and heat, and becoming too thin to burn the fuel. The longer it can maintain that balance, the less oxidizer it needs to carry. But, eventually we do want it to actually get into space where there is no atmosphere. So, some oxidizer is needed for that last push.

Now, let’s look at the engines. One could say that the entire Screamer fuselage is one big engine, although one could just as easily argue that there are thousands of little tiny engines, unlike the Blackbird’s massive ramjet engines. Ramjets “ram” the incoming air against the sides of the engine, superheating it (that compressive heating again), and mixing it with jet fuel which ignites and blows the air out the back of the engine.

So, what’s the problem? Why can’t a ramjet keep going faster and faster? Because once the air flowing through the engine reaches the speed of sound, strange things begin to happen. For example, turbulence becomes cavitation. Turbulence is present to some degree in any fluid, and it can be accounted for and even utilized to thoroughly mix the air and fuel. But, when the waves of turbulence move faster than sound, they leave pockets of vacuum in their wake. This is a process called cavitation, which starves the fuel of the oxygen it needs to burn. Other problems arise at supersonic airspeeds, as well. Together, they represent a natural governor on the effectiveness of a ramjet.

Supersonic combustion ramjets, called “scramjets”—engineers are not known for clever nicknames—have been proposed and experimented with, mainly at the Skunk Works (go ahead, look it up). The Screamer, though again takes an impediment and makes it an enabler. The Screamer’s stubby wings are riddled with holes that run from the leading to the trailing edges, providing channels for the air to slide through. Rather than a traditional wing, whose leading edge splits the air sending some over the top of the wing and some under the bottom, the Screamer’s leading edges channel most of the air into the wing itself, leading to much less compressive heating of the wings. Since the channels are quite narrow, the airflow is much smoother leading to less turbulence. The heat generated from the parts of the wings which do slam through the air, as well as the heat from the rest of the airframe is conducted by the fuel into the wings. There the heat is transferred to the air flowing through channels, speeding it up even more.

The channels constricting shape further heats the air creating a standing wave of extremely high pressure and temperature at designated locations throughout the channel. This is where the fuel is introduced and ignited. By designing the resonance of the channels to create this standing wave, the natural turbulence is controlled and combustion can take place even in hypersonic airflows. The side effect, of course is a high-pitched whistle that each of the channels produces. Their frequencies need to be staggered to prevent runaway resonance, but the resulting dissonant scream gives the engine its name.

I’ve started a science fiction story in which my protagonist, Nick Arms, rides a Screamer with a group of colonists to begin their training in orbit. It’s just one scene, but it puts to bed any reservations the reader might have about the possibility of routine access to space.

Rob Johnson

May 19, 2020


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