Adventures from the Technology Underground: Catapults, Pulsejets, Rail Guns, Flamethrowers, Tesla Coils, Air Cannons, and the Garage Warriors Who Love Them - Rilegato

William Gurstelle is the author of Backyard Ballistics, Building Bots, and The Art of the Catapult. He lives in St. Paul, Minnesota.

Chapter 1.

high-power rockets

Your finger hovers over the red button, and you move the microphone close to your mouth. You test the public-address system and are relieved to find that it works: When you speak, your voice is clearly heard all over the firing range.

Several hundred feet away is the launch pad, and on it stands the culmination of many hundreds of hours of labor and many thousands of dollars of your hard-earned discretionary income. It is your rocket, a 15-foot-tall accurate scale model of an American early 1960s solid-fuel Pershing I nuclear ballistic missile. It is a machine that you designed and built from scratch.

Your rocket is loaded with two stages of powerful chemical engines. Like the original Pershing, your motive power comes from two stages of precisely packed chemical fuel arranged in solid form. Each rocket engine is designed such that after it ignites, the gas from the burning chemicals will issue rearward in a high-velocity, high-temperature stream from the ceramic nozzle and propel the rocket up toward the stratosphere. Your rocket will reach empyreal heights, tens of thousands of feet—if all goes well.

You pay rigid attention to the preflight checklist. So far, everything looks like a go. There are small indicator lamps on the firing controls that signal launch status, and the ignition lamp shows green. This means that you have a working circuit, and so when the Fire button is pushed, enough current will be sent through the thin metal wire rammed into the motor to heat it red hot and thereby initiate the self-sustaining chemical reaction that occurs within the main motor’s combustion chamber.

The countdown begins. Ten. Nine. Eight . . . At zero, you push the button and instantly great plumes of white smoke surround the base of the rocket. For a moment, the rocket doesn’t move, and you too hold your breath. Then suddenly it leaps toward the sky with neck-jerking acceleration. The noise from the launch comes a split second after you see it leave, and when the noise does come, it is nearly deafening. The rocket climbs 100, 200, 500, 1,000 feet, its speed escalating logarithmically as it ascends. It climbs and climbs, and it becomes difficult, then nearly impossible, and then totally impossible to see the rocket itself, although the smoke and nozzle fire remain visible.

Everyone congratulates you on a successful launch. There is applause and backslapping, high fives all around.

But the celebration is cut short by the sound of the range safety officer’s warning horn: Whoop! Whoop! Whoop! The RSO’s voice is plainly heard over the public-address system. “Attention! Look up! Look up! We have a rocket coming in hot!” This is not good for you. This is not good for anybody. In fact, this is trouble with a capital T.

What has happened is this: your rocket has two stages. The first stage consists of several large chemical rocket engines that lift the entire rocket for the initial or “booster” phase of the flight. When expended, the booster rocket falls away, and a second engine, mounted above it, is supposed to automatically ignite and continue powering the remaining components upward.

But the second stage, powered by its own very large engine, has ignited later than it was supposed to. In fact, it ignited after the rocket reached apogee and had already turned and begun to head back to earth. So the engine is not powering the rocket to fly up higher. Your rocket is being driven back down to earth not only by gravity, but also by the second-stage engine. There is a real danger that the rocket will reach the ground and your launch area before this engine is burned out and triggers the timed ejection charge that deploys the recovery parachutes.

The current situation is this: There is a very large and heavy rocket coming your way on an unpredictable descent path, and not just in free fall, but pushed by the thrust of a high-impulse, high-velocity, solid-fuel rocket engine.

This is LDRS, the country’s—and probably the world’s—largest annual gathering of high-power amateur rocket enthusiasts. From all over the world, eager rocketeers come for a long weekend’s worth of home-brewed acceleration and conversations about rocketry.

LDRS is an acronym for Large Dangerous Rocket Ships. It’s the place where the people who started out as boys and girls experimenting with Estes and Centuri model rockets go when they want to build really, really big rockets of their own.

LDRS is sponsored by a group called Tripoli, which is the largest organization of high-power rocket makers. There are scores of local chapters or “prefects” in locations across the world. This year, Tripoli has chosen the Panhandle of Texas Rocketry Prefect to host the big event. The local leadership has been busy for months turning a large patch of cow pasture into the nation’s most active rocket launching area.

Rocketeers both need and love wide-open spaces—the wider the better. Amateur rocket builders, especially those who specialize in building the largest and most powerful rockets, want only a couple of things: a lot of flat, open, unpopulated land in which to recover their rockets after flight, and clear, sunny skies. This makes places such as Texas, Kansas, and the Canadian prairie provinces ideal spots for LDRS gatherings.

The launch site is south of Amarillo, straight down the Interstate to the tiny hamlet of Happy, Texas. At that point, the route to LDRS follows Texas Ranch Road 287 east, a long, straight, and uncrowded chunk of pavement that goes through territory so flat you can practically see the curvature of the earth.

At the end of the long drive is the LDRS launch site, a sprawling temporary compound of tents, launch pads, electronics, and people. The level, open venue is perfect for facilitating the retrieval of the hundreds of rockets that will eventually drift back to earth during the event, attached by elastic shock cords to large white parachutes. This particular site has the additional and highly valued quality of being well outside all commercial air lanes, so the airspace above it has no scheduled flights. Even so, the Tripoli organizers had to apply for a certificate of special clearance from the Federal Aviation Administration, allowing very-high-altitude rocket flights during the three days of the event.

Central Texas can be brutally hot and bright in July, and the tents and E-Z Ups set up by the rocketeers and vendors provide the only shade. This meet has the air of a large crafts fair, except that the vendor booths contain recovery chutes, rocket engine casings, altimeters, and launch towers instead of decorated ceramic pots and fabric wall hangings. The east side of the area is dominated by rows and rows of missile launching pads.

In this heat, people are not inclined to exert themselves if they can help it, so most simply wander around the dusty field, working on their projects, talking to one another, and pointing. Spectators at a large-scale high-power rocket launch do a lot of pointing—always toward the sky, arms extended at about 70 degrees to the horizon. Their fingers trace out the rocket’s acceleration skyward and then fall back down to their sides as they watch it float down on the end of a parachute or two.

Temperature notwithstanding, for a few days the formerly sleepy area becomes an energetic beehive of activity: smoke plumes and contrails constantly hanging like puffy ropes over the ranch, rockets roaring up, then silently floating down.

The great number of participants keeps several launching pads active. The pads with the biggest rockets are placed the farthest away from people, for it is not unusual for a rocket to blow up, or in rocket lingo, “CATO,” on the pad, producing a shrapnel rainstorm.

On the afternoon of the second day, a really big rocket, two and a half stories tall, stands erect on the far launch pad. It is a gracefully proportioned and aerodynamically shaped rocket and it is beautiful, at least to a high-power rocket enthusiast. Spectators and rocketeers alike press toward the safety fence to get into position for the best view.

This is the Athos II rocket, built by the Gates brothers of California. Athos II is a very large rocket with high-specific-impulse engines and will likely attain great heights. This launch is obviously going to involve significant velocity, complexity, and power. Athos’s launch has been anticipated for quite some time, so the crowd near the safety fence is thick. People reach for their binoculars and position their cameras on tripods. Over the facility’s loudspeaker, the launch control officer begins the countdown for one of the highlights of LDRS-21.

THE TECHNOLOGY OF HIGH-POWER AMATEUR ROCKETRY

In the typical solid-fuel rocket, the rocket maker builds a fiberglass shell that houses the motor, the recovery system, and whatever sensors, cameras, or other payload is placed within.* But the bulk of the rocket’s weight is contained in its powerful chemical engines. In and of themselves, rocket engines are marvelous things. Their most basic form goes back to first-millennium China, when crude black powder was stuffed into bamboo rockets and used to frighten the enemy’s horses. A simple rocket engine is straightforward and easy to understand. There is chemical propellant packed inside a metal casing. The chemicals inside the motor burn, and as they do so, hot, expanding gas is produced. This gas rushes out the back of the motor through a nozzle and, as described in Isaac Newton’s Third Law of Motion, the backward g...