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 gush of the gas results in an equal and opposite forward thrust of the rocket body. Simple, yes. But hey, this is rocket science, and things get complicated quickly.
Small, commercially available model rocket motors consist of black-powder propellant pressed under tons of pressure into a hard, dense matrix called "grain." When the grain is ignited, the motor starts burning linearly, like a very fast-burning cigarette, from its back to its front. As it does so, it pushes hot gas out through a clay nozzle, and the rocket zips forward until the propellant is all burned up.
The world of high-power rocketry is different and much more complicated. Instead of using a simple black-powder chemical rocket motor, the experienced flyers most often use engines made out of "composite propellant"--a combination of an oxidizer chemical such as ammonium perchlorate (AP) and a synthetic rubber binder material to hold the oxidizer in a desired shape and provide fuel. In addition, the rocket engine maker may mix in plasticizers, catalysts, and crosslinkers, all of which can make the propellant burn stronger, longer, slower, or hotter, depending on the goals of the rocket designer. Composite motors are formed into various shapes with voids and holes precisely designed into the motor in order to shape the direction and velocity of the exiting gas. Such complex contours and figures are complicated to fabricate, requiring great quantities of heating, molding, curing, machining, and, above all, attention to detail.
The most extreme rocket makers spend days on end experimenting with rocket designs and motor formulations. There are so many variables that the maker can adjust to affect the performance of the rocket. A quick list of their concerns includes the shape of the rocket body, fin design, the shape of the nozzle, the geometry of the motor's core, the combination of various chemicals that make up the propellant mixture, the rate of burn, and the ignition method. It takes a lot of scientific, mechanical, and seemingly alchemical knowledge to become a really good rocket maker. There is also an element of danger working with toxic and flammable chemicals such as ammonium perchlorate, potassium nitrate, and liquid oxygen.*
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What rocket makers care about most is the physics quantity called "total impulse." Total impulse is the product of the force acting on a rocket (the thrust) multiplied by the amount of time the thrust is applied. Expressed mathematically, it is:
Total Impulse = Average Thrust ´ Burn Time
An engine that applies a lot of thrust, for a long period of time, is a high-performance engine. To a rocket engine maker, the goal is lots and lots of impulse.
The size of a rocket motor and the amount of total impulse it produces are described by assigning the motor a letter of the alphabet. The smallest rocket motor is an A and is commonly sold in hobby stores without need for a permit. The B motor is twice as big as an A, and a C is twice as big as a B. Each increase in letter size denotes a doubling of the engine's rocket-lifting ability, or total impulse. The total impulse of an A-motor is about 2.5 newton-seconds (N-s), which is enough to lift a small rocket a few hundred feet. A B-motor provides 5 N-s, C-motors provide 10 N-s, and so on. The largest commercially produced rocket motor available to certified amateur flyers, the mammoth N motor, provides a muscular 41,000 N-s. Custom engines are available from a number of boutique rocket engine designers. Some of these go into the O and P range and even beyond. They are large and energetic enough to power a half-ton rocket to jet-fighter altitudes. (Using this scale, the NASA space shuttle's 8.3 million Newton-second booster rockets are about two letters beyond a Z-motor.)
Although there are many variations in the design and construction of homemade rocket engines, one of the clearest differentiating factors is the type of chemicals used to provide the energy and hence the impulse. The two most common general categories of chemicals are those involving variations of black powder and those that use composite propellant. Composite engines are, pound for pound, significantly more powerful than black-powder engines, that is, they have a higher specific impulse.
Every rocket engine, from black powder to solid fuel composite to liquid fuel to hybrid systems, works in similar fashion and is subject to the same basic physical laws: The propellant is ignited. It burns. Hot and expanding gases are produced and then stream out of a nozzle. Thrust is produced and the rocket and whatever is attached to it goes forward.*
The force produced by the gas issuing out of the nozzle is called "momentum thrust." Imagine that a rocket engine builder constructs an engine with a burn rate of 10 pounds of fuel per second. Now further assume that the builder's rocket engine handbook tells him that his choice of rocket fuels will result in the gases leaving the rocket nozzle at a velocity of around 3,000 feet per second.
The thrust produced is equal to the propellant burn rate multiplied by the exhaust velocity. So the momentum thrust is:
Momentum Thrust = Propellant Burn Rate ´ Exhaust Gas Velocity
So, in the example above,
Thrust = (10 lbs/sec) ´ (3,000 ft/sec)/(32.2 ft/sec2)*
Momentum Thrust = 932 pounds of force
So far, so good. But momentum thrust is only part of the reason rockets go up. The other reason is pressure thrust.
Inside a rocket engine, there are unbalanced forces at work. The rocket engine has an open end (the nozzle where the gases come out) and a closed end. During the burn time, the combustion of rocket engine chemicals results in a pressure buildup inside the engine. But since one end is closed and one end is open through the exit nozzle, there is a net force pushing against the closed end.
Excerpted from Adventures from the Technology Underground by William Gurstelle. Copyright © 2007 by William Gurstelle. Excerpted by permission of Potter Style, a division of Random House, Inc. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.