(U.S. Air Force)

That's Why They Call It Rocket Science

Edward Ruth

Why is it so hard to launch a rocket into space with absolute assurance of success?

We've all seen the images. The silver and white rocket is thrusting majestically across the sky. It seems the very embodiment of power and technology and the fulfillment of an age-long dream of reaching the heavens. Suddenly, something goes terribly wrong. The rocket seems to tear itself apart and in an instant is transformed from an icon of humanity's great achievement to a scattering of high-tech trash raining down from the sky in flaming bits.

In the early days of spaceflight, this scenario was all too familiar, as one launch attempt after another ended in failure. Today, after nearly 45 years of development, launch systems have grown reliable enough to permit civilians and other noncareer astronauts to fly in space on a somewhat routine basis. The experience and expertise of The Aerospace Corporation has helped the U.S. Air Force attain some of the highest launch success rates in the world. Still, even with these achievements, each launch requires a dedicated team of experts working many hours to achieve success—and even then, too many launches come to a tragic end.

Why don't modern launch systems have the same reliability as other comparable technologies? Is there something about the fundamental physics behind launch vehicles that makes them inherently challenging to operate? What's so hard about launch systems that the expression "it's rocket science" is still used to signify a difficult activity?

The Rocket Equation

To answer these questions, we need to look at the engineering of launch vehicles, beginning with the so-called "rocket equation," which shows what parameters affect launch vehicle design and what impact they have on launch system reliability.

The rocket equation states that the velocity imparted to a payload by a given rocket stage can be found by:

∆v = gI_spln(M).

Here, ∆v is the change in velocity, and g is the gravitational constant (i.e., the groundward acceleration caused by Earth's gravity, 9.8 meters per second per second). The quantity M, the mass ratio, is the ratio of the mass of a fully fueled rocket stage (including any upper stages and payload) to its mass without fuel. The specific impulse, I_sp, is a measure of engine performance.

Failed launch of a Titan IV carrying a National Reconnaissance Office payload on August 12, 1998. The vehicle was lost at an altitude of 5330 meters, approximately 39 seconds into flight. Electrical transients in the power supply caused the guidance computer to reset, thereby losing attitude reference. Upon power recovery, the guidance system, responding to erroneous attitude information, commanded a maneuver that exceeded the vehicle's structural limits. (U.S. Air Force)

To get better performance from a rocket stage, we must increase either the specific impulse or the mass ratio. Today, the best engines can achieve a specific impulse on the order of 450 seconds, and this is probably about the best we are ever going to see from a chemical rocket. Specific impulse is mainly determined by the chemistry of the propellants, and it seems likely that after decades of experimenting, we are already using the most energetic propellant combinations that are technically useful.

This limitation on specific impulse leaves mass ratio as the only parameter that can be modified to improve performance. Mass ratio is simply a function of how much propellant can be loaded into the lightest possible structure—a property known as structural efficiency. The more efficient the launch vehicle is, the lighter it will be without fuel. A modern rocket stage will be 10 to 20 times more massive when it is fully fueled then when it is empty.

Balancing Mass and Safety

The implications of the rocket equation can be summed up in this one simple fact: A high-performance rocket must be extremely light when empty. Unlike designers of highway bridges, nuclear reactors, or other large terrestrial structures, the rocket designer does not have the luxury of using more material than is needed to satisfy the minimum design requirements. Every item on the stage—engines, electronics, thermal protection system, etc.—must be as light as possible. Otherwise, the gross liftoff mass will become impractically large. This means that every part must be just strong enough to withstand the stress of flight and no more. Therefore, every part of a rocket is so close to its breaking point that any imperfection or flaw can lead to catastrophe.

This need for minimum design margins is a particular concern for the propulsion systems, which generate extremes of pressure, temperature, shock, and vibration. Rocket engines represent a significant fraction of the overall mass of the empty rocket stage, so they must achieve a high thrust-to-weight ratio to lift themselves—and the rest of the rocket and payload—into orbit. Given their high-energy output and low weight, it's not surprising that the propulsion systems are to blame in the majority of launch vehicle failures (see sidebar, Driving Home the Point).

Complex Physics

The designer's job is further complicated by the specialized physics underlying the load-generating phenomena on the vehicle. The physics of launch vehicles involve such disciplines as aerodynamics, heat transfer, and combustion chemistry. These are difficult branches of engineering, and solutions to problems in these areas are often known only approximately, and then obtained only at great expense.

A good example of this complexity can be seen in determining the transient aerodynamic loads on the launch vehicle caused by a transonic flow over its exterior. The transonic flow regime—which is reached as the vehicle's velocity approaches and overtakes the speed of sound—is particularly difficult to model. There are no closed-form mathematical solutions to the complex equations governing the physics of transonic motion. Solutions can only be obtained using computationally intensive techniques that must be verified by testing in large and costly wind tunnels. Often, true measurements are only obtained in flight, and sometimes only after many flights.

This Delta II rocket failed during a January 1997 launch of a GPS IIR satellite. Approximately seven seconds after ignition, one of the solid rocket motors developed a long split in its casing. The motor exploded about five seconds later. The casing failure prompted the first-stage automatic destruct system. The second stage, third stage, and payload separated, but remained largely intact. Flight controllers then sent destruct commands to control the disintegration of the vehicle. These commands destroyed the second and third stages, which in turn released the payload fairing with the payload largely intact. The payload and fairing exploded on impact with the ground. Fortunately, there were no injuries. (Jim Gazur)

Despite these challenges, the designer must ensure that an expendable launch vehicle will work right the first (and only) time it is used. Yet, problems with a launch vehicle might only become apparent when its components—including the software—are operated together as a complete system. Frequently, important system-design characteristics such as the aerodynamic loads or heat-transfer rates can only be determined with sufficient fidelity in flight. Simply put, the only way to thoroughly test an expendable launch vehicle is to expend it in a launch.

The Limits of Reliability

Putting it all together, we can summarize the designer's primary difficulties: Expendable rockets have minimal design margins, are governed by complex physics, and cannot be completely tested before flight. The launch vehicle designer must achieve as lightweight a structure as possible with only approximate knowledge of the loads it will encounter in its first and only flight. That modern rockets have achieved their current level of reliability is a testament to the dedication and skill of the teams that design, build, and launch them.

Simple graph plotting an increase in wall thickness of a rocket propellant tank versus an increase in the launch vehicle's gross liftoff mass. As wall thickness increases, the margin of safety goes up—but so does the mass. The curve is exponential, so a small improvement in margin of safety causes a large increase in liftoff mass.

Have we reached the limit of launch vehicle reliability? We certainly have not. For one thing, there is a growing understanding that it is okay to sacrifice some performance in exchange for increased robustness. If we can accept an increase in gross liftoff mass while still getting the same payload into the desired orbit, then we can use significantly more mass to enhance structural durability.

There are, of course, practical limits to how much performance can be sacrificed while maintaining a launch system that is both affordable and operable. An increase in mass implies an increase in manufacturing cost. Historical expendable launch vehicle data suggest that hardware costs range from 50 to 75 percent of total launch costs, and all launch vehicle cost-estimating tools assume that mass and unit hardware costs are positively correlated. Therefore, limiting mass (beyond that which is necessary to provide acceptable margins) makes the system more affordable. Larger vehicles are more difficult to erect and launch, requiring larger cranes, bigger trailers, and more extensive hardware in general.

Recall also that the rocket equation is exponential. Thus, a small increase in safety margins leads to a large increase in gross liftoff mass. In fact, if safety margins are increased too much, liftoff mass approaches infinity! At a certain point, an increase in reliability simply becomes impractical.

What's Next

As we move into the age of reusable launch vehicles, we move away from the paradigm of launch vehicles as ammunition—hardware used once, then discarded. A reusable vehicle can be flown repeatedly and, possibly, recovered from an aborted launch intact so flaws can be detected and corrected before the next flight.

Reusable launch systems may be able to take advantage of air-breathing propulsion, in which a rocket uses air from the atmosphere as oxidizer, rather than carry an oxidizer onboard. This scenario could provide a vastly improved specific impulse and thus a reduced emphasis on high mass ratios, with their attendant decreased margins.

Reusable systems may finally allow us to achieve launch-system reliability on a par with that of commercial aircraft. Until then, though, our launches really will remain rocket science.

To Winter 2004 Table of Contents