The Sciences

Rocket on a Round Trip

Blasting off is easy--any rocket can do that. Climbing to Orbit in a single stage is harder. Coming back and landing on your feet is harder still. Can the Delta Clipper do all that? Nasa hopes so.

By Jeffrey KlugerMay 1, 1995 5:00 AM


Sign up for our email newsletter for the latest science news

Pete Conrad was a happy guy on the surface of the moon. Even 25 years later, that’s the best way he can think of to describe it. Standing in the soil of the Ocean of Storms, toeing the lunar dust that only two men had trod before, Conrad, the commander of Apollo 12, felt neither the weight of history nor the exhilaration of exploration nor a profound appreciation for the sweep of human experience. He just felt . . . happy. People always ask how you can enjoy yourself on the moon knowing that if the lunar module doesn’t fire the next day, you’re going to be marooned there forever, he says. Hell, I quit worrying about that sort of thing the first time I went into Earth orbit.

Ever since Conrad returned from his fourth and last trip into space in 1973, though, his professional equanimity had not been put much to the test--not until last June 27, anyway. On that day, Conrad found himself in a familiar place--the White Sands missile range in New Mexico--having a familiar experience: waiting out the final seconds before a fueled and flight-ready rocket blasted up from its pad and headed for the sky. In a modest trailer almost lost in the bleached stretch of desert, Conrad and six other men crowded around a bank of video screens to monitor the launch. The rocket, a 42-foot-tall cone-shaped affair known as the DC-X, for Delta Clipper Experimental, smoldered more than three miles away--close enough to be easily tracked as it moved through its flight; far enough to present no danger if that flight turned into a fall.

As the countdown reached zero, the technicians to Conrad’s right turned to the video monitors and watched the rocket rise from the ground. Conrad himself looked elsewhere, fixing his attention on a screen full of less-dramatic plots and readouts that would reveal whether the flight was adhering to its planned profile. And for several seconds after liftoff, the DC-X did stick to the plan. Then engineer Jim French noticed something alarming. Squinting at the dwindling image of the missile on the monitor, he saw scorched bits of aeroshell--the carbon-fiber rind that gave the rocket its aerodynamic shape--flaking away and falling to Earth. On Conrad’s screen, the trajectory continued to read true.

We were fortunate that one of the cameras happened to be looking at the correct side of the vehicle, Conrad recalls. French said there were pieces coming off it, and the deputy flight director, Tom Ingersoll, took a look. I was looking at the gauges. Ingersoll said, ‘Autoland!’ -- that’s the emergency procedure that would bring the rocket promptly back to the ground--and I said, ‘Why? The vehicle is perfectly on track.’ Then I looked up and saw the hole in its side.

Conrad turned back to his console and keyed in the autoland command. For a second nothing happened--and then the rocket did a remarkable thing. Climbing at about 70 miles per hour, it slowed a bit, shuddered a bit, and then, quite simply, stopped. Poised atop its tail of clear flame, the DC-X hovered eerily for a few seconds, establishing its footing in a place where there was none, and slowly began descending toward Earth. For the better part of a minute the rocket backed itself down. At last it settled into the desert gypsum, standing upright on four insectile legs.

Conrad, who had been watching French’s screen, now looked back at his own. His rocket, the numbers told him, had reached an altitude of barely 2,800 feet and had landed just 800 feet from the spot where it took off. The entire flight had lasted 78 seconds.

The last thing we thought is that we would damage the outside of the vehicle, Conrad says. But the best thing about it is that all the emergency procedures worked as advertised. I was accused of laughing after the autoland was executed, and I know I was giving the guys in the trailer my normal giggles when it was going well. A lot of people had said to us, ‘If you land in that gypsum, you’re going to be in deep yogurt.’ We were supposed to land back on the flight stand, but the autoland program causes the rocket to stop whatever it’s doing and land wherever it is. It was very quiet for a while afterward because there was this huge cloud of gypsum. Then it cleared and I saw the ship.

The missile whose flight ended so abruptly that day was the latest engineering expression of one of the aeronautics community’s most elusive machines: the wholly reusable one-part space vehicle. Known popularly as the single-stage-to-orbit spacecraft--or, in the acronymic argot of the engineer, the SSTO--the ship is based on a single hard-to- quarrel-with premise: a spacecraft ought to be able to put itself into orbit and bring itself back to Earth without shedding parts along the way.

For decades that simple dream has dogged engineers. Wernher von Braun’s groundbreaking V-2 may be considered an early prototype--but it could barely make it to Leicester Square, let alone low Earth orbit. The space shuttle was originally envisioned as a modern-day SSTO, until budget constraints and technical obstacles ruled out that possibility. Lacking the technical and financial wherewithal to build a single self-contained engine powerful enough to carry the shuttle all the way to space, engineers instead grafted a whalelike external tank to the underside of the ship and, when that proved insufficient, twin solid-rocket boosters to its sides.

Now the men in Pete Conrad’s trailer, along with their employers at McDonnell Douglas Aerospace in Huntington Beach, California, may be doing what Von Braun and all the rest could not. The wounded ship they flew in New Mexico is a one-third-scale demonstration of a craft that may someday have what it takes to earn the SSTO distinction. So far their craft has flown five times, never climbing higher than a mile or staying aloft longer than two minutes, but each time doing more than any aspiring SSTO had done before. All at once, it appears, NASA may be taking notice. Pressed to find a next-century alternative to its overly expensive shuttle fleet, the agency has been casting about for a new workhorse launcher. The Delta Clipper could well be it. Says Paul Klevatt, the DC-X program manager at McDonnell Douglas: Given the success we’ve had so far and the plans we have to improve our prototype further, we’re well on our way.

While the shuttle was being built by Rockwell International, McDonnell Douglas engineers were busy gathering experience that prepared them well for designing an SSTO. In the past few decades, the aerospace giant earned itself a formidable reputation as the manufacturer of the stalwart Delta rocket, which put more than 200 satellites into space. It also worked on rapid prototyping, a program to explore ways of building complex new rockets in a hurry, for the Ballistic Missile Defense Organization (BMDO)--né the Strategic Defense Initiative, also known as Star Wars. When Star Wars officials began looking for an affordable way to launch their hoped-for fleet of antimissile satellites several years ago, they turned to the idea of a wholly reusable one-stage booster and chose McDonnell Douglas to help them build it.

McDonnell christened the as-yet-unbuilt ship the Delta Clipper-- an attempt to wed the solid reputation of its own missile with that of the clipper ships of the nineteenth century. The engineers--including Conrad, who’d been a company vice president for the better part of two decades, and Klevatt, who led the group--started designing the rocket in August 1991. They knew from the outset how demanding the job of building an SSTO would be. The lowest orbit a satellite can attain and not be pulled back to Earth by the drag of the upper atmosphere is about 100 miles, but a useful orbit can be as high as 24,000 miles. To orbit a planet with the mass of Earth, a payload has to travel at least 17,500 miles per hour. Any slower, and once again it falls back home. Flinging a spacecraft that high and fast takes a whopping load of rocket power, which requires a whopping load of hardware and fuel--and that adds weight.

As every rocket scientist from Robert Goddard on down has discovered, it does you no good at all to build a booster that can generate even millions of pounds of thrust if the overall weight of the ship is greater still. The multistage booster was conceived to solve this problem. By stacking rocket stages on top of one another and giving each one its own fuel supply and engine, and by discarding each stage as its fuel is exhausted, you can make your booster lighter and lighter as you push your payload higher and faster. That gets the payload into orbit--but at a breathtaking cost in machinery.

The alternative is to build a ship so light and efficient that no matter how huge a load of fuel you had to pour into its tanks, it could still make it into space in a single jump. But how? When the McDonnell Douglas engineers toted things up, they saw that the fuel load of a single- stage-to-orbit booster would be so great that the weight of the vehicle itself would have to be pared down to almost nothing at all. Payload, tanks, and rocket hardware would constitute only 10 percent of the ship’s weight; fuel would make up the other 90 percent. Those are the proportions- -the mass fraction, as rocket engineers say--of an egg, which is 90 percent yolk and white and only 10 percent hard-shell packaging, and building an Earth-to-orbit-and-back egg isn’t easy. It’s really the materials that have been available in the past that have prevented SSTOs from being built, says Bill Gaubatz of McDonnell. You couldn’t make them light and rugged enough to get into orbit in the first place, never mind being reusable.

Gaubatz and his colleagues still haven’t solved the materials problem, but they quickly reached a consensus on another fundamental question--what shape their rocket would have. Nothing, they decided, beats the simple cone. Cones are the most effective of all shapes--and far better than winged ships--in dissipating the blistering heat of reentry. They require the least thermal shielding and thus the least carry-along weight. Whereas the overall size of the ship and the angle of its reentry would determine just how low and stout or tall and slim its final body dimensions would be, the conical shape was set almost from the start.

A related question involved the ship’s flight profile: should it take off and land vertically, like a rocket, or horizontally, like an airplane? In theory, single-stage-to-orbit vehicles can fly in either of these orientations. The early plans for the space shuttle called for a horizontal takeoff and horizontal landing like a conventional airplane, but the added fuel tank and boosters made the shuttle too heavy and awkward to take off horizontally. For the Delta Clipper, the McDonnell team decided to scrap horizontal flying altogether and build the ship so it would both leave the Earth and return to it standing bolt upright. At the end of a mission, small thrusters on the bottom would slow the spacecraft down, allowing it to drop out of orbit and plunge into the atmosphere. At an altitude of 10,000 feet, the steadily thickening air would begin to slow the ship from its 17,500-mile-per-hour orbital speed to barely 250 miles per hour, at which point the main engines would relight, allowing the ship to lower itself gently to the ground.

This flight profile would give a conical rocket additional advantages over a winged vehicle besides the weight saved on heat shielding. One of the McDonnell team’s biggest considerations was building a ship that was both simple to fly and cheap to operate, and nothing works against those two goals more than trying to take off and land like an airplane. Airplanes require runways, and runways require land, equipment, and dozens or even hundreds of support technicians. What’s more, winged vehicles on the whole are more vulnerable to variables like inclement weather and high winds than unwinged vehicles. When you take off and land vertically, you can fly under many more conditions, and you can lift off and touch down in many more places.

Of course, nobody had ever built a rocket that could land vertically on the Earth. The DC-X’s engines would have to be powerful enough to blast it into orbit but capable of being throttled down to 28 percent of full thrust to bring the ship in for a soft landing on its feet. The Apollo lunar module had done that on the moon, but soft landings are a lot easier in the moon’s weak gravity.

The moon’s lack of atmosphere also made the job simpler. Unlike the lunar module, the DC-X would have to withstand a veritable whirlwind of air currents and pressure waves threatening to topple the rocket or throw it out of control. In the face of such turbulence, rocket engines designed to swivel would have to work in concert with an onboard gyroscope to shift the position of the ship, keeping it stable like a ball balanced on a seal’s nose.

For two years, Klevatt, Conrad, Gaubatz, and their team worked on their new rocket, and when they at last rolled it out to the pad in August 1993, they indeed appeared to have come up with something remarkable. Milled to its promised conical shape, the ship measured 42 feet in height and just under 13 feet wide at the base. Resting on a simple five-foot launch stand, it had four legs that would retract into the body during liftoff and extend just before landing, easily supporting the craft’s relatively light 21-ton weight. Four oxygen-hydrogen engines, firing for close to two minutes, would propel the ship off the ground and bring it slowly back down again. The ship was not intended to fly into space; a real working model, able to place a crew or a satellite into orbit, would have to be about three times the size and have many times the rocket power. But this mini-ship looked like a good first step.

Although the McDonnell engineers had built most of the ship that was unveiled that day in-house, they had farmed out the aeroshell to aeronautical engineer Burt Rutan. He had designed and built the ultralight Voyager airplane, which his brother, Dick Rutan, and Jeana Yeager had flown on the first nonstop circumnavigation of the globe. That flight was made possible by the plane’s hollow, honeycombed fuselage and wings, which weighed next to nothing and maximized space for storing fuel. Rutan brought the same engineering frugality to bear on the DC-X, designing a carbon- fiber aeroshell that was no thicker than a credit card and added only 3,120 pounds to the gross weight of the ship.

For the first test, the McDonnell and BMDO engineers trundled their creation out to the New Mexico desert, hunkered in their trailer, and, with the aid of a launch computer, lit, so to speak, the craft’s fuse. The result--photographed, filmed, and made available to newspapers, magazines, and TV stations around the country--was impressive. The booster lifted itself slowly off its pad, rose to a height of 300 feet, and hovered for several seconds. Then, with the help of its swiveling engines, it seemed to slide 350 feet to its right, hovered once again, and slowly descended to the ground. The entire flight lasted 66 seconds.

It’s real hard to describe what we felt after that first launch, Klevatt says. As soon as we caught our breath, there was a great sense of accomplishment that swept over the whole crew. We felt like a small corner had finally been turned.

Over the coming months, Klevatt had reason to grow even more optimistic: the DC-X made three more flights, its altitude inching steadily up toward half a mile and its flight time filling the better part of two minutes.

As with so much of the Star Wars program, however, even such modest success did not come cheap. By early 1994 the Pentagon had spent a total of $58 million. Meanwhile the Republican administrations that had heavily sponsored the Star Wars program had left office, and the Soviet empire that inspired them had long since disintegrated. In late January of that year, in spite of the Delta Clipper’s promising start, the Pentagon put the rocket on indefinite hold.

But the hold turned out to be brief. Daniel Goldin, NASA’s administrator, had been keeping an eye on the developments at the BMDO. One of his main priorities was to find a twenty-first-century replacement for the agency’s quartet of space shuttles, which are very costly to launch. The Delta Clipper, he decided, might just fit the bill. Whereas a shuttle flight costs taxpayers roughly $400 million according to NASA, a skeleton crew should be able to launch the Clipper for a fifth of that cost. Shortly after the ax fell at the Department of Defense, Goldin and NASA stepped in with an offer to put up $17.6 million to keep the Delta Clipper in business. McDonnell Douglas agreed to pitch in $7.6 million of its own. Then came last June’s test launch.

After that near disaster, the DC-X retreated to dry dock, occupying a tiny corner of the same McDonnell Douglas hangar once used to assemble the mammoth Saturn V’s second stage. Engineers studied the videotapes and telemetry from the aborted flight and discovered, happily, that the damage to the ship’s aeroshell was due not to the ship itself but rather to the launchpad. At launch, a series of ducts and fans built into the concrete pad are supposed to vent gaseous oxygen, hydrogen, and water safely away from the vehicle. In June the system malfunctioned and an explosive brew collected at the base of the ship. The main engines then detonated the gases, sending a pressure wave up one side of the vehicle. That wave collapsed part of the aeroshell.

While they repaired the blast damage and checked out the pad fans, engineers were also trying to figure out how to improve the DC-X with new components--converting it into the DC-XA. Their biggest concern remains weight. Built with an eye less toward actual orbital flight than toward short, puddle-jumping test hops, the DC-X is something of an aeronautical anvil, with a fuel-to-rocket ratio of 50-50 rather than 90-10. To get their vehicle into space, the McDonnell engineers are going to have to rethink some of the materials they used to build their experimental rocket.

The fuel tanks, Gaubatz says, are the likeliest spot to start this retooling. Sitting near the top of the rocket, an oxygen tank wrapped with a fiberglass blanket for insulation holds 16,000 pounds of liquid oxygen at -297 degrees Fahrenheit. Below the oxygen tank are two smaller tanks filled with helium, an inert gas used to pressurize the ship’s fuel tanks and push the oxygen and hydrogen into the combustion chamber. The highly explosive hydrogen fuel is stored in an 8-by-16-foot tank at the bottom. Lined with common balsa wood--an unglamorous but cheap and light insulation--the vessel keeps 3,500 pounds of hydrogen at -423 degrees Fahrenheit.

Currently the tanks themselves are made largely of aluminum--a remarkably light material if you’re building, say, a motorboat, but a remarkably heavy one if you’re building an SSTO. Manufactured by the Chicago Bridge and Iron Company, the cylindrical vessels add as much ballast to the ship as the name of their manufacturer implies. For the DC- XA, McDonnell Douglas plans to replace the original oxygen tanks with new ones made of a lightweight alloy of lithium and aluminum. They also want to replace other aluminum parts, such as the hydrogen tanks and the struts and ribbing between the tanks that make up the internal architecture of the spacecraft, with parts made of carbon fiber--a material that has never been used in a tank that holds cryogenic fuel under the extreme stresses and vibrations of a rocket takeoff. Putting these new components into the DC- XA gives us a real good way of testing all of them at once, says Klevatt.

The DC-XA doesn’t exist yet, and even if it did it would be a long way from a full-scale rocket that could make it to orbit. NASA is thus hedging its bets: while supporting the Delta Clipper program, it is entertaining other proposals from other manufacturers who believe they can beat McDonnell Douglas into space. A winged, airplane-like SSTO is on the drawing board at Rockwell International, and a so-called lifting body--an odd, wedge-shaped vehicle that looks like a piece of pie with its corners rounded--is taking shape at Lockheed. Both vehicles are being designed to take off vertically and land horizontally. Sometime in the middle of 1996, NASA will pick--or down-select, as the space agency puts it--what it considers the most promising design for a full-size SSTO. So far, though, McDonnell has a big jump on the competition: it at least has a model that flies. And by the time the down-select takes place, Klevatt, Gaubatz, and Conrad hope to have completed their DC-XA and flown it to an altitude of 8,000 feet and a velocity of about Mach .4--nowhere near orbit, but a far piece better than what the DC-X is capable of.

If the Clipper is indeed anointed by NASA, the McDonnell Douglas team will have a demanding schedule to meet: the agency wants the winner to start building a full-size, fully functioning ship by the year 2000. The engineers would have to tackle some tough problems, not least of which is a redesign of perhaps the most fundamental part of the spacecraft: the engine.

The oxygen and hydrogen that produce the controlled explosion that makes the missile fly are among the most powerful rocket fuels known, but they alone may not be enough to carry a fully functioning SSTO, complete with astronauts and life support, into orbit. Once again it’s the nonnegotiable physics of rocket flight that presents the problem. Along with the 90-10 mass fraction that’s necessary to make an SSTO fly, Klevatt says, we have to concern ourselves with another important ratio: 70 to 1. This number, Klevatt explains, is the ratio of the thrust an engine produces to its own unfueled weight (not that of the whole rocket). If the combustion system isn’t capable of producing an explosive oomph at least 70 times greater than the weight of the engine itself, the Delta Clipper won’t make it into space and back. At the moment, the DC-X engine operates at a popgun ratio of just 42 to 1.

To improve this performance, the Delta Clipper engineers will try a number of engine configurations and fuel recipes. Although hydrogen and oxygen make a very reliable fuel, they have a drawback: they take up a lot of volume, even when compressed into a frigid liquid state. Bigger volume, of course, means bigger tanks feeding the engine, and bigger tanks mean more weight for the entire engine system. One possible solution is to replace hydrogen with a far more compact, far less elegant fuel: kerosene. Since kerosene is denser than hydrogen, engineers can pack more by weight into smaller, lighter tanks.

But kerosene has its own drawback: pound for pound it delivers less thrust than hydrogen. With kerosene, you save weight on tanks but have to add weight on fuel to get whatever thrust you need. McDonnell engineers believe some type of hybrid engine that combines the advantages of kerosene and hydrogen might do the job. Such an engine would burn the heavier but more compact kerosene for the early part of the flight, thus shedding weight faster, and save the more powerful but voluminous hydrogen for later. The trick in designing such a hybrid rocket will be to strike just the right balance of fuels while somehow contriving to feed both fuels, along with the flame-sustaining oxygen, successively to one set of engines.

Another component of the DC-X that will have to be replaced in a space-ready Clipper is Rutan’s carbon-fiber aeroshell. Ingenious as it is, it was never intended as more than a stopgap--it let the McDonnell engineers build a model that could fly to a few thousand feet and thereby test all the other pieces of their dream. Carbon fiber may work for the fuel tanks of an orbiting Clipper, but it would not do as the skin because it would never survive the inferno of atmospheric reentry. The most likely material for the aeroshell is some version of the flame-resistant ceramic that is now used by the space shuttle. But no one has yet made a ceramic light enough to satisfy the uncompromising weight demands of an SSTO.

The success of the venture, then, is far from assured--as the test last June demonstrated rather dramatically. The better part of a decade will pass before the McDonnell Douglas engineers, or any other team, know whether they can solve the problems inherent in building a round-trip rocket. For now, Conrad and his colleagues are concentrating on getting the model they have back on the pad for a few more test flights. The excitement of designing an entirely new type of spacecraft helps keep them going. Getting into space and back--let alone going to the moon--has always been an enterprise that required months of work by a small standing army. The DC-X is a first step toward changing all that. Normally it takes only three people to fly it, says Conrad. But if the other guys were ever late, I’d just tell them not to worry--I’d do it myself.

1 free article left
Want More? Get unlimited access for as low as $1.99/month

Already a subscriber?

Register or Log In

1 free articleSubscribe
Discover Magazine Logo
Want more?

Keep reading for as low as $1.99!


Already a subscriber?

Register or Log In

More From Discover
Recommendations From Our Store
Shop Now
Stay Curious
Our List

Sign up for our weekly science updates.

To The Magazine

Save up to 70% off the cover price when you subscribe to Discover magazine.

Copyright © 2023 Kalmbach Media Co.