Space: The Late-Starting Rocket

For years, the loudest noises in the aerospace business have been the rumble of liquid-fueled boosters blasting spacecraft into orbit, the sharper roar of solid-fuel military missiles climbing into their long trajectories, and the continuing, wordy battles between the promoters of each type. Now, back of the racket, can be heard the insistent voice of still another competitor in the rocketry race—the hybrid that manufactures its power by combining liquid oxidant with solid fuel. Detractors may scoff that the hybrid combines all the dangers and difficulties of both solids and liquids. Its champions are confident that it is better than either. “Within about one year,” says Douglas Ordahl, director of hybrid development at United Technology Center of United Aircraft Corp., “a hybrid will perform in orbital flight and prove that it can do all the things we claim for it.”

Fierce Fluid. A typical hybrid is a comparatively simple product of a complex science. It is basically a pressurized tank for liquid oxidizer, behind which sits a combustion chamber lined with solid fuel. The two propellants are usually hypergolic—they ignite spontaneously when they come in contact with each other. A valve permits a jet of oxidizer to squirt into the rear chamber, combustion begins, and a hot, high-temperature flame roars out of the nozzle. The oxidizer valve can be used as a throttle to reduce the thrust and the engine can be stopped and started any number of times, a maneuver that is difficult or impossible with almost all other types of rockets.

In all-solid rockets the oxidizer and fuel must be mixed together intimately, a requirement that rules out many high-energy combinations that would start reacting as soon as they came in contact. Hybrids have no such difficulty. Their fuel can be hopped up with a high proportion of high-energy powdered metals or metallic hydrides, but in the absence of an oxidizer, they remain as inert as an auto tire. The oxidizer may be a fiercely reactive fluid such as OF2 (oxygen difluoride), but since it stays in its own tank until needed, it behaves itself. When the two components come together, they burn with extremely high specific impulse, the measure of rocket power. Best of all, say their boosters, hybrids are free of the complicated plumbing demanded by the liquids, and their inert fuel cannot cause an explosion. A solid-fuel rocket, on the other hand, has a “grain” (propellant mixture) that burns all over its surface. If the grain is cracked, the sides of the crack may start burning too; pressure may increase until the rocket explodes. The solid propellant can be riddled with cracks or even stacked up like bricks without increasing the burning rate.

Beautiful Flame. With so many advantages, what can be wrong with hybrids? A great deal, say their critics. Worst of all is low-combustion efficiency; the fuel and oxidizer tend to escape through the exhaust nozzle before they have reacted. They burn as a long, beautiful flame, which wastes much of its energy on empty air. When the hybrid is throttled down by partially closing the oxidizer valve, its efficiency gets worse, sometimes falling as low as 20% .

But all such objections are out of date, insists U.T.C.’s Ordahl. He admits that early hybrids spat unburned propellants out their nozzles, but he claims that the habit has been cured by baffles that keep the propellants from escaping before they have mixed and burned. Baffles and other improvements, says Ordahl, have boosted combustion efficiency to 90% in some U.T.C. hybrids. Aerojet-General Corp. has a different kind of baffle that is said to get 95% . The Reaction Motors Division of Thiokol Chemical Corp. believes that good burning under variable conditions can best be had by injecting extra oxidizer near the rear of the combustion chamber or downstream from the baffles.

Looking into the future, the hybrid men see big hybrid space boosters using extremely cheap fuels: liquid oxygen and ordinary rubber. For extra performance in upper-stage rockets, they have high hopes for hybrids using fuel that is almost entirely powdered metal.

If a little hydrogen is burned along with the metal and its oxidizer, the hybrid will become a “tribrid.” Its specific impulse will rise into the range of the yet to be built nuclear rockets. But there will be nothing like the “nukes’ ” penalties in cost and danger.