Solid Boosters: How Far Have We Come? How Far Can We Go?

Oct. 1, 1961

Solid rockets have now attained a really solid position in our space planning.

A year ago, few people familiar with the US space effort would have bet much money that solid propellants would ever be used as “superboosters,” developing upward of fifteen million pounds of thrust. By last fall, in fact, the large solid-rocket program apparently was stuck for good on the treadmill of end­less study and research.

Since Sputnik I opened the space age, the nation’s solid-rocket experts had contended that multimil­lion-pound-thrust solid-propellant boosters would be cheaper, more reliable, and ready sooner than large liquid-fuel boosters. These contentions had fallen on deaf ears, however. Large solids remained in the “re­search” stage. No plan existed to put them into devel­opment.

Today, the situation has been completely reversed. The big solid rocket is going to get its chance. Devel­opment of these boosters has been made an Air Force responsibility, and about $60 million has been pro­vided to begin the job in earnest during this fiscal year. All signs point toward a maximum-effort devel­opment program that will be pushed as fast as tech­nology will allow.

Several factors contributed to this sudden change of fortune. Yuri Gagarin’s orbital flight in April—in which the Red flyer became history’s first spaceman—certainly was instrumental. It changed the entire course of the US space program by overcoming the go-slow philosophy of President Kennedy’s scientific advisers. Strong elements in the Air Force had pushed for development of large, solid-fuel boosters. But one factor undoubtedly contributed more than any other to the rise of solid rockets. This was the fight that solid-propellant manufacturers put up against the “research” status they had been given by the National Aeronautics and Space Administration, which was in charge of all booster development until last May.

The solid-propellant companies carried their argu­ments to the Congress and to the public in every man­ner available to them. Company officials made in­numerable speeches, appeared on radio and TV, and wrote or assisted with articles for mass-circulation magazines. They clearly regarded publicity and public education their main job in the fall of 1960 and early months of 1961. As a result, Congress probably is as thoroughly briefed on the arguments for solid-pro­pellant rockets as on any technical subject. Large numbers of congressmen and senators on both sides of the aisle are convinced that solid boosters offer a possible means of getting ahead of the Soviets in booster power.

In principle, no overwhelming technical opposition has been registered against large solids, even by the partisan supporters of liquid-fuel rockets. However, at budget time the paper proposals for solid-fuel boosters have always been pitted against established projects for large liquid-fuel systems such as the Saturn booster and the 1.5 million-pound-thrust F-1 engine. These have been in the works for several years. These projects soaked up all of the available booster-development money in the first three years of the space age when urgency, parallel programs, and technical insurance were not in vogue. Small sums of research money were then spread around to keep promising ideas going for possible development in the future.

The solid booster was clearly lodged in the research stage last fall. In the Eisenhower Administration’s FY 1962 budget, $3 million was allotted for NASA to study large, solid-fuel rockets. Liquid-fuel rocket programs on the other hand received more than $65 million.

Congressional testimony showed that most NASA officials and liquid-propellant rocket experts from in­dustry believed that time was against the large solid booster. The argument, in essence, was that the solid booster would eventually work but that it would take many years of research and development to bring it to a usable state. According to this theory, the solid booster would follow the liquid-fuel rocket booster into operational space use, just as the solid-fuel ICBM followed the liquid.

The change in Administrations last January brought no change in the solid-fuel booster situation. As Presi­dent Kennedy’s advisers reviewed the space program, NASA asked for an additional $5 million in solid-rocket research money, but this was refused and the appropri­ations request remained at $3 million.

Intense congressional pressure in the wake of the first Soviet orbital flight succeeded in reversing the picture. The solid-rocket booster now has the status of a parallel development to the liquid-fuel booster program. It is a backup for the ten-million-pound­thrust-plus, liquid-fuel Nova booster called for in the Kennedy Administration’s moon program. It is prob­ably also the main contender for the military space booster role.

The hard sell has brought the solid rocket equal status, so to speak, but it has also given wide pub­licity to a number of ambitious claims and predictions. If they can be realized as soon as the solid-rocket in­dustry predicts, then the US space program will receive an almost miraculous speedup.

These predictions include:

• Demonstration of the reliability of a 3.5 million-­pound-thrust booster, composed of a cluster of seven motors, within eighteen months, and its first flight test in two years.

• Flight test of a first-stage booster of twenty-one million pounds’ total thrust, within thirty-seven to thirty-nine months. With the program pushed to the maximum, the first flight, it is said, could take place within two years.

• Payload of the largest solid booster, when com­bined with the proper upper stages, to be two million pounds in a 300-mile orbit, or about 600,000 pounds to the moon.

The optimism displayed by all solid-rocket manu­facturers was based primarily on four facts prior to May of this year. They were:

• Almost perfect reliability. Thiokol Chemical Corp., as a typical example, could boast that their units had shown a 99.66 reliability in 875 Nike-Hercu­les flights and 99.98 in 5,000 Falcon firings.

• Scaling-up or increasing the size of solid rockets had not given any trouble in the past. The weight of propellant in a solid rocket is of more interest than the thrust it produces. The weight of propellant charges had been increased about nine times within seven years, from the neighborhood of 5,000 pounds to about 45,000 pounds. Solid-rocket engineers believed that the successful tests with 45,000-pound engines proved that it was possible to go directly to single motors holding more than 700,000 pounds of propellant.

• Most of the technical know-how needed to build large solid rockets had been made available in the Minuteman and Polaris programs. These two military rockets have done more than anything else to advance the solid-rocket state of the art. When the Minuteman and Polaris entered development, no one was abso­lutely certain that they could be built. Their perform­ance specifications called for propellant efficiencies and structural excellence that were not possible at the time. Together they have caused the creation of a new solid-rocket technology. They have made it possible for the solid-rocket superbooster almost to equal the over-all performance of a similar type liquid booster using kerosene and liquid-oxygen propellants. The performance advantage, but not the cost advantage, would go back to liquids if liquid hydrogen and liquid oxygen were used as first-stage propellants as now being considered.

There is one major developmental unknown. It was not solved in the Minuteman and Polaris programs. This is the mechanism for controlling the vehicle by moving the thrust vector. Many engineers believe that gimballing nozzles will not be efficient on large motors. They point to small-scale experiments that show a fluid injected into the nozzle will cause the flow to cant. Others doubt the effectiveness of this method. These are questions that probably can be answered early in a development program.

• Clustering of large solid rockets is believed to be feasible on the basis of experience with the X-17 re­search vehicle, the Nike-Hercules booster, and the Little Joe test vehicle used in the Mercury program.

Obviously the people in charge at NASA and all of the authorities in the rocket industry did not agree with these estimates of the state of the art in solid-rocket technology. Solid-rocket specialists contend that popularly held notions of the state of their technology usually has been a state of mind among people who are usually about two years behind the fact.

These arguments between solid- and liquid-rocket enthusiasts are not subsiding. The solid-booster de­velopment program being readied by the Air Force will have more than the usual number of Monday morning quarterbacks.

The first big series of tests that supposedly will set­tle some of the arguments began in May and extended over a four-month period. Four motors of record size were fired as part of the research programs financed by NASA and the Air Force over the past few years. They carried propellant charges weighing from 50,000 pounds to more than 125,000 pounds and produced up to 500,000 pounds of thrust. All of the tests were completely successful.

Coming so soon after the Kennedy Administration’s decision to develop solid-fuel “superboosters,” these firings have provided a great reinforcement to the optimism of solid-rocket proponents.

The four test motors are shown in the scale draw­ings on page 34. The United Technology P-1 motor was constructed under NASA contract and Aerojet-­General’s SS-B, TW-1, and FW-1 motors were financed by the Air Force. For comparison, the Minuteman first stage is shown on the left of the drawing. This motor designed and manufactured by Thiokol is the largest solid rocket flight-tested to date.

Hopefully, the Aerojet FW-2 and the United Tech­nology P-2 motors will be fired before the end of the year. If these units are successful, it is possible that no more “subscale” motors will be tested, and the solid-rocket program will move immediately into the con­struction of motors similar to the ones shown on the right of the illustration.

In addition to improving the position of solid-pro­pellant boosters in general, the four motors tested this summer apparently settled a basic argument among solid-rocket engineers. They proved that segmented motors were practical.

All four of the test motors used segmented construc­tion. The motor cases were manufactured in segments, the segments were filled with propellant separately, and finally they were joined together just before firing at the test site. (See page 35 for picture of engine segments.)

In the opinion of many experts, segmented manu­facture is the key to low-cost construction and opera­tion of very large solid rockets. This design would allow relatively small segments of big engines to be prepared at any of a large number of existing facili­ties. The segments could be transported by rail to launch sites where they could be inspected with ex­isting equipment and then assembled into large boosters.

Some respected solid-propellant engineers have strongly disagreed with the segmented concept. They believed that the joints between the segments could never be made completely leakproof. If hot-gas leaks developed, the motor in all probability would fail. Therefore, it was theorized that the segmented motor would not have the high reliability of the one-piece or monolithic type in service today.

If the large motors weighing more than one million pounds and more than 100 feet long had to be built in one piece, then they would have some unique con­struction and transportation problems. Proposals were made either to load propellant into the case at the launch pad or to float completed engines from the manufacturing plants to the launch complex. This technique would require new types of cranes to hoist the motors, new furnaces to heat-treat the cases, and new inspection equipment to make sure that the pro­pellant charge had no cracks.

Both the segmented and monolithic designs have been studied by Air Force and NASA contractors. Each approach had its industry and government ad­herents.

Many planners at NASA and the Air Force want to limit the diameter of segmented motors to 160 inches even though diameters of more than twenty feet are possible. If the diameter stays below 160 inches, the motor segments can be transported on the railroads, greatly easing the logistical problems.

The diameter question and a host of others are now being settled by a subcommittee of a DoD-NASA long-range planning group known around Washington as the Gollovin Committee. These experts began meeting early in August and were asked to develop within ninety days a definite set of specifications for large solid boosters agreeable to both NASA and DoD.

The board, it can be seen, is playing a key role in the development of a national launch-vehicle program. Current ground rules call for civil and military agree­ment on the design of every booster that enters devel­opment.

This agreement is rather difficult to reach at this stage of the game. As one Air Force expert put it, “Our requirements only call for putting up footballs, and NASA has a mandate to orbit the whole stadium.” It is hoped that a solid-rocket booster that can have a varying number of motors in its cluster and a varying number of segments in its motors will be adaptable over this very wide range of payloads.

One objective is certain. The maximum thrust of the solid-rocket booster to be developed by the Air Force will be somewhere between twenty and thirty million pounds. This is to allow the booster to be used as the base element in a moon rocket if NASA so desires after it sees the finished product. The payoff for the Air Force is that such a booster system would allow military space operations to step quickly out of the “football” class.

As one keeps time on the solid-booster program and the solid-rocket manufacturers to see if they can de­liver all they have claimed, the “management” factor should not be neglected. Officially, the Air Force was given responsibility over the program last May. The subcommittee didn’t meet until early August. It will not have an acceptable specification prepared until the beginning of November. After that, three months at the very minimum will be needed for USAF to ask for bids from industry, evaluate the bids, and select a contractor. So if the development contracts are let next February, there are no management delays, and the contractors are allowed to move as fast as they can, the first large solid-rocket booster should begin flight test early in 1964.—End