The exact timetable for US manned spaceflight during the next decade is subject to so many political and technical uncertainties that it is difficult to predict more than a year in advance. However, the vehicles which will be used and the objectives of the program are pretty well understood today in the government and by industrial groups who will carry it out.
There are nine major objectives in the program, one of which has been accomplished. The objectives are:
— Suborbital shots.
— Orbit.
— Landing on a prechosen spot on the earth.
— Maneuvering in space.
— Rendezvous of two space vehicles in orbit.
— Development of highly reliable launch techniques.
— Flights to increased altitudes—along orbits that get closer and closer to the moon.
— Moon orbit flights.
— Moon landing.
The encouraging point about these objectives is that the United States apparently will be able to accomplish the first six of them within the next two or three years. The original Mercury capsule will be used as planned for simple orbital flight. Then a new, improved Mercury which can maneuver in space and control its trajectory to some degree during reentry will be able to perform the third, fourth, and fifth objectives. If the Administration heeds congressional and Air Force pressure, the Dyna-Soar will be accelerated and could orbit in three years or a little more.
While the improved Mercury program has not been given the final go-ahead and probably won’t be authorized until the first orbital flights have passed, the current planning schedule calls for it to be ready during the summer of 1963. By extending the capability of the Mercury and accelerating the Dyna-Soar the US will be able to fill an undesirable four- or five-year void in its manned spaceflight program. If the US is limited to simple, uncontrolled orbiting during this period, there will be no doubt about the Soviets maintaining their lead in manned-space capability. With the new Mercury and the accelerated Dyna-Soar the US can keep the pressure on the Russians and possibly move ahead in some phases of the manned-space race during the next five years.
The preliminary design phase of the new Mercury has been completed. It will weigh in the neighborhood of 1,500 pounds more than the present version. Most of the extra weight will be invested in a restartable rocket engine and propellant load. This engine will be fired in short bursts to change the vehicle’s orbital plane and altitude in a precise manner, the two basic requirements for maneuvering in space. Rendezvous of two space vehicles is the first practical application for space maneuverability. Its feasibility will be demonstrated by having the new Mercury intercept a small satellite.
The other new feature of major importance will be a number of small flaps spaced evenly around the vehicle just behind the heat shield. During reentry these flaps will be deflected individually or in small groups to develop lift, so that the capsule’s trajectory can be controlled. The asymmetrical deflection of the flaps creates lift by unbalancing the airflow around the vehicle. If all of the flaps were opened simultaneously, the capsule would still be a pure drag or ballistic shape because the airflow would remain balanced.
Only a small amount of lift can be developed by using flaps on a symmetrical vehicle like the Mercury. Also, structural considerations may not permit the pilot to develop lift during the maximum heating portion of the reentry and may also limit the length of time that he holds lift. Regardless of the crudeness and comparable inefficiency of the new Mercury’s lifting system, the vehicle will provide valuable data on the capabilities of human pilots. It is hoped that the advanced Mercury can be brought down by parachute into a rectangular area of a few hundred square miles. The Dyna-Soar objective is to land on a preselect runway.
The key question about using the improved Mercury or any of the advanced vehicles planned in the US is the availability of adequate boosters. By current estimates, the Atlas-Centaur can be man-rated and ready to launch the improved Mercury within two years. This booster will have a modified Atlas ICBM for the first stage and two 15,000-pound-thrust liquid-hydrogen engines in the second stage. It will be the first vehicle to use high-energy liquid hydrogen for launching men.
A year or so after the Centaur is man-rated, the US booster situation will begin to improve at a rapid rate. The Titan II, with more than 400,000 pounds of thrust in the first stage, and capable of sending the Dyna-Soar on very high speed suborbital flights is being developed by the Air Force. Possibly other boosters with slightly increased performance, will be built to orbit the Dyna-Soar. NASA’s 1.5 million-pound-thrust Saturn booster will be ready to carry men and send large vehicles like the Apollo and Dyna-Soar into close orbits about the earth late in 1964, if all goes well. Another probability as a booster for manned vehicles is the large solid-propellant rocket now being developed by the Air Force. NASA’s giant Nova rocket, whose size is still undetermined, possibly will be ready for use in 1967 if Congress grants the President’s current requests for space funds.
To take full and immediate advantage of the plentiful supply of boosters promised after 1964, two different types of advanced, man-carrying aerospace vehicles must be ready for use.
In this context, aerospace vehicles are those able to maneuver in space and reenter the atmosphere. Pure space vehicles such as large satellites and long-life manned stations form another category. However, the reentry aerospace vehicle must be developed before permanent manned-space installations can be constructed, resupplied, and maintained.
The two basic types of aerospace vehicles are represented by the Apollo and the Dyna-Soar. The Dyna-Soar is designed to reenter the atmosphere at orbital speeds up to about 18,000 mph. The Apollo will be able to strike the atmosphere at parabolic or escape speeds of about 25,000 mph.
Design philosophy for the vehicles varies considerably because of the difference in top speed. Aerodynamically, the Apollo and the Dyna-Soar are both advances over the Mercury because they can develop sustained lift. The principal aerodynamic objectives are different in each case, however. On the Dyna-Soar the objective is to develop highest possible lift/drag ratio, so it can maneuver extensively in the atmosphere and achieve maximum glide range. High lift/drag ratio, the measure of aerodynamic efficiency, requires the use of a wing and a long, thin fuselage, similar to that on the Dyna-Soar drawing on page 42. It is estimated that this configuration will give the Dyna-Soar a lift/drag ratio from 1.5 to 2.0, depending upon the flight speed.
The main aerodynamic objective on the Apollo and all escape-speed reentry vehicles is to develop the maximum possible lift with little regard for drag. Maximum lift occurs at very high angles of attack for all vehicles at hypersonic speeds. Therefore, the Apollo will reenter the atmosphere with its nose up at a thirty-degree angle of attack or greater. Its drag as well as lift will be very high so that its lift/drag ratio is low. For this reason, escape-speed reentry vehicles are often called high-lift high-drag configurations.
High lift is necessary on the Apollo to widen its reentry corridor and to relieve the requirements on its guidance system. The dramatic effect of lift in widening the corridor and increasing the safety of moon vehicles returning to earth is best illustrated by first considering the ballistic vehicle which cannot develop lift.
The ballistic vehicle returning to the earth from the moon must fly down a corridor only eight miles high just above the atmosphere, if the maximum deceleration load is limited to ten Gs. If the vehicle comes in slightly above the corridor, it will travel past the earth and begin to orbit out through the Van Allen radiation belts. If it comes in below the corridor, the reentry angle will be too steep and the vehicle will go into dense air too quickly where the deceleration will reach overload proportions very quickly.
The only method of widening the corridor for the pure-drag vehicle is to increase the deceleration load. If this limit is raised to twenty Gs for a very short period, just about the most the pilot can take, the safe corridor becomes twenty-two miles high. The guidance tolerances of this corridor are almost the same as those required to send an ICBM nose cone 5,500 miles and have it hit within one mile of its target. In other words, the ballistic reentry vehicle is marginal for the trip to the moon.
Theoretically, the proper use of lift will allow the reentry corridor depth to be increased more than twenty times over the ballistic depth. However, the reentry problem is so involved—with such a large number of aerodynamic, structural, and control restraints—that under practical conditions it would be impossible to achieve this much improvement. Apparently, there will be little trouble in increasing the corridor depth to fifty miles for a ten-G vehicle, so that the trajectory precision required during a reentry from the moon will be well within the capability of existing systems.
Elaborate studies by NASA, the Air Force, and various industry groups have made it clear that the best reentry procedure is to complete the reentry in less than one circuit of the earth. It has been shown that slowing down gradually, by grazing the atmosphere several times in gradually shrinking orbits, increases the guidance requirement as well as exposing the crew to undue radiation. As a result of these studies, reentries at superorbital speeds will be made quickly down corridors near the poles where the radiation level is low.
To meet the requirement for the maximum possible lift, it is not necessary for the Apollo or other escape-speed vehicle to have wings. Even wings in the Dyna-. Soar sense are not needed. Lift on hypersonic vehicles varies primarily with the area of their lower surfaces and their inclination to the air stream. Optimum escape-speed vehicles will have large flat bottoms if such construction proves to be structurally possible.
Two designs are now receiving close consideration from NASA for the three-man Apollo. One is a modified Mercury shape. Some of the proposed modifications have canted heat shields on the front end to improve lift. Their control flaps would be located on the rear portion probably around the parachute container as shown on page 42.
The other design in strong contention for the Apollo is a so-called Eggers high-lift, high-drag shape, also shown on page 42. This flat-topped shape is named for its originator, A. J. Eggers, Jr., of NASA, who also played an instrumental role in the B-70 design. Aerodynamically, the Eggers shape and the modified Mercury are just about at a standoff. Each develops a maximum lift coefficient of less than one and a maximum lift/drag ratio of around one. The choice between them probably will hinge on other considerations.
The area of greatest concern on both the Dyna-Soar and the Apollo is the structure. Each vehicle is pioneering an entirely new concept in high-temperature structures for manned vehicles. The Dyna-Soar has a radiation-cooled hot structure. Its nose cap, which must take the highest temperatures, will be made of a graphite-ceramic material. The leading edges will be thick sections of coated molybdenum. Most of the remainder of the structure will consist of a thin molybdenum skin backed up underneath by a layer of insulation. Below this there will be an intermediate structure of Rene 41 nickel alloy to hold the skin panels in place. Underneath the intermediate members there will be a heavy truss work of the nickel alloy Rene 41 to carry the main loads.
Basically this is the same type of structural design used on canvas-covered aircraft. The skin is unstressed and does not assist in carrying the main loads.
The Dyna-Soar’s molybdenum skin panels efficiently radiate to the atmosphere most of the heat load trans-miffed to the vehicle during reentry. Structural temperature will stabilize out at about 1,600 degrees Fahrenheit. The pilot’s compartment probably will have a double wall filled with a jellied substance consisting mostly of water. Heat entering the compartment will be absorbed by boiling the water away. It will also be necessary to use a small refrigeration unit in conjunction with the water wall to keep the crew compartment at seventy degrees Fahrenheit. The equipment and instrumentation bay adjoining the pilot’s compartment will require a simpler cooling job because its temperature probably will be kept at about 200 degrees Fahrenheit.
Structurally, the Apollo presents just about double the problem of the Dyna-Soar. Even though the Apollo’s reentry speed is only about fifty percent greater than the Dyna-Soar, it has to dissipate twice the kinetic energy. It is not possible for a radiation-cooled structure made of available materials to dissipate this he load. It now appears that a thick ablating skin oil entire outer surface will provide adequate heat protection for the Apollo and other superorbital reentry vehicles. The basic technology for lightweight high temperature ablating skin was developed during ballistic missile program.
The entire outer surface of the moon vehicles will have to be replaced after each flight. They will not be able to maneuver at very high speeds for as long a period as radiation-cooled structures such as the Dyna-Soar. However, this is not expected to create an operational problem for a number of years. The ultimate hope is to have an outer surface which will ablate at superorbital speeds and radiate at orbital speeds and below.
The takeoff problems of the Apollo and the Dyna-Soar closely approximate those of the Mercury. Safety during launch by large rocket boosters is of critical concern with all manned space vehicles. NASA and the Air Force will undoubtedly continue to work together on this problem.
All vehicles will use auxiliary rocket systems to carry the manned vehicle clear of the booster rocket if trouble develops on the launch pad or in flight while the booster is firing. A small solid-propellant rocket is carried in the rear of Dyna-Soar primarily for this purpose. The rocket has enough power to allow the Dyna-Soar to take off from the top of its booster and to orient itself over Cape Canaveral for a power-off landing on the skid strip there. If it isn’t possible for the pilot to land on a prepared surface, he will use his ejection seat and come down by parachute.
It is probable that the Apollo will use an escape rocket tower similar to that on the Mercury. These vehicles have a more difficult center-of-gravity problem than the winged Dyna-Soar during periods of 1ow-speed flight. Placing the weight of the escape rockets forward moves the center of gravity to a more manageable position.
Both NASA and the Air Force are interested in giving the crew of manned vehicles more control over the booster rockets during launch. The crew won’t try to guide the rocket vehicles but will monitor its systems. In case of trouble, they will not have to rely completely on automatic safety devices and can cut out the malfunctioning system and switch to backups. Numerous plans of this type are being considered. One the main objectives of the improved Mercury program is to get as much flight time as possible on the various schemes for launch safety.
The ultimate objective of the launch safety effort and all of the orbital flights of the next few years is to make spaceflight at least as safe as flying in high-performance military aircraft. There is definite hope that this can be accomplished, eventually. However, men will have to learn to fly in space by doing so. Simulators and training equipment on the ground will play an important part in the learning process, but there can be no substitute for flight time—in large quantities and way out.—End