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established it. is impossible to define the best trajectory. For a ballistic missile to reach a given range from the burnout point there are infinite combinations of burnout angle and burnout velocity, but there is an optimum burnout angle for any range which produces the least burnout speed requirement for that range. A limitation might accordingly be placed on velocity or on angle. The other unknown could then be determined. Because the magnitude of the burnout velocity is a function of the amount of propellant carried relative to the payload, propellant economy and hence economy in initial launch weight'will be obtained by minimizing the magnitude of the burnout velocity vector; One way of attacking the problem is to determine what is the maximum range that can be obtained for a given burnout velo­ city. There is a maximum range trajectory for any given burnout velocity, and this is called the minimum energy trajectory. For a given burnout velocity there is only one direction of the burnout velocity vector which can produce the minimum energy trajectory. However, it may not be desirable to choose the. mini­ mum energy trajectory from the standpoint of guidance require­ ments.

Consequently, if the missile design is such that the range required is less than the maximum range for a given burnout

velocity, there are two solutions for	the burnout angle.	They
yield a high trajectory — a lofted	trajectory — and a	low
trajectory.

23. Servo and Guidance Systems of Long-Range Ballistic Missile

Two servo systems are used during the powered flight to control the attitude, position, and velocity of the ballistic missile. A control system governs the attitude and the rate of change of attitude. It controls turning forces which are generated by swivelling the rocket thrust chamber to direct the thrust vector at some angle to the longitudinal axis of the missile or by placing vanes into the exhaust stream to deflect the jet .and

thereby	change the direction of	the thrust vector. When the
ballistic	missile has several chambers making up its rocket
engine,	swivelling of the "thrust	chambers can be used to gene­

rate roll forces. If a single chamber is used in the power plant, roll forces are usually generated by small, gimbal-mounted rocket engines, known as vernier engines, or by the use of the turbine exhaust.

The second servo system is termed the guidance system. This introduces steering signals into the control system so that the

ballistic		missile	is guided into the required ballistic	trajectory
to	reach	the programmed target. During powered flight'energy
is	being	given	to the missile.. This energy comes	from the

combustion of the propellant. The total energy expended during the propulsion phase is the sum of the kinetic energy of the

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missile at the time of burnout and the potential energy at the point of the trajectory, and the energy that has been expended to overcome the drag forces and to carry some of the propellant

upward	through the gravitational field of the Earth.	Energy
used to	lift propellant which is subsequently ejected as	part of

the exhaust jet, and energy used to overcome drag are wasted energies. The design of the missile and the thrust programming has to be planned so that these lasted energies are kept as low as possible. Actually two effects work against each other. To avoid carrying propellant to high altitude in the gravitational

field the propellant should be ejected	as quickly	as possible
during the powered flight. This would,	however,	produce high

velocities for the missile when low in the atmosphere and would increase the drag losses. A compromise has to be made and it is obtained by detailed computations for the propulsion phase of the trajectory.

Two types of guidance system are used to ensure that the payload of a ballistic missile is placed in the correct free-space trajectory that will carry it to the target. The first main type is the automatic dead-reckoning system which is called an inertial guidance system, or is sometimes referred to as an all-inertial system. This latter phrase is used because the systetn is entirely contained within the missile. During flight it operates indepen­ dently of the outside environment, that is, it does not need to receive signals from a ground source. It is, therefore, almost immune to counter-measures.

The other main type of system is the radio-inertial system. This relies upon commands and upon ground-based equipment stich as radars and computers, and it is, therefore, susceptible to enemy counter-measures. The great advantage of the ballistic missile is that the guided flight takes place relatively close to the launch pad and forms such a small part of the whole trajectory. It is hence very difficult for an enemy to detect the missile rising from the pad and immediately apply counter­ measures to upset the guidance system. Not only is the guidance system required for only a few minutes after liftoff but also the pad is hidden from the target by the curve of the Earth’s surface.

The guidance system of a ballistic missile is a system which measures and evaluates flight information, correlates this with target data, and converts the resultant information into parame­ ters which are required to achieve the desired flight path. It then communicates this data in the form of commands to the flight control system of the vehicle. A guidance system can be selfcontained within the missile, or the function may be jDerformed by various combinati.ons of ground and airborne equipment. Guidance is thus essentially a process of measurement and subsequent correction. Measurement is the critical part of the

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process	and has to be made with high precision if the warhead
is to impact on its target.
First	a decision has to be made on what are to be the special

measuring instruments for guidance purposes. Then the imple­ mentation of the actual guidance manoeuvre has to be considered. There are several ways in which the missile can be manoeuvred to correct the trajectory after an error has been detected and measured. Each of these ways will be capable of bringing the

warhead	to	within lethal distance of the target. The way which
is chosen	is	referred to as the guidance scheme.

A typical guidance scheme is to use a computer carried within the ballistic missile. The computer has a ‘memory’ in which are stored the elements of a standard trajectory to the desired target. This trajectory is sometimes known as the ideal reference trajectory. The guidance system then operates by continually comparing the actual trajectory of the missile with the reference trajectory. If an error is detected a correction is immediately made to return the missile to the reference trajec­ tory. The obvious disadvantage with this type of guidance scheme is that any slight change from the reference trajectory always results in a correction being applied irrespective of whether or not the slight change would result in the warhead falling within lethal range of the. intended target.

24v Problems of Nuclear-Rocket Engine

A nuclear-rocket engine that produces one hundred tons of thrust runs at a reactor power level of several million kilowatts. In a well-built nuclear-rocket engine, only about one percent of the energy escapes in the form of gamma and neutron radiation, but, for a certain power level, that still amounts to tens of thousands of kilowatts! Think of the heat radiated even by a puny 100-watt lamp, and it is easy to understand why parts near a nuclear-rocket engine may need active cooling by suitable routing of the flow of liquid hydrogen to the engine.

A nuclear-rocket engine cannot be abruptly started and shut down. The tremendous difference between the original tempera­ ture of the liquid hydrogen (—423 degrees F.), and the white-hot temperature at which the reactor operates under full power, makes it necessary to start a nuclear-rocket engine relatively slowly. Otherwise, cracks in the reactor’s brittle graphite-uranium core are unavoidable. Moreover, the increase in hydrogen flow through the reactor (controlled by the liquid-hydrogen feed pump)

must be carefully synchronized with	the	increase	in	the
reactor’s power level (controlled by the	position of its		neutron-
absorbing control rods).	more severe		problem.
Reactor shutdown poses an even
During power operation, the reactor has		contaminated		itself

with highly radioactive fission products. For a few minutes these

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keep emitting such a strong “decay radiation” that the reactor core would soon be heated to destruction unless one kept up an adequate flow of hydrogen through its passages.

The “aftercooling” of nuclear-rocket engines is not necessarily wasteful. How much hydrogen will be needed to prevent the core from overheating is known beforehand. Thus, on a typical space mission, one would shut down the nuclear engine shortly before the required flight velocity had been reached. The missing balance of the speed would be produced by the exhaust of after-coolant heated by the decay radiation.

The most promising uses for nuclear rockets seem to be:

For upper stages of large chemically-boosted rockets, parti­ cularly in missions requiring very high-final velocities.

For supply vehicles shuttling back and forth between a low earth orbit and an orbit around the moon (with hydrogen reloading during each stopover in earth orbit).

For planetary space vehicles beginning their voyages in earth orbit. These vehicles may be assembled in orbit from parts and propellants brought up by several chemically powered rockets.

25. The USSR Investigates the Moon
The Soviet Union is successfully	advancing its	programme
of Moon research. This programme	is characterized	by the use

of automatic vehicles. According to estimates made by specialists the flights of automatic space vehicles are much cheaper than manned expeditions, namely one-fiftieth or even one-sixtieth of the cost of the latter, in pursuit of identical goals.’ At the same time they ensure a high degree of reliability and are free from

risks.

In the course of the last years the Soviet scientists solved many important problems in the study of the Moon and nearlunar space.

The Luna-3 and Zond-3 automatic probes photographed, the far side of the Moon. Early in 1966 the Luna-9 probe made a soft touchdown on the Moon and for the first time transmitted to the Earth a panoramic view of a moonscape. The studies of the density and other mechanical properties of lunar rocks carried out by the Luna-13 probe played a big role in the further designing of lunar spacecraft.

In the 1970’s new lunar probes appeared. These were designed to make a landing on the Moon or for conducting lunar research from a circumlunar orbit. An outstanding design feature of these was a unified rocket stage. The development of a unified rocket stage is an engineering and technological achievement of equal value to the creation of a self-propelled vehicle for the Moon, known as the lunokhod or moon rover or a Moon-Earth complex

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with a return vehicle. Lt is a self-contained unit which alters the trajectory on a portion of the flight path, ensures injection into a,near-Moon orbit, correction thereof and finally a soft touch­ down on the surface of the Moon on highly broken ground.

In the recent period three main areas have emerged in Moon

research. The first consists in	the geochemical	investigation	of
the surface layer of the Moon	through the delivery of lunar
rocks to the Earth. The second	consists in the	investigation	of

the Moon with the aid of long-life artificial lunar satellites. The third consists in the study of the Moon with the aid of mobile scientific laboratories controlled from the Earth.

26. Research Satellites in a Selenocentric Orbit

The broad scope of .scientific problems confronting cosmonau­ tics has made it necessary to carry on further research of the

Moon with the aid	of artificial lunar satellites.	The flights of
the first automatic	probes — Luna-10, Luna-11,	Luna-12, and

Luna-14 — in selenocentric orbits between 1966 and 1968 showed that such research was highly effective.

A	new stage	in long-term experiments in near-Moon space
set in	September	1971 with the launching of the Luna-19 probe.

The probe functioned in a selenocentric orbit for more than a year, making about four thousand circuits round the Moon. It studied the gravitational and magnetic fields in near-lunar space, cosmic rays, micro-meteorites and interplanetary plasma. The' Luna-19 probe held one thousand radio communication sessions with the Earth transmitting a large volume of scientific information.

In the scientific programme of the Luna-19 considerable attention was given to the study of the gravitation field of the Moon. A knowledge of the characteristics of the. Moon’s gravita­

tional field will make it possible to	effect	reliable control over
space vehicles in circumlunar orbits	and in	the landing portion
of the flight before touchdown.

Regular path .measurements which were conducted in the entire lifetime of Luna-19 helped study the evolution of its orbit in detail. Measurement of the orbit parameters in the first two

months of operation of	Luna-19 enabled the scientists to make
a quantitative appraisal	of the effect produced	by the asymme­
trical character of the	Moon’s Northern and	Southern Hemi­

spheres on the movements of the satellite.

The probe was us.ed for conducting experiments in the study of near-lunar space by the method of radio-wave propagation. The laws governing the reflection of radio waves from different areas of the lunar surface, were also studied by the probe. The results obtained have led the scientists-to the conclusion about the presence of plasma which is formed as a result of interaction of cosmic radiation with the surface of the Moon.

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In keeping with the research programme the Luna-19 probe conducted television photography. This was carried out both for the purpose of exploration of separate areas on the lunar surface and for the establishment of the possibility of using orbital panoramic views for space navigation.

Special instruments measured the space density of meteoric particles in near-lunar space. It has been established in particular that the space density of particles which do not belong to meteo­ ric fluxes is the same as that of interplanetary space.

It is an important result of the flight that systematic data were obtained on thecorpuscular fluxes of solar and galactic radiation. More than ten considerable rises were known to have

been registered, in solar cosmic-ray	proton	fluxes	with	energies
of more than one million electron-volts.		This	has	been the
consequence of powerful solar flares.

The Luna-19 probe studied the magnetic fields to find the answer to the question of whether the Moon deformed the mag­ netic loop of the Earth at the height of satellite flights and the interplanetary magnetic fields of solar origin, which are carried

b^ solar plasma streams.	measurements in
The Luna-19 satellite conducted magnetic
46 portions of the trajectory over the visible	side of the Moon

at full Moon when the natural satellite was in the magnetic loop of the Earth together with the artificial satellite, and at waxing and waning half-Moon and when the Moon was “swept” by the solar wind. The magnetograms obtained at heights of 80 to 385 kilometres in medium latitudes confirmed the disturbing effect of the Moon on the surrounding medium..

The investigations started by the Luna-19 probe were conti­ nued by the Luna-22 probe which was put into a selenocentric orbit on June 2, 1974.

The initial altitude of the probe’s orbit over the lunar’surface was considered to be 220 kilometres. However, in thecourse of many months flight it was repeatedly decreased to a height of the order of several dozen kilometres. This was done to conduct detailed television photography of selected areas of the lunar surface.

The functioning of two Soviet lunar probes over a conside­ rable period of time has again confirmed the high effect derived from the use of artificial satellities in the study of the physical nature of the Moon.

27. Luna-9 Flight

The lunar rocket with its booster stage was first put on a parking orbit round the Earth. After the rocket’s engines were turned off it orbited the Earth like any other satellite. At the preset moment the booster stage was activated. At the end of the

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boost phase when the rocket had developed the required speed the engine was shut off and the lunar rocket separated from the booster stage, continuing its power-off flight towards the Moon. Radio-communication centres on Earth carried on telemetric

control of the on-board				systems and		measured		the	parameters
of the rocket’s movement tp determine its actual trajectory.										and
This,	in	turn,	made	it possible to		calculate		the	value
direction	of	the	correcting		impulse	and at	a	given moment
correct the trajectory.					the rocket	hitting	the		Moon	at'a
The correction			ensured

definite place. After that another series of trajectory measure­ ments was carried out, the execution of the command for correc­ tion checked and the actual parameters of the movement verified. On approaching the Moon the on-board control system was prepared for effecting deceleration. The flight ended in the probe’s slowdown and landing on the Moon’s surface.

Analysis of various methods of deceleration showed that the most reliable and, therefore preferable one, for the first experi­ ment was a vertical slowdown. Flight time to the Moon was largely pre-determined by optimal power consumption. Compu­ tations showed that the most advantageous trajectory in this respect was one which required a threeto four-day flight to reach the target.

Another thing that was taken into account in making the final choice of trajectory was favourable visibility of the Moon from

certain	points on Soviet territory during	the rocket’s decelera­
tion and immediately after the landing.		between lines drawn
For	the. selected trajectory the angle

from the Moon’s centre to the landing point and to the Earth would be 60°—70°, assuming an almost vertical deceleration. Thus, for an observer on Earth the landing point would be in the western part of the Moon’s visible disk. Even when decele­ ration is exactly vertical the landing point shifts within an area having the selenographic coordinates of 50°—70° west longitude and 0°—16° north latitude, depending on the Moon’s position in

its orbit as influenced by libration (irregularities					in its move­
ment due tp the attraction of celestial bodies)				and the eccentricity
of its orbit.				for photographing
To have the most favourable conditions
the lunar surface and maintaining		the	required		temperature
conditions in the probe, the landing was			to	take	place in	the
neighbourhood of the morning terminator.
This determined the launching date			and, consequently,			the
date of reaching the Moon.	orbit		determinations			were
Trajectory measurements and
indispensable.elements of flight control.
One of the most critical stages	in	the flight was correction
of the trajectory. This was carried out,		in keeping with the flight

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programme, on February 1, 1966, at 22 hours 29 minutes, Moscow

Time, when the probe was. at a distance	of	233,000 kilometres
from the Earth and 190,000 kilometres	from	the Moon. This

particular moment was chosen for a number of reasons. On the one hand, the greater the distance of the probe from Earth, the greater the accuracy in determining the trajectory and, consequ­

ently, in making the correction. Moreover,		the	closer		the	probe
to the Moon when the correction	is made,	the		less	deviation
from the calculated trajectory will	be caused		by	errors		in -its

execution. The closer the probe to the Moon, however, the greater the power required to correct the same mistakes.

Orientation of the engine is considered to be an essential factor in making the correction and in deceleration. The rocket was equipped-with an astro-navigation system, the reference bodies being the Sun, the Moon and the Earth. In addition, the system enabled measurements to be made of the rocket’s coordi­ nates with regard to the Moon.

The astro-navigation system was adjusted on Earth. The mutual position of the mentioned celestial bodies was determined for the planned time periods of orientation and the sensor telescopes were positioned accordingly.

Final adjustment of the system was carried out in flight. The operating principle of the navigation system was such that readjustment according to certain parameters was always necessary, and provision was made for possible readjustment according to others. As soon as the actual trajectory was deter­ mined on the basis of measurement data, new computations were made which enabled readjustment if necessary. During the Luna-9 flight, however, no readjustments were called for.

Preparation for the correction began by an automatic orien­ tation of one of the rocket’s axes towards the Sun. Signals from

the	rocket’s system of sensors were received			by	a	computing
device which		controlled the ^operation of	the	micromotors.
the	Rough orientation brought the Sun into the optical sight of
	precision solar orientation system. After the position of one
of	the rocket’s axes towards the Sun was stabilized, search for
the Moon began by rotation of the rocket			on	this	axis.
	The actual	parameters of the rocket’s	movement,			calculated

on the basis of the trajectory changes after correction, made it possible.to determine the initial parameters for adjusting the system so as to enable autonomous landing control.

Six hours before the landing these data were relayed to the probe over the command radio channel. Preparation of the rocket for deceleration was begun over the command radio channel some two hours before the rocket reached the Moon’s surface. The sequence of further operations was determined by the on-board, automatic equipment. The first stage in orienting the rocket was similar to its orientation during correction. As a

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result, the rocket was stabilized relative to the Moon and the Sun.

A special .optical sight then began seeking .the Earth and as soon as it was in its field of vision the system’s logical device kept constant control over Earth tracking throughout further flight.

These operations were completed some ten minutes before the lunar rocket passed a point at a distance of 8,285 kilometres from the Moon’s centre.

The problem of deceleration was to reduce the rocket’s velo­

city with regard to the Moon (the speed	in approaching the
Moon was some 2,600 metres per second)	practically to nought.

Analysis showed that if the engine were orientated in space during the slowdown according to the calculated trajectory the value of the lateral component of the velocity could be very great by the end of deceleration. It would be roughly proportio­ nal to the deviation of the trajectory’s actual point'of intersection of the lunar surface from the calculated one. A deviation of one hundred kilometres would mean a lateral velocity of some 40 metres per second. In other words, this method of deceleration would require very great precision in determining the trajectory after the correction.

Therefore the method of orienting the engine by the lunar vertical was made use of. The essential idea was to orient the engine towards the Moon’s centre at a pre-determined distance (in the case of Luna-9 the distance was 8,285 km) and to main­ tain this orientation up to the beginning of deceleration.

After the rocket had passed that height, tracking of the Moon ceased' and the orientation was maintained by appropriate optical sensors following the Earth and the Sun.

For deceleration to begin at the proper time it was necessary to know with the. highest precision the rocket’s altitude above the Moon’s surface at the final stage of its approach trajectory. For this purpose the rocket' was supplied with a radio altimeter, the axis of whose aerial was oriented parallel to the axis of the engine. The device gave the signal for beginning deceleration as soon as the rocket had passed a pre-determined altitude above the landing point (about 75 kilometres). As during the correc­ tion a special gyroscopic system controlled the rocket’s stabilization after the engine had been activated within the calculated time from the signal. The equipment which was no

longer	needed	for	further	flight was separated from thus
reducing the weight			of the landing vehicle by 300 kilogrammes.
	28. Self-Propelled Lunar Laboratory
The first self-propelled				laboratory — Lunokhod-1	(moon
rover) — controlled			from the	Earth/ opened a fundamentally new
stage in	lunar	research. For		the first time it has been	possible

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to explore, with the aid of an automatic vehicle not only the point of landing, but also the nearby areas.

The Lunokhod-1 was delivered to the surface of the natural satellite of our planet by the Luna-17 j)robe which made a landing in the region of the Sea of Rains on November 17, 1970. The Lunokhod-1 is a self-propelled vehicle weighing 756 kilo­ grammes, comprising an instrument compartment and an eightwheel running gear.

The sharp temperature changes in daytime and night on the Moon necessitated the development of an elaborate system of thermal regulation with the use of an isotopic source of heat.

The mobility of the moon rover was ensured by a self-propel­ led running gear comprising eight driving wheels with a separate drive for each wheel. The design of the running goar enabled the vehicle to move forward and backward in two gears, to make turns on the spot and in motion.

The moon rover was operated by a special crew consisting of captain, driver, navigator, “flight” engineer and aerial opera­ tor. The spfeed of movement was chosen on the basis of an estimate of TV information on terrain features and continuously supplied telemetry data on rolling and pitching, the path traver­ sed and operation of wheel drives. While the vehicle was controlled in motion the problem of navigation, i. e., the charting of the route in the chosen direction, was being solved too.

The first self-propelled vehicle, Lunokhod-1 , blazed a trail in the Sea of Rains 10,540 metres long. This made it possible-to explore the lunar surface across an area of 80,000 square metres. To this end more than 200 panoramic views and over.20,000 photographs of the lunar surface were taken. At more than 500 points on the route of the moon rover the physical and mechani­ cal properties of the surface layer of the ground were investiga­ ted and at 25 points chemical tests were conducted. The functioning of the automatic vehicle over a period of many months made it possible to conduct prolonged measurements of cosmic X-ray radiation and studies of the radiation situation on the Moon.

The Lunokhod-1 carried an angular reflector which had been developed and produced by French specialists. The Soviet scien­ tists of the Crimean Astrophysical Observatory, USSR Academy of Sciences, and the French scientists of Observatoire de Pic du Midi conducted joint experiments in laser ranging of the angular reflector.

The unique lunar experiment was continued	in	1973 when
the Luna-24 probe made a soft touchdown on	the	Moon on

January 16. The probe delivered to the eastern boundary of the Sea of Serenity, in the region of the crater Lemonnier, another self-propelled vehicle the Lunokhod-2,

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