a friendly receiving station did. Successful interception would require that the enemy know almost every detail of the system and its operation.

Interference and Other Countermeasures. The television link can be relatively easily jammed by an enemy who knows the approximate locations of the ground receiving station and the frequency of transmission and who is able to get a jammer within line-of-sight range of a ground station. Even though the ground station's receiving antenna is highly directional (peak gain probably in excess of 20,000) and tracks the satellite, so that the jamming signal will be discriminated against by a factor ranging from a minimum of 1000 (for 30 db peak-side lobes) to an average of more than 20,000, the jammer can take tremendous advantage of the pulse transmission. For example, an air-borne pulse jammer of 10- to 100-kw peak output worked into an antenna [37] of modest gain (100 to 10) carried by an aircraft at 20,000 ft to within -200 mi of the receiving station could prevent reception of a usable picture. Such jammer powers (peak pulse, at a duty cycle of about 1 per cent) and antenna sizes are comparable with, or modest compared with, those of ordinary airborne radars, and spot-frequency jamming is therefore quite feasible.

If the enemy can be denied access to within line-of-sight range of the ground stations, the television system will be relatively invulnerable to interference by the enemy. Counter-measures applied at the satellite-end of the circuit presume possession by the enemy of adequate search and tracking facilities (the difficulties of which were previously discussed) and can be directed only against the satellite's tracking receiver.

Reception

Reception and Presentation of the Television Signal

A description has already been given of the ground station's receiving antenna and tracking system. Consideration is now devoted to the assimilation of the TV pictures after they have arrived at ground level.

Concurrently to read and interpret information on a single television screen at the rate of 10 completely different frames per second is obviously impossible. Furthermore, each ground station receives only a piece of the target system under scrutiny. Thus it appears necessary to record the transmitted data with as little loss in resolution as possible and to forward it to a central evaluation center.

At a first glance, it would seem that a prodigious amount of film would be required to record all the pertinent television frames transmitted. However, analysis reveals that 2.9 hr/day, at most, are spent over USSR and her satellites, China included. At 10 frames/ sec, 2.9 hr are equivalent to 1.0 X 10' frames/day. It has been shown previously that one satellite will observe a given area only on alternate 35-day periods in daylight. Thus, for the first 35 days' operation, 3.5 X 10' frames would be recorded. This is 265,000 ft of 35 mm camera film, or about that used in filming several feature-length movies.

[38] It is believed that during the first 30 to 40 days' operation a fairly comprehensive picture of the USSR would be obtained, and subsequent operations would be concentrated on specific target systems or areas, perhaps with the narrow-scanning-width lens system previously described.

The equipment required at any forward receiving station is not complex. The receiving antenna has already been discussed. An ordinary television receiver will probably suffice for monitoring purposes (to see if the picture quality is satisfactory). Its viewing scope, however, must have a high-persistence screen which will project about one frame out of a hundred.

For recording, a second television receiver is needed. Its scope must be as large as possible and its electric beam spot size must be reduced to a minimum; in short, the whole set must be tailored to the criterion of putting the image on the screen with as little loss in resolution as possible.

The image will then be reduced by camera optics to the appropriate film size; 35 mm

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may be adequate, but if a significant amount of detail is lost, then 70 mm can be employed. The film does not have to be very "fast," but should be of a fine-grain variety. The camera will be similar to a movie camera but will operate at about one-half the frequency.

Each forward station will be furnished with a time schedule for operating the cameras computed on the basis of the satellite's orbit. Such a schedule will vary from day to day, as mentioned in the discussion on orbits. Some sort of time coding will be included with each frame; a feed-back from the tracking-antenna control will also be fed into this coding, but this is only a crude location device to show up any gross errors in evaluation. Presentation

The central evaluation station will receive the composite films from the forward stations and assemble the story into an integrated whole. Standard photogrammetric techniques call for synchronizing one or more sets of films, together with overlays, etc., [39] to aid in interpretation of results. In such a device, the frames are projected in a mosaic form and compose as the scenes appear on the earth. Also projected could be a master overlay made up of geographical coordinates and, later, after a number of films are taken, of that area of the ground already filmed. The over-all area can be enlarged to any extent necessary for rapid determination of the worth of the films being evaluated. For instance, if a large area is covered by clouds, then just those frames having glimpses of the ground could be separated for subsequent addition to the master mosaic of the USSR.

The cloud pictures would be placed on a larger-scaled photomap so that daily weather maps could be made and preserved.

The entire presentation system should be simple, rapid, reliable, and amenable to standard evaluation techniques.

Summary

To summarize, a 350-mi altitude satellite, having an f/10, 2-in. aperture, 20-in. focal length, Image Orthicon TV camera of 1000 TV lines/in. with a speed of 10 frames/sec, would be capable of resolving scenes of contrast greater than 20 per cent to about 200 ft. Transmitting and receiving antennas for the described system will require careful analysis and design, but their accomplishment does not present any serious research problems. Presentation of the viewed scenes by photographic and photogrammetric methods appears within the limits of known, practiced techniques.

Such a system, employing presently available equipment, is considered satisfactory for both weather and pioneer terrestrial reconnaissance. However, in order to obtain acceptably detailed target evaluation of bomb-damage assessment, the minimum resolvable surface dimension will have to be improved; several possible methods are suggested.

For example, by keeping the frame speed constant but optically reducing the field of view and thereby reducing the scanned bandwidth on the ground, acceptable values for most terrestrial reconnaissance can be attained with present television tubes. This results in not having a daily coverage of the entire target area.

Other means of improvement of the resolvable surface dimensions are an increase in the inherent tube resolution (an increase of about 50 per cent is visualized at this time) and an increase in the frame frequency to 30/sec (about 45 per cent improvement of resolvable surface dimension).

The estimated over-all power, weight, and space requirements of the electronic transmitting system are 350 watts, 300 lb, and 2.25 ft3, respectively....

309. Such expedients as, for example, using blue sensitive film with a blue cathode-ray screen can be used to bring out certain details in the viewed scene.

Document II-4

Document title: R.M. Salter, “Engineering Techniques in Relation to Human Travel at Upper Altitudes," Physics and Medicine of the Upper Atmosphere: A Study of the Aeropause (Albuquerque: University of New Mexico Press, 1952), pp. 480-487.

Few scientists and engineers outside of the narrow field of rocketry took human space travel seriously until the mid- to late 1950s. Many felt that it was little more than science fiction fantasy unworthy of serious study; they frowned upon their peers who devoted time and effort to such a frivolous topic. Robert Salter's article was most likely the first serious treatment in American academic and engineering circles of the problems of human spaceflight.

The subject now under consideration is the current and predicted status of engineering techniques related to the travel of man in the upper atmosphere. In other words, we are to discuss the "how" and "when" of manned space flight. The "why" of human participation in such a venture should also be examined since it is not immediately obvious that we cannot always substitute electronic equipment in place of a pilot.

In order to correlate properly the various data available it is necessary to distinguish between physical limitations, those imposed by actual physical laws (or absolute limits), and purely engineering constraints. Quite often an operation is said to be impractical when actually it is only infeasible on the basis of current engineering techniques. On the other hand, physical considerations usually furnish a clear-cut indication that the particular problem in question either can or cannot be solved. This is not a completely rigorous limitation, since some phases of physics, notably nuclear physics, are currently in a rather fluid state so that our ideas may change in the future.

Thus on the basis of the laws of motion, etc., as we now know them, the various allowable regimes of operation in the aeropause can be enumerated. It can be said that, without the employment of a rather unique release of nuclear energy, certain modes and areas of space travel must be excluded. For example, long-duration flights at 50 mi altitude would be excluded. On the other hand, it now appears that a large portion of space travel of interest can be accomplished with present-day types of propulsion and energy sources! The day for successful interplanetary travel awaits only the decision of man to provide the [481] prodigious and concerted effort required. It might be mentioned in passing that nuclear fuels are not necessary to such an operation and that the basic techniques required have been known for centuries. It is pertinent to note, for instance, that a two-stage rocket was successfully tried in 1855-nearly a hundred years ago.

Regimes of Flight in the Aeropause

Some of the first questions to be answered are, "how high," "how fast," and "how long" can flight be sustained in the upper atmosphere? Emphasis must be given the last item if a pilot is carried in the vehicle. Obviously, other than for the purposes of physiological experimentation or establishing a record, one would not conceive of a manned sounding rocket. Here, there is not time or need to supplant electronic equipment for making observations. However, in cases where flight duration is of sufficient length that electronic reliability is a problem, where computer operations (such as having adequate "memory" included) are too complex, and, in particular, where judgment in unforeseen circumstances is needed-then the participation of man will be required. It may be seen that the first two requisites are limited only by prevailing engineering development, while the last is clearly a basic constraint.

The employment of pilots in supersonic rocket planes and in balloons is an example of present approaches to the problem, and has been covered in previous papers. In the case of air-borne vehicles (those using forward motion to derive lift from the atmosphere) we must consider duration of flights as well as altitude. It is convenient to subdivide this class of vehicles into those using rocket engines and those using air-breathing power plants. This latter type is represented by the various jet propelled aircraft and missiles. In order to fly at very high altitudes it is necessary for such a vehicle to operate at supersonic speeds, not only to provide sufficient lift but also for adequate thrust. At an altitude of 20 mi (32.2 km), for example, the required Mach number for a ramjet is over 5 and the resultant incoming air has a stagnation temperature of the order of 2000° F. Since energy must be imparted to this air at higher temperatures it may be seen that a present engineering limitation on suitable fuels and materials is approached. This is particularly true with the use of nuclear heating. Thus the air-breathing vehicle is limited in altitude. As for duration, a nuclear ramjet might be capable of cruising for indefinite periods around the earth and comprises a very interesting possibility for travel in the near aeropause.

[482]

Physics and Medicine of the Upper Atmosphere

With rocket vehicles, higher altitudes can be attained. At 50 mi (80.6 km) and at a near-satellite speed of 4 mi/sec a vehicle can support its weight in gliding. However, in the region of 20 to 100 miles (32.2 to 160.9 km), flights with propulsion that can be envisioned at the present time will be of short duration (an hour or so) and less than a revolution about the earth. In this altitude region, the justification for incorporating a pilot is doubtful.

The gliding trajectory mentioned above naturally leads us to a satellite. The feasibility of establishing a vehicle in a stable orbit around the earth has been theoretically demonstrated both here and abroad. Placing the vehicle in the direction of the instaneous tangent to the earth at the proper speed will result in a circular orbit. At 150 mi (241.4 km) altitude nearly 5 mi/sec is needed; at 500 mi (804.6 km) this speed is a little slower at about 4 1/2 mi/sec. At 150 mi the duration would be of a day or so, drag slowing the vehicle down. At 500 mi, it is estimated that several decades would elapse before a satellite would come down of its own accord.

It is apparent that automatic operation of complicated scientific observational equipment is a tenuous proposition for long periods of time. The temptation for employing a human observer on a one-way basis is ever present, and it is probable that a number of volunteers willing to devote their lives to science could be obtained. However, it is physically possible to bring a satellite back without a great additional source of power.

This is not easy and would require considerable development in control equipment. In launching a satellite, a long, coasting (elliptical) trajectory is indicated, with a small additional kick provided to pull it into orbit. This same kick in reverse will put the vehicle back into the original ballistic flight path, but the vehicle might burn on the way down. By using a carefully selected and maintained gliding trajectory it is believed possible to enter the atmosphere without disastrous skin temperatures and high landing speeds. In fact, terminal speeds slightly over sonic are indicated, at which point parts of the vehicle could be landed with parachutes.

The main problem, then, of the returnable satellite is that it requires a very accurate control during the descent phase-automatic programmed control at the least, and possibly the continuously computed variety.

The satellite, especially a returnable one, is important as a step in the direction of interplanetary space travel. The principle of orbiting [483] about a given planet will undoubtedly be incorporated in future vehicle operations. For example, computations have shown that the establishment of an elliptical orbital path about the earth and moon will require not appreciably more energy than that of a low-altitude satellite-in fact, the energy required is less than that needed to establish a circular orbit at 6400 mi (10,299.9 km) height.

To escape from the earth's gravitational field, a velocity of 40 percent more than the low-altitude satellite or about 7 mi/sec is required. By properly timing the trajectory it should be possible to arrive at a near planet in a reasonable length of time. Some additional control thrust will probably be required to allow the vehicle to be captured by this planet as a satellite. At this point the vehicles can elect to land, or return to earth by orbital escape at the proper time.

Motivating Techniques Required

It is apparent from the foregoing that, as more complicated operations are visualized, an effectively greater thrust impulse is necessary for a given vehicle. Staging is one expedient, and an important one, for effecting space travel. However, if it requires a million pounds take-off weight to put 10,000 pounds on a distant planet, it will take a billion pounds initial weight for a round trip (to a planet of similar gravitational pull and return). This is the weight of ten battleships. This condition prevails if all the fuel must be carried with the vehicle.

Instead of multiplying stages the vehicle might carry a pilot plant for making its own fuel for the return voyage. This would, say, double the pay load and thus only double the gross weight.

The launching of a one-way space vehicle is felt to be possible with highly developed structural techniques and chemical fuels. What gains, then, can be made with the use of nuclear fuels?

Two possibilities exist with nuclear energy, using the fission particles directly, and degrading the energy into heat for increasing the momentum of a secondary fuel. At first sight the direct use of fission fragments and neutrons seems attractive. However, at the vehicle speeds where most of the thrust force is expended, many orders of magnitude greater energy release is required for the direct employment of fission particles to attain a given thrust than is needed for sufficient heating of a secondary fuel. This is because the mass of the particles is so small. One can consider the “dilution" of the particle momentum with inert particles of greater mass. The energy required is proportional to mV, [484] while the momentum change for thrust is proportional to mV if the velocity of the particle is greatly different from that of the vehicle, the low-mass, high-energy propellant is inefficient. (In the above discussion it has been assumed that it is possible to direct the fission particles rearward, which, in itself, is unrealistic considering the tremendous amount of heat generated in an absorbing chamber designed for such purposes.)

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It is, of course, well known that such orders of energy production are probably physically realizable, but to accommodate such an energy release in a vehicle is beyond the scope of present engineering thinking.

On the other hand, the use of a nuclear reactor to heat a secondary propellant does not impose a large strain on the imagination. The impulse for a given energy release is roughly proportional to the square roots of the temperatures and to the inverse of the atomic weights of the propellant components. Thus the ability to use hydrogen or methane alone, and/or higher temperatures, indicates possibilities for nuclear propulsion. However, it is not immediately apparent that an improvement over a chemical-fuel system will result or, if so, that the amount of improvement will be significant.

Engineering Limitations

We have explored, in a qualitative fashion, the allowable regimes of upper atmosphere flight from the standpoint of that which is physically conceivable. How much then is realizable on the basis of engineering? In other words, if say, a satellite vehicle can be 298. The optimum velocity of the particle is approximately twice that of the vehicle. For particle speeds significantly greater than vehicle speeds the propulsive efficiency is proportional to Vv/Vp.