The discovery that a salty sea once covered party of the surface of Mars will have lasting effects on the future exploration of the red planet, according to scientists and policy experts inside and outside NASA.

Project Orion:

Its Life, Death, and Possible Rebirth

Michael Flora

The race to the moon, in the forms of Project Apollo and the still-shadowy Soviet lunarprogram, dominated manned space flight during the decade of the 1960's. In the United States, the project sequence Mercury-Gemini-Apollo succeeded in putting roughly sixty people into space, twelve of them on the moon. Yet, during the late 1950's and early 1960's, the U.S. government sponsored a project that could possibly have placed 150 people, most of them professional scientists, on the moon, and could even have sent expeditions to Mars and Saturn. This feat could conceivably have been accomplished during the same period of time as Apollo, and possibly for about the same amount of money. The code name of the project was Orion, and the concepts developed during its seven-year life are so good that they deserve serious consideration today.

Project Orion was a space vehicle propulsion system that depended on exploding atomic bombs roughly two hundred feet behind the vehicle (1). The seeming absurdity of this idea is one of the reasons why Orion failed; yet, many prominent physicists worked on the concept and were convinced that it could be made practical. Since atomic bombs are discrete entities, the system had to operate in a pulsed rather than a continuous mode. It is similar in this respect to an automobile engine, in which the peak combustion temperatures far exceed the melting points of the cylinders and pistons. The engine remains intact because the period of peak temperature is brief compared to the combustion cycle period.

The idea of an "atomic drive" was a science-fiction cliche by the 1930's, but it appears that Stanislaw Ulam and Frederick de Hoffman conducted the first serious investigation of atomic propulsion for space flight in 1944, while they were working on the Manhattan Project (2). During the quarter-century following World War II, the U.S. Atomic Energy Commission (replaced by the Department of Energy in 1974) worked with various federal agencies on a series of nuclear engine projects with names like Dumbo, Kiwi, and Pluto, culminating in NERVA (Nuclear Engine for Rocket Vehicle Application) (3). Close to producing a flight prototype, NERVA was cancelled in 1972 (4). The basic idea behind all these engines was to heat a working fluid by pumping it through a nuclear reactor, then allowing it to expand through a nozzle to develop thrust. Although this sounds simple the engineering problems were horrendous. How good were these designs? A useful figure for comparing rocket engines is specific impulse (Isp), defined as pounds of thrust produced per pound of propellant consumed per second. The units of Isp are thus seconds. The best chemical rocket in service, the cryogenic hydrogen-oxygen engine, has an Isp of about 450 seconds (5). NERVA had an Isp roughly twice as great (6), a surprisingly small figure considering that nuclear fission fuel contains more than a million times as much energy per unit mass as chemical fuel. A major problem is that the reactor operates at a constant temperature, and this temperature must be less than the melting point of its structural materials, about 3000 K (7).

A number of designs were proposed in the late 1940's and 1950's to get around the temperature limitation and to exploit the enormous power of the atomic bomb, estimated to be on the order of 10 billion horsepower for a moderate-sized device (8). The Martin Company designed a nuclear pulse rocket engine with a "combustion chamber" 130 feet in diameter. Small atomic bombs with yields under 0.1 kiloton (a kiloton is the energy equivalent of 1000 tons of the high explosive TNT) would have been dropped into this chamber at a rate of about one per second (9); water would have been injected to serve as propellant. This design produced the relatively small Isp of 1150 seconds, and could have yielded a maximum velocity change for the vehicle of 26,000 feet/second. The vehicle would have been boosted to an altitude of 150 miles by chemical rockets, and the extra 8000 ft/sec or so thus provided would have allowed it to escape the Earth's gravity (10). The Lawrence Livermore Laboratory produced a similar although much smaller design called Helios at about the same time (11).

In a classified 1955 paper (12), Stanislaw Ulam and Cornelius Everett eliminated the combustion chamber entirely. Instead, bombs would be ejected backwards from the vehicle, followed by solid-propellant disks. The explosions would vaporize the disks, and the resulting plasma would impinge upon a pusher plate. The advantage of this system is that no attempt is made to confine the explosions, implying that relatively high-yield (hence high-power) bombs may be used. Such a system is neither temperature- nor power-limited. Ulam may have been influenced by experiments conducted at the Eniwetok proving grounds, where graphite-covered steel spheres were suspended thirty feet from the center of an atomic explosion. The spheres were later found intact; a thin layer of graphite had been ablated from their surfaces (13).

Project Orion was born in 1958 at General Atomics in San Diego. The company, now a subsidiary of defense giant General Dynamics, was founded by Frederick de Hoffman to develop commercial nuclear reactors. The driving force behind Orion was Theodore Taylor, a veteran of the Los Alamos weapons programs. De Hoffman persuaded Freeman Dyson, a theoretical physicist then at the Institute for Advanced Study in Princeton, New Jersey, to come to San Diego to work on Orion during the 1958-1959 academic year. Dyson says that Taylor adopted a specific management model for the project: the Verein fur Raumschiffahrt (VfR), the German rocket society of the 1920's and 1930's which numbered among its members Werner von Braun. The VfR had little structure: no bureaucracy and essentially no division of labor between its members; it accomplished much before it was taken over by the German army. Orion at first was similar: scientists did practical engineering and engineers built working scale models, all on a shoestring budget (14).

Taylor's specialty at Los Alamos had been the effects of atomic weapons. He was an expert at making small bombs at a time when the drive was toward ever-bigger superweapons. He was also aware of techniques for shaping explosions, for making bomb debris squirt in one particular direction. Taylor adopted Ulam's pusher-plate idea but instead of the propellant disks he combined propellant and bomb into a single pulse unit. The propellant material of choice was plastic, probably polyethylene (15). Plastic is good at absorbing the neutrons emitted by an atomic explosion (i.e. it couples well with the prompt radiation energy) and in addition it breaks down into low-weight atoms such as hydrogen and carbon which move at high speeds when agitated. There are indications that a plastic similar to Styrofoam is used inside hydrogen bombs to "channel" the energy from the atomic trigger into the fusible material (16). The advantage of the pusher plate design, as Taylor and Dyson saw it, was that it could simultaneously produce high thrust with high exhaust velocity. No other known propulsion system combined these two highly desirable features. The effective Isp could theoretically be as high as 10,000 to one million seconds (17). The calculated force exerted on the pusher plate was immense; it would have created intolerable acceleration for a manned vehicle. Therefore, a shock absorber system was placed between the plate and the vehicle itself. The impulse energy delivered to the plate was stored in the shock absorbers and released gradually to the vehicle.

The Orion workers built a series of models, called Put-Puts or Hot Rods, to test whether or not pusher plates made of aluminum could survive the momentary intense temperatures and pressures created by chemical explosives. Several models were destroyed, but a 100-meter flight in November 1959, propelled by six charges, was successful and demonstrated that impulsive flight could be stable (18). These experiments also proved that the plate should be thick in the middle and taper toward its edges for maximum strength with minimum weight (19).

The durability of the plate was a major issue. The expanding plasma bubble of each explosion could have a temperature of several tens of thousands of Kelvins even when the explosion occurred hundreds of feet from the plate. Following the lead of the Eniwetok tests, a scheme was devised to spray grease (probably graphite-based) onto the plate between blasts (20). It is not known if this scheme was retained in later versions of the Orion design. Extensive work was done on plate erosion using an explosive-driven helium plasma generator. The experimenters found that the plate would be exposed to extreme temperatures for only about one millisecond during each explosion, and that the ablation would occur only within a thin surface layer of the plate (21). The duration of high temperatures was so short that very little heat flowed into the plate; active cooling was apparently considered unnecessary. The experimenters concluded that either aluminum or steel would be durable enough to act as plate material.

The Orion workers realized early that the U.S. government had to become involved if the project was to have any chance of progressing beyond the tinkering stage. Accordingly, the Advanced Research Projects Agency (ARPA - later DARPA with "D" standing for "Defense") was approached in April1958. In July, it agreed to sponsor the project at an initial funding level of $1 million per year; it was at this time that the code name of Orion was assigned (22). Work proceeded under ARPA order 6, task 3, entitled "Study of Nuclear-Pulse-Propelled Space Vehicles" (23).

Taylor and Dyson were convinced that the approach to space flight being pursued by NASA (which had just been created in January 1958) was the wrong one. Von Braun's chemical rockets in their opinion were very expensive, had very limited payloads, and were essentially useless for flights beyond the moon (24). The Orion workers wanted a spaceship that was simple, rugged, capacious, and above all affordable. Taylor originally called for a ground launch, probably from the U.S. nuclear test site at Jackass Flats, Nevada (25). The vehicle has been described as looking like a bishop's miter or the tip of a bullet, sixteen stories high and with a pusher plate 135 feet in diameter (26). Intuitively it seems that the bigger the pusher plate, the more efficiently the system would perform. For a derivation of a formula for the effective specific impulse of a nuclear-pulse rocket and for the relations between pusher plate diameter, pulse energy, and Isp, the reader should consult Reynolds (27). The launch pad would have been composed of eight towers, each 250 feet high. The mass of the vehicle on takeoff would have been on the order of 10,000 TONS (28); most of this mass would have gone into orbit. The bomb units ejected on takeoff would have yielded 0.1 kiloton; initially the ejection rate would have been one per second. As the vehicle accelerated the rate would slow down and the yield would increase until 20-kiloton bombs would have been going off every ten seconds (29). The idea seems to have been for the vehicle to fly straight up until it cleared the atmosphere so as to minimize radioactive contamination.

At a time when the U.S. was struggling to put a single man into orbit aboard a modified military rocket, Taylor and Dyson were developing plans for a manned voyage of exploration through much of the solar system. The original Orion design called for 2000 pulse units, far more than enough to attain Earth escape velocity. "Our motto was 'Mars by 1965, Saturn by 1970'", recalls Dyson (30). Orion would have been more akin to the rocket ships of science fiction than to the cramped capsules of Gagarin and Glenn. One hundred and fifty people could have lived aboard in relative comfort; the useful payload would have been measured in thousands of tons (31). Orion would have been built like a battleship, with no need for the excruciating weight-saving measures adopted by chemically-propelled spacecraft. It is unclear how the vehicle would have landed; it is reasonable to assume that specialized chemically-powered craft would have been used for exploration. Taylor may have anticipated that a conventional Space Shuttle-type vehicle would have been available to transport people to and from orbit. Dyson gives the astounding figure of $100 million per year as the cost of the proposed twelve-year program (32); surely this does not include development costs for the thousands of items from spacesuits to scientific instruments that such a program would require. The Orion program would have most likely "piggybacked" on the military weapons programs and the existing civilian space projects. Still, even if Dyson underestimated the cost by a factor of 20, the revised total would have been only $24 billion, roughly the same as the accepted cost for the Apollo program.

The times were changing, however. The fledgling space administration began to acquire all civil-oriented space projects run by the federal government; the Air Force got all projects with military applications. ARPA was left with Orion as its only space project, for two reasons. The Air Force felt that Orion had no value as a weapon, and NASA had made a strategic decision in 1959 that the civilian space program would be non-nuclear for the near future (33). NASA was and is a very publicity-conscious organization, and it is hard to overcome the negative perception of atomic devices of any kind on the part of the public. In addition, NASA was filled with engineers who had spent their careers building ever-larger chemical rockets and either did not understand or were openly opposed to nuclear flight. In this situation the Orion workers were truly outsiders.

A crisis came in late 1959, when ARPA decided it could no longer support Orion on national-security grounds. Taylor had no choice but to approach the Air Force for funds. It was a hard sell. A common reaction from both military and civilian officials is displayed by the quote: "...you set off one big bomb and the whole shebang blows up."(34) The Air Force finally decided to take on Orion, but only on the condition that a military use be found for it. Dyson says that his Air Force contacts, although sympathetic to the goal of space exploration, felt that their hands were tied (35). One immediate result of the change of management was that all model flight testing was stopped (36). The freewheeling era was over; Taylor's dream of a company of "men of goodwill" exploring the solar system had died.

One can imagine that Orion could be used as a weapon platform, in a polar orbit so that it would eventually pass over every point on the Earth's surface. It could also protect itself easily, at least against attacks by small numbers of missiles. However, this idea has the same disadvantages as the early bomb-carrying satellite proposals. Terminal guidance would have been a problem (assuming that hardened, high-value installations were the intended targets), since the technology for steering missile warheads accurately had not yet been developed. Both the U.S. and the Soviet Union were deploying missiles that were capable of reaching their targets in fifteen minutes with multi-megaton warheads, making orbiting bomb platforms irrelevant.

Robert McNamara, Defense Secretary under the Kennedy Administration, realized that Orion was not a military asset. His department consistently rejected any increase in funding for the project, effectively limiting it to a feasibility study (37). Taylor and Dyson knew that another money source had to be found if a flyable vehicle was to be built. NASA was the only remaining option. Accordingly, Taylor and James Nance, a General Atomics employee and later director of the project, made at least two trips to Marshall Space Flight Center (MSFC) in Huntsville, Alabama (38). MSFC was von Braun's domain and it was where most of NASA's space propulsion research and development took place. Von Braun was hard at work on the Saturn project, which NASA had inherited from the old Army Ballistic Missile Agency. The Saturn V would eventually transport men to the moon. The Orion workers had produced a new, "first generation" design that abandoned ground launch and instead would have been boosted into orbit as a Saturn V upper stage. The core of the vehicle was a 200,000-pound "propulsion module" with a pusher-plate diameter of 33 feet, limited by the diameter of the Saturn. This design limitation also restricted Isp to from 1800 to 2500 seconds (39). While disappointingly low by nuclear- pulse standards, this figure still far exceeded those of other nuclear rocket designs. The shock absorber system had two sections: a primary unit made up of toroidal pneumatic bags located directly behind the pusher plate, and a secondary unit of four telescoping shocks (like those on a car) connecting the pusher plate assembly to the rest of the spacecraft (40).

How many Saturn V's would have been required to put this vehicle into orbit? Dyson says one or two (41); a simple inspection of published drawings indicates at least two, possibly three if the crew module (with crew aboard) was intended to be flown separately (42). In this case, some assembly would have been done on-orbit. Several mission profiles were contemplated; the one developed in greatest detail appears to have been a Mars flight. Eight astronauts, with around 100 tons of equipment and supplies, could have made a round trip to Mars in 125 days (43); most modern plans call for one-way times of at least nine months. Another impressive figure is that as much as 45% of the gross vehicle weight in Earth orbit could have been payload (44). Presumably the flight would have been made when Mars was nearest to the Earth; still, so much energy was available that almost the fastest-possible path between the planets could have been chosen. Inspection of the drawings indicates that a lander may also have been carried.

What about the cost? Pedersen's 1964 estimate of $1.5 billion for the project (45) suggests the superior economics of nuclear pulse spaceships. Dyson felt that Orion's appeal was greatly diluted by the chemical booster restriction: the Saturns would have represented over 50% of the total cost (46).

Von Braun became an enthusiastic Orion supporter, but he was able to make little headway among higher-level administration officials. In addition to the general injunction against nuclear power, very practical objections were raised: what if a Saturn bearing a propulsion module with hundreds of bombs aboard should explode? Was it possible to guarantee that not a single bomb would explode or even rupture? NASA's understandable fear of a public-relations disaster contributed to its reluctance to provide money (47); however, its Office of Manned Spaceflight was sufficiently interested to fund another study (48).

A hammer blow was delivered in August 1963 with the signing of the nuclear test-ban treaty by the U.S., U.K., and U.S.S.R. Orion was now illegal under international law. Yet the project did not die immediately. It was still possible that an exemption could be granted for programs that were demonstrably peaceful. Surely the treaty reduced Orion's political capital even further, though. Yet another problem was that, because Orion was a classified project, very few people in the engineering and scientific communities were aware of its existence. In an attempt to rectify this, Nance (now managing the project) lobbied the Air Force to declassify at least the broad outline of the work that had been done. Eventually it agreed, and Nance published a brief description of the "first generation" vehicle in October 1964 (49).

The Air Force, meanwhile, had become impatient with NASA's temporizing. It was willing to be a partner but only if NASA would contribute significant funds. Hard-pressed by the demands of Apollo, NASA made its decision in December 1964 and announced it publicly the following month: no money would be forthcoming (50). The Air Force then anounced the termination of all funding, and Orion quietly died. Some $11 million had been spent over nearly seven years (51).

Overshadowed by the moon race, Orion was forgotten by almost everybody except Freeman Dyson and Theodore Taylor. Dyson in particular seems to have been deeply affected by his experience. The story of Orion is important, he says, "...because this is the first time in modern history that a major expansion of human technology has been suppressed for political reasons"(52). His 1968 paper (53) gives more physical details of nuclear pulse drives, and even suggests extremely large starships powered by fusion explosions. Ultimately he became disillusioned with the concept, primarily because of the radiation hazard associated with the early ground-launch idea. Yet he says that the most extensive flight program envisaged by Taylor and himself would have added no more than 1% to the atmospheric contamination then (circa 1960) being created by the weapons-testing of the major powers (54).

Does it make any sense to even think of reviving the nuclear-pulse concept? Economically the answer is yes. Pedersen (55) says that 10,000-ton spaceships with 10,000-ton payloads are feasible. Spaceships like this could be relatively cheap compared to Shuttle-like vehicles due to their heavyweight construction. One tends to think of shipyards with heavy plates being lowered into place by cranes. How much would the pulse units cost? Pedersen gives the amazingly low figure of $10,000 to $40,000 per unit for the early Martin design (56); there is reason to think that $1 million is an upper limit (57). Primarily from strength of materials considerations, Dyson (58) argues that 30 meters/second (about 100 feet/second) is the maximum velocity increment that could be obtained from a single pulse. Given that low-altitude orbital velocity is about 26,000 feet/second, around 350 pulses would be required (59). Using $500,000 as a reasonable pulse-unit cost, this implies a "fuel cost" of $175 million, cheaper than a Shuttle launch. Whereas the Shuttle might carry thirty tons of payload, the pulse vehicle would carry thousands. If one uses the extreme example of spending $5 billion to build a vehicle to lift 10,000 tons (or 20 million pounds) to orbit, the cost if spread over a single flight is $250 per pound, far cheaper than the accepted figure of $5,000 to $6,000 per pound for a Shuttle flight.

Efficiency improvements could be made by improving the design of the pulse units. Considerable progress has been made in nuclear bomb design over the past thirty years. Neutron bombs, for instance, demonstrate that it is possible to change the form of the energy emitted by the explosion. Recent work on X-ray lasers bears on the important problem of shaping the explosion into a beam. Yet it is impossible to prevent the formation of radioactive fission fragments. For a ground launch, choosing a very remote site such as a floating platform in the extreme southern Atlantic or Pacific would minimize the radiation hazard to humans. The chemical-rocket imperative to launch as close to the equator as possible disappears when such an abundance of energy is available. Even this might be judged environmentally unacceptable, though; perhaps ANY release of radiation into the atmosphere is wrong. In this case the option of a space launch remains open. Even this has been criticized on the grounds that it would leave a radioactive debris trail in space. However, interplanetary space is a very dangerous environment to begin with, being periodically saturated with fast charged particles from solar flares and with extremely energetic cosmic rays occasionally blasting through. The notion that the bomb debris would form a trail is challenged by the fact that the velocity of most of the debris would exceed solar escape velocity (60).

Although the Saturn V no longer exists, U.S. engineers are currently studying several heavy-lift systems. Given the recent reduction in world tensions, even the Russian Energia could be considered. Russian nuclear scientists, unemployed after the Cold War, might collaborate with Americans on nuclear-pulse space projects. Fast flights to the planets might be made in ten years or less, at reasonable expense, instead of thirty to fifty years.

Unfortunately, the Orion concept is inherently "dirty" because it uses fission fuel. It is also inefficient; this is acceptable only because of the vast amounts of energy available. A much better alternative is fusion, since a fusion rocket would not leave a wake of heavy radioactive ions. The British Interplanetary Society's Daedalus project (61) was a study of an unmanned interstellar probe. It would have been driven by fusion "microexplosions" caused by irradiating fuel pellets with electron beams at pulse rates up to 250 Hz, in a magnetic "combustion chamber". Confinement and shaping of the plasma with a magnetic field would make Daedalus vastly more efficient than Orion. Daedalus would work just as well in the solar system as between the stars, and one can imagine that in 75 to 100 years fusion freighters will be sailing regularly between the planets. An important point is that no one has yet produced controlled fusion energy with electron beams or anything else, while the technology required to build an Orion-type spaceship has existed for over thirty years. Nuclear propulsion will get into space eventually. Orion might be the device that makes possible human occupation and economic exploitation of the solar system.


Notes

1.		Erik S. Pedersen, Nuclear Propulsion in Space (Englewood Cliffs, NJ: 
		Prentice-Hall Inc.,1964), p. 275.

2.		William R. Corliss, Nuclear Propulsion for Space (U.S. Atomic Energy
		Commission: Division of Technical Information, 1967), p. 11.

3.		Corliss, pp. 1-16.

4.		James A. Dewar, "Project Rover: The United States Nuclear Rocket Program",
		in History of Rocketry and Astronautics (John L. Sloop ed. - San Diego:
		American Astronautical Society Publications Office, 1991), p. 123.

5.		"Specific impulse", article in McGraw-Hill Encyclopedia of Science and 
		Technology, vol. 17, p. 204.

6.		"Specific Impulse", p. 204

7.		Corliss, pp. 13-14.

8.		Pedersen (p. 276) gives 4.2 x 1019 ergs per kiloton; exploding one such
		bomb per second yields 4.2 x 1012 joules / sec (i.e. watts) or roughly
		5 billion average horsepower.

9.		Pedersen, p. 276.

10.	Pedersen, p. 276.

11.	Eugene Mallove and Gregory Matloff, The Starflight Handbook (New York:
		John Wiley and Sons, 1989),  p. 60.

12.	Mallove and Matloff, p. 61.

13.	John McPhee, The Curve of Binding Energy (New York: Farrar, Straus and
		Giroux, 1974), pp. 167-168

14.	Freeman Dyson, Disturbing the Universe (New York: Harper and Row, 1979),
		pp. 109-110.

15.	Mallove and Matloff, p. 63.

16.	The Ground Zero Fund, Inc., Nuclear War: What's In It for You? (New York:
		Simon and Schuster Inc., 1977), p. 27.

17.	Mallove and Matloff, pp. 60-61.

18.	Dyson, Disturbing, p. 113

19.	McPhee, p. 175.

20.	McPhee, p. 175

21.	J.C. Nance, "Nuclear Pulse Propulsion", IEEE Transactions on Nuclear
		Science (Feb. 1965), p. 177.

22.	McPhee, p. 170.

23.	DARPA letter to the author dated October 7th, 1992.

24.	Dyson, Disturbing, pp. 109-110.

25.	McPhee, pp. 173-174.

26.	McPhee, pp. 173-174.

27.	T.W. Reynolds, "Effective Specific Impulse of External Nuclear Pulse
		Propulsion Systems", Journal of Spacecraft and Rockets 10 (Oct. 1973),
		pp. 629-630

28.	The volume of a cone 200 feet high with a base diameter of 135 feet (the 
		approximate dimensions of the proposed Orion vehicle) is about 1.5 million 
		cubic feet.  If the average density is 10 pounds per cubic foot (about 1/6
		that of water) the weight is 15 million pounds or 7500 tons.

29.	McPhee, pp. 173-174.

30.	McPhee, pp. 180-181.

31.	McPhee, p. 158.

32.	Dyson, Disturbing, p. 111.

33.	Dyson, Disturbing, p. 113.

34.	Nance, p. 177.

35.	Freeman Dyson, "Death of a Project", Science (9 July 1965), pp. 141-144.

36.	Dyson, Disturbing, p. 113.

37.	Dyson, "Death", p. 142.

38.	McPhee (p. 183) says that Taylor traveled to MSFC in 1961; Dyson
		("Death", p. 142) says that Taylor and Nance established relations with
		MSFC management in 1963.

39.	Nance, pp. 181-182.

40.	Nance, p. 182.

41.	Dyson, Disturbing, p. 115.

42.	Nance, p. 182.

43.	Dyson, "Death", pp. 141-142.

44.	Nance, p. 179.

45.	Pedersen, p. 276.

46.	Dyson, "Death", pp. 141-142.

47.	Dyson, "Death", pp. 143-144.

48.	Dyson, "Death", pp. 143-144.

49.	Dyson, "Death", pp. 143-144.

50.	Dyson, "Death", p. 142.

51.	Dyson, "Death", p. 144.

52.	Mallove and Matloff, p. 61.

53.	Freeman Dyson, "Interstellar Transport", Physics Today (Oct. 1968),
		pp. 41-45.

54.	Dyson, Disturbing, p. 114.

55.	Pedersen, p. 275.

56.	Pedersen, p. 276.

57.	Kenneth A Bertsch and Linda S. Shaw, The Nuclear Weapons Industry
		(Washington D.C.: Investor Responsibility Research Center, 1984),
		on p. 55 state that warheads for 560 ground-launched cruise missiles 
		were expected to cost $630 million.  Not only were these military
		weapons but they were quite likely fusion devices as well and so would
		be significantly more expensive than simple fission bombs.

58.	The figure of 350 pulses was arrived at as follows: if the net 
		acceleration during the initial vertical phase is about 2 g's,
		about 100 pulses are required to reach an altitude of 60 miles (at
		an average of one pulse per second).  The velocity at this height is
		about 6400 ft/sec.   If the spaceship then performs an attitude
		correction and accelerates to orbital velocity at about 3 g's, roughly
		260 pulses are required, at which time the altitude is roughly 300 miles.
		This is a very crude estimate and the actual number of pulses might be
		much lower.

59.	Dyson, "Interstellar", p. 44.

60.	McPhee, pp. 167-168.

61.	British Interplanetary Society, Project Daedalus (London: British
		Interplanetary Society Ltd., 1981)


References

	Bertsch, Kenneth A. and Shaw, Linda S. The Nuclear Weapons Industry
	(Washington D.C.: Investor Responsibility Research Center, 1984)

	British Interplanetary Society, Dr. A.R. Martin ed. Project Daedalus
	(London: British Interplanetary Society Ltd., 1981)

	Corliss, William R. Nuclear Propulsion for Space (U.S. Atomic Energy
	Commission: Department of Technical Information, 1967)

	Dewar, James A. "Project Rover: The United States Nuclear Rocket Program",
	in History of Rocketry and Astronautics, John L Sloop ed. (San Diego:
	American Astronautical Society Publications Office, 1991)

	Dyson, Freeman	"Death of a Project", Science 9 July 1965

	___.		Disturbing the Universe (New York: Harper and Row, 1979)

	___. 		"Interstellar Transport", Physics Today  October 1968

	Ground Zero Fund Inc., The Nuclear War: What's In It for You?
	(New York: Simon and Schuster Inc., 1977)

	Letter from Defense Advanced Research Projects Agency to the author,
	dated October 7th, 1992

	Mauldin, John Prospects for Interstellar Travel (San Diego: American
	Astronautical Society Publications Office, 1992)

	McPhee, John The Curve of Binding Energy (New York: Farrar, Straus and
	Giroux, 1974)

	Pedersen, Erik S. Nuclear Propulsion in Space (Englewood Cliffs, NJ:
	Prentice-Hall, Inc., 1964)

	Reynolds, T.W. "Effective Specific Impulse of External Nuclear Pulse
	Propulsion Systems", Journal of Spacecraft and Rockets 10  October 1973)

	"Specific Impulse", article in The McGraw-Hill Encyclopedia of Science
	and Technology, 6th ed., vol. 17 (New York: McGraw-Hill Inc., 1987)

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