Nuclearpulse Propulsion Orion Daedalus And Medusa

NEP may never be capable of true interstellar flight. However, the same cannot be said for nuclear-pulse propulsion (NPP), which is illustrated in Figure 6.3.

Project Orion - probably the first serious nuclear-pulse proposal - began its life in the early Space Age as a secret USAF project. It was later incorporated in NASA plans for the first Moon-landing missions, in case the Saturn V failed. Before its cancellation in the mid-1960s, several Orion prototypes were tested in the Earth's atmosphere using chemical (not nuclear) charges. An Orion prototype is on display in the Smithsonian Air and Space Museum, and a photographic sequence of an Orion prototype in flight appears in The Starflight Handbook. After the project's cancellation, Freeman Dyson published (in 1968) a design for an interstellar Orion.

It is perhaps just as well that the original Earth-launched Orion was never constructed, because it would have been an environmental nightmare. NPP should be used only in space, hopefully far from any inhabited planet.

In the basic Orion concept, the payload and fuel tank are separated from a combustion chamber (or pusher plate) by enormous shock absorbers. The reason for this separation (which from the crew's point of view should be as large as

Project Orion

Fuel tank

Combustion chamber of pusher plate Shock absorbers

Project Orion

Fuel tank

Payload

Direction of spacecraft acceleration

Payload

Direction of spacecraft acceleration

Exhaust from exploding nuclear 'device'

Project Daedalus

Payload

Laser or electron beams

Project Daedalus

Laser or electron beams

Medusa Nuclear Propulsion

Exhaust from exploding fusion micropellet

Payload

Combustion chamber

Exhaust from exploding fusion micropellet

Combustion chamber

Direction of spacecraft acceleration Figure 6.3. Two approaches to nuclear-pulse propulsion.

possible) is the nature of the fuel. This is not simply a nuclear reactor; the fuel consists of nuclear bombs (often called 'devices' to sanitise the concept). To prevent its evaporation, the pusher plate would be coated with an ablative material. Unless shaped nuclear charges are applied, no more than half the bomb exhaust actually interacts with the pusher plate or combustion chamber.

With kiloton-sized fission devices, a Project Orion vehicle launched from Earth might have an exhaust velocity as high as 200 km s-1, and much greater thrust than a NEP propelled spacecraft. The Orion pusher plate assembly would also be considerably less massive than a NEP reactor.

With Dyson's (1968) interstellar Orion, the spacecraft is much larger and can operate only in space. The kiloton-sized fission devices are replaced by 1-megaton hydrogen bombs. These operate by the fusion of deuterium atoms, which are heated by energy released from a small fission 'trigger'.

Without violating national security protocols, Dyson (1968) was unable to accurately predict performance of his interstellar Orion. He did, however, conclude that for a mass ratio of 4, if fuel is used for both acceleration and deceleration, the terminal (interstellar cruise) velocity of a thermonuclear Orion is between 0.0035 c and 0.035 c. If all of the hydrogen bombs in humanity's arsenals were devoted to the task, small populations could be transferred to a Centauri on voyages of 100-1,000-year duration. What a lovely use for the bombs!

Exercise 6.4. Calculate the exhaust velocity range for Dyson's (1968) interstellar thermonuclear Orion. First double the interstellar cruise velocity to obtain the total velocity increment (since the spacecraft must both accelerate and decelerate); then apply equation (4.4) to determine the range of exhaust velocities. From the value for the fusion mass/energy conversion fraction in Table 6.1 and equation (6.3a), calculate the range for enf, the fraction of nuclear energy transferred to the kinetic energy of the nuclear exhaust stream (also called the burn fraction).

In an effort to reduce the radioactive emissions from an Orion spacecraft, Winterberg has investigated the possibility of triggering a thermonuclear explosion by means other than a fission device. In 1977 he considered igniting a deuteriumtritium fusion reaction with chemical explosives. In an earlier 1971 contribution he had considered another mode of igniting a fusion reaction - intense, relativistic electron beams. This earlier contribution led directly to Project Daedalus, a starship study conducted by the British Interplanetary Society during the 1970s and 1980s (Bond et al., 1978).

Instead of megaton-sized H-bombs, Daedalus uses fuel micropellets of fusable isotopes. The most easily ignitable fusion-fuel combination is a mixture of deuterium and tritium, each of which are heavy isotopes of hydrogen. This fuel combination was rejected by the Daedalus design team because much of the fusion energy is released in the form of thermal neutrons, which result in nuclear radiation after absorption in the combustion chamber walls.

The Project Daedalus fuel combination of choice was a mixture of deuterium and helium-3, a light isotope of helium. Deuterium is rather common in nature, and this isotope mix produces many fewer neutrons than the deuterium-tritium mix. A drawback is the extreme rarity of helium-3 in the terrestrial environment.

Helium-3 is, however, found in the solar wind and atmospheres of the giant planets, and Daedalus designers considered a number of options to obtain the millions of kilogrammes required for a full-scale starprobe or starship. These included strip-mining the upper layers of lunar soil (where the solar wind has deposited small concentrations of this isotope), producing helium-3 on Earth, mining the solar wind directly and mining the atmospheres of the outer planets.

The option selected was to insert robotic helium-mining packages below balloons suspended in the jovian atmosphere. Periodically, the separated helium isotope would be rocketed to fuel-processing stations orbiting Jupiter. Obtaining fuel for Daedalus would indeed be a major task!

The Orion pusher plate would be replaced by a combustion chamber in which electromagnetic (EM) fields would reflect the high-velocity charged particle exhaust from the fusion reaction. Calculations revealed that the exhaust velocity of a Daedalus probe would be as high as 0.03 c (about 9,000 km s-1).

Exercise 6.5. Assuming an exhaust velocity of 0.03 c, estimate the effective burn fraction (enf) for a Daedalus probe. For an unfuelled spacecraft mass of 106 kg and an (undecelerated) interstellar cruise velocity of 0.1 c, how much fusion fuel is required? If 50% of the fuel mass is helium-3, how much of this rare isotope is required to propel this interstellar spacecraft?

As Hyde et al. (1972) discussed, fusion micropellets can be compressed and ignited by electron beams. Such 'inertial' fusion reactors are already in operation in defence laboratories, simulating (on a small scale) thermonuclear explosions. The Daedalus team instead adopted Winterberg's suggestion of electron-beam ignition.

A number of Daedalus follow-on concepts exist. We might consider lithiumproton or boron-proton fusion reactions. Although more difficult to ignite, these reactions use very common reactants and are aneutronic. In 1977, Winterberg suggested staged microexplosions in which a small helium-3-deuterium pellet ignites a larger boron-proton or lithium-proton pellet after it is ignited by electron or laser beams. (See M. L. Shmatov (2000) for a review of staged fusion microexplosion literature.)

But like Orion, Daedalus would be huge. Both a 10-kg and a 100,000-kg payload would probably require spacecraft with unfuelled masses in the multi-million-kg range. A surprising concept which might greatly reduce spacecraft mass is Solem's (1993) 'Medusa', which combines elements of NPP and solar sail propulsion. Medusa replaces the massive combustion chamber/pusher plate with a gossamer canopy joined to the payload by high-tensile strength cables. Calculations revealed that if the canopy is sufficiently strong and radiation-resistant, it can withstand the nearby ignition of micropellets or even small nuclear devices. If a low-mass field generator could be implaced in this canopy, charged particle Daedalus exhaust can be reflected with little or no canopy degradation.

Medusa would move through space in a manner analogous to a jellyfish's transit through the ocean - thus the inspiration for the name. Although much analysis remains to be carried out, the Medusa concept might allow great reduction in the mass of a nuclear-pulse starprobe.

Genta and Rykroft (2003) have discussed a proposal of the Italian Nobel laureate Carlo Rubbia that has certain commonalities with NPP. Rubbia's proposed nuclear rocket would use fission fragments from the reaction of americium-242 to heat hydrogen gas that would be expelled at an exhaust velocity as much as 55kms_1.

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