The vortex separation method requires a system of cylindrical cavities in the reactor core (5), each cavity containing a vortex flow field. The gaseous mixture of propellant and fissionable material is injected at the periphery of the tubes with a large ratio of tangential to radial velocity. Ideally, the mixture spirals into the center to be drawn off axially and discharged through one end. Under the enormous centripetal acceleration possible within such flows, the heavy fuel molecules experience an outward diffusion with respect to the lighter propellant.

Vortex flows can be generated hydrodynamically, by an array of tangentially directed jets at the tube periphery, or magnetohydrodynamically, by externally imposed electric and magnetic fields. Theoretical analyses of the hydrodynamically induced vortex approach have been performed by several investigators (5-7). A detailed calculation (5) based on the assumption of an inviscid vortex which includes the volume heating by the fissionable species, indicates that rather high concentrations of fuel (10~1018 nuclei/cm3) can be retained within the tube while considerably lower concentrations (~101) appear in the vortex core and are lost in the propellant exhaust.

The realization of the vortex separation scheme awaits the production of a vortex flow with large tangential velocities but low radial mass flows. Preliminary experiments (8–10) have revealed a number of difficulties in obtaining suitable flows with even a single gas :

The tangential velocity of a viscous gas does not follow the ideal inverseradius law; the departure from the ideal increases with decreasing radial mass flow rates.

The flow may be turbulent, giving rise to shear stresses many times greater than the viscous stresses.

The tangential flow of the vortex results in a boundary layer at each endwall of the vortex tube.

At least at low Reynolds' numbers, axial velocities in the vortex core can be large compared to the local radial velocities.

Two sources are thought to be responsible for the turbulence in the flow: the turbulent boundary layer on the cylindrical walls and the tangentially directed inlet jets that produce the flow. This turbulence might prevent steep concentration gradients in the fuel distribution.

In spite of these complications, the experiments have produced a modest vortex (with maximum tangential mach number of about 1.5) with fairly low radial flow rates and with turbulence levels of less than 1% (10). The separation experiments reported (8, 9) suggest some enrichment (~ a factor of 2 between maximum and vortex center) of the heavy species is possible, but the very large amounts required (at least 100 to 1) for the vortex reactor have yet to be demonstrated.

The idea of an MHD mechanism to drive the vortex has been suggested several times but (11), very little study has been devoted to the problem. The scheme sometimes proposed (but not yet analyzed) is a rotating magnetic field that would drive an ionized fissionable species in a vortex motion. An alternative scheme (12-14) is a radial electric field between a pair of cylindrical electrodes combined with an axial magnetic field. An ionized species contained within the cylindrical annulus would be subjected to a Lorentz force causing it to rotate. An idealized treatment of the hydrodynamic problem which includes the viscous forces (but does not include the separation phenomenon) has been reported recently (13). A sample calculation for a pair of infinite, concentric electrodes 100 and 10 centimeters in radii, with a magnetic field of 100 gauss, a total current of 10 amperes per centimeter, an average viscosity of 7×10-* gm/cm sec, and an average gas temperature of 2,400° Kelvin, indicates that a maximum tangential mach number of about 8 can be achieved. The results of this preliminary analysis encourage further study.

Preliminary attempts to explore magnetic field containment of fissioning plasmas have been directed to cylindrical magnetic mirror configurations. The plasma of fissionable material is preferentially trapped in a central region by a magnetic field because of its relatively low ionization potential. The space around the plasma is occupied by the essentially neutral propellent gas. The fission energy is first deposited in the plasma by atomic collisions as the fission fragments slow down, and subsequently ejected by the plasma as thermal radiation, which is attenuated by the blanket of propellant.

The factors that determine the effectiveness of the scheme as a separation device include the particle density in the plasma, its temperature, the magnitude

and the mirror ratio of the magnetic fields, and the nuclear requirements of the reactor. From an engineering point of view, the objective is to minimize the weight by having a high fuel (plasma) concentration and small cavity size. However, the prolellant must be optically thick if it is to absorb the bulk of the fission energy; this means a large cavity. Also, for efficient magnetic confinement, the plasma density should be low.

Unfortunately the very simple configurations selected for the first analyses do not appear attractive because these various conflicting requirements lead to systems with slightly outlandish characteristics.

Estimates of confinement times with representative figures (14, 15) indicate that plasma leakage by radial diffusion across the magnetic field is not the crucial problem. The problem of axial confinement appears much more difficult. Estimates of the mirror loss rate indicate that the entire plasma would escape in milliseconds.

Apparently the simplest scheme is not feasible. Perhaps the most promising course at this point would be to relax the requirement of high fuel density in the plasma and consider systems which carry additional fuel elsewhere in solid form, as in the vortex reactors discussed earlier.

BIBLIOGRAPHY

1. H. S. Tsien. "The Science and Engineering of Nuclear Power,” vol. 2, p. 177 (Addison-Wesley Press, Inc., 1949)

2. R. V. Meghreblian, "Gaseous Fission Reactors for Spacecraft Propulsion," J. P. L. Technical Report 32–42, (July, 1960)

3. R. V. Meghreblian, "Gaseous Fission Reactors for Booster Propulsion,” J. P. L. Technical Report 32–56, (March, 1961)

4. R. W. Bussard, R. DeLauer, “Nuclear Rocket Propulsion,” p. 322 (McGrawHill Book Co., Inc., New York, 1958)

5. J. L. Kerrebrock, R. V. Meghreblian, “An Analysis of Vortex Tubes for Combined Gas-Phase Fission Heating and Separation of the Fissionable Material," ORNL-CF 57-11-3 (1957); J. L. Kerrebrock, R. V. Meghreblian. Vortex containment for the gaseous fission rocket, J. Aero/Space Sci. (in press)

6. J. Grey, "A Gaseous-Core Nuclear Rocket Utilizing Hydrodynamic Containment of Fissionable Material," preprint 848–59 (Am. Roc. Soc., San Diego, 1959)

7. R. G. Deissler, M. Perlmutter, "An Analysis of the Energy Separation in Laminar and Turbulent Compressible Vortex Flows" (Univ. of California, Berkeley, Calif., 1958)

8. J. J. Keyes, Jr., "An Experimental Study of Flow and Separation in Vortex Tubes with Application to Gaseous Fission Heating,” paper presented at 15th meeting of Am. Roc. Soc. (Washington, D.C., December, 1960) ; J. J. Keyes, Jr., R. E. Dial, “An experimental study of vortex flow for application to gas-phase fission heating," ORNL-2837 (April, 1960)

9. R. G. Ragsdale, “Analysis and Experiment on an Air-Bromine Vortex System," NASA D-288 (1960)

10. J. M. Kendall, Jr., "Vortex Separation Experiments," J. P. L. Research Summary 36–3, vol. 1, part 2, p. 31 (California Institute of Tech., 1960); “Experimental Study of a Compressible Viscous Vortex with Applications to Gaseous Separation,” J. P. L. Technical Release 34–168.

11. J. Grey. Gaseous-core nuclear rockets, J. Am. Rocket Society 4, 23 (1959) 12. G. V. Gordew, A. I. Gubanov. Acceleration of plasma in a magnetic field, Soviet Phys. JETP 3, 1880 (1958)

13. C. S. Wu, "Transient Characteristics of a Rotating Plasma," J. P. L. Technical Release 34-122 (October, 1960)

14. S. T. Nelson, “The Plasma Core Reactor," GM 60–7630-2-9 Space Tech. Lab. (June 1960)

15. H. Wahlquist, Research Summary 36-6, vol. 2 (Jet Propulsion Lab., Dec., 1960)

THRUST FROM NUCLEAR EXPLOSIONS

(By J. W. Eerkens,1 University of California, Berkeley, Calif.)

The ultimate vehicle for space travel may be one propelled by the periodic explosion of small nuclear bombs (or bomblets). Such a scheme would make feasible gigantic spaceships in the same weight class as the NS Savannah (displacement weight=11,000 tons) and with a similar "cargo" capacity (9,000 tons). With these space "liners" many people could travel in comparative comfort on long voyages taking with them complete laboratories.

Pulsed-thrust system studies have been the subject of two major projects— Project Orion, in being at General Atomics since 1958, and Martin's NuclearBomb Pulsed Rocket studies. The table gives typical parameters for three different Martin models; parameters for the Orion vehicle are of the same order of magnitude.

The principle of operation of an Orion vehicle (see figure) is as follows: small nuclear bombs (~0.01 kiloton) are dropped at the rate of 0.1-1 per sec from the vehicle and exploded at distances of 100-1,000 ft. The bomb is loaded with propellant; when exploded it imparts its energy to this propellant (e.g., plastics or other low-molecular-weight materials). The energized propellant propagates from the explosion center to the vehicle where it hits a pusher plate that propels the vehicle. The pusher plate is made of ablative material (to absorb thermal and impact shocks) and is connected to the vehicle by watercooled springs.

2

The Martin proposal is similar in concept except that the explosion takes place in a 130-ft dia spherical explosion chamber connected to a nozzle and water is injected into the chamber for propellant. In the Martin scheme, before starting the nuclear-bomb propulsion, the vehicle would first be boosted by a chemical rocket cluster to a 150-mile altitude at 8,000 ft/sec.

1 Present address: Aerospace Corp., Los Angeles, Calif.

2 Aviation Week, p. 34 (Jan. 25, 1960).

Alternatively the vehicle could be launched directly from the earth solely with bomb propulsion. Orion calculations conclude that contamination of the atmosphere for this kind of operation would be quite small. The first phase of propulsion to achieve orbit or escape would last only a few minutes during which occupants would experience a series of mild 2-9 g pushes. With its remaining supply of propulsion bomblets the spaceship could roam around the solar system almost at will.

The feasibility of bomb propulsion has been checked out by “Project Put Put" a 3-ft-dia 300-lb scale model of the Orion vehicle in which 3-lb highexplosive charges simulated the nuclear explosions. Because of the international moratorium on nuclear tests, no plans have been made for trials with nuclear charges. The total development costs for an operational Orion vehicle have been estimated at $1.5 billion.

Project Orion was given special attention earlier this year by an ad hoc committee on space headed by Jerome Wiesner, President Kennedy's special adviser on science (see p. 187). "Above all," the committee said in discussing U.S. space policy, “we must encourage entirely new ideas which might lead to real breakthroughs. One such idea is the Orion proposal. * * * This proposal should receive careful study with a realization of the international problems associated with such a venture."

Officials at Los Alamos, where the idea was born, said recently that Orion looks "very promising"; they asserted that a flight-test vehicle could be ready "in 5 years."

General Atomic's contract for Orion started in 1958 with an initial $2.5 million covering roughly the first 2 years of work; subsequently the project has received an additional $1.7 million covering roughly the present fiscal year.

Harold B. Finger, Manager, AEC-NASA Space Nuclear Propulsion Office and Assistant Director for Nuclear Applications (NASA)

Any time now, an attempt will be made to launch a manned Mercury capsule on a ballistic trajectory from Cape Canaveral. We will not be first. It is clear that in terms of manned space flight, the Russians lead us. There are other areas of science, however, in which we were not first and about which no real concern was expressed. For example, an Englishman discovered penicillin, an Italian invented the telegraph, an Englishman climbed Mount Everest, the possibility of releasing energy from the fission of the uranium atom was discovered in Germany and Austria, Faraday and Maxwell developed the relation between magnetism and electricity. We were not first in these areas but we have never been concerned about that fact. Why then are we so deeply disturbed by the fact that we are behind in our ability to deliver large payloads, and particularly manned payloads, into space?

I believe the reason lies very clearly in a statement that was made by the President in his speech before the newspaper publishers in New York last week. He was talking about the challenge to our way of life when he said, "Whatever our hopes may be for the future-for reducing this threat or living with itthere is no escaping either the gravity or the totality of its challenge to our security and survival-a challenge that confronts us in unaccustomed ways in every sphere of human activity."

As has been pointed out frequently by NASA and other Government officials, the Soviet Union has chosen the area of space exploration as the most visible form in which to demonstrate its technological strength. It is using this area of science to win the high technological position that we have held, and thereby to win the support of the world. Do we have any alternative but to meet this challenge? The President indicated in a recent news conference that the best ways of meeting this space challenge will be the subject of study by the National Aeronautics and Space Council under the chairmanship of the Vice President.

1 Luncheon talk at ARS-Oak Ridge National Laboratory meeting, Gatlinburg, Tenn., May 3, 1961.