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that the biological effects of the various fuels utilized be carefully scrutinized. Many of these so-called exotic fuels can be extremely toxic when absorbed or inhaled. Some of the visual symptoms which may result can be quite severe. The absorption of methyl alcohol, for example, has been shown to produce within 24 hours such visual symptoms as retinitis, retrobulbar neuritis, and optic atrophy. If inhaled, the same fuel can produce blindness. Ethyl alcohol, although not as toxic, can produce marked drowsiness and irritation of the eyes. Ammonia, a more commonly encountered agent, is very irritating to the eyes and in high concentrations can cause conjunctival and corneal injuries. The fumes of certain jet fuels have been found to have an effect similar to that of ethyl alcohol. Other fuels which have toxic symptoms associated with their fumes are the boranea, nitric acid, fluorine, hydrogen peroxide, ozone, and liquid oxygen. A detailed discussion of the above fuels can be found in Stumpe (1958). In view of the toxic effects of some of these fuels, great care must be taken to provide adequate fume detectors for the protection of the crew. The Visual Effects of Gravitational Stress2 One factor of space travel which cannot be avoided or excluded, either at present or in the future, is accelerative and decelerative force. Accelerative force will be encountered in attaining orbital and For a more detailed discussion of the effects of gravitational forces on vision, see White (1958). 11
escape velocity; decelerative force in re-entry and recovery. Fortu- nately, due to extensive studies in aviation medicine, a large body of information concerning the effect of these forces on the human body is already available. In addition to numerous investigations pertaining to systemic effects, many studies have been concerned directly with associated visual phenomena. Early studies performed at the Naval School of Aviation Medicine in Pensacola (Cochran, Card, k Northsworthy, 1954) were designed to determine the G-tolerance levels with respect to loss of peripheral vision, blackout, and unconsciousness. The subjects (1,000) were seated in an upright position. They experienced loss of peripheral vision at 4. 1 - 0.7 G, visual blackout at 4.7 - 0.8 G, and unconscious- ness at 5.4 - 0.9 G. Although visual blackout occurs at about 4.7 positive G, as just mentioned, similar visual symptoms do not occur as readily with transverse G (chest to back) until the force is in excess of 12.0 G (Gauer k Ruff, 1939). Recent unpublished studies at Johns- ville indicate however, that there are visual symptoms present during transverse G if the trunk is inclined forward. Other studies concerned with the effects of transverse G (Duane, Beckman, Ziegler, & Hunter, 1953) did not reveal serious visual symptoms at even higher G levels, i.e., 13-17 G. An additional study by Bondurant et^ al (Bondurant, Clark, Blanchard, Miller, Hessburg, & Hiatt, 1958) was designed specifically to examine the acceleration patterns anticipated in space flight. Again, positioning the body so that the G force was transverse 12
was found to be optimal. It was found by these authors that, "The optimal body position for exit appears to be a seated position with a 20 degrees inclination of the trunk in the direction of acceleration, with the legs fully flexed (seated, forward-facing). Three-stage ac- celerations sufficient to reach orbital velocity, with peaks of either 8, 10, or 12 G, are tolerable in this position." It was found also that "Accelerations of less than 4 G are tolerable in either chest-to- back, back-to-chest, or foot-to-head direction for long enough to exceed escape velocity. Prolonged low G acceleration patterns, while not feasible with current propulsion systems, have the potential ad- vantage of enabling man to reach very high velocity (200,000 m.p.h.) and to retain a degree of mobility sufficient to perform limited control functions during the boost phase." In addition to the visual symptoms discuaaed with respect to the dimming of vision and blackout, there are other aspects of vision af- fected by G forces. White and Riley (1958) found that the number of errora in dial-reading increased as the G level approached 3-4. These authors point out that the effect of acceleration at 4 G is comparable to reducing the luminance of the dial by one logarithmic unit, and that this factor should be considered in the design of instrument panels. Visual acuity also has been found to be affected by acceleration (White k Jorve, 1958). White and Felder (n.d.) found, for example, that at a luminance of 0.01 millilamberts the minimum visible angle increased from 4.0 minutes of arc at 1 G to 7.59 minutes at 4 G. 13
