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A Simple Classification of the Present and Future Stages of Manned Flight BY H. STRUGHOLD, M.D., PH.D.

HENEVER great inventions terms, lies at an altitude of about 600 or discoveries are made, there miles. As an environment for the W is public concern with their flyer and the vehicle, however, the at- potentialities and the length of time mosphere shows conditions typical of required for their full realization. This space at much lower altitudes. We en- has been notably true in the applica- counter within our , be- tion of the principle of pro- ginning at 50,000 feet, a region which pulsi.on to flight. Rocket flight and becomes increasingly space-equivanem space flight symbolize trips to the with regard to one or more of the or to . As long as these conditions important for manned trips do not materialize, in the public flight. These conditions are anoxia view there is no such thing as space (50,000 feet), the boiling point of flight. This all-or-nothing attitude is body fluids (63,000 feet), the necessity often found in conversations, in radio for a sealed cabin (80,000 feet), and television programs, and in print. meteors (75 miles), and the darkness It is true that the development ,of of space (100 miles).9 the rocket principle of is Above 120 miles we find space- an achievement of revolutionary signi- equivalent conditions in almost all ficance. Yet its complete exploitation respects. The atmospheric region from will probably follow the pattern of a ~0,000 feet (about 10 miles) up to gradual evolution. The stages of this 120 miles may be considered partially evolution can be understood best if we space-equivalent, and the region above examine three factors : (1) the physio- 120 miles totally space-equlvalent, if logical and mechanical properties of we ignore certain environmental effects the environment; (2) the at- which are caused by the solid body of tained by ; (3) and the , its magnetic field, and its they travel over and away from the own and reflected radiation s (Fig. 1). earth. Above the 120 mile level, the atmos- ENVIRONMENT phere is unrecognizable in manned It is well known that the border flight. It is imperceptible to the flyer, between the atmosphere and space, in although for the astronomer it exists meteorological and astrophysical up to 600 miles.

From the U. S. Air Force School of This being true, the rocket powered Aviation Medicine, Randolph AFB, Texas. plane which has carried man to 90,000 Dr. Strughold is head of the Department of . feet and the rockets which have carried Presented on March 21, 1955, at the animals up to 36 miles, * have flown luncheon during the annual meeting of the Space Medicine Association, Washington, in a region which is space-equivalent D.C. to a high degree. A two-stage rocket, 328 AVIATION ME~ICr',Is MANNED FLIGHT--STRUGHOLD the WAC corporal mounted on the breaks away from the earth's gravita- nose of a V-2, which attained an tional pull and escapes into the depths altitude of 250 miles, was flying under of interplanetary space. It is called conditions of total space-equivalence the escape . even though for a few minutes only. Do not these achievements in very high MI. KM altitude flying, justify the statement 140 ILl 220 that we have already reached the era TOTAL SPACE-EQUIVALENCE 200 of space fllght? From the standpoint 120 of the environment we are at present ,(:Elid t~ ~ 180 in the partial space-equivalent phase of 100 ~r 160 -r manned flight. ~.v// 140

80 m ,o~ I00 Just as we find levels that are 60 v/ 0 vr~q/ 8 0 characteristic in the environment 40 around the earth, so too do we find certain characteristic points in the ~0" factor of speed. They also mark 20 distinctive stages in the development 0 ,,o ...... EARTH I"---.,,HZ...... H.~. 0 of flight (Fig. 2). Fig. 1. The space-equivalent regions with- The first of these is the speed of in the earth's atmosphere. sound (760 m.p.h, at sea level). The present record in the supersonic speed The highest speed so far attained in range exceeds Mach 2. In the higher a two stage rocket (the WAC Corporal range of Mach 3 or 4, in horizontal mounted on a V-2) is 1.4 miles per flight, centrifugal forces begin to second. This is 30 per cent of the counteract gravitation to an in- orbital and 20 per cent of the escape creasingly noticeable degree. This velocity. This is where we stand to- brings on the phenomenon of decreased day with regard to speed. The three- weight or subgravity. Theoretically, stage rocket 1'~ or the atomic rocket 2,' at about 18,000 m.p.h, or 5 miles per may ,bring in the remaining percentage. second, in a horizontal flight the state DISTANCES of , or the gravity-free state, is finally reached. This speed of The various stages of flight can also 5 miles per second or 8 kin. per second, be classified by the factor of the where centrifugal force equals the they cover. The craft may gravitational pull of the earth, is fly from one point on the globe to known as the orbital, or better, circular another point on the globe, in a certain velocity. It is the speed which will distance around the globe, or far away enable a craft to circle about the earth from the globe into space. in a fixed circular like an If we combine the factors of en- artificial . At 7 miles per vironment, speed and distance with second or 11 km. per second, the craft their characteristic levels into one AUGUST, 1956 329 MANNED FLIGHT--STRUGHOLD picture, we see an evolutionary course next stage. Rocket powered planes in the development of manned flight will take us at supersonic speed under that looks somewhat as follows: The subgravitational conditions and in long distance flights of today take us sealed cabins through the space-

KILOMETERS PER SECOND 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 I I I I I I I I I I I I I I I

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ESCAPE VELOCITY

ORBITAL VELOCITY I

SPEED OF SOUND

THREE STAGE ROCKETp

TWO STAGE ROCKET

ONE STAGE ROCKET

I I I I I I I I I , I ,, I I , ,, ! I I .I I ! 0 2 4 6 8 10 12 14 16 18 MILES PER SECOND Fig, 2. Speed records in rocket flight compared to sma:d of sound, orbital velocity and escape velocity.

at subsonic speed, under normal equivalent regions of the atmosphere gravitational conditions, in pressurized from one point on the globe to another cabins through the lower regions of even more distant point in a matter of the atmosphere from one point on the hours) Still bound to the earth, they globe to another distant point on the will fall into the category of global globe, across a number of time zones spece-equivalent flight. The precursors and/or across zones of different sea- of this long distance space-eqMvatent sons in a single day. These are global space flight are seen in the short dis- atmospheric flights. This in tance, short time flights of rocket pow- flying began when Lindberg crossed ered planes and unmanned rockets of the Atlantic Ocean in 1927. today. They can be termed local space- We are now on the threshold of the equivalent flight. 330 AVIATION MEDICINE MANNED FLIGHT--STRUGHOLD

As soon as the orbital velocity (5 psychological power of attraction of miles per second) is reached, flights these objects as the final goal, how- of long duration around the globe in a ever, must not ,be underestimated. satellite orbit under conditions of zero They are extremely valuable as a back- gravity and in an environment equiva- TABLE I. CLASSIFICATION OF DEVELOP- lent to space will become possible. MENTAL STAGES IN MANNED FLIGHT But these craft still will operate within the gravitational hold of the earth and II I Global III will remain within the earth's vicinity. Global Spa.ce- Circum- IV Atmos- mv- plane- , Inter- This eventual stage may be called heric e~aent tary planetary light Flight Space Space circumplanetary space flight. Flight Travel Distance Geo- Geo r Vicinity Inter- The next step will follow as the graphic ~raphic of planetary Dimen- Dimen- Earth ] Dimen- escape velocity (7 miles per second) sions 3ions sions is reached. When, one day, a manned Envi- Lower Space- Cireum- Inter- ronment Atmos- equiv- plane- I planetary rocket leaves the earth, attains this phere alent tary I Space Regions Space speed and moves freely in space, then r~f the Atmos- we will have arrived at interplanetary phere space flight or what we may now call Speed Sub- am ~uper- Orbital i Escape Super- sonlc Velocity Velocity "space travel. ''1'~'~ sonic Speed Speed This classification (Table I) gives Gravi- Normal ~ub- Zero Zero us, I believe, a clearly defined and tational Gravi- gravity Gravity i Gravity Condi- tation realistic picture of the stage at which tion (1 g) we stand today and of the possibilities we may expect in the future. At ground stimulus for our efforts to- present we are actually in the first wards advancement of human flight. phase of space flight, the phase defined REFERENCES as global space-equivalent flight. Solu- 1. BRAUN, YON. W. : The Mars Project. Urbana, Ill.: University of Illinois tion of the medical problems in this Press. 1953. stage is, therefore, of immediate con- 2. BURGESS, F. : Rocket Propulsion. Lon- don: Chapman & Hall, 1954. cern to the physiologist, the engineer, 3. DORNBERGER, W. : Flights at the border and the flyer. Incidentally, most of the of the atmosphere (unpublished). 4. HENRY, J. P., BALLINGER, E. R., 'medical problems involved in the final MAHER, P. J., and SIMONS, D. G.: stage (space travel) are encountered Animal studies of the subgravity state during rocket flight. J. Av. Med., 23: in the stage of globul space-equivalent 421, 1952. flight. ,5. OBERTH, H. : Wege zur Ra~m~chlffahrt. Muenchen: R. Oldenburg. 1929. This step by step approach to the 6. RYAN, C. : Conquest of the Moon. New possibilities of rocket-powered flight York: Viking Press, 1953. 7. SAENCER, E. : Fbrschung Zwischen Luft- by human beings is perhaps more fahrt und Raumfahrt, Weltrauanfahrt, stimulating, and more fruitful for re- 1:1, 1955. 8. STRUGHOLD~ H. : Space equivalent condi- search and development, than the all- tions within the earth's atmosphere. or-nothing attitude displayed by those Mstro~autica ,4eta: 1:32. 1955. 9. STRUGHOLD, H., HABER, H., BUETTNER, who constantly gaze upon remote K., HABER, F.: Where does space celestial bodies like the moon or Mars begin; functional concept of the boundaries between atmosphere and with a kind of space fascination. The space. J. Av. Med., 22:342, 1951. AUGUST, 1956 331