Summary Session, 19 February 1957
Participants and Their Topics
William H. Dorrance (Panel Leader), Assistant to the Director of Scientific Research, Convair
Carl Gazley, Jr., Aerodynamicist, Rand Corporation "Descent from Circular Orbits for Various Atmospheres"
Alfred A. Eggers, Jr., Research Scientist, NACA Ames Aeronautical Laboratory, Moffett Field, Calif. "Motion and Heating During Atmospheric Re-Entry"
Lester Lees, Guggenheim Aeronautical Laboratory, California Institute of Technology, Pasadena "Aerodynamic Solutions of the Manned and Unmanned Re-Entry Problem"
George Solomon, Assistant Director, Hypersonics Staff, Guided Missiles Research Division, Ramo-Wooldridge Corp. "Planetary Atmospheres"
Antonio Ferri, Head, Aerodynamics Laboratory, Polytechnic Institute of Brooklyn, Freeport, N. Y. "Use of Lift During 'Re-Entry' "
Fred R. Riddell, Senior Research Engineer, AVCO Research Laboratory, Everett, Mass. "Re-Entry and Real Gas Effects"
Edward van Driest, Chief Scientist, Missile Development Division, North American Aviation, Inc., Downey, Cal. "Transition and Turbulent Heating, Including Possible Real Gas Effects"
Joseph Charyk, Director, Aeronautics Laboratory, Aeroneutronics Systems, Inc., Glendale, Calif. "Shock Layers in Different Planetary Atmospheres"
This summary of the re-entry panel discussions is presented in the light of the stated objectives of this symposium, viz: "To formulate and define, in technical terms, the scientific problems associated with near solar system spaceflight."
One of the first items of business was a re-definition of the "re-entry" problem to include the broader aspects of entry into the atmosphere of a foreign planet. It was proposed by Mr. Dorrance that the problem area be called the "problem of entry into an atmosphere." He stated the factors the designer of an atmospheric entry vehicle must always keep in mind :
1. weight must be minimized
2. high reliability
3. high safety, and
4. minimum cost.
Mr. Dorrance also stated related technological factors which bear on the design philosophy which would be used to design the atmospheric entry vehicle:
1. heating of the vehicle
2. aerodynamic loads on the vehicle
3. communication with the vehicle, and
4. controllability and maneuverability of the vehicle.
It was pointed out that data bearing on all of these technological factors must be available in order for the atmospheric entry vehicle designer to proceed. As the discussion developed it was apparent that all the presentations contributed to or raised questions about these technological design factors. It was further apparent that most of the panelists regarded an earth satellite as the first step towards space exploration. Several discussions centered on the atmospheric entry problem associated with the decay of an earth satellite orbit into the atmosphere of Earth.
In organizing and summarizing the remarks of the panel members, it seemed logical to group them under the following categories:
1. planetary atmospheres
2. flight mechanics and aerodynamic loads
3. entry vehicle heating, and
4. data needed
The remainder of the summary will be organized into these categories.
I. Planetary Atmospheres
It was apparent to all members of the panel that calculations for entry to the atmospheres of foreign planets required knowledge of the composition, density, temperature, proportions of the constituents, and thickness of the atmospheres.
Dr. Solomon reported that planetary atmospheric data is sparse and is obtained by telescopic and spectroscopic means. He regards the temperatures commonly accepted to be fairly accurate but the density and composition of the atmosphere to be only order of magnitude estimates. He pointed out the limitations of spectroscopic techniques in identifying compounds by band spectra.
Dr. Charyk pointed out that the presence or absence of various gases in the planetary atmospheres would depend upon atmospheric temperature and whether or not the root-mean-square thermal velocity was less than or greater than the planetary escape velocity. He reported that more data was available on Mars' atmosphere than for other planets (excluding Earth) and that the density at sea level appears to be about 1/4 that of Earth's sea level density falling off with altitude at such a rate that at about 40,000 ft. altitude the density equaled earth's density and was greater than than earth's density above that altitude. He speculated that the atmosphere of Jupiter consisted primarily of hydrogen.
Dr. Charyk and Dr. Solomon mutually agreed that considerable data in the planetary atmospheres was lacking.
2. Flight Mechanics and Aerodynamic Loads
It was apparent in these discussions that including a man in the atmospheric entry vehicle placed more stringent requirements on the flight mechanics than would apply if the vehicle was unmanned. The aerodynamic loads will have to be tolerable to man; the vehicle must be steered; and it must be kept comfortable and cool.
Dr. Eggers reported on an analysis he made of the descent of a spherical earth satellite through the Earth's atmosphere with the purpose in mind of recovering the satellite intact. He reported that if the inclination of flight path with the horizontal is kept low during initial entry to the atmosphere, then the aerodynamic loads can be kept to a tolerable low value and the skin could be cooled by radiation at such a rate as to keep the skin temperature below 2500 degrees F. For such a trajectory the Reynolds number during the period of high heat transfer to the satellite will be low enough that laminar flow over the satellite could reasonably be expected during this period of descent. He suggested that a parachute could be deployed once the satellite had decelerated to the subsonic velocity of about 400 feet per second at 35,000 feet for the example he chose. He further suggested that retarding or decelerating rockets might be used advantageously early in the descent of the satellite.
Dr. Gazley reported an analysis of the flight mechanics of entry to foreign planet atmospheres including Mars and Venus. He pointed out that the maximum decelerating aerodynamic force on the descending satellite was independent of the drag coefficient and occurred at a velocity about 60 percent of the re-entry velocity.
Dr. Gazley described a "braking ellipse" approach to a planet which involves the progressive decay of an orbital elliptical flight path to a near circular near the top of the atmosphere followed by a descent through the atmosphere. In this case the maximum decelerating force is also independent of the aerodynamic drag coefficient of the vehicle.
Dr. Gazley compared the heating rates which might be encountered in entering the atmospheres of Earth, Mars and Venus using the best information available on the atmospheres of these planets. The heating and deceleration rates of Venus were comparable to those for Earth and those for Mars appeared to present much less of a problem.
3. Entry Vehicle Heating
Heating of atmospheric entry vehicle is always one of the more severe problems facing the designer. This problem was dealt with at considerable length by re-entry panelists.
Professor Lees reported on the analysis of the possibilities of using a "sweat" or transpiration cooling technique to reduce heat transfer to the skin of an entry vehicle. Such a scheme would involve forcing a liquid or gas through the porous skin of the vehicle. He also suggested the use of lift during entry to control the flight path in such a way as to minimize aerodynamic loads and heating of the vehicle.
Professor Ferri reported on his study of the possibilities of using aerodynamic lift to control aerodynamic loads and heat transfer to the vehicle. His proposal q involves decelerating the entry vehicle at sufficiently high altitudes by using lift and drag to keep it at high altitude where the air density is low and, consequently, the aerodynamic heating is low. He believes that lift to drag ratios of the order of one can be obtained by designing sufficiently low density entry vehicles. He demonstrated that heating rates are considerably reduced using this technique.
Dr. Riddell reported on the effects of dissociation and ionization on high speed flight in the atmosphere. Dissociation at a stagnation point becomes appreciable above 8,000 to 10,000 f.p.s. while ionization is sufficient to cause radio communication problems above 12,000 to 13,000 f.p.s.
Dr. Riddell reported on results of experiments run using high pressure shock tubes to generate short duration, high temperature, airstreams. He showed comparisons of heat transfer rates measured in a shock tube at the stagnation point of a hemisphere with a theory taking into account the imperfection of a* at high temperature. The agreement between experiment and theory was gratifyingly good. This lends some confidence to the belief that entry vehicle heating can be predicted fairly accurately if the composition of the atmosphere is known. Dr. Riddell noted that theoretically the effect of finite relaxation times does not seriously affect the heat transfer rates provided the surface of the body is catalytic to atom recombination. At high enough altitudes use of a noncatalytic surface may reduce the heat transfer rates appreciably.
Dr. Riddell reported on his analysis of radiative equilibrium temperature of an Earth satellite sphere descending through Earth's atmosphere. It was apparent, for example, that using proper design precautions the Vanguard satellite might survive re-entry intact.
Dr. van Driest was concerned with the possible serious consequences of turbulent boundary layer flow over an entry vehicle since turbulent boundary layer heat transfer rates can exceed laminar rates by a factor of about 10 at entry speeds. Van Driest showed results of his experiments using liquid nitrogen cooled cones in a supersonic wind tunnel. It was shown that decreasing the surface temperature or cooling the cone delayed the onset of turbulent flow significantly. Van Driest also showed that the sensitivity of the boundary layer to roughness of the surface apparently deaeased with increasing Mach number.
Van Driest dwelled at some length on the analysis of turbulent boundary layer velocity profiles with the view in mind of detecting a significant parameter of transition to turbulent flow from laminar flow. He suggested a further avenue of research on this problem.
Dr. van Driest also presented his theory for calculating turbulent heat transfer rates to the surface of a hemisphere near the stagnation point. It appears that the theory predicts a maximum in the heat transfer rate to an isothermal hemisphere at some point which is not the stagnation point.
Dr. Farber presented a graph showing the results of calculations to determine the components and their percent composition in the hot gas mixture behind the strong shock wave preceding an entry vehicle. He pointed out that, depending on the composition of planetary atmospheres, heat transfer to the surface of an entry vehicle by radiation from the hot gas could take place. Also the presence of free electrons and radicals in the hot gas layer could influence and prevent electromagnetic wave propagation to and from the vehicle.
Dr. Farber also suggested that the danger exists of possible interaction of charged components in the atmosphere of foreign planets with the surface materials of entry vehicles in such a way as to cause ablation of the surface.
Without referring to the person making the suggestion the following data, research and information requirements are presented.
1. Data on the atmospheres of foreign planets is needed including:
b. density versus altitude
d. ion concentration
e. dust intensity
f. winds, and turbulence, and
2. Data on the atmosphere of Earth is needed including:
a. composition of atmosphere above about 100,000 ft.
b. winds above 100,000 ft.
c. ion density above 100,000 ft., and
d. dust intensity
3. Aero-thermodynamic problems suggested include:
a. research on the aerodynamic regime' between continuum flow and free molecule flow (slip flow)
b. further research on transpiration cooling
c. further research into the turbulent boundary layer and transition
d. development of high temperature, high energy, high velocity gas dynamics test facilities
e. research into the effects of non-equilibrium of the gas species in the hot gas flowing around a body in hypersonic flight, and
f. lift devices at hypersonic speeds.
4. Materials problems suggested include:
a. the interaction of materials with strange gas species present in the shock layer in foreign atmospheres.
b. research into methods of protecting or preventing common structural materials from oxidizing, melting or spalling under the heating conditions present during the entry to foreign or Earth's atmosphere.
In conclusion, it can be stated that the unanimity of opinion on the general problem areas and approaches to the atmospheric entry problem was quite remarkable considering the fact that the presentations were prepared independently. It is also remarkable that considerable thought has been devoted to the problems of entry to foreign atmospheres by responsible and recognized members of the scientific community.