Occurrence of aerodynamic flight
Planetary atmospheres comprise a very small part of space; however, these atmospheres give rise to some very vital problems in space flight.1 During exit from and entry into planetary atmospheres, hypersonic speeds will be characteristic of all space planetary vehicles. Thus, hypersonic aerodynamics will be involved in important phases of operation.
Strictly speaking, we should use the term "gas-dynamics" rather than "aerodynamics," since our consideration is not limited to the Earth's atmosphere. Furthermore, due to chemical effects, the "air" we are concerned with in hypersonic aerodynamics may be quite different from our customary ideas of air. However, we shall use "aerodynamics" as a term that does not preclude gases other than air.
The vehicle for sustained atmospheric flight at conditions of interest in astronautics is the hypersonic glide rocket.2 Although not a true cruising-type vehicle, the glide rocket is much more closely related to the conventional airplane than is the ballistic rocket. It is a promising vehicle for manned hypersonic flight and for manned return from space. The glide rocket is boosted to initial speed and altitude by a rocket, much like a ballistic vehicle. It is then tilted over into a glide path and glides back to Earth, losing speed and altitude as it descends. To enhance accuracy and to avoid low impact speeds, the flight path of an unmanned glide bomber would probably terminate in a near-vertical dive. A schematic flight path is shown in figure 1. Note that the vertical scale is considerably expanded, and that the entire flight path is within the atmosphere.
A likely intercontinental-range glide-rocket configuration is shown in figure 2. It is a long, thin, streamlined vehicle because of the importance of high lift and low drag. The flat-bottom body and drooped nose provide the best lift-drag ratio in the hypersonic regime.3 The flares at either side of the body are essentially wings which increase the lift-drag ratio. The rate of aerodynamic heating along a glide path is low enough to permit the use of thin-skin construction except for hotspots near the nose and leading edges.
1 Willlams, E. P., and Carl Gazley, Jr., Aerodynamics for Space Flight, The RAND Corp., Paper P-1256, February 24, 1958.
2 Willlams, E. P., et al., Long Range Surface-to-Surface Rocket and Ramjet Missiles-Aerodynamics, The RAND Corp., Rept. R-181, May 1,1950.
3 See footnote 2.
The greater efficiency of the intercontinental glide rocket, as compared with that of the ballistic rocket, is illustrated by the fact that for two vehicles of the same gross weight and payload the ballistic rocket will go only about one-third as far as the glide rocket, or, for the same range and payload, the glide rocket will weigh only one-third as much. If an ICBM were converted to a glide rocket of the same range and initial gross weight, the payload could be 8 to 10 times as great. There are, of course, many other factors that influence a choice between ballistic missiles and gliders.
A manned glide rocket involves essentially the same design considerations for the glide portion of its path, with the added requirement for a compartment suitable for human occupancy. However the rocket ascent and the landing phase must be modified to be compatible with human tolerance and safety. The takeoff acceleration of a typical unmanned glide rocket or a ballistic rocket might start one-half g above normal gravity and increase to perhaps 10 g as the propellant is burned. For an inhabited vehicle, the maximum acceleration could be reduced to a tolerable limit - perhaps 4 g - by extending the time of powered flight, i.e., accelerating more gradually.
The terminal portion of a manned flight must involve essentially a conventional airplane type of landing or a parachute recovery, so that tolerable decelerations occur. Ballistic reentry, except for very shallow descents, however are accompanied by very low decelerations - usually less than 1g. While the power-on landings appear to present no insurmountable problems, as has been demonstrated by current VTOL aircraft, the weight penalty for a rocket vehicle would probably be excessive.
THe glide rocket, which was introduced conceptually in this country nearly ten years ago,4 is closely related to the skip rocket which was first proposed by Sänger and Bredt.5 The skip path is similar to that of a flat stone skipping over the surface of a pond; a lifting-type vehicle descends on a ballistic path from above the atmosphere; upon reentry into the atmosphere, lift builds up with dynamic pressure, causing the vehicle to take an upward turn and be ejected from the atmosphere again. Thus, the skip path consists of a succession of ballistic trajectories each followed by a pullout.
4 See footnote 2, p. 85.
5 Sänger, E., and I. Bredt, Über einen Raketenantrieb für Fernbomber, ZWB, UM Nr. 3533 Berlin 1944. ( Available as Navy translation CGD-32, A Rocket Drive for Long Range Bombers. )
From a pure flight-mechanics standpoint, the skip rocket is superior to both ballistic and glide vehicles. However, the increased structural weight resulting from the higher load factors and greater peak aerodynamic heating rates of the skip rocket reduce its net range below that of the glide rocket. Furthermore, the inherent high load factors of the optimum skip path rule out its use for manned vehicles.
As indicated by these brief considerations, feasible hypersonic airplanes are primarily of the boost-glide type. It is expected that development of a hypersonic cruise aircraft must wait the advent of an efficient sustaining powerplant, such as a hypersonic ramjet.
The initial ascent flight paths of spacecraft will be almost identical with those of ballistic and glide rockets.
The distinction between the supersonic and hypersonic flight regimes is not clear-cut; but, for most interesting vehicles, the dividing line falls close to Mach 5, five times the speed of sound. In hypersonic flow the shock wave lies close to the surface of the vehicle body; whereas, in supersonic flow the nose shock wave is fairly far from the body.
In the flight of a vehicle through the atmosphere, some of the body's kinetic energy is continuously being converted to thermal energy in the air, and some of this thermal energy is transferred to the body. The rate of conversion of kinetic energy to thermal energy and the rate of heat transfer to the vehicle surface increase approximately directly with air density and very sharply with increasing vehicle velocity. Surface heating rates are thus most severe when high velocities occur at low altitudes, and can become more severe than any heating rates experienced in current heat-transfer technology.
Because air does not behave as a simple fluid under hypersonic flight conditions, hypersonic aerodynamics is much more complex than lower-speed aerodynamics. Unusual chemical and physical phenomena occur in the violently heated air adjacent to a hypersonic vehicle. The high air temperatures cause excited molecular states, radiation, chemical reactions, ionization, and so forth, resulting in effects that further complicate the overall heat-transfer problem, and may also cause difficulties for radio transmissions to and from the vehicle.6-8
The problem of atmospheric penetration arises for any vehicle which approaches a planetary atmosphere and for which physical recovery or survival is desired at the pLanetary surface. The cases of interest run from simple-sounding rockets to manned vehicles returning from interplanetary trips. Several types of atmospheric entry paths are illustrated in figure 3.
6 Goldberg, P. A., Electrical Properties of Hypersonic Shock Waves and Their Effect on Aircraft Radio and Radar, Boeing Airplane Co., Report No. D2-1997, July 2, 1957.
7 Sisco, W. B., and J. M. Fiskin, Effect of Relatively Strong Fields on the Propagation of EM Waves Trough a Hypersonically Produced Plasma, Douglas Aircraft Co., Report No. LB-25642, November 22, 1957.
8 Roberts, C. A., W. B. Sisco, and J. M. Fiskin, Theory of Equilibrium Electron and Particles Densities Behind Normal and Oblique Shock Waves in Air, Douglas Aircraft Co., report No. LB-25872, September 1,1958.
It can be seen that the glide vehicle just discussed descends through the atmosphere in a more gradual fashion than the ballistic missile.
Descent from a satellite orbit may be accomplished either by waiting for the orbit to decay under the action of aerodynamic drag or by using rocket braking ("dump") to shift from the satellite orbit to a descent path. Depending upon the aerodynamic characteristics of the descending vehicle, the entry path may range from the gradual one of a glide vehicle to the steeper path of a ballistic vehicle.
A vehicle arriving from outer space will approach a planet with a velocity that is at least equal to the escape velocity characteristic of the planet. (In the vicinity of the Earth, the escape velocity is about 37,000 feet per second.) Several kinds of approach orbits are of interest. These are illustrated in figure 4. A direct hit on the planet would involve an entry path similar to that of a ballistic rocket, but with a higher velocity. A more gradual penetration can be accomplished by either approaching the planet tangentially or by maneuvering into a satellite orbit before descending. Shifting into a satellite orbit can be accomplished either by reaction control or by the aerodynamic braking procedure illustrated in figure 5.
Although the heating and deceleration accompanying atmospheric entry bring about severe design problems, the presence of a planetary atmosphere is advantageous in that it acts as a cushion to reduce a space vehicle's velocity to a safe landing speed. Without an atmosphere, as in the case of a landing on the Moon, one is forced to the weight-consuming expedient of rocket braking.
Deceleration and heating
A body approaching a planetary atmosphere possesses a large amount of energy; and one of the most important of the problems of atmospheric penetration is the dissipation of this energy in a manner that will not prove disastrous to the vehicle, either during penetration or on landing. If all of the vehicle's energy were converted into heat within the body itself, it would in most cases be more than sufficient to vaporize the entire body. The survival of many natural meteorites however, is an obvious indication that not all of the energy goes into the body. Actually, the body's initial energy is transformed, through the mechanism of gas-dynamic drag, into thermal energy in the air around the body; and only part of this energy is transferred to the body as heat. The fraction of the original energy that appears as heat in the body depends upon the characteristics of the flow around the body.
37162° - 59 - 7
The chief effects accompanying atmospheric entry are reduction of vehicle velocity accompanied by appreciable deceleration loads and by appreciable heating. Both deceleration and heating are most severe when there is a combination of high atmospheric density and high vehicle velocity, i. e., when high velocities are allowed to persist down to low altitudes. This condition is most apt to occur when the approach velocity is very high and/or when the entry is at a high angle. On the other hand, a lower initial velocity or a shallow entry angle (tangential approach) tends to restrict high velocities to higher altitudes. The initial entry velocity is determined by the planet's gravitational characteristics and by the type of vehicle mission, i. e., return from satellite orbit, return from outer space, etc.; and, therefore, one must usually just accept the initial entry velocity involved. The angle of entry is, however, open to selection to relieve the severity of entry conditions.
Deceleration can also be caused high in the atmosphere by the use of a body having high drag and/or some aerodynamic lift. High drag produces deceleration at high altitudes, while aerodynamic lift allows a more gradual descent. The slender, low-drag body shown in figure 6a would experience more severe heating and deceleration loading than the blunt body in figure 6b. If, however, the latter body were oriented in the position shown in figure 6c, a lift force would be developed; and it would assume a more shallow path of descent with less heating and loading.
Influence of properties of the atmosphere
The physical and chemical characteristics of the planetary atmosphere also strongly influence entry characteristics. To obtain a first approximation to the gas-dynamic forces and heating, a knowledge of the density variation in the atmosphere is sufficient. The approximate density variation in the Earth's atmosphere and the estimated density variation in the atmospheres of Venus and Mars are shown in figure 7. The atmosphere of Venus, estimated to consist of about 10 percent nitrogen and 90 percent carbon dioxide, is somewhat more dense than the Earth's atmosphere, but varies in a similar way with altitude. The atmosphere of Mars, estimated to contain about 95 percent nitrogen and 5 percent carbon dioxide, is appreciably less dense than the Earth's atmosphere at surface level, but drops off much more gradually with increasing altitude and is actually more dense at high altitudes. The more gradual density variation in the Martian atmosphere effectively makes it "softer," so that it would involve a comparatively less severe entry.
A simple analogy
The effects of some of these factors on atmospheric entry condition can be visualized with the aid of a simple analogy. Imagine the problem of crash-landing a light airplane in a dense forest (fig. 8) Close to the ground, the trees have thick trunks; farther up, the trunk and limbs are more slim; and at the treetops, only slender twigs and branches occur. The forest, then, is analogous to the atmosphere-dense at low altitudes and tenuous at high altitudes. When the airplane enters the trees, it suffers deceleration due to impacts with part of the trees (aerodynamic drag) and suffers surface damage due to abrasion by twigs and branches (aerodynamic heating). If the airplane dives vertically at high speed into the forest, it will penetrate the thin upper branches without much deceleration and will still be moving at high speed when it reaches the heavy lower branches. Consequently the deceleration and surface abrasion would be great. However, if an attempt is made to reduce speed and glide into the treetops at a low angle, the plane will decelerate more slowly in the thin upper branches and will be moving at a relatively lower speed when it finally reaches the heavy lower trunks. A still better approach could be accomplished by pulling up just before striking the treetops so that the plane's altitude tends to keep it on the tops of the trees (i. e., aerodynamic lift) .
The effects of the drag characteristic of the body can be visualized by imaging the landing of two different airplanes, e. g., a modern fighter and a World War I fighter. The heavy, low-drag modern fighter would penetrate at high velocity into the heavy lower branches. The relatively light, high-drag, obsolete airline would be decelerated with comparative comfort in the light upper branches.
The effects of the density distribution in the planetary atmosphere can be visualized by considering a different type of tree. For example, the "softer" Martian atmosphere, with a lower sea-level density and a more gradual variation of density with altitude than the Earth's atmosphere, can be visualized as a forest of taller trees with smaller trunks and a more gradual variation of branch size with height.
The three general types of atmospheric penetration illustrated in figure 3-1 the steep descent path of a direct entry from space, the more gradual descent path of a satellite orbit decay, and a very gradual glide or lifting descent-are accompanied by differing patterns of deceleration.9
The influence of entry angle on this deceleration pattern is shown in figure 9 for a body like the Vanguard satellite. An entry angle of 90° indicates a vertical descent. ( is the angle between the vehicle path at entry and the local horizontal.) The maximum deceleration during direct entry is independent of the drag characteristics of the body. It is dependent only upon the path angle, the initial velocity, and the atmospheric characteristics. Only the altitude at which maximum deceleration takes place is dependent upon the drag characteristics of the body.
Fig.9-Velocity and deceleration during direct entry into the Earth's atmosphere from space at various angles
|= 10 lb / sq ft|
The pattern of velocity and deceleration for the same body is shown in figure 10 for vertical penetration of the atmospheres of Venus, Earth, and Mars. The different gravitational attractions account for the different initial velocities, which are equal to the escape velocities. The effect of atmospheric density variation is apparent In the shape and position of the curves. The more dense atmosphere of Venus results in deceleration at a higher altitude; however, the velocity variation with altitude and the maximum deceleration is about the
9 Gazley, Jr., C., Deceleration and Heating of a Body Entering a Planetary Atmosphere from Space, Vistas In Astronautics. Pergamon Press, 1958 (proceedings of the First annual Air Force Office of Scientific Research Astronautics Symposium).
same as for Earth because of the similar density variation. The more gradual variation of density in the Martian atmosphere results in a more gradual variation of velocity with altitude and a lower peak deceleration.
|= 10 lb / sq ft|
The phase of satellite orbital decay of interest here is the very last portion where heating and deceleration are appreciable-say, about the last 2,000 miles and the last few minutes of the satellite lifetime. This phase is preceded by a much longer period covering many revolutions in which the satellite executes a very gradual spiral that becomes more circular. The rate of energy loss by the vehicle due to aerodynamic drag is small enough so that the vehicle's kinetic energy (energy of motion) and potential energy (energy of height) adjust themselves to a momentary ''equilibrium '' orbit. In this process, the potential energy decreases, and the kinetic energy increases. Thus the satellite's velocity actually increases in the initial phases of orbital decay.
The deceleration pattern in the last phase of descent is very similar to that for direct descent at a very shallow angle. For the Vanguard example used above, the peak deceleration would be about 9 g.
A lifting descent involves a still more gradual atmospheric penetration, and here again the path angle adjusts itself to the forces acting on the vehicle and is generally quite small-of the order of a few
tenths of a degree. In this case, the deceleration does not go through a sharp peak, but increases gradually to a maximum. Decelerations can be limited to rather small values in a lifting descent.
Range and time of descent
It should be noted that the more gradual descents involve longer times and cover greater ranges than the steeper descents. For example, starting at the same altitudes and velocities, a direct descent may traverse a distance of only a few hundred miles and be accomplished in about one-half minute; an orbital decay might cover a range of a few thousand miles in 5 or 10 minutes; and a lifting descent might extend over 5,000 to 10,000 miles in about 2 hours. A gradual descent involves velocity reduction and consequent energy dissipation over a long period of time.
Examples of deceleration loads Some examples of deceleration loads that would be experienced in various kinds of entries are listed in table 1. These are to be compared with an allowable load tolerance of roughly 10 to 15 g for manned vehicles.
|" Planet "||
Direct entry at escape
| Direct entry at orbital
| Entry of lifting vehicle
at orbital velocity
|= 5||20||90||= 5||20||90||L/D = 1||2||5|
Heating during atmospheric penetration
The reduction of the vehicle's kinetic and potential energy during descent is accompanied by an increase in thermal energy in the surrounding air, some of which is transferred to the vehicles surface. The fraction of this energy which reaches the vehicle surface as heat is of primary concern to the designer. This fraction, or conversion efficiency, depends upon the vehicle's shape and upon its velocity and altitude-and ultimately upon the mechanism of heat transfer between the hot gas and the vehicle surface. At very high altitudes, the heat energy is developed directly at the vehicle's surface, and one-half the vehicle's lost energy appears as heat in the body. At lower altitudes, thermal energy appears in the air between the shock wave and the body. Heat is transferred from this hot air to the body by conduction and convection through a viscous boundary layer. Radiation from the hot gas may also contribute appreciably to the surface heating.
The heating of a vehicle in a given application determines the type of surface-protection system it needs.10,11
Supersonic aircraft and hypersonic glide missiles operate with essentially constant skin temperatures, and the vehicle must be so designed that heat is carried off at the same average rate as it is acquired.The temperature attained by various parts of the body will depend on the radiative characteristics of the vehicle surface and the local heating rates.
During the flight of a ballistic missile, the skin temperature experiences a large variation. During ascent through the atmosphere, the skin experiences moderate heating-about like that for a supersonic aircraft. Heating drops to zero upon exit from the atmosphere, and the skin is cooled by radiating its heat while the missile is going over the top of its trajectory. During descent, heating increases enormously and the skin reaches its maximum temperature sometime during this reentry part of its flight.
Surface cooling systems
Methods for protecting a payload from high external heating include-
Some idea of the spectacular nature of a high-speed reentry can be gained from the following excerpt from an account of the reentry of
10 Gazley, Jr., C., Heat-Transfer Aspects of the Atmospheric Reentry of Long-Range Ballistic Missiles, The RAND Corp., Rept R-273, August 1, 1954.
11 Masson, D. J., and Carl Gazley, Jr., Surface-Protection and Cooling Systems for High-Speed Flight, Aeronautical Engineering Review, vol. 15, No. 11, November 1956.
the nose cone and associated structures from the firing of an Army Jupiter missile on May 18,1958 :12
Within 3 seconds after the first reentry light was observed the phenomena had blossomed into 3 distinct objects. The brightest object appeared similar to a huge magnesium flare, which was assumed to be the booster. The light emitted by this object definitely pulsated as if the body were tumbling through space. At one time, the body's trajectory was nearly in line with the planet Jupiter. It was estimated that the brightness was at least 1,000 times that of the planet.
The second brightest visual object was a beautiful blue-green. This was assumed to be the instrument compartment. The blue-green light may have resulted from the copper and magnesium in this section. The actual trajectory of the instrument compartment during its burning stage was much shorter than the booster and nose cone. Slightly past the midpoint of the visible trace, the blue-green light turned nearly white and then burned out.
The nose cone never reached a white color. The radiation in the visible spectrum was orange-red in color. Visibility of the nose cone, spacewise, began slightly behind the booster, and then moved ahead of the booster. During the last few seconds of the visible flight, the booster and nose cone moved behind the large cumulus cloud to the south of the U. S. S. Stickell. The radiation was so intense that the whole cloud became illuminated. It was in this section of the flight that the booster ceased to glow and became invisible. The nose cone was seen to appear from behind the cloud and was tracked for another couple of seconds before it cooled enough to become invisible. The total time of visibility from the position of the U. S. S. Stickell was approximately 27 seconds.
12 Woodbridge D. D., and R. V. Hembree, Operation Gaslight, Jupiter Missiles AM-5 Army Ballistic Missile Agency, Huntsville, Ala., June 5,1958.