5.5.2.1 Sustained Linear Acceleration Regimes

For space systems, sustained linear accelerations include the following (Table 5.5-2):

Table 5.5-2 Sustained Linear Acceleration Regimes

5.5.2.2 Effects of Sustained Linear Acceleration on Humans

Space flight can create significant changes to the cardiovascular system, starting on the launch pad and continuing on orbit, but are usually of most concern during reentry and landing when astronauts are reintroduced to gravity and acceleration.

The microgravity, also called 0g, environment causes cardiovascular deconditioning due to the reduced load on the heart and vasculature since blood does not need to pump against gravity, and because physical workload is markedly reduced. While the body’s adaptation is appropriate for the 0g environment, it is maladapted for exposure to gravity and acceleration. During reentry, the body’s fluid is pulled downward towards the legs, but with reduced blood volume and diminished cardiovascular capacity, hypotension can occur, which is the primary cause for Gravity-Induced Loss of Consciousness (G-LOC). G-LOC is often preceded by visual symptoms progressing from tunnel vision to grey-out prior to complete blackout, and is accompanied by deficits in motor and cognitive function. These problems can create a dangerous situation in flight, reducing the ability of the crew to perform piloting tasks. While it has not been an issue for space flight to this point, the potential for G-LOC should still be considered, especially if returning from planetary missions under higher reentry decelerations than are encountered during orbital flights such as the Space Shuttle. During the deorbit burn (~1g) of Skylab 2, two crewmembers experienced grey-out for about 10 seconds, after being on orbit for 28 days. While Shuttle reentry forces do not exceed 1.5G, they last up to 17 minutes, and are much more provocative than normal due to the cardiovascular deconditioning caused by adaptation to 0g. The commander of STS-1 did fly the Shuttle manually from Mach 6 to touchdown in roll control, but the mission was only 56 hours in length, likely not allowing for full adaptation.

Also of great concern for returning from space flight is orthostatic intolerance, which is characterized by a variety of symptoms that occur upon standing, including lightheadedness, increase in heart rate, altered blood pressure, and fainting. This condition would make rapid egress difficult, especially during an emergency.

There are also effects on the vestibular system associated with sustained linear acceleration. Stimulation of the otoliths in the inner ear can create disorienting sensations when the head is tilted in flight. When moving the head in hypergravity, an otolith signal is generated that corresponds to a greater change in attitude than has actually occurred. Nausea, vertigo, sensations of tumbling, as well as an apparent change in attitude, can also be evoked by a head movement during acceleration.

The following is a summary description of the combined human responses to specific sustained linear acceleration vectors in a relaxed, unprotected individual adapted to Earth’s gravity. It is important to note that physiological effects of sustained acceleration will be exaggerated for 0g-adapted astronauts returning from space, and will occur at lower thresholds, which is not taken into account below. It is unclear how adaptation to partial-gravity, such as on the Moon or Mars, might affect acceleration tolerance. Accelerations may be accompanied by complex oscillations and vibrations. It should be noted that the physiological effects described below depend on the onset rate (e.g. gradual onset < 1 G/sec; rapid onset 1-2 G/sec; very high onset > 6 G/sec). The following are primarily derived from studies involving gradual onset exposures (REFERENCES).

Table 5.5-3 – Physiological Effects of Sustained Linear Acceleration

Effects of Sustained +Gz Acceleration (eyeballs down)
+1 Gz	Equivalent to the erect or seated terrestrial posture.
+2g to +2.5Gz	Increased weight; increased pressure on buttocks; drooping of face and body tissues; hypotension; difficult to raise oneself at 2.5g
+3 to +4 Gz	Impossible to raise oneself; difficult to raise arms and legs; movement at right angles extremely difficult; progressive dimming of vision (grayout) after 3-4 seconds; progressive tunneling of vision
+4 to +6 Gz	Total loss of vision (blackout) after about 5 seconds; hearing and then consciousness lost if exposure continued; mild to severe convulsions in about 50% of the subjects following unconsciousness, frequently with dreams; occasionally paresthesias (abnormal nerve sensations, such tingling or burning), confused states; pain not common, but tension and congestion of lower limbs with cramps and tingling; inspiration difficult; loss of orientation of time and space for up to 15 seconds post-acceleration; following unconsciousness, return to purposeful action takes an average of 24 seconds
> +6 Gz	Protection needed to preserve health
Effects of Sustained -Gz Acceleration (eyeballs up)
-1 Gz	Tolerable; sense of pressure and fullness in the head; congestion of eyes
-2 to -3 Gz	Severe facial congestion; bradycardia; dysrhythmia; throbbing headache; blurring, graying, or occasional reddening of vision after 5 seconds; congestion disappears slowly; may leave petechial hemorrhages, swollen eyelids
>-3 Gz	The five second tolerance limit is rarely reached; causes mention confusion and unconsciousness
Effects of Sustained +Gx Acceleration (eyeballs in)
1 Gx	Slight increase in abdominal pressure; respiratory rate increases
+2 to +3 Gx	Difficulty in spatial orientation; +2g tolerable for at least 24 hours
+3 to +6 Gx	Progressive tightness in chest and abdomen; cardiac rhythm disturbances; loss of peripheral vision; difficulty in breathing and speaking; blurring of vision, effort required to maintain focus; 4g tolerable up to at least 60 minutes
+6 to +9 Gx	Chest pain and pressure; shallow respiration from position of nearly full inspiration; decreased oxygen uptake during acceleration; pulmonary vascular pressures increase towards the dorsal part of chest and fall in alveolar pressure on the ventral part; arterial oxygen saturation falls below 85%, which can lead to cognitive impairment; further reductions in visual acuity and depth perception, increased blurring, occasional tunneling, great concentration required to maintain focus; occasional lacrimation (tears); body, legs, and arms cannot be lifted at +8g; head cannot be lifted at +9g; precise manual control compromised
+9 to +12 Gx	Increased severity of symptoms; severe breathing difficulty, increased chest pain, marked fatigue, loss of peripheral vision, diminution of central acuity, lacrimation
> +12 Gx	Extreme difficulty in breathing and speaking, severe viselike chest pain; loss of tactile sensation; total loss of vision possible
Effects of Sustained -Gx Acceleration (eyeballs out)
All levels	Similar to those of forward acceleration with modifications produced by reversal of the force vector. Chest pressure reversed. Total body restraint system is critical and has a direct relationship with the ability to tolerate rearward acceleration exposures.
Effects of Sustained +/-Gy Acceleration (eyeballs left/right) *Little information available
+/- 1 to 2 Gy	Difficulty maintaining head and shoulders upright without restraints; Difficulty of precise manual control
+/- 3 Gy	Discomfort after 10 seconds; pressure on restraint system; feeling of supporting entire weight on clavicle; inertial movement of hips and legs; yawing and rotation of head toward shoulder; petechiae and bruising; engorgement of dependent elbow with pain; total body restraint system is critical.
+/- 5 Gy	Conjunctival hemorrhage has been reported; severe headache after exposure