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Gravity and Acceleration

This is an attempt to cover the effects of varied gravitation, acceleration protection (and its lack) on characters.

General Considerations


Acceleration due to gravity less than 0.5ms^(-2)
Acceleration due to gravity greater than 12ms^(-2)
Positive g's (+g)
Acceleration vector from head to feet
Negative g's (-g)
Acceleration vector from feet to head

All g tolerances are in the absence of special conditions and equipment, unless otherwise noted.

Adapting to acceleration in the short term (seconds to hours)

This happens every day when we wake up in the morning and stand up. On standing, blood pools in the legs. A combination of vaso- and venoconstriction, increased heart rate, and activation of renal fluid retention mechanisms maintain cardiac output within tolerable limits. Above the level of the heart, the circulation dilates to guarantee an adequate blood flow to the brain. These changes take place within two seconds, but are insufficient to maintain brain blood flow for more than an hour or so.

Walking helps keep venous pressures low in the legs ('muscle pump' action), preventing further falls in effective blood volume.

'Parade ground' fainting associated with prolonged quiet standing is the inevitable consequence of cardiac output falling below the level required to maintain brain perfusion.

Despite maximal compensation, cardiac output (the volume of blood pumped by the heart per unit time) falls by 25% on standing. The fall in effective blood volume is about 5-7% of the total blood volume (70-80mL/kg in an adult), equivalent to a minor bleed.

Increasing levels of positive g multiply these effects. With maximal physiological compensation, 5+ +g causes impairment of blood flow to the eyes within 5 seconds and unconsciousness shortly thereafter - 'blackout'. It is functionally equivalent to an acute blood loss of 50%.

Negative g's cause blood to pool in the upper part of the body. The increased blood volume delivered to the heart leads to an increase in cardiac output and blood pressure. The blood vessels of the head and neck become engorged. Vision is impaired due to engorgement of the retinal vasculature. This usually occurs with accelerations in excess of -2g's. Headache is a common prodromal symptom prior to the occurrence of 'redout' - delirium and psychosis due to impairment of cerebral blood flow and oedema (swelling). This usually takes place at about -5g's.

Acceleration is much better tolerated if it is in the lateral (side to side) or transverse (chest to back/back to chest) directions. 11g back-to-chest can be tolerated for 3 minutes, 17g chest-to-back for 4 minutes - the 'muscle control limit'.

Microgravity is relatively innocuous in the short term. The loss of gravitational cues for the vestibular organs of the middle ear may lead to motion sickness.

Big transient accelerations

Transients (of a second or less in duration) come and go too quickly for any compensatory reflexes to assert themselves. Injury is common due to the tremendous forces involved.

Human tolerance limits for whole body impact (duration 0.1 second):

Acceleration vector directed from front to rear of vehicle :

Forward seating (shoulder belts) 40g
Forward seating (lap belts only) 27g (max 0.002 second)
Rearward seating 80g
Sideward seating 9g

Unless noted, these values assume full head protection, neck bracing and torso immobilisation and represent the threshold for injury with these precautionary measures in use.

Injury is typically due to:

Acceleration vector directed along head -> foot axis (0.1 second):

Upward (negative g) 20g
Downward (positive g) 15g

These values represent the threshold value for compression or burst fractures of the vertebral column, femurs and pelvis, in the presence of full torso/neck immobilisation (ejector seat), or a parachute landing (legs and pelvis).

Source: US Army Field Manual 1-301, Aeromedical Training for Flight Personnel, May 1987 (courtesy Christopher Thrash).

Longer term Adaptation (days+)


Cardiovascular (and blood)

The increased upper body blood volume (+5-7% over standing 1g 'controls') leads to a modest increase in cardiac output. Stretch on the atria of the heart leads to release of atrial naturetic peptide and diuresis (increased urine volume). This leads to a decrease in blood volume of up to 10% within 24-48 hours, due to a fall in plasma volume.

The fall in circulating blood volume is limited by regulation by the kidneys to around 15%. The total volume of red cells falls over a period of 3-4 months, in response to the reduced oxygen demand as a result of muscle atrophy.

Cardiac muscle also atrophies as the work required of the heart falls. Blood pressure falls, due to a decline in both cardiac output and peripheral vascular resistance.

No further change in these parameters appears to take place after 4 months of exposure.


Muscle atrophies due to reduced load and reduced feedback from sensory nerves in the joints and muscle spindles (length and force sensors). Muscle cross-sectional area falls by up to 25% within 5 days, leading to a proportionate decrease in strength. Within a month, strength losses of 40% are common (studies of bed-bound humans). There is a decrease in endurance as the fibres change from the 'slow twitch' (aerobic) type to 'fast twitch' anaerobes. There is no further decline in muscle strength.

The rate of bone loss is 12-25 times that at one gravity (0.4-1% loss in bone calcium per month regardless of age vs. 0.3-0.5% per year after age 40-50). This loss is not uniform, and affects primarily load-bearing bones - the vertebrae and femurs, for example. Unlike the cardiovascular and muscle changes, bone loss continues with ongoing exposure to microgravity.

Osteoporosis is defined as a fall in bone density 2.5 standard deviations below the average young mean. After 12 months of exposure to microgravity, the 'osteoporosis threshold' will be crossed given the loss rates above. The risk of fracture is greatly increased.

The calcium is excreted in the urine. Precipitation of insoluble calcium salts can lead to kidney stones, and obstructive renal failure. High levels of calcium in the plasma can lead to constipation, abdominal pain, and delirium.

Interestingly, the decreased load on the vertebral column leads to an increase in intervertebral disc height. People get taller in microgravity as a result.


The primary problem is motion sickness due to the conflicting data received by the visual, vestibular, and joint position sensing systems. Onset is generally prompt, and is characterised by nausea, vertigo and vomitting. It takes about 3-4 days for the vestibular system to pass through this 'hypersensitive' phase. Resetting of threshold leads to a transient 'hyposensitive' period lasting about a week.

Motion sickness has an incidence of 50%. Risk factor data is sketchy. Change in acceleration can lead to recurrence.


Ventilation (gas delivery) and perfusion (blood flow) are not equal in the lung. The upper portions are better ventilated; the lower, better perfused due to hydraulic effects. Microgravity reduces the level of inequality between lung regions, but overall there is little change in lung performance.


Fluid retention mechanisms are inhibited, most notably the renin-angiotensin-aldosterone axis and the secretion of vasopressin (AKA antidiuretic hormone). Reduced levels of the adrenal steroid aldosterone and vasopressin, with an increase in the stimulus threshold required to release them, leads to a vulnerabilty to fluid depletion. The reserve required to withstand insults like blood loss, acceleration or infection is therefore decreased.


Cell-mediated immunity appears to be impaired by microgravity. The mechanism is unclear. Paradoxically, some studies have shown an improvement in humoral (antibody-mediated) immunity. The long-term effects of decreased adrenal function in combination with this haven't been investigated.

Reproductive and Developmental Biology

There is a lack of data about the effects of microgravity on fertility and growth. Certainly plants need gravity to sense which way is up - germination failure is more common. Animal fertility appears little affected.

Subsequent development hasn't been well studied. Periods of microgravity appear to be well tolerated by young animals with no adverse effects on the nervous, cardiovascular or musculoskeletal systems.


Beyond 3g's, usual activity (walking around, etc.) is almost impossible without special support.


The challenge is to maintain adequate cardiac output in the face of the increased tendency to venous pooling and the increased metabolic requirements of the muscles with loading. Cardiac output and blood pressure increase. Activation of fluid retention mechanisms will lead to ongoing volume expansion; effective volume returns to a 1g normal value within 12-72 hours, and ongoing expansion slowly occurs for one to two weeks thereafter. Red blood cell volume will increase over two weeks to a month, due to an increase in bone marrow activity mediated by erythropoietin.

Cardiac work is increased, and hypertrophy of the cardiac muscle is inevitable. The muscle may outstrip the ability of the coronary circulation to perfuse it, especially in the presence of atherosclerosis. Heart failure may occur without angina or infarction (muscle damage from limitation of blood flow) due to the effects of high blood pressure alone.

Oedema - accumulation of fluid in the tissues due to high venous pressures - is increasingly likely, as is the development of varicose veins and haemorrhoids, as +g increases.


The increased load on bone and muscle is a potent cue to growth. Within 8 weeks, strength increases by 30%. There is an associated increase in muscle mass and both aerobic and anaerobic endurance. The change in muscle endurance may be masked by fatigue caused by continuous exertion.

Bone mass and density increases under the increased load for up to three months. Height decreases due to vertebral disk compression.


Collapse of small airways occurs more readily when upright. Mismatch of ventilation and perfusion is increased due to hydrostatic effects. The overall effect is a decline in functional capacity with increased +g.


The increased levels of circulating adrenal hormones required to maintain blood volume, vascular tone and cardiac output also cause increased production of glucose. Diabetes is therefore more likely, with its adverse effects on all systems.

Blood and immune

If blood sugar levels are chronically raised, immunosuppression ensues. Mild impairment of immune function has been demonstrated with centrifuge experiments on cell cultures.

Reproductive and Developmental Biology

Fertility is likely to be decreased. Pregnancy will be more risky due to the increased cardiovascular stress, especially during labour.

Growth velocity of children (height/weight with time) is likely to be slower due to the stress imposed by the environment.

Equipment and rules (draft v1.0)

  1. Protective equipment
    1. G-suits :- First devised for aviators in the TTL5 era to prevent acceleration related loss of consciousness (G-LOC), these garments apply positive pressure to the legs and abdomen to return blood to the heart. Initially, they do not protect against the effects of -g's.

      More advanced suits (TTL8+) are able to apply negative pressure to the lower limbs, and small amounts of positive/negative pressure to the chest to assist in maintaining cardiac output. The latter effect is achieved by mask ventilation - 'cuirass' style pumps are too bulky.

      Tech Level Compensation Mass (kg) Price (Cr)
      5 +2 6 5000
      6 +3 5 3000
      7 +4 4 2000
      8 +5/-2 4 3000
      9 +5/-3 3 5000

      G-suits can be fitted in vacc suits, combat armour and battledress.

    2. G-tanks: These rely on the fact that liquids are incompressible to protect the occupant from acceleration. Up to 10G of protection is possible using gaseous ventilation. Beyond this, the presence of gas in the lungs could lead to barotrauma or damage to the heart, great vessels and diaphragm.

      Liquid ventilation is the only way to get around this problem. Perflurocarbon liquids are available from TTL 6 which could deliver enough oxygen to permit survival. There are problems with carbon dioxide retention and an increase in the work of breathing. Liquid ventilation enables G-tanks to provide up to 25G of compensation.

      To initiate and establish liquid ventilation :-
      Difficult, Med, Edu

      Time taken is 30 minutes.

      To wean from liquid ventilation :-
      Difficult, Med, subject's End

      The task can be performed every half an hour. It takes a further two hours for the subject to cough all the liquid up; all tasks are at +1 Difficulty level for 12 hours thereafter.

      Tech Level Type Volume (cu.m) Mass (t) Power (MW) Price (MCr)
      6 Passenger 5 2 0.001 0.1
      6 Crew 6 2 0.001 0.5

      The volume figure allows for access, fluid pumps and reservoirs and the necessary control electronics, cable space, etc. for the crew version.

    3. Inertial compensation: With the advent of gravitics, vehicles tend to be fitted with inertial compensators to protect the occupants. While these are useful with ordinary movement, the information in the first post shows that they are of little use in crash situations.

      I propose that inertial compensators should be capable of providing 50X their G-rating in crash protection. This leads to burnout of the compensator system, but may shield personnel from damage.

      e.g. A TTL 10 air/raft crashes travelling at 300km/h (83.3m/s). The occupants are in forward facing seats with full immobilisation, and thus have 40g of deceleration protection. The compensators kick in, providing another 50g worth.

      So 90m/sec of velocity shielding protects the crew. The air/raft is completely written off, but the occupants can walk away from this otherwise horrendous crash.

    4. Spin habitats and centripetal pseudogravity: In settings where gravitic tech is unavailable, simulating gravity with circular motion is an alternative, to prevent the adverse effects of microgravity on travellers. John Cramer has a good descriptive article on his website (http://www.npl.washington.edu/AV/av_index_sub.html). It's 'Alternate View' #18.
  2. Coping with acceleration

    Increasing acceleration is equivalent to wounds from other causes.

    Wound Levels

    Number of Characteristics at 0

    Wound level and Damage

    None (1D) Superficial
    1 (2D) Minor
    2 (4D) Major
    3 (6D) Destroyed


    1. Estimate acceleration
    2. Apply damage based on severity level in table
    3. Apply other effects (cumulative).
    4. Further wound checks if acceleration is still being experienced.
    1. Acute acceleration in head-foot axis, uncompensated :-
      [Multiply g's X 3 for chest-to-back or back-to-chest acceleration].

      Acceleration* Wound Level Effects
      <=2 Superficial Check for motion sickness[1]
      <=2.5 Minor Visual impairment (+/-g)[2]
      <=4.5 Major Consciousness check required
      4.5+ Destroyed Blackout (+g) ; redout (-g)[3]

      in addition to the usual wound checks :-
      Wound Level Needs treatment in If not treated
      Superficial 2D hours hourly catastrophe check
      Minor 1D X 10 minutes cat. check every 15min
      Major 2D minutes cat. check every 5min
      Destroyed 1D minutes Death[4]

      * - absolute value of uncompensated g's.
      [1] - 50% chance, unless modified by drugs (see below). -4 to Dex,
      all tasks 1 difficulty level higher.
      [2] - visual impairment, all tasks 1 difficulty level higher
      [3] - blind, then unconscious within 5 seconds with blackout ; blind,
      then delirious with redout (Int 0 for tasks).
      [4] - (+/-g - hypovolaemic shock/cerebral oedema)

      Catastrophe check :- Difficult, Medical/First Aid,
      patient's End. Failure - 1D more damage.

      Consciousness check :- (Difficulty), End

      Wound Severity Difficulty
      Superficial Easy
      Minor Average
      Major Difficult
      Destroyed Formidable

      Recovery: Non-lethal damage is recovered at the rate of 1 point per hour.

      Motion sickness: The chance of developing motion sickness can be reduced with drug prophylaxis. +2 DM to 1D roll if any medications are used (sedatives, etc.)

      Check daily to see if symptoms settle :-
      Difficult, Zero-G or Environmental Combat, End
      DMs :-
      +1 if sedatives taken
      +2 if specific treatments taken (e.g. scopolamine, cyclizine) TTL 8-
      +3 if specific treatments taken TTL 9+

      If check fails, nausea and vertigo persists.
      If check succeeds, task penalty is lifted and Dexterity returns to normal.

      Redout: Consciousness check every 15 minutes based on current End to check for resolution of delirium.

      Blackout: Consciousness check every 2 minutes. This task also applies for return of visual acuity.

    2. Effects of large transients

      For the purposes of these rules, all transient accelerations are assumed to be 0.1 seconds or less in duration.

      Damage is resolved as above, but points incurred are wounds instead of stun damage.

      To recap :-
      [Velocities are included to ease calculation].
      Acceleration from front to rear of vehicle:

      Forward seating (shoulder belts) 40g 40 m/s 140 km/h
      Forward seating (lap belts only) 27g (max 0.002 second) 0.5 m/s 1.8 km/h
      Rearward seating 80g 80 m/s 280 km/h
      Sideward seating 9g 9 m/s 32.4 km/h

      Acceleration along head to foot axis:

      Upward (negative g) 20g 20 m/s 72 km/h
      Downward (positive g) 15g 15 m/s 54 km/h

      For each increment of 20km/h speed, 3D more damage is sustained.

  3. Chronic adaptation

    G's Attribute effects*

    Gs First Week Next 3 Weeks Thereafter
    0 -2 STR/DEX/END -1 STR/END per week; +1 DEX -1 STR/END per month next 3 months; +2 DEX
    0.2 -1 STR/DEX/END +1 DEX; -1 END per week -1 STR/END per month next 3 months
    0.5 -1 STR/END -1 STR/END per 2 weeks -2 STR/END over 3 months
    0.7 -1 STR -1 STR/END -1 STR, -2 END over 3 months


    1.3 -1 END, +1 STR +1 STR/END over 3 months
    1.5 -1 END -1 END, +1 STR +2 STR END over 3 months
    1.8 +1 STR, -1 DEX/END -2 END, +1 STR/DEX +1 STR/END per month next 3 months
    2.0 +1 STR, -1 DEX/END -3 END, +2 STR, +1 DEX +1 STR/END per month next 3 months
    2.2 +1 STR, -1 DEX/END -3 END, +2 STR, +1 DEX +3 STR, +4 END over 3 months
    2.4 +1 STR, -1 DEX/END -4 END, +2 STR +3 STR, +5 END, +1 DEX over 3 months
    2.6 +1 STR, -2 DEX, -1 END -4 END, +3 STR +4 STR, +5 END, +1 DEX over 3 months
    2.8 +1 STR, -2 DEX, -1 END -5 END, +3 STR +4 STR, +5 END, +1 DEX over 3 months

    * for low g, if not undertaking exercise/drug regime ; for high g, if performing usual daily activities.

    To resist attribute loss (low gravity) :-
    Difficult, (target attribute)

    DMs :-
    +2 if resistive exercise regime is adhered to (3+ hours/day at zero G)
    +1 for calcium and iron enriched diet, fluid loading
    +1 if steroids/growth hormone + erythropoietin used
    +2 if TTL 9 regrowth agents used
    +3 if TTL 12 regrowth agents used

    High gravity attribute losses can be neutralised with High-G or Environmental Combat skill (Dex), by lying still and doing very little (Str/End), or by using rerowth agents.

    Attribute changes take 6 months to return to baseline in the absence of exercise/drugs, etc.

    If any attribute drops to zero over the acclimation period, a crisis has occurred, and medical attention should be sought.

    Effects on aging:

    Gs Aging Roll DM
    less than 0.2 +2
    0.2 - 0.5 +1
    0.5 - 1.2 0
    1.2 - 1.5 -1
    1.5 - 1.8 -2
    more than 1.8 -3

Additional notes

Work has been recently published by a French group that suggests that the proteins that make up the 'skeleton' of cells may require gravity to orient themselves. This has very important effects on cell division. Infertility and an increased rate of malignancies may be a long-term consequence of exposure to microgravity.