Why has nobody built rotating spacecraft to simulate gravity?
A whole spacecraft is expensive to rotate, but small spaces on space stations or ships could easily be rotated.
Can those small spaces be big enough to provide meaningful and healthy artificial gravity?
From my physics knowledge I recall that gravity and acceleration are the same.
If I remember correctly, 1 g is equal to an acceleration of 9.81m/sec per second. In other words, a wheel with a circumference of 10 meters would have to be spun about once per second to simulate 1 g in Zero gravity? Not quite.
Alas, it’s a bit more complicated than that, and thankfully we don’t have to spin the wheel quite so fast. That’s a bonus!
Here are a few handy calculators to work out wheel sizes and rotation rates to simulate Earth gravity:
SpinCalc, solves for gravity, radius and rotation rate,
Circle Calculator, solves for diameter, radius and circumference.
A wheel with a circumference of 10 meters would have a diameter of 3.18 meters. This would be a handy size for artificial gravity experiments, even on Earth.
Would it be comfortable to spend any time in this? The wheel should rotate at about 24 RPM to simulate 1 g. It could be compartmentalized in to 1 x 2 meter beds, holding ten crew.
So at least during their rest period spacefarers would have the benefit of normal gravity. The astronauts are lying on the inside of the wheel, a bit like in the fairground ride illustration but with more privacy.
Is it feasible to built such small Drum Gravity units?
How would the human body react? (Artificial Gravity by Centrifuge).
We know that the negative effects of zero gravity are really serious and numerous. Even 2.5 hours of daily treadmill exercise are insufficient to prevent these effects:
- fluid redistribution: Bodily fluids shift from the lower extremities toward the head. This precipitates many of the problems described below .
- fluid loss: The brain interprets the increase of fluid in the cephalic area as an increase in total fluid volume. In response, it activates excretory mechanisms.
- electrolyte imbalances: Changes in fluid distribution lead to imbalances in potassium and sodium and disturb the autonomic regulatory system .
- cardiovascular changes: An increase of fluid in the thoracic area leads initially to increases in left ventricular volume and cardiac output. As the body seeks a new equilibrium, fluid is excreted, the left ventricle shrinks and cardiac output decreases.
- red blood cell loss: Blood samples taken before and after American and Soviet flights have indicated a loss of as much as 0.5 liters of red blood cells.
- muscle damage: Muscles atrophy from lack of use. Contractile proteins are lost and tissue shrinks. Muscle loss may be accompanied by a change in muscle type.
- bone damage: Because the mechanical demands on bones are greatly reduced in micro gravity, bones essentially dissolve.
- hypercalcemia: Fluid loss and bone demineralization conspire to increase the concentration of calcium in the blood.
- immune system changes: Loss of T-cell function may hamper the body’s resistance to cancer — a danger exacerbated by the high-radiation environment of space .
- interference with medical procedures: Bacterial cell membranes become thicker and less permeable, reducing the effectiveness of antibiotics.
- vertigo and spatial disorientation: Without a stable gravitational reference, crew members experience arbitrary and unexpected changes in their sense of verticality.
- space adaptation syndrome: About half of all astronauts and cosmonauts are afflicted. Symptoms include nausea, vomiting, anorexia, headache, malaise, drowsiness, lethargy, pallor and sweating.
- loss of exercise capacity: This may be due to decreased motivation as well as physiological changes.
- degraded sense of smell and taste: The increase of fluids in the head causes stuffiness similar to a head cold.
- weight loss: Fluid loss, lack of exercise and diminished appetite result in weight loss. Space travelers tend not to eat enough.
- flatulence: Digestive gas cannot “rise” toward the mouth and is more likely to pass through the other end of the digestive tract “very effectively with great volume and frequency” .
- facial distortion: The face becomes puffy and expressions become difficult to read, especially when viewed sideways or upside down.
- changes in posture and stature: The neutral body posture approaches the fetal position. The spine tends to lengthen.
- changes in coordination: Earth-normal coordination unconsciously compensates for self-weight. In weightlessness there is a tendency to reach too “high” .
Compared to these adverse effects of zero gravity, here are some studies by a psychologist named Graybiel from 1977 on the effects of rotating a human on his own axis here on Earth, like on a spit (from https://psycnet.apa.org/record/1980-22567-001).
GRAYBIEL ROTATION COMFORT ZONES
Graybiel concluded that
1.0 RPM: even highly susceptible subjects were symptom-free, or nearly so
3.0 RPM: subjects experienced symptoms
5.4 RPM, only subjects with low susceptibility performed well
10 RPM, adaptation presented a challenging but interesting problem. Even pilots without a history of air sickness did not fully adapt in a period of twelve days.
The “adaption” that Graybiel is talking about is the getting used to the absence of the rotation, after the body had been spun.
What that feels like we all remember from childhood.:
I must say that spit rotating a human on his own axis in the horizontal under the influence of Earth gravity is most likely to be very far removed from what a human may experience in an artificial gravity drum in weightless space.
I’d go as far as to say that Graybiel’s rotation comfort zones have absolutely nothing to do whatsoever with artificial gravity by centripetal force. All he proved in his paper “Somatosensory motion after-effect following earth-horizontal rotation about the Z-axis” is that the after effect of spinning someone rapidly is disorientation of the vestibular system of the ear, leading to dizziness, aka vertigo.
But let’s see if these Graybiel’s comfort zone figures can be applied.
The SpaceX Mars rocket is going to have a diameter of 9 meters. Would it be possible to create a comfortable habitat for sleeping or resting spacefarers within the confines of this rocket?
A 9 meter drum would need to rotate at 14 RPM to simulate 1 g, or at 8 RPM to achieve 1/3 of Earth gravity. Graybiel’s findings would indicate that the space available on the SpaceX Mars rocket would be too small.
However, I believe that the gravity (centripetal force) acting on the body as it lies down, not spinning about itself and on one level , will be more comfortable than twirling rapidly around one’s own axis.
In Drum Gravity Bed Units there would be no head-to-foot acceleration gradient.
DRUM GRAVITY BED UNITS
The drum gravity bed units are conceived of as an add-on module to a spacecraft or space station, be it in transit, orbit or on the Moon, Mars or asteroids to provide more natural gravity.
Have prototypes of this concept been built?
In a certain way: Yes! The first picture in this post is a fairground attraction from the 1950’s.
Did humanity really forget from the ’50s how easy and fun it is to enjoy artificial gravity? Apparently the fairground visitors subjected themselves to the experience voluntarily and enjoyed it.
Simple gravity devices like this could help spacefarers to maintain their health, after the device is tweaked.
A BIGGER MODEL
Here are the calculations on the von Braun wheel from 1952 used in the movie 2001: A Space Odyssey:
They envisioned a rotating wheel with a diameter of 76 meters (250 feet). The 3-deck wheel would revolve at 3 RPM to provide artificial one-third gravity. It was envisaged as having a crew of 80.
Fast forward 70 years (not much has happened since the 1950’s):
SAHC HUMAN CENTRIFUGE
The SAHC human centrifuge began testing and operations in about 2020. It’s to investigate the tolerability and use of artificial gravity on astronauts and their health, to counter the effects of weightlessness. What’s taken so long?
The machine measures 5.6 meters in diameter.
It would be small enough to put in the SpaceX Mars rocket. But it needs a few more seats.
Centrifuge with lying test person
With the Short-Arm Human Centrifuge (SAHC) in Cologne — provided by the ESA — artificial gravity will be created to afford fundamental research in medicine and human physiology. The main focus is on the possibility to extend e.g. bed-rest studies to test methods of artificial gravity based counter-measures for medical risks due to weightlessness.
Max. radius at outer perimeter: 2,8 m
Max. overall payload: 550 kg
Max. centrifugal acceleration
(foot level, test subject height 185 cm): 4.5 g
Max. revolution of centrifuge rotor
(software limit): 39 rpm
- Development of effective countermeasures for neuromuscular and skeletal degeneration of astronauts using Artificial Gravity, etc…
This is an article by Erich Habich-Traut for the Contact Project,