Introduction

Currently, science knows of precious few methods of simulating gravity on a spacecraft.

These boil down to: using acceleration by thrusting the ship, spinning the ship (or sections of the ship) to utilize "centrifugal force", or placing a large mass under the ship (generally by landing on a planet).

Centrifugal force is the method of choice for obvious reasons. Nothing short of a freaking torchship can do 1 g of acceleration for longer than a few minutes, and it is highly inconvenient to cart along a planet the size of Terra just so you can have some gravity.

Science fiction authors find all these choices to be confining, so they have invented all sorts of technobabble ways of generating gravity with the flip of a switch.

The small reason to put artificial gravity on your spacecraft is because it makes things like preparing food and urinating easier. But the big reason is that microgravity does hideous long term damage to the human organism.

With centrifugal gravity, the direction of "down" is in the opposite direction of the spin axis of the centrifuge (in a direction at 90° to the spin axis, pointing away from it). Unless you are doing something silly like using rocket acceleration at the same time with a centrifuge that is not gimbaled.

Spin Grav Math

For 1.0 g of Artificial Gravity
Rotation
Rate
(rpm)
Radius
(m)
1895.47
2223.87
399.50
455.97
535.82
624.87
718.27
813.99
911.06
108.95

How fast will the ship have to spin in order to provide acceptable gravity?

Ca = 0.011 * Cr2 * Cl

Cl = Ca / (0.011 * Cr2)

Cr = sqrt( Ca / (0.011 * Cl))

where

  • Ca = centrifugal artificial gravity acceleration at point X (m/s2)
  • Cl = distance from point X to the center of rotation (m)
  • Cr = rotation rate at point X (rotations per minute)

Remember that 1.0 g is 9.81 m/s2. Notice that as point X is moved further from the center of rotation the artificial gravity increases.

Instead of doing the math yourself, you can cheat and use SpinCalc.

CALCULATING SPIN

      "Soup's on," announced Lopez. "This is your messroom. Lunch in a few minutes."
     Behind Lopez, secured firmly to the far wall, were mess tables and benches. The table tops faced Matt -- under him, over him, or across from him -- what you will. It seemed an impractical arrangement. "I'm not very hungry," one youngster said faintly.
     "You ought to be," Lopez answered reasonably. "It's been five hours or more since you had breakfast. We're on the same time schedule here as Hayworth Hall, zone plus eight, Terra. Why aren't you hungry?"
     "Uh, I don't know, sir. I'm just not."
     Lopez grinned and suddenly looked as young as his charges. "I was just pulling your leg, kiddo. The chief engineer will have some spin on us in no time, as soon as we break loose from the Bolivar. Then you can sit down on your soft, round fanny and console your tender stomach in peace. You'll have an appetite. In the meantime, take it easy."
     Two more squads filtered in. While they waited Matt said to Lopez, "How fast will the ship spin, sir?"
     "We'll build up to one gravity at the outer skin. Takes about two hours to do it, but we'll eat as soon as we're heavy enough for you groundhogs to swallow your soup without choking."
     "But how fast is that, sir?"
     "Can you do simple arithmetic?"
     "Why, yes, sir."
     "Then do it. The Randolph is two hundred feet through and we spin on her main axis. The square of the rim speed divided by her radius — what's the rpm?"
     Matt got a faraway look on his face. Lopez said, "Come, now, Mr. Dodson — pretend you're heading for the surface and about to crash. What's the answer?"
     "Uh-I'm afraid I can't do it in my head, sir."
     Lopez looked around. "All right — who's got the answer?" No one spoke up. Lopez shook his head mournfully. "And you laddies expect to learn to astrogate! Better by far you should have gone to cow colleges. Never mind — it works out to about five and four-tenths revolutions per minute. That gives one full gravity for the benefit of the women and children. Then it's cut down day by day, until a month from now we're in free fall again. That gives you time to get used to it — or else."

Ed note: 200 foot diameter = 61 meters diameter. 61 / 2 = 30.5 meter radius. 1 g = 9.81 meters per second.

  • Cr = sqrt( Ca / (0.011 * Cl))
  • Cr = sqrt( 9.81 / (0.011 * 30.5))
  • Cr = sqrt( 9.81 / 0.3355)
  • Cr = sqrt( 29.24)
  • Cr = 5.4 minutes
     Someone said, "Gee, it must take a lot of power."
     Lopez answered, "Are you kidding? It's done by electric-braking the main axis flywheels. The shaft has field coils wound on it; you cut it in as a generator and let the reaction between the wheel and the ship put a spin on the ship. You store the juice. Then when you want to take the spin off, you use the juice to drive it as a motor and you are back where you started, free for nothing, except for minor losses. Savvy?"
     "Er, I guess so, sir."
     "Look it up in the ship's library, sketch the hook-up, and show it to me after supper."

From SPACE CADET by Robert Heinlein (1948)

Problems with Spin Grav

Coriolis Effect

The Coriolis Effect is due to "rotating frames of reference", the latter means that if you are spinning around but think you are stationary, the universe looks weird. The Coriolis effect is one of the three "fictious forces" that rotating frames of reference is prone to (one of the other fictious forces is the centrifugal forces being used for artificial gravity).


But you really do not need to know all of this. The point is that inside a centrifuge or other spinning method of creating artificial gravity, moving objects appear to move in curves instead of straight lines. I mean other than the ordinary curve you see when you throw a ball and gravity tugs it down to the ground.

The practical point is the list of moving objects whose path curves due to the coriolis effect includes the fluids in one's inner ear. Which can cause nausea.


Refer to Figure 1. It is a large spinning disk like a merry-go-round. There is a person standing on the red dot. There is a black ball at the center which moves to the rim.

To an outside observer, they see the disk and the person spinning, and the black ball moves in a straight line.

But to the person on the red dot, they see themselves and the disk as stationary, and the black ball moves in a curve.

You can see this yourself if you go to a children's playground that still has an old fashioned merry-go-round, sit on it, spin it up, then start throwing some balls. Weird, huh?


The amusing effects are the crazy trajectories of thrown objects, such as the whisky being poured in the image above. However it is not so amusing if the moving object is a bullet. If you fire a slugthrower inside a spinning habitat you will miss every time until you learn to correct for the Coriolis effect.

Refer to Figure 2. In a spinning space habitat, tossing a ball towards the spin axis makes it travel to the opposite side. But from the person spinning inside, the ball appears to loop-the-loop. The ball is traveling at such a speed that the time it takes to go from one side to the other is the same time as one-half the habitat's spin rate. If a bullet was traveling at the same rate you could inadvertently shoot yourself (in practice the habitat would have to be spinning rather rapidly and bullet traveling rather slow).

Other moving objects with their trajectories curved by the Coriolis effect include your arms and legs.

Spin RPM Limit

As it turns out, there are limits on the rotation rate. The Coriolis effect can induce nausea. Sort of like spin motion-sickness. You do not want a bunch of green-faced astronauts/star-liner passengers/space habitat colonists moaning that they are going to die and vomiting everywhere.

The only way to increase gravity (Ca) without increasing the RPMs (Cr) is to increase the spin radius (Cl). What this means is you take the required gravity and the maximum rotation rate allowed, plug it into the Cl = Ca / (0.011 * Cr2) equation, and you'll see what sort of spin radius you will have to deal with.

If the spin radius is too huge (I don't wanna blasted spaceship with a centrifuge two-hundred freaking meters radius!), you'll have to decrease the amount of gravity, increase the rotation rate, or both. That or put up with a lot of vomiting astronauts.

According to Space Settlement Population Rotation Tolerance the safe spin limits are:

  • Up to 2 rpm should be no problem for residents and require little adaptation by visitors.
  • Up to 4 rpm should be no problem for residents but will require some training and/or a few hours to perhaps a day of adaptation by visitors.
  • Up to 6 rpm is unlikely to be a problem for residents but may require extensive visitor training and/or adaptation (multiple days). Some particularly susceptible individuals may have a great deal of difficulty.
  • Up to 10 rpm adaptation has been achieved with specific training. However, the radius of a space colony at these rotation rates is so small (under ~20 m for seven rpm) it’s hard to imagine anyone wanting to live there permanently, much less raise children. But military personnel could be trained to tolerate it.
Discovery Radius 5.5 m
Rotation RateGravity
7.5 (rpm)0.347 gs
8 (rpm)0.395 gs
9 (rpm)0.500 gs
10 (rpm)0.617 gs

However, the data on artificial gravity is a bit out of date. The original research into it had subjects sick at 3 RPM and incapacitated at 6 RPM+.

However, more recent research suggests that, by using incremental increases in rotation and making a few limb movements, adaptation can occur with almost no feelings of nausea. The old research (done on about 30 subjects) simply went from zero to full rotation.

Moreover, the adaptation can be simultaneous with non- rotational adaptation. So, moving in and out of the rotating habitat for maintenance or whatever is no problem. It's thought that rotation rates of up 7.5 to 10 RPM are possible.

This makes Discovery's 5.5m radius centrifuge a real possibility. In fact, with 10 RPM, you could crank it up to a handsome 0.61 G, or 0.34G if you want to play it safe at 7.5RPM.

Troy Campbell

Which Way Is Down?

The main design problem when adding artificial gravity to a spacecraft is that the direction of "down" while under thrust is not the same as the direction of "down" while under spin gravity. And the direction of "down" while under both thrust and spin gravity was at an angle between the two (the vector sum of the two accelerations). This can get confusing.

Why is this a problem? What was a floor under thrust might turn into a wall under spin gravity. So which surface do you mount the toilet on? If the designer is not careful, half the time the toilet will be sideways and pouring water all over the floor.


A similar problem happens with belly landers: the direction of "down" while under thrust is at ninety degrees to the direction of "down" when sitting on the runway impersonating an aircraft.


The brute force solution is to force the crew to detach all the furniture from the floors that are now walls and put them on the walls that are now floors whenever the spacecraft changes mode. This is quite a chore. And there will be further problems with floor and wall mounted control consoles and related items. Not to mention the toilets.

The alternative is rotating the entire room on gimbals, or using a gimbaled centrifuge.

NASA's old Space Shuttle had the belly lander problem. They dealt with the problem by mostly ignoring it. The Shuttle's habitat module was laid out for "belly is down" mode. It was only subjected to "thrust is down" mode while sitting on the launch pad and during the boost into orbit. That period of time was only a fraction of the total mission time, and the astronauts were to spend that time strapped into their acceleration couches anyway. They made do with a few ladders to climb into their couches.

The pilots just had to learn how to deal with flying the shuttle on their backs with the control panels above them during lift off, and flying the shuttle on the seat of their pants with the control panels in front of them during the dead-stick landing.

REPAIR UNDER SPIN

      The Rocket Ship Valkyrie was two hundred and forty-nine days out from Earth-Luna Space Terminal and approaching Mars Terminal on Deimos, outer Martian satellite. William Cole, Chief Communications Officer and relief pilot, was sleeping sweetly when his assistant shook him. ‘Hey! Bill! Wake up — we’re in a jam.’
     ‘Huh? Wazzat?’ But he was already reaching for his socks. ‘What’s the trouble, Tom?’

     Fifteen minutes later he knew that his junior officer had not exaggerated; he was reporting the facts to the Old Man — the primary piloting radar was out of whack. Tom Sandburg had discovered it during a routine check, made as soon as Mars was inside the maximum range of the radar pilot. The captain had shrugged. ‘Fix it, Mister — and be quick about it. We need it.’
     Bill Cole shook his head. ‘There’s nothing wrong with it, Captain — inside. She acts as if the antenna were gone completely.’
     ‘That’s impossible. We haven’t even had a meteor alarm.’
     ‘Might be anything, Captain. Might be metal fatigue and it just fell off. But we’ve got to replace that antenna. Stop the spin on the ship and I’ll go out and fix it. I can jury-rig a replacement while she loses her spin.’

     The Valkyrie was a luxury ship, of her day. She was assembled long before anyone had any idea of how to produce an artificial gravity field. Nevertheless she had pseudogravity for the comfort of her passengers. She spun endlessly around her main axis, like a shell from a rifled gun; the resulting angular acceleration — miscalled ‘centrifugal force’ — kept her passengers firm in their beds, or steady on their feet. The spin was started as soon as her rockets stopped blasting at the beginning of a trip and was stopped only when it was necessary to maneuver into a landing. It was accomplished, not by magic, but by reaction against the contrary spin of a flywheel located on her centerline.

     The captain looked annoyed. ‘I’ve started to take the spin off, but I can’t wait that long. Jury-rig the astrogational radar for piloting.’
     Cole started to explain why the astrogational radar could not be adapted to short-range work, then decided not to try. ‘It can’t be done, sir. It’s a technical impossibility.’
     ‘When I was your age I could jury-rig anything! Well, find me an answer, Mister. I can’t take this ship down blind. Not even for the Harriman Medal.’
     Bill Cole hesitated for a moment before replying. ‘I’ll have to go out while she’s still got spin on her, Captain, and make the replacement. There isn’t any other way to do it.’
     The captain looked away from him, his jaw muscles flexed. ‘Get the replacement ready. Hurry up about it.’

     Cole found the captain already at the airlock when he arrived with the gear he needed for the repair. To his surprise the Old Man was suited up. ‘Explain to me what I’m to do,’ he ordered Bill.
     ‘You’re not going out, sir?’ The captain simply nodded.
     Bill took a look at his captain’s waist line, or where his waist line used to be. Why, the Old Man must be thirty-five if he was a day! ‘I’m afraid I can’t explain too clearly. I had expected to make the repair myself.’
     ‘I’ve never asked a man to do a job I wouldn’t do myself. Explain it to me.’
     ‘Excuse me, sir — but can you chin yourself with one hand?’
     ‘What’s that got to do with it?’
     ‘Well, we’ve got forty-eight passengers, sir, and —‘
     ‘Shut up!’

     Sandburg and he, both in space suits, helped the Old Man down the hole after the inner door of the lock was closed and the air exhausted (remember under spin the airlock will be in the floor). The space beyond the lock was a vast, starflecked emptiness. With spin still on the ship, every direction outward was ‘down’, down for millions of uncounted miles. They put a safety line on him, of course — nevertheless it gave him a sinking feeling to see the captain’s head disappear in the bottomless, black hole.
     The line paid out steadily for several feet, then stopped. When it had been stopped for several minutes, Bill leaned over and touched his helmet against Sandburg’s. ‘Hang on to my feet. I’m going to take a look.’ (apparently these space suits have no radio)
     He hung head down out the lock and looked around. The captain was stopped, hanging by both hands, nowhere near the antenna fixture. He scrambled back up and reversed himself. ‘I’m going out.’
     It was no great trick, he found, to hang by his hands and swing himself along to where the captain was stalled. The Valkyrie was a space-to-space ship, not like the sleek-sided jobs we see around earthports; she was covered with handholds for the convenience of repairmen at the terminals. Once he reached him, it was possible, by grasping the safe steel rung that the captain clung to, to aid him in swinging back to the last one he had quitted. Five minutes later Sandburg was pulling the Old Man up through the hole and Bill was scrambling after him.

     He began at once to unbuckle the repair gear from the captain’s suit and transfer it to his own. He lowered himself back down the hole and was on his way before the older man had recovered enough to object, if he still intended to.
     Swinging out to where the antenna must be replaced was not too hard, though he had all eternity under his toes. The suit impeded him a little — the gloves were clumsy — but he was used to spacesuits. He was a little winded from helping the captain, but he could not stop to think about that. The increased spin bothered him somewhat; the airlock was nearer the axis of spin than was the antenna — he felt heavier as he moved out.

     Getting the replacement antenna shipped was another matter. It was neither large nor heavy, but he found it impossible to fasten it into place. He needed one hand to cling by, one to hold the antenna, and one to handle the wrench. That left him shy one hand, no matter how he tried it.
     Finally he jerked his safety line to signal Sandburg for more slack. Then he unshackled it from his waist, working with one hand, passed the end twice through a handhold and knotted it; he left about six feet of it hanging free. The shackle on the free end he fastened to another handhold. The result was a loop, a bight, an improvised bosun’s chair, which would support his weight while he man-handled the antenna into place. The job went fairly quickly then.

     He was almost through. There remained one bolt to fasten on the far side, away from where he swung. The antenna was already secured at two points and its circuit connection made. He decided he could manage it with one hand. He left his perch and swung over, monkey fashion.
     The wrench slipped as he finished tightening the bolt; it slipped from his grasp, fell free. He watched it go, out and out and out, down and down and down, until it was so small he could no longer see it. It made him dizzy to watch it, bright in the sunlight against the deep black of space. He had been too busy to look down, up to now.
     He shivered. ‘Good thing I was through with it,’ he said. ‘It would be a long walk to fetch it.’ He started to make his way back.
     He found that he could not.
     He had swung past the antenna to reach his present position, using a grip on his safety-line swing to give him a few inches more reach. Now the loop of line hung quietly, just out of reach. There was no way to reverse the process.


     He hung by both hands and told himself not to get panicky — he must think his way out. Around the other side? No, the steel skin of the Valkyrie was smooth there — no handhold for more than six feet. Even if he were not tired — and he had to admit that he was, tired and getting a little cold — even if he were fresh, it was an impossible swing for anyone not a chimpanzee.
     He looked down — and regretted it.
     There was nothing below him but stars, down and down, endlessly. Stars, swinging past as the ship spun with him, emptiness of all time and blackness and cold.
     He found himself trying to hoist himself bodily onto the single narrow rung he clung to, trying to reach it with his toes. It was a futile, strength-wasting excess. He quieted his panic sufficiently to stop it, then hung limp.
     It was easier if he kept his eyes closed. But after a while he always had to open them and look. The Big Dipper would swing past and then, presently, Orion. He tried to compute the passing minutes in terms of the number of rotations the ship made, but his mind would not work clearly, and, after a while, he would have to shut his eyes.
     His hands were becoming stiff — and cold. He tried to rest them by hanging by one hand at a time. He let go with his left hand, felt pins-and-needles course through it, and beat it against his side. Presently it seemed time to spell his right hand.
     He could no longer reach up to the rung with his left hand. He did not have the power left in him to make the extra pull; he was fully extended and could not shorten himself enough to get his left hand up.
     He could no longer feel his right hand at all.
     He could see it slip. It was slipping — The sudden release in tension let him know that he was falling falling. The ship dropped away from him.

     He came to with the captain bending over him. ‘Just keep quiet, Bill.’
     ‘Where —,’
     ‘Take it easy. The patrol from Deimos was already close by when you let go. They tracked you on the ‘scope, matched orbits with you, and picked you up. First time in history, I guess. Now keep quiet. You’re a sick man — you hung there more than two hours, Bill.’

(ed note: unfortunately Bill has to resign, since he has contracted a bad case of pathological basophobia and agoraphobia. Luckily he is eventually rescued by a cat.)

From ORDEAL IN SPACE by Robert Heinlein (1948)

First Men to the Moon

This design is from a book called First Men to the Moon (1958) written by a certain Wernher von Braun, aka "The Father of Rocket Science" and the first director of NASA. The book came out shortly after the Sputnik Crisis.

Their solution to the "which way is down?" problem is to put the crew's seats on tracks. The track was shaped like a letter "L", with one track at 90° to the ship's tail (thrust axis) and the other at 90° to the ship's belly. While landing on its belly the seat would be on the belly track. While being a tail lander on the lunar surface, the seat would be on the tail track. During lift off for some odd reason the seat would be on the tail track but tilted back at 45°.

Space Angel

In Space Angel (1962), illustrated by the legendary Alex Toth, the pilot's chair and attached controls rotate on gimbals independently of the ship. Of course there is no need to rotate the chair on gimbals since the Star Duster never ever lands on its belly like an aircraft, but the unsophisticated audience demands it.

R/P FLIP

The Scripps Institution of Oceanography's FLIP ship does things the brute force way. Notice the two sinks at ninety degrees to each other. It features doors in the floor, portholes in the ceiling, tables bolted sideways to walls, and stairs leading to nowhere.

BIS Lunar Spaceship

Below is a crude but clever arrangement. Under thrust, "down" is in the direction of the red arrows and the green chairs feel like they are prone. When thrust is off, the ship is spun on its long axis for centrifugal gravity so "down" becomes in the direction of the yellow arrows. The green chairs abruptly feel like they are upright, and the crew can walk on the blue "floor". In other words, they deal with the problem by making the layout usable under either orientation. Due to the small diameter of the spacecraft, it will have to spin exceedingly fast to produce appreciable gravity.

Polygon Centrifuge

This clever design solves the problem of how to quickly assemble a wheel space station. Details can be found in Self-deploying space station final report .

But there is one tiny little drawback. You see, there is a reason that wheel space stations are shaped like, well, wheels and not like hexagons.


The amount of centrifugal gravity experienced is determined by the distance from the axis of rotation (the greater the distance, the stronger the gravity). So if you want the amount of gravity to be the same in all parts of the wheel, the station has to be a circle. That is the only shape where the all parts of the rim are the same distance from the axis.

The point is, with a hexagon, different parts of the rim are at different distaces from the axis, and so have different gravities.

Now, look at the first image below. The segment labeled "SPACE STATION RIGID MODULE" is one of the hexagonal sides. The green lines lead to the axis of rotation (i.e., that is the direction of "up". Note the little dark men figures, they feel like they are standing upright). And the red lines are lines of equal gravity. You will note that they do not align with the module.

In the module, centrifugal gravity will be weakest at the center of the module, and strongest at the ends where it joins with the neighbor modules (i.e., the longer the green line, the more intense the gravity). Even though the module is straight, the gravity will feel like it is a hill. If you place a marble on the deck in the center, it will roll "downhill" to one of the edges.

As you see, the designers tried to compensate for this by angling the decks, but it really doesn't work very well.


I was curious as to how much of a problem this actually was. After doing some trigonometry (with no help from my parents) my questionable results are that for a hexagonal station, the gravity will vary between 100% and 113%. This is only about ten percent, which is annoying but probably not a show-stopper.

At point A gravity is at 100%, whatever the station is spun up for.

Point B is 15 degrees counter-clockwise from A, so in right triangle ABX if adjacent side (line AX) is of length 1.00, then the hypotenuse (line BX or distance from the spin axis) is 2.00 - cosine 15° or 1.03. Therefore gravity is 103% (because according to the equation the gravitational acceleration is proportional to the distance from the spin axis).

Point C is 30 degrees counter-clockwise. This hypotenuse is 1.13 so the gravity is 113%

Again this is only a difference of 13%, but things dropped on the floor are going to accumulate at the station hexagon vertices as they roll downhill.

Unexpected Spin

If a spacecraft (with spin gravity or not) was slammed by a foreign object hard enough to start the spacecraft tumbling, this will generate unexpected spin gravity. This is pretty much guaranteed to create an emergency situation inside the ship.

This is highly unlikely to occur naturally.

But when Zane Mankowski started working with his simulation game, he discovered that this was rather common when low mass combat spacecraft were struck.

LIFE IN THE LONELY VOID

Another consideration mentioned a few times in previous posts is that crew modules are put close to the center of mass in case of fast rotations. Spinning a multi-kiloton spacecraft around fast enough to produce 9 g’s or more, enough to cause fatal damage to the crew, is rare, but it does happen in game. Keeping the crew near the center of mass reduces the centripetal acceleration on the crew in such cases.

It is very difficult to knock a multi-kiloton spacecraft into a fast spin, and if you have enough firepower to do so, you generally don’t need to slush the crew in this manner.

On the other hand, for smaller spacecraft, under a kiloton fast attack spacecraft, knocking them into a tailspin is actually rather common. To exacerbate this, small spacecraft with enormous projectile weapons can often knock themselves into unpleasant spins through recoil alone. As such, keeping the crew near the center of mass is most important on smaller spacecraft.

Spin Grav Types

FIRE BATON

Mars Crew Transfer Vehicle Artificial Gravity

The entire point behind this study was to discover the optimum way to give artificial gravity to an ion-drive spacecraft. Prolonged microgravity missions do horrible things to the health of the crew. Mars missions tend to have over-long wait times to start with. Limiting Mars missions to the ones with the shortest duration drastically reduces the available mission trajectories in a given decade. Ion and other electric powered drives only exacerbate the problem with their absurdly low accelerations. This particular design is going to take three extra months just to accelerate up to Terra's escape velocity (chemical and nuclear thermal propulsion reaches escape velocity in a matter of minutes). That is long enough for the crew to lose 4.5% of their bone mass.

The problem is that the standard artificial gravity architectures have problems on a spacecraft that uses rockets for propulsion. And these problems are also exacerbated by low-acceleration drives.

It all boils down to Thrust Vector Control (TVC).

Each of the mission's maneuvers contains a specifed Axis of Acceleration. To perform the maneuver the spacecraft's thrust axis has to be exactly on the axis of acceleration. Before the maneuver the spacecraft has to be rotated so the thrust axis is in the proper orientation, and during the burn the thrust axis must be monitored and corrected if it drifts off the specified acceleration axis.

The problem is that the spacecraft's spin-gravity section acts like a gargantuan momentum wheel. This gyrostabilizes the ship and will fight your attempts at TVC tooth and nail. This is referred to as the Rotational Angular Momentum problem.


Spin Gravity Concepts
ConceptFeaturesAdvantagesDisadvantagesExample
(Dependent Centrifuge)
  • In spin section (entire ship) the long axis is the spin axis

  • The entire ship rotates as a unit, there are no segments without rotation.
A. C. Clark
FIRE BATON
(Dependent Centrifuge: Tumbling Pigeon)
  • In spin section (entire ship) the habitat is counterweighted by the reactor and power conversion system

  • The entire ship rotates as a unit, there are no segments without rotation.

  • The majority of TVC is by pointing the entire vehicle
  • No rotating problematic joints, megawatt power connections or fluid piping

  • Power conversion can take advantage of operating in a gravity field
  • The vehicle angular momentum must be continuously vectored during TVC in order to deal with the rotational angular momentum problem.

  • Heat radiators have to be designed to operate in a gravity field.

  • It is challenging to design methods for crew ingress, crew egress, and ship docking to a spinning object.
Mars NEP with Artificial Gravity
OX CART
(Independent Centrifuge Hab+Power)
  • In spin section (everything but engine modules) the habitat is counterweighted by the reactor and power conversion system

  • Thrusters are de-spun and gimbaled for TVC
  • TVC is decoupled from rotational angular momentum, thus avoiding the rotational angular momentum problem.

  • Power conversion can take advantage of operating in a gravity field
  • Design is faced with the daunting problem of transferring megawatts of electricity and kilograms of propellant across a rotating joint. For months.

  • Potential cyclical loading of rotating joints can shatter them.

  • Heat radiators have to be designed to operate in a gravity field.

  • It is challenging to design methods for crew ingress, crew egress, and ship docking to a spinning object.
Boeing STCAEM Mars NEP
BEANIE CAP
(Independent Centrifuge Hab Only)
  • Habitat modules spin for gravity, the rest of the spacecraft is stationary.

  • The two habitat modules act as counterweights.

  • The thrusters are gimbaled for TVC.

  • TVC is decoupled from rotational angular momentum, thus avoiding the rotational angular momentum problem.

  • Heat radiators can take advantage of operating in a zero-gravity field
  • Crew ingress, crew egress, and ship docking can be easily done to a stationary docking port.
  • There are inefficiencies in duplicating habitat modules.

  • Allowing crew to transfer between two spinning modules is a problem.

  • Potential cyclical loading of rotating joints can shatter them.

  • Power conversion have to be designed to operate in a zero-gravity field.

  • Design is faced with the problem of transferring kilowatts of electricity across a rotating joint.

Hedrick Fusion Spacecraft


The problem can be avoided by de-spinning the spin-grav section of the ship for the duration of the thrust. Sadly, since the thrust is more or less on for the entire trip, this kind of defeats the point of giving the ship spin-grav in the first place.

Ox Cart and Beanie Cap avoid the rotational angular momentum problem by de-spinning the engines from the spin gravity sections of the ship. The spin plane is aligned with the interplanetary trajectory plane. The main draw-back is the engineering and maintenance nightmare of the rotation joints.


Fire Baton is trying a new approach. The entire ship spins in order to avoid those nasty rotation joints. Instead it tries to precess the entire ship in order to aim the thrusters for TVC.

To lock a spacecraft or other object solid with gyrostabilization you actually need three spinning gyros at 90° angles to each other. A spin-grav ship only has one spinning object. So instead of being locked in place, if you push on it the spinning thing will undergo precession. Which is a fancy word meaning the object rotates unexpectedly at a right angle to the direction you push it. Try playing with a spinning gyroscope and you'll quickly discover this.

The report did an analysis and discovered that thrust vector adjustments came in two classes: very slow rates and moderate rates. The very slow rates were changing the vector less than two degrees per day (during the heliocentric trajectory). The moderate rates were changing the vector fifteen degress per day (during Terra departure and during midcourse thrust reversals). This means that two different steering strategies can be used. For flipping the main engines to point the opposite direction (for braking) a third strategy can be used.

In both strategies, the mechanism is to thrust in a direction at right angles to the desired steering direction, to precess the thrust axis in the desired direction (see "resulting precessional yaw rate" in diagram above). The thrust has to be done intermittently, when the thruster is pointed in the correct direction by the ship's spin (see "thrusting arc" in diagram above). If the thrust is appled every 180° of a spin-grav rotation, very slow rates require 3 Newtons and moderate rates require 15 Newtons.

The three strategies are:

  1. firing the control thrusters (RCS)
  2. differentially throttling the main ion thrusters (the two banks are throttled in an unbalanced manner)
  3. firing tangential RCS to spin the ship 180° on its long axis

The report tried all the combinations, and concluded that the lowest propellant consumption was if:

  1. Very slow rates precession (∼2°/day, 3 n) was performed with differentially throttling the ions engines ±5%
  2. Moderate rates presession (15°/day, 15 n) was done with the RCS
  3. Spinning 180° on the long axis was performed with tangential RCS

Over the entire mission the report calculates these strategies will require approximately one extra metric ton of propellant (1,074 kg). And no nasty rotation joint needed.

Dependent Centrifuge

DEPENDENT CENTRIFUGE 1

The landing boat overtook Discovery from below and behind, giving Drake a good look at his ship. The battle cruiser consisted of a torpedo-like central cylinder surrounded by a ring structure. The central cylinder housed the ship’s mass converter, photon drive, and jump engines — the latter needing only an up-to-date jump program to once more hurl the ship into the interstellar spacelanes. In addition, within the cylinder were fuel tanks filled with deuterium and tritium enriched cryogen; the heavy antimatter projectors that were Discovery’s main armament; and the ancillary equipment that provided power to the ship’s outer ring.

The surrounding ring was supported off the cylinder by twelve hollow spokes — six forward and six aft. It contained crew quarters, communications, sensors, secondary weapons pods, cargo spaces, and the hangar bay in which auxiliary craft were housed.


Drake listened to the communications between the landing boat and the cruiser all through the approach. As they drew close, he noticed the actinic light of the ship’s attitude jets firing around the periphery of the habitat ring. When in parking orbit, the cruiser was spun about its axis to provide half a standard gravity on the outermost crew deck. The purpose of the attitude jets was to halt the rotation in preparation for taking the landing boat aboard.


There is a common belief among the uninitiated that a spaceship’s control room is located somewhere near the ship’s bow. In truth, that is almost never the case. Discovery, with its cylinder-and-ring design, was particularly unsuited to such an arrangement. Like most warships, the cruiser’s control room was located in the safest place the designers could find to put it — at the midpoint of the inside curve of the habitat ring.

Actually, Discovery possessed three control rooms, each capable of flying or fighting the ship alone should the need arise. For normal operations, however, there was a traditional division of labor between the three nerve centers. Control Room No. 1 performed the usual functions of a spacecraft’s bridge (flight control, communications, and astrogation); No. 2 was devoted to control of weapons and sensors; and No. 3 was used by the engineering department to monitor the overall health of the ship and its power-and-drive system.


An hour later, the ship was accelerating along a normal departure orbit at one standard gravity while crewmen rushed to convert compartments from the “out is down” orientation of parking orbit, to the “aft is down” of powered boost. The only compartments that did not need conversion were the control rooms (which were gimbaled to automatically keep the deck horizontal) and the larger compartments (hangar bay, engine room), which had been designed to allow access regardless of the direction of “down.”


Their destination was Alexandria’s main ballroom. Situated on the outermost level (where gravity was highest), the ballroom was large enough for the deck underfoot to show a perceptible curve. In order to use the large compartment, however, it was necessary that ship’s spin provide the pseudo-gravity. During powered flight, when gravity was ‘aft’ rather than ‘out’, the compartment was a deep arc-shaped well of limited utility. Since no spaceship can afford to waste that much space, the architects who designed the liner had installed four levels of retractable decks. Once extended from their recesses in the walls, they turned the oversize compartment into a series of smaller spaces.

From ANTARES DAWN by Michael McCollum (1998)

Tumbling Pigeon

In Heinlein's The Rolling Stones, some spacecraft are classified as "tumbling pigeons". They rotate end over end to provide artificial gravity (i.e., they spin on the short axis instead of the long axis). The idea is to increase Cl, that is, if you spin on the long axis Cl is the relatively tiny width of the ship, while if you spin on the short axis Cl is the length of the ship.

Of course, this means that in all the crew spaces in the "top" of the ship, the floor will become the ceiling ("top" is defined as the half of the ship that does not contain the engines, with the dividing line along the axis of rotation). In the novel, such ships were generally limited to passenger liners for tourists with weak stomachs.

However, there were a couple of real-world designs usings the tumbling pigeon technique (e.g., Mars NEP with Artificial Gravity and Stuhlinger Ion Rocket). In the designs a dependent centrifuge would not provide a long enough Cl unless the habitats were put on long radial arms. This increases the structural mass and cuts into the payload mass. By using the tumbling pigeon arrangement, the ship's spine does double duty as framework and as radial arms. No extra structural mass needed.

Martin Marietta Study

A variety of Artificial gravity (AG) / Mars transfer vehicle (MTV) concepts were developed by the Martin Marietta Astronautics Group for NASA’s Mars Exploration Case Studies in 1988 – 89. Each of these concepts used a large diameter (~39 – 46 m) aerobrake (AB) with a low lift to drag (L/D) ratio of ~0.2 for Mars orbit capture (MOC). These large ABs required assembly in LEO before being outfitted with habitation, auxiliary PVA power and chemical propulsion system elements within their protective envelope. By rotating the AB about its central axis at different spin rates and mounting the habitat modules near the outer perimeter of the AB to increase the rotation radius, a range of centrifugal forces can be generated for the crew during the transit out to Mars and back. A sampling of these AB concepts (minus their multiple expendable trans-Mars injection (TMI) stages) is shown in Fig 1.

Concept 1 was developed for a large crew of 12 – 18 astronauts. It carried eight cylindrical Space Station Freedom (SSF)-type habitation modules arranged in a ring to provide a 100-m long circular jogging track. The modules were mounted to a large 45.6-m diameter AB sized for aerocapture at both Mars and again at Earth for spacecraft recovery and possible reuse. This very large spacecraft had an initial mass in low Earth orbit (IMLEO) > 1500 metric tons (1 t = 1000 kg).

Concept 2 carried 8 astronauts and used four SSF habitat modules arranged in a “Bent-I” configuration inside a 41-m diameter AB. Two pressurized tunnels connected the four habitat modules to a central logistics and docking hub to which the Mars Descent / Ascent Vehicle (MDAV) was attached. The IMLEO for Concept 2 was ~1091 t.

Concept 3 utilized a deployable flexible fabric AB (~39-m in diameter) and carried two cylindrical hab modules each with five separate floors arranged perpendicular to the modules’ long axis. The modules were attached to the central logistics and docking hub using swivel joints allowing them to swing outward to increase their rotation radius during AG operation. The modules were cranked back inside the protective envelope of the AB prior to MOC. The modules housed 5 - 7 crew and the total mission IMLEO was ~687 t including the four expendable TMI stages.

Concepts 4 and 5 used dual retractable tethers to separate paired or individual hab modules from the AB and primary propulsion system. With tether lengths of approximately several hundred meters, rotations rates as low as 2 revolutions per minute (rpm) could provide ~1-g of centrifugal acceleration for the crew.

Each of the above concepts had a number of drawbacks. Concepts 1 and 2 were very large, required significant orbital assembly for the AB and overall vehicle, and had large IMLEO requirements (>1000 t). Concept 3 required an internal arrangement for the hab modules that differed from that of the SSF habitation modules used in the other designs. It also required movement of two major pressurized mechanical joints. With tethered Concepts 4 and 5, the reaction control system (RCS) propellant requirements to initiate and stop vehicle rotation were larger, and the dynamic control problems more severe during the deployment and retraction process, as well as during vehicle spin up and spin down. A tether break or reel freeze-up could also be a critical failure mode. From an operational standpoint, once deployed, the crew in Concept 4 would be isolated from the systems enclosed within the aerobrake (e.g., MDAV) and in Concept 5, isolated from each other as well.


To avoid the deficiencies of the above concepts, Martin Marietta proposed Concept 6, an AG/MTV design that used chemical propulsion and carried twin cylindrical SSF habitation modules whose long axes were oriented perpendicular to the longitudinal spin axis of the MTV – referred to as the Dumbbell B configuration (Fig. 2). The hab modules were connected to a central logistics and docking hub by two pressurized tunnels each ~12.5 m long. Each hab module – designed to accommodate 2 - 3 crewmembers – had excess capacity so that either could serve as a safe haven for the entire crew in case of an emergency. Attached to the Sun-facing side of each tunnel and hab module were ~30 m2 and 75 m2, respectively, of PVAs producing ~26 kWe of electrical power for the spacecraft’s various systems. Once fully assembled, the rotation radius from the center of the logistics module to the floor of each hab module was ~17 m allowing centrifugal acceleration levels ranging from 0.38-g to 0.68-g for vehicle spin rates of 4.5 to 6 rpm. At a slightly higher spin rate of 7.25 rpm, 1-g could be achieved. The pressurized logistics hub also provided a shirt-sleeve environment and anytime crew access to the MDAV docked to the front of the vehicle.


General Design Principles

  • Hab Modules should have their major dimension / traffic pattern parallel to the vehicle's spin axis
  • Radial traffic flow should be minimized
  • Orient command / work station displays vertically to minimize left-right head rotations — lateral axis through ears should be parallel to spin axis
  • Orient sleeping bunks parallel to spin axis
  • Minimize visual stimuli (windowless crew cabins)

Radial Orientation

  • Multi-level vertical design
  • Large gravity gradient
  • Ladders between levels
  • Tangential and Transverse Coriolis effects
  • Potentially simpler payload (PL) & SC design Earth Orbit Rendezvous and Dock (EOR&D)

Axial Orientation

  • Uni- or bi-level horizontal layout design
  • Minimal gravity gradient
  • Long transfer tunnel to central connecting hub
  • Minimal Coriolis effects
  • More complex PL & SC design (on-orbit assembly)

Tangential Orientation

  • Uni- or bi-level horizontal layout design
  • Minimal gravity gradient
  • Long transfer tunnel to central connecting hub
  • Coriolis effects
  • More complex PL & SC design (on-orbit assembly)

In designing an AG spacecraft, a number of important human factors must be taken into account in selecting the rotation radius, angular velocity, and g-levels. These factors include the gravity gradient effect, Coriolis forces and cross-coupled acceleration effects. These human factor effects also come into play when considering the orientation of the habitat module or modules (shown in Fig. 5) relative to the spin axis of the vehicle. Orientation options include:

  1. Radial (used by Concept 3, the von Braun and Copernicus-B)
  2. Tangential (used in Concepts 1, 2, 6 and the A. C. Clark) – also referred to as the Dumbbell B configuration
  3. Axial (used in Concepts 4, 5, and a variant of the A. C. Clark) – referred to as the Dumbbell A configuration

The radial habitat module is by definition a multi-level vertical design. Because the centrifugal acceleration varies directly with the radial distance from the center of rotation, a vertical gravity gradient will exist between the different levels of the hab module(s) and even along the human body itself. Crewmembers climbing “up” a radial-oriented ladder toward the vehicle’s center of rotation would lose weight with each step. Awkward materials handling problems and uneven g-loadings on the body are also possible but are not expected to be significant on AG vehicles with reasonable rotation radii.

Tangential Coriolis forces will also expose the crew to pseudo weight changes depending on their direction of motion with respect to the spin axis of the vehicle. While no Coriolis force occurs when walking parallel to the spin axis, astronauts will feel heavier when moving in the direction of vehicle rotation and lighter when walking in the reverse direction. Transverse Coriolis forces will be experienced by astronauts moving vertically between habitat levels. When climbing “up” toward the vehicle’s center of rotation, the astronauts will be pushed sideways in the direction of spacecraft spin. A sideways push away from the spin direction will be felt when climbing “down” the ladder (refer to Fig. 6).

Lastly, cross-coupled angular acceleration effects will be experienced by astronauts early on due to head movement in the directions transverse to the axis of rotation and the primary direction of spacecraft flight. With time and use of distinctive interior color schemes or wall-mounted designations to help identify spin direction (depicted in Fig. 6), astronauts should be able to compensate for and acclimate to these AG effects. General design principles for habitats onboard rotating AG spacecraft were provided by Loret and are summarized in Fig. 5.

Despite the lack of current experimental data needed to establish accepted g-threshold requirements and other operational characteristics (e.g., rotation radius and ω), previous experts in this area used existing physiological and/or human factors data coupled with reasonable assumptions to identify representative operational regions for AG vehicles. Stone and Thompson recommended a rotation radius ≥ 14.6 m and a spin rate ≤ 6 rpm, while Shipov thought that the minimum radius should be ~20 m.

During the 1960’s, researchers at the Naval Medical Research Laboratory in Pensacola, Florida used their Slow Rotating Room (SRR) to study the acute rotation effects phenomena at rates as high as 10 rpm. Their results indicated that a judicious restriction of head motions and progressive adaptation through stepwise increases in spin rate (1 rpm every 2 days during 16 days of rotation) allowed most human subjects to adjust quickly to avoid the adverse physical symptoms of higher rotation rates. “Later studies have expanded on the experience from that time and demonstrated that complete adaptation to rotation rates as high as 10 rpm can be achieved within minutes if repeated voluntary movements are made. Such movements were avoided in the early Pensacola studies.”

Bola

There are some ship designs where the ship separates into two sections connected by cables and spun, in a desperate attempt to increase Cl. Please note that such a spacecraft is "spinning like a bola," not "spinning like a bolo."

Independent Centrifuge

An independent centrifuge is where only part of the ship is spun for gravity while the rest stays stationary. As opposed to dependent centrifuges where the entire ships spins.

Usually it takes the form of a large spinning ring with its axis coincident with the thrust axis (i.e., it looks like a pencil stuck through a doughnut). But there are some types where the centrifuge is internal, e.g., the Discovery from 2001.

INDEPENDENT CENTRIFUGE 1

After lunch, from 1300 to 1600 Bowman would make a slow and careful tour of the ship — or such part of it as was accessible. Discovery measured almost four hundred feet from end to end, but the little universe occupied by her crew lay entirely inside the forty-foot sphere of the pressure hull.

Here were all the life-support systems, and the Control Deck which was the operational heart of the ship. Below this was a small "space-garage" fitted with three airlocks, through which powered capsules, just large enough to hold a man, could sail out into the void if the need arose for extravehicular activity.

The equatorial region of the pressure sphere — the slice, as it were, from Capricorn to Cancer — enclosed a slowly rotating drum, thirty-five feet in diameter. As it made one revolution every ten seconds, this carrousel or centrifuge produced an artificial gravity equal to that of the Moon. This was enough to prevent the physical atrophy which would result from the complete absence of weight, and it also allowed the routine functions of living to be carried out under normal — or nearly normal — conditions.

The carrousel therefore contained the kitchen, dining, washing, and toilet facilities. Only here was it safe to prepare and handle hot drinks — quite dangerous in weightless conditions, where one can be badly scalded by floating globules of boiling water. The problem of shaving was also solved; there would be no weightless bristles drifting around to endanger electrical equipment and produce a health hazard.

Around the rim of the carrousel were five tiny cubicles, fitted out by each astronaut according to taste and containing his personal belongings. Only Bowman's and Poole's were now in use, while the future occupants of the other three cabins reposed in their electronic sarcophagi next door.

The spin of the carrousel could be stopped if necessary; when this happened, its angular momentum had to be stored in a flywheel, and switched back again when rotation was restarted. But normally it was left running at constant speed, for it was easy enough to enter the big, slowly turning drum by going hand-over-hand along a pole through the zero-gee region at its center. Transferring to the moving section was as easy and automatic, after a little experience, as stepping onto a moving escalator.

From 2001 A SPACE ODYSSEY by Sir Arthur C. Clarke (1969)

Canfield Joint

One of the nagging problems with an independent centrifuge is trying to transfer power and/or fluids across the hub. You cannot just put a cable or hose across because the spin will just keep twisting it until it snaps. Many designs try to use slip-rings (to transmit power) and rotary unions (to transmit fluids) but these are maintenance horror-shows and have limited capacity. Try sending megawatts over a slip ring and see how long it lasts. And if you are trying to send multiple fluid types (e.g, drinking water and sewage) the problem multiplies.

One possible solution is to use a Canfield Joint. It can move a linkage over 360° without twisting any number of hoses and cables. Watch the video, especially the part about the solar panels. Note that while the spacecraft is spinning, the orientation of the solar panels will be constantly changing but they will always be perpendicular to the sun. The same will apply to an array of ion drive rockets, the orientation will constantly change but they will always be aimed at the mandated thrust axis.

CANFIELD JOINT 1

The Canfield joint is a pointing mechanism that allows for full hemispherical motion from whatever connects to it. Invented by Dr. Stephen Canfield of the Tennessee Tech University, this joint was developed specifically for spacecraft thrusters and solar panels. Its gimbal mount simplifies the roll program performed when the space shuttle launches and allows for greater overall manoeuvrability from the reaction control system. Unlike other joints, which can only transmit rotational motion up to a constant 70° (not 0 ° to 70°), this joint can transmit rotary motion from 0° and in increments one 1° to 90°. This joint also has higher stability due to its unique cage-like design. By making use of appropriate actuators (hydraulic/pneumatic), the joint can be moved with surprising speed and accuracy.

When applied to solar panels, the Canfield joint tracks the Sun more of the time and will not tangle the power cords attached to them. This is especially valuable to space flight when the spacecraft is performing complicated manoeuvres. Its application was expected to be incorporated into the now-defunct Constellation Program as a key element.

Advantages Over Fixed Thrusters

  • Fewer parts resulting in fewer mechanical failures and less weight
  • Twelve fewer thrusters
  • Simplifies movement for roll maneuver
  • Allows greater maneuverability
From the Wikipedia entry for CANFIELD JOINT
CANFIELD JOINT 2

Solar arrays, communications systems, and thrusters would all need to transfer power, data, and fluids across rotating joints or slip rings if they were to be mounted near the pressurized module and yet fulfill their duties. These fluid, data, and power transfers could become very unpleasant development problems. Fortunately, several recent innovations make such operation possible without any sort of rotating connections.

The key is a unique mechanical joint developed by Dr. Stephen Canfield of Tennessee Technological University. The “Trio Tri-Star Carpal Wrist Robotic Joint”, mercifully referred to simply as the Canfield joint, is a “parallel” mechanism that allows a structure to point at an inertial target while being mounted to a rotating platform. Fluid, power, or data connections can be made from one side of the joint to the other because the two sides of the joint do not differentially rotate, even though the joint itself may be bent 90 degrees or more.

The kinematic behavior of the Canfield joint is extremely difficult to describe in words, and not much easier to discern in still pictures, but suffice it to say, seeing is believing—the joint is truly capable of smooth pointing and orientation within a hemisphere.

Figure 12: A series of still pictures showing a typical reaction-control thruster mounted on a Canfield joint, capable of pointing any direction within a hemisphere.

The parallel structure of the joint not only gives it tremendous strength, but three degrees of freedom—altitude, azimuth, and “plunge”. The first two degrees are sufficient to allow it to point to any direction within a hemispheric workplane. And unlike other structures, the joint can access this entire workspace without encountering mathematical “singularities” that make solution of the base angles impossible. The joint is driven by three actuators on each of the three base legs. Any commanded direction corresponds to a unique solution of the three angles between the base and the base legs.

Figure 13: A unique arrangement of solar arrays mounted on Canfield joints enable constant solar tracking at any beta angle on a rotating facility with a minimum of bending loads across the array.

CANFIELD JOINT 3

I told the story of how I had gotten involved with the JSC study of an artificial-gravity/nuclear-electric propulsion (AG-NEP) Mars vehicle study. I came into the study near the end (January 2003) and right before the Columbia disaster.

As near as I could tell, after Columbia happened, nobody kept working on the AG-NEP design, or even on Mars studies for that matter. If they did, I certainly didn’t know about it.

But for some reason, the whole idea kept rattling around in the back of my head. There were a few reasons that the JSC guys had given me that were compelling for AG-NEP as a Mars vehicle.

1. You solve the muscle and bone loss problem through artificial gravity. You don’t have to worry about hours of exercise or fret whether their bones will snap when they re-enter the Earth’s atmosphere. They’re going to be good and strong when they get home because you made sure that their bodies felt a normal level of gravity throughout the trip.

1a. Because you’ve solved the muscle and bone loss problem, the pressing need to fly the mission quickly is tremendously diminished. You can go to Mars and come back in the three-year time frame that is more astrodynamically “natural”, in other words, the time frame that aligns with the Earth and Mars’s movements around the Sun.

2. By using nuclear-electric propulsion, you actually have a credible propulsion system to execute a mission abort if you need to, for some reason along the way. You’re not going to get back quickly, but you can get back.

3. By using nuclear-electric propulsion, you actually have a credible story for vehicle reuse. You could refuel the vehicle and go again. Or you could go somewhere else like an asteroid. You have a lot more flexibility than in a chemical or NTR vehicle.

I liked the basic idea. Here was a vehicle that might actually be a true “spacecraft” as we like to think of them, with the ability to go and come from a variety of destinations and be reused. I imagined that this might be the kind of vehicle that would be in Captain Picard’s ready-room a few centuries from now as a little model, with him saying, “This is a model of the vehicle man used to explore the solar system in the early days.”

But there were definitely residual technical problems with the design as it stood when I was exposed to it. The biggest one had to do with getting the body-mounted electric thrusters to point in the right direction as the vehicle moved around the Sun, and the problem got so bad when you got to a spiral-in, spiral-out scenario around a planet that it was practically a no-go. It came down to the architectural decision to orient the thrusters so that they were firing in the same direction as the vehicle’s angular momentum vector (orthogonal to the rotation plane). That approach certainly solved any problem of plume impingement, but since the inertial direction of the thrust vector was going to change by >180 degrees during the transit to Mars, and by that much or more on the way back, you had to continuously move the angular momentum vector of your spacecraft around, and there was a non-trivial cost associated with doing that. During spiral-in or spiral-out the cost became prohibitive.

The other problem concerned spin up and spin down of the system. The JSC design assumed that spinup and spindown would be done by dedicated thrusters on the habitat module end of the vehicle. That meant a duplication in thrusters and tankage for a capability that you would want to utilize as little as possible.

Despite these problems, I recognized that the JSC design as it stood had also solved a great many problems, and that perhaps it represented a minimum in the design space of overall difficulty. I’m fond of saying “you have to pick your pain” when it comes to system optimization, and that the “best” system always involves residual problems. Perhaps this was as good as it got.

Or maybe it could be even better.

One day I was driving down the street in the pouring rain and a simple sequence of thoughts formed in my brain:

1. I had spent a whole bunch of time trying to figure out how to get solar panels on a MXER tether to point at the Sun while the tether rotated.

2. I had been lucky enough to meet Steve Canfield and had figured out how to use the Canfield joint to fix the problem of pointing the panels at an inertial target (the Sun) while the overall structure (the tether) rotated.

3. The basic problem that the AG-NEP vehicle faced was the need to point its electric thrusters at an inertial target (its thrust vector) while it rotated, much like the MXER tether needed to do with its solar arrays.

4. The reasons that JSC had rejected rotating machinery for the AG-NEP vehicle had to do with the difficulty of moving propellant and electric power across a rotating connection like a rotary union or slip-ring, and these were good and valid reasons.

5. The Canfield joint had no such problems because provided that propellant lines or power cables were flexible, they could transmit fluids and power across a Canfield joint.

thus…maybe a Canfield joint was the answer to the problems of the AG-NEP vehicle!

I couldn’t believe that I had known about the Canfield joint for so long and hadn’t put these utterly compatible ideas together.

If we were to use the Canfield joint on the AG-NEP vehicle, the overall geometry would change substantially. The logical location for the thrusters moved from the center of the vehicle, on a cross-brace, to the reactor end of the vehicle. This kept the high-power lines short since they didn’t have to run all the way to the middle of the vehicle to reach the engines. You could also place the propellant tanks on the reactor end of the vehicle as well.

This in turn led to several other vehicle advantages:

1. The moment-arm from the reactor module to the hab module is shortened (or alternatively the moment arm from the CM to the hab module can be lengthened) because now there is much more mass counterbalancing the hab module. The thrusters and the propellant constitute a lot of mass.

2. The truss between the reactor module and the hab module now doesn’t need any “cross-brace” on it or any other body-mounted structures. It can be a strong but simple extensible structure, like a CoilABLE boom, with nothing more than the power connection between the reactor and the hab module integrated into it.

3. The main thrusters can be used for spin up and spindown operations. By placing them on the end of the moment arm, they now have the ability to change the angular momentum of the vehicle, by simply remaining fixed relative to the vehicle during spinup and spindown. In fact, spin rate can be changed during thrusting simply by changing the fraction of the spin arc during which the thrusters fire.

4. The angular momentum vector of the vehicle doesn’t have to point along the thrust vector (like in the JSC design) but can point orthogonal to the spacecraft’s orbital plane. This means that the angular momentum vector’s direction doesn’t have to be altered during flight. This also means that spiral-in/spiral-out maneuvers at planets are no problem.

5. If you wanted to use the AG-NEP vehicle for asteroid missions, the electric thrusters might even be able to be used as “descent engines” provided some “landing gear” were provided on the habitat module.

6. Propellant could be used for additional reactor shielding during the flight.

Gimbaled Centrifuge

Gimbaled Centrifuges are attempts to deal with the "which way is down?" problem. As a general rule they are hideous engineering challenges and maintenance nightmares.

Pilgrim Observer
Pilgrim Observer
LevelDist from
center
Centrifugal
Accel
Gravity
Level 610.16 m0.447 m/s0.05g
Level 512.70 m0.559 m/s0.06g
Level 415.24 m0.671 m/s0.07g
Level 317.78 m0.782 m/s0.08g
Level 220.32 m0.894 m/s0.09g
Level 122.86 m1.006 m/s0.10g

Another possibility is an arrangement like the Pilgrim Observer. This is a variant on rotating the room on gimbals, it is actually a "gimbaled centrifuge". The three living quarters are held parallel to the spacecraft's long axis when under acceleration. At other times, they are extended at ninety degrees, and spun like blades of a propeller to act as a centrifuge. The direction of "down" is always the same, whether under thrust or spin. This also allows the hub to remain stationary, providing a mounting for all the telescopes and other sensors which would otherwise have to cope with being spun around. Not to mention simplifying the docking of small craft. In the diagram, the three living quarters arms and the thin ring they are attached to are the only parts of the spacecraft that spin, the rest of the spacecraft is stationary.

The blades can be spun up by attitude jets, or by a flywheel. The advantage of a flywheel is that the blades can be stopped by stopping the flywheel.

In the picture of the Pilgrim Observer, it looks like there are six levels on each blade. Use the distance from the floor to the center of rotation as "point X" in order to calculate the artificial gravity for each level. The Pilgrim spins at a rate of 2 revolutions per minute, the maximum radius at the bottom of the blade is 22.86 meters (75 feet), each deck is 2.54 meters tall (100 inches).

Dream Pod 9

The innovative people at Dream Pod 9 have an elegant version of the gimbaled centrifuge. Their ships have a centrifuge ring which pierces the centers of a series of rectangular habitat modules. The habitat modules can pivot on their axis. There are two habitats visible in the image above. In the head-on view to the right, they are at one o'clock and seven o'clock, with the pivot located where they intersect the ring. As in the Pilgrim, they pivot so that down is aft when the ship is under thrust, then pivot so that down is sideways while the ring starts rotating.

The pivot is the weak point, obviously. The pivots on the US Air Force B-1 bomber are only rated for about three gravities of acceleration.

Graham Baxter points out that the diagram pictured to the right makes more sense if the labels for "In Flight" and "In Free Fall" are swapped.

It strikes me that the design featured has the angle of the habitat modules backwards; instead of having the modules in a vertical 'office building' layout as shown, wouldn't it be better to lay them out horizontally, with the decks running parallel to the long axis of the module? With the decks laid out in this fashion, the module would have far less variation in centripetal force between decks when under rotation and depending on how the module was locked into place during acceleration, more/stronger anchoring points to the rotating frame as well?

At the very least, you cut down on the need to climb all those damn stairs.

Graham Baxter

Both Nick Dumas and Christopher Moore have pointed out to me that Graham Baxter's arrangement is actually the way the designers intended. The problem is that the diagram is confusing. Please find below a modified diagram that makes things clear.

Ezekiel's Wheel

I have thought up a not terribly original variant on the gimbaled centrifuge that I call "Ezekiel's Wheel". Be told that when I show this design to real engineers they laugh themselves silly, mostly due to the reasons given by Ken Burnside.

Remember the basic problem is that the direction of "down" is different under thrust than it is under spin gravity.

A gimbaled centrifuge uses long slabs for the centrifuge, rotating them to change from thrust mode to spin mode. The trouble is that slabs are a very inefficient use of habitat space. It would be nice if you could somehow rotate the orientation of a doughnut shaped centrifuge.

Imagine that the doughnut is actually sliced through to form six or so radial segments (i.e., cut the doughnut like it was a pie). See the green parts in the diagram to the right. For gimbaling, rotate each segment along their long axis to switch modes. Note red arrow for "direction of 'down' while thrusting" and yellow arrow for "direction of 'down' while spinning".

The hard part is the mechanism that allows the segments to pivot. However, that has been solved by Josef F. Blumrich, formerly of NASA. His patent 3,789,947 is for a "unidirectional wheel", but for our purposes it includes a design for the pivot mechanism.

The gold wheels in Blumrich's mechanism allow the green centrifuge segments to rotate for mode changing. Yes, this is a monstrous Rube Goldberg contraption but you can't have everything.


The main problem is sometimes the floor is not level. In other words, the blasted thing is a polygon centrifuge.

In the diagram, note that the floor (which the little man is standing on) is curved. As in all centrifuges, this ensures that the floor will seem flat while under spin (that is, if you place a ball on the floor it will stay put). Unfortunately, when the spin stops and the habitat modules rotate down for thrust gravity, the floor will seem like valleys. If you place a ball on the edge of the habitat floor, the ball will roll "downhill" to the center floor of the module.

Or you could have it the other way. Make the floor straight instead of curved. This way it will seen flat while under thrust. However, under spin, the floor will seem curved like a hill. If you place a ball on the center of the habitat floor, it will roll "downhill" to one of the two edges. This is because the strength of spin gravity depends upon the distance to the spin axis, and the two edges are farther away from the axis than the habitat floor center.

The only solutions I've manage to think up involve dynamically altering the contour of the floor, or making each habitat segment narrow enough that the slope is manageable (which is the solution used by the Pilgrim Observer and Dream Pod 9).

Centrifuge Problems

Adding a centrifuge to a spacecraft add entire new categories of headaches for ship designers.

Now, one would think that such a centrifuge would act as a titanic gyroscope, doing its best to prevent the ship from changing its orientation. Aerospace Engineer Bill Kuelbs Jr points out that if the centrifuge is a sufficiently large percentage of the ship's total mass, it will not prevent turning. What it will do is alter the axis of any turning force by ninety degrees. Rev up a toy gyroscope and try to turn it to see what I mean.

The solution is fairly simple. The turning thrusters will have to be effectively at ninety degrees to where you'd expect. In reality, this means that when the centrifuge is spinning, the "pitch the nose downward" control button will actually fire the "yaw to the left" thruster. An alternative solutions is to have two centrifuges that are spinning in opposite directions.

Another problem is Spin Balancing.

Yet another minor problem is the Coriolis effect. A thrown ball or other object will have its trajectory bend to the side, as if being blown by a cross-wind.

CONSERVATION OF ANGULAR MOMENTUM

(ed note: these comments are about independent centrifuges)

If a spaceship has a spinning section to provide simulated gravity, and lest the whole ship is intended to spin, that section would need to be set in motion somehow.

If it is done by reaction engines, the connection between the non-spinning part of the ship and the spinning section would need to be literally free of any friction, otherwise, that part of the ship would eventually begin to spin also, and slow down the spinning section. Or another set of reaction engines would be needed to keep it from spinning.

But what I find more problematic is the above mentioned conservation of momentum. In a vessel with a spinning section set in motion by mechanical means or even contact-less linear electric motors strung around the circumference of the non-spinning section and the hub of the spinning section, the non-spinning section would obviously instantly spin once the spinning section is fired up, only in the opposite direction.

There would need to be a counter-rotating flywheel to counter this, spinning in the opposite direction of the main spinning section, or even a complete second spinning section turning the opposite way. Since a spinning section would need to be large not just to provide adequate room but also to have the necessary diameter to not necessitate a very high frequency of revolution, a flywheel would have to be substantial, and unless it has a double use, such as a radiation shield or the like, would be just a big lump of dead weight. Logically, a second habitable spinning-section would be preferable. Of course, a constantly burning reaction engine might be used, much in the same way as the tail-rotor of a helicopter, but that would just be a massive waste of fuel.

Also, there is the question of sealing the non-spinning section, like on the Discovery from 2001. Shaft-seals and the like would have to be huge lest the only way to get from the spinning section to the non-spinning section be so small as to necessitate contortionist-midget-astronauts. Huge seals are problematic, creating huge friction and needing to be kept clean. Labyrinth seals would be friction-less but not airtight (bad in a spaceship, unless one wants to include the entire spinning-section in the pressurised area of the ship). One could, of course, create to separate habitable environments for the two sections of the ship, but that would entail suiting up and cycling through two airlocks every time one has to rush from ones bunk to the control room.

Guido Lißmann
INDEPENDENT CENTRIFUGE PROBLEMS

(ed note: some of these problems apply to the independent centrifuge, others to the independent gimbaled centrifuge)

I have a very basic problem with spin sections in spaceships.

1) The more things are meant to move, the more they wear out. Making a gimbaled joint that doesn't become the maintenance nightmare of the F-14 swing-wing is a tricky proposition — particularly in space where there's vacuum welding, thermal differences from ambient solar radiation, and worse.

2) How the heck do you get the plumbing to work? I guarantee you that rotating your typical bathroom through 90 degrees will make every water conveyance run wrong. Not to mention the times in transition when it'll be at zero gee.

Or do you put in twice as many bathrooms?

The reason why ships in Ten Worlds don't have spin sections come from the maintenance issues, the fact that the trips themselves are months, not years in duration and that most of the waypoints have spin gravity on can cities.

And the "90 degree off-axis aggressive maneuvers with the portapotty" problem.

On a related note, when your spin section arms are extended out, where do your radiators go?

(ed note: Guido Lißmann notes that the plumbing problem has a couple of solutions. Route the pipes through the pivot center with some kind of rotating sleeve, have self contained plumbing systems totally within the centrifuge, or resort to chemical toilets like used in passenger aircraft. )

Ken Burnside

Spin Balance

Another problem is Spin Balancing. The spinning ship or centrifuge will have to make sure its mass is evenly balanced around the circumference, or it will start acting like an unbalanced clothes washer on spin cycle.

Without load balancing, the simple act of the crew walking around could be a disaster. The centrifuge could wobble: making the gravity uneven, causing structural damage/centrifuge destruction, and causing the center of gravity to oscillate.

The latter means the center of spin will wander around instead of being rigidly located at the designed spin axis. Which can be a real problem if the spin axis has the docking port. The situation will be just as nasty if the centrifuge is a wheel space station with a spacecraft landing bay at the spin axis. Generally the landing bay is de-spun by spinning contrary to the centrifuge, so incoming spacecraft can land in something stationary. If the space station is wobbling, so will the landing bay. Docking will be much more dangerous than a fighter jet trying to land on the deck of a sea-going aircraft carrier during a typhoon.

Even worse, spacecraft tend to have optimized structural support because every gram counts. The ship's spine will be very strong along the thrust axis, but being jerked sideways by an oscillating centrifuge could snap the spine like a piece of uncooked spaghetti.

Load balancing could be accomplished by a series of ballast tanks and a network of pipes to pump water from one tank to another. In NASA-speak, these are called ballast trim tanks.

BALLAST TRIM TANKS 1

Twenty years later (1949) Wernher von Braun independently came up with a very similar design. Though he had never read of the Wohnrad, his station also was wheel-shaped, 250 feet across, and rotating to give 1/3 g artificial gravity. He proposed the wheel have two or three floors and the atmosphere be oxygen and helium at one-half sea level pressure. He also devoted attention to pumping water into ballast tanks so the station would continue to spin evenly as people went about their business inside, redistributing the masses within the wheel. Von Braun proposed to use this station as a base for the outfitting of expeditions to the moon and Mars.

BALLAST TRIM TANKS 2

The complexities inherent in the momentum control system arise from the limits for center of gravity offset and for wobble allowed by the rotating torus. The overall inertial balance of the torus wlll tolerate crew members moving about within a section and individuals traversing the circumference. On the other hand, the shifting of a significant weight (200 kg) would require a balance trim such as the transfer of ballast water between holding tanks. A major shift of weight (all the crew to dinner at one sitting) would require a programmed and controlled counter-movement of ballast.


In addition, the (support) area includes tankage which will accommodate as much as 10,000 kg of water for trim ballast distributed and transferred among four locations In response to inertial control requirements.


The Large Rotation Transfer Unit provides the means to exchange spacecraft sub-assemblies or items of equipment between the central tube and the rotating portion of the Space Station. The unit accepts the freight elevators from Spokes 2 and 4 plus the variable gravity facilities from Spokes 1 and 3.

In an operation to transfer items from the torus to the central tube, the transfer unit would be rotating at torus speed relative to the central tube and be locked into a position that aligned the elevator guide rails. As the elevator moved into the spoke-extension sections, a ballast transfer would occur that put water into the counterbalance tank on the transfer section. In the same motion, the elevator would leave the carrier for the force equalizer and complete the travel against the partial gravity present at the hub.

With the trim completed, the drive systems engage to slow the rotation of the transfer unit, such that the energy goes into the rotating torus. A shift of trim ballast from hub to torus maintains a constant rotation (e.g., constant angular momentum). The transfer section comes to a rotational stop with the elevator registered to the main transfer port in the central tube.

The equipment is then transferred by manipulator into the bays of the central tube for use as intended (e.g., Spacecraft assembly or service). The transfer of equipment or elements for service from the central tube into the torus involves a reversal of the procedure. In all cases, the acceleration or deceleration of the transfer unit occurs with a balanced system. The adding or subtracting of rotational momentum interacts with the torus, and the rotation of the torus is maintained by a radial transfer of ballast.


7.1.2.1 The Personnel Unit Concept

The most frequent transit requirement will involve crew members and small items of equipment moving between the central tube and the torus. The personnel transfer unit has been configured to minimize the effects of gravity change Coriolis forces and differential motions. The personnel carriage element becomes a 3 m (10 ft) cube which travels along the "shade" side of a spoke. The carriage has two sets of local-vertical opening doors. One set faces the wall of the spoke, and the other faces radially inward.

In traversing the spoke, the elevator follows a pair of rails which include a rack gear on one face that engages the drive mechanism on the carriage (e.g., an electric motor with a worm drive into a reduction gear). Coriolis effects and power demands effectively limit the traversing velocities to 2 m/sec maximum.

In traversing operations, the elevator will require two independent counterbalances. The balance required for maintaining the angular momentum and center of gravity for the torus requires shifting an equivalent mass of ballast water in a direction opposite to the motion of the elevator. Ballast transfer tanks must be emplaced within both the torus and the hub together wlth pumps and controls.

The effective suspended weight of the carriage will vary in proportion to the radial position along a spoke. A realistic drive system (less than 5 kW) dictates a force balancing system which inherently follows the force profile; a pneumatic accumulator has the appropriate characteristics. The force counterbalance system for the personnel transfer elevator utilizes a pair of pneumatic cylinders of 20 cm diameter operating wlth a 3 m stroke through a set of pulley blocks (distance multiplier) and cables up the guide rails.

In operation, the pneumatic cylinders would be pressurized to a level which matched the weight of the elevator and its load at the torus. The hoist requirement then would only need to overcome the friction and drag effects. Each llft or lowering would have the equalizing pressure tailored to that particular load. The configuration, as summarized, places these equilibrator cylinders and cable blocks in the torus.

Spin Power

Like everything else, spinning a centrifuge up or down will require power. Most NASA designs use attitude jets at the rim of the centrifuge regardless of whether it is a dependent or independent centrifuge. But some fancier designs feature an independent centrifuge designed like a titanic electric motor, with the spinning centrifuge as the rotor and the stationary ship core as stator. If you want to spin a dependent centrifuge using electricity, use a reaction wheel.

And a couple of science fiction authors realized that the only difference between an electric motor and an electric generator is which side the power is applied to, the mechanism is pretty much the same. Input electricity, the output is mechanical energy, and you have a motor. But if take the same device, input mechanical energy, receive electricity as output, then it is a generator.

SPIN POWER 1

      Someone said, "Gee, it must take a lot of power."
     Lopez answered, "Are you kidding? It's done by electric-braking the main axis flywheels (which is a reaction wheel, not a momentum wheel or control moment gyroscope). The shaft has field coils wound on it; you cut it in as a generator and let the reaction between the wheel and the ship put a spin on the ship. You store the juice. Then when you want to take the spin off, you use the juice to drive it as a motor and you are back where you started, free for nothing, except for minor losses. Savvy?"
     "Er, I guess so, sir."
     "Look it up in the ship's library, sketch the hook-up, and show it to me after supper."

From SPACE CADET by Robert Heinlein (1948)
SPIN POWER 2

His face didn't change. "Experienced cadets, eh? Well, we'd best be down to it. Mr. Haply will show you what we've been able to accomplish." They'd done quite a lot. There was a lot of expensive alloy bar-stock in the cargo, and somehow they'd got a good bit of it forward and used it to brace up the bows of the ship so she could take the thrust. "Haven't been able to weld it properly, though," Haply said. He was a young third engineer, not too long from being a cadet himself. "We don't have enough power to do welding and run the life support too."

Agamemnon's image was a blur on the screen across from my desk. It looked like a gigantic hydra, or a bullwhip with three short lashes standing out from the handle. The three arms rotated slowly. I pointed to it. "Still got spin on her."

"Yes." Ewert-James was grim. "We've been running the ship with that power. Spin her up with attitude jets and take power off the flywheel motor as she slows down."

I was impressed. Spin is usually given by running a big flywheel with an electric motor. Since any motor is a generator, Ewert-James's people had found a novel way to get some auxiliary power for life-support systems. (basically they are converting attitude jet fuel into electricity)


Agamemnon didn't look much like Slingshot. We'd closed to a quarter of a klick, and steadily drew ahead of her; when we were past her, we'd turn over and decelerate, dropping behind so that we could do the whole cycle over again.

Some features were the same, of course. The engines were not much larger than Slingshot's and looked much the same, a big cylinder covered over with tankage and coils, acceleration outports at the aft end. A smaller tube ran from the engines forward, but you couldn't see all of it because big rounded reaction mass canisters covered part of it.

Up forward the arms grew out of another cylinder. They jutted out at equal angles around the hull, three big arms to contain passenger decks and auxiliary systems (the three arms are part of the independent centrifuge). The arms could be folded in between the reaction mass canisters, and would be when we started boosting. All told she was over four hundred meters long, and with the hundred-meter arms thrust out she looked like a monstrous hydra slowly spinning in space (design based on the Pilgrim Observer).


     The fuel transfer was tough. We couldn't just come alongside and winch the stuff over. At first we caught it on the fly: Agamemnon's crew would fling out hundred-ton canisters, then use the attitude jets to boost away from them, not far, but just enough to stand clear.
     Then I caught them with the bow pod. It wasn't easy. You don't need much closing velocity with a hundred tons before you've got a hell of a lot of energy to worry about. Weightless doesn't mean massless.
     We could only transfer about four hundred tons an hour that way. After the first ten-hour stretch I decided it wouldn't work. There were just too many ways for things to go wrong.
     "Get rigged for tow," I told Captain Ewert-James. "Once we're hooked up I can feed you power, so you don't have to do that crazy stunt with the spin. I'll start boost at about a tenth of a centimeter. It'll keep the screens hot, and we can winch the fuel pods down."
     He was ready to agree. I think watching me try to catch those fuel canisters, knowing that if I made a mistake his ship was headed for Saturn and beyond, was giving him ulcers.
     First he spun her hard to build up power, then slowed the spin to nothing. The long arms folded alongside, so that Agamemnon took on a trim shape. Meanwhile I worked around in front of her, turned over and boosted in the direction we were traveling, and turned again.
     The dopplers worked fine for a change. We hardly felt the jolt as Agamemnon settled nose to nose with us. Her crewmen came out to work the clamps and string lines across to carry power. We were linked, and the rest of the trip was nothing but hard work.

From TINKER by Jerry Pournelle (1975)
SPIN POWER 3

      Daddy and Uncle Wes began trying to figure out why the power was out. As near as they could tell, there was a break in the power bus somewhere between the passenger spheres and the biggest part of the solar arrays. This meant an EVA repair, which meant it wasn't going to get fixed anytime soon.
     Uncle Wes was stroking his broad chin, and thinking. "Hey, you know those big electric motors up at the axis, the ones for overcoming frictional losses in our spin?"
     "Yeah?" said Daddy.
     "When I was looking over the specs for this ship, it'd occurred to me that if there ever was an interruption of power, you could swap the leads on those motors, making them generators. We'd be using our spin to crank out juice. We could do it for quite a while before we noticed ourselves getting any lighter. We can always make up the difference later."
     "Get right on it," Daddy ordered. "Momma and I are going to suit up, get to the Docking Bay, and try to patch our breach. Miranda, you stay here. When Wes gets the power back up, you watch the monitors, and keep us all apprised of our situation."
     "OK, Daddy."
     Everyone headed up the ladder. I had to wait in the unusually-quiet, dimly-lit Bridge for about fifteen minutes. Then the overhead lights flashed to brilliance, and the monitors winked on. I began punching up camera views of the axis, and the Docking Bay. Uncle Wes was in low-G, and was finishing up his re-wiring job on the spin motors. Daddy and Momma were in space suits, floating over to inspect a hole the size of a washing machine in one of the walls.

From MIRANDA AND THE SPACE PIRATES OR HOW HAVEN ROCK WAS SAVED
by Mike Combs (1997)

Making Do Without

It is tempting to just forget about spin gravity, and just have everybody float around while the ship is not under thrust. While it is true that after about a year in free-fall the human body starts to suffer bone decalcification and other damage, one can assume that future medical will have discovered a treatment. Marshall Savage suggests electro stimulation therapy of the muscles (Ken Burnside says rocket crewmen will have to wear their "jerk-jammies" when they sleep). One would hope that a medical cure will be found for the nausea induced by free fall, or "drop sickness" (they say that the first six months are the worse).

FREE-FALL CAT 1

(ed note: this is science fact, not science fiction)

And prior to (the) Mercury (program) we hadn't any real experience at all. We flew transport planes in parabolic courses that might give as much as 30 seconds of almost-zero-g, and that was all we knew. I will not soon forget some of our early low-g experiments. Some genius wanted to know how a cat oriented: visual cues, or a gravity sensor? The obvious way to find out was to take a cat up in an airplane, fly the plane in a parabolic orbit, and observe the cat during the short period of zero-g.

It made sense. Maybe. It didn't make enough that anyone would authorize a large airplane for the experiment, so a camera was mounted in a small fighter (perhaps a T-bird; I forget), and the cat was carried along in the pilot's lap. A movie was made of the whole run.

The film, I fear, doesn't tell us how a cat orients. It shows the pilot frantically trying to tear the cat off his arm, and the cat just as violently resisting. Eventually the cat was broken free and let go in mid-air, where it seemed magically (teleportation? or not really zero gravity in the plane? no one knows) to move, rapidly, straight back to the pilot, claws outstretched. This time there was no tearing it loose at all. The only thing I learned from the film is that cats (or this one anyway) don't like zero gravity, and think human beings are the obvious point of stability to cling to...

(ed note: the Lockheed F-94C Starfire was developed from the Lockheed T-33 "T-bird", and they look remarkably alike. So I am pretty sure the photo is from the movie Dr. Pournelle mentions.

On the other hand, here they say general free-fall experiments were started using a Lockheed T-33 "T-bird", went to using an F-89, then ended up using a Lockheed F-94C Starfire. So Dr. Pournelle might have actually seen a T-bird after all, and the photo is from a totally different test. )

From A STEP FARTHER OUT by Jerry Pournelle, 1979.
FREE-FALL CAT 2

Actual unethical experiment story here, complete with results:

My dad was a skydiver back in the sixties. There was a guy in his club that was a nut. He had the idea that he could test the axiom that "cats always land on their feet" from free fall altitude, where he would fall with them and observe their self-righting behavior. He had no interest in aiding their descent, just wanted to see how they behaved in free fall. In his plan, landing was the cats' problem, not his. Scientific impartiality, or some such thing.

He took four stray cats up in a pillowcase for the jump. After exiting the plane, he turned the pillowcase inside out, releasing the cats. To his great surprise, all four cats attached themselves to his body immediately. With their claws. Given that cats have 18 claws each, he was punctured at least 72 times. More, probably, because he struggled vainly to remove the cats as he fell, but they were having none of it, and would reattach with even more conviction with every effort he made to pull them off.

Presently, he was out of altitude, and had to turn his attention to opening the chute. Let's pause to do some math. A chute opening can generate as much as 3 Gs of force. The average cat weighs 8 lbs at 1 G. At three Gs, this becomes 24 lbs per cat. So when the chute opened, for a moment this guy had 72 razor sharp claws in his skin, each one being pulled down with a force of about one and a third pounds. That's 96 pounds of cat. He was sliced to ribbons, basically.

All four cats hung on through the chute opening, although the skydiver's shredded flesh allowed each one to slip several inches. Bleeding and in misery, the skydiver managed to make a safe, if rather rough, landing in a farm field.

As soon as he hit the earth, all four cats ran off across the field, leaving him to lie there bleeding from his hundred or so wounds.

He was the only member of the skydiving club that was displeased with the results of his experiment.

(ed note: RocketCat is of the opinion that the experimenter got off way too lightly. RocketCat then left to hunt down the experimenter and teach him the error of his ways...)

TRAPPED IN MID-AIR 1

      Gibson was sitting in the observation gallery, trying to see how many of the Pleiades he could resolve with the naked eye, when a small projectile whispered past his ear and attached itself with a “thwack!” to the glass of the port, where it hung vibrating like an arrow. At first sight, indeed, this seemed exactly what it was and for a moment Gibson wondered if the Cherokee were on the warpath again. Then he saw that a large rubber sucker had replaced the head, while from the base, just behind the feathers, a long, thin thread trailed away into the distance. At the end of the thread was Dr. Robert Scott, M.D., hauling himself briskly along like an energetic spider.
     Gibson was still composing some suitably pungent remark when, as usual, the doctor got there first.
     “Don’t you think it’s cute?” he said. “It’s got a range of twenty metres—only weighs half a kilo, and I’m going to patent it as soon as I get back to Earth.”
     “Why?” said Gibson, in tones of resignation.
     “Good gracious, can’t you see? Suppose you want to get from one place to another inside a space-station where there’s no rotational gravity. All you’ve got to do is to fire at any flat surface near your destination, and reel in the cord. It gives you a perfect anchor until you release the sucker.”
     “And just what’s wrong with the usual way of getting around?”

     “When you’ve been in space as long as I have,” said Scott smugly, “you’ll know what’s wrong. There are plenty of handholds for you to grab in a ship like this. But suppose you want to go over to a blank wall at the other side of a room, and you launch yourself through the air from wherever you’re standing. What happens? Well, you’ve got to break your fall somehow, usually with your hands, unless you can twist round on the way. Incidentally, do you know the commonest complaint a spaceship M.O. has to deal with? It’s sprained wrists, and that’s why. Anyway, even when you get to your target you’ll bounce back unless you can grab hold of something. You might even get stranded in mid-air. I did that once in Space Station Three, in one of the big hangars. The nearest wall was fifteen metres away and I couldn’t reach it.
     “Couldn’t you spit your way towards it?” said Gibson solemnly. “I thought that was the approved way out of the difficulty.”
     “You try it someday and see how far it gets you. Anyway, it’s not hygienic. Do you know what I had to do? It was most embarrassing. I was only wearing shorts and vest, as usual, and I calculated that they had about a hundredth of my mass. If I could throw them away at thirty metres a second, I could reach the wall in about a minute.
     “And did you?”
     “Yes. But the Director was showing his wife round the Station that afternoon, so now you know why I’m reduced to earning my living on an old hulk like this, working my way from port to port when I’m not running a shady surgery down by the docks.

     “I think you’ve missed your vocation,” said Gibson admiringly. “You should be in my line of business.”
     “I don’t think you believe me,” complained Scott bitterly.
     “That’s putting it mildly. Let’s look at your toy.”
     Scott handed it over. It was a modified air pistol, with a spring-loaded reel of nylon thread attached to the butt.
     “It looks like——”
     “If you say it’s like a ray-gun I’ll certify you as infectious. Three people have made that crack already.”
     “Then it’s a good job you interrupted me,” said Gibson, handing the weapon back to the proud inventor.

From SANDS OF MARS by Arthur C. Clarke (1952)
TRAPPED IN MID-AIR 2

      They started for the control room, much relieved. Arcot dived first, with Wade directly behind him. Wade decided suddenly to go into his room and stopped himself by grabbing a handhold. Morey, following close behind, bumped into him and was brought to rest, while Wade was pushed into his room.
     But Fuller, coming last, slammed into Morey, who moved forward with new velocity toward the control room, leaving Fuller hanging at rest in the middle of the corridor.
     "Hey, Morey!" he laughed. "Send me a skyhook! I'm caught!" Isolated as he was in the middle of the corridor, he couldn't push on anything and remained stranded.
     "Go to sleep!" advised Morey. "It's the most comfortable bed you'll find!"
     Wade looked out of his room just then. "Well, if it isn't old Weakmuscles Fuller! Weighs absolutely nothing and is still so weak he can't push himself around."
     "Come on, though, Morey—give me a hand—I got you off dead center." Fuller flailed his hand helplessly.
     "Use your brains, if you have any," said Morey, "and see what you can do. Come on, Wade—we're going."

     Since they were going to use the space control, they would remain in free fall, and Fuller would remain helplessly suspended in mid-air.
     The air of the ship suddenly seemed supercharged with energy as the space around them became gray; then the stars were all before them. The ship was moving forward again.

     "Well, old pals," said Fuller, "at least I have traffic blocked fairly well if I feel like it, so eventually you'd have to help me. However—" He floundered clumsily as he removed one of his foam-rubber space-boots, "—my brains tell me that action is equal and opposite to reaction!" And he threw the boot with all possible velocity toward Morey!
     The reaction of the motion brought him slowly but surely to a handhold in the wall.
     In the meantime, the flying boot caught Morey in the chest with a pronounced smack! as he struggled vainly to avoid it. Handicapped by the lack of friction, his arms were not quite powerful enough to move his mass as quickly as his legs might have done, for his inertia was as great as ever, so he didn't succeed in ducking.
     "Round one!" called Arcot, laughing. "Won by Kid Fuller on a TKO! It appears he has brains and knows how to use them!"
     "You win," laughed Morey. "I concede the battle!"

From ISLANDS IN SPACE by John W. Campbell jr. (1931)
TRAPPED IN MID-AIR 3

      Dorothy floated away from him, frantically clutching at a brace just beyond reach.
     "Pull me down, Dick!" she called, laughing gaily.
     Seaton, seizing her instinctively, neglected his own anchorage and they hung in the air together, while Crane and Margaret, each holding a strap, laughed with unrestrained merriment.
     "Tweet, tweet—I'm a canary!" chuckled Seaton. "Throw us a rope!"
     "A Dicky-bird, you mean," interposed Dorothy.
     "I knew that you were a sleight-of-hand expert, Dick, but I did not know that levitation was one of your specialties," remarked Crane with mock gravity. "That is a peculiar pose, Dick. What is it supposed to represent — Zeus sitting on his throne?"
     "I'll be sitting on your neck if you don't get a wiggle on with that rope!" retorted Seaton, but before Crane had time to obey the command the floating couple had approached close enough to the ceiling so that Seaton, with a slight pressure of his hand against the leather, sent them floating back to the floor, within reach of one of the handrails.

From THE SKYLARK OF SPACE by E. E. "Doc" Smith (1928)
BROOMSTICK

... This was where the broomsticks came in.

Commander Doyle had invented them, and the name, of course, came from the old idea that once upon a time witches used to ride on broomsticks. We certainly rode around the station on ours. They consisted of one hollow tube, sliding inside another. The two were connected by a powerful spring, one tube ending in a hook, the other in a wide rubber pad. That was all there was to it. If you wanted to move, you put the pad against the nearest wall and shoved. The recoil launched you into space, and when you arrived at your destination you let the spring absorb your velocity and bring you to rest. Trying to stop yourself with your bare hands was liable to result in sprained wrists.

It wasn't quite as easy as it sounds, though, for if you weren't careful you could bounce right back the way you'd come.

From ISLANDS IN THE SKY by Arthur C. Clarke (1952)
SPACESICKNESS

What is so funny about a man being dropsick? Those dolts with cast-iron stomachs always laugh — I'll bet they would laugh if Grandma broke both legs.

I was spacesick, of course, as soon as the rocket ship quit blasting and went into free fall. I came out of it fairly quickly as my stomach was practically empty — I'd eaten nothing since breakfast — and was simply wanly miserable the remaining eternity of that awful trip. It took us an hour and forty-three minutes to make rendezvous, which is roughly equal to a thousand years in purgatory to a ground hog like myself.

I'll say this for Dak, though: he did not laugh. Dak was a professional and he treated my normal reaction with the impersonal good manners of a infight nurse — not like those flat-headed, loud-voiced jackasses you'll find on the passenger list of a Moon shuttle. If I had my way, those healthy self-panickers would be spaced in mid-orbit and allowed to laugh themselves to death in vacuum.

Despite the turmoil in my mind and the thousand questions I wanted to ask we had almost made rendezvous with a torchship, which was in parking orbit around Earth, before I could stir up interest in anything. I suspect that if one were to inform a victim of spacesickness that he was to be shot at sunrise his own answer would be, "Yes? Would you hand me that sack, please?"


"Dak?" I said as he signed off.

"Later," he answered. "I'm about to match orbits. The contact may be a little rough, as I am not going to waste time worrying about chuck holes. So pipe down and hang on."

And it was rough. By the time we were in the torchship I was glad to be comfortably back in free fall again; surge nausea is even worse than everyday dropsickness.

From DOUBLE STAR by Robert Heinlein, 1956

A possible compromise is the personal centrifuge. This is a centrifuge a few meters long, just big enough for one man to strap in, spin up about 30 RPM, and do some exercises. Yes, this will probably give them severe motion sickness, but it will only be for the duration of the exercise period.

Magnetic Boots

In lieu of a habitat module on a centrifuge, under acceleration, or equipped with technobabble paragravity; science fiction novels often equips the crew with magnetic boots/sandals and a ferromagnetic floor.

Magnetic boots sometimes appear on space suits as well, assuming the hull is constructed out of something ferromagnetic. But magnets do not work very well on hulls composed of titanium, aluminum, magnesium or other space age materials.

If one does have a ferromagnetic hull, it might be best to have magnets just in the boot heels but not the toes, to facilitate walking. The idea is that if a boot is attached to the hull, you can release it by pushing down with your toes and lifing your heel, using a natural walking motion to detach the magnetic heel. Then the boot moves forward, approaching the hull heel-first. This allows the magnet in the heel to attach. At least that's how I remember it, maybe it is the other way around.

In The Expanse, the magnetic boots were electromagnets, so you could turn them on or off. They had red indicator lights and a switch one could operate by tapping boots.

In the movie 2001 A Space Odyssey the stewardess wore velcro footies to walk on the velcro floor.

But it is good to keep in mind that they do not use any of these on the International Space Station. They just float everwhere and to heck with walking. Besides, magnetic fields interfere with navigation and communication systems.

VELCRO SHOES

The stewardess came walking up the narrow corridor to the right of the closely spaced seats. There was a slight buoyancy about her steps, and her feet came away from the floor reluctantly as if entangled in glue. She was keeping to the bright yellow band of Velcro carpeting that ran the full length of the floor — and of the ceiling. The carpet, and the soles of her sandals, were covered with myriads of tiny hooks, so that they clung together like burrs. This trick of walking in free fall was immensely reassuring to disoriented passengers.

From 2001 A SPACE ODYSSEY by Arthur C. Clarke (1969)
MAG BOOTS 1

As they walked thorugh a maze of corridors, the ship started a slight vibration, and gravity slowly reappeared. They were under thrust. Holden used his heels to touch his boots' slide controls, turning the magnets off.

From LEVIATHAN WAKES by James Corey (2011)
MAG BOOTS 2

“Click on,” directed the instructor, and placed his boots gently against the side of the lock. Matt did likewise and felt the magnetic soles of his boots click against the steel. “Follow me and stay closed up.” Their teacher walked along the wall to the open door and performed an awkward little squatting spread-eagle step. One boot was still inside the door, flat to the wall, with the toe pointing inboard; with the other he reached around the corner, bent his knees, and felt for the outer surface of the ship. He withdrew the foot still in the lock and straightened his body-with which he almost disappeared, for he now stuck straight out from the ship, his feet flat to her side.

Following in order, Matt went out through the door. The ninety degree turn to get outside the lock and “standing” on the outer skin of the ship he found to be tricky; he was forced to use his hands to steady himself on the door frame. But he got outside and “standing up.” There was no true up-and-down; they were still weightless, but the steel side was a floor “under” them; they stuck to it as a fly sticks to a ceiling.

Matt took a couple of trial steps. It was like walking in mud; his feet would cling stickily to the ship, then pull away suddenly. It took getting used to.

From SPACE CADET by Robert Heinlein (1948)
MAG BOOTS 3

Last of all, Torwald took Kelly to the rear of the shop, where the footwear was kept. They rummaged around for a few minutes while Torwald gave him a running lecture on the virtues of good boots.

"You might not think so, kid, but boots are more important than any other item of a spacer's equipment. That's because you never know when you may be set afoot, or in what terrain, or in what climate." Kelly didn't like the sound of the expression "set afoot."

"Besides," Torwald continued, "a spacer has very little to do with space, any more than a sailor has with water. It's just something to get across to reach the planets, where the jobs are. And on the ground, you need boots. Aha, jackpot!" With that exclamation, he pulled a pair of boots from a bin. "Genuine pre-War unissued Space Marine boots!"

"How can you tell they're pre-War?" Kelly asked, sorting through the bin to find a pair that fit. Torwald turned a boot sole-up.

"See those little threaded holes? That's where they used to screw in the magnetic plates. They haven't used those plates in fifty years, but the Navy required that the mounts be left there in case of equipment failures. When the War came along, they dropped that reg, and a lot of quality, to cut costs. These boots will last you a lifetime."

From SPACE ANGEL by John Maddox Roberts (1979)

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