This is where the spacecraft's pilot flies the ship. In fiction it is often the dramatic focus, even though without help from the astrogator, engineer, and ship's captain one will find that the pilot is helpless.
Each spacecraft mission is composed of several "maneuvers". In between maneuvers, the spacecraft is falling along a trajectory to its next maneuver. In between maneuvers, the pilot has nothing to do.
A maneuver consists of using attitude controls to aim the thrust axis in the prescribed direction, and using the thrust controls to do an engine burn for the prescribed amount of delta V. And this maneuver must be performed at the prescribed time. Maneuvers are calculated by the astrogator and given to the pilot. For the most part maneuvers are to  insert the spacecraft into a trajectory to the destination  upon arrival insert the spacecraft into orbit around the desination, and  mid-course corrections when the ship is off track.
The two main controls are the ship's attitude controls and the thrust controls (i.e., similar to the steering wheel and the accelerator pedal on an automobile). They will probably look like a joystick and a throttle.
In most science fiction, it is assumed for dramatic purposes that the spacecraft is sufficiently automated so the pilot can fly the entire spacecraft like it is a huge jet fighter, all by themselves. See Han Solo at the controls of the Millennium Falcon. The pilot's room is more like an aircraft cockpit.
If the spacecraft is more like a wet navy vessel, instead of a pilot you have a "helmsman" who works in a room called the "bridge." Keep in mind that this is nothing like the "bridge" you see in Star Trek. What Trek calls a bridge is actually a Combat Information Center, the bridge is that red console in front of Captain Kirk's chair where helmsman Sulu sits. In real world wet navy vessels the bridge is commonly in a totally different part of the ship from the CIC, they are merged in Star Trek for strictly dramatic reasons.
The helmsman uses the attitude controls and the thrust controls. In some cases, the thrust controls are so complicated that they are delegated to the ship's engineer. "Sufficiently complicated" usually means there is a fission or fusion reactor involved in the thrust. Holms Cronch has a pilot's console and an engineer's console. On many US wet navy vessels the helmsman steers the ship by controlling the rudder (attitude controls), while the "Lee Helmsman" sets ship's speed by controlling the engine and the propeller (thrust controls).
On a US Naval vessel, the current Conning Officer is the only person the Helmsman and Lee Helmsman listen to when it comes to direction of the ship (because a back-seat driver can cause a disaster). The officer assuming the role of Conning Officer does so by announcing "I Have The Conn".
The most important controls to the pilot are of course the piloting or flight path controls. These are the 3D equivalents of the steering wheel and accelerator in an automobile. To get an idea of what the bare minimum is, we will unashamedly be taking a good look at the solution in the computer game Kerbal Space Program. Since that is a game, the designers were forced to distill the controls to the very essentials (because the players will quickly get fed up and leave if they think the game is too complicated). As a matter of fact, that game is so wonderfully educational yet fun, you might be better off if you skipped this section of the website and instead started playing the Kerbal game.
The bottom line is that piloting boils down to aiming the ship's nose in the proper direction, then at the proper time performing a burn of the proper amount of delta V. This will put the ship on a trajectory to its destination. Calculating the values of all these proper parameters is the job of the astrogator. It is the captain's job to tell the astrogator the destination to be calculated for. But I digress.
The combination of burn direction, delta V, and timing is called a "maneuver."
The pilot will have to look over the parameters the astrogator passes to them and squawk if there is a problem, such as the spacecraft not having enough remaining propellant to create enough delta V. The astrogator is supposed to avoid problems like that but mistakes will happen.
The technical term for the direction the astrogator wants the ship's nose to pointing at is "Axis of Acceleration." Why the ship's nose? Because it points in the same direction as the engine's thrust axis. For most spacecraft, when they do an engine burn, the exhaust goes directly aft (out the rear of the ship) and the thrust goes forward in the direction the ship's nose is pointing. To do the maneuver properly, the thrust axis and the acceleration axis must be the same and stay the same. This is half of the pilot's job. Remember: the acceleration axis is where the astrogator wants the ship to be pointed, the thrust axis is where the ship is actually aimed at any given point in time.
During the flight while the ship is coasting, in between maneuvers, the astrogator will keep track of the spacecraft's course and timing. If the spacecraft starts to move off track, the astrogator will calculate a mid-course correction maneuver to fix things.
Since under acceleration "down" feels like it is in the direction the exhaust is going, the ship's nose will feel like it is directly overhead.
The rotation control spins the ship on one or more of its three axes, it is used to aim the ship's nose in the proper direction (i.e., controls the thrust axis).
The translation control move the ship laterally on the three axes, it is only used for docking but it is generally located next to the rotation control. Sometimes the translation control and the thrust control were combined into one, with a selector switch. You generally never need to translate while doing a burn, neither do you need to thrust while docking. Combining the controls saves mass, and every gram counts.
The thrust control turns on the rocket motor and sets the thrust level, it is used to control the delta V.
The pilot needs feedback in order to full fill all the proper parameters. Where is the ship's nose currently pointing? Is it time for the burn yet? How much delta V has been created? The pilot uses the display instruments for the necessary feedback.
The attitude display tells where the ship's nose is currently pointing (the thrust axis). Feedback for the rotation control.
The time display tells the current time with split-second accuracy. Feedback for the thrust control.
The delta V display tells how much delta V has been generated so far by the current burn. Feedback for the thrust control.
When a spacecraft is falling along a trajectory to its destination, there is no need for a pilot. The ship's course is determined by Newton's Laws of Motion and the effect of gravity, the pilot can go play poker with the atomjacks. It is when the ship has to have its nose pointed in the proper direction so that a scheduled engine burn gives the desired vector that the pilot earns their pay. Or when the ship needs to be docked to a space station or something.
The pilot moves the spacecraft via rotations and translations. A rotation spins the ship around its center of gravity, the ship's orientation in space changes but its position does not. A translation, on the other hand, moves the ship's position but does not affect its orientation. So if you were standing up and pivoted in place clockwise, you would be doing a rotation. But if you took a step to the right, you would be doing a translation. Aircraft can do rotations, but they generally do not do non-thrust-axis translations (with exceptions like helicopters, Harrier jump jets, and unfortunate aircraft in the process of augering in).
And please try and remember that Rockets are not Arrows, there is no law that says the spacecraft has to be traveling in the direction its nose is pointing (i.e., the thrust axis and the ship's trajectory are independent). While the Polaris' vector is a Hohmann transfer to Mars, no law of physics will prevent Tom Corbett from using the controls to make the ship's nose point anywhere he pleases. So the Polaris is now traveling sideways through space, so what?
For a given maneuver, the pilot will use the rotation control to aim the nose in the required direction. The translation control is not needed for a burn, it generally is required only for a docking maneuver. Both controllers will have a "trim" control. The trim will set whether each tap of the grip will move the ship's nose by a large amount (for big coarse movements) or by a small amount (for tiny high-precision movements). The trim control might be on the hand controller proper or might be on the main control panel.
The pilot might need to make rotational corrections during the burn, if the ship's nose wanders off-target due to engine irregularities, crewman Joe Idiot walking around during the burn, or something like that (which will earn Joe Idiot a free trip out the nearest airlock when the angry pilot catches up with him). Generally the spacecraft will have stabilization gyros or automatic attitude jets to keep the nose from wandering, but those can only go so far.
Rotation spins the ship around one of its (imaginary) axes. A "yaw" pivots the ship's nose to the left or right, spinning around the Z axis. A "pitch" pivots the ship's nose up or down, spinning around the Y axis. And a "roll" makes the ship spin like a old-style propeller prop, spinning around the X axis.
In most NASA vehicles, pushing the control grip left or right does roll, forwards and back does pitch, holding it upright and twisting left or right for yaw. In other words, imagine that there is a little model spacecraft glued on the top of the hand grip with the nose pointing forwards. Move the hand grip in such a way that the model's nose moves the way you want the spacecraft's nose to move.
According to NASA human factor design, the hand controller should be set to operate according to the viewpoint of the operator. So if you had a control for the pilot facing in the direction of the ship's nose, and a second control for an operator facing in the direction of the ship's tail, the "pitch" control for each operator will be the reverse of the other. For instance a pull back for the nose controller will pitch the nose upwards, while a pull back for the tail controller will pitch the nose down. This is because according to human factor design, the operator at the tail controller expects to pitch the tail upwards when they pull back the hand grip.
In NASA designs, pushing the control up or down translates in the Z direction, left or right translates in the Y direction, push or pull translates in the X direction. The rotation clockwise/counterclockwise by 17° is to engage/disengage the autopilot. Not shown is the "push-to-talk" button on the top of the T control.
In the game Kerbal Space Program, rotation and translation is handled by the computer keyboard. A and D are for yaw, W and S are for pitch, Q and E are for roll, H and N translate x, J and L translate y, I and K translate z, and the coarse/fine trim control is the caps-lock key.
The rotation and translation controls have to be hooked up to some mechanism that actually turns the spacecraft. These are the Attitude Acutators. The three main types are Attitude Jets aka Reaction Control Systems (RCS), Thrust Vectoring, and Flywheels.
Translations are only done by attitude jets or thrust vectoring, never by flywheels. This is because converting rotary motion into linear motion is impossible (the Dean Drive notwithstanding).
Attitude jets are tiny rocket engines mounted such that they can torque the spacecraft around to new orientations. They are often mounted in tiny clusters, with each cluster having jets for yaw, pitch, and roll. The clusters are mounted so each of its jets is paired with an opposite jet on another cluster. By "opposite" I mean a jet pointing in the opposite direction and part of a cluster located at the same distance from the ship's center of gravity but 180° around the axis of rotation.
Attitude jets are also called a Reaction Control System (RCS). It is also possible to do mild yaw and pitch by gimbaling the engine. If you have two or more engines that are off-axis, gimbaling can also do a mild roll as well.
Translations are only done by attitude jets, never by momentum wheels. As mentioned before this is because converting rotary motion into linear motion is impossible.
If you need to changes the ship's attitude rapidly, you should look into thrust vectoring.
Hop David has an exceedingly clever arrangement of attitude jets on his Tetrahedral spaceship concept. This takes the "jet on a long lever arm" arrangement of Babylon-5 Starfuries to the logical ultimate.
What do the jets sound like as the pilot rotates the ship? In Larry Niven and Jerry Pournell's classic The Mote in God's Eye the space taxi's attitude jets make a sound like popcorn popping.
You may or may not need thermal protection on the hull to shield it from the thruster exhaust. Depends upon how the thrusters are angled, and how hot the exhaust is.
Or what the exhaust is. The Space Shuttle thrusters used nitrogen tetroxide as the oxidizer, and monomethyl hydrazine as the fuel. Nitrogen tetroxide is not particularly healthy for human beings, but monomethyl hydrazine is hideously toxic. Back when the Space Shuttle was still flying, whenever it approached the International Space Station it used a complex nautilus-shell shaped approach trajectory in order to ensure that the RCS thruster jets exhaust never hit the station. Otherwise unburnt monomethyl hydrazine could be deposited on the space station hull, just waiting for an astronaut on EVA scrape some off by accident and bring it back inside.
Why do they use such nasty stuff for RCS fuel? It does have some advantages. Nitrogen tetroxide and monomethyl hydrazine are hypergolic, which means the thrusters do not need a failure prone maintenance nightmare ignition system. As soon as the two chemicals hit each other they go boom, no pilot light required.
Thrust Vectoring is basically using the spacecraft's main engines as attitude jets. The two main techniques are gimbaling the engine, or using cascade vanes. If the spacecraft design can handle multiple engines, it can thrust vector by throttling engines down in an unbalanced fashion. For example if you throttle down the -Z engine while keeping the +Z engine steady, the spacecraft will pitch its nose in the -Z direction.
Rapidly changing the ship's attitude with a RCS is a problem unless you have unreasonably powerful attitude jets. Thrust vectoring is a solution, since the ship's main engines are generally always huge compared to an attitude jet. Of course exploration and merchant spacecraft generally do not need to rapidly change attitude, this is only needed with warships.
Conventional spacecraft have the engines firmly welded to the ship's thrust frame, such that the engine's axis of thrust passes precisely through the ship's center of gravity. With gimbaled engines, the rocket engine is attached to a tilting device so that the engine's axis of thrust can be pivoted off center. This allows the engine to act like a powerful attitude jet.
The old design for NASA's Reusable Nuclear Shuttle had a gimbaled NERVA nuclear engine. The engine can be pointed up to three degrees off-center in any direction. The maximum rate it can change the pivot in preparation for thrust vectoring is 0.25 degrees per second, but it takes time to get up to speed. It can only accelerate to maximum rate at 0.5 degrees per second per second.
In a 1969 US Atomic Energy Commission pamphlet on NERVA, it had this to say:
Dr. Crouch is unsure about this. He notes that unlike a chemical rocket, the poor gimbal is trying to swing an entire reactor so the actuation loads will be large. Even worse: the gimbal is subject to cryogenic temperatures, nuclear radiation, and thrust loads. Simultaneously. He is of the opinion that a more optimal solution would be secondary injection, jetavators, or a movable nozzle.
Having said that, here are the gimbals designed for the NERVA project.
In the images here, the gimbals are the network of blue rods emerging from the bottom of a hydraulic piston. The wasp-waist in the center is the pivot. The green "mushroom caps" are the control-drum actuators, perched on top of the control drums protruding from the reactor.
Note that the gimbals are a nasty trouble spot. If one jams, that engine cannot be steered. The proximity to the dangerously radioactive engine makes repair difficult. This is a good place for a waldo.
Cascade vanes are used to redirect the engine's thrust axis by redirecting the exhaust without gimbaling the engine. The vanes are inserted into the exhaust right where it emerges from the engine. The vanes change the direction of the exhaust gases. The new direction is controlled by which vanes are inserted and how deep they go.
Cascade vanes are used in Dr. Crouch's design for the Basic Solid Core NTR. They were intended on solving the problem of maneuvering around a space station or space craft without exposing them to the deadly radiation from the NERVA's reactor. You have to change the exhaust direction without altering the orientation of the engine (and its anti-radiation shadow shield).
The plug nozzle lends itself well to thrust vectoring, thrust throttling, and nozzle close-off. This is because of the short shroud and the configuration of the cowl lip. Unlike a conventional bell nozzle there is no fixed outer boundary. While the cowl lip defines the outer periphery of the annular throat, there isn't an outer boundary. So all you have to do is alter the cowl lip angle to adjust the throat area, which will vector the thrust (that's what Mr. Crouch meant when he was talking about varying rβ and β).
In the diagram at right, variable throat segments A, B, C, and D are sections of the cowl which are hinged (so as to allow one to alter the lip angle). This will allow Yaw and Pitch rotations.
If the pilot wanted to pitch the ship's nose up, they would decrease the mass flow through segment A while simultaneously increasing the mass flow through segment C. Segment A would have its lip angle increased which would choke off the throat along its edge, while Segment C's lip angle would be decreased to open up its throat section. The increased thrust in segment C would force the ship to pitch upwards.
It is important to alter the two segments such that the total thrust emitted remains the same (i.e., so that segment A's thrust lost is exactly balanced by segment C's gain). Otherwise some of the thrust will squirt out among the other segments and reduce the amount of yaw or pitch thrust. With this arrangement, it is also possible to do yaw and pitch simultaneously.
The moment arm of thrust vectoring via a plug nozzle is greater than that of thrust vectoring from a conventional bell nozzle. This is because the thrust on a bell nozzle acts like it is coming from the center, along the thrust axis. But with a plug nozzle, the thrust is coming from parts of the annular throat, which is at some distance from the center. This increases the leverage.
Nozzle close-off means when thrusting is over, you can shut the annular throat totally closed. This keeps meteors, solar proton storms, and hostile weapons fire out of your reactor.
Pivoting each section of cowl lips is a problem, because as you pivot inwards you are reducing the effective diameter of the circle that defines the edge of the lips. The trouble is that the lip is not made of rubber. The solution used in jet fighter design is called "turkey feathers" (see images above). It allows the engine exhaust to dialate open and close without exposing gaps in the metal petals.
With chemical rockets, retrothrust is achieved by flipping the ship until the thrust axis is opposite to the direction of motion, then thrusting. This is problematic with a nuclear rocket, since it might move another object out of the shadow of the shadow shield and into the radiation zone. For example, the other object might be the space station you were approaching for docking. Ideally you'd want to be able to perform retrothrust without changing the ship's orientation. What you want to do is redirect the primary thrust stream.
Jet aircraft use "thrust reversers." These are of two type: clam shell and cascade vanes. For complicated reasons clam shell reversers are unsuited for nuclear thermal rockets so Mr. Crouch focused on cascade vanes reversers. The main thing is that the actuators for cascade vanes are simpler than clam shell, and unlike clam shells a cascade vane reverser surface is segmented. There are five to ten vanes in each surface.
Note that the maximum reverse thrust is about 50% of the forwards thrust.
Each vane is a miniature partial nozzle. It takes its portion of the propellant flow and bends it backwards almost 180°. In the "cascade reverser end view" in the right diagram above, there are eight reversers, the wedge shaped surfaces labeled A, A', B, B', C, C', D, and D'. Each reverser is normally retracted out of the propellant stream, so their rear-most edge is flush with the tip of the cowl lip. When reversal is desired, one or more reversers are slid into the propellant stream. At maxmimum extension, the rear-most edge makes contact with the plug body.
Vane segmentation of the reverser surface eases the problem of center-of-pressure changes as the reverser's position is varied in the propellant stream.
Inserting all eight reversers causes retrothrust (see "Full Reverse" in below left diagram). Inserting some but not all reversers causes thrust vectoring. You'd expect that there would be a total of four reversers instead of eight (due to the four rotations Yaw+, Yaw-, Pitch+, Pitch-), but each of the four were split in two for reasons of mechanical alignment and the desirablity of shorter arc lengths of the vanes. This means the reversers are moved in pairs: to pitch upward you'd insert reverser A and A' (see "Thrust Vectoring" in below left diagram).
I am unsure if using reversers means that it is unnecessary to use the variable throat segments for yaw and pitch rotations, Mr. Crouch is a little vague on that. And the engineering of reversers that can withstand being inserted into a nuclear rocket exhaust is left as an exercise for the reader. There will be temperature issues, supersonic vibration issues, and edge erosion issues for starters. These are desgined for a solid-core NTR, where the propellant temperatures are kept down so the reactor core remains solid. This is not the case in a gas-core NTR, where the propellant temperatures are so high that the "reactor core" is actually a ball of hot vapor. The point is that a gas core rocket might have exhaust so hot that no possible material cascade vane could survive. There is a possibility that MHD magnetic fields could be utilized instead.
But the most powerful feature of cascade vanes is their ability to perform "thrust neutralization". When all the reversers are totally out of the propellant stream, there is 100% ahead thrust. When all the reversers are totally in the propellant stream, there is 50% reverse thrust. But in the process of inserting the reversers fully in the propellant stream, the thrust smoothly varies from 100% ahead, to 75% ahead, to 50% ahead, to 25% ahead, to 25% reverse, and finally to 50% reverse.
The important point is that at a specific point, the thrust is 0%! The propellant is still blasting strong as ever, it is just spraying in all directions, creating a net thrust of zero.
Why is this important? Well, ordinarily one would vary the strength of the thrust while doing maneuvers. Including stopping thrust entirely. Trouble is, nuclear thermal rocket reactors and turbopumps don't like having their strength settings changed. They lag behind your setting changes, and the changes put stress on the components.
But with the magic of thrust neutralization, you don't have to change the settings. You put it at a convenient value, then leave it alone. The cascade vanes can throttle the thrust to any value from 100% rear, to zero, to 50% fore. And do thrust vectoring as well.
Mr. Crouch also notes that while using thrust vectoring for maneuver, the rocket will have to be designed to use special auxiliary propellant tanks. The standard tanks are optimized to feed propellant while acceleration is directed towards the nose of the ship. This will not be true while manuevering, so special "positive-expulsion" tanks will be needed. These small tanks will have a piston or bladder inside, with propellant on the output tube side of the piston and some neutral pressurized gas on the othe side of the piston.
I was having difficulty visualizing the cascade reversers from the diagrams. I used a 3D modeling program called Blender to try and visualize them.
Momentum wheels are also called flywheels or reaction wheels.
A control moment gyroscope (CMG) is a large precessing flywheel on a gimbal. Momentum wheels are fixed (no gimbal, they are bolted onto the spacecraft frame), which is why you need three or six of them.
Aim the axis of the CMG so it is parallel to the desired axis of rotation, start spinning it, and the spacecraft will start to yaw, pitch, or roll in the opposite direction. Stop the CMG and so will the ship. Be sure to unclutch the gyros first, if the ship has any. Trying to use the CMG while the gyros are clutched is like trying to drive a car with the emergency brake on.
Since the spacecraft has far more mass that the flywheel, the ship will rotate far more slowly than the flywheel does. So if you want the ship to rotate faster than the hour hand on an analog watch the flywheel will have to spin like a x100 CD-ROM drive. It might be prudent to put an armored cage around the flywheel, in case of "explosive delamination". This will ensure that the deadly shrapnel from the delaminating flywheel will shred the armored cage instead of shredding the unlucky crewmembers who happened to be in the plane of the flywheel. Unfortunatly the mass of the armor cuts into payload mass.
A flywheel is too slow to be used during a burn. For that one will use gyros, either massive ones to prevent tumbling by brute force or a tiny ones connected to gimbaled nozzles on the propulsion system.
How do you keep the ship from spinning, tumbling, or otherwise stationary? Equip the ship with a massive stabilizing gyroscope. If you cannot construct or otherwise use a single massive gyroscope, you can use a series of smaller ones.
I have seen such gyros in science fiction by Robert Heinlein and Larry Niven, always used in tail-sitting rockets to keep then steady once they have landed. I am unsure if any real-world spacecraft has or needs such a thing.
Once you spin up a given gyro, the inner framework will stay in one orientation. If the gyro frame is "unclutched", the inner frame can freely rotate (actually it stays stable while the ship rotates around it). When you "clutch" the gyros the inner frame is clamped onto the outer frame (which is firmly attached to the spacecraft's spine). Thus the gyro cage will do its best to keep the ship from changing orientation.
Aerospace Engineer Bill Kuelbs Jr corrected an error on an earlier version of this page. I mistakenly stated that the control moment gyroscopes had to be mounted at the center of gravity of the ship. Mr. Kuelbs pointed out that due to a force known as a 'moment couple' the the translative forces are balanced out (i.e., you can mount the gyros anywhere inside the ship and they will work).
Some spacecraft designers will try to economize by specifying gyros that are too light for the spacecraft's moment of inertia (i.e., the rotational analogue to mass ). Such ships will tend to wobble under acceleration. This will also happen if a gyro's bearings start to go bad.
Since gyros heavy enough to stabilize the entire spacecraft are rather massive, a more elegant solution is to use tiny gyros to detect changes in the spacecraft's orientation and connect this to an attitude control system to automatically counteract it (generally a RCS). In the old Heinlein novels ships had gyros massive enough to keep a landed ship from tipping over, but this might not be realistic.
If you want your space warships to be unreasonably maneuverable, and are willing to stray perilously close to the boarder between scientifically accurate and ludicrous space opera, there is an example to use.
First off you make your spacecraft in the form of a sphere. Ordinarily you'd take advantage of the minimally low moment of intertia possessed by a sphere to increase the effectiveness of your attitude jets and/or flywheels, allowing one to quickly point the ship's engine in the desired direction.
But if spinning around the ship takes too long, you have to forego that and instead allow the engines to thrust in any direction. Short of some sort of technobabble reactionless thruster, antigravity carrot-on-a-stick drive, or the vectored thrust nozzle from hell; the only option is multiple engines pointing in all directions. And to heck with "every gram counts."
Let us trace the history of this, which surprisingly starts back in 1919.
In that bygone year Franz Abdon Ulinski more or less invented the concept of the ion drive. Electrical power could ionize atoms, then be used to expell the electrons and ions to create thrust. A year later he expounded upon his ideas in a journal of aeronautics in Vienna called "Der Flug" in an article titled "Das Problem der Weltraumfahrt" (The Problem of Space Travel). It was way ahead of its time and was mostly ignored.
The first design had a spherical habitat module, ion drives located around the module's equator, and a large disk shaped solar thermocouple to supply power to the ion drives. A curious detail that will be important in a minute is one of the propellants proposed for the ion drive was Bismuth.
The second design was more ambitious. He theorized that atomic decay (i.e., a nuclear power plant) could be harnessed for power, so you could get rid of the unwieldy solar disk and make the habitat module larger.
Note that the ion drives are still located in a ring bulge around the sphere's equator.
In 1928 Valentin Gluško (https://translate.google.com/ is your friend), the founder of the Soviet liquid-propellant rocket engine, more or less invented the resistojet rocket engine. While its specific impulse was abysmal compared to an ion drive, it was much less of a technological challenge to build.
His "Helioraketoplan" (planetary pilot) was very similar to Ulinski's first spacecraft, including the huge solar thermocouple disk. It used resistojets instead of ion drives, but they were still arranged around the equator of the habitat module. The Helioraketoplan was considerably smaller than Ulinskis, the habitat module was only 2.6 meters in diameter (Ulinskis is three stories tall), and the disk was only 20 meters in diameter.
I'm just spitballing it, but to my eye the disk is going to need a slot about 2.6 meters wide cut at 90 degrees to the disk's spin axis or at certain angles the resistojet exhaust is going to burn the disk.
The ring bulge around the sphere's equator lives on in this spacecraft from "Prisoners of Alpha Centaurians", a Buck Rogers sunday newspaper comic strip that ran from 11/1/1931 to 1/24/1932 featuring Buddy Deering and Alura.
Finally we come to Perry Rhodan. For all you ugly Americans who have never heard about it, this German-language science fiction saga is about the longest-running series ever. It was created in 1961 by K. H. Scheer and Walter Ernsting.
And at least one of the two had read about Ulinski, since the mightly battleship starships of the Arkonide interstellar empire are spheres with engines in the ring bulge around the equator. Not to mention using Bismuth for fuel.
And as it turns out Walter Ernsting was friends with the infamous Erich von Däniken, author of the fraudulent pseudoscientific fiction Chariots of the Gods? The latter was quick to seize upon the idea of ball-shaped spaceships with girdles.
Some propulsion systems only have "on" or "off", there is no fine control over the amount of thrust. The amount of delta V is set by controlling the duration of the engine burn. This can prove uncomfortable to the hapless astronauts, since the gees of acceleration will go up as the mass of the ship goes down (as propellant is expended). Acceleration is thrust divided by ship mass, as the mass goes down the rising acceleration tries to mash the crew into a pulp.
More advanced systems have variable thrust levels, they can be throttled. This gives more fine control. Or at least a way to keep the acceleration from rising and turning the astronauts into chunky salsa. The amount of thrust will have to be reduced in proportion in order to keep the acceleration constant. There may be some sort of control that will do this automatically.
The throttle may be a direct control link to the engine, or it may be a glorified engine order telegraph. In the latter case, the pilot pushes the 50% thrust button, and down in the reactor room the atomjack's control panel lights up the indicator for "GET OFF YOUR BLASTED BACKSIDE AND MAKE THE REACTOR SPIT OUT 50% THRUST!". The nuclear engineer on watch throws down the poker hand, scoops up their winnings, and then proceeds to perform all the delicate complicated procedures required to make the reactor thrust as required without exploding into a nuclear fireball. You find this set-up when the task of piloting the ship and the task of controlling a touchy reactor simultaneously is too much multi-tasking for one person to do.
In the Apollo command module, the thrust was computer controlled. The flight crew would type the start time and duration of the thrust into the computer, the computer would do the rest. The computer would initiate the burn at the start time, then act as a brennschluss timer, automatically cutting off the engine.
In the Apollo lunar module things were a little more like flying by the seat of your pants. They did not have any moon maps with fine detail. The flight commander had to run the throttle manually, to take control in case the lunar module was trying to land on top of a bolder or something equally stupid. So the lunar module translation controller had a switch that would change it into a thrust controller. You never needed to do both functions at the same time, and combining the two controls saved payload mass.
Burn start and stop might be under autopilot control. But the pilot will still keep their hand hovering over the manual start key (or cut-off switch), as they never quite trust the auto-pilot. The co-pilot and the power officer will also have their hands hovering over their manual keys, since they never quite trust the auto-pilot nor the pilot.
There might be a brennschluss timer. When a burn is initiated, this is pre-set to count-down to burn stop time. The term is from German Brennschluß, or "End of burn". Brennen is 'to burn', schluß is 'end' or 'finish'. This was popularized in the 1950's by former German rocket engineer Willy Ley.
In the game Kerbal Space Program, thrust control is handled by the computer keyboard. In the game, most propulsion systems have variable thrust settings instead of just a rocket on/off setting. Shift key increases the throttle, ctrl key decreases the throttle, and X sets the throttle to zero. If the thrust is zero the engine is off, otherwise it is on. The current thrust setting is displayed on a dial on the left side of the nav ball.
The direction the ship's nose is pointing is displayed by the attitude indicator, also known as gyro horizon, artificial horizon or attitude director indicator. In Kerbal Space Program it is called the Nav Ball. This was originally developed for aircraft, and was adapted for spacecraft. An artificial horizon for a spacecraft is actually a pretty poor display for the data, but the NASA astronauts were mostly former test pilots who demanded a familiar instrument.
At the center of the display is the miniature airplane icon, indicating the postion of the ship's nose. It is painted on the glass cover since it does not move. Underneath is the sky ball, with a grid painted on. The sky sphere does rotate in place. The position of the miniature airplane on the sky ball shows what position on the celestial sphere the ship's nose is aimed at.
Primitive displays are vulnerable to the dread horror of gimbal lock, but modern ones are immune. If the ship is really primitive it might have to make do with a coelostat instead of an attitude indicator.
The attitude indicator has two modes, only one of which we are interested in.
The first mode is where the ball mirrors the celestial sphere around the ship, and is only of interest to astrogators (the ball is slaved to the inertial guidance platform). The miniature airplane will be over the part of the sky ball corresponding to the point on the celestial sphere the ship's nose is aimed at. The three scales around the ball show the rate at which the spacecraft is yawing, pitching, and rolling.
The second mode is where the pilot dials in the required ship's nose position for the next burn (on the main control panel), and the ball shows how far off the actual ship's nose is from where it should be (the ball is slaved to the gyro display coupler). When the ship's nose is on target, the miniature airplane will be over the sky ball's north pole. The three scales around the ball show how far off the nose is from being on target (the technical term is "attitude errors"). The pilot then uses the rotation controls while watching the attitude indicator, and moves the ship's nose into position. During the burn, the pilot keeps an eye on the attitude indicator, ready to correct things if the nose drifts off target.
In more sophisticated control boards, the attitude errors can be slaved to the attitude jets, so the spacecraft will automatically put and keep the ship's nose on target. Which of course gives space pilots anxiety about job security.
In the game Kerbal Space Program the display is slightly different, and a lot of the work is done automatically for you. The miniature aircraft is called the "level indicator". The astrogator will use Map View with maneuver nodes to set up the maneuver. A "maneuver icon" will magically appear on the nav ball. This is the point where the ship's nose should be pointed. So the pilot's job is to keep the level indicator on the maneuver icon. A variety of other informative icons will also be automatically added to the nav ball as needed.
The sky ball does not correspond to the celestial sphere, nor is is zeroed in on the burn target. Instead it is always set so the center of the brown "ground" hemisphere is aimed at the nearest planet or moon. This makes it easier for players to enter into orbits around planets.
The prograde icon is where the ship's current vector is pointing. The retrograde icon is the exact opposite direction. Since the ship's trajectory is generally a curve, these icons will move with time. If the level indicator is over the prograde and a burn is made, the ship will maximally accelerate. If the level indicator is over the retrograd and a burn is made, the ship will maximally decelerate. If the level indicator is anywhere else and a burn is made, it will do something in between.
If the astrogator enters Map View and selects another ship as a "target", the two target icons will appear on the nav ball. The target prograde icon the point in the sky where the target appears, from the ship's point of view. The target retrograde icon is the opposite point in the sky. Placing the level indicator over the target prograde and burning will start altering the ship's trajectory towards the target. And the opposite for the target retrograde icon. And both icons will be moving, since both the ship and the target are too.
Heinlein's short stories have rockets that use "coelostats" for an attitude display. This is the old-school clunky percursor to the artificial horizon. G. Harry Stine calls this instrument an "astrostat".
The coelostat is an astronomical instrument invented in the early 1900s by Gabriel Lippmann. The purpose was to allow long term observation or long photographic exposures of astronomical objects through a telescope, despite the fact that all the blasted things in the sky are moving due to Terra's rotation. Instead of the telescope looking directly at the sky, it instead looked at a large mirror (or prism used as a mirror). The mirror is rotated on an axis parallel to Terra's axis of rotation by a clock motor, cancelling out the effects of Terra's rotation.
Heinlein and Stine use the coleostat in a different way. They get rid of the clock motor, and have three separate coleostat mirrors feeding one telescope (the "pilot’s periscope"). The telescope has cross-hairs.
When the astrogator has plotted a given maneuver, they have the required axis of acceleration for that maneuver. Then they take three bright stars (e.g., Vega, Antares, and Regulus), one for each coleostat mirror. The idea is to angle each of the three mirrors such that when the nose of the spacecraft is properly aimed along the axis of acceleration, the view through the pilot's periscope will show all three stars dead on the center of the cross-hairs. If the nose is not pointed properly, the guide stars will be all over the screen.
This means the astrogator has to do some extra calculations to determine the correct angle for each coleostat mirror, but that's their job.
The pilot has to know their constellations with enough precision to identify the three guide stars, and will have to use a wide field spotting scope and the attitude actuators to roughly orient the ship so each guide star is visible in the proper coleostat mirror (i.e., get all three stars visible through the periscope). Then the pilot can use the pilot's periscope to fine tune the ship's position, getting all three stars in the cross-hair.
An artificial horizon is much easier to use, but requires a much higher level of technology to construct. As it turned out, NASA never used coelostats, they had artificial horizon technology by the time they had manned spacecraft. But in a science fiction novel, things might have turned out different on Planet Steampunk.
The time is displayed by the ship's chronometer. Both the astrogator and the pilot are obsessed with keeping the chronometer accurate, because mistiming a burn is a good way to doom the ship to a slow journey to oblivion.
In the game Kerbal Space Program there are two substitutes for a chronometer.
 The pilot can enter Map view and see the planets, moons, and ships like an animated diagrame of the solar system (an orrey in other words). The pilot can fly by the seat of their pants by starting the burn when the ship reaches certain marked points, such as apoapsis and periapsis.
 The astrogator can use Map view and maneuver nodes to create a maneuver. The maneuver parameters are automatically sent to the nav ball display in front of the pilot. Next to the nav ball, a count-down timer will tell the pilot when to start the next burn (and they had better have the ship's nose pointed properly by then).
The burn start time as given by the astrogator should actually occur at the mid-point of the burn. First the pilot will figure the total burn duration needed to generate the specifed delta V. Divide burn duration by 2, and subtract this from the astrogator-supplied burn start time. In Kerbal, the burn duration is helpfully displayed next to the nav ball, 12 seconds in the case of the illustration.
The delta V is displayed by a pendulous integrating gyroscopic accelerometer, or by a laser gyroscope attached to a microprocessor. On the Apollo command module, the delta V was displayed on the entry monitoring system (see illustration). The use of the EMS is an example of redundancy in the Apollo design. Delta V could be measured via the main command module guidance and nav inertial system, but this is more complex and liable to errors due to drift. The fixed EMS accelerometer provides a simple, reliable means of making this critical measurement.
In more primitive rockets they have no fancy delta V displays. Instead they do it by dead reckoning. The duration of thrust required to create the needed delta V is calculated, and a brennschluss timer is used to keep the engine thrusting for exactly the corrent amount of time.
In the game Kerbal Space Program the amount of delta V required for a maneuver is displayed as a green arc to the right of the nav ball.
This is from Study of One-man Lunar Flying Vehicle. Volume 1 - Summary Final Report (though the title page misspells it as "vechicle"), a 1969 study from the space division of North American Rockwell. It is for a single-astronaut lunar hopper. But the important part is the stripped-down reduced-to-bare-minimum console and controls.
The idea is that when you design your own spacecraft control panel and controls, you can start with the items here and be sure you have the basics covered. Then you can add more items as needed (like an FTL drive control).
The rotational hand controller looks pretty standard. The thrust hand controller rotates, instead of moving it up and down like an Apollo controller. The rotation from 0% to 100% is 150°. In order to prevent a single point of failure there is another control on the console to turn off the engines. Just in case the thrust hand controller malfunctions.
The console is the interesting part. It is all the pilot needs as far as instruments and extra controls, squeezed into a 28x21 centimeter panel with a chunk taken out of the bottom for the thrust hand controller. The instruments and controls are:
This amusing example of 1960's style user interface design is from NUCLEAR SPACE PROPULSION by Holmes F. Crouch (1965). This complements the Engineer's console from the same book. This design assumes that it is for a solid-core nuclear thermal rocket. Mr. Crouch decided that controlling the rocket's trajectory while simultaneously juggling the power levels of the nuclear reactor was a little too much to ask of a single human being, so he split it into two jobs. Each subsystem has too many displays, control functions, and automatic interlocks.
According to Mr. Crouch, there are four independent subsystems involved with flying a nuclear thermal rocket:
- Thrust vectoring (Engine exhaust nozzle)
- Spacecraft orientation and stability (Attitude jets)
- Heat generation for specific impulse (Reactor)
- Propellant flow (Turbopump)
The pilot will be controlling the thrust vectoring and spacecraft orientation, the engineer will be controlling heat generation and propellant flow. So the pilot is flying the rocket, while the engineer is flying the reactor and turbopump.
The "space scanner" is an array of displays showing TV field of vision views fore, port, starboard, dorsal, and ventral; plus radar views.
Above is the collision detector. If anything is on a collision course, the light will flash, the buzzer will buzz, the linear range will display how far away it is, and the range rate will display how fast it is approaching. The way to avoid collision is to do a short thrust in any direction. For reasons explained in more detail here, the simple way to detect a collision is to have the radar watch for any object that maintains a constant bearing while having a range that decreases.
Around the space scanner are panels displaying astronomical data, navigational data (including an accelerometer, chronometer, coelostat, integrating accelerograph, brennschluss timer, and gyroscopic artificial horizon. Not to mention radar plotted trajectories of all other spacecraft and objects in the vicinity), astrophysical data (including solar storm warnings), and radio communications.
The pilot has two 3-axis joysticks, sorry, Translational Hand Controllers and Rotational Hand Controllers. The left is a rotational controller. It activates the attitude jets in order to control the spacecraft orientation (basically which way the nose is pointing and thus the direction of thrust). The right is a translational controller. It controls the thrust vectoring of the engine. This allows "translation control", which is a fancy term for moving the ship left or right without turning the nose in that direction. This also allows thrust neutralization. This means letting the engine blast but with no thrust. You need this because a nuclear thermal rocket relies upon the propellant to cool off the reactor, sometimes the reactor needs coolant when the ship does NOT need to be thrusted. Please not that for translations, an engine is limited to vectoring the thrust to no more than ten degrees or so off-axis.
Each hand controller would be fitted with step and trim buttons to throttle and vernier the maneuver commands as desired. Note that the hand controllers are analogous to the Ship's Wheel on a sea going vessel. The compass and the windows are like the other displays.
Finally there is the Thrust Mode Selector. This is basically a glorified Engine Order Telegraph from the age of steam. The pilot uses it to tell the engineer what sort of thrust is required. It is then the engineer's job to juggle the reactor control rod and the propellant turbines to produce what is requested. When the engineer has the engine configured to the requested thrust mode, they turn on the appropriate yes/no light on the pilot's console (next to the thrust mode line) to indicate the state of nuclear readiness.
On an old-time engine order telegraph, the pilot uses the lever to set the desired engine setting. The engine crew acknowledge the order on their own telegraph. At the pilot's telegraph, the acknowledgement moves the tiny inner arrow. This should move so it matches the pilot's setting, otherwise Something Is Wrong. This includes both no acknowledgement and incorrect acknowledgement. In that case, the pilot repeats the setting on the telegraph. If things are still wrong, the situation is immediately reported to the officer of the deck (unless the officer of the deck is also the current pilot, of course).
Sometimes a situation will develop in the engine, and the engineer will have to alter the thrust mode due to the measures taken to prevent the reactor from melting down or doing something else unfortunate. The engineer will probably not bother manually changing the thrust mode yes/no lights (as you would with an order telegraph on a steam ship), instead they will hit the "discoverer" button and the big red nuclear disaster alarm on the pilot's console will start screaming.
What else is in the control room? A radarscope, accelerometer, gyroscope platform, periscope, and chronometer. And maybe an integrating accelerograph. This will display elapsed time, velocity, and distance in dead-reckoning for empty space. If the spacecraft is under programmed controls, the programmed values for the three items will be displayed below the actual values, so the pilot can see how results matched prediction.
Another important item is the control panel lock. When the lock engaged, all the other controls are locked in place. So the pilot can sleep in their chair and not have to worry about accidentally brushing a toggle switch. This also comes in handy if the pilot is forced to allow into the control room a bratty kid who just happens to be the son of the boss.
A control of dubious utility is the three-position control switch. It is available if one has duplicate sets of controls for pilot and co-pilot. The control switch is labeled "Pilot & Co-Pilot", "Pilot only" and "Co-Pilot only". It determines which sets of controls are live. One would expect to find this only on a training spacecraft, or if you would commonly expect a non-pilot to be occasionally riding in one of the control seats.
There may also be repeater displays. Such as a red indicator light from the power room which will change to green when the power officer unlocks the safety on the reactor damper. Or maybe the colors will be the other way around, depending upon how much you trust the reactor.
The three types of instrument displays are Analog, Digital, and Binary. Analog are typically circular like a clock with hands, semicircular like a multimeter or some automobile speedometers, or tape-like similar to a ruler. Digital displays numbers, such as an automobile odometer or a pocket calculator. Binary are "idiot lights" that are either on or off.
The advantage of analog is in displaying the relationship between the current reading and any "red-line" minimum or maximum. The gas (petrol) gauge on an automobile typically has a red area adjacent to "Empty" as a warning that you'd better fill your tank soon. Analog displays are also good at showing the rate of change. You can tell at a glance if the temperature is rising too quickly. The disadvantage of analog displays is that they can seldom be read with more than three figures of accuracy.
The advantage of digital displays is that it can be read with as many figures of accuracy as there are digits in the display. Disadvantages include having memorize what the red-line values are, and not being able to read the display if the figures change so rapidly as to be a blur.
The advantage of binary displays is the simplicity of an immediate warning. Disadvantages include the necessity of a test mode (so you can tell if an indicator light has burnt out) and the lack of extra information. Airplane pilots have many worries when they hit the "lower the landing gear" button and the "landing gear down" binary display fails to light up. Is the gear still up, or is gear actually down but the light is burnt out or the sensor wiring connection loose? All you can do is make a low pass by the control tower so they can look at the status of your landing gear. An analog or digital display with the angle of gear would avoid that worry.
A computer keyboard is a commonly used computer input device, but it sure ain't compact. Most have a bit more than 100 keys, multipled by the use of the shift, ctrl, and alt keys into something like over 300. Smartphone designers quickly ran into this barrier as they tried to cram all those keys into a tiny screen.
If only there was a way to drastically reduce the number of keys.
A common solution that never seems to catch on is the Chorded Keyboard.
The idea is to press several keys simultaneously, as if you were playing a chord on a piano. As a crude example, if the keyboard had seven keys corresponding to bits in a byte, only seven keys with seven fingers can chord any of the 128 ASCII characters.
The reason this never caught on is due to the unfortunate fact that memorizing all 128 chords is quite difficult. In practice, keyboard designers arrange the chords such that the simpler ones map to the most commonly used characters. That way if the user forgot a more complicated chord it would be something rarely used like the left curly bracket.
Neuroprosthetics is connecting electronic equipment to the human nervous system. The most common example is the cochlear implant, but a lot of work has been done recently on connecting artificial arms to be controlled by nerves in the stump of the arm. In Samuel R. Delany's novel Nova, starship crew have neuroprothetic sockets in their wrists and at the base of their spine to issue commands to the starship equipment they operate. In David Drake's Counting the Cost military officers activate their implanted radio transceivers by willing their left little finger to crook. The finger does not move, the nerve impulse is re-routed to the transceiver.
A Brain–computer interface (BCI) uses electronics to directly communicate with the human brain itself, instead of just some nerve endings. This allows the pilot to issue commands to the spacecraft, mecha, or whatever. Sometimes the BCI can communicate back, with sensory information. In extreme cases the BCI can give the pilot the illusion that the entire spacecraft is their body. Also known as mind-machine interface (MMI), brain–machine interface (BMI), or direct neural interface.
The advantages are that the pilot can control the ship with the speed of thought instead of with slow clumsy hands, and the ship can be controlled while under such high acceleration that the g-force prevents lifting the hands to the control panel. Sometimes they are used to pilot man-amplifiers instead of spacecraft.
It can also be used to give the operator a "math coprocessor for their brain", that is, a way that the operator can simply think of the desired equation and the computer will instantly solve it and report it back.
There are draw-backs of course. The best control comes when the operator actually has the BCI surgically implanted in their brain instead of wearing an external headset (which is kind of invasive). If the interface is sensitive, a stray thought on the part of the operator can inadvertently send a catastrophic control command to the ship (Pilot: "Gee, the ground looks pretty down there..." BCI controlled ship instantly puts the ship into a crash dive and augers into the ground). And if the BCI sends back information to the brain, certain circumstances can trigger a disorienting feedback loop. Finally there is the "Monsters from the ID" problem.
BCI feedback is analogous to audio feedback.
Audio feedback can happen when you connect a microphone to an amplifier connected to a speaker. If you aim the mike at the speaker, you'll created an agonizing high-pitched feedback whine. What happens is that [a] random external noise enters mike [b] amplifier makes random noise louder and sends it out the speaker [c] amplified random noise leaves speaker and enters mike where it feeds back into another loop through the system. The noise rapidly becomes louder until you snatch the mike away from the speaker.
BCI feedback is when one is using the BCI for a math coprocessor input (or similar) and the math coprocessor sends the answer back. Otherwise the mechanism is much like with audio feedback. Operator thinks of some random garbage, coprocessor turns it into super-garbage and feeds it back, operator thinks about super-garbage, coprocessor turns it into super-duper-garbage and feeds it back, feedback look repeats until screaming operator yanks out the plug or goes insane from the flood of mental garbage. The solution is the mental discipline to keep a tight rein on the thoughts sent to the coprocessor.
Another danger is the "Monsters from the ID" problem. If your conscious mind can use the BCI to control the spacecraft, there is a danger that your subconscious mind can use the BCI as well. This is a problem since the subconscious is a lot more barbaric and impulsive than the conscious mind. This appears in the movie Forbidden Planet (see quote below) which is where the phrase "Monsters from the ID" originated.
It also appears in the 1963 episode of The Outer Limits titled "The Man With the Power", where the poor hen-pecked and demeaned professor has implanted in his brain a device that can manipulate objects through mind power. The US space agency wants this for astronaut to use to move asteroids and other objects. Unfortunately the man is constantly humiliated and bullied, as he swallows his frustration his angry subconscious uses the device to slay his persecutors. The man does not realize that he is the cause of the freak "accidents" that are killing everybody. Of course, neither did Dr. Morbius.
In Ben Bova's As on a Darkling Plain (1972), a mission is sent into the atmosphere of Jupiter, using a spacecraft that is also a high-pressure submarine. The pilot uses a BCI to control the vessel. Unfortunately, the pilot has mental issues and subconsciously wants to commit suicide. When it is time to leave Jupiter, the submarine diving planes are somehow locked into the "down" position, making it impossible to leave Jupiter. There are some tense times with the crew, until the pilot realizes that his subconscious is secretly locking the diving planes. The pilot disconnects from the BCI, puts in a substitute pilot, and the ship escapes Jupiter.
In Daniel Galouye's Lords of the Psychon (1963), the alien invaders use "psychon plasma" as their machines, a weird substance that can be controlled by conscious thought. Unfortunately it can be controlled by unconscious thought as well. Psychon plasma will instantly manifest all of a person's deep seated psychoses and other horrors lurking in the depths of their subconscious, sometimes with lethal results. Actually, the psychon plasma merely creating a visual image of the observer's subconscious fears is enough to drive a person into insanity. It can only be safely handled by a person who has somehow psychologically purged their subconscious to become perfectly mentally balanced. The novel does not mention it, but I'm sure the psychon plasma is perfectly capable of lashing out at other people besides the controller, and the dangerous aspects can be amplyfied by BCI feedback (see quote below from Invaders from the Infinite).
BCI have made an appearance in many works of science fiction, such as the movies Forbidden Planet, Pacific Rim, The Matrix, Ghost in the Shell, "The Man With the Power"; and in novels such as Neuromancer, Nova, Forever Peace, The Genesis Machine, As on a Darkling Plain, Skylark of Valeron, Invaders from the Infinite, Crown of Infinity, and The Halcyon Drift.
For complicated maneuvers, one programs the controls with an autopilot. Nowadays one uses computers.
In pre-computer days, they "cut a cam". A cam operates in a similar fashion to the paper roll on a player piano. When I was little they were all the rage in motorized toys, to program various movement patterns. But those have gone the way of eight track tapes and slide rules.
Currently the only place one is likely to encounter a cam in on the camshaft in the engine of your automobile. Each cam is a "program" that controls the state of the intake and exhaust valves, synchronizing them to the position of the pistons.
Last but not least is the pilot's logbook in the corner. Or log tape. Or floppy disk. Or CD-ROM. Or DVD. Or holographic crystal. Or whatever.
Think about Captain Kirk on the bridge of the Starship Enterprise, starting a recording with "Captain's Log, Stardate [fill in random number here]".