Advanced Spacecraft Design
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Volume and Mass
The Polaris is 792.6 tons of propellant and 396.3 tons of everything else. How big is this, exactly?
When comparing the spacecraft to other vehicles, just use the "everything else" value, ignore the propellant mass. This is because few earthly vehicles have total masses dominated by fuel mass as much as rockets are. How does 396.3 tons stack up?
Rick Robinson notes that is pretty small compared to "wet-navy" vessels. It's under the size of a coastal corvette. But compared to aircraft, it's huge. A Boeing 747 is only 180 tons empty. If you want to get an idea of other sizes, go check out Jeff Russell's huge Starship Dimensions website and Florian Käferböck's impressive Rockets and Space Ships Size Comparison.
|1||Giraffe||6 meters/20 feet|
|2||City Bus||12 meters/40 feet long|
|3||Small Orion Drive ship||21 meters/70 feet|
|4||Millennium Falcon||27 meters/90 feet||Star Wars|
|5||Polaris||43 meters/140 feet||Tom Corbett, Space Cadet|
|6||Moonship||44 meters/144 feet||Chesley Bonestell, Conquest of Space|
|7||Luna||46 meters/150 feet||Destination Moon|
|8||Arc De Triomphe||49 meters/160 feet|
|9||Orion Drive Mars Exploration Vehicle||50 meters/165 feet|
|10||United Planets Star Cruiser C-57D||51 meters/170 feet wide||Forbidden Planet|
|11||Nautilus||51 meters/170 feet long|
|12||Space Shuttle stack||56 meters/180 feet|
|13||Absyrtis||60 meters/197 feet||G. Harry Stine, Contraband Rocket|
|14||Boeing747||72 meters/232 feet|
|15||RS-10||73 meters/240 feet||Andre Norton Star Born|
|16||Ferry Rocket||84 meters/280 feet||Collier's Magazine, 22 March, 1952|
|17||Statue of Liberty||93 meters/300 feet|
|18||DE-51 Destroyer Buckley||93 meters/306 feet|
|19||Saturn V||102 meters/335 feet|
|20||DY-100 Botany Bay||111 meters/365 feet||Star Trek|
|21||California Redwood||112 meters/367 feet|
|22||Discovery||113 meters/370 feet||2001, A Space Odyssey|
|23||Romulan Bird of Prey||131 meters/430 feet||Star Trek|
|24||Great Pyramid of Cheops||140 meters/500 feet|
|25||Oscar class submarine||154 meters/500 feet|
|26||Galactic Cruiser Leif Ericson||169 meters/554 feet|
|27||Washington Monument||170 meters/560 feet|
|28||Klingon D7 battlecruiser||228 meters/750 feet||Star Trek|
|29||LZ-129 Passenger airship Hindenburg||245 meters/800 feet|
|30||BB-62 Battleship New Jersey||270 meters/887 feet|
|31||NCC 1701 Starship Enterprise||289 meters/950 feet||Star Trek|
|32||Eiffel Tower||324 meters/1060 feet|
|33||CVN-65 Carrier Enterprise||336 meters/1101 feet|
|34||Empire State Building||444 meters/1500 feet|
|35||Al Rafik||102 meters/335 feet||Attack Vector: Tactical|
Calculating Volume and Mass
The Hard Way
The following is derived from a document at Christopher Thrash's web site. He bases his analysis on data from the book all the pros in astronautics use, Space Mission Analysis and Design. There is some additional information here.
Lucky you, Eric Rozier has implemented the algorithm below as an on-line calculator.
Assumptions: as a first approximation, the spacecraft is modeled as a free standing column resting upon the engines. The column is "thin-walled", that is, the column radius divided by the hull thickness is less than 0.1. The column is only supported by its walls (monocoque construction). The column has its mass uniformly distributed along its length. The ratio of column's length to its diameter is 3.2 : 1.0. The hull is assumed to be capable of withstanding forces equal to its mass times gs of acceleration on any axis: axial, lateral, or bending.
This means that the following formula only work for a cigar-shaped rocket, not a spherical one.
Decide upon the volume, or total displacement of the hull in cubic meters (m3). This will boil down to volume for reaction mass plus volume for the crew and cargo. Calculate the volume for your reaction mass by
Vpt = Mpt / Dpt
- Mpt = mass of propellant (kg)
- Dpt = density of propellant (kg/m3) = 71 for liquid hydrogen, 423 for methane, 682 for ammonia, and 1000 for water
- Vpt = volume of propellant (m3)
If you don't know the mass of the propellant, it can be calculated from the dry mass and the mass ratio:
Mpt = (R * Me) - Me
- R = mass ratio (dimensionless number)
- Mpt = mass of propellant (kg)
- Me = mass of rocket with empty propellant tanks (kg)
Add the volume of the reaction mass to the desired living space volume to get the spacecraft's volume. Later you can figure the approximate spacecraft dimensions by using the formula for the volume of a cylinder ( v = π r 2h ), keeping in mind that it should be about 3.2 times as high as it is wide (although you can get away with larger values).
Now comes the fun part. This is going to be what they call an "iterative process". This means you do the calculations, take the results and do the calculations again on the results.
M = M~st + Mst
- M = mass of spacecraft (kg)
- M~st = sum of mass of all spacecraft components except structure (kg)
- Mst = spacecraft's structural mass (kg)
Since this is an iterative process to calculate Mst, the first time through Mst will be equal to zero.
D = (M/1000) / V
- D = density of spacecraft (ton/m3)
- M = mass of spacecraft (kg)
- V = volume of spacecraft (m3)
Note that here density is in tons, not kilograms per cubic meter
Vsr = (V4/3 * Apg0 * D) / (1000 * Thm)
- Vsr = volume of structural mass needed to support spacecraft (m3)
- V = volume of spacecraft (m3)
- Apg0 = maximum acceleration of spacecraft (Terra gs)
- D = density of spacecraft (ton/m3)
- Thm = "toughness" of hull material. Hard steel = 2.86.
Vsb = (V1.15 * (Apg0 * D)0.453) / 300
- Vsb = volume of structural mass needed avoid buckling (m3)
The actual volume Vs is equal to the larger of Vsr and Vsb.
(Note: Mr. Thrash informs me that an aeronautical engineer of his acquaintance is of the opinion that while the equation in step 4 works fine for a small rocket with a ten ton payload, the equation does not scale well if used for a larger rocket. The engineer is sure that Vsr will almost always be enough to resist buckling as well. In other words, just use Vsb = Vsr).
Mst = Vs * Dhm
- Mst = spacecraft's structural mass (kg)
- Vs = volume of structural mass (m3)
- Dhm = density of hull material (kg/m3) (7,850 for steel, 4,507 for titanium, 1,738 for magnesium)
Use the new value for Mst and start over. Repeat until the value for Mst stops changing (or you get tired).
When you have your final value for Mst, and M, use M to check and see if the spacecraft's mass ratio is still acceptable. If not, reduce the value for M~st and do some more iterations.
Now you know why rocket scientists use computers to do all the grunt work.
Remember that the mass of the propellant tanks will be approximately equal to full propellant mass times 0.15. The tank mass will be included in the structural mass, if the ship designer is not totally incompetent.
The shortcut is to stop at step seven, reduce M~st by Mst, and everything will add up.
The Easy Way
If you just want something really quick and dirty, decide upon the hull volume of your spacecraft, and multiply it by the average density of a spacecraft. Of course there is some differences of opinion on the exact value of the average density of a spacecraft.
One easy figure I've seen in various SF role playing games is a density of 0.1 to 0.2 metric tons per cubic meter. That corresponds to average pressure compartments being cubes 10 meters on a side, with pressure bulkheads averaging 17 to 33 kg/m2.
Ken Burnside did some research when he designed his game Attack Vector: Tactical. He found that jet airliners have an average density of about 0.28 metric tons per cubic meter, fighter aircraft 0.35 tons/m3, wet navy warships from 0.5 to 0.6 tons/m3, WWII battleships 0.7 tons/m3 (it don't take much excess mass to send them straight to Davy Jones locker), and submarines 0.9 tons/m3. For the combat spacecraft in AV:T, Ken chose a density of 0.25 tons/m3.
Figuring the hull volume is a bit more tricky.
By way of example, a Russian Oscar-II submarine is an oval cylinder about 18 meters wide by 9 meters tall by 154 meters long. It has an internal volume of about 15,400 cubic meters. It has a density of about 0.9 metric tons per cubic meter, so it has a mass of about 15,400 x 0.9 = 13,900 metric tons.
There are equations for calculating the internal volume of various geometric shapes. What you have to do is approximate your spacecraft design using only these shapes. A sphere is easy. A classic cigar shape is sort of a cylinder with a cone on each end. You'll find a crude example of that here.
If your spacecraft is a complicated shape like the Starship Enterprise, you have a real problem.
If you have a physical model of your spacecraft, you can try estimating its displacement by caulking it water-tight, immersing it in a container of water, and measuring the water it displaces. Alternatively, fill a box with sand, dump the sand into measuring cups to measure the volume of sand, put the model in the box and fill it with sand, dump the sand out into measuring cups, and finally subtract the two volumes to discover the volume of the model.
Designing with CGI Modeling
Alternatively, you can proceed like graphic artist Myn.pheos, creating your mesh in the amazing free program Blender and using the Quantities Bill script to calculate the volumes. Myn.pheos also has some techniques to find the center of gravity of various components, and to discover optimal placement of heat radiators.
The following tips are specific to the Blender software, but an artist skilled with another 3D computer modeling program could adapt the tips to their software. Myn.pheos is a native of Slovakia, and English is his second language. Myn.pheos:
Area and Volume
Guessing the volume of spacecraft isn't accurate in most cases. Boxy shapes aren't the most pleasing, and computing volume or area of curved surface by hand is tedious and hard. So the best approach is to let the computer [do the] work for you. In Blender, there is no build-in way to compute volume of objects. But there exist scripts than can do this. One of the is Quantities Bill by Yorik. It computes length, area or volume depending on the topology of mesh. If you have the shape of the spacecraft in your mind, let it pass the test. Roughly model the hull, propellant tank or crew compartment (it must be one object, with no holes in it) so you can get the volume. If you want to know the area of hull, simply remove the smallest face from the mesh and run the script. The figures aren't exact (this depends on how precisely you modelled the hull), but they are obtained fast, and it's easy to [re-calculate the figures if you alter the shape of the hull].
Where is the Center of Gravity?
This is easy to guess in case of homogeneous objects. But spaceships aren't that case. When you know the mass of spacecraft, rough location of components and their estimated weight, you can try to search for the center of gravity (COG). In Blender, it is possible to find the COG easily, just place vertexes in COG of each component. Decide the weight of each vertex, and then add as many as you'll need. Logically, the sum of them should be equal to total mass of ship. To get the COG, simply select all vertexes and make sure the pivot is set to Median point.
(ed note: in Blender, if the pivot control is set to "Median", when you select a group of vertexes the pivot control will automatically appear at the mathematical median point. Myn.pheos is saying that at the center of gravity of each component, place a number of vertexes proportional to that component's relative mass. Select all the COG vertexes of all the components, and the pivot control will indicate the COG of the spaceship as a whole. Keep in mind that the ship's axis of thrust must pass through the COG)
Where to place radiators?
That depends on the shape of the ship. If you have several spots where they look good, you can test the placement. This involves rendering the image and then using histogram to interpret the rendered result. First create two materials. For hull, create fully transparent material (Alpha = 0.0), with no specularity (Spec=0.0), don't forget to check the Ztransp button on. For radiator, use total white material (Col = R 1.00, G 1.00, B 1.00), with again without specularity (Spec=0). Make sure both receive all ambient colour (Amb = 1.0). Now to the environment settings. As background, use total black color (HoR = 0.0, HoG = 0.0, HoB = 0.0, ZoR = 0.0, ZoG = 0.0, ZoB = 0.0), and ambient perfect white (AoR = 1.0, AoG = 1.0, AoB = 1.0). Turn on Ambient Occlusion, make the Sub button pushed (so it darkens occluded spots), ensure that Energy is 1.0 and Plain button pushed.
Now only to set the camera (the best to be perpendicular to the radiator) and render.
Open the rendered image in an image editor. I use GIMP, but only the histogram is important. Now set the lower value in histogram to the lowest non-zero number (remember the pitch black background?), and read the statistical data. The most important is Mean value, this is the average value of all pixels on radiator. Divide this number by 255 to get the percentage of unoccluded area. There rest is probably heating up the ship, so change try with another radiator position.
This method has some weak points, but it is good enough for some decisions. The fully occluded pixels aren't taken into account, the precision increases with samples, the edges aren't treated well (they are not full white, if antialiasing is on).
I must say that I am very impressed with Myn.pheos' technique. I am reasonably skilled with Blender, but it never occurred to me that it could be used to find centers of gravity and optimal heat radiator placement. Myn.pheos is a genius.
Something like Myn.pheos technique for placing heat radiators was used to solve the mystery of the Pioneer Anomaly. The trajectory of space probes in general and the Pioneer probes in particular should follow precisely Newton's Laws of Motion. Once you've accounted for all the extra factors, of course. So scientists were quite upset when the probes started to gradually diverge from their calculated trajectory. There are all sorts of proposed explanations, ranging from observational errors to new laws of physics.
Dr. Frederico Francisco (Instituto Superior Técnico, Lisbon) and colleagues believe they have the answer. Others have tried and found wanting the hypothesis that heat radiated from the probes could be the culprit. But Dr. Francisco et al submit that this is because the radiation mathematical models are too simplistic. Using the 3D CGI rendering technique known as "Phong shading", they have shown this will account for the Pioneer Anomaly. Phong shading takes into account not just the heat radiated, but the heat that hits parts of the probe's structure and is reflected from it.
As you can see, this is very similar to the technique used by Myn.pheos.
The traveling-public gripes at the lack of direct Earth-to-Moon service, but it takes three types of rocket ships and two space-station changes to make a fiddling quarter-million-mile jump for a good reason: Money.
The Commerce Commission has set the charges for the present three-stage lift from here to the Moon at thirty dollars a pound. Would direct service be cheaper? A ship designed to blast off from Earth, make an airless landing on the Moon, return and make an atmosphere landing, would be so cluttered up with heavy special equipment used only once in the trip that it could not show a profit at a thousand dollars a pound! Imagine combining a ferry boat, a subway train, and an express elevator. So Trans-Lunar uses rockets braced for catapulting, and winged for landing on return to Earth to make the terrific lift from Earth to our satellite station Supra-New York. The long middle lap, from there to where Space Terminal circles the Moon, calls for comfort-but no landing gear. The Flying Dutchman and the Philip Nolan never land; they were even assembled in space, and they resemble winged rockets like the Skysprite and the Firefly as little as a Pullman train resembles a parachute.
The Moonbat and the Gremlin are good only for the jump from Space Terminal down to Luna . . . no wings, cocoon-like acceleration-and-crash hammocks, fractional controls on their enormous jets.
The British Interplanetary Society (BIS) in general, and Sir Arthur C. Clarke in particular figured that there were three main types of spacecraft needed for the exploration of space. Each is optimized for their own particular area of use. More recently, orbital propellant depots and their related tanker ships also seem like a good piece of infrastructure. There are some sample realistic designs here.
However, space warships are an entirely different kettle of fish.
Type: Space Ferry
The space ferry concept is what evolved into the NASA space shuttle. Its function is to boost payload into orbit, though you can think of it as an "atmospheric lander." Refer to the section on Surface To Orbit. The idea was to re-use as much of the rocket as possible, which is why the upper section has wings and the lower stages had parachutes. In Robert Heinlein's Space Cadet, the rocket is launched from a rocket sled going up the side of Pike's Peak. Nuclear powered rockets could boost more massive payloads, but a space elevator could boost so much more cheaply and efficiently. Hop Davis estimates that space ferries launching from Terra will require a delta-V budget of around 10 kilometers per second (with orbital propellant depot) and require a thick atmosphere for aerobraking. It will require a bit more if there is no orbital depot, but not much more because coming down it uses aerobraking instead of propellant. The delta-V budget means they will probably have to be multi-stage if they are chemical rockets (good luck getting permission to use nuclear rockets). They will require a propulsion system with a thrust-to-weight ratio above 1.0.
Orbit-to-orbit spacecraft never land on any planet, moon, or asteroid. Therefore they are free to use efficient propulsion systems with a thrust-to-weight ratio below 1.0, such as ion drives or VASIMR. They require no landing gear or parachutes. If there ain't no landing gear, it is an orbit-to-orbit. No streamlining is required either. They require no ablative heat shields unless they are designed to perform aerobraking to burn off delta-V without requiring propellant (like the Leonov in the movie 2010 The Year We Make Contact). Hop Davis estimates that orbit-to-orbit spacecraft will require a delta-V budget of from 3 to 4 kilometers per second, if orbital propellant depot are available. Otherwise it will be twice that, with along with a dramatic reduction in payload capacity. The old image of orbit-to-orbit ships look like dumb-bells, the front ball is the cargo and habitat module, the rear is the propellant and radioactive atomic drive.
Type: Airless Lander
These are designed for landing on bodies that have no atmosphere, but you probably could get away with using them on Mars. They evolved into NASA's Apollo Lunar Module. So they will require some sort of landing gear. But no streamlining. They will require a propulsion system with a thrust-to-weight ratio near 1.0, depending on the surface gravity of the bodies they are designed to land on. This probably means chemical propulsion, maybe a solid-core NTR. Hop Davis estimates that airless lander spacecraft will require a delta-V budget of around 5 kilometers per second if orbital and surface propellant depots are available. Otherwise it will be twice that, with along with a dramatic reduction in payload capacity.
Many aerospace engineers have pointed out that all of these spacecraft can be far more cheap and efficient if there were orbital depots of propellant and/or fuel established in various strategic locations where space travel is desired. This will necessitate some sort of tanker-type spacecraft to keep the depots supplied. They will be a species of orbit-to-orbit spacecraft optimized to carry huge amounts of propellant, and hopefully be unmanned drones or robot controlled. They can use an efficient propulsion system with thrust-to-weight ration below 1.0, ion or VASIMR. Like standard orbit-to-orbit, probably a delta-V budget of 4 km/sec, unless they are in a real hurry.
There will also be a species of airless lander optimized to carry propellant to planetary based depots, this is called a "lighter". As all landers the propulsion thrust to weight ratio will have to be near 1.0, probably chemical propulsion. As standard airless lander, probably a delta-V budget of 5 km/sec. The lighter will probably be designed to land a single modular tank from the cluster carried by the tanker.
This is far more speculative, since as far as we know there have not been any space warships created yet. Refer to Warship Design, Space War: Intro, Space War: Detection, Warship Weapons Intro, , Warship Weapons Exotic, Space War: Defenses, Space War: Tactics, and Planetary Attack .
Fundamentally they are weapons platforms, so by definition they will be carrying various weapons systems. They may or may not have armor or other defenses, they may or may not have human crews. They probably will have an over sized delta-V capacity, and a large thrust capacity so they can jink around and complicate the enemy's targeting solution (i.e., dodge around so you are harder to hit). Lasers will require large amounts of power, and huge heat radiators and heat sinks to cope with waste energy. They will probably be carrying little or nothing that cannot be used to attack the enemy.
(ed note: this system assumes the presence of propellant depots. Otherwise the the delta-V budgets will have to be more or less doubled)
I imagine 3 types of vehicles for space development.
The yellow vehicles have a nearly 10 km/sec delta-V budget and a thick atmosphere to contend with. It is possible these will always be multi-stage expendable vehicles. (ed note: Space Ferry)
The red vehicles move between locations in different orbits. They need no landing mechanism, no thermal protection or ablation shields, parachutes, etc. They have delta V budgets between 4 and 3 km/sec. It is my belief such vehicles could be single stage, reusable vehicles. (ed note: Orbit-to-Orbit)
The green vehicles (lander/ascent vehicles) move between orbital locations and a surface of a substantial body, but not as substantial as earth. Their delta V budget is around 5 km/sec. I believe these vehicles could also be single stage, reusable vehicles. (ed note: Airless Lander)
It would take some investment to build infrastructure to maintain and supply the propellant depots pictured here. Wouldn't it be cheaper to just send ships directly from Earth to Mars? That depends. If your goal is flags and footprints sortie missions, disposable mega rockets are the way to go. But if you wanted genuine development of Mars, it would take many, many trips. If infrastructure could enable these trips to be done with smaller, reusable vehicles, the infrastructure would return the investment many times over.
Reduced to fundamentals, there are two basic shapes for your atomic rocket: the cylinder (cigar shape) and the sphere. Both have advantages and disadvantages. Of course matters are different in the totally unscientific world of media science fiction.
Flying saucers are not atomic rockets and are therefore beyond the scope of this website. If you want the absolute best information (including blueprints) of the most famous flying saucers from movies and TV, run, do not walk, and get a copy of The Saucer Fleet by Jack Hagerty and Jon Rogers. For rocket-like spacecraft, the last word is Spaceship Handbook by the same authors. Both books are solid gold.
Any Freudian symbolism is the responsibility of the reader.
The cylinder is more aerodynamic (for take-off and landing on planets with atmospheres), and allows the use of a smaller anti-radiation shadow shield (because from the point of view of the reactor the body of the ship subtends a smaller angle). It also lends itself well to the tumbling pigeon concept since it does not have to spin as fast as a sphere of the same volume in order to generate the same centrifugal gravity.
Drawbacks include a larger surface area, and a larger "moment of inertia" for yaw and pitch maneuvers (but a lower moment of inertia for roll maneuvers). This means it takes forever to point the ship's nose in different directions as compared to a sphere, which means poor maneuverability (See short story "Hide and Seek" by Sir Arthur C. Clarke for details). Larger gyros or stronger attitude jets will be needed. A faster roll rate is actually not of much use, unless you are trying to get a weapon turret to bear on an enemy ship (See the wargame Attack Vector: Tactical for details).
Cylinder shapes are also better if your ship has a so-called "spinal mount" weapon, that is, where instead of mounting a weapon on your ship you instead build the ship around the weapon. Such weapons are typically long and skinny, which fits the profile of a cigar more than a sphere.
Spheres have the largest enclosed volume for the smallest surface area of any shape, which is a major advantage where every gram of structural mass is a penalty. They also have a smaller moment of inertia for yaw and pitch maneuvers. Drawbacks are the opposite of the cylinder: they are only slightly more aerodynamic than a brick, they don't shadow shield well, and they are lousy tumbling pigeons.
Spheres also require more internal support structure than cylinder to handle the same acceleration load, particularly if you're going to be putting decks inside of it that rely on the structural framework of the spheroidal hull for rigidity. Cylinders under acceleration support themselves in the same manner as a skyscraper building, spheres need extra bracing to keep the equator from sagging. Of course this only becomes a problem if the acceleration is greater than a tenth of a gee, neither spheres nor cylinders have any problem coping with milligee acceleration.
On the other tentacle, if the shape has to be pressurized, like a fuel tank or a crew compartment, non-spherical shapes require more bracing mass and are more expensive to construct than spherical shapes.
Ken Burnside noted that another drawback of a sphere is that your internal volume is going to have a lot of "wasted dead spaces" near the hull. Odd shaped volumes that are what happens when you have an interior wall sectioning off part of the curved surface of the sphere. Anybody who has tried to lay out a floor plan inside a Buckminster Fuller geodetic dome house knows the problem.
Yet another thing to keep in mind is that using current manufacturing techniques, constructing a cylindrical hull costs about 70% of the cost of constructing a spherical hull with the same volume.
Why? Because it is more difficult to manufactured girders and plates that are bent compared to straight ones. A cylinder is constructed using straight stringers. The frames are circular, but all the frames have the same radius and radius of curvature. A sphere on the other hand uses curved stringers and circular frames all of different sizes (well, there are actually two frames of each given radius, but you understand the point I'm trying to make).
On most modern wet-navy warships, the hull plates are mostly straight, with a few bent in one dimension, and only a couple bent spherically in two dimensions. Bending is expensive. Eliminating the bending cost will require one and perhaps two breakthroughs in manufacturing technology.
Other ship geometries are possible. In Sir Arthur C. Clarke's Islands in the Sky there is an Terra-Mars passenger liner shaped like a doughnut (torus). The power plant and propulsion system is in the hole, and the ship spins for centrifugal gravity.
And there is also the open-frame design, where components are attached wherever is convenient and braced by girders. The von Braun Moonship from the Collier's article is an example.
Which Way Is Up?
Remember that in a spacecraft under acceleration, "down" is in the direction the exhaust is shooting (i.e., under acceleration the ship will seem like it is landed, sitting on its tail fins with the nose pointed straight up). The spacecraft living quarters will be arranged stacked like floors in a skyscraper, not sideways like an aircraft. (The latter arrangement is the "Confusing-a-spaceship-with-an-airbus" school of spacecraft design, found mostly in bad SF TV shows and in old "Space Ghost" cartoons). For a compromise solution, one can mount things on gimbals. Note that it is allowed to use an airbus arrangement for a spacecraft that actually does act like an aircraft at some point, e.g., the Space Shuttle.
There was an interesting hybrid in Larry Niven's World of Ptavvs. The "honeymoon special" was laid out sideways like an aircraft. The spacecraft resembled a huge arrow. It sat on the takeoff field like any aircraft while the passengers boarded. It would taxi down the runway and take off with JATO units, the "tail feathers" acting as wings. Once aloft, the scramjets kicked in, boosting the ship into Terra orbit. In space, the main fusion propulsion system was in the belly, not the tail. The ship flew through space sideways, which kept the direction of "down" still pointed at the floor. The wings also contained the heat radiators.
GURPS Traveller: Starships defines the following terms:
- Drive Axis: a line from the center of thrust in the engines passing through the ship's center of gravity. One end points in the direction the exhaust goes, the other end points in the direction the ship moves.
- Tail Lander: a spacecraft whose decks are perpendicular to the drive axis. All the ships described in this website are tail landers.
- Belly Lander: a spacecraft whose decks are parallel to the drive axis. Space Ghost's ship is a belly lander.
- Fore: in the direction of the drive axis towards the ship's nose.
- Aft: in the direction of the drive axis towards the ship's tail.
- Port: a line perpendicular to the drive axis passing through the spacecraft's main airlock. Ship's "left."
- Starboard: a line perpendicular to the drive axis 180° from Port. Ship's "right."
- Dorsal: a line perpendicular to the drive axis 90° from Port, counterclockwise when looking aft. Ship's "top" or "back."
- Ventral: a line perpendicular to the drive axis 90° from Port, clockwise when looking aft. Ship's "bottom" or "belly."
- Outboard: away from the drive axis.
- Inboard: towards the drive axis.
The problem with the definition of port is that in a nuclear powered spacecraft, the logical place for the main airlock (and the ship docking point) is the ship's nose.
You serve "in" a ship, not "on" one. "Abaft" means "behind", "forward" means "in front of." It is a "deck", not a "floor".
Pressure-tight walls are "bulkheads", pressure-tight doors are "hatches." Non-pressure tight doors are just doors. Generally they are pretty flimsy (in some traditions "hatches" are openings in the deck while "doors" are openings in the bulkheads).
It's not a "restroom" it's a "head", it's not a "kitchen" it's a "galley." It's not the "dining room", it's the "mess deck" (unless it's for officers, then it's the "wardroom"). The "mess" refers to the crewmen currently eating on the mess deck. It's not a "bunk" its a "rack", it's not a "ceiling" it's an "overhead." It's not a "hallway" it's a "companionway" or "passageway", it's not the "stairs", it's a "ladder." And the "brow" is any walkway or catwalk leading to the main airlock.
These are all from the naval tradition, the air force jargon is totally different.
If the ship has some sort of centrifugal gravity where spin gravity does not match thrust gravity, there will be some sort of jargon for "thrust gravity downward direction" and "spin gravity downward direction." The wet navy won't help you with this one, make it up yourself. Inside a centrifuge the directions "spinward" and "trailing" will be used.
Hatches have "dogs", which are individual fasteners that put pressure on the hatch to maintain the seal with the hatch coaming. Doors do not have dogs, and cannot be "dogged down". Some hatches have a clever arrangement where a single handle can close all the dogs simultaneously (a "quick acting" hatch). Otherwise the dogs have to be turned individually. Naturally the clever hatches require more scheduled maintenance than the standard kind. A hatch is a damage control barrier, while a door is an access control barrier.
If spacecraft commonly have nuclear propulsion systems and/or nuclear power systems, ship design will more or less force ships to dock bow-to-bow (nose-to-nose). Here's why. Radiations shields by their very nature are massive, and thus cut into the payload capacity. So instead of coating the entire reactor, ships will use "shadow shield" as the smallest possible shield. In the left diagram above, the white area is safe, and the blue area is filled with the deadly radioactive shine from the reactor.
Now say that a lunar shuttle vehicle arrives, and wants to dock. It does not want to wander into the blue radiation zone, or its crew will be irradiated. The crew of the nuclear ferry vehicle does not want the lunar shuttle in the radiation zone either, because the shuttle's metal structure could scatter (reflect) radiation from the ferry's reactor into the ferry's crew.
If you examine the situation, the only safe way seems to be bow-to-bow. Even more so if two nuclear spacecraft want to dock. You may remember this is how the Apollo command and service module docked to the lunar module.
This does throw a monkey wrench into Traveller's definition of "Port", but that's just too bad.
In all the crew's "blastoff stations", they will have acceleration couches. As most space fans know, the human body can tolerate more gravities of acceleration when lying horizontal than when sitting upright in a chair. Crew members who will have to operate controls while under multi-gravity acceleration will have fancy chairs which hold their bodies horizontal, vital controls at their fingertips, and critical dials, telltales, repeaters, and read-outs mounted above them in easy view. The rest of the crew will be lucky to get glorified cots or hammocks (They will probably be stuck with using whatever it is that they sleep in. Tough if they are using a "hot bunk" system.). In the movie DESTINATION: MOON, the pilot had the important controls located on a sort of lap-board for easy access. For real high gravity acceleration, the crew will have to use couches that are high-tech waterbeds.
There may be a "docking control station" with all-around viewports, either for guiding small craft to docking ports or for bringing the ship itself up to dock to another ship or a station. You could use video screens, but a viewport is simpler, and less likely to go to "snow" at the worst possible moment. The docking control station might be out on a boom or otherwise elevated to give a better field of view.
The corridors will have cables, pipes and ducting either exposed or behind easily removable panels. This is to facilitate repairs. The panel brackets can double as hand-holds. The main function of panels is to protect the cables from clumsy crew members flying in free-fall. Of course all the cables and pipes will be color-coded.
The corridors will become instantly dark if the power goes off (since port-holes are often more trouble than they are worth). In James Blish's SPOCK MUST DIE, shuttlecraft have "glow-pups", which are tubes filled with (imaginary) "ethon" gas excited by a built-in radioactive source. They will glow with no power for millions of years.
As with so many other things, high tech items predicted by Star Trek have come to pass. The modern version is called a "Gaseous Tritium Light Source", and is used in submarines. A tube of borosilicate glass is internally coated with a phosphor. It is filled with a trace amount of radioactive Tritium gas and sealed. It will glow for about 10 to 20 years, and is not particularly radioactive. Even if the tube breaks, the gas is too rarefied to be a health hazard. They sell these things in England as glow-in-the-dark keychain fobs.
Glow-pups will be in strategic places for lighting, and will also be placed to indicate hatches and sharp corners of equipment. Anywhere to help getting around in the dark.
Rick Robinson notes that the corridors will probably not be cramped like those on a submarine. The main reason subs are so claustrophobic is because the entire sub has to have, on the average, exactly the density of water. Spacecraft don't have to. (spacecraft designers do have to worry about how much air it takes to pressurize the lifesystem, and the mass of the bulkheads enclosing the interior space.)
While not cramped, the interior will probably be similar to the inside of a conventional Naval vessel. That is, it will be full of sharp corners and hard girders to bark your shins or to give you a concussion. The rule in the U.S. Navy is "one hand for the ship, one hand for you." In other words, always keep a hand free, and when moving through the corridors, you put you hand on the thing sticking out into the passageway as you reach it.
The duty stations of the crew members will probably be cramped. In NASA speak the "work envelope" will be small.
Ladderways may be offset between decks. You don't want to have a five story fall awaiting somebody who slips off the ladder. Especially if the spacecraft is pulling three gees. If they are offset, the farthest one can fall is one deck's worth. However, Rick Robinson has an interesting alternate solution. He notes that moving equipment and supplies through a ship is always a problem, and will be exacerbated by offsetting the ladderways. His solution is to have the ladderway openings in a straight line, but while the spacecraft is under thrust, the ladders will be inclined to become stairs. The stairs will prevent fall-through. When the spacecraft enters free-fall, the stairs are rotated to a vertical position, becoming a ladder again and allowing the ladderway to become a fast route for moving equipment. The stair/ladders can be secured in either position by cotter pins. Don't forget to attach the pins to the ladders with wires to prevent them from floating away while the ladders are rotated. And obviously places where the ladderway penetrates a pressure bulkhead will have large hatches.
has some important observations:
Another thing you might want to think about, based on my naval engineering days: how big are the biggest parts in the engineering spaces? That is, what's the size of the biggest thing you might have to move in and out of the craft for repairs or replacement? The radiators are already on the outside. Are there reactor vessels, fusion containment cells, or some other nifty big bits that cannot be broken down into smaller parts? How about tanks (for algae, fuel, water, sewage, recycling, air)? You're going to need a way to get that stuff on and off, and a way to handle the large mass safely.
In the movie Forbidden Planet, there is a small crane mounted over a deck hatch to facilitate moving equipment between decks. It is shown in the scene where the invisible monster enters through the hatch into the bunkroom full of sleeping enlisted men. It is the long metal arm that the invisible monster bumps out of the way.
The interior walls will be flimsy with flimsy doors (for "flimsy" read "low mass"). All except for the pressure bulkheads dividing the lifesystem into compartments. These will be solid, airtight, and contain emergency hatches that will automatically slam shut into their gaskets if they detects a pressure drop (or if that much automation is too much of a maintenance nightmare, there will be a regulation requiring the hatches to be sealed at all times, only opened long enough to allow passage). The hatches will be substantial, but not as massive as those on a submarine. Sub hatches have to handle several atmospheres worth of pressure, while spacecraft hatches just has to manage one.
The hatches will probably have a numeric label stenciled on (note that the link describes a 1941-era ship). This helps when reporting emergencies, as it provides a standard way to describe a location on the spacecraft. On a World War II LST ship, it was in the form of three numbers separated by hyphens. The first number is the deck, the second is the frame it is abaft, and the last indicates the number of the opening from the inboard out (port even numbers, starboard odd). A different system will be needed for spacecraft, since they do not really have a port or starboard and the frames are parallel to the decks instead of perpendicular to them. In GURPS Traveller: Starships, they use the following system. Odd numbers are port, even numbers are starboard. Numbering is consecutive in order from inboard to outboard, fore to aft, dorsal to ventral.
The hatches will have a pressure gauge indicating whether there is any air on the other side. Just in case of meteors...
The section of the spacecraft that the crew lives and works in is called the Habitat Module (Larry Niven calls it a "Lifesystem"). It is pressurized with a breathable atmosphere, and protects the crew from extremes of temperature and from radiation. Unlike spacecraft in TV and movies, most of a spacecraft is not pressurized. The vast majority of the ship is composed of the propellant tanks, rocket engine, and power plant. The habitat module is sort of tucked into some convenient corner.
Because every cubic meter of habitat module has to be pressurized and protected from the space environment, interior volume will be at a premium. Due to mass constraints, spacecraft designers will have no choice but to minimize the volume. Which will of course make them very cramped.
The TransHab concept was a NASA project to create an inflatable space station, which is not quite as insane as one would think. The walls include layers of Kevlar, and are probably harder to puncture than the metal walls of the International Space Station. The private company Bigelow Aerospace has purchased the rights to TransHab patents, and is in the process of developing a commercial space station. Bigelow already has launched two prototypes into orbit and they are working just fine.
In the Mars Reference Mission, they had a bimodal nuclear thermal rocket on a Mars mission. The rocket could deliver the mission to Mars, come back to approach Earth but with dry propellant tanks. So the rocket would go sailing past Earth into the abyss while the crew bailed out to be rescued. Bye-bye rocket.
However, if you replaced the relatively massive hard-shell habitat module with a lightweight inflatable TransHab module, the increase in delta-V was enough so that the rocket would have enough extra delta-V to be able to brake into Earth orbit and be re-used.
There is an online calculator for TransHab modules here.
Troy Campbell pointed me at a fascinating NASA report about spacecraft design (warning, 2 MB PDF file). The report shows how much easier it is to design a habitat module if it for a one gravity environment instead of free fall (surprise, surprise). It has the spacecraft separate into two parts connected by tethers, spinning for artificial gravity.
Some of the details of this design cannot be used with, say, a warship. You do not want to used an inflatable habitat module on a ship going into battle. But the lists of required equipment are very useful for your ship designs, as are their masses, volumes, and power requirements.
For its habitat module, the report take a TransHab inflatable habitat module, and modifies it for one gravity. TransHab modules are low mass since the walls are made of woven Kevlar instead of metal. For the report design, interior suspension cables are added to support the decks (since the basic TransHab is designed for free fall), and an anti-radiation storm cellar added to the core. The other main reason for using a TransHab is because the proposed launch vehicles used to boost the module into orbit had severe payload size limits. The TransHab could fit into the limits while collapsed, then inflated to full size when in space. For your design, you probably will not have such payload size limits, so you will not need to use an inflatable habitat.
To cool off the module, a small heat radiator is wrapped around the exterior. This radiator can only collect and reject 15 kilowatts of heat, since it is only for life support. The propulsion system and power system will require a much larger radiator (read the report for more details).
The report gave a sample set of deck plans. The first floor is the lowest, at the 1.03g level. For some odd reason the first floor deck plan is rotated 45 degrees counterclockwise with respect to the other two deck plans, as you can see if you try to match up the ladder and pass throughs on the three plans.
Note how all the crew beds are inside the storm cellar.
The module is designed to house a crew of six for eighteen months. According to the report, the bare minimum internal volume for a crew of six is 101 cubic meters (about 17 m3 per crewperson). This design has more than that. The TransHab has 350 cubic meters of internal volume, and of that 193 is habitable (about 32 m3 per crewperson). Please note that this is the total habitable volume, the crew's personal volume is much smaller (basically their bunk and their desk).
The module has an exterior surface area of 233 m2. Just the cylindrical exterior surface has an area of 153 m2.
Again remember that this is for a crew of six and an endurance of eighteen months. The values for mass and volume of all the components will have to be scaled up or down with the size of the crew and the amount of endurance.
|System||Mass (kg)||Stowed Vol. (m3)|
|Power Management and Distribution||625||1.05|
|Displays & Controls||14||0.01|
|Environmental Control & Life Support||5030||31.50|
|Temperature and Humidity Control||113||6.32|
|Fire Detection and Suppression||13||0.05|
|Water Recovery and Management||2199||6.02|
|Thermal Control System||576||2.43|
|Internal Thermal Control System||135||0.34|
|External Thermal Control System||167||0.13|
|Galley and Food System||8063||31.35|
|Waste Collection System||327||8.83|
|Recreational Equipment and Personal Stowage||150||3.00|
|Operational Supplies and Restraints||120||0.01|
|Vehicle Support for EVA||291||0.40|
|EVA Translation Aids||123||3.36|
|Structure and Mechanism||12941||84.51|
|Human Research Facility||289||2.50|
|Crew Health Care Systems||759||3.67|
From the report (which goes into this in much greater detail):
|Mass (kg)||Stowed Vol. (m3)||Quantity|
|Fiber Li-Ion Battery||0.17||335||1|
|Battery Charge/Discharge Unit||0.09||50||3|
|Main Bus Cable||0.84||7.5||3|
|Secondary Power Distribution
|Wiring Harness Secondary
|Power Management and Distribution|
|Galaxy Inverter Boxes||0.04||28||3|
|Custom Built 400 Hz, 115 Vac
|Unitron PS-95-448-1 400 Hz
to 60 Hz Frequency Converter
|Vikor AC/DC Rectifiers||0.0007||2||9|
The primary power system for the spacecraft is a pair of nuclear reactors on the other end of the boom. Since they are external to the habitat module, their mass and volume are not included here.
The secondary power system is internal to the module. It consists of three main subsystems:
- Secondary Power
- Power Management and Distribution
These three subsystems can be further broken down to the component level as shown in the table to the right.
The assumption was made that the power entering the habitat would be 115 Vac, delivered at 400 Hz. A final assumption that was made was that the habitat would nominally use 15 kW of power. The final subsystem that needed to be sized for this habitat was the secondary power source. Upon analyzing the architecture and the type of primary power sources, a decision was made to supply 24 hours of emergency power to the habitat that will accommodate 50% of the nominal load (180 kW-h).
Includes a communication system; a guidance, navigation and control system; a crew interface system; and an integrated vehicle management system. It has a peak power consumption of 864 watts. It provides for the command, control, communications, and computation required for the carrying out the mission including insertion into transit orbits. This involves provisions for crew displays; data, voice, and video communications home base, other orbital assets, and EVA crewmembers; an integrated health management system for onboard and ground monitoring of all systems; and a full flight system capability for Guidance, Navigation, and Control. The flight system must also integrate requirements for data communication and computational support for remote commanding of the spacecraft during any uncrewed phase as well as ground commanding during crewed phases. The crew interface must be integrated with data communications and computational support for remote commanding of visiting vehicles.
Environmental Control and Life Support System
The Air Management Subsystem is characterized by a 4-Bed Molecular Sieve (217.7 kg, 0.6 m3, 733.9 W), a Sabatier CO2 Reduction Unit (26 kg, 0.01 m3, 227.4 W), an Oxygen Generation Subsystem (501 kg, 2.36 m3, 4,003 W), and high-pressure storage tanks for O2 (20.4 kg, 0.78 m3, 6 W) and N2 (94.4 kg, 3.6 m3, 6 W). The Water Management Subsystem uses a Vapor Phase Catalytic Ammonia Removal system (1,119 kg, 5.5 m3, 6,090.7 W) and potable water storage tanks (145.9 kg, 0.54 m3, 5 W). The Waste Management Subsystem uses a Warm Air Dryer (527.2 kg, 11.2 m3, 2,043.7 W).
Thermal Control System
|Internal TCS||External TCS||Radiators||Total|
|Fluid mass, kg||0.0||34.4||n/a||34.4|
|Dry mass, kg||111.0||131.0||243.8||485.8|
|Volume, m3||0.158||0.129||1.742||2.0 m3|
|Power, kw||0.000||1.109||0.000||1.1 kw|
The TCS system concept makes use of flexible lightweight body mounted radiators, which are attached to the outer surface. The TCS system has been sized to collect and reject 15.0 kW of heat. Mass, power, and volume are listed below. ITCS refers to coldplates, heat exchangers, and plumbing located inside Transhab, while ETCS refers to similar equipment mounted on the outside. Radiators are listed separately.
A propylene glycol/water coolant is circulated inside the module to collect heat from heat exchangers and coldplates and this heat is rejected to space through the body mounted radiators mounted on the outer shell of the module. Radiator size was determined for the warmest case (0.5 A.U. orbit). The results indicate a required area of 78 m2. This represents 51% of the available area of the cylindrical portion of the shell.
Two other sizing exercises were also conducted for the module. The first determined the radiator area needed to reject twice the average load of 15 kW. Assuming the warmest environment temperature at 0.5 A.U., the analysis indicated approximately 157 m2 was required. This is just slightly over the total cylindrical area of the shell of 153 m2, therefore rejecting just under 30 kw on average is the maximum amount of heat rejection possible without adding something like a heat pump to raise the radiator temperature.
Another sizing exercise determined the heat rejection given the following scenario: The module is in Mars orbit and the crew has left the module for the Martian surface leaving the AG module uninhabited. If the heat loads are reduced and the TCS fluid is allowed to approach its freezing temperature of -50°C, the question becomes how much heat can be rejected. The analysis indicated that the radiators could still reject up to 11 kW of heat with the TCS fluid just above its freezing temperature. This is in part due to the much colder environment at the low Mars orbit assumed. At the 0.5 A.U. orbit location heat rejection would be approximately zero because the radiator and sink temperature would be identical for this scenario.
Propylene glycol was selected for the working fluid. The relevant options are water or 60% propylene glycol with 40% water or some other working fluid. While water is non-toxic and has greatest thermal capacity per mass of working fluid, it also freezes at 273.2 K and thus may not allow sufficient radiator availability for some mission phases. 60% propylene glycol with 40% water is also non-toxic but, compared to water, it is a less desirable thermal working fluid. However, 60% propylene glycol with 40% water freezes at roughly 223 K, a significant advantage over water. Thus, tentatively the working fluid for the thermal control fluid loops is 60% propylene glycol with 40% water. As above, complete resolution of this issue also requires in-depth thermal environment modeling focusing on radiant rejection from the habitat.
This provide crew accommodations systems and layout to make an 18-month mission habitable for six crewmembers. Functions covered include the following: crew support (meal preparation, eating, meal clean-up, full-body cleansing, hand/face cleansing, personal hygiene, human waste disposal, training, sleep, private recreation and leisure, small-group recreation and leisure, dressing/undressing, clothing maintenance), and operations (facilities for meetings and teleconferences, planning and scheduling, general housekeeping). It is also responsible for configuring work and personal stations such that traffic congestion are minimized. Work efficiency, space use, crew comfort, and convenience should be maximized.
The EVA system is designed to be used for three planned, two person EVA days per mission. The airlock will transfer two crewmembers per cycle. If full crew transfer is required in LEO, this system assumes all three EVAs are used to transfer crew out of the habitat. EVA days are sized to be 8 hrs, and are accomplished with a personal life support system (PLSS) that is sized for eight hours. The system includes a single flexible airlock with umbilical support and PLSS recharge system; no gas reclamation is planned due to the minimal number of EVAs (3). Two EVA tools boxes are provided. Translation aids are provided to aid crew transportation about the vehicle. EVA system spares are also provided.
Included in the airlock arrangement is a single flexible airlock that allows two persons to egress the AGH at one time. A staging area by the inside airlock door is included in the concept. This area provides volume to store all space suits as well as space suit spares and expendables. Provisions for donning, suit expendables recharge, and checkout are included as well. An unpressurized area by the outside airlock doors is included in the concept. It provides a place for EVA tool storage and allows handling of large objects.
EVA tools provided consist of two toolboxes containing mechanical, electrical, and storage/tie downs. The tools are stowed in the unpressurized area just outside the airlock. EVA system spares as needed to support the six suits and airlock suit recharge provisions are stowed in the AGH in the EVA staging area and remain stored there until needed.
Structure and Mechanism
|Unpressurized End cone||650|
|Pressurized End cone||800|
|Internal fixed structure||2,120|
|Internal deployable structure||1,870|
|Crew Quarters Radiation Insulation||1,500|
The structure and shell are to provide a safe habitat for the crew and the necessary space to store supplies and equipment to sustain them for the duration of the entire mission. The inflatable module design was chosen because it is the best means to effectively increase the habitable volume of a spacecraft while keeping the diameter of the core within acceptable payload size limits set by current launch vehicles. The airlock system is to provide the crew with the capability to perform extravehicular activities. It is to be located atop the habitat module, so as to allow the fully suited EVA astronauts to take advantage of a slightly lower gravitational pull.
The medical operation capabilities onboard the artificial gravity habitat during transit will provide medical contingencies to promote successful mission completion, crew health, safety, and optimal crew performance.
The potential medical contingencies that are to be addressed include those currently required for International Space Station and additional procedures unique to a continuously rotating spacecraft. Following the convention for classification of medical contingencies onboard ISS, the artificial gravy habitat will enable the practice of emergency medicine, environmental medicine, countermeasures or preventive medicine, rehabilitation, and dentistry. Emergency medical procedures will provide for Advanced Cardiac Life Support (ACLS), Basic Cardiac Life Support (BCLS), and trauma. Additionally, emergency medical contingencies may include shock, behavioral, compromised airway or breathing, drug overdose, and smoke inhalation. Environmental medicine will enable treatment for exposure to toxic and hazardous materials. Countermeasures/Preventive Medicine and Rehabilitation will enable countermeasures to prevent neurovestibular dysfunction resulting from the Coriolis effect induced by the rate of rotation of the spacecraft. Coriolis effects induced by rotation of the spacecraft develop within the neurovestibular system and impacts motor performance, behavior, and motion sickness. Exposure to partial gravity, 0.38G, may greatly impact musculoskeletal and cardiopulmonary systems. Dentistry onboard the artificial gravity habitat will enable basic cleaning, crown replacement and treatment of exposed pulp.
And we can't forget lifeboats. There are some nifty lifeboat and one man re-entry vehicles detailed here. There is a good description of them in the eponomously named novel Lifeboat (AKA Dark Inferno) by James White. His lifeboats are inflatable spheres. They are launched perpendicular to the stricken nuclear propulsion ship with three persons per sphere. After the ship has moved out of radiation range the life boats burn a pre-measured solid fuel thruster to move back to the central point to await the arrival of the rescue vessel.
Christopher Weuve says that a merchant ship's primary piece of damage control equipment is a lifeboat.
Please note there are two different types of rescue craft here that are being lumped together. "Lifeboats" are long duration devices that generally are not re-entry capable. "Re-entry capsules" are short duration devices that allows an astronaut to bail out of a spacecraft in orbit around a planet and safely land on the surface. In other words, the lifeboat is more like a wet-navy lifeboat, while the re-entry capsule is more like a parachute on an aircraft.
The escape pods were lenticular ceramic heat shields with thermoplastic covers, hardly bigger than Porta Potties. Stored inside each was a parachute and an inflatable raft in an ejection rig and -- the only really specialized gadget -- a hand-aimed, gyro-stabilized, solid-fuel retrorocket. An astronaut who had to leave orbit in a hurry was supposed to climb in, lie back, clutch the retrorocket to his or her chest, adjust position and attitude with its gas jets, then take aim at an easily identifiable star specified by mission control and pull the trigger.
The impulse from the solid-fuel rocket would gradually slow the pod until orbital velocity was lost, whereupon the astronaut threw away the rocket, closed the flimsy hatch with its little bubble window, and tried to relax while falling through the atmosphere, on fire, decelerating at five gees plus. Below about 7,000 meters or so the pod's cover would pop off, spilling the astronaut and deploying the chute.
In seconds he had the nearer pod free of its straps. Lifting the thermoplastic lid, he found all the neat packages of equipment nestled where they should be. He ripped open Velcro fastening of yellow webbing, yanked at cotter pins festooned with red warning strips. One of them activated a SARSAT radar beacon...
...Flipping over to squat on the pod, he shrugged off his life-support backpack and hooked into the pod's portable emergency oxygen supply. He wrestled himself onto his back and tugged the parachute straps across his chest and shoulders, pulling the life raft package up under his rump. The strap edges scrunched thick layers of suit material into an oppressive lump in his crotch. It was exhausting work, and he heated up fast without the coolant flow from this abandoned backpack, but it had to be done right; parachutists had dismembered themselves with loose harnesses.
However, Jim Cambias raises an important point:
I've never understood the purpose of life pods. Why abandon a spaceship, however shot up or meteor-damaged it may be, just to hang around in a flimsy balloon or cramped pod? You're still on the same course, since no life pod can carry much delta-v, and the life-support problems are considerable. Why not include some kind of pressure balloon to provide temporary airtight containment in a hulled compartment and use the ship's own life-support? That way you get the ship's radiation shielding, power, etc.
If it's a reactor emergency you're worried about, don't eject the crew in pods, EJECT THE REACTOR!
(Actually, I realize perfectly well the purpose of life pods: it lets sf writers tell lifeboat stories in space.)