For certain spacecraft emergencies, it is best if the crew and passengers abandon ship in some type of rescue craft.
A "lifeboat" or "life pod" is a long endurance device carrying many castaways that generally is not reentry capable. It is much like a wet-navy lifeboat. It is discussed here.
A "Reentry capsule" or "escape pod" is a short duration devices carrying few or one castaway that allows them to bail out of a spacecraft in orbit around a planet and safely land on the surface. It is much like a parachute on an aircraft. It is discussed here.
To reiterate: "lifeboat" or "life pod" is a long endurance device carrying many castaways that generally is not reentry capable. It is much like a wet-navy lifeboat. The main difference between a liferaft and a lifeboat is that the liferaft does not have a motor.
There are some nifty lifeboat and one man reentry vehicles detailed here.
Christopher Weuve says that a merchant ship's primary piece of damage control equipment is a lifeboat.
If the lifeboat is designed for prolonged use, it would be useful for it to contain equipment to put the people into suspended animation. This will reduce the consumption of air, food, and sanity. The lifeboat Narcissus from the movie Alien had a suspended animation capsule.
However, Jim Cambias raises an important point:
There is a good description of lifeboats in the eponomously named novel Lifeboat (AKA Dark Inferno) by James White.
The NTR passenger rocket's habitat module spins on its axis for artificial gravity. Since the rocket's designer failed to consult with Mr. Cambias, in the event of a nuclear engine disaster the crew and passengers escape in lifeboats.
The hatch to each lifeboat are set in the floor of the habitat module. The lifeboats are cylindrical but inflate into spheres once they are clear of the ship. At the top is the pressure hatch. 2.4 meters below the hatch is a plastic bag containing lightweight screens used for dividing the inflated pod. Below that is the service module and food store. While uninflated, the walls are folded with convolutions projecting inward. When inflated each lifeboat is three meters in diameter. The upper half of the sphere is transparent, the lower half is covered in reflective foil as a sun-screen.
The service module contains a two shot pre-measured solid rocket, a radio, one heavy set of sunglasses, and a lifesupport system. The lifesystem contains the breathing mix equipment, thermal control, toilet, and water reclamation.
The first passenger jumps into the pod. They then press backward into the side wall and raise their hands to help the next passenger into the pod. The second passenger does not jump, instead they sit on the edge of the hatch with legs dangling down while gripping the hatch coaming with both hands. The first passenger grabs the second's legs and helps them down. They both press backward and the second passenger helps the third in the same way. Three passengers is what the lifeboat lifesupport system is rated for. Passengers are warned to leave behind anything made of metal or having sharp edges, which could puncture the lifeboat walls.
The lifeboats are ejected radially perpendicular to the habitat module's spin axis, at a velocity of 2.45 m/s (1/4 g). Under normal conditions, each lifeboat's umbilical power line is remotely severed before ejection. In the event of a ship control failure, the umbilicals will not be severed, and will give each lifeboat an off-center tug as they separate. This will cause the lifeboats to slowly tumble. If there is an crewperson available, they can manually sever the umbilicals.
The crew cabins eject as four wedge shaped sections. Since they are closer to the spin axis than the floor of the passenger module, the ship's spin has to be increased so they too will be ejected at 2.45 m/s. The medical officer's cabin has a radio powerful enough to reach all the passenger lifeboats, since the officer will have to offer medical advice.
A radio beacon is left behind to designate the rendezvous point.
If a lifeboat is tumbling, the passengers can arrest the tumble by crawling. Lie flat on the transparent section of the lifeboat skin while holding the moulded finger-grips. Rotate their body until the Sun appears to be coming from the top of your head, passing in front of you, and then moving under your feet. Then start crawling in as straight a line as possible. When you come to the lock section or the services panel, or when you are crawling over plastic, which is not transparent, try to keep your line of movement straight by looking ahead to the next transparent section to see where the Sun is. Gradually the tumble will stop. If the Sun is moving too fast to see, blink as fast as you can to visually slow it down. If there is more than one passenger in the pod, they can help by crawling along the same line, evenly spaced around the interior wall. Or even hold on to each other with feet on the walls and heads near the center, and walk in the proper direction.
The desired attitude of each lifeboat is with zero tumble, and the silver section of the wall aimed at the Sun.
The ship proper, still under thrust, leaves the rendezvous point. Once the ship is safely away (after a few days), each lifeboat burns a pre-measured solid rocket to reverse their vector. The engine is oriented so that the thrust axis is aimed at the rendezvous point. The "A" pre-measured rocket is burned for 4.9 m/s of delta V. This cancels the 2.45 m/s outward vector, and gives a 2.45 m/s inward vector in the direction of the rendezvous point.
If the radio beacon is operational, orienting a lifeboat in the proper direction is easy, using fancy electronics.
If the disaster renders the beacon inoperable, the passengers in the lifeboats have to do orientation the hard way. The navigation officer will calculate the reference stars for each lifeboat. The stars will be in a plane perpendicular to the desired vector. The boat's passengers will have to orient such that the line of demarcation between the transparent and the silvered skin hemispheres touches each of the reference stars. They use the same technique used to arrest lifeboat tumble: by crawling on the skin. The officer will have to teach the passengers enough constellations so they can identify the reference stars. Failure to properly orient the lifeboat will probably doom the occupants.
The navigational officer will give each specific lifeboat a precise count-down to igniting the "A" pre-measured rocket.
When the lifeboats near the rendezvous point, the lifeboat will orient itself so that the thrust axis point away. Then on command from the navigational officer they burn the "B" pre-measured rocket and come to a halt. The rocket gives a delta V of 2.45 m/s, cancelling their vector. In this case the proper orientation of the lifeboat is secondary to igniting the B rocket at precisely the correct time. Improper orientation will merely result in a small amount of lateral drift. Improper timing means the lifeboat could stop way short or way past the rendezvous point, possibly even far outside the rendezvous area.
At the rendezvous point they will meet the rescue ship.
The life support section can supply breathable atmosphere enough for three people for two weeks (42 person-days).
The life support section can handle the body heat of up to three adults. Past that the environment will become hotter. Passengers should avoid exertions and remove some clothing in order to prevent heat build up.
The food supply is low-residue and highly concentrated. This is to avoid straining the toilet. The lack of bulk will mean the passengers will always be hungry even though there is enough nourishment to keep them alive. There is enough food for three persons to last two weeks (also 42 person-days).
Water is reclaimed from the toilet and atmospheric humidity. It is more pure than most tap water, but passengers might detect a psychosomatic stink from it if they dwell too much on its source.
For you ugly Americans who have not heard of Perry Rhodan, it is a German science fiction series that has been steadily published installments since 1960 (more than 2,850 as of April 2016). Pretty much the most successful science fiction book series ever written.
To reiterate: A "Reentry capsule" or "escape pod" is a short duration devices carrying few or one castaway that allows them to bail out of a spacecraft in orbit around a planet and safely land on the surface. It is much like a parachute on an aircraft.
Note that all of the reentry capsules shown here rely heavily upon aerobraking, they would not work on an airless planet or moon.
Under the heading of "some people have too much free time", there are a few science fiction writers who talk about bored people using reentry capsules as a sport, much like sky-diver do today. Adrenaline junkies will always be with us.
A deluxe reentry capsule will also contain a survival kit. There are two basic types:
Hostile environment survival kit: those kits that assist survival on nasty deadly planets that will kill unprotected humans in a few seconds. Non-habitable planets, in other words.
A habitable planet survival kit is going to be more or less identical to a conventional survival kit you can purchase right here on Terra.
A first-aid kit and other medical gear which are useable by an untrained person. You can assume a nurse or doctor will have their own full medical kits.
Hostile environment survival kits will contain pretty much everything in a shirt-sleeve environment kit, plus extra equipment to cope with such problems as the lack of a breathable atmosphere.
Making a survival kit for a planet with an enviroment lethal to an unprotected human is incredibly difficult. An castaway using a reentry capsule to land on Terra might wind up in unpleasant places such as a jungle, desert, or ocean; but at least they will have access to unlimited amounts of breathable air. Maybe even food and water. A castaway landing on Mars will not be so lucky. Just ask Mark Watney.
Basically the kit will have to include a pup tent sized habitat module with an entire life support system (Poul Anderson called them "sealtents"). Might as well just make the reentry capsule into a freaking spacecraft. It will probably take the form of some type of inflatable habitat module.
Alternatively you'll need compact equipment to re-fill your space suit's oxygen tanks (and make more O2). Meaning your space suit will be your habitat module. I hope the suit has sanitary facilities or it is going to be really nasty as the suit fills up with poop.
In a paper entitled An overnight habitat for expanding lunar surface exploration by Samuel S. Schreiner et al is described a piece of equipment that could be adapated into a hostile enviroment pup-tent. The item was intended only to be an overnight habitat used for eight hours or so, but it is a start.
The system is intended to enable two astronauts, exploring with an unpressurized rover, to remove their space suits for an 8-h rest away from the lunar base and then conduct a second day of surface exploration before returning to base. This system is composed of an Environmental Control and Life Support System (ECLSS) on the rover, an inflatable habitat, a solar shield and a solar power array...
...The mass, volume, and power analyses of each subsystem are integrated to generate a total system mass of 124 kg and a volume of 594 L, both of which can be accommodated on the Apollo Lunar Roving Vehicle with minor improvements.
The rover ECLSS connects to the habitat via umbilical cables to maintain the atmosphere in the habitat. A thermal control unit on the rover is connected to the liquid cooling garments worn by the astronauts to provide active thermal control, while a solar shield is used to provide passive thermal control to minimize the load on the thermal control unit. The ECLSS contains a carbon dioxide scrubber, a “slurper” to remove humidity, an oxygen tank for respiration, and a water tank for the sublimator, crew hydration, food preparation, hygiene or medical use. The solar array provides power for the overnight system and recharges the rover batteries for the second day of exploration...
2.1. Inflatable ribbing concept
When the astronauts first enter the inflatable habitat, the airlock volume requires a support structure in order to retain its internal shape while at zero internal pressure. Inflatable ribbing may be chosen for structural support. This ribbing consists of a frame of small-diameter inflatable tubes that, when inflated to high pressure, provide a rigid structure for the habitat. Thus, the astronauts can inflate the ribbing prior to entry without filling the interior of the habitat with O2. Then the astronauts can enter the habitat, close the airlock, and fill the interior of the habitat to the desired pressure. A similar inflatable ribbing concept is used in commercially available inflatable camping tents.
2.2. Flexible membrane airlock design
To reduce the total size of the habitat, a novel flexible membrane was designed such that the same internal volume could function as both an airlock and habitat. As shown in Fig. 3, a thin, flexible, airtight membrane divides the internal habitat volume into an airlock side (left) and a habitat side (right). The membrane material is similar to the habitat outer surface without the micrometeorite protection, resulting in approximately 1/4 the surface density. The membrane is sized such that at any given time the entire volume can be used either as an airlock or as a habitat.
The concept of operations for entering the habitat using the flexible membrane airlock is illustrated in Fig. 3. After the astronauts have entered the habitat and pressurized the airlock, they remove their suits on the airlock side (top row of Fig. 3). Next, the airlock membrane is moved manually by the astronauts to its neutral transition configuration (middle row of Fig. 3). A valve in the membrane is used to regulate air flow from one side of the habitat to the other while the membrane is moved and the flow path is filtered to ensure that lunar dust is not transferred from the airlock side to the habitat side. After moving the membrane to the neutral position (in which it divides the habitat into two equal halves), the astronauts unzip an airtight zipper and proceed to the habitat side through the hole in the airlock membrane. Equipment is passed through in the same way. Next, the airtight zipper is closed and the membrane is manually moved towards the airlock side, maximizing the volume of the habitat side (bottom row in Fig. 3). The astronauts close the valve to the airlock side and then conduct activities within the habitat...
...When considering entry and exit of the habitat, a net could potentially be used to restrain the airlock membrane in its neutral transition position in circumstances where the habitat side is pressurized but the airlock side is not. With a net in place, only partial venting of the volume would be required for entry and exit. The flexible membrane concept makes efficient use of the available space and reduces the total required internal volume of the habitat.
2.3. Inflatable geometry optimization
A preliminary geometric analysis was used to select a cylindrical geometry with hemisphere end caps and a flat floor. The cylinder was designed to accommodate the two astronauts standing vertically to don/doff their suits and the two astronauts sleeping side-by-side. The design was further constrained to a minimum interior volume of 12 m3 as a conservative estimate , and was required to have a flat cylinder wall between the end caps that was long enough to accommodate a door 0.75 m wide for entry and egress. The radius of the cylinder, the width of the flat floor, and the length of the cylinder were optimized to ensure that these requirements were met while minimizing the total mass of the inflatable skin and ribbing...
Table 1. Optimal inflatable pill mass and volume Component Mass
Support ribbing 1.10 0.09 0.0007 Adjustable airlock 4.04 1 0.0160 Wall/ceiling 26.27 5 0.1117 Floor 3.60 4 0.0155 Packed volume – – 0.2879 Total 35.01 – 0.1440
Table 2. The optimized geometry of the inflatable cylindrical flat-floor habitat Cylinder radius 1.29 m Cylinder flat side length 0.75 m Maximum floor width 1.80 m Maximum floor length 2.55 m Maximum height 2.21 m Interior volume 12.00 m3 Door height 1.84 m
2.5. Inflatable deployment and stowing
...Using the packaged volume of 0.2879 m3 determined in Section 2.3 (packing factor of 2), the folded inflatable can be expected to fit into a rectangular prism of dimensions 1.4 m×0.7 m×0.294 m...
4. Environmental Control and Life Support Systems
To enable an overnight stay on the lunar surface, the system needs to provide a suitable environment and consumables such as water and food. To meet this need, an Environmental Control and Life Support System (ECLSS) was designed to support two astronauts during the overnight stay and to recharge the astronauts׳ Portable Life Support Systems (PLSS) for a second day of exploration. The demands of the first day were not included in the system design because they would be met by the astronauts׳ (PLSS)...
Table 5. Oxygen and water requirements and storage system sizing for the overnight mission Consumables storage Oxygen Water Quantity required (kg) 8.1 23.9 Tank mass (kg) 6.8 4.0 Tank volume (L) 99.9 23.9
6. Results: system mass and volume estimates
Mass Breakdown ECLSS 28.5 kg Emergency Power 2.0 kg Emergency Inflatable Seat 6.0 kg Power 13.2 kg Thermal Shield 4.6 kg Inflatable 35.0 kg ECLSS Consumables 34.1 kg Total 123.4 kg
ECLSS Consumables Mass Breakdown H2O - Med, Hygeine, Food Prep 7.8 kg H2O - Crew Hydration 7.1 kg H2O - Rover Sublimator 5.9 kg H2O - Refill Suit Sublimator 3.9 kg Oxygen for Ribs 1.5 kg Oxygen for Habitat + Respiration 6.6 kg Food 1.3 kg Total 34.1 kg
Volume Breakdown Inflatable Habitat
287.9 L Thermal Shield 4.1 L Power 62.6 L ECLSS 186.0 L Emergency Inflatable Seat
51.2 L Total 34.1 kg
Castaways will need survival skills or they will be facing a real short life-span. They will get to see how good they are at playing Robinson Crusoe or Swiss Family Robinson. Which could be a real challenge if the planet does not have a human-habitable biome.
As the duration on the planet without rescue drags on, the line between castaway and colonist becomes blurred. A few old-timey science fiction stories postulate a "castaway's code" where people marooned on a habitable planet with no hope of rescue must marry each other and found a colony, by law. Left unstated is why such a bizarre law would have been passed in the first place. Rampant imperialism, I guess.
Sometimes space explorers won't crash but will discover a shipwrecked spacecraft or life boat and will do a search for castaways. That is, of course, if it is a Terran spacecraft. If it is extraterrestrial, more caution is needed. If it from an unknown extraterrestrial species, call out the marines and the first contact specialists. And do keep in mind the movie "Alien."
If explorers discover lots of shipwrecked spacecraft, go to red alert because you have apparently discovered a "Sargasso of Space planet". And if you are not real careful you'll be the next shipwreck. Whatever wrecked all those other spacecraft might still be active.
Old pulp science fiction stories sometimes take the slant that deliberately marooning another human on a wilderness planet for the rest of their life is an unspeakable act, the crime of crimes. It doesn't matter if they are your worst enemy, it just isn't done.
More recent science fiction is a bit more cynical. Vaporizing your enemy with a laser pistol is too merciful, marooning them ensures they suffer your maximum revenge.
Novels and short stories that cover the shipwrecked spaceship and castaway theme include:
- The Martian by Andy Weir
- The Moon Is Hell by John Campbell
- Transit of Earth by Arthur C. Clarke
- Enemy Mine by Barry Longyear
- Five Against Venus by Philip Latham
- No Man Friday (aka First on Mars) by Rex Gordon
- The Man Who Lost the Sea by Theodore Sturgeon
- Shipwreck by Charles Logan
Movies that cover the shipwrecked spaceship and castaway theme include:
Sometimes in an emergency situation, the crew will have to deal with people who cannot wear a space suit. This includes people who are too wounded, too unconscious, too untrained, or too stupid to use a suit (or even put one on). It will be useful to have some kind of basic no-frills life support equipment that you can shove the people into and trust it to keep them alive without your attention.
|Oxygen Supply||1 hour|
|Habitable Volume||0.33 m3|
It will also be useful to supplement one's supply of space suits with Personal Rescue Enclosures aka emergency life support balls. These are basically bare essential spherical suits with no arms, legs, or heads for use by people who are injured or untrained in suit operations.
The ball had three layers: urethane inner enclosure, Kevlar middle layer, and a white outer thermal protective cover. The user enters the ball, puts on the oxygen mask, cradled in their arms a carbon dioxide scrubber/oxygen supply box, and a crewperson outside zips it up. The ball would be connected by an umbilical to the shuttle to supply air until the airlock depressurized. Then the oxygen box gives the user one hour of breathable air, while a crewperson tows the ball to safety.
Mercifully the ball included a tiny Lexan window to prevent total sensory deprivation.
When a passenger liner has a problem, the crew members will stuff the passengers into these balls, zip them up, and tow them to safety. And even a person highly skilled in space suits can be a problem if they are unconscious and suffering from a broken arm. It will be much quicker to slip them into a ball instead of trying to suit them up.
For passengers, one would be wise to use balls that cannot be opened from the inside. Passengers can do remarkably silly things at the worst possible moment.
A morbid but necessary fixture that nobody talks about will be the "C-Chute" (from the Isaac Asimov story with the same name). "C" is short for "Casualty". A dead body will quickly contaminate the air of the lifesystem, so there has to be a way to jettison the dear departed. Also of concern is the effect on crew morale. Personnel will be prone to morbid thoughts while their crewmate(s) mortal remains are lying in the next cabin. There will probably be a tradition of laying the dead to rest within twenty-four hours of death.
It will be important to have an already established protocol for laying the dead to rest. In the movie Conquest of Space they did not have such an established protocol, and the results were ugly. During an EVA astronaut Andre Fodor is killed by a meteor. Not knowing what to do, they leave the body out there still on the safety line.
You can see the surviving crew start to freak out as they try to ignore their dead friend floating outside the porthole. Finally one of them cracks and starts to scream at the body. That's when the captain suddenly wakes up to the vital necessity of laying to rest the dear departed. Say a few words, and push the body off into space.
Don't bother trying to push it into collision course with the Sun, it takes far too much delta V and if the course is only a tiny bit off the body will just sling-shot around and head off to the Oort cloud.
As it turns out, NASA does not have an established protocol for dealing with unexpected dead bodies. They are going to be faced with the "Conquest of Space" scenario if they don't quit pretending that it will not happen. This is complicated by the fact that the UN space debris mitigation guidelines forbid space littering, which includes dumping dead bodies.
- Place the body of the dear departed into a special bag
- Hang the bag out in vacuum, where in an hour it will freeze-dry into the consistency of florist foam
- Bring the bag in and place it in a high-frequency vibration unit
- The body shatters into fine powder. You now have a bag full of about 23 kilograms of dust.
- Attach the bag to the outside of the spacecraft until it can be returned for proper burial
In far future and alien cultures there can be all sorts of methods for the disposal of the dead.
If the spacecraft uses a closed ecological life support system and is on a very long mission, they may be forced to recycle the body back into the system. However you'll probably find this more on a generational starship on a hundred year trip or in a space colony.
On military spacecraft, the death of crewmen will be a more common occurrence than on a NASA ship. For purposes of morale, a ship-board military funeral will be part of The Book of military regulations. Military ships cannot afford "Conquest of Space" scenarios, not with the high mortality rates common to battles. Science fiction authors are fond of such ships using their torpedo tubes to launch the dear departed to their eternal rest. In the webcomic Schlock Mercenary they use "coffinpedoes".
This was done with a lot less dignity in the movie Enemy Mine. In that movie the military was so callous and cynical about the many combat deaths that the space stations had a "funeral launcher" fed with dead soldiers on a conveyor belt, equipped with a side-magazine loaded with cheap funeral wreaths and Muzak-style funeral parlor music playing in the background. The current body in the launcher stops just long enough for the bored technicians on duty to check the deceased's religion and play the appropriate prerecorded last rites.
Somebody suggested using the spacecraft's rocket exhaust to cremate the body. Tuyu explains why this is not a good idea:
Damage control facilities are generally only found on military vessels. One room will be Damage Control Central (DCC), often near or in the engineering section. This is where the Damage Control Officer coordinates the damage control parties. Generally you want the DCC to be in the section of the ship that is hardest to damage (actually, the second hardest spot to damage. The hardest spot should be occupied by the bridge/CIC).
There may be small damage control lockers sited at strategic locations throughout the ship. Locker contents may include hull patches, emergency power cables (i.e., glorified extension cords), short range radios, testing and sensing instruments, portable emergency power generators, fuses, fire extinguishers and tools. They may also have first-aid kits.
Lockers near the reactor or drive will also include geiger counters or other radiation detection and monitoring gear. The detectors will be mounted on long telescoping rods, so one can poke the detector around a corner or near a suspicious breach without exposing oneself.
On wet-navy ships there is a special damage-control deck, which is the lowest deck with longitudinal breaks in the watertight bulkheads. This allows quick access to all parts of the ship. However, since our ships are tail-landers instead of belly-landers, in place of a damage-control deck might be one or more special ladderways running along the core of the spacecraft.
The cables, pipes, and duct work will either be exposed along the corridors, behind removable panels to protect them from clumsy crew, or accessable via manholes.
If the ship's power grid goes dead, the emergency lighting will go on. This will be red to preserve the night vision of the damage control parties. This means the cables and pipes will be labeled in black text since red lighting makes color coding ambiguous.
Christopher Weuve says that a merchant ship's primary piece of damage control equipment is a lifeboat.
The Jovian Confederation ship books have spacecraft designs that are remarkably scientifically accurate and will repay careful study. The accuracy is due to precise oversight by Marc Vezina.
Damage Control Gallery
Primitive spacecraft (like we make today) tend to use lightweight power supplies. Since the one-lung propulsion systems cannot cope with anything massive, not without savagely cutting into the payload mass. But once the state of the art advances, ships become electricity hogs. Especially if they are warships.
While plentiful power is always welcome, it does come at a cost. Besides the fact that they are aglow with lethal radiation, such plants can occasionally become — how can we put it — unstable. Which is real exciting if the plant is using fission, fusion, or antimatter reactions.
Alternatively, a ship could be inexorably heading for a crash landing and you'd just as soon not share the crash site with a reactor going all China Syndrome on you. Or with magnetic cannisters of antimatter fuel, which are much more touchy than nitroglycerin bottles and contain orders of magnitude more bang.
Remember, Jim Cambias said If it's a reactor emergency you're worried about, don't eject the crew in pods, EJECT THE REACTOR!
The point is there has to be some mechanism to quickly quench the power reaction (whatever it is), and render both the reactor and the fuel inert and safe. Or a mechanism to eject the blasted thing and get it as far away as possible.
There will be a SCRAM button to shut down the power plant and a JETTISON button to eject the power plant. Paranoid designers will also have computerized monitoring systems to watch the power plant and automatically push the appropriate button in a fraction of a second.
The term "Scram" means "the sudden shutting down of a nuclear reactor usually by rapid insertion of control rods." Urban myth alleges it came from "Safety Control Rod Axe Man" but this is incorrect.
In many cases powerful rocket engines incorporate dangerous power technologies integral to their design. Just like dangerous power generators, you will need the ability to SCRAM or eject them in emergencies. A good example is solid-core nuclear thermal rockets, which are literally nuclear reactors with the hot working fluid piped to an exhaust nozzle instead of a generator turbine.
NASA had even more worries during the NERVA project. Instead of just worrying about the crew, they also has to worry about the unfortunate inhabitants on Planet Terra who lived near the (radioactive) engine crash site.
I found two interesting reports: Nuclear Rocket Destruct System Requirements by W. H. Esselman of Westinghouse Electric Corporation (Astronuclear Laboratory) and A Destruct System For the NERVA Engine by K. N. Kreyenhagen, W. H. Thiel, and S. K. Yoder of Aerojet-General Corporation. You can find them in this report along with more scary reading. I'll try to give you an executive summary.
For NASA's purposes, there are actually two separate types of engine jettison: pre-operational ("anti-criticality") and post-operational ("disposal"). Or "before you power-up the reactor" and "after you power-up the reactor". Pre-op happens when the chemical booster lofting the nuclear spacecraft into orbit fails mid-flight. Post-op happens when the nuclear spacecraft has been delivered into orbit, is flying around under nuclear power, and suddenly starts to crash on Terra.
You see, a brand-new nuclear reactor that has never been powered-up is actually not very radioactive. After you power-up the little monster it creates all sorts of hideously radioactive radioisotopes in the fuel rods, and neutron-activates nearby structural members exposed to the neutron flux.
What's the difference? Well, for pre-op jettison you eject the engine and use explosive shaped charges to coarsely chop it into sub-critical bits. Bits that will not undergo nuclear fission even if they land in the ocean (water is a great nuclear moderator). A relatively chunky 0.205 grams of U235 per square centimeter (750 grams within a 27 inch diameter circle). The idea is that uranium is relatively harmless, you just want to prevent the blasted stuff from gathering in a critical mass and undergoing nuclear fission. Even if the fuel elements dissolve into goo and start flowing around.
Post-op is different. Now the engine is full of dangerous radioisotopes. To have less than quote "acceptable" unquote levels of contamination, you have to use explosives to finely pulverize the reactor into itty-bitty fragments that will ablate down to less than 25 microns (9.84×10-4 inches) in size by the time they fall down to the 30 kilometer altitude level. The report figures the bits will have to start out at 1 mm in diameter to ablade enough. The report helpfully defines "acceptable" as "no excessive radioactivity returns to a populated area."
They did lots of math that you can read all about in the report to analyse various reactor fragment sizes, see what size it will ablade down to, and calculate the radiation dose it emits. They assumed the nuclear engine operated for 30 minutes at 1120 megawatts. The results are in the table below. The important parts are the last two columns. They figured a dose rate of 0.018 Rads per hour (1.8×10-4 Grays per hour) was acceptable. This translates to an initial fragment size of 1/32 inch (0.79 mm or "about 1 mm").
Now, since the post-op minimum fragment size is smaller than the pre-op fragment size, one would assume that you could use the same post-op explosives system for either type of engine destruct. But you'd be wrong. You see, pre-op the nuclear engine is perched on top of the chemical booster, a gigantic thin-walled tank jam-packed with chemical fuel. NASA safety experts concluded that you want to use the smallest explosive system possible because detonating the chemical booster will make everything worse. And the post-op explosives system is much larger that the pre-op, it will detonate the chemical booster for certain. Bottom line is you'll need two separate explosive systems, one for pre-op and one for post-op.
Shaped charged explosive systems were selected for the design because they had the lowest mass of all the reactor disassembly systems. (It might be worth while to review the difference between a shaped charge and a self-forging projectile, they are similar enough to be confused together, but are quite different in end result. The report tends to use the two terms interchangeably.)
PRE-OP DESTRUCT CONCEPTS
Concept 1 consists of a girdling array of linear shaped charges. When detonated, they cut through the crunchy outside reactor casing and neutron reflector layers, to get at the chewy core in the center. The shock pulverizes the core, and the sliced and diced reactor casing allows the core to disperse. Simple and reliable.
However, the girdle cannot withstand the radiation or the intense heat of normal engine operation. Before you fire up the reactor you have to somehow dismount or discard the girdle, or it will unexpectedly blow up the engine.
Concept 2. My apologies, the image is almost worthless. I think the original was in color. It is supposed to show a conical shaped charge inside the nozzle. Upon detonation it sends a self-forging hypervelocity jet of metal upward through the throat of the nozzle, scoring a direct hit on the bottom of the reactor core. This shatters the core, and hopefully also ruptures the reactor casing so the fuel rods can escape. While this concept is lighter than Concept 1, it is unclear if the shock will be enough to rupture the casing.
Obviously the pilot had better eject the conical shaped charge before firing up the engine or they will get a very rude surprise. The pilot will find the hot exhaust ignites the shaped charge and
shoots them in the a... destroys the engine.
Concept 3 is merging Concept 1 and Concept 2. You reduce the power (and mass) of C1's girdle so it is just strong enough to rupture the casing. C2's up-the-nozzle shot only has to take care of the core. The researchers actually tested this using a steel rocket casing, magnesium bars to simulate reflector segments, and a Titan nozzle. It flew into pieces like a champ. They used 57 kilograms of C-4 plastic explosive for the girdling charge with a cross-sectional area of 29 square centimeters. This actually proved to be over-kill, they wanted to try even smaller charges.
POST-OP DESTRUCT CONCEPTS
This is a challenge. You have to take a 1,360 kilogram core of fuel-enriched graphite and pulverize it into 1 mm particles.
Since the core is surrounded by pyrolitic graphite tiles, support tiles, a lateral support system, graphite reflector barrel, steel barrel, shim rods, coolant channels, tie bolts, beryllium reflectors, control rods, and the aluminum pressure hull, designers have focused on somehow introducing an explosive charge into the core and detonating it in the center. Otherwise the explosive force has to waste energy cutting through all the crap surrouding the core. This is known as the "central burster" concept.
Obviously the explosive charge cannot be resident inside the core during normal operation, for the same reason you do not store crates of dynamite inside a furnace. You have to somehow quickly get the explosive charge into the core and trigger it.
Concept 4 has the explosive charges inside a series of long projectiles. This are stored in launcher tubes above the engine, behind the radiation shadow shield. The latter is because radiation is bad for the explosives. Upon command, the projectiles are launched, penetrate the shield, enter the core, then blow up. They require an impact velocity of 300 meters per second.
The guns or launchers have to be lightweight, reliable, capable of delivering the projectiles simultaneously and capable of detonating the projectiles simultaneously. This is going to cost you lots of mass, "lightweight" is a relative term. If the explosive charges are 14 kg apiece and there are four projectiles, the total mass will be a whopping 680 kg, not counting the control and power source circuitry.
Concept 5 has a series of shaped charges with self-forging warheads that are attached to vertical bars. These are stored above the radiation shadow shield. Upon command, the bars are slowely lowered so they surround the core. Sort of a three dimensional circular firing squad. When detonated they fire hypervelocity jets of molten metal through the stuff surrouding the core and shred the core.
The advantage over Concept 4 is much lower system mass, it is trival to deliver them simultaneously and it is relatively easy to trigger the charges to go off simultaneously. For the same 680 kg system mass, Concept 5 can utilize a hundred or more shaped charges.
Concept 6 uses slabs of plastic explosive instead of racks of shaped charges. The idea is for the explosion to implode the core, crushing it.
RADIATION DAMAGE TO EXPLOSIVES
Nothing really enjoys radiation, and explosives are touchier than most. You do not want the radiation from the engine degrading the explosive's punch nor do you want them to detonate prematurely. A NERVA engine typically produces a dose rate of 105 rad/sec at the side and 103 to 104 rad/sec in the shadow of the radiation shield. So over an operating time of 1,200 seconds the total dose will be from 106 to 108 rads.
Premature detonation happens when the radiation flux heats the explosive by gamma absorption and inelastic scattering. Typically explosives blow up when they reach a temperature of 150 to 200°C. They may not actually explode, but it is almost as bad if the stuff undergoes decompostion or deflagrates. They will not be able to perform their duty. Some coolant may be required.
Explosive degradation happens as radiation breaks chemical bonds in the explosive's molecules. This gradually turns the plastic explosive into just plain plastic. This seems to happen at about 107 to 108 rad which means the radiation shadow shield might provide enough protection. There is some suggestive evidence that cooling helps slow degradation, which is a good thing. Coolant weighs less than radiation shielding.
Self-destruct is a mechanism (protocol or device) that can cause an object to destroy itself on command. The object can be totally blown into smithereens or merely render the object useless if captured by the enemy (the latter is called scuttling). It is rather common in media science fiction since it is so dramatic. That agonizing count-down really ratchets up the tension.
Reasons for including such a device on a spacecraft, space station, or planetary base include:
Most real-world boosters and spacecraft include self destructs to prevent lawsuits and massive negative publicity if the rocket goes off course. Manned rockets generally have some sort of launch escape system to propel the habitat module clear of the blast radius (with the notable exception of the Space Shuttle).
The range safety officer with their finger on the big red button are usually located at some distance from the object they are blowing up. So they will have some objectivity (i.e., not hesitate because they are scared of committing suicide).
If civilian owned spacecraft have propulsion systems frightful enough to be weapons of mass destruction then by law all such ships will be equipped with destruct devices controlled by the Launch Guard. Just in case a tramp freighter with an antimatter engine has a drunk pilot and starts heading towards a major metropolitan area.
Military ships do not have self-destructs for range safety reasons, but they might have them for scuttling purposes. Or because the civilian goverment does not trust the space navy.
The data banks are a treasure trove of valuable information: space navy secret code books, battle plans, task force compositions, etc. If the enemy gets their hands on any of that, the results could be more damaging than losing a battle. All data stores will need some kind of explosive charge or whatever to render the data unreadable. With the charges capable of being detonated on remote command from the CIC or manually by the crew stationed nearby. In the old wet navy the code books had covers made out of lead, to help speed them to Davy Jone's Locker when the captain throws them overboard. That won't work in space.
Building space warships takes such an inconveniently long time. If the enemy captures one of your warships intact they will gleefully replace the crew, hastily paint on their national insignia, and thus instantly have a new (slightly used) unit in their space navy. To prevent that you want to scuttle your ship. You don't have to atomize it, just damage it enough so that its major contribution to the enemy's war effort is as a load of scrap metal.
This is a specialized form of scuttling a captured warship's data banks, where the emphasis is on destroying any star charts you have on board.
If you are super paranoid you might have to destroy the entire ship with crew. It is surprising how much aliens can learn about your home planet by examining seemingly innocent details of the ship. For instance, they can learn clues to your homeworld star's spectral class by analyzing the frequencies emitted by the ship's lamps and track lighting. And the crew can be tortured for information, especially the astrogators (to get them to cough up your homeworld's coordinates) .