If Fission Solid is a nuclear steam boiler, Fission Gas is a nuclear furnace. To improve performance, higher temperatures are needed, but this was limited by the physical properties of the engine's components. So, thinking outside of the box, designers skipped the meltdown and created an engine where the uranium has turned to vapor. This allows the engine designs to have operating temperatures up to ten times that of solid core engine...
There are two major configurations: open cycle and closed cycle.
Open cycle has the uranium gas inside a reaction chamber directly above a nozzle. The uranium will escape, along with the propellant. A major design challenge is reducing this outcome to a minimum.
|A spherical-chamber design for an open-cycle nuclear rocket.|
|A design for an overly optimistic Mars mission.|
Most fission gas designs have uranium operating at impressive temperatures of 55000K or more. No physical material can hold this vapor, and the heavy uranium atoms are hard to hold in using magnetic fields, so these designs have to rely on gas flow. The hot uranium is kept within a sheath of cold propellant. The flow of the propellant exerts a force on the uranium vapor within, and if moved in the right directions, it can greatly limit the leakage of uranium through the nozzle.
Understandably, trying to contain a dense gas at temperatures much hotter than the surface of our sun using something as tenuous as gas flow leads to multiple failure modes.
The first and most destructive is a change in the momentum of all gasses in the reaction chamber. This can be caused by acceleration of the spaceship or an external impact. Gas is compressible, so it will shift around inside the engine. The uranium inside the engine is very dense, so it will shift disproportionately more compared to the propellant containing it. If it moves too much, it will pierce the rest of the gasses and strike the inner wall of the reaction chamber.
As you would expect from a ball of ultra-hot radioactive gas, it will turn the walls to slag, splash through and coat the engine's internals in molten metal embedded with uranium particles. Packed closely enough, they'd remain critical and glow red hot for years.
Acceleration of the entire engine creates buoyancy. The hot uranium will gradually 'fall' within the much less dense propellant and coolant gasses. Eventually, it will just be ejected in its entirety through the nozzle. A big, dumb ball of radioactive death shining in the ultraviolet, just dumped into space.
Spacecraft designed to handle milligee accelerations would have to account for this. If they hit the edge of an atmosphere and suffer drag, the buoyancy effects could make them lose all of their hot uranium. The same goes for military spacecraft that have to perform emergency maneuvers.
The handling of gas flows creates an opening for a second failure mode. Like any machine with moving parts, it only takes a small failure to make the entire process grind to a halt. Except if its an open-cycle gas core engine, where 'grinding to a halt' is replaced by a 55000K ball of uranium plasma being set loose inside your spaceship.
If the propellant flow is reduced, the uranium will expand and increase the heat load on the engine walls. The forces that keep the uranium together are also reduced, increasing leakage through the nozzle. If it is cut off entirely, and the pressure on the uranium removed entirely, it'll explode. It probably won't produce enough pressure to blow up anything, but it will ruin the engine.
If there propellant flow against the uranium ball is too fast, the boundary between the gasses will drag along uranium particles and increase the losses. In the vortex designs, it might spin up the uranium too quickly and particles are ejected by centrifugal forces.
The velocity of the gasses involved guarantees turbulent flow. In some cases, this can create shockwaves that compress, isolate or pinch off uranium from the hot core and move to the walls or out of the nozzle.
All of these problems are exacerbated during the startup and the shutdown of the engine, since the balance between gas flows is intentionally unstable and changing. They also come with the failure modes you'd expect from thermal stress, pressure vessels, uranium and hydrogen embrittlement and so on, mostly analogous to those of the solid core nuclear rocket.
If all this sounds too much, you might want to try a nuclear lightbulb.
The closed-cycle nuclear gas-core rocket tries to completely contain the hot uranium within physical vessels, thereby preventing nuclear material from leaking out of the nozzle.
To do this, uranium is allowed to reach a temperature of about 25000K. There, it emits its energy as ultraviolet rays.
Fused quartz has a high temperature tolerance and is very transparent to ultraviolet light. Place the uranium at 25000K inside quartz vessels, and nearly all of its energy with shine through the quartz and into the propellant contained inside the reaction chamber.
This design has the major advantage that the hot uranium isn't going anywhere... but reintroduces the disadvantage of physical components directly in contact with hot stuff.
Since the quartz is not 100% transparent, it will absorb a fraction of the uranium's energy and heat up. To counter this, it is built like a double-glazing window. The gap between the two panels run through with transparent coolant.
So how does this precarious contraption fail?
Well, let's start small. The coolant flow through the quartz vessels is insufficiently cold. The fluid heats up and puts pressure on the quartz. The quartz is necessarily thin to increase transparency, so it might fissure and burst open. This releases hot uranium into the engine and ruins it.
What if the uranium itself gets too hot... or too cold? Its emissions can shift along the electromagnetic spectrum into something other than UV. X-rays, for example, would be fully absorbed by the quartz and melt it in seconds.
The uranium has the distributed equally within the quartz vessels. This is especially important during reactor startup. If the uranium gas being injected accumulates somewhere or is compressed during its injected, it might create hot spots or sharp increases in pressure. These would damage or crack open the quartz vessels.
The entire assembly is especially vulnerable to external influences. The quartz vessels are nothing more than special glass, so a sudden acceleration or impact can break them. The propellant flow exerts pressure, and creates vibrations, and if the engineers controlling the reactor do not compensate, the resonance can shatter the quartz vessels.
The quartz is in direct contact with radioactive material, hot hydrogen and coolants. This means it has a short lifetime. If left unattended, and combination of thermal stress and embrittlement can mean an unexpected failure.
During shutdown, here is an additional danger of the uranium solidifying on the walls. This creates a coating that, during the next startup, would absorb nearly all of the UV rays. Its temperature increases and is conducted onto the quartz, which promptly melts. This is worrisome when the quartz vessels are expected to handle pressures of 500-2000atm.
So yes, the closed-cycle gas core designs do explode.
|Uranium Tetrabromide apparently dissolves pretty well in water.|
The nuclear salt-water rocket design is either ridiculous or brilliant.
Imagine a continuous nuclear detonation inside your spaceship. If things run smoothly, you'll dump enough water onto the nuclear explosion to smother it. If you mess up, the explosion travels upstream....
The number of failure modes for such an engine are innumerable. The water propellant has to ejected by jet turbines to maintain sufficient flow, and if they falter, your reaction chamber vaporizes. The nuclear saltwater has to be directed within this flow so that it has a sudden increase in concentration just outside of the nozzle, thereby achieving criticality.
These requirements are complicated by the fact that trying to move a liquid within another liquid is rather difficult, and even more so when you're trying to push several swimming pools of it out of the nozzle every second.
Turbulent flow is deadly. Pockets of saltwater will detonate. If the water freezes at the edge of the reaction chamber, it will reduce your neutron moderation capability, as does the formation of bubbles.
Changes in temperature are deadly. If the water cools too much, the uranium will precipitate out of it and detonate. If it is too hot, you won't be able to achieve the necessary concentration at the right place to ignite the engine.
The entire fuel tank is essentially a nuclear bomb. If the fuel lines clog up with salts, the increase in pressure and concentration can detonate the uranium. This creates a pressure wave through the fuel tank, with the edge producing criticality within the salts. The same goes with a leak, as the nuclear saltwater accumulating just about anywhere creates explosions.
The nozzle is a firehose of radioactive death.
The neutrons emitted by the NSWR are also dangerous to itself, as in addition to the slow neutron embrittlement, it can activate uranium that hasn't reached the designated detonation zone and cause it to go critical prematurely. It also has a tendency to create heavy water that messes up the engineer's density and flow calculations.
Considering the power levels of NSWR design proposals, a single hiccup in the thrust can rip the spaceship apart. As water is generally incompressible, pressure variations are transmitted efficiently to the reaction chamber walls, and can resonate to deadly effect.
In the end, the NSWR can be seen as the first nuclear reactor in the 1940s. A dangerous technology that can go wrong in a million ways, but holds the promise of amazing performance. We've managed to handle nuclear reactors after a few decades of research and several mishaps along the way. Maybe, one day, we'll master the NSWR equally well. If not, it'll be relegated to the infamy of Lithium fluoride rockets and Superdeep drilling as being too dangerous to be practical.
By comparison to the nuclear saltwater rocket, riding on a chain on nuclear detonations sounds relatively tame.
The oldest and best-developed nuclear pulse propulsion system is the Orion.
|NASA Orion design.|
|Casaba Howitzer design.|
Testing of the pusher-plate design has revealed that it is surprisingly durable. Unless the redundant suspension mechanisms are damage, it can handle pretty impressive forces and continue to work.
The nuclear pulse units, however, are a liability.
If they detonate too close to the plate, they can vaporize a big chunk off it. If they detonate and an angle, they can create asymmetric forces that stress the suspension mechanisms and flip the spaceship.
Having thousands of nuclear bombs in a spaceship is a nightmare for many people. Anywhere it crashes will become a diplomatic incident. Units that fail to detonate can crash and smash open, becoming dirty bombs, or be recovered by the wrong people.
If the pulse units are manufactured defectively, such as with insufficient 'channel filler', they could bathe the spaceship in penetrating radiation.
The pulse units also produce fallout. In an atmosphere, this would drift on air currents and affect places far from the launch site. In the upper atmosphere, it would produce EMP that would knock out electronics. In space, it is mostly harmless, but a spacecraft performing a retro-burn will have to fly through a cloud of radioactive debris. The outer surfaces would become contaminated and might prevent spacewalks.
Worldbuilding tips: Following a retro-burn insertion into orbit, an Orion spaceship might become a prime target for a hijack. The robots used to clean up the exterior surfaces can be jammed or shot off, leaving the crew trapped inside. Another spacecraft can then approach, cut a hole to the fuel tanks and start stealing pulse units to be re-purposed as nuclear warheads...
This technique can be extended for use with fusion pulse units, with even greater performance and less radioactive products.
The advantage is that misfired nuclear pulse units are no longer a liability. Each unit can also be much smaller, and the acceleration smoother The disadvantage is that you now have to rely on a bank of capacitors for every single detonation. Those capacitors have to be recharged by an onboard nuclear reactor, with a host of new problems.
The MM-Orion just trades a political problem for a mechanical problem.
Pulsed fusion propulsion usually uses a variant of inertial confinement. This is when the pellet of fusion fuels are dropped into the reaction chamber, and it is detonated by use of lasers or electromagnetic effects. These ignition mechanisms heat up the pellet's external surface so quickly that it explodes. The pressures and temperatures generated compress the interior of the pellet until it undergoes fusion.
The fusion products are them captured by magnetic fields. Propellant can be injected alongside the pellets to increase thrust.
This type of engine is very complex, involving the simultaneous use of several systems that have to be synchronized on atomic timescales.
The lasers, for example, have to all aim at the exact center of a tiny ball of frozen fusion fuels. Any deviation, such as, jitter from the previous detonation, can cause the pellet to fly off to the side and fail to detonate.
The lasers also need a clean path. Using propellant to increase your thrust creates an obstruction that reduces the energy delivered to the pellet. If too much propellant is in the chamber, it might absorb the lasers and heat up. Hot propellant would change the frozen pellet's trajectory and render the engine un-usable until it has cleared. This problem is exacerbated by the fact that propellant, until it has been blasted by a fusion detonation, is not ionized and thus uncontrollable by magnetic of electric fields.
In a zeta-pinch device, if ionized propellant from a previous detonation remains, it might conduct electricity.
The pellets themselves are rather delicate. Some can be naked fusion fuel, other are contained within hohlraums to better direct the ignition energy. In any case, they are susceptible to contamination, damage, neutron activation and other space travel nasties while being held in the fuel tanks. It is very likely that the fusion fuel will be prepared in batches specifically for use in a single burn. They'd have a short shelf-life, and be thrown out or recycled if they sit in a 'ready state' for too long.
Another possibility is antimatter-catalyzed pulsed fusion.
|The ICAN-II proposal for fusion spacecraft|
Instead, it adds the risks and dangers of storing size-able quantities of antimatter in addition to the usual fusion fuels.
Also, a mis-firing of the ignition mechanism doesn't just waste a fusion pellet, but inserts a stream of antimatter into the reaction chamber. If it is not evacuated quickly enough by magnetic fields, it will prematurely ignite the next pellet or hit the reactor walls.
|The antimatter containment failed?|
Well, that's all for this rather speculative list. Hopefully, you'll look up dramatic plot-points of your story and think about whether an imminent explosion is really necessary to create tension....