You could use a stirring fan. (In my second mention of Apollo 13 in this thread, it was a short in the wiring of a fan used to stir liquid oxygen that caused the explosion.)
This. Look at a picture of a nuclear plant, specifically at the cone things with steam coming out. It’s very, very important that the steam keeps coming out that end, therefore it’s very, very important that water keeps coming in to the other end. Where’s the water going to come from?
The first thing that came to mind (after the obvious “lack of a heat sink” problem) was the gravity issue alluded to by others.
This was the downfall of my silly “can you roll a sub” question I asked one of my instructors in nuclear power school. He didn’t even dignify my question with a response.
The problem is that every last bit of the power plant expects heavy stuff to go to the bottom. There are “head tanks” that are high, intended to provide sufficient head (pressure) to pumps lower down in the engine room. There are lube oil tanks that are expected to catch the drippings from the reduction gears and so forth. There are vapor separators and other oddities like the deaerating feed tank (removes air from feed water). Modern pressurized water reactors have a pressurizer vessel at a point higher than the coolant loop, with a steam bubble at the top.
Besides all that, the instant the condensate touches the turbine blades everything would fail spectacularly.
While I am not going to pretend to be an expert in space-based nuclear fission power, I have worked on studies and a proposal proposal for a solar orbiting communications and positioning satellite constellation in which the use of nuclear fission reactors was considered and ultimately recommended, and we consulted with people who were recognized experts in space-based nuclear thermal power and propulsion, so I have some cursory knowledge in this area.
First, I’ll affirm the statements of previous posters (including a couple of Navy nucs) that the nuclear power plants used in attack and ballistic missile submarines are in no way suitable for space applications. Aside from the issues which have already been addressed (thermodynamics, designed to operate in an Earth gravity field, the mass and size of the systems), these reactors are designed to use highly enriched uranium (HEU, >90% [SUP]235[/SUP]U) and they cannot be refueled in situ; they are designed to operate for a lifetime of ~10 years after which the entire core is removed and replaced during a planned refit cycle. There is no practical way that a conventional pressurized water reactor could be assembled in orbit without some kind of massive orbiting facility, and all extant and proposed reactors are assembled prior to launch, which means they are limited in size to a payload that could be launched by existing or proposed launch vehicles.
Given that a space-based fission reactor operating in freefall could not rely on any gravity-based natural convection cycles or operating/safety features, the trade space ends up either going to a high temperature gas phase system, an actively pumped liquid salt, and/or solid state thermoelectric conversion. The gas phase (likely helium, although neon or argon could be used as the working fluid) can be used in a direct Stirling cycle heat engine, while liquid salt will require a separate heat transfer to a working medium, and solid state thermoelectric is limited by the essential physics of statistical mechanics. The boiling and pressurized water reactors so favored in Navy reactors, and in generally in all terrestrial reactors, albeit more for convenience and safety (in marine environments) than maximum performance, pretty much fall out of the study by default given the mass of the fluid and difficulties in handling steam in freefall. Conventional systems fielded by the United States (under the SNAP-10 program) and the former Soviet Union (TOPAZ) used direct thermoelectric conversion with a liquid phase NaK coolant, and had poor energy conversion efficiency.
As other posters have alluded to and Chronos addressed specifically, the system will have waste heat to be rejected exclusively by radiation, which requires outward facing surfaces (and not facing the Sun). This limits the power output of a reactor by its efficiency and the mass required for rejecting waste heat. A space-based reactor is going to necessarily be focused on thermal efficiency at the expense of power output, while reactors used in submarines are optimized for high specific power output even if they produce a lot of waste heat (which, as noted, is readily carried away by the liquid medium around them).
In the specific application we looked at for communications systems, the original intention was to use solar power since the satellites would be orbiting at about 0.85 AU. However, the initial study demonstrated that the mass of a solar array and deployment systems necessary to provide sufficient power requirements would be vastly beyond the launch capability of the Delta IV Heavy SLV, we looked to nuclear fission systems to meet power requirements to support outer planets exploration mission support capability. However, the resulting options were only marginally better and had a limited operational lifetime owing to fuel depletion/contamination with a massive increase in costs that even in the most optimistic case bumped up against the budget limitations. I think if we redid the study today with advances in flexible solar arrays and inflatable membranes we’d probably go to solar electric power generation as the default.
It is very easy to gloss over the complexities of nuclear fission power production if you don’t know much about the details, but anyone who has worked in or around the field even in the terrestrial power production field understands that there are a lot of nuances that are not evident in simplistic diagrams and concepts, and moving into space presents novel hazards and issues that are not resolved without fundamental design changes.
Stranger
Space based nuclear performs so poorly that one concept I have seen is just using solar power to run your VASIMR engines. You have few of the drawbacks of solar power on earth - no need for batteries, as once you leave earth orbit, you are going to have perpetual light on your panels. They can be very, very thin and with conversion efficiency around 40% (you use the most expensive multi-junction panels for this) you get about 400 watts per square meter of panel. The surface area taken up is irrelevant, and your thrust from your engine is so tiny that very little structural bracing is needed.
Out in Mars orbit your insolation drops to 590 watts per square meter.
I think I read that the magic 1 kg/kW (enough to reach Mars in 30 days?) is technically achievable with solar - basically a thin wafer of silicon that is 2.5 square meters would only weigh 1 kilogram, and the whole array would be suspended by cables or something.
So this was intended to be sort of TDRSS + GPS for the rest of the Solar system?
Is there any public info on the project, the RFP, etc.? Timeframe? Did it just die undone or did some later project implement some parts of the original mission?
Bottom line: this is pretty interesting and I’d like to learn more abut anything up this general alley. Any info or Google-fodder would be appreciated.
As always, thanks for an informative post in an esoteric area.
Well, he did say that it was a proposal proposal. So if the proposal proposal proved to be unfeasible, they never moved on to the actual proposal (which really bummed out those who worked on the preliminary proposal proposal proposal.)
It’s proposals all the way down.
As somebody I knew back on college used to say: “It’s time to prepare to begin to commence to proceed to start to schedule a meeting to discuss plans to maybe talk about possibly potentially doing something.”
Or something like that; it’s been a long time since I said the whole recitation.
“Or we can just spend some of my vast fortune and go blow up a rocket” - Elon Musk’s method.
This is obviously a big reason why SpaceX can actually get a rocket built that they are then going to blow up - all that reduced bureaucratic overhead. Of course, the reason it blows up so often is probably from some of those skipped meetings, but that’s the tradeoff.
In a nutshell, yes. The unsolicited proposal was named the Planetary Telemetry and Positioning System (PlaTePoS) and was conducted at the behest of The Planetary Society (TPS), presumably to be submitted to NASA Jet Propulsion Laboratory as a proposal to support future planetary exploration capability with greater bandwidth and capability than the current terrestrial Deep Space Network as well as provide solar orbiting observatories for surveying potentially hazardous objects (PHOs) which could pose impact risk to Earth. I have not seen it published publicly anywhere and given some of the drama within the proposal team I suspect TPS has probably disowned it.
As far as I know it never went anywhere; with the dearth of interplanetary missions on the current manifest would not justify it and CNEOS has found other ways to search for PHOs (including the Arecibo Observatory in Puerto Rico which the NSF has been trying to defund even though it costs a pittance to operate and its data still produces leading work in radio astronomy despite being over fifty years old). There were numerous technical challenges but nothing that was unsolvable, and with modern membrane deployables and cheaper heavy lift capability (one of the largest cost line items was buying Titan IV or Delta IV-H launches) the costs could be reduced substantially, but it would still be a multibillion dollar effort to deploy even the minimum constellation of three satellites. It is not in any part of the 2013-2022 Decadal Survey and while the concept is occasionally discussed at conferences there is no expectation that it will be recommended in the next DS so I doubt it will happen in the next twenty-odd years, or indeed, until there is some pressing demand for it justified by regular interplanetary exploration missions.
Stranger
The other issue I see with anything resembling nuclear power - besides the gravity/convection, lack of heat sink, weight of reactor and coolant, nuclear accident on launch/de-orbit issues, radiation danger to astronauts, etc. etc. - is simple basic maintenance. A nuclear steam generation system is a Rube Goldberg contraption compared to some solar panels and a battery. You have the reactor core, mechanical movement to regulate the core temperature, coolant pumping, steam regulation and pressure containment, a turbine, cooling module using pumped coolant again, etc. etc. Way too many moving parts, too many points of failure.
If this thing is sitting in space, a cracked pipe or micrometeor could allow your entire coolant to escape; or a pump failure or rod control failure could leave you also (very) high and dry, assuming the safety factors worked. How do you repair things? You would now need a welding robot, or some guy in a bulky vacuum suit to do repairs; or you encase the whole thing in a pressure container so you can work on it is shirtsleeves?
The trouble is, in space, any repair is non-trivial, even the trivial ones. Out in a vacuum instead of indoors, doubly so.
Meanwhile, you have no power. One of the truly necessary things in space is power.
The only real solution I see that would work is ganging multiple small modules, so you have an array of say, 4 or 5 of these devices minimum. Any particular component is small enough to be disconnected from teh whole and returned to earth(!!!) for repairs
Realistically, you’d want a space-based fission reactor to be as simple in design as possible, autoregulating, and require no maintenance throughout its operational lifespan. This is why RTGs are used for interplanetary space probes to outer planets and for the Mars Science Laboratory; with a completely solid state design and the ability to operate for years (in the case of the Voyager probes, now decades) without servicing, they’re ideal for space probe applications. That they are of relatively poor thermal efficiency is actually a benefit; since they only produce a modest level of power, the waste heat is small enough to be rejected by completely passive systems. However, RTGs are very expensive because the material used to power them (for American systems the plutonium isotope [SUP]238[/SUP]Pu) is very expensive to produce and dangerous to handle before it is encapsulated, and they don’t produce enough power for high power applications, nor can they be used for propulsion beyond low power Hall effect or electrostatic thrusters.
For terrestrial applications, boiling water and pressurized water reactors have become the default, in part because water is cheap and accessible (and most commercial power generating reactors are adjacent to a river or large body of water which is used as a cold temperature reservoir) but largely because this is what has been used for nearly all US naval vessel nuclear power production. Admiral Hyman Rickover insisted on using pressurized water reactors rather than liquid metal (sodium an NaK) cooled reactors after experience with SSN-575 Seawolf demonstrated the maintenance problems and safety issues using a coolant that would react violently with water. Pressure-driven NaK or high temperature gas, using only moderately enriched uranium or even a thorium or mixed oxide fissile fuel are probably the best approaches.
It is not practical nor safe to plan for a return to Earth reactor maintenance or servicing. The potential hazard during launch is unavoidable but at least it can be mitigated through mission assurance and trajectory selection to minimize potential for loss of vehicle and dispersion of fissile material in inhabited regions. The potential for loss upon reentry, however, is more significant and likely, and just not necessary. And frankly, most applications where nuclear power would be necessary are interplanetary missions to outer planets that are never going to return to Earth. At Earth orbit, the abundant amount of solar radiation (1.3 to 1.4 kW/m[SUP]2[/SUP]) makes it unnecessary to rely on nuclear power unless there is some reason not to deploy a large solar array.
Stranger
You see the advantage of solar, then. You have only 60% power at the orbit of Mars - which is still plenty to maneuver. PV Solar arrays have exactly the kind of redundancy you are talking about and no moving parts.
You need nuclear if you want to travel beyond Mars orbit. Proposed missions like a mission to the Oort Cloud would require insane dV, so you need some kind of lightweight nuclear power source and a heat engine.
The Advanced Sterling Radioisotope Generator is something like what you’d need. That’s what a spacecraft nuclear plant would probably actually look like. The heat engine would be the same, just larger, but the power source would be something like a cylinder of highly enriched U-235 with a control rod going through it. It would launch cold. A single control rod would retract after launch and the fission reaction would start.
It would probably be designed to run at continuous high power for the entire reactor lifespan, with the control rod slowly retracted as reactivity decreases over time.
Basically, the least parts possible. Hydrogen or helium gas would be the coolant. There would be minimal radiation shielding as this would not be a manned mission - instead, radiation hardened electronics would be used and the reactor core would probably use a reserve propellant tank or something as shielding. At the end of the mission, you’d start burning propellant from that tank and the electronics would begin to fail from neutron exposure, but some of the instruments would probably still work to give you one final bit of data on your last bit of fuel.
It doesn’t make sense to send humans out beyond Mars orbit and we will probably not do this for a very, very long time.
Too true… from a maintenance point of view, it would probably be simplest to launch a replacement and drop a faulty reactor into the sun, rather than in the Pacific by accident on the downward trip.
The spaceship in 2001 was supposedly designed to be excessively long to so the reactor was far away from the crew compartment, rather than having a lot of shielding. Presumably, like most Hollywood, it was more artsy than practical.
Yes, something with zero moving parts makes the most sense, anyway.
Don’t forget, for radiant cooling, you have to pump that coolant out to the fins; and unlike cooling fins on earth, where air flows through them so you can space them closely, in space the only way to get the heat to the fins is pumping coolant or have some really really good heat conductors. So now your fragile, hard to replace coolant, is pumping through hundreds of yards of exposed piping.
Drop into the sun? You do realize that is not possible. You’re not totally wrong though. A nuclear-fission spacecraft, you would launch and place it into a high orbit where the reactor is cold. The fuel at that point with be inert and in some way protected so that there’s not going to be a criticality event if the rocket blows up.
Once the reactor + spacecraft is in a high orbit, one stable for centuries, only then do you order whatever packaging devices to uncouple, freeing up the control rod and allowing the reactor to operate. It would then provide power to send the spacecraft on a slow, nuclear-electric powered journey to the outer planets or beyond.
The thing about radiant cooling is that it uses EM (IR) to shed heat. Fins are an inefficient way to spend IR, because one fin just throws part of its heat onto the next fin and versa-vice. The most efficient way to shed heat is a spherical body, because it comports with the normal spread of EM without any crossing interference.
And what happens if the core fails? A terrestrial core melts, burns through the floor, burn through the ground, hits the water table and generates a steam explosion. In zero-G, the core fails, melts down, and then what? You have a big blob of hot fissionable material floating in space. The heat causes the core to expand until it stops being critical. Then you have a hot mass of actinides, lanthanides and low-level contaminated metals floating in a soft ball. Where is it going to go? Make the containment vessel large enough (space is full of space), and any meltdown will just be a localized hot mess.
You can’t drop an object into the sun from Earth orbit, as the object is also in solar orbit and will just keep orbiting the sun even if you manage to get it out of earth’s orbit. It actually requires a lot of energy to move an object from first from earth orbit, then from solar orbit, into a trajectory that goes into the sun. Firing things off into the sun is just not a practical means of getting rid of objects.
This. Earth orbits the sun at about 66,000 MPH. Disregarding the energy requirement to escape earth’s gravity, you then need to shed most of that 66,000 MPH to get something to drop straight into the sun (or at least close enough to it that its own corona/atmosphere will cause enough drag to slow it down the rest of the way).
Yes, to drop something into the sun, you need to negate the orbital motion of the earth, plus add the additional energy to reach earth escape velocity. The question is whether the cost of doing so is worth it. As others mention, alternatives are designating a “dead reactor orbit” zone, or alternatively, I would suggest designating a particular point on the moon as the elephant graveyard of nuclear reactors, just crash everything onto there. The sun is the most effective way to ensure there is no future problem. An orbit has to be high enough that decay is not a problem; by the time you loft something past geosynchronous and then circularize it, you may as well aim for the moon. The problem with orbits is that when there’s too much stuff up there in one zone, collisions become a risk; and a collision at high speed would then create random debris going every direction…
Why would you even have a nuclear reactor in Earth orbit, anyways? Solar is lighter and simpler. The reason you need nuclear is for missions that are going out to Jupiter and beyond - so just fire up the reactor when the spacecraft is already on an escape trajectory. Problem solved - the solar orbits are so unfathomably vastly larger than earth orbit that you’ll never encounter the spacecraft again, even if the reactor fails right at startup, unless you’re intending a rendezvous.
Manned missions could be done with NERVA but it’s not really worth it. Jupiter is a deadly cyclotron of radiation belts, and I’m not sure what living astronauts can accomplish out there in terms of meaningful objectives. Maybe 10 years ago it would have seemed like a good idea - it would have seemed like you could never program a rover to be smart enough to handle any situation without human help. Turns out, that’s incorrect and we in fact can do something almost that good. (machine learning solutions still have edge cases, but they would be very rare, and the rover could pause and wait for a software update if this happens)