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NASA Funds Designs for a Nuclear Thermal Propulsion Rocket (space.com)
"Dangerous radiation. Overstuffed pantries. Cabin fever. NASA could sidestep many of the impediments to a Mars mission if they could just get there faster," writes Space.com, which reports NASA is now exploring an alternative to chemical rockets.
In August, NASA announced an $18.8-million-dollar contract with nuclear company BWXT to design fuel and a reactor suitable for nuclear thermal propulsion (NTP), a rocket technology that could jumpstart a new era of space exploration. "The strengths with NTP are the ability to do the very fast round trip [to Mars], the ability to abort even if you're 2 to 3 months into the missions, the overall architectural robustness, and also the growth potential to even more advanced systems," Michael Houts, principal investigator for the NTP project at NASA's Marshall Space Flight Center, told Space.com. NTP rockets would pull all that off by offering about twice the bang for the buck that chemical rockets do... "Nuclear thermal propulsion can enable you to get to Mars faster, on the order of twice as fast," said Vishal Patel, a researcher involved in subcontract work for BWXT at the Ultra Safe Nuclear Corp. in Los Alamos, New Mexico. "We're looking at nice 3- to 4-month transit times."
You heat water or hydrogen (probably hydrogen,since it gives you a higher Isp at lower temps). And squirt it out the back...
"I do not agree with what you say, but I will defend to the death your right to say it"
Read up on Projext Orion and on Aircraft Nuclear Propulsion (both closed-loop and open-loop designs).
Extrapolate.
Profit.
For a split moment, I thought I’d just read: “Dangerous radiation. Overstuffed panties. Cabin fever.”
The point of the "nuclear" is you heat up the exhaust hotter than you can get by just burning fuel and oxidizer. Exhaust velocity (the Isp part of the rocket equation) goes like the square root of energy released per unit of fuel burned. That sets a point of diminishing returns on chemical rockets. You can get go faster if inject more energy into the exhaust, but not if that energy comes from more fuel you have to carry with you and burn. Nuclear gets you orders of magnitude more energy density that you can dump into the exhaust with effectively no penalty other than the weight of the reactor, which grows much more slowly than an equivalent weight of chemical fuel would. That said, ain't no one building anything to go into space for under 30 million. This is more like research. I'd be surprised if any metal gets cut for anything other than individual component bench testing. TRL 1 type stuff.
More like crib from NERVA, and per a friend of mine who's father was one of these, in the 1980s someone had the bright idea of gathering the surviving team members and getting an infodump from them. Click on the Project Timberwind link, that was likely how it happened, the timing is right.
As in Robert Heinlein's book The Rolling Stones, copyright 1952
TRL 1 type stuff.
Well, the Rover and Pee Wee projects built and tested nuclear rocket engines, so it's already beyond Technology Readiness Level (TRL) 1. Right now nuclear thermal rockets are TRL 4: Module and/or subsystem validation in laboratory environment; standalone prototype implementations.
The trick was to get them to TRL 5 and beyond.
Project Pluto as well.
That was a crazy idea - cruise missiles that could stay airborne for months spewing radiation along the countryside.
Hydrogen has the advantage that it's available anywhere you can find frozen methane. Xenon can be a bit harder to scavenge. What I'd really like is a "high" power ion rocket that could use rocks for exhaust. This, though, is a big problem because rocks aren't a simple element, but a complex mix that varies. (By "high power" I'm thinking of about 30 pounds thrust, but that's probably dreaming.)
I think we've pushed this "anyone can grow up to be president" thing too far.
You can look these things up, Chris.
https://en.wikipedia.org/wiki/...
Material limits set what we can do with conventional rockets. Not just melting points but thermal shock and fatigue.
No. Chemical rockets are limited by the energy content of the chemical fuel. They haven't been limited by materials for well over fifty years.
Those material limits are the same for a nuclear power source - and shoving water through a barely sub-critical reactor to heat it seems like a laughable idea. Water is hellish corrosive at high temperatures so odds are you'd be leaving a trail of reactor guts behind you before the engine had been running long.
Nobody proposes using water as reaction mass in a nuclear thermal rocket-- Specific impulse (Isp) is not high enough; you might as well use chemical propellants.
Hydrogen isn't a lot better. See "hydrogen embrittlement"
Hydrogen is a lot better. It is pretty much what everybody (or at least, everybody who knows the technology) would use for a NTR.
Since nuclear engines were designed, built and tested with hydrogen reaction mass back in the 1960s and early 1970s, your belief that they couldn't work is quaint.
A nuclear powered ion-drive seems a lot more likely to work.
Yes...and no. Ion drives put out a very low thrust per unit power. Thermal rockets are high thrust. There are some applications where you can get there slowly but efficiently, but it's definitely an engineering trade-off.
"A nuclear powered ion-drive seems a lot more likely to work." Indeed. Hydrogen is sort of stupid, but that is where they started decades back, and it is perhaps best if they continue along those lines and develop it properly.
Yes, nuclear thermal is very simple, and it has been demonstrated.
Then load up with Xenon and get serious. Get the Reactor nice and toasty, both heat and ionize the Xenon with it, and then work with what Plasma Physicists have been doing, in accelerating the product and shooting it out the back end.
Really you want to do one or the other, not both. Either use the thermal energy, in which case you want hydrogen, or convert the thermal energy to electrical power and use an ion engine, but not both at once.
(Accelerators are notoriously thirsty, but that is another thing Reactors are good at providing.) 135Xe is a particularly good candidate, as it has a very high Thermal Neutron Cross Section, and can carry some of those pesky Neutrons away with them.
That makes little sense. If you're using the reaction mass for neutron shielding, basically you want the lowest atomic mass you can get. And a fuel that decays with a half life of 9 hours means you'd have to breed the fuel in situ.
Look up NERVA, NASA BUILT and TESTED Nuclear Thermal Reactors in the 60's and 70's, they were almost flight ready when the program was cancelled
We had this tech in the 60s. No Nukes in Space treaty killed that. An engine isn't a weapon.
No, it didn't. The Nuclear test ban treaty banned nuclear explosions in space. It didn't ban nuclear reactors, and in fact several have been flown (primarily in the old Soviet Union's RORSAT program, but one-- SNAP-10A—by the US.)
Nuclear thermal is particularly interesting for Venus ascent stages. It lets you do them single stage, and while you essentially have to use hydrogen, it doesn't take that much. It reduces the habitat lift requirements dramatically; while the dry mass is high, the vastly reduced propellant requirements outweigh that many times over. It aslo makes it plausible to launch to high elliptical orbits rather than LVO; this cuts the Dv requirements down on the interplanetary transfer stage significantly, meaning either faster transfers or larger payloads. Some NTR designs offer hover capability, which would enable (effectively) limitless, propellentless hover time during docking without needing a balloon stage. And lastly, since a Venus ascent stage would never operate anywhere even near Earth, NIMBY concerns would be greatly reduced.
The most exciting phrase to hear in science, the one that heralds new discoveries, is not âEureka!â(TM), but
Unfortunately (orrather fortunately) we'll almost certainly be sliding backwards on a a TRL perspective. There have been a lot of major improvements in NTR design since then - not just for higher peak ISPs, but in particular to deal with the poor T/W of previous designs. The first big improvement was the LOX afterburner concept, wherein you burn the hot hydrogen with LOX early in the flight fie greatly augmented thrust, then revert to pure H2. Since then a lot of designs have also called for bringing atmospheric air into the mix, ranging from simple ram air thrust augmentation all the way up to designs with nuclear thermal-driven compressors and NTR scramjets.
Its hard to say at what point the added complexity ceases to pay off, but at the very least, the afterburner offers a huge leap forward at little cost. And of course you want modern ISPs too. To a point ; some of the more exotic reactor designs can theoretically provide crazy ISPs, but they do so by keeping the hydrogen hotter than the rest of the reactor with tricks like fission fragment reactors, which are anything but mature. Then again, if the craft could also double as a pure fission fragment rocket as well, that would certainly be pretty keen...
The most exciting phrase to hear in science, the one that heralds new discoveries, is not âEureka!â(TM), but
That's all there is to it. The rector is very hot, you take hydrogen from your tank, run it through the reactor where it boils and heats up to however hot you can run your reactor (much hotter than hydrogen and oxygen burns), and let it flow through a nozzle. You'll need a pump to push the hydrogen into the reactor, but that will just be a turbopump running off some of the hot gas. You can then use the turbopump exhaust to keep the pressure up in your propellant tank.
Prediction for end of Universe #42: Fencepost error in Quantum_bogosort.cpp
Almost correct. The temperatures are not actually hotter than a chemical rocket, but you can use pure hydrogen as fuel. Since hydrogen molecules are lighter than typcal exhaust gasses (water, CO2 etc), at the same temperature they are moving faster. That means you need less mass for the same velocity change in the rocket, or you can go faster on the same fuel.
The best chemical fuels are around 4500 M/s exhaust velocity. Storable chemicals are more like 3000 M/s. Nuclear thermal rockets get to around 10,000M/s So in principal you can go 2X as fast with the same fuel to mass ratio.
There are lots of caveats. The reactor is heavy. The radiation shielding is heavy - these both mean that you need a very large spacecraft before you have a net win in performance.
You probably don't want to turn one of these on before you are in orbit due to the potential problems with an accident (and the thrust to weight is pretty small anyway).
An additional problem is that its difficult to store hydrogen for long periods of time - you would need a complex and heavy refrigeration system. Or you can just use the nuclear rocket for leaving earth, and conventional storable chemicals for arrival.
Its a reasonable idea but with a lot of engineering tradeoffs that need to be considered. Its .... rocket science.
Until you start up the reactor, you just have uranium fuel (half life about 1 billion years), not lots of nasty highly radioactive stuff. If it fails on launch and the fuel is not contained you only have chemical heavy metal toxicity to worry about. I expect they can do a good job of containing the fuel in any case.
Once the reactor starts up, you are safely in orbit. The biggest danger would be on return from Mars to Earth orbit. You'd certainly want to design these things not to ever attempt reentry. It would take a lot going wrong to cause accidental reentry.
I'd want there to be a high quality risk assessment, but I think it wouldn't be hard to reduce the risk of atmospheric contamination to very low levels.
Quattuor res in hoc mundo sanctae sunt: libri, liberi, libertas et liberalitas.
If you are going with a high specific impulse and also greater-than-micro thrust propulsion system, you will need some kind of thermodynamic cycle to generate the required electric power, and that cycle will need to reject heat. Furthermore, the heat rejection for the cold side of that cycle into vacuum involves Stefan-Boltzmann T^4 limited radiators -- the "radiator" in your aging apartment building benefits from convection of air that is not on option in space.
Even a photovoltaic cell is subject to the Carnot limit on efficiency. The solar cell has the advantage that the hot side is surface-of-the-Sun hot in terms of the radiation spectrum of the impinging light whereas you have large surface area of the panels to radiate from the cold side. However clumsy and bulky solar panels are, you will need something almost as clumsy and bulky for radiators for a nuclear energy cycle to generate electricity venturing farther out from the Sun.
Is Discovery a nuclear-electric craft? In the 2001 A Space Odyssey genre of science fiction, you still get to wave your hands a lot even though it was meant to portray a plausible near-term future rather than warp drives and Star Trek transporters. Early concepts of Discovery had large space radiators making it dragonfly-like in appearance, but that wasn't "cool" so it ended up with this thin spine with the habitat at one end and presumably the nuclear power plant way at the other end. I never did figure out what those "pods" or "bunkers" were along the spine -- too small for cryogenic propellant storage, too small for proper Stefan-Boltzman fourth-power-of-surface-temperature radiators.
There are crazy concepts for more effective space radiators involving spraying water or pellets to get enormous surface area and then somehow recapturing the water or solid pellets so you don't end up losing them. Discovery didn't seem to depict that system.
And then there is nuclear thermal, but those are much lower specific impulse, not that much better than chemical rockets, especially when you consider the bulk of liquid hydrogen tanks and the weight of the nuclear reactor. Your "radiator" (Carnot-cycle cold side) is to blast H2 molecules out your rocket nozzles, a lot of H2 molecules. We have come full circle from the NERVA project of the 60's to VASIMIR or whatever kind of much higher impulse nuclear or solar-electric propulsion back to nuclear thermal, again?
While we're at it, why don't you work on ways to eradicate the biggest source of dangerous radiation in our solar system - the sun.
Uneducated environmentalists and like-minded Hollywood script writers have got you convinced that zero radiation is the natural state of things, and any radiation is aberrant. It's actually the other way around - radiation is everywhere. Even your own body is radioactive. The only reason the sun's radiation doesn't kill everything on Earth is because of its magnetic field, which directs most of the solar radiation into the polar regions (where it collides with air molecules and ionizes them to create the aurora, instead of ionizing your DNA and causing cancer and genetic defects). It's actually one of the biggest problems that need to be overcome for a manned Mars mission. Any additional radiation due to nuclear rockets will be negligible.
you're overestimating how much mass we can move to Mars by conventional rockets. We can't send the ISS to Mars, our biggest rocket can send 8 tons to Mars transfer orbit. We're going to send up 57 of those to push the ISS to Mars? thousands of them for your 100 year colony's supplies.
No, we need powered fusion rockets.
Project Timberwind was a far more advanced system design than NERVA (although, unlike NERVA, they never built a prototype). The thrust-to-weight ratio of the NERVA engine was 1:1, for Timberwind it was 30:1. The notion that Timberwind is derived from NERVA does not stand up to the slightest bit of scrutiny. The designs are entirely different (other than, you know, both using a nuclear reactor and hydrogen propellant). Any new effort in this direction is likely to use Timberwind as a reference design for a jump-off point.
Starships were meant to fly, Hands up and touch the sky - Nicky Minaj
Even if you don't discard the LOX tanks and do it as a SSTO, the mass fraction is still greatly improved versus a pure hydrogen NTR. And even in the portion of the flight where you're burning LOX with the hot H2, it's significantly higher performance than a regular hydrolox engine, because the hydrogen has already taken on a lot of energy; if I recall the numbers correctly, designs predict somewhere around 550 sec sea level.
Adding an afterburner doesn't increase the total system mass much, but greatly increases thrust for early in the flight when you really need it. And in many ways, you're facing a much easier task than a regular hydrolox engine. Your hydrogen is gaseous and has enough energy to vaporize the LOX, so you're dealing with gas phase combustion, as well as self ignition.
It's interesting thinking about how far you can take nuclear thermal designs. Picture, for example, the afterburner case, with a fission fragment reactor as the heater. You can transition all the way from super high thrust for liftoff and atmospheric flight, to moderate thrust / high ISP for attaining orbit and performing orbital maneuvers, all the way to ISPs in the hundreds of thousands via direct fission fragment propulsion (note: requires large radiators). A single system could provide you access to every flight mode needed for missions ranging from the surface Earth out to the Oort Cloud and even potentially beyond - as well as effectively unlimited onboard power. And you can refill it anywhere you can get water; given the very high temperatures capable in the core, the primary loop should readily function for thermolysis (it happens on its own at those temperatures), with hydrogen-selective membranes leading to the hydrogen tank and a chiller for liquefaction (needed regardless to deal with boiloff).
The most exciting phrase to hear in science, the one that heralds new discoveries, is not âEureka!â(TM), but