Personally I am excited to see this, but I wonder if the optics will kill it?
People get upset enough about reactors that don’t move and live inside huge structures of reinforced concrete. Can we convince this segment of the population that launching a small device (with necessarily less shielding) is safe?
Even the most reliable launch vehicles (Falcon 9, Atlas V) are probably not more than ~99% likely to succeed. Can the payload be made safe in the event that it fails to make orbit?
NASA has already used multiple radioisotope thermoelectric generators in space probes without many people getting upset. An RTG recently went to Mars. So this new engine doesn't seem like much of a stretch from a political and public relations standpoint.
This reply deserves more upvotes; NASA routinely launches big hunks of plutonium, and a nuclear thermal reactor at launch (before it's been turned on) is far less radioactive.
The key point that needs to be related to the public is that the reactor is turned off while in Earth's atmosphere, and a reactor poses very little danger when it's turned off.
> Even the most reliable launch vehicles (Falcon 9, Atlas V) are probably not more than ~99% likely to succeed.
Falcon 9 is it at this point. All of the remaining stock of Atlas Vs and Ariane 5s have been sold.
Of course Vulcan and Ariane 6 will come online soon; buy they don't have anywhere close to the safety record that Falcon 9 does.
I'm not a statistician, but I wouldn't be surprised if Falcon 9 had better than 99% chance of success. It has more than a 150 long success streak. And no rockets in the current configuration have ever failed.
> Can we convince this segment of the population that launching a small device (with necessarily less shielding) is safe?
This isn't obviously no to me. NASA has launched a bunch of RTGs in the past. Those have the same sort of safety concerns in the event of a failed launch. Probably less material, though.
If p is the probability of success, then probability of 1500 successes in 150 independent trials is p^150. With p = 0.99, the chance you see 150 successes in a row is 0.22. If we take significance level to be the standard 0.05, then we cannot reject the hypothesis that the chance of success of Falcon 9 is less than 99%. We can, however, reject the hypothesis that it's less than 98%: if it was 98%, then in 150 trials, probability of all successes is 0.048, which is smaller than 0.05.
Most of them have been, but Perseverance and Curiosity were exceptions. I believe the original motivation was for Curiosity to be able to land at higher latitudes.
There's been a clear shift in nuclear power through the last decade. I've seen people that previously protested for the closure of plants turn their tune. Really all this is because we've learned that the fact that it produces zero carbon during operation and has a lot of other environmental advantages (like a low footprint), that many are considering the benefits vs the costs. Previously people only cared about the costs but now we see the picture is more complicated. So not only do I have hope for this project, but I also have hope for nuclear power in general. (remember, the argument isn't "nuclear vs renewables" it is "nuclear + renewables vs fossil fuels" vs "renewables vs fossil fuels". Anyone making another argument is not coming from the scientific community).
Also, an interesting aspect of nuclear engines is that astronauts will likely receive lower dosages of radiation as opposed to using traditional engine designs. This is because even with increased radiation from the engine (which can be shielded, but that's more weight) the reduced travel time means that the astronauts are exposed to lower amounts of solar (directional) and cosmic (omnidirectional) radiation. The NASA article mentions reduced weight of supplies but shielding also matters and is quite heavy.
It doesn't matter at all: they'd only be lifting unused nuclear fuel, which isn't a hazardous material. This isn't comparable to the 238Pu radioisotope generators; it's much safer.
NASA understands this well and if they feel they need to (considering the current political climate, it's likely) then they will likely start a years-long PR campaign soon to get ahead of it.
I'm sure someone's crunching the numbers on this and other articles shared across social media.
Representative democracies need to stop caring about what the most outrageous media-baiting people in a society can say about safe practices and just do it.
They can’t, they’re representative democracies, caring about the vocal minority (or being perceived to) is the key function needed for incumbents to stay alive.
If the flight plan is layed out intelligently, I suppose less people would experience radiation than in the cold war days with the many above-ground detonations.
Nice, at least this doesn't seem like one of those horrible 60s designs that spewed radiation everywhere by basically detonating a small nuclear bomb against a pusher plate.
I think nuclear propulsion is there only way forward for interplanetary colonisation because we've long reached the limits of chemical. And ion is too slow.
NERVA in the 1960s operated under a similar principal to the one just announced. Orion (what you're describing) was not the only proposal in the 1960s.
As for nuclear propulsion being the only way forward, I doubt that, I suspect that chemical will be more cost efficient for a lot of the trips, especially for unmanned cargo trips.
So I've been working on a sci-fi book that involves interplanetary governance, and one of the things I've realized is that if one wanted to have a civilization that worked on an interplanetary scale nuclear is probably the only way to go for meaningful power production.
Chemical means, based on carbon products that we've used up till now, would be very rare on remote planets, solar wouldn't work because planets could be various distances from a star they orbit, or could have different atmospheric, or magnetospheric conditions that would make that impractical, wind suffers a similar issue.
The only power source that could be guaranteed to work at the large scale, both in terms of space and time, reliably, and is likely to be available is nuclear power. With your other options being the quantum vacuum energy or anti-matter, but both fall more into the fiction part of sci-fi right now than the science part.
it was just a fun interesting little thought exercise. I'd love to hear if anyone has a criticism of the reading though.
Naively I'd expect that you would need fusion specifically. Uranium isn't plentiful unless you're on a rocky planet, and isn't very high energy density compared to hydrogen.
Hydrogen is everywhere by comparison.
The other reasonable option might be solar power near stars, and forwarding the power on to your ships/distant planets using some form of directed energy (lasers, lenses, etc).
Isaac Arthur on YouTube has some interesting commentary on antimatter, and other advanced proposed propulsion systems. It seems that for the moment, at least, antimatter is strictly in the realm of science fiction, until some very difficult problems are solved.
Probably easier to generate chemical fuel on the moon/mars too then to refine nuclear fuel, although if you could carry enough nuclear fuel in the first place maybe that would be moot.
Nuclear rockets are a bit funny: they have a separate fuel (something that produces heat, like U-235, Thorium, etc) and propellant (something that gets heated and pushed out the back, like Hydrogen, Nitrogen, etc) .
I think the idea is to carry a lifetime supply of nuclear fuel. Nuclear rockets aren't necessarily super particular about what propellant they use in conjunction with that fuel, so you can probably easily either directly scoop or otherwise easily refine/distill usable propellants anywhere you can find gas or something that is liquefiable.
It sounds like you’re referring to nuclear pulse propulsion, a proposed method of reaching Mars from the Earth’s surface in 2 weeks.[1]
I’m not sure what’s “horrible” about it. Research was halted because of the Test Ban Treaty, but if a safe way could be found to get the nuclear material into orbit, why not try it?
2 weeks is far better than 45 days which is what Nasa estimates nuclear thermal propulsion would achieve.
Zero gravity and space radiation are deadly to humans, and the more exposure we can cut, the better.
In the 60s we were much too cavalier about nuclear pollution. Barfing out nuclear isotopes everywhere is just not feasible. The whole spacecraft would get polluted with it.
The same attitude also gave us the Russian RORSATs, many nuclear reactor cores are still in a parking orbit and will come down at some point in the future.
I don't think the launch issue is the main problem: after all the nuclear thermal system also requires nuclear materials in orbit.
Apparently a nuclear drive could half the transit time to Mars for astronauts, which has the additional benefit of less hard radiation exposure (assuming the nuclear reactor doesn't have any issues in transit).
There were projects to develop nuclear thermal designs at the Nevada Test Site (Area 25, 'Jackass Flats') run by Los Alamos, 1955-1973. They had three designs (Kiwi, Phoebee, Pewee), all using highly-enriched uranium (bomb grade, as with nuclear engine reactors). I think these have longer lifetimes and are more efficient, but the new NASA report makes a point of using low-enriched designs this time around.
I can't imagine it would be plausible to launch something like that without first testing it at a DOE/NNSA facility of some kind however, which might be a little problematic, as blowing up a nuclear reactor during a rocket test would be bad optics.
Given the increased speeds, how does arrival at the destination work? I assume the rocket is rotated 180° and fired. Is the plume the rocket flies through radioactive?
There are some other options like aerobraking in the thin Martian atmosphere, and the gravity of Mars helps capture the vehicle at the other end, so not that much braking thrust is needed, and it's going to be fired at some complex angle:
These types of rockets ideally shouldn't produce much radiation, as the working fluid (propellant) is just heated up by passage through the reactor, although I don't know if there's a primary coolant loop and a heat exchange or if it's just basically the primary coolant being blown out to space, in which case maybe there'd be some 3H-tritium formed? Probably not much of a concern, though it might be for a Earth-based test firing.
Definitely a common sense step in the right direction. Humans going anywhere beyond the moon using chemical propulsion seems quite problematic due to all the unsolved issues related to radiation and micro-gravity.
Now talking speculative fiction, the real breakthrough will come if we figure out a way to induce acceleration without an action-reaction process. Just an energy source, and no propellant. Being able to sustain 1g for a few months on a heavy spacecraft means interstellar travel (proxima centaury) would be within grasp.
Meanwhile, having engines that have higher specific impulse helps quite a bit as well. That is to say: a nuclear engine needs less propellant to provide the same amount of delta V (actual work). Since the majority of a rocket's mass is in the propellant, this scales non-linearly.
A nuclear rocket is a big improvement over chemical rockets already.
It would take 6 years to get there (earth time) or 3.6 years (spacecraft time) if you could have constant 1G acceleration (in the opposite direction for the second half of the journey).
With a spacecraft big enough to live in, we are talking of a time frame not too different from the one from the age of exploration by sea. I have no doubt many people would want to take such a trip even if it takes 10 years to get back to earth.
Unmanned spacecrafts would be able to go much faster, sustaining more dramatic accelerations. So getting cargo and robots there for support would be way faster.
Sadly, not very feasible because of the hydrogen atoms floating around in the empty space, about 1 per cubic meter.
At speeds about 0.5C a collision with such an atom produces X-rays, and harder gamma rays at higher speeds. They are pretty hard to insulate against, and are actively harmful. For a spaceship of a considerable size, enough collisions would occur to be dangerous. The paper: https://www.scirp.org/journal/PaperInformation.aspx?paperID=...
To make it even worse, that's based on an average density 1.8 atoms/c3. The real values would probably vary wildly, and you won't be able to "break" in advance in order to go through a high density area.
And then there's the problem of hitting an interstellar grain of sand at 0.5c.
Only 0.95 per the source linked above. I don't know.
Speculating you'll substantially blue shift the light coming from in front of you, and I suppose that can't be good for materials (or people), but I'm not sure if there is enough to matter. Any dust you collide with is also going to have ridiculous amounts of energy, but you'll be in interstellar space when you're at high speeds so there shouldn't be much of it (even for space) either.
The rocket equation says that the fuel mass of a rocketship is higher than the cargo mass by exp(delta_v/v_exhaust).
When the final velocity is relativistic, delta_v should be replaced with delta_rapidity. In our case this would introduce a factor of 2.65, but the results are so ridiculous that we can ignore that.
So, let's simply say that delta_v is the speed of light, or 300000 km/s.
The exhaust velocity for a nuclear thermal rocket is about 9 km/s.
The ratio between delta_v and the exhaust velocity is about 33000. The exponential of that is roughly speaking 1 followed by 15000 zeros.
There are less than 10^100 atoms in the known universe.
So, even if you want to accelerate just one single atom to 99% of the speed of light, you would need more fuel than the entire universe. Many, many, many times more.
Yeah well, we don't really know what happens at high speeds. Probably biochemistry stops working at 0.3c? It doesn't seem structural materials would remain solid at 0.95c... who knows.
I am not a physicist but I suspect that we are already moving at significant speed relative to other bodies. We have no special frame of reference that defines our "real" speed.
> You are absolutely right in that the CMB becomes Doppler shifted when you have some relative velocity. But Earth is not in the CMB rest frame: when we observe the CMB from Earth, we observe a dipole component to the CMB caused by the Earth's motion orbiting the Sun, the Sun's orbit around the Milky Way, and any velocity the Milky Way as a whole has. I was at a talk about the new Planck results last week, and saw a plot in which you can clearly see the dipole component in the raw data. You need to correct for this motion before you can even remotely see the anisotropies that are the interesting science goals of Planck.
> There is a rest frame in which the CMB is closest to isotropic (no dipole component), and this rest frame is special but not 'absolute'. This frame is effectively the 'center-of-momentum' frame of the observable universe, in which we expect the total momentum to be zero. We know from classical mechanics that for any system of objects, we can construct such a frame, and that it sometimes has useful properties for solving certain types of problems. But there is nothing 'absolute' about this rest frame, the laws of physics operate entirely the same.
> And so this is fine, because ultimately what relativity requires is that the laws of physics operate the same in every rest frame, not that every rest frame looks the same. Because the CMB is itself physical (made of photons) and was emitted by matter, it is entirely natural that it should be affected by frame transformations, and should look different if you shift to a frame that is moving differently than the emitting medium.
The frequency of light changes depending on how fast you are moving relative to it. Move away from a light source and the light still approaches you from the same speed, but is lower frequency. Rest here is where the frequency becomes "uniform" (ish) regardless of direction.
> NASA and DOE are working another commercial design effort to advance higher temperature fission fuels and reactor designs as part of a nuclear thermal propulsion engine. These design efforts are still under development to support a longer-range goal for increased engine performance and will not be used for the DRACO engine.
Anyone have info on how they’re improving on NERVA?
I think the differences are less dramatic than that. You're conflating the nuclear fuel internal temperature (inl.gov link) with hydrogen exhaust temperature (NERVA / 2,300 K). There should be a sizable temperature gradient between those two.
Exactly. The temperature limit for the exhaust (and therefore the specific impulse) is set by the materials used to make the engine. You can run a reactor as hot as you want, but it will want to vaporize itself and the spacecraft around it.
Basically a controlled thermo nuclear reaction blasting out the back.
"One design would generate 13 meganewtons of thrust at 66 km/s exhaust velocity (or 6,730 seconds ISP compared to ~4.5 km/s (450 s ISP) exhaust velocity for the best chemical rockets of today)."
You certainly wouldn't want to use this to take off from Earth. But we could use it for deep space travel.
Practically all the nuclear propulsion designs are ideally launched from space. You want to get the "fallout" as far from the Allen Belts as you can from what I remember reading of General Atomics's Project Orion.
Since nuclear rockets are far more practical for all major interplanetary travel, that's why a moon base or captured asteroid habitat will be the first real step to a "space civ".
Of course I'm nutso enough to think that SpaceX should launch antimatter collection arrays in orbit to grab it from the solar wind, right now.
Getting to Mars otherwise is really just a big marketing exercise.
It now kinda seems unrealistic that most sci fi interstellar empires have lots of planet based settlements (well, the ones that have to deal with gravity). Gravity wells are a huge PITA once a reasonably closed-loop space civ gets moving. A nice asteroid belt seems a lot more valuable or a planet with a crapton of low-gravity moons, than a planet with 1G gravity well you have to spend millions to escape.
On the moon, you could probably setup maglev launch or assisted launch with just solar panels.
Asteroids may make up the vast majority of available system mass. Kind of hard to mine the interior of planets, so that mass is irrelevant. The asteroids are bits of broken up planet internals, right there for the taking?
Use for heavy lifting? Depends on efficiency. If it 'burned' its fuel completely, then maybe not as dirty as imagined.
Further the oceans already contain dissolved uranium salts. Returning them to the ocean is 'net neutral' in a sense. Especially if we harvested them from there to begin with!
The dirty part is the fission products, not the unspent uranium. You wouldn't want to launch a nuclear salt water rocket from the surface even if it achieved 100% fuel burnup.
Ion thrusters have a much higher specific impulse, but nuclear thermal engines are high thrust and have about twice the specific impulse of chemical rockets.
There's a concept called 'Bimodal Nuclear Thermal Propulsion' where you use the reactor for nuclear thermal propulsion when you need high thrust, and as a low-thrust, high specific impulse nuclear electric rocket the rest of the time.
People get upset enough about reactors that don’t move and live inside huge structures of reinforced concrete. Can we convince this segment of the population that launching a small device (with necessarily less shielding) is safe?
Even the most reliable launch vehicles (Falcon 9, Atlas V) are probably not more than ~99% likely to succeed. Can the payload be made safe in the event that it fails to make orbit?