But concretely, what is the reaction mass made of and what is the most reasonable mechanism for propelling that mass with the energy of the nuclear reaction?
Normally liquid hydrogen. Just pass the hydrogen past the reactor. Cooling the reactor and heating the hydrogen. The expansion due to heat send the hydrogen out of the nozzle at high speed, creating thrust.
More efficient than a chemical hydrogen and oxygen rocket due to the high energy to weight ratio you get from nuclear fuel.
Good design would either reuse the reactor for other purposes, or ensure it runs out of fuel at roughly the same time as the rocket runs out of reaction mass.
If it's out of fuel then it can no longer generate any heat. The position of the control rods wouldn't matter, and it couldn't melt down. If you're referring to reaction mass (the hydrogen gas), then yes, you would need to insert the control rods to stop the reaction. Or land the reactor and use it to power your Mars colony.
If you do the latter, then you will have had to build it with another cooling mechanism in mind. That could increase the mass of the reactor and reduce your payload capacity, so you might not do it. Instead, you might include just enough uranium to make the trip, and no more.
How much more efficient is it? The fact that you have to carry propellant with you anyway seems like a big factor in favour of traditional rockets, where the fuel becomes the propellant. And burning the hydrogen way hotter seems like a big engineering problem - rocket motors already run at temperatures which pose materials science problems, increase that a few thousand degrees and you might as well say we should run the spaceship on fusion power.
I don't doubt that the physics works, and it's important to note that say a 2x gain in thrust-to-mass ratio would lead to enormous advances in trips out of Earth's gravitational well (10x? 100x?) due to the tyranny of the rocket equation. But I'm curious whether this is the 2x gain of replacing oxygen+hydrogen with hydrogen, plus a big complex reactor, or the millionfold gain of replacing hydrogen fuels with uranium.
Edit: TFA says twice the specific impulse, and presumably that's the optimistic estimate. But still very good!
NTR is not anywhere near 2x chemical because of parasitic losses, in fact it may only barely offer higher net DeltaV.
NTR requires heavy engines, heavy shielding and heavy radiators to keep cool. The final NERVA prototype was as close to a functional NTR as ever built, and it massed 40,000 lbs while only generated 55,000 lbs of thrust, with a maximum ISP of 710 seconds.
A SpaceX Raptor only has a Mac ISP of 380 seconds, but masses only 3,000 lbs, and produces 500,000 lbs of thrust. Add another 50,000 lbs dead mass to the NTR for shielding and cooling, and you see why Raptor will get humans to Mars well before any NTR and just as quickly.
TWR is only important when you have to climb up the gravity well. Once you're up in orbit, high ISP is king to get you anywhere. Other comments already highlighted that NTRs are a middle ground between superefficient, ultra-high ISP electric drives, and very high TWR chemical rockets. So that's where this sits: high enough TWR to be practical, but way better ISP than chemical (though still far from electrical).
NTR is not competitive with chemical rockets in any actual application. Dry mass has to triple at a minimum, your propellant evaporates over long trips, and NTR engines don’t have enough thrust to land on Mars or even the moon, requiring the additional dry mass and complications of specialized landers.
We need a huge step forward in NTR before it’s going to be useful at all.
This sounds like a matter of scale. A nuclear spaceship (a theoretical one, like all of them) with twice the thrust wouldn't be carrying twice the dead weight.
So at some level of scale this outperforms a traditional rocket, and your oddly impassioned argument about why SpaceX is so much better is just relevant to particular use cases rather than spaceship design in general.
Scaling up low thrust to weight engines with large cooling and shielding requirements doesn’t create many mass efficiencies. Maybe in the shielding, but that’s the least of your concerns.
The other problem is that low thrust to weight means an NTR spade ship can’t land on Mars, or on any body with a significant gravity well. So you need to bring chemical rocket landers, increasing your mass duplication and tech complications.
A multipurpose chemical rocket powered space ship Luke Star Ship is far more practical and nearly as fast.
Most likely the reaction mass is hydrogen. The NERVA project [1] used it, and the mechanism was quite straightforward: heat the hydrogen to 2250 deg Celsius and let it go. The vacuum specific impulse obtained by NERVA was 841 s (the Space Shuttle main engine had a specific impulse of 453 s)
This is essentially the what you are doing. Like if you sat on a chair and aimed a fire extinguisher you'd go flying (cold gas thruster, typically used in RCS). You'd go faster if you super heated the gas before it exited the nozzle.
This article didn't explain it at all, so you're right to ask.
tl;dr: Gaseous propellant (I'm guessing hydrogen) is heated with fission then pointed in the opposite direction of intended travel.
The uranium in this design is not a propellant, but a heat source. Aside: you can use photons/heat as a propellant, but it's thrust is very low https://en.wikipedia.org/wiki/Pioneer_anomaly. Ideal propellants typically have a high exit velocity and low mass. That gives you the longest amount of "burn" time, and the greatest amount of control for the weight. https://en.wikipedia.org/wiki/Specific_impulse
Back in the day when the US was building more of these nuclear rockets, the propellant of choice was typically hydrogen https://en.wikipedia.org/wiki/NERVA. Old timey video explaining it https://youtu.be/eDNX65d-FBY?t=238. I'm assuming this proposed design would also use hydrogen, but I couldn't find any sources on the propellant for their design.
Liquid hydrogen served to keep the reactor cool as it transitioned from liquid to gas as that phase change absorbs energy. The gas is the directed through the reactor core where the gas heats up. As gases heat up, they absorb energy, their average particle velocities increase.
Eventually, the hydrogen molecules (mostly H2 or H-H gaseous hydrogen), makes it to the nozzle and is ejected. The high-velocity hydrogen is what actually provides the bulk of the thrust to the spacecraft.
Compare this to Project Orion (https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...) which intended to detonate nuclear warheads and the craft essentially rode the shock wave into the stars. I would classify this method of propulsion, not safe.
The primary failsafe mode for an NTR would be to insert control rods to stop fission. Without the hydrogen the core wouldn't be able to cool itself and would melt down. There are however NTR core designs with closed circuit cooling. The core would be kept at a low critical state (hot but not melting) and circulate a coolant through the core and into a generator and from there to radiator panels. When the NTR wasn't providing thrust it would provide electrical power. When thrust is needed the coolant loop would cut off and hydrogen would be pumped through the core. Provided no mechanical breakdown in the coolant/generator loop an NTR could provide power for years.
I guess one could still build single burn/single use reactors. The thing would be a bit lighter than one that can survive multiple burns & it should not pose a Hazzard as long as you plan the resulting orbit of the discarded reactor accordingly.
For a Hohmann transfer orbit you need at least two burns, the perigee burn to put you into the elliptical transfer orbit and the apogee burn to circularize that orbit at your destination. Even free return trajectories can require a secondary burn. So in many situations throwing your engines away is not a great idea.
An NTR can be designed such that the engine and spacecraft "chassis" are reusable over multiple missions. NASA has/has an NTR concept with such a reusable vehicle. The fuel tanks are disposable and slot into the central frame like AA batteries. The crew portion would be a TransHab-like habitation module with a docked crew capsule and Mars lander. Propellant tanks would be disposed of during the mission and the vehicle parked in Earth orbit between missions. For a new mission propellant tanks would be fitted along with a new crew and off it goes. It's an interesting design but a little passed the current bleeding edge of in-orbit construction.
Uhh space itself is about 2.7K. Seems like cooling shouldn’t be a problem. There’s probably all sorts of ways to avoid a problem. Even just using a different element altogether.
You'd think cooling would be no problem, but it turns out that when your only option is to radiate away heat, that's _very_ slow. We get spoiled here on Earth by conduction and convection, both much easier and faster.
This is sarcastic, right? Regardless of whether nuclear is safe or not, the name still rubs me the wrong way, in the same way that "PATRIOT act" or "clean coal" does.
Starship can get to Mars in 3-4 months. It depends mainly upon how much payload you carry. Elon has discussed crewed Starships making the trip in 3-4 months to minimize radiation exposure, and cargo Starships maximizing payload mass and taking the 9 month Hohmann low energy orbit.
They claim the temperature of the active zone to be about 2000°C. This is lower than most rocket exhaust. Still they claim twice the specific impulse of a chemical rocket; this means that the reactor is very-very lightweight.
I wonder how much radiation protection does it have, and whether the exhaust would be acceptable to use for a launch from Earth surface.
One clarification: thrust-to-weight says something about an engine’s mass (by definition), but specific impulse only says something about the exhaust velocity of the propellant (i.e. the impulse per unit of mass), not the mass of the engine.
So the high Isp is due to the very light exhaust (hydrogen, not water or CO₂) which gives the higher velocity to the gas molecules at a lower temperature?
In a sense, yes. For a given temperature, hydrogen, with 1 atomic mass unit, will have a much higher velocity than water (18 amu, so √18 ≈ 4.2 times as slow) or carbon dioxide (44 amu, so √44 ≈ 6.6 times as slow).
Yes, having lighter exhaust also helps you raise your ISP. Scott Manley has a few videos on this topic, where he explains the science behind this much better than I ever could.
I believe a tank full of liquid hydrogen would be a reasonable neutron radiation shield. Generally things like water or concrete are used for neutron radiation shielding due to the large amount of light nuclei (hydrogen) in those.
On NTRs (and space reactors in general) you only need a "shadow shield" that casts a radiation shadow in the direction of thing to protect (crew, sensitive systems).
This makes the necessary shielding much less than a full sphere, reducing the mass to a mere fraction.
Thing get a bit hairy though when you need to dock with something or even when running multiple engines (the neutron balance would apparently be totally wrong).
You want to be careful to avoid heating your hydrogen fuel, it’s slowly evaporating through the tank even when near absolute zero, any significantly higher temperature is going to increase that rate.
I envision the tank being between the engine and the capsule, not wrapped around the engine or something like that. As another commenter points out, you only need a shadow shield. I don't think there would be much more heat transferred from the engine to the fuel tank than in traditional engines; the neutrons would warm up the fuel some but perhaps naively I assume that effect would be fairly small. Now that I think about it more, I'm not sure. Is it the neutrons that heat the water in a nuclear reactor?
The specific impulse does not take the engine (or reactor) mass into account, but it's (approximately, if you neglect pressure effects) proportional to the exit velocity of the gas flying out of the nozzle. The exit velocity in turn is proportional to the square root of (temperature divided by molar mass of the exhaust gas).
Thus, if you use cooler hydrogen gas rather than hotter water+hydrogen, you can still make it a lot more efficient given that water has 9 times the mass of hydrogen per molecule.
Is the nuclear engine thrust only usable from the orbit forward or could it be used in the first escape stages? I don't understand the engine's lifecycle and how it combines with other rocket stages.
Can the engine produce the peak power needed at liftoff? Or are its main benefits realized over the course of the trip instead?
No, nuclear thermal rockets could be launched from the ground. That was the plan with NERVA, which was a pretty serious attempt to develop the technology in the early seventies. A nuclear thermal rocket engine 'looks' like a rocket engine, but there's energy from fission in the combustion chamber.
NERVA was targeting thrust to weight ratios of <5, though - which isn't high enough to lift off of the ground with. Makes a fantastic upper stage engine, but a rather useless first one.
And more specfically, it was being a proposed as a replacement for the S-IVB, the third stage of the Saturn V. Early maps (like the one shown at https://www.hq.nasa.gov/office/pao/History/SP-4204/ch11-8.ht... ) show the Nuclear Assembly Building, along with three pads, at Launch Complex 39, which launched the Saturn and Space Shuttle, and now launches the Falcon rocket.
Nuclear rockets are almost exclusively proposed for use as upper stage engines or for interplanetary transfers. One major reason for this is "no one wants their launch complex to glow", which is a major downside of the currently feasible nuclear rocket engines with >>1 TWR. (Orion drive, which is throwing nukes out the back and riding the shockwave, and the Nuclear Salt Water Rocket, which is like that but continuous via a standing nuclear detonation in streams of enriched Uranium salts in water)
Once you're in space, ISP matters a lot more than TWR, and is the main limiting factor on how far you can get with a given mass fraction. Nuclear rockets generally have ISPs at least double that of the current best high-thrust engines, hydrolox. (hydrogen+liquid oxygen)
Its really a shame - I've read somewhere Orion is actually even more efficient in an atmosphere! Oh well, we can still use that somewhere else where the atmosphere is not breathable or worse (Jupiter, Titan, Venus, etc.).
A nuclear thermal rocket works just fine in atmosphere. It has a similar profile to chemical rockets, in that you can tune the reaction or the bell nozzle to work well in atmosphere or in vacuum. The biggest problem with using this as a first stage is dealing with the legal and social logistics of running a nuclear reactor that isn't yet on an escape trajectory.
A solid core NTR exhaust might even be rather clean. Liquid or gas core NTR, not so much.
And if someone is planning to launch a nuclear salt water rocket inside the atmosphere of an inhabited planet, you certainly have bigger issues to worry about.
You can deeplink into PDFs by the way: at least Firefox's integrated PDF viewer jumps directly to the page when you append #page=37 and I assume that Chrom(ium) will do the same. There are other parameters. e.g. for highlighting or searching words. You can use several parameters at once by appending the following ones similiar to parameters in an URL with &<parameter>=<value> (obviously replace <parameter> and <value> with what you want).
Based on other comments here, I did a little digging on Wikipedia, in case others with zero background (like me) are curious.
This appears to be a nuclear thermal rocket [1] which would use a nuclear reaction to directly heat propellant (e.g. hydrogen) that is ejected.
This is different from a nuclear electric rocket [2] which would produce electricity which would then generate propulsion [3] using a smaller quantity of propellant, e.g. using an ion thruster [4].
It is also different from a fission-fragment rocket [5] where the nuclear fission products themselves are ejected for thrust directly.
Also different from nuclear pulse rockets [6], where you throw an atomic bomb out the back and use the blast wave to push you forward. And repeat the process a couple hundred times.
Don't forget the ever lovely Nuclear salt-water rocket [1] which forgoes the throw bomb out the back part and simply has a continuous nuclear explosion inside the ship. Highly efficient in theory.
This is one of the best Wikipedia entries I have read yet. Concise and relatively plain English. Clear description of benefits, shortcomings, and possible tests.
I have to disagree, the intro is not very concise, and includes needless details without giving a really clear picture of what the thing is. The parent comment gives a better summary!
Terrestrial testing might be subject to reasonable objections; as one physicist wrote, "Writing the environmental impact statement for such tests [...] might present an interesting problem ..."
Well, presumably you wouldn't be accelerating just before you want to slow down, so the atomic bombs wouldn't be recently detonated and their debris wouldn't be nearby.
If you flip around and fire the engines to decelerate, you would be flying into the exhaust.
Edit: Actually this hurts my brain. If you're going 1,000,000MPH and fire propellant at 500MPH in your direction of travel, it would travel at approximately 1,000,500MPH and you would decelerate below 1,000,000MPH, so you actually wouldn't run into it. There's no drag on your exhaust in space like there is in an atmosphere...
According to A. Einstein you shouldn't be able to tell the difference between two inertial frames of reference. Physics would behave exactly the same. So if you don't hit your exhaust in the first instance you won't in the other either.
You don’t fly into your exhaust until you accelerate enough to start going in the opposite direction. If you’re driving down the road and throw an apple out the window behind you, you don’t run over it if you put on the brakes, you’d have to accelerate to cross zero velocity and then start going backwards to catch up. An apple will just sit on the highway, rocket exhaust will be moving with quite a bit of speed as well.
Your analogy isn’t really valid, because in the rocket situation you’re flipping the rocket and throwing exhaust forward to slow down. That would be analogous to throwing apples forwards, not backwards.
There’s no real concept of “faster” exhaust compared to yourself. The speed of the exhaust is relative to the frame of reference of your rocket which is an accelerating frame, that speed is always the same when measured against the stationary rocket in that frame. When measured from a fixed inertial frame the speed changes as your rocket accelerates.
For a more concrete example, for a moon rocket the speed of the rocket exhaust is around 3 or 4 km/s. On the ground the speed of the rocket is obviously 0, in low earth orbit the speed of the rocket is 7 or 8 km/s, and to initiate the transfer orbit to the moon you have to accelerate to about 10 km/s. (these would all be in earth centered, nonrotating frame speed measurements)
The rocket exhaust doesn’t have to get faster to get you to those higher speeds because you’re taking it with you.
The more you can increase the rocket exhaust relative to yourself though, the more efficient your rocket is.
Say a ship in 1D is flying at 50km/s, exhaust velocity is -25km/s, relative to a static frame.
Last bit of departure burn or correction burns will fly at 25km/s towards the ship, so if the ship decelerated to below that, the plume could catch up. Meanwhile, plume from deceleration continues at 75km/s away from the ship.
I was thinking that high Isp engines generally have insane exhaust velocity, like hundreds of km/s or more, that problems like this is not an issue even for interplanetary transfers. But interstellar is a bit different, depending on other factors such as dispersion, I guess?
A one dimensional rocket traveling from A to B: all of the exhaust emmitted after the rocket speed exceeds the exhaust nozzle speed will end up hitting B.
In three dimensions though in a hard vacuum, particles coming from a fluid with a bulk velocity of kilometers per second are clearly not going to be nearby the path of the rocket for very long at all
In an atmosphere, yes, with the engine pointing forwards to slow you down, the head wind would blow nuclear dust all over you as you were braking.
In space, the exhaust just flies off into the distance no matter which direction you are facing.
A different but related problem: if you arrive at my space house for space dinner in your nuclear space car, it would be quite rude for you to shower me with nuclear space dust (from when you hit the nuclear space rocket brakes to stop at my house) moments before you arrive.
This isn’t an issue if you go into orbit around a planet, where the braking maneuver is at right angles to the direction to the planet surface.
I imagine spaceships would need specially designated lanes and directions where they can accelerate and brake, so everybody knows which places to avoid if they don't want to get a blast of radioactive material in their face.
That's what the red and green buoys in the shipping lanes are for. It's a near universal agreed upon standard. It's just those guys from NGC7835 that refuse to accept as they are unable to see that particular shade of green, and are waiting for the intergalactic disabilities act to be ratified and used.
> This isn’t an issue if you go into orbit around a planet, where the braking maneuver is at right angles to the direction to the planet surface.
Unsure. Sounds like your exhaust could enter the planet's orbit. It seems prudent to have a secondary engine for use near planets, to avoid filling their upper atmosphere with radioactive garbage.
There is one case where it would be bad: if the velocity of the exhaust was lower than the escape velocity required to leave the gravitational field of your (massive) spaceship.
> By rotating the craft to point opposite the direction of travel and actuating the engine again.
IIRC, one of Heinlein's juvenile novels referred to this as a skew-flip maneuver; I was quite impressed reading about it at about age 12, several years after its publication date.
[Just remembered the title: Have Space Suit - Will Travel, a play on the title of the TV show Have Gun - Will Travel]
In the Expanse books/show they use this all the time and call it "flip and burn". Pretty cool to see what nearly-plausible everyday spaceflight might look like.
In the paper, he recommends using a magnetic sail to slow the craft down from drag in the interstellar medium. Pretty fun stuff on page 6, 60 years to Alpha Centauri! Linked in the wiki or here, http://path-2.narod.ru/design/base_e/nswr.pdf
The most important parameter of an engine like that is velocity of particles emitted as reaction mass (translate to specific impulse). Thermal rocket is going to loose to well designed electric one. Electric rocket can propel ions to huge velocities basically functioning as a small particle accelerator.
The issues currently are ability to get enough thrust (density of the particle stream) and preventing materials from decaying due to energetic particles.
Thermal rocket will be limited to very small, thermal velocities.
You still need to get rid of excess heat, which scales linearly with power. The more engines, the bigger the radiators you need to carry, which will increase your total mass.
> Thermal rocket is going to loose to well designed electric one.
Depends on your metric. For a given tech level, you'll get higher thrust/weight out of a Nuclear Thermal Rocket than you would from a Nuclear Electric Rocket, even if the specific impulse is lower.
And 'very small velocities' here is still double the ISP of hydrolox, so not exactly shabby...
I assume neither design is going to lift off Earth's surface. Most likely any nuclear engine is going to be lifted cold and disassembled and we'll packaged in case of mishap.
Since you are going to have months to run the engine, thrust to weight is less important than ISP. Small accelerations do wonders if you can run them continuously.
Oberth effect is for long term missions where you have time to swing by other planets or moons just to get some free velocity. This is fantastic, but costs huge amount of mission time.
The nuclear engine is about getting so much delta v that you can cut the crap and power directly to your destination.
Oberth effect is useful for every reasonable trip - since every reasonable trip either originates or ends deep within a gravity well. (And being in orbit deep within a gravity well implies being at high velocity) It's why you make an escape burn at periapsis and not apoapsis.
Not really. An 85 ton Starship fueled in LEO can use Oberth effect, get to Mars just as fast, and land directly on Mars.
Your deep space NTR has engines that weigh way at least ten times more, while providing only a fraction of the thrust, and also has to push not only hundreds of tons of radiators, shielding and heavier tanks, but also a lander since NTR doesn’t have the thrust to climb out of any significant gravity well. And also is going to have substantial propellant evaporation by the time it reaches Mars since it can only achieve that ISP with hydrogen.
Small accelerations over long periods are great! But when you're using electric propulsion, how small starts to become an issue. As far as I'm aware ion thrusters have TWRs in the 1/1000 range - NERVA's were in the ~1 range. This means you're taking a 3000 second burn and replacing it with a 3,000,000 second burn - add in efficiency losses and things start getting interesting. (assuming a constant mass fraction devoted to engines, and that the electric propulsion TWR includes power generation and cooling)
Ion thrusters = technical problem. Thermal rocket = fundamentally limited by rocket equation.
With thermal rocket you need huge amounts of reaction mass because it is expelled at slow speeds (it gives little push relative to its mass) and then you need more reaction mass to push that reaction mass and so on. This hugely limits what you can do.
Ion thrusters are largely technical problem of erosion. Current designs have trouble withstanding continuous load because ions hit electrodes and erode them. But there is no physical limitations. Superconducting electromagnets, maybe something else. Somebody hopefully gets a good idea and gets reasonable thrust from ion engine.
Ion engines are still limited by power density - higher ISPs take quadratically more power input at reasonable ISPs. (As in, at non-relativistic velocities) Thermal rockets have a lower upper bound, but come with higher thrusts. Everything is tradeoffs, in the end.
And why on earth would the kind of newtonial engine mean that you wouldn't be limited by the rocket equation? The only escape that is to have reaction mass outside of your reference frame somehow. (picking up fuel from interstellar space, having thrust beamed to you via laser, some form of reactionless drive...)
No body are talking about plasma rockets (VASIMIR). Elctric rockets that have variable ISP on demand (low-thrust, high–specific impulse exhaust or relatively high-thrust, low–specific impulse exhaust). Also, VASIMR does not use electrodes so not erosion problem.
Hydrogen arcjets are going in between 1300-2000 seconds ISP, and are 30%-40% efficient, which is huge by electric standards of electric propulsors, and are can be done with tens of newtons thrust, with 100+ newton per engine deemed possible.
And you can get 1300-1500s ISP from a liquid core NTR, too - or 3-5000s from a gas core NTR. Sure, no one's ever built a liquid core NTR, but there have been designs made.
And needless to say, "tens of newtons" is not the projected thrust of a liquid core NTR. More like a few hundred thousand.
Scalability, and scale matter too. I believe that solar electric thrust to weight ratio is very favourable with modern solar cells, and scalable.
Arcjets can be really tiny, and you can have hundreds of them. Given that you will also have to get huge amount of electricity for crew needs, you will have to pack solar cells anyway. A bimodal NTR will be even heavier, and require even bigger vehicle to legitimise its use.
Only minimum scale really matters - past that you can just cluster engines to get the desired acceleration. Minimum scale for any sort of nuclear thermal rocket is below what you'd want for a manned interplanetary mission, and is thus not relevant.
Solar power plus ion engines is in the 1/2000 TWR range as far as I'm aware. That means millimeters per second squared acceleration of your total craft at best, and means you basically don't save any time over a standard minimum-delta-v Hohmann transfer - 0.001 m/s^2 continuous acceleration gets you 2 AU in ~400 days. It could also do 66,717,283km - the Earth-Mars distance at time of writing - in ~189 days. A Hohmann transfer from Earth to Mars is 259 days. And, of course, the above numbers don't take into consideration matching velocities or escaping Earth's gravity well in the first place. [1] does a good job of describing why the power supply is the primary limiting factor here.
Liquid-core NTRs [0] aren't bimodal, and I'm sort of confused what you'd mean by bimodal here in the first place.
6.4kg 50N 30%-40% efficient hydrogen arcjet, and 1kg/kw solar panels will probably scale up until a 1.5-2kn, which is really a lot of for a relatively efficient, 1000s+ ISP engine made using existing material science.
Current hall-effect engines can produce thrust for 50,000 hrs (~5.8yrs) before needing refurbishment. This is enough for a few trips to Mars.
Even if they cut a week or two off the travel time, they might be worth it on craft with humans as an extra week or two of life support is fairly heavy (~2kg/day/person).
From what I read, electric engines have some pretty big limitations.
- Very low thrust makes it hard to use the oberth effect
- Low thrust to weight ratio makes the actual wet/dry mass ratio harder to get down
- You are not just thrust limited, but also thermal limited. Many other rockets expel a lot of the heat they generate through the exhaust. Electrical engines do not, which means needing to get rid of that heat in other ways.
It's really not the most important parameter, though. Unless you get the necessary thrust levels, the efficiency (Isp) of electric propulsion will get you very efficiently nowhere.
And don't get me wrong - electric is amazing and well working, but if you want to move serious payload to Mars, that's not going to work for a while.
> The most important parameter of an engine like that is velocity of particles emitted as reaction mass (translate to specific impulse).
Hum... That's true for simple designs. Once you start transforming one kind of energy into another, you have to deal with a more complex form that is how much total momentum you can eject from a fixed amount of fuel (and weight it by the mass of the engines that stays on the rocket).
Powering those ion thrusters is a bit of a problem at the mass ranges you need for peopled interplanetary flight. Either you have an absolutely enormous solar array, or an absolutely enormous radiator array for your nuclear reactor.
I think STR is the best possible competitor to chemical rockets for trips inside Jupiter’s orbit. NTR is what we will need out past Jupiter unless a few brave souls use chemical rockets for the fame of being first to make those extremely long expeditions to Jupiter and beyond. Much like early explorers in the age of sail.
Those aren't necessarily all that massive, though. Enormous in size, yes, but the mass would be a lot less than the mass of the fuel for less efficient rockets like chemical or nuclear thermal.
Either way, any vessel travelling beyond Mars or Venus in a time short enough to be safe for humans is going to require in orbit assembly, so size in terms of volume becomes less of a constraint. But as long as we're boosting all of our mass from Earth, that's what costs the money.
Don’t be silly. Nuclear pulse rocketry is very well understood from an engineering perspective, the forces it generates are easily managed, and the radiation it produces easily shielded for.
Without a doubt it’s by far the most practical candidate for sending manned expeditions to the nearby stars.
I was replying to a comment whose parent said anything sent would end up as soup. The joke might’ve been bad, but it certainly is on the reader for not seeing that obvious connection.
In the same way we're removing master/slave and terms like 'blacklist' and 'whitelist' we should also take the opportunity to remove the de-facto sexism in our language. Just because the definition is in the dictionary, doesn't mean it can't be updated.
Could you please stop with this ideological political activism? There is no inherent sexism in any language, ancient historical origin of the words isn't carried over to modern meaning, semantics of the words and doesn't create any bias against women or men. Meaning is created by mass media, people, world around you through propagandistic rhetorics. If you hang out with someone expressing "sexist" attitude or exposed to them through media, it really doesn't matter what words they use to call things they want to be for men or for women, you will still develop "sexist" associations, for example, you still won't consider babysitting a manly task, no matter how politically correct gender-neutral it is called.
Words like “manned”, “no-man’s land”, and “mankind” are derived all the way back to proto Germanic “mann” through to old English, which meant “person” or “human”. Mistaking it as gendered is like mistaking “history” as gendered because it has “his” in it.
Let's not beat around the bush. There is inherent sexism in our vocabulary. The word "man" did uniquely mean "human" in the old English (and the words wæpman and wifman meant male human and female human, respectively), but it doesn't anymore. In the same way that males aren't the defacto representatives of the species, the word man should not refer to the species and the sex at the same time.
By the way, the verb to man, as in to man the decks, comes from military and nautical contexts, which used to be male-only occupations. To continue to use the verb "man" in that context is just unnecessary baggage.
Clearly discussing propulsion methods of going to another planet is less important that percieved sexism of vocabulary. So what of it? Does it help us get to Mars faster?
> Let's not beat around the bush. There is inherent sexism in our vocabulary.
Yes there absolutely is. That doesn’t mean we should knee jerk react to things without any actual understanding of them. Over time, words like mankind will be used less and less and become more anachronistic as our language evolves. But that is not the same as them being, in actuality, sexist and definitely doesn’t warrant sarcastic comments about there being no women on board due to the word being used.
> The word "man" did uniquely mean "human" in the old English (and the words wæpman and wifman meant male human and female human, respectively), but it doesn't anymore.
But they both do. “Wer” survives in werewolf and wif survives in Wife. Just because the originals did not survive, it doesn’t mean that all words derived from them didn’t as well. Mann did not survive, but it doesn't mean all words derived from it didn't as well.
I choose not to use words like mankind and manned in my writing and they already feel a bit anachronistic, but it just strikes me as petty to try and “call out” other people for using words that are perfectly acceptable.
What's acceptable is subjective. What should be accepted is subjective too.
Languages, just like software (both are symbolic systems), require maintenance. If either is used without conscious intent, it accumulates debt. We know very well that technical debt can be a PITA.
Faster ships are good, yes, but doubling the specific impulse also means you can move twice as much for the same amount of fuel (roughly, don't get pendatic on me).
Twice as much mass may not sound that important, but there's always these limits wherein it's not worth building it at all because it would take too much fuel. The life support equipment humans need is also heavy- double the lsp is what we need.
So many options open up with an efficient and powerful rocket like this.
Those options are already opening up without Nuclear Thermal.
The problem with NTR is the engines are heavy, and the necessary shielding and heat radiators are very heavy. So the high ISP is mostly lost to higher dead mass requirements.
If (when) Starship proves it can be fully refueled in-orbit, it obviates every potential advantage of Nuclear Thermal rockets. Starship will be able to make 90 day trips to Mars, carry heavy payloads in excess of 100 tons to the Martian or lunar surfaces, and do this all without requiring specialized landers and with zero radiation and other regulatory concerns.
Sure, starship is a perfect first step (finally!) but nuclear propulsion in some form will almost certainly be used in mid to long term - you can do a lot with orbital refueling but the double plus good ISP can't be ignored.
I’m still skeptical. Nuclear just has too many characteristics that limit its ability to make use of that ISP.
Starship will have a dry mass around 85 tons including about 9 tons of engines, payload of 100 tons, and 1,200 tons of methane/LOX fuel. At an ISP of 380 that gives it a deltaV of around 9.8 Km/sec.
A similar NTR based Starship with a 750 ISP requires a lot more dry mass. The engines will mass at least 100 tons just to provide 1/15th the thrust of Methane based Raptors. That’s ok in space, given you can just fire them longer but they will need hundreds of tons of radiators and shielding. And more mass to insulate the hydrogen tanks.
A dry mass of 500 tons would give the NTR a deltaV of 10.7 Km/sec. That’s better, but it comes at a price. Your NTR Starship can’t land on Mars, it doesn’t have enough thrust. So you need dedicated landers, increasing dry mass further. And it’s hydrogen leaks, so you lose some of that DeltaV on a long voyage. And you can’t use anything other than hydrogen, or your ISP drops precipitously, and you can’t use any reactive fuels that will corrode your engine.
NTR designs need dramatically higher thrust to Weight ratios to ever become competitive with chemical rockets, even in deep space.
> but they will need hundreds of tons of radiators and shielding.
NTRs do not need radiators, they can keep cool using the prop. And the mass of shielding is measured in hundreds of kg to single-digit tons, not hundreds of tons. (You can very effectively reduce your shielding needs by putting the engine on a spar.)
The rest of your concerns are still valid. NTRs seem like they would work better if you are going to asteroids, or other such targets that don't have an atmosphere you can use to brake with, but I do not see the appeal for trips to Mars, where a more compact atmospheric-capable ship can shed half the trip Δv using a heat shield.
They stay hot after shutdown and you'll still need to cool them to avoid damage. If you use your propellant/coolant for that, the ISP will drop further.
You plan for that in your burn, essentially doing a very low-powered long tail in it. Yes, doing this will slightly reduce Isp, but assuming you can alter the size of the throat on your bell, this effect is just single-digit seconds on the entire burn, compared to the "hundreds of tons of radiators" the GP referred to.
A variable geometry nozzle would help a lot here, by keeping a lower volume of propellant to still be ejected at a high speed. May be a good idea after all.
You need heavy radiators or your entire ship quickly becomes uninhabitable when the engine is running. We’ve never built a NTR capable of shedding all its heat through its propellant. In fact, we’ve never operated one in space, only on Earth where cooling is trivial.
But why not use starship to carry a heavy but efficient ntr engine up, and use that for the trips to and from Mars?
Build a special-purpose starship (like is planned for the moon landing) that replaces the rap-vacs with NTRs instead. Boom, you don't need an oxygen tank anymore, creating more living space.
This ship can't land, but if you're heading to the belt for mining or on a manned trip near Venus or Jupiter, you weren't going to anyway.
You can’t replace Raptors with NTRs because of size/thrust mismatch. A NTR with the same vacuum thrust as a Raptor would weigh at least 80 times more, and likely be far larger. Then you have shielding and radiators, it’s never gonna be able to lift off the Moon.
And you aren’t saving space losing the oxygen tank. First you will need that space for hydrogen propellant which doesn’t have the density of Methane. Worse is it also offset by requiring expensive (in energy and cost) additional cooling for Hydrogen.
F. Incomplete. Missing plans for sustainable food and water supply, shelter, energy, sanitation, healthcare, etc. Unless the plan is just to make it as cheap as possible to dump Earth resources into the Martian colony, but that would be more like Mars colonizing Earth rather than the other way around. In that case the Mars plan would get less than an F.
This engine is meant to be used purely outside of the atmosphere, as a means for a ship to transit space efficiently. It's for traveling between planets.
There's some risk during takeoff, as we put it into orbit, but that risk can be handled- make it that the radioactive bits are protected even during an explosive launch failure.
Nuclear reactor have been safely put into space many times before.
I'm sure the same can be done for nuclear reactor fuel. Even better actually, as reactor fuel is basically just a very expensive and pure heavy metal & only slightly radioactive. Only once the reactor is first started all sorts of unstable radiation releasing elements are formed in the fuel.
So if you only start the reactor once it is in space & pack the fuel securely for launch, all should be good to go! :-)
The difference is that a Voyager from 70's nuclear engine if it fails and explodes on launch you'll get the equivalent of Beirut explosion at worse. This one looks like Hiroshima instead. I don't think people living in nearby cities would appreciate that.
Nuclear reactors don't explode. The risk is the chemical rocket exploding and spreading the nuclear fuel. The fuel doesn't get very radioactive until the reactor is started and proper packaging can protect against that.
It was not a nuclear explosion. If it were a nuclear explosion there wouldn't be anything left of the building and other reactors around it and capping it with a concrete and steel structure would be rather pointless.
Quote from said wiki: "The reactor explosion killed two of the reactor operating staff."
Yeah, I guess it wasn't an explosion </sarcasm>
And North Korea underground explosions that were detected and consequently destroyed some of their underground facilities were too nuclear explosions. You know, there is such big explosion and smaller explosions.
The claim upthread was not “nuclear reactors don't have nuclear explosions” (which would also be overgeneralized), but “nuclear reactors don't explode”. The reactor at Chernobyl did explode. The fact that it was a steam explosion induced by energy from a nuclear chain reaction and followed by a reactor core fire does not change the fact that it was a nuclear reactor, and it did explode.
> If it were a nuclear explosion there wouldn't be anything left of the building and other reactors around it and capping it with a concrete and steel structure would be rather pointless.
You seem to be confusing “nuclear explosion” with “nuclear explosion whose yield is maximized via explosive containment, in the manner typical of deliberately engineered nuclear weapons”.
Let me be more clear: powered off nuclear reactors don't explode. You can blow them up and make a mess, but you can't have a critical excursion without first turning the reactor on, which would be very hard to do with an explosion. Even if you pulverized the fuel elements on the explosion (that's one hell of an explosion), it wouldn't be as bad as Chernobyl or Fukushima because until you start the reactor, the nastiest isotopes won't be there.
I believe a bomb is nuclear fuel surrounded by explosives in order to trigger nuclear fission through compression, which in turn will generate the nuclear explosion. Powered off nuclear reactor that sit on top of a lot of fuel, fuel that has many times the explosive power than the explosives that are surrounding the bomb core, can become an atomic bomb.
Personally I would prefer such a rocket to lift off from a secluded location way out there in the ocean, in case something goes wrong and becomes an atomic bomb instead. And I like to think that people with common sense think like me as well. Do you personally have common sense?
Typical nuclear engine designs are safed during launch so even a launch explosion isn’t dangerous, they use fuels that won’t become heavily radioactive until the engine is activated.
One of the problems with using nuclear reactors in spacecraft is that it's difficult to get rid of excess heat in a vacuum. How do nuclear thermal rockets deal with that? Presumably a lot of the heat is transferred to the propellant, but is that energy transfer efficient enough that there isn't an excessive amount of leftover heat?
IIRC the wast majority goes out with the propellant & propelant being cold liquid hydrogen also helps.
Still after a burn you would definitely need some cooling until the fision byproducts decay. How much & for how long I have no idea.
If a couple minutes/ hours just running some more hydrogen through the shutdown reactor might be enough. Otherwise passive or active cooling loops and radiators could be needed.
Or maybe not enough fission byproducts form during the tens of minutes the reactor is active and cooling after a burn is not a major concern?
Heavy radiators are definitely needed if you don’t want your space ship becoming uninhabitable. And running extra propellant through the engine to cool will further reduce usable ISP.
A simple way to think of a nuclear thermal rocket is that it's a nuclear reactor where the coolant is vented into the vacuum. (At very high temperatures, through a rocket nozzle, therefore it produces thrust.)
So yes, it pretty much by definition has no cooling issues. If you'd stay too hot, you'd just pump more fuel to cool your reactor better, and get more thrust.
(One flipside to this is that you cannot just turn it off. After a long burn, there will be decay heat you will have to dispose of, potentially for weeks. NTRs design around this with complex, slow shutoff sequences, or in the case of disposable stages, by making sure the stage will not be near anything important as it melts down.)
This is where the effective Isp of an engine is enhanced if the burn is conducted deep in a gravity well. This effect makes chemical rockets perform better than you might otherwise think, when compared to low thrust high Isp engines.
I saw even some slightly crazy studies where they launched a shielded probe & high thrust for a very close Solar flyby to achieve crazy high delta-v via Oberth effect so that the probe can catch the next unannounced extrasolar asteroid.
Do you have links for these studies? This sounds very interesting, I've been trying to figure out how we could be ready for this kind of thing. I envision some sort of craft that has a combustion engine on it that uses frozen hydrocarbons present on such a body to power the craft long beyond the life span of solar or nuclear power sources to continue sending data as it leaves the solar system.
Jupiter flyby first that drops the perhelion to 3 solar radii (!) and the a high thrust SRB burn for maximum Obert effect on reaching perhelion. Looks like it can give you up to 70 km/s of delta-v. :)
Life on earth is heading for an ecological apocalypse and all we can do is obsess about taking a holiday on Mars (bangs head against wall). Talk about fiddling while Rome burns.
To be honest, I don’t get the rush to move to different planets? With the absolute plethora of benefits earth gives us, is it really hard to just save what we have than think about these far fetched ideas. Suppose that due to some way we actually figure out a way to move to Mars, would it be better than leaving what we have? I would rather prefer living rest of my life on the dying planet that is earth than move to other planet.
I think the real economic case is for asteroid mining. On earth a lot of the rarest elements are trapped in the core, but for asteroids, my understanding is that this same issue doesn't exist.
Seems like that's how humanity works. Just throw more stuff on the problem and hope it goes away. If some thought is put into it, it might actually work.
Garbage landfill? Cover it with some soil and build buildings on top!
Trash in the oceans? Gather it with (more) automated ships!
Water levels rising? Build dams!
Too much CO2 in the air? Use more energy to get rid of it!
Global temperatures getting too high? Build a solar shade for the whole planet (bonus points if it also generates electricity)!
Actually hoping that last two become a reality soon, because we all need it, and the ideas are actually feasible.
The increase in cancer rate for a two year trip to Mars and back is about 4% over your lifetime. The only actual radiation risk is from solar storms, which can be ridden out in storm shelters aboard ship and in Mars habitats.
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[ 2.7 ms ] story [ 252 ms ] threadImagine the same with a nuclear rocket (a reactor getting hot) and then firing out something very quickly through the exhaust.
Same F=ma law applies.
More efficient than a chemical hydrogen and oxygen rocket due to the high energy to weight ratio you get from nuclear fuel.
If you do the latter, then you will have had to build it with another cooling mechanism in mind. That could increase the mass of the reactor and reduce your payload capacity, so you might not do it. Instead, you might include just enough uranium to make the trip, and no more.
I don't doubt that the physics works, and it's important to note that say a 2x gain in thrust-to-mass ratio would lead to enormous advances in trips out of Earth's gravitational well (10x? 100x?) due to the tyranny of the rocket equation. But I'm curious whether this is the 2x gain of replacing oxygen+hydrogen with hydrogen, plus a big complex reactor, or the millionfold gain of replacing hydrogen fuels with uranium.
Edit: TFA says twice the specific impulse, and presumably that's the optimistic estimate. But still very good!
NTR requires heavy engines, heavy shielding and heavy radiators to keep cool. The final NERVA prototype was as close to a functional NTR as ever built, and it massed 40,000 lbs while only generated 55,000 lbs of thrust, with a maximum ISP of 710 seconds.
A SpaceX Raptor only has a Mac ISP of 380 seconds, but masses only 3,000 lbs, and produces 500,000 lbs of thrust. Add another 50,000 lbs dead mass to the NTR for shielding and cooling, and you see why Raptor will get humans to Mars well before any NTR and just as quickly.
We need a huge step forward in NTR before it’s going to be useful at all.
So at some level of scale this outperforms a traditional rocket, and your oddly impassioned argument about why SpaceX is so much better is just relevant to particular use cases rather than spaceship design in general.
The other problem is that low thrust to weight means an NTR spade ship can’t land on Mars, or on any body with a significant gravity well. So you need to bring chemical rocket landers, increasing your mass duplication and tech complications.
A multipurpose chemical rocket powered space ship Luke Star Ship is far more practical and nearly as fast.
https://en.wikipedia.org/wiki/Direct_Fusion_Drive
[1] https://en.wikipedia.org/wiki/NERVA
- Send gas through nuclear reactor to make it hot
- Send hot gas through nozzle
This is essentially the what you are doing. Like if you sat on a chair and aimed a fire extinguisher you'd go flying (cold gas thruster, typically used in RCS). You'd go faster if you super heated the gas before it exited the nozzle.
tl;dr: Gaseous propellant (I'm guessing hydrogen) is heated with fission then pointed in the opposite direction of intended travel.
The uranium in this design is not a propellant, but a heat source. Aside: you can use photons/heat as a propellant, but it's thrust is very low https://en.wikipedia.org/wiki/Pioneer_anomaly. Ideal propellants typically have a high exit velocity and low mass. That gives you the longest amount of "burn" time, and the greatest amount of control for the weight. https://en.wikipedia.org/wiki/Specific_impulse
Back in the day when the US was building more of these nuclear rockets, the propellant of choice was typically hydrogen https://en.wikipedia.org/wiki/NERVA. Old timey video explaining it https://youtu.be/eDNX65d-FBY?t=238. I'm assuming this proposed design would also use hydrogen, but I couldn't find any sources on the propellant for their design.
Liquid hydrogen served to keep the reactor cool as it transitioned from liquid to gas as that phase change absorbs energy. The gas is the directed through the reactor core where the gas heats up. As gases heat up, they absorb energy, their average particle velocities increase.
Eventually, the hydrogen molecules (mostly H2 or H-H gaseous hydrogen), makes it to the nozzle and is ejected. The high-velocity hydrogen is what actually provides the bulk of the thrust to the spacecraft.
Compare this to Project Orion (https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...) which intended to detonate nuclear warheads and the craft essentially rode the shock wave into the stars. I would classify this method of propulsion, not safe.
An NTR can be designed such that the engine and spacecraft "chassis" are reusable over multiple missions. NASA has/has an NTR concept with such a reusable vehicle. The fuel tanks are disposable and slot into the central frame like AA batteries. The crew portion would be a TransHab-like habitation module with a docked crew capsule and Mars lander. Propellant tanks would be disposed of during the mission and the vehicle parked in Earth orbit between missions. For a new mission propellant tanks would be fitted along with a new crew and off it goes. It's an interesting design but a little passed the current bleeding edge of in-orbit construction.
...isn't it USNT (not USNC)?
I've never played Halo though so I just assume I'm missing something.
EDIT: I was just referring to the abbreviation for what GP said, "U"nited "S"tates "N"uclear "T"echnologies
Does anyone know what it was reduced from?
[1] https://www.spacex.com/human-spaceflight/mars/
More detail: https://news.cgtn.com/news/2020-07-19/What-is-a-Mars-launch-...
I wonder how much radiation protection does it have, and whether the exhaust would be acceptable to use for a launch from Earth surface.
This makes the necessary shielding much less than a full sphere, reducing the mass to a mere fraction.
Thing get a bit hairy though when you need to dock with something or even when running multiple engines (the neutron balance would apparently be totally wrong).
Thus, if you use cooler hydrogen gas rather than hotter water+hydrogen, you can still make it a lot more efficient given that water has 9 times the mass of hydrogen per molecule.
Can the engine produce the peak power needed at liftoff? Or are its main benefits realized over the course of the trip instead?
Once you're in space, ISP matters a lot more than TWR, and is the main limiting factor on how far you can get with a given mass fraction. Nuclear rockets generally have ISPs at least double that of the current best high-thrust engines, hydrolox. (hydrogen+liquid oxygen)
And if someone is planning to launch a nuclear salt water rocket inside the atmosphere of an inhabited planet, you certainly have bigger issues to worry about.
https://www.spacex.com/media/making_life_multiplanetary_2016...
See https://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdf... and try the different parameters to see which work with your browser.
Here is another link to the SpaceX PDF, showing a few more options in action (in Firefox at least):
https://www.spacex.com/media/making_life_multiplanetary_2016...
This appears to be a nuclear thermal rocket [1] which would use a nuclear reaction to directly heat propellant (e.g. hydrogen) that is ejected.
This is different from a nuclear electric rocket [2] which would produce electricity which would then generate propulsion [3] using a smaller quantity of propellant, e.g. using an ion thruster [4].
It is also different from a fission-fragment rocket [5] where the nuclear fission products themselves are ejected for thrust directly.
[1] https://en.wikipedia.org/wiki/Nuclear_thermal_rocket
[2] https://en.wikipedia.org/wiki/Nuclear_electric_rocket
[3] https://en.wikipedia.org/wiki/Electrically_powered_spacecraf...
[4] https://en.wikipedia.org/wiki/Ion_thruster
[5] https://en.wikipedia.org/wiki/Fission-fragment_rocket
[6] https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion
[1] https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket
I'm assuming that it wouldn't be able to eject the exhaust in the reverse direction because the exhaust material is still fissioning!
Edit: Actually this hurts my brain. If you're going 1,000,000MPH and fire propellant at 500MPH in your direction of travel, it would travel at approximately 1,000,500MPH and you would decelerate below 1,000,000MPH, so you actually wouldn't run into it. There's no drag on your exhaust in space like there is in an atmosphere...
For a more concrete example, for a moon rocket the speed of the rocket exhaust is around 3 or 4 km/s. On the ground the speed of the rocket is obviously 0, in low earth orbit the speed of the rocket is 7 or 8 km/s, and to initiate the transfer orbit to the moon you have to accelerate to about 10 km/s. (these would all be in earth centered, nonrotating frame speed measurements)
The rocket exhaust doesn’t have to get faster to get you to those higher speeds because you’re taking it with you.
The more you can increase the rocket exhaust relative to yourself though, the more efficient your rocket is.
Last bit of departure burn or correction burns will fly at 25km/s towards the ship, so if the ship decelerated to below that, the plume could catch up. Meanwhile, plume from deceleration continues at 75km/s away from the ship.
I was thinking that high Isp engines generally have insane exhaust velocity, like hundreds of km/s or more, that problems like this is not an issue even for interplanetary transfers. But interstellar is a bit different, depending on other factors such as dispersion, I guess?
In three dimensions though in a hard vacuum, particles coming from a fluid with a bulk velocity of kilometers per second are clearly not going to be nearby the path of the rocket for very long at all
In space, the exhaust just flies off into the distance no matter which direction you are facing.
A different but related problem: if you arrive at my space house for space dinner in your nuclear space car, it would be quite rude for you to shower me with nuclear space dust (from when you hit the nuclear space rocket brakes to stop at my house) moments before you arrive.
This isn’t an issue if you go into orbit around a planet, where the braking maneuver is at right angles to the direction to the planet surface.
Unsure. Sounds like your exhaust could enter the planet's orbit. It seems prudent to have a secondary engine for use near planets, to avoid filling their upper atmosphere with radioactive garbage.
IIRC, one of Heinlein's juvenile novels referred to this as a skew-flip maneuver; I was quite impressed reading about it at about age 12, several years after its publication date.
[Just remembered the title: Have Space Suit - Will Travel, a play on the title of the TV show Have Gun - Will Travel]
https://en.wikipedia.org/wiki/Have_Space_Suit%E2%80%94Will_T...
The thermal one will reach usable velocity within your lifespan.
A lot less than the fuel for chemical or propellant for nuclear thermal, though
> and has cooling requirement.
That's for the electricity generation, not the thrusters
> Also wear and tear if you run them preatty much continuously.
The whole point of using thrusters is to run them continuously. No moving parts.
Depends on your metric. For a given tech level, you'll get higher thrust/weight out of a Nuclear Thermal Rocket than you would from a Nuclear Electric Rocket, even if the specific impulse is lower.
And 'very small velocities' here is still double the ISP of hydrolox, so not exactly shabby...
Also without high trust you can't really make use of the Oberth effect: https://en.m.wikipedia.org/wiki/Oberth_effect
The nuclear engine is about getting so much delta v that you can cut the crap and power directly to your destination.
Your deep space NTR has engines that weigh way at least ten times more, while providing only a fraction of the thrust, and also has to push not only hundreds of tons of radiators, shielding and heavier tanks, but also a lander since NTR doesn’t have the thrust to climb out of any significant gravity well. And also is going to have substantial propellant evaporation by the time it reaches Mars since it can only achieve that ISP with hydrogen.
With thermal rocket you need huge amounts of reaction mass because it is expelled at slow speeds (it gives little push relative to its mass) and then you need more reaction mass to push that reaction mass and so on. This hugely limits what you can do.
Ion thrusters are largely technical problem of erosion. Current designs have trouble withstanding continuous load because ions hit electrodes and erode them. But there is no physical limitations. Superconducting electromagnets, maybe something else. Somebody hopefully gets a good idea and gets reasonable thrust from ion engine.
And why on earth would the kind of newtonial engine mean that you wouldn't be limited by the rocket equation? The only escape that is to have reaction mass outside of your reference frame somehow. (picking up fuel from interstellar space, having thrust beamed to you via laser, some form of reactionless drive...)
And needless to say, "tens of newtons" is not the projected thrust of a liquid core NTR. More like a few hundred thousand.
Arcjets can be really tiny, and you can have hundreds of them. Given that you will also have to get huge amount of electricity for crew needs, you will have to pack solar cells anyway. A bimodal NTR will be even heavier, and require even bigger vehicle to legitimise its use.
Solar power plus ion engines is in the 1/2000 TWR range as far as I'm aware. That means millimeters per second squared acceleration of your total craft at best, and means you basically don't save any time over a standard minimum-delta-v Hohmann transfer - 0.001 m/s^2 continuous acceleration gets you 2 AU in ~400 days. It could also do 66,717,283km - the Earth-Mars distance at time of writing - in ~189 days. A Hohmann transfer from Earth to Mars is 259 days. And, of course, the above numbers don't take into consideration matching velocities or escaping Earth's gravity well in the first place. [1] does a good job of describing why the power supply is the primary limiting factor here.
Liquid-core NTRs [0] aren't bimodal, and I'm sort of confused what you'd mean by bimodal here in the first place.
0: http://www.projectrho.com/public_html/rocket/enginelist2.php...
1: http://www.projectrho.com/public_html/rocket/enginelist.php#...
6.4kg 50N 30%-40% efficient hydrogen arcjet, and 1kg/kw solar panels will probably scale up until a 1.5-2kn, which is really a lot of for a relatively efficient, 1000s+ ISP engine made using existing material science.
Even if they cut a week or two off the travel time, they might be worth it on craft with humans as an extra week or two of life support is fairly heavy (~2kg/day/person).
- Very low thrust makes it hard to use the oberth effect
- Low thrust to weight ratio makes the actual wet/dry mass ratio harder to get down
- You are not just thrust limited, but also thermal limited. Many other rockets expel a lot of the heat they generate through the exhaust. Electrical engines do not, which means needing to get rid of that heat in other ways.
And don't get me wrong - electric is amazing and well working, but if you want to move serious payload to Mars, that's not going to work for a while.
Hum... That's true for simple designs. Once you start transforming one kind of energy into another, you have to deal with a more complex form that is how much total momentum you can eject from a fixed amount of fuel (and weight it by the mass of the engines that stays on the rocket).
Either way, any vessel travelling beyond Mars or Venus in a time short enough to be safe for humans is going to require in orbit assembly, so size in terms of volume becomes less of a constraint. But as long as we're boosting all of our mass from Earth, that's what costs the money.
[1] https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion
Without a doubt it’s by far the most practical candidate for sending manned expeditions to the nearby stars.
staffed expeditions...? crewed expeditions...?
Unless you are not planning on sending any women?
manned /adjective/ (of an aircraft or spacecraft) having a human crew.
"a manned mission to Mars" is even given as an example use.
By the way, the verb to man, as in to man the decks, comes from military and nautical contexts, which used to be male-only occupations. To continue to use the verb "man" in that context is just unnecessary baggage.
Yes there absolutely is. That doesn’t mean we should knee jerk react to things without any actual understanding of them. Over time, words like mankind will be used less and less and become more anachronistic as our language evolves. But that is not the same as them being, in actuality, sexist and definitely doesn’t warrant sarcastic comments about there being no women on board due to the word being used.
> The word "man" did uniquely mean "human" in the old English (and the words wæpman and wifman meant male human and female human, respectively), but it doesn't anymore.
But they both do. “Wer” survives in werewolf and wif survives in Wife. Just because the originals did not survive, it doesn’t mean that all words derived from them didn’t as well. Mann did not survive, but it doesn't mean all words derived from it didn't as well.
I choose not to use words like mankind and manned in my writing and they already feel a bit anachronistic, but it just strikes me as petty to try and “call out” other people for using words that are perfectly acceptable.
Languages, just like software (both are symbolic systems), require maintenance. If either is used without conscious intent, it accumulates debt. We know very well that technical debt can be a PITA.
The medusa style can do better shock absorption.
Also to travel to other planets you would need to work out
- Anti Gravity system
- Radition shielding
- Reclying Water
- Growing food without gravity or artifcal gravity (Aquaponics ?)
- Recyling air
- Non tradational fuel sources
- Long Distance Radio commication without stations/repeaters
https://toughsf.blogspot.com/2019/10/the-expanses-epstein-dr...
...
Alright, alright! I’ll show myself out.
Twice as much mass may not sound that important, but there's always these limits wherein it's not worth building it at all because it would take too much fuel. The life support equipment humans need is also heavy- double the lsp is what we need.
So many options open up with an efficient and powerful rocket like this.
The problem with NTR is the engines are heavy, and the necessary shielding and heat radiators are very heavy. So the high ISP is mostly lost to higher dead mass requirements.
If (when) Starship proves it can be fully refueled in-orbit, it obviates every potential advantage of Nuclear Thermal rockets. Starship will be able to make 90 day trips to Mars, carry heavy payloads in excess of 100 tons to the Martian or lunar surfaces, and do this all without requiring specialized landers and with zero radiation and other regulatory concerns.
Starship will have a dry mass around 85 tons including about 9 tons of engines, payload of 100 tons, and 1,200 tons of methane/LOX fuel. At an ISP of 380 that gives it a deltaV of around 9.8 Km/sec.
A similar NTR based Starship with a 750 ISP requires a lot more dry mass. The engines will mass at least 100 tons just to provide 1/15th the thrust of Methane based Raptors. That’s ok in space, given you can just fire them longer but they will need hundreds of tons of radiators and shielding. And more mass to insulate the hydrogen tanks.
A dry mass of 500 tons would give the NTR a deltaV of 10.7 Km/sec. That’s better, but it comes at a price. Your NTR Starship can’t land on Mars, it doesn’t have enough thrust. So you need dedicated landers, increasing dry mass further. And it’s hydrogen leaks, so you lose some of that DeltaV on a long voyage. And you can’t use anything other than hydrogen, or your ISP drops precipitously, and you can’t use any reactive fuels that will corrode your engine.
NTR designs need dramatically higher thrust to Weight ratios to ever become competitive with chemical rockets, even in deep space.
NTRs do not need radiators, they can keep cool using the prop. And the mass of shielding is measured in hundreds of kg to single-digit tons, not hundreds of tons. (You can very effectively reduce your shielding needs by putting the engine on a spar.)
The rest of your concerns are still valid. NTRs seem like they would work better if you are going to asteroids, or other such targets that don't have an atmosphere you can use to brake with, but I do not see the appeal for trips to Mars, where a more compact atmospheric-capable ship can shed half the trip Δv using a heat shield.
They stay hot after shutdown and you'll still need to cool them to avoid damage. If you use your propellant/coolant for that, the ISP will drop further.
Build a special-purpose starship (like is planned for the moon landing) that replaces the rap-vacs with NTRs instead. Boom, you don't need an oxygen tank anymore, creating more living space.
This ship can't land, but if you're heading to the belt for mining or on a manned trip near Venus or Jupiter, you weren't going to anyway.
And you aren’t saving space losing the oxygen tank. First you will need that space for hydrogen propellant which doesn’t have the density of Methane. Worse is it also offset by requiring expensive (in energy and cost) additional cooling for Hydrogen.
This engine is meant to be used purely outside of the atmosphere, as a means for a ship to transit space efficiently. It's for traveling between planets.
There's some risk during takeoff, as we put it into orbit, but that risk can be handled- make it that the radioactive bits are protected even during an explosive launch failure.
Nuclear reactor have been safely put into space many times before.
I'm sure the same can be done for nuclear reactor fuel. Even better actually, as reactor fuel is basically just a very expensive and pure heavy metal & only slightly radioactive. Only once the reactor is first started all sorts of unstable radiation releasing elements are formed in the fuel.
So if you only start the reactor once it is in space & pack the fuel securely for launch, all should be good to go! :-)
Quote from said wiki: "The reactor explosion killed two of the reactor operating staff."
Yeah, I guess it wasn't an explosion </sarcasm>
And North Korea underground explosions that were detected and consequently destroyed some of their underground facilities were too nuclear explosions. You know, there is such big explosion and smaller explosions.
A steam explosion. That's very different from a nuclear explosion.
> And North Korea underground explosions
Those were not reactors exploding. Those were bombs, which were pretty much designed to explode and, as expected, exploded.
The claim upthread was not “nuclear reactors don't have nuclear explosions” (which would also be overgeneralized), but “nuclear reactors don't explode”. The reactor at Chernobyl did explode. The fact that it was a steam explosion induced by energy from a nuclear chain reaction and followed by a reactor core fire does not change the fact that it was a nuclear reactor, and it did explode.
> If it were a nuclear explosion there wouldn't be anything left of the building and other reactors around it and capping it with a concrete and steel structure would be rather pointless.
You seem to be confusing “nuclear explosion” with “nuclear explosion whose yield is maximized via explosive containment, in the manner typical of deliberately engineered nuclear weapons”.
Personally I would prefer such a rocket to lift off from a secluded location way out there in the ocean, in case something goes wrong and becomes an atomic bomb instead. And I like to think that people with common sense think like me as well. Do you personally have common sense?
Still after a burn you would definitely need some cooling until the fision byproducts decay. How much & for how long I have no idea.
If a couple minutes/ hours just running some more hydrogen through the shutdown reactor might be enough. Otherwise passive or active cooling loops and radiators could be needed.
Or maybe not enough fission byproducts form during the tens of minutes the reactor is active and cooling after a burn is not a major concern?
So yes, it pretty much by definition has no cooling issues. If you'd stay too hot, you'd just pump more fuel to cool your reactor better, and get more thrust.
(One flipside to this is that you cannot just turn it off. After a long burn, there will be decay heat you will have to dispose of, potentially for weeks. NTRs design around this with complex, slow shutoff sequences, or in the case of disposable stages, by making sure the stage will not be near anything important as it melts down.)
https://en.wikipedia.org/wiki/Oberth_effect
This is where the effective Isp of an engine is enhanced if the burn is conducted deep in a gravity well. This effect makes chemical rockets perform better than you might otherwise think, when compared to low thrust high Isp engines.
Jupiter flyby first that drops the perhelion to 3 solar radii (!) and the a high thrust SRB burn for maximum Obert effect on reaching perhelion. Looks like it can give you up to 70 km/s of delta-v. :)
Did we ever need to criss the Bering land bridge? Or leave Africa?
When Europeans arrived in North America it was already inhabited.
Garbage landfill? Cover it with some soil and build buildings on top!
Trash in the oceans? Gather it with (more) automated ships!
Water levels rising? Build dams!
Too much CO2 in the air? Use more energy to get rid of it!
Global temperatures getting too high? Build a solar shade for the whole planet (bonus points if it also generates electricity)!
Actually hoping that last two become a reality soon, because we all need it, and the ideas are actually feasible.