New nuclear engine concept could help realize 3-month trips to Mars | Hacker News Reader

New nuclear engine concept could help realize 3-month trips to Mars

newatlas.com

425 pointsbookofjoe252 comments

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.

[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

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.

[6] https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion

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.

[1] https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket

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 ..."

How would such a rocket slow down?

I'm assuming that it wouldn't be able to eject the exhaust in the reverse direction because the exhaust material is still fissioning!

By rotating the craft to point opposite the direction of travel and actuating the engine again.

(same way as all space craft)

And then just fly into your still-fissioning exhaust? Wouldn’t that be bad?

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...

Oh wow nice explanation. Thanks.

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.

Yeah, I'm just so used to watching Falcon 9 landings where it's falling through the exhaust because of atmospheric drag.

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.

Aren’t the plume generally supposed to be faster and in opposite direction compared to the vehicle?

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.

The good news is that If it's space winter those rude space guests won't need to bring a space heater.

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]

https://en.wikipedia.org/wiki/Have_Space_Suit%E2%80%94Will_T...

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.

The electric will finally reach much higher velocity, yes.

The thermal one will reach usable velocity within your lifespan.

Just run a lot of ion thrusters in parallel to get your thrust. They're tiny, that's feasible for any ship trying to go further than Mars/Venus.

This works to a degree - each thruster weights something and has cooling requirement. Also wear and tear if you run them preatty much continuously.

> This works to a degree - each thruster weights something

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.

IIRC some ion thrusters experience grid erosion over very long runs - stuff can wear out even without moving parts.

Adding more of them doesn’t meaningfully change the very low thrust to weight ratio, which is what makes it hard to get to closer places quickly.

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.

Thrust to weight still will matter. NTR and NERs still have to drag all that dead mass of engine, radiators and shielding wherever they go.

Not to forget NTR dumping most of the heat during the burn kinda by definition while NER has to radiate lots of heat continuously to power the engine.

Also without high trust you can't really make use of the Oberth effect: https://en.m.wikipedia.org/wiki/Oberth_effect

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.

0: http://www.projectrho.com/public_html/rocket/enginelist2.php...

1: http://www.projectrho.com/public_html/rocket/enginelist.php#...

Above I meant arcjets under solar-electric.

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).

Build on the Moon and launch with a mass driver. Dock with a shuttle on HEO. Easy!

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.

Or even the Age of the Black Friday Sale! ;-)

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.

The hotter your reactor is, the higher is your efficiency, and the smaller radiators you need.

Don't forget Nuclear Pulse Propulsion [1]. 3 Months? Hah, how about 2 weeks!

[1] https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion

And everyone arrives at mars, two weeks later, a fine soup of person bits inside their space suit.

probably better to send equipment that way... let the humans go the scenic route.

Forget equipment. The only thing that survives that mode of transportation is soup.

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.

Clearly beign silly isn’t appreciated.

With only text to go on, your joke has to be pretty good to not be mistaken for an, shall we say, "uninformed" comment.

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.

I got your joke even if no-one else did.

Only if the shock absorbers and their backups fail.

Yeah, but it can transport a million tons of cargo.

The medusa style can do better shock absorption.

This stuff isn't new is it? Wasn't their a project called NERVa back in the late 60ies doing the same thing?

Worth pointing out that you can already do 3 month trips to Mars with chemical propulsion, it just requires refueling: Page 37 of the pdf: https://www.spacex.com/media/making_life_multiplanetary_2016...

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).

https://www.spacex.com/media/making_life_multiplanetary_2016...

TIL. Does that only for for page numbers?

There is more. You can also add zoom=<value> to show the PDF with a zoom level of <value> percent, search="<search term>", ...

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...

"Ultra Safe Nuclear Technologies" is a tremendous name for a company that does nuclear stuff.

All I can think of is 30 Rock, when they order takeout from a place called “American Sub Restaurant Very Clean Come In”

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.

Do you want to go to Mars or not?

Are we gonna pollute outer space like we did our own planet?

It's full of nastier shit than whatever we can dump out there

There is not enough material on earth to pollute outer space in any significant way. A greater source of pollution is radio signals.

You made me laugh :D

Their acronym sounds like a Halo reference

You still want to have a fairly high thrust/weight ratio to take advantage of the Oberth Effect.

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.

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.

Here it is: https://www.centauri-dreams.org/2017/11/10/project-lyra-send...

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. :)

Check my thinking here, from a lay perspective:

So the fastest way to point B in space (Mars, Europa, whatever) would be to accelerate constantly until you're halfway there, and then flip around and start slowing down, right?

OTOH, a more efficient approach is to achieve some velocity V via an initial burn and/or gravity assist, and cruise along via Newton with no additional burn until time to slow down and drop into orbit at your destination.

I further assume that, if you live passengers you'd prefer not to render into soup, you must labor under an acceleration limit.

Realistically, what IS that limit? And would would the acceleration plan look like for a long flight like Mars with people aboard?

Lots of hard-ish SF (notably the Expanse) handwaves this away with drugs or whatever, which is fine for storytelling, but I'm sort of curious what we could actually tolerate.

Hard to say with the G limit. Fighter pilots routinely tolerate 9 Gs with training and pressure suits (or more, for very short periods of time). That's extreme, but if you're strapped on your back, the Gs don't pull blood down out of your skull as much (but you also don't want to send too much pressure IN to the head). I think for a while at least, one can assume astronauts are comparable in physical shape and training to elite pilots. Previous spacecraft and roller coasters routinely do around half that. If the required acceleration would take a very long time, even at 9 Gs, you'd need to lessen the force.

Edit: Some back of the napkin math: It's It's between 30 and 250 million miles to Mars. Let's call it 100 million on average. And let's say we want to make the trip in about 3 months. That means we need about 20km/s of velocity, and at 9Gs that only takes you around 4 minutes. I would probably lower the acceleration and just stretch it out longer. The bigger acceleration might be needing to match Mars's velocity once you get there. But I think those are all pretty reasonable numbers for humans to tolerate. The really hard problem is still the landing, rendezvous, and launching a spacecraft big enough to do all that and then come back.

Continuous acceleration on the order of 1G is solidly in the realm of fiction today (see: The Expanse series). The actual engines we have today capable of continuous thrust for weeks at a time only produce less than a Newton of thrust. The practical approach is starved for both energy and propellant to achieve such levels of acceleration.

We don't have good data for long term high G tolerance, but estimates I've seen are usually around 1.5 G maximum. However, engine efficiency usually comes at the expense of thrust; there are few theoretical designs even that could provide 1.5 G for the amount of time needed to get to Mars.

For this engine, for example, the specific impulse is twice that of a chemical rocket - so around 800. At an acceleration of 1.5 G, this engine could only run for around forty minutes before exhausting all the fuel in the vessel.

So we are not yet at the technology level necessary for Brachistochrone trajectories (accelerate halfway there, flip over, decelerate the rest of the way); this would merely allow for a shorter Newtonian transfer orbit.

You're describing a brachistochrone trajectory. The "magic" in sci-fi isn't the drugs, it's rocket engines that are actually that efficient. If you accelerate/decel at a sustained 1 g, you can make it to Mars in about 2 days.

> If you accelerate/decel at a sustained 1 g, you can make it to Mars in about 2 days.

This is the key calculation where you realize that quick inter-planetary travel is totally within the realm of physics.

> Using Days and AU (astronomical units) we can see 3 days will get about 2.5 AU (halfway to Jupiter). 4.5 days will get you 5 AU (halfway to Saturn). 9 days will get you 20 AU (more than halfway to the Kuiper belt)

Obviously inter-stellar travel seems like a totally different matter, but actually at 1g constant acceleration it works out to about “1 year + the number of light years” to an outside observer. Less time passes for the traveler.

Invent an engine efficient enough to allow constant 1g acceleration for years at a time and humans really could become interstellar.

The best ion engines get around 200,000 Isp. If you had a 500t ship burning 1t of mass per day, you would need an Isp of ~4.5m to achieve 1g over that first day (it gets easier as you get lighter).

The theoretical limit (ejecting mass out the back at 1c) for specific impulse is 300 million.

https://space.stackexchange.com/questions/840/how-fast-will-...

At first I was like: Nah, it can't be right...

High-school math, napkin, and 5 minutes later: Wow, it's only 42 hours!

Thanks for providing that term. I did not know it.

I'm surprised to learn that Mars is (or can be) so "close" in terms of a sustained 1g acceleration.

We just need to work some more on the dark matter repulsion drive, or the parallel universe reaction mass extraction drive. Or, the "belch of god". Still figuring out what the belch or the god part of that one actually are.

Or you fire off refueling packs ahead of the trip.

When we say "something accelerates" what we mean is "its velocity increases over time". Its acceleration could also increase over time, but that is not a requirement. When you ask "what is the acceleration limit" it seems like you are not clear on that fact. The velocity of the rocket relative to the Sun could be increasing for the first half of the journey, while its acceleration (or deceleration, on the second half) could remain constant. At 9.8 m/s² the crew would cruise at a comfortable 1G.

But let's consider that the acceleration increases over time. The maximum number of Gs the human body can withstand has been studied in astronauts and fighter pilots. They can withstand 8 or 9 Gs for some time with special training and compressive clothing. That puts the biggest acceleration at about 99.8 = 88 m/s², but this would have to be done in bursts, with special clothing and training. The maximum acceleration for being able to perform regular human tasks like walking or taking readings would be much less however. I don't have figures for that, but I think the risk of accidental bone fracture increases dramatically when we go above our specs. 2Gs would already be too much.

All these questions are kind of moot for now because with the fuels we currently have we can only sustain acceleration for a very limited amount of time.

I also want to point out that the fastest point from A to B in space must* take into account that space is not always empty. A third object C can be used to give a "gravitational assist" to a spacecraft. Or its influence might have to be counteracted by spending extra fuel (making an alternative to the straight line between A and B preferable).

What I meant was "what sustained acceleration could humans tolerate? Others have answered that question, and also provided the surprising (to me, at least) bit of data that a 1G constant acceleration -- currently not remotely possible -- would put the entire solar system within easy reach.

And yes, I understand about gravity assists and the like.

High accelerations require bigger heavier engines, or more engines, so they chew through fuel quickly. Essentially your trading away some of your payload for the luxury of burning your engines for a shorter period.

Burning your engines continuously during a long journey isn’t practical with most of our propulsion technologies as they can’t carry enough fuel for that long a burn. The exceptions are mostly electric propulsion systems such as Hall Thrusters, which are very low thrust and aren’t useful for quick transits.

Note that the source is a press release.

This is very similar to the Peewee nuclear rocket of the late 1960s. A small version of the better-known Kiwi reactor, that used zirconium carbide coatings like this new plan. Those 1960s projects got far enough along that the next step was flight hardware.

These things are certainly buildable, but the risks are a problem. These are upper stage engines. The risk comes from a failure of the booster used to get them clear of the Earth, in which case you have a nuclear reactor re-entering in pieces.

You should probably ship them without fuel or as cargo, and assemble / fuel the interplanetary rocket in orbit.

You would still have to rocket the fuel up at some point, though, how would that be different then sending it prefueled

But since you're not starting them up before you're in orbit, if the launch would fail it wouldn't be particularly radioactive. Sure, uranium is a toxic heavy metal, like many other heavy elements like lead, but not very radioactive.

Also, since they're using 20% enriched fuel and not 90%+ like Peewee/Kiwi, I guess the fuel design is somewhat different although there certainly are similarities.

I agree with this "what if it blows up before it reaches orbit" view generally.

I do though now wonder how this view will change with the likes of SpaceX. Sure they still have the occasional accident, but it feels (I don't have any data) that reliability is improving dramatically? It felt before like the rocket blowing up on take off was a bit of a high-odds roll the dice thing, but already even SpaceX landings have become somewhat routine.

I think USNT's fuel canisters are inert outside of the reactor and something small like 2cm-diameter each (1). Maybe stick the fuel canisters in a Dragon-type capsule complete with escape system for extra peace-of-mind? Send the reactor up in a cargo launcher, then have astronauts load the fuel in orbit?

1 - https://usnc.com/fcm-fuel/

For everyone interested in alternate concepts here and if you can handle the 90's web design: Project Rho is probably the most comprehensive resource on the internet.

http://www.projectrho.com/public_html/rocket/engineintro.php

That 90's web design it loads faster and looks way cleaner than "modern" crapware I get fed all over the place.

For real, I had a very long page open the other day (was linked probably from another page) and it just loaded straightforward.

Meanwhile I'm developing a single page webapp and my system seems to struggle. Mind you, the unuoptimized version is already (apparently) 35MB (uncompressed) JS. Should see if I can improve that.

edit: built version is ~600KB, of which ~70KB is actual application code. Seems like I'm using a 'big' version of lodash somewhere.

Edit 2: Fixed, shaved off down to ~730KB. Biggest factors now are React itself, react-bootstrap, i18next, yup and lodash-es.

https://usnc.com/ultra-safe-nuclear-technologies-delivers-ad... for the company page And https://usnc.com/fcm-fuel/ for their fuel (not propellant).

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?

I’m not an expert but I’m pretty sure the only way to orbit via nuclear is riding the shockwave of one or more nuclear explosions. There was a program to develop one called Project Orion. https://en.m.wikipedia.org/wiki/Project_Orion_(nuclear_propu...

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.

I didn’t know about NERVA, thanks for the correction!

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)

Yea but NTR’s gains in ISP is mostly wasted by all its large dead mass requirements and its requirement to use hydrogen as a fuel.

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.

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.

In this case you're passing the heat into the fuel. Excess heat you can use for ship or to power your vehicle.

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.)

You need a rough complete rocket design to compare propulsion systems.

If your Mars mission has a 10 km/s delta vee, and your exhaust velocity is 10 km/s (ISP = 1000 s), then your velocity ratio is 1 and your fueled to empty mass ratio is e^1 = 2.7. Let's round that to 3. Ie your rocket is 2 parts fuel and 1 part other stuff like engines and tank and payload.

If you need initial acceleration of say 1 km/s per day, then in SI units your acceleration is 0.01 m/s^2. If you have a nuclear thermal engine with a thrust to weight of 3, that means 30 m/s^2 or 30 N per kg.

For a 100 ton empty mass, you need a 300 ton initial fueled rocket. To accelerate, you need 300000 x 0.01 = 3000 newtons of thrust, meaning only a paltry 100 kg of nuclear rocket engine!

If your tanks have a mass ratio of 10 (hydrogen means this might not be trivial), you have 20 t of tanks. That leaves 100-20=80 t for payload. (The engine is in the noise.) This would be excellent.

With a chemical rocket, your specific impulse would be a lot worse, your propellant mass would be a lot more (3-4x), but the tanks would contain mostly liquid oxygen by weight and thus could be lighter per contained propellant mass so it might not be as bad. Engine thrust to weight might be 10x so total engine mass could still be lower, but it doesn't dominate in this kind of slow acceleration mission.

With a nuclear or solar electric rocket, you might have some weird propellant - availability and tank mass depends on that. The engine is going to be really heavy.

To me, a nuclear thermal makes sense for Mars missions - it is a good fit for the problem. You still need the chemical rockets for takeoff and landing at each end. Also if you assemble in LEO, there's the question of passage through the radiation belts if you accelerate really slowly.

It might work for lunar work as well, haven't looked at it.

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 can keep cool using the prop

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.

I'd bet at least one person who helped come up with the name "Ultra Safe Nuclear Technologies" "UNSC" name had played Halo before.

USNC not UNSC.

> USNC not UNSC.

...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

In the article it says it’s written USNC-Tech

Oh, I wasn't correcting the Halo reference. I just meant the initials of the company are USNC.

Oops.

I'd like to find the marketing genius who invented the term "high-assay low enriched uranium", and buy them a high-energy diet beer.

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.

yeah but you "burn" that shielding as you go... slowly depleting it as you get faster and further away.

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.

Can someone explain how uranium turns into thrust?

https://en.wikipedia.org/wiki/Nuclear_thermal_rocket

When fuel burns, it's the velocity of the exhaust gas that propels the rocket (F=ma).

Imagine 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.

I think they were specifically asking about the "firing out something very quickly" part.

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.

What happens when the rector runs out of fuel? Control rods permanently insert or it just, you know, proceeds to melt down once arrived at Mars?

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.

Nobody's going to make a reactor that can't turn off.

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!

The gains from an NTR over chemical rocket has to do with the temperature, thus velocity, of the exhaust.

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.

Would they make sense as a means to transport the already heavy parts a of nuclear reactor to Mars?

Those already take the slow boat Hohmann transfer orbit to maximize payload capacity. Starship might be able to send 150 tons that way to the surface itself. And building bigger starships is easier than building NTRs.

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.

There actually is a concept for fusion rocket in work.

https://en.wikipedia.org/wiki/Direct_Fusion_Drive

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)

[1] https://en.wikipedia.org/wiki/NERVA

This is another shortcoming of NTRs, as Hydrogen is tiny and slippery, leading to significant amounts of leakage over long trips.

- Pressurize gas (typically hydrogen)

- 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.

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.

What happens when the hydrogen runs out? Control rods to slow the reactor? Or eject it?

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.

Cooling is the Achilles heel of NTR, requiring massive radiators to remain functional over a long burn.

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.

The big problem with nuclear propulsion is always: What happens if a rocket with such an engine explodes at the start? What happens if it explodes in the upper atmosphere or accidentally reenters the atmosphere?

A "simple" nuclear engine might just contaminate a huge area in such a scenario. What does this new engine do, to prevent that from happening?

Its very simple. Build it in space. Use non-nuclear landing crafts.

You still need to get those materials into space somehow.

In small doses, not under 'explosion' condition?

u mine the materials in space

> The big problem with nuclear propulsion is always: What happens if a rocket with such an engine explodes at the start?

The big problem is that most people don't realize that isn't a problem at all.

We've been launching radiation-powered devices into space for decades in the form of RPS's, Radioisotope-powered systems. In one launch, the rocket exploded and even then the radioactive parts were unharmed[0]

And 'accidentally reenters the atmosphere' isn't really a thing. Once an object is in orbit above a certain height, it takes a lot of energy to get it out of orbit. The only alternative is waiting for the very faint amount of atmosphere to drag on it and bring it down- and anything about 1000km, that would be decades of waiting.

We launch the radioactive components safely, as we have lots of experience doing. We then assemble the nuclear engine in-orbit at an orbit above 1000km. Then we go to Mars!

[0]https://rps.nasa.gov/about-rps/safety-and-reliability/

Would there be any way to do a test flight of this technology without the in-orbit infrastructure?

Maybe we should just blow up a nuclear bomb in space just to make sure there isn’t any weird interaction of physical forces we didn’t anticipate for an explosion of that magnitude in space

do this on the orbital and moon bases please! so much space out there!

What if there is an accident and the nuclear reactor blows to pieces in the atmosphere? Will we have radio-active rain?

Not likely.

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.

For the record an RTG was recovered from the seabed after a launch failure intact - and then relaunched! https://rps.nasa.gov/about-rps/safety-and-reliability/

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 voyager probes from the '70s are both nuclear powered.

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.

Nuclear reactors don't explode? I suppose then Chernobyl was just another bomb and this: (https://en.wikipedia.org/wiki/Chernobyl_disaster) is just the official cover-up, eh? </sarcasm>

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.

No? Then what was it?

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.

> No? Then what was it?

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.

> It was not a nuclear explosion.

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.

Sounds like the Epstein Drive, perhaps version 0.1:

https://toughsf.blogspot.com/2019/10/the-expanses-epstein-dr...

The Epstein drive is great, but it only lets one travel to the private island.

...

Alright, alright! I’ll show myself out.

One would have thought the name was already "radioactive" when the Expanse books were written.

Those Sweathogs really got around!

Not even close both thrust & ISP wise. "Initial commit!" version of it at most. ;-)

> reducing Earth-Mars travel time to just three months

Does anyone know what it was reduced from?

SpaceX projects six months to get to Mars[1]. I think it’s always between 4-9 months with current tech due to orbital physics.

[1] https://www.spacex.com/human-spaceflight/mars/

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.

Depends on when you leave. Orbital conditions are optimal every 26 months, when it's a 260 day trip (~8.6 months).

More detail: https://news.cgtn.com/news/2020-07-19/What-is-a-Mars-launch-...

I still think plasma rockets like VASIMR could make the difference http://www.adastrarocket.com/aarc/VASIMR

Going to Mars faster isn't a good idea, but I hope Nuclear Thermal Rockets will develop further so the payload can be greatly increased.

Hydrogen cannot travel faster than the speed of light neither is it replenish-able in space. There is no magnetic field nor electric field propagating through spacetime. However there is a continuous gravitational field that can be tapped into. Even our solar system rotates around the milky wave using this gravitational field. It is of interest to pursue the development of 'Field Propulsion' engine.

Unfortunately, unless our understanding of physics changes radically, that won't be possible any time soon. Gravity is not a force, and we do not have the means to distort space. If we did, you might as well go for making a wormhole for the fastest trip times :)

Is something like Epstein drive (from Expanse) possible in real life?

The idea of magnetic acceleration for the exhaust particles is probably taken from VASIMR [1], while the specs are a tad higher than the NSWR someone else mentioned above [2]. Probably not outside the realm of possibility if we ever devise powerful enough energy sources :)

[1] https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magn...

[2] https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket

This is the same company doing the Micro Modular Reactor ? amazing

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.

bruv we've got enough people to work on setting up the lifeboats while bailing out the ship

I think it's easier to terraform Earth than Mars.

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.

Did we really need to go to the Americas? When first discovered by Europeans North America was nearly uninhabitable.

Did we ever need to criss the Bering land bridge? Or leave Africa?

When first discovered by Europeans North America was nearly uninhabitable.

When Europeans arrived in North America it was already inhabited.

Didn’t I cover both groups?

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 optimist in me hopes that if we can learn to live on mars then perhaps we can use that technology to live on earth sustainably.

Radiation levels in Mars were two to three times greater than at the International Space Station https://en.wikipedia.org/wiki/Mars_Radiation_Environment_Exp...

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.

blah blah blah. nobody figured out the radiation part yet. So... it's just blah blah blah

Why not use dilithium crystals /joke attempt

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.

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