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Comment by crazygringo | Hacker News Reader

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

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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

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.

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.

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

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