Feasibility study of a mission to Sedna – Nuclear propulsion and solar sailing

Sedna's perihelion is ~76 AU - more than twice as far as Pluto, which took New Horizons nearly a decade to reach.

Sedna's apehelion is over 500 AU.

> The Direct Fusion Drive rocket engine is under development at Princeton University Plasma Physics Laboratory

Is it ... is it actually working? How close are they? And even if they get it to work next year, will it be something well-engineered & reliable enough to send it into space for 10 years and expect it to work?

In any case, it certainly cannot be ready next year, and would require large amounts of 3He.

> Modelling shows that this technology can potentially propel a spacecraft with a mass of about 1,000 kg (2,200 lb) to Pluto in 4 years.

They're apparently targeting an in-orbit test in 2027. Even if this were to slip to 2030, and becomes commercially available in 2040, I expect that would be plenty of time for a rendezvous with Sedna's perihelion

This sounds significantly more feasible than nuclear pulse propulsion ("project orion" style) which I used to think was the only feasible approach to get to another star.

One thing that was unclear from the paper to me: How does the fusion drive "pick" D/He3 fusion over D/D? Can this be "forced" by just cranking the plasma temperature way up? Or do you still just have to deal with a bunch of neutrons from undesired D/D fusion?

https://ia800108.us.archive.org/view_archive.php?archive=/24...

https://www.wolframalpha.com/input?i=4.189%C3%9710%5E9+km%5E...

Not very. That said, DFD is a technology with tremendous moonshot potential.

Fusion propulsion is inherently easier than fusion power on Earth because you don’t have to worry about converting heat to electricity and the breakeven threshold is far lower; depending on the mission, even Q < 1 could be fine.

Was that the fossil fuel lobby's doing?

But I don't see us putting a a 1000 kilometer lens into orbit anytime soon, and that multi-terawatt (sustained!) laser system sounds like a bit of a headache, too...

Hopefully this time round it goes a bit better than that.

If the DFD takes 10 years to get there it means it would need to be launched in 40 years. That's quite a timeline.

Amazing that an organization can keep budgeting and planning for such a long project.

My hope with Pulsar Fusion is that their existing thruster business provides the necessary revenue to both keep them solvent, and attract continued investment, until they're able to get their Fusion Drive off the ground.

I still carry a torch for project Orion, it's impossible to not love.

* Feasible 50 years ago, not 50 years from now.

* No ultra lightweight fancy space age materials, steel and lots of it.

* Seriously, lots of it, let's launch a battleship to to Mars,

* or Jupiter,

* or Alpha Centauri.

* Gives everyone something way better to do with all those nuclear bombs they have laying around.

The counterpoint there is it gives lots of reasons to make so many more, increasing proliferation worries.

However, there's an SF novel that just came out that features nuclear pulse: Fenrir, by Ryk Spoor and (posthumously) Eric Flint. I enjoyed it.

Very cool.

https://en.m.wikipedia.org/wiki/King_David%27s_Spaceship

It was bad enough that Richard Branson discredited private orbital spaceflight with the overly long development process for a vehicle that made the Space Shuttle look like a paragon of safety and low costs -- Skylon was so much worse.

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

but the accelerator needs like 100 barrels that are each 1 km. Maybe you can build a generation starship with that but whatever it is it's going to be big.

Though with how SpaceX has been blowing up rockets left and right, probably a good idea to not have nuclear materials launching until that's been resolved entirely.

Boca Chica beach is a mess now, I can only imagine what new Fallout installment we'd get if South Texas became irradiated from a failed launch.

This isn't an issue at all: fission reactors aren't hazardous until after they first start up (go critical), which in the space electric-propulsion context means after (if) they've successfully launched, and are no longer in the vicinity of Earth.

At any rate, China is apparently[0] moving in this direction, regardless of what the US does.

[0] https://www.scmp.com/news/china/science/article/3255889/star... ("Starship rival: Chinese scientists build prototype engine for nuclear-powered spaceship to Mars" (2024)) (mirror: https://archive.is/sGUJr )

I guess this will be the Niven-Pournelle thread.

> Detailed spectroscopic analysis has revealed Sedna's surface to be a mixture of the solid ices of water (H2O),[15] carbon dioxide (CO2), and ethane (C2H6), along with occasional sedimentary deposits of methane (CH4)-derived,[16] vividly reddish-colored organic tholins,[15] a surface chemical makeup somewhat similar to those of other trans-Neptunian objects.[17]

This is only true if the fission reactor's fuel isn't scattered over square kilometers after a launch failure.

But, yeah, you probably don't want to be launching these routinely. People generally badly underestimate the number of nuclear explosions that have been set off on Earth and overestimate the badness of nuclear explosions. Putting one or two of these into orbit might be justifiable. It's certainly not a bad emergency plan to have in your pocket in case of emergencies. But you still certainly wouldn't want an entire industry routinely lighting these things off.

Still... the romance of it all...!

Of course there was 'the shadow of the Bomb'. From bold, almost reckless experimentation (Mercury, Gemini, early Apollo, things shifted to safety-optimized, cost-constrained engineering. And there was Cost and Politics; the post-Apollo world didn’t want to colonize the solar system. It wanted low Earth orbit, and safe returns. Budgets followed.

Kinda sad.

Edit: The latter is "Fusion enhanced"[3]

  The company’s the FireStar Drive uses is a water-fueled pulsed plasma thruster that uses a form of aneutronic nuclear fusion to boost its performance.

[1] https://www.nanosats.eu/sat/otp-2

[2] https://celestrak.org/NORAD/elements/graph-orbit-data.php?CA...

[3] https://www.aerospacetestinginternational.com/news/space/roc...

But yeah, it's not dangerous like the P238 in a radioisotope thermal generator (RTG). To put off enough heat to power a spacecraft just through natural decay you need something ferociously radioactive.

Perhaps there are some solid or non-cryogenic liquid fuels that could take place of the liquid hydrogen and make fission based systems far more feasible in the near term.

[1] https://en.wikipedia.org/wiki/Project_Rover#Phoebus

https://www.newscientist.com/blogs/shortsharpscience/2009/03...

'Trying to build a spaceship by making aeroplane fly faster and higher is like trying to build an aeroplane by making locomotives faster and lighter - with a lot of effort, perhaps you could get something that more or less works, but it really isn't the right way to proceed. The problems are fundamentally different, and so are the best solutions.

As Mitch Burnside Clapp, former US Air Force test pilot and designer of innovative launcher concepts, once commented: "Air breathing is a privilege that should be reserved for the crew".'

Absolutely. I’ve just noticed that a lot of people think, correctly, that fusion power is hard and space is hard so doing them together is stupidly difficult. Not so in this application—the relaxation of requirements on fusion outweigh the difficulties of doing it in space.

Put another way, the dollars going into fusion power might be better spent on DFD.

It's bizarre to suggest that the same strategy would be used with nuclear materials onboard. Developing the "can not fail" rocket is the sort of thing NASA does well, and kind of highlights how we've squandered them.

(The original link says "Page is Gone")

And here's some more quoting

Could a single-stage-to-orbit spaceship, something that could operate rather like an aeroplane, be built with just rocket engines? Well, actually, yes. In the 1980s, NASA and the US Air Force spent about $2 billion trying to build the X-30, a single-stage spaceship powered by scramjets (with help from rockets, of course). It never flew. At the same time, for comparison, NASA's Langley Research Center studied building a single-stage pure-rocket spaceship. The results were interesting.

The pure-rocket design was more than twice as heavy as X-30 at takeoff, because of all that LOX. On the other hand, its empty weight - the part you have to build and maintain - was 40% less than X-30's. It was about half the size. Its fuel and oxidiser together cost less than half as much per flight as X-30's fuel. And finally, because it quickly climbed out of the atmosphere and did its accelerating in vacuum, it had to endure rather lower stresses and less than 1% of X-30's friction heating. Which approach would be easier and cheaper to operate was pretty obvious.

The Langley group's conclusion: if you want a spaceship that operates like an aeroplane, power it with rockets and only rockets.

There have been some other discussions of this lately, but I would say the pursuit of SSTO resulted in a lost decade for spaceflight in the 1990s.

SSTO is just barely possible, the problem is that you have a big rocket that carries a tiny payload so you are driven to exotic engines, exotic materials, and various risky technologies.

If Musk had any good idea it was not only falling back to two-stage-to-orbit reusable rockets but also recognizing that it was worth just reusing the first stage. A SSTO gets closer to aircraft-like operations in that you don't need to stack two stages on top of each other, but given how much TSTO improves everything else it's probably worth just optimizing the stacking.

The real question "is there actually fund this engine and mission to bring that to completion in the next 40 years" than whatever the completion and reliability is today.

The word 'just' is doing a whole lot of work in that sentence!

New Horizons, to use your example, weighed a thousand pounds and used a 2 meter dish transmitting at something like 12 watts to compensate for the fact that the receivers are billions of miles from earth and hidden beneath a blanket of RF noise. The inverse square law can't really be beaten at that kind of distance so everything becomes inefficient by design.

If we can pick up that tiny 12 watt whisper of a signal from billions of miles away, surely we we could design much lower power omnidirectional signals that relay between mesh nodes closer together using far less power?

Imagine a string of probes that are all within a few thousand miles of each other with clear line of sight. Yes, we might need six million of them to cover that same distance, but if they were cell phone sized devices produced using what we've learned about consumer electronics it should be feasible to just keep launching them forever, for a few hundred bucks apiece, until we eventually build a large network that could assemble high resolution data by combining multiple sources.

We keep trying to fight the rocket equation, but that's not a battle that can be won. Mass is always going to be the limiting factor for space exploration, so maybe we can just start launching lots of intelligent low mass things regularly instead of the occasional big dumb thousand pound lump of metal.

First, you can't say that any of this propulsion tech is remotely mission-ready. It's all very speculative. There's been no real-world testing of any kind. You'd need to at least test-fire it in orbit and prove a solar sail in particular. Any kind of nuclear propulsion adds whole new levels of proof-of-solution (yes I know RTGs exist but those are technically quite simple being just radioactive decay rather than something utilizing fission or fusion).

Second, it's not clear what kind of speed this could reach. At New Horizons speed, assuming you can find the right launch window, you're looking at 18-25 years transit. That's a long time for a probe to survive.

If you do adopt a solar sail, what happens to it over 20+ years? What happens from long-term damage of hitting dust and micrometeors? Could you need to course corret if it gives uneven thrust?

And all this for... a flyby. Obviously Sedna is too far and too slow for anything else. Just like Pluto.

But if we're talking 2j0-30 year missions, I'd rather send an orbiter to Uranus. About 20 years is I believe the time frame for an orbital insertion to Uranus. IIRC Neptune is closer to 30.

I think we're also getting better and faster at iteration and design. CAD, modelling, even wind tunnels from 50 years ago made a massive difference over jumping off a cliff with a glider for tests.

I guess my point is, I don't see 50 years as validation of it being hard. And some of those designs were likely dismissed due to tech limits at the time.

Each one needs whatever sensors, but more importantly, the auxiliary stuff to last years or decades in transit: in particular power supplies, heaters (=power!) if you can't make your electronics survive constant cryogenic temperatures, as well as comms amongst themselves to organise the mesh and high-gain comms back to Earth.

Maybe the worst of that could be solved with a nuclear power supply and then it's basically "just" radio and software design.

I also don't think you'd use onmidirectional mesh comms, you can get a lot of milage (literally) out of a phased array that can steer the beam at each target, plus it also becomes a bonus multistatic radar network.

1) Orbital velocity is FAST. VERY fast. In KSP orbital velocity for a low orbit is about 2,200 m/s. For earth its about 7,600 m/s 2) An air-breathing engine, by definition can only be used inside the atmosphere. 3) You will struggle to get anywhere close to orbital velocity while still in the atmosphere, due to drag, and heating.

At best, your air-breathing engine will only get you to a small fraction (less than 1/4th) of orbital velocity. Then you will have to a) climb higher, and b) use a different engine to accelerate to the required orbital velocity.

Yes, you will potentially save some weight by not having to carry oxidizer for while you gain that first 1/4 or so of your final velocity. But once your air-breathing engines, and wings and everything else are useless, you still have to carry their weight

55 years from Apollo 11 to Katy Perry

While it would be preferable due to the immense weight, you can either lift it by conventional means or possibly build it from local resources in the long run.

Once in space Orion is much less problematic & might be even easier to dock and maintain than normal nuclear thermal rockets, where the unshielded reactor will just put out insane amounts or radiation in all directions outside of its shadow shield.

Correctly engineered pusher plate should be much easier to deal with.

Would need around 1km/s on top of its average orbital velocity to escape, but the mass is probably roughly in the 10^22kg range, so thats like 10^28 Joule.

Significantly more than a billion of the biggest nuclear bombs we built.

I suppose that's my point. It's really just a series of maybe-not-that-easily solvable engineering problems, and it would allow us to not only explore further but to do it at a relatively low cost and with the ability to "upgrade" the network gradually as each generation of probe improves. More importantly, it would allow us to finally get around that pesky rocket equation and do it cheaply enough that we might actually get political buy-in.

> nuclear power supply

This was my exact thought, a small RTG power supply in each could provide enough power for hundreds of years with no moving parts. Not enough for billion mile transmitters anymore, but now they don't have to be.

> phased array that can steer the beam at each target

That's a great idea, and like most of the individual pieces of the plan it's kind of a thing we already know how to do. Sure, you could dedicate the next decade to solving it well, but you COULD solve it today with variations on off the shelf systems.

Really, the power source and antenna design are just a few of several hundred (thousands?) of engineering problems that would need to be solved, but all of the engineering challenges I can think of are solvable with variations on current tech.

The only reason it isn't being done is that nobody is doing it.

However I believe your point holds more generally for nuclear-based space propulsion. That we fear "NUKULAR!" by about two to three orders of magnitude more than is justified has kept us from having halfway decent space travel for at least a good two decades, most likely. There are a number of nuclear propulsion mechanisms that would make things like going to Mars halfway feasible instead of flights of fancy, or doing science missions in months instead of years or even decades, but people hear that you're thinking of lifting nuclear material into space and all rationality goes flying out the launch window. Nuclear is so bad that it basically reaches out through outright magic and guarantees explosions and there's no conceivable amount of preparation that could be done in people's minds to prevent the evil radiation!!!!1! from escaping and eating people's puppies.

The funny thing is that even so quite a bit of nuclear material has been lifted into space, but hearing that doesn't make people go "oh, well, maybe it's less dangerous than I thought".

I mean, I know this isn't the safest stuff in the world but I sure hope all that anti-nuclear propaganda in the 20th century actually did help prevent nuclear war because it has certainly had massively negative impacts in energy generation, environmental damage, space exploration, and who knows what else.

I also suggested a variation of this to him. But (IIRC) he said Orion was pretty pointless if you didn't use it to lift you out of Earth's gravity well.

Best estimates are that Chernobyl and Fukushima killed maybe ~5,000 (including long term).

The 1975 Banqiao Dam failure in China resulted in ~171,000 deaths.

We saw the hyperreactivity over Fukushima. I even know some very educated people who should know better like not wanting to eat any seafood caught in the Pacific.

A nuclear reactor in space would require an enormous heat sink to get useful energy out.

As an idea, yeah. But if somebody actually tried to build it, the entire world would oppose for very good reasons.

Still, it's something that maybe we should build on space (outside of Earth's magnetic field).

They don't launch space probes out of cannons because they don't make it out of the atmosphere. According to [1], muzzle velocity of a cannon is about 1685 ft/sec, which is 0.51 km/s. Delta-v to orbit is around 10 km/s. This is a feature, though, because launching your cannon shell into orbit means it isn't hitting it's target.

But let's suppose you have some propellant that is 20 times more potent. A cannon imparts all the energy at the beginning, with the acceleration happening as the expanding gasses push the projectile out of the tube. Assuming that the probe survives the initial explosion (unlikely), it is going to accelerate to 10 km/s very rapidly. Once calculation [2] put the g-force on a cannon shell to be 15 g, but lets say 10 g to be conservative. So we need 20 times more acceleration, so 200 g. Even if your probe is not smooshed in the acceleration, it is unlikely to be functional. (Note that, in comparison to cannons, rockets avoid this problem by providing the acceleration over a long period of time)

Now if you managed to engineer it for 200 g, air friction is going to burn it up. We know this because when spacecraft come down they have to lose all the velocity they got going up, and they tend to burn up. Heat shielding is almost certainly going to put you over the weight limit.

What, you say? This is a space cannon? Okay, well leaving aside how this cannon is going to burn the propellant without oxygen, the delta-v to Pluto from LEO is 8.2 km/s, so Sedna will be a little bit more. This is still an order of magnitude larger than the cannon, and still has acceleration problems. Plus, you had to use a rocket to get the payload to the cannon, so putting a second stage on the rocket.

You still have the issue that it's going to take a couple of decades to get there, which is what this paper is trying to address.

[1] https://www.arc.id.au/CannonBallistics.html

[2] https://math.stackexchange.com/questions/3249185/calculate-g...

More importantly, I would like to point out that while all of your concerns are valid, many of those problems were already solved in the 1960's. Project HARP[1] was able to use a 400 lb projectile to launch a 185 lb payload to a height of 111 miles... in 1966. We don't need anything close to 185 lbs of payload.

You'll note that 111 miles up is considered suborbital space. HARP was built mostly of 1950's era technology, and cost between $1000-$3000 per payload to fire. It had a 16" barrel and could be reloaded in about an hour. The payloads were encapsulated within a "sabot" to protect them, and the sabot seemed to do it's job, because primitive electronic instrument packages were deployed without being destroyed and weather balloons were deployed with success.

The long term plan for that project was to add a second stage which would push the payload into orbit, or beyond. There is no reason to believe it wouldn't have worked, but the Vietnam war happened and people lost the taste for funding space exploration. It was shut down. The enormous gun is still there, rusting where it was abandoned after firing nearly 100 ballistic payloads into suborbital space

Now, if we could fire ballistic payloads into suborbital space in 1966 what do you think we could achieve today? Honestly, the engineering isn't even that difficult, it's just a matter of figuring out how to pay for it. The rest is an incremental improvement over something we could already do in the 60's.

Sure, I'm glossing over a ton of minor issues (like the entire second stage), but those problems are also basically solved and we've learned a few things in the last 60 years. I not only think it's possible, I think someone should give it a shot (pun intended).

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