hckrnws

> Sedna is expected to pass through the perihelion of its orbit in 2075--2076 and then move again away from the Sun. Considering the distances involved, a mission targeting the object would need to be launched "relatively" soon, especially if using conventional propulsion systems, which could require up to 30 years of deep-space travel.

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?

There's also Pulsar Fusion, a UK company currently building a Dual Direct Fusion Drive (DDFD). They claim:

> 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

When it comes to the UK space industry all I can think of is Skylon and Reaction Engines Ltd. Or more how they spent 20 years working on an engine that never left the ground until going bankrupt.

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

Yeah, the British space industry has struggled; principally with investment. Reaction Engines largely went under because they ran out of money and their investors declined to put more money in.

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 remember when Reaction turned down relocating to America in favour of some minor support from London. It was around 2014 and we all figured it was D. O. A.

Frankly it seemed like an idea that made no sense for multiple reasons. For one thing the density of atmospheric oxygen is a fraction of the density of liquid oxygen so it's hard to picture getting enough oxygen in the thing to make a difference. If you're liquifying it you're going to slow your rocket down by bringing O2 as well as 4 times as much N2 on board, then there is the weight of the liquification plant. Investing in Skylon is like investing in cold fusion.

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.

Henry Spencer on air breathing launchers (New Scientist, 2009):

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".'

https://web.archive.org/web/20090727013542/http://www.newsci...

(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.

See https://en.wikipedia.org/wiki/Lockheed_Martin_X-33

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.

And I strongly suspect Henry knew the "don't turn an airplane into a launcher" extended to using wings for landing and takeoff as well, although in 2009 that maybe wasn't quite as inescapable a conclusion as it is today.

I really wanted that thing to fly. Anyone know the fate of the IP?

It was ~15 years between the V-2 rocket crossing the Karman line to a human walking on the moon. 15 years from now we will have time for a 10 year break followed by another 15 years before we'd need to launch such a 10 year mission to be there by 2075-2076.

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.

> How close are they?

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.

"Easier" in this context is still ridiculously hard. Fusion rocket designs were first seriously researched 50 years ago and not a single one of the countless designs proposed since then has reached readiness for in-space use.

Note the economics might be better than for terrestrial fusion energy because you're not paying for watts you're paying for thrust and something like D-He3 has a great exhaust velocity.

> "Easier" in this context is still ridiculously hard

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.

So 75-76 for closest approach. How far away will it be in 2100? Given that orbit size I think we have some slack in the launch date.

And tragically, nuclear propulsion at NASA has been aggressively singled out for the axe so humanity will be counting on more advanced countries to finish that research.

Was that the fossil fuel lobby's doing?

It makes sense for USA to not want to be the only ones pushing a solution to a challenge like this. America is the trailblazer in this domain and that's impressive enough.

I always figured it was from Nuclear pearl-clutching and genuine fear about launch disasters. Especially after the various Apollo and shuttle disasters.

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.

> "probably a good idea to not have nuclear materials launching until that's been resolved entirely"

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 )

>fission reactors aren't hazardous until after they first start up (go critical)

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

It's not radioactive enough to matter.

Generally the sort of lightweight reactors NASA is looking at for space power use highly enriched uranium. U234 isn't particularly radioactive (it's lasted since the Earth was formed) and far less toxic than the hydrazine propellant our ships carry but it's a significant proliferation risk if it should all into the wrong hands.

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.

Actually spreading it out over a large area is much safer. What you don't want is a big hunk of highly enriched uranium landing somewhere. Not that it is very likely to harm anyone, but it becomes quite a nightmare to deal with it.

SpaceX let rockets explode because they're using chemical propellants and the consequences of that are low provided no one gets hit by debris.

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.

It's a scheme based on rotating magnetic field drive (RMF) of field reversed configurations. The claim is that they can preferentially accelerate and recover energy from 3He ions, greatly reducing DD fusion and associated neutrons. I question the recovery part (how is the entropy that is introduced by ion-ion collisions removed?), but do not have the expertise to fully evaluate the claim.

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

https://en.m.wikipedia.org/wiki/Sedna_(dwarf_planet)

They know its radius is ~1000km but interestingly, there is no way to determine its mass without a flyby or other gravitational interaction. I guess you could swag it by using the lunar density, which gives ~~~ 10^22 kg.

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

Wouldn't it's composition be of more ice and rock (like pluto), therefore lower density than the moon?

That's a reasonable assumption, but given Sedna's unusual orbit, its origin could also be quite different from Pluto's.

In the wikipedia post you are replying to has the chemical composition of the surface of the planet, obviously we can't know what is beneath that, but to me, indicates this is closer to Pluto than it is to our Moon.

> 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]

Why dont they just launch a string of hundreds or thousands of tiny mesh probes out of a canon over a longer period of months or years? They dont need to be powerful if you have a big mesh network and each probe would only need to cost about what a cell phone does.

> Why dont they just launch a string of hundreds or thousands of tiny mesh probes out of a canon over a longer period of months or years?

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

Check out the antenna size, path loss, link budget and modulation and FEC that were required for new horizons to send back data at the equivalent of about 2400 bps. Tiny cubesat size things have no hope of sensing useful data to earth.

I'm not talking about launching a cubesat, I'm talking about launching a mesh network of thousands or millions of mass produced devices half or a quarter the size of a cubesat.

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.

I was surprised there were no references to past nuclear (fission) efforts, including a long test (more than 12.5 minutes) at 4000 megawatts of Pheobus 2A.[1]

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

Hydrogen is really the only propellant that makes sense for a nuclear thermal rocket. A nuclear reactor can't get substantially hotter or higher pressure than a chemical rocket engine, the reason it offers high specific impulse (basically efficiency) is because since you don't need chemical energy from the propellant to heat the propellant, 100% of your thrust can come from low molecular weight propellants (ie hydrogen). Helium will also give you better performance than chemical rockets, but substantially worse than hydrogen, and it's even more deeply cryogenic. Anything heavier than helium is going to provide little to no advantage over a chemical rocket, certainly nothing to justify the atrocious thrust to weight ratio and the extreme engineering challenges.

Already in orbit is OTP-2, which has 2 novel drive systems, one based on non-Newtonian thrusters, and the other based on an ION drive.[1]

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...

*IVO – Quantum Drive Propellantless Thruster - *The objective of the IVO Quantum Drive is to test the system in the LEO environment and qualify the drive’s ability to provide thrust utilizing proprietary quantum technology with no required propellant. Estimated Thrust: 1.75mN.

Why does this give me EM-Drive vibes? Haven't we established that some kind of propellant is required for conservation of momentum?

Similar thing... it shouldn't work, according to the established laws of physics, though it does seem to work on the ground. If it actually does work in space, then our rules of physics need tweaking.

Already in orbit around Earth, notably. Not Sedna.

Non-Newtonian drives have to prove they work outside the influences of a laboratory, if they work in low earth orbit, they should work anywhere. The Semi-Major Axis Altitude (SMAA) is a great proxy for orbital energy, and if they can make that number go way up, we should all take note, and start looking for new physics.

This direct fusion drive is a really interesting concept. Maybe something like this could be used for interstellar travel in a century (or five), it is very encouraging that there is active research on it. ~5kg of thrust is not a lot, but over time...

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?

> 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.

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.

I once spoke to Freeman Dyson at a book signing and asked him if Orion would work. He said he thought it would. And I asked him if it should be launched. He said probably not (IIRC due to the amount of radiation that would be put into the atmosphere).

It is almost the epitome of steampunk romance. Launch an entire mid-20th century city and economy into space! And it might even work!

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...!

He also made the interesting point that pretty much every big engineering project kills people.

> Gives everyone something way better to do with all those nuclear bombs

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.

Yeah, if I'm being really honest, I don't want to give anyone an excuse to put a 1000+ nuclear bombs in orbit. Plus the few dozen you'd have to detonate in quick succession to even get it above the karman line.

My favorite SF along those lines is King David's Spaceship, by Jerry Pournelle.

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

I should re-read Footfall, by Larry Niven. Quite a few banger lines in there.

The electron beam ignition they talked about doesn't work. Heavy ion probably does

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.

Project Orion was the promise of my youth [70/80s]. It speaks to both the technological courage and the philosophical optimism that once characterized space exploration — and how that momentum seems to have faded. By all accounts, it was technically feasible. And yet...

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.

Roundtrip Interstellar Travel Using Laser-Pushed Lightsails

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

> This paper discusses the use of solar system-based lasers to push large lightsail spacecraft over interstellar distances. The laser power system uses a 1000-km-diam. lightweight Fresnel zone lens that is capable of focusing laser light over interstellar distances. A one-way interstellar flyby probe mission uses a 1000 kg (1-metric-ton), 3.6-km-diam. lightsail accelerated at 0.36 m/s2 by a 65-GW laser system to 11% of the speed of light (0.11 c), flying by a Centauri after 40 years of travel. A rendezvous mission uses a 71-metric-ton, 30-km diam. payload sail surrounded by a 710-metric-ton, ring-shaped decelerator sail with a 100-km outer diam. The two are launched together at an acceleration of 0.05 m/s2 by a 7.2-TW laser system until they reach a coast velocity of 0.21 c. As they approach a Centauri, the inner payload sail detaches from the ring sail and turns its reflective surface to face the ring sail. A 26-TW laser beam from the solar system, focused by the Fresnel lens, strikes the heavier ring sail, accelerating it past a Centauri. The curved surface of the ring sail focuses the laser light back onto the payload sail, slowing it to a halt in the a Centauri system after a mission time of 41 years. The third mission uses a three-stage sail for a roundtrip manned exploration of e Eridani at 10.8 light years distance.

Very cool.

This is very interesting. Apparently beam collimation is much less of a show-stopper than I would have assumed.

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...

See: https://en.wikipedia.org/wiki/Breakthrough_Starshot

The Mote in God's Eye

I guess this will be the Niven-Pournelle thread.

The easiest way (perhaps the only practical way) to favour the aneutronic reaction is to run a helium-rich mixture. The trade-off is lower power density.

Based on my experiences with Kerbal Space Program, this object seem to be almost being pushed off from solar orbit. Given its 'small' size, how much energy would be required to push it off the solar system?

> relatively soon

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.

Very fascinating mission idea. Given how Sedna reaches so far away (>500AU), I wonder if the flyby would also reveal some details about conditions that distant. Maybe the surface contains some unexpected molecules that could shed light on its origin and what it's like that far out.

Sound like something out of 3 Body Problem