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Comment by sillysaurus3 | Hacker News Reader

Happy to see this getting some attention. It's seemed like one of the only viable propulsion methods for interstellar purposes, though nuclear is unfashionable at the moment.

I remember seeing a chart showing that it was possible to reach other stars using a technique like this. Unfortunately the problem is slowing down. You could get there, but you'd be going too fast to stop. And since the more fuel you carry, the harder it is to reach that speed, it ends up becoming completely impractical to carry enough fuel to both get there and slow down.

Maybe it'd be viable to stop by smashing into a planet, but it's hard for anything to survive that kind of impact force.

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> nuclear is unfashionable at the moment

It's more than that - this isn't just hippies, it's every major power on the planet. To fly Orion you have to put hundreds of nuclear explosives in orbit and pinky-swear that you're not going to use them to obliterate your enemies. Nobody is going to accept that.

Orion's a fun paper rocket but will never be more. Engineers and they "yay nukes!" crowd will drag it out every couple of months for a repost, though.

If you read the book (by Freeman Dyson's son) or even the wikipedia page, you'd know the bombs were quite specialized. And it would certainly be possible to allow some verification (the trigger would not need inspection, that would probably be the really sensitive part, from a technology secrets point of view).

[Edit: The bombs would be directed, throwing light weight molecules up against the pusher plate.]

Also, there is no direct lack of ways to distribute nuclear devices to capitals today, for the countries able to build an Orion. Compare that with the possibility to launch enough material to e.g. start on getting an industrial infrastructure going outside the atmosphere.

The "directed" bomb designs used tungsten - in "The Curve of Binding Energy" I seem to remember there is a comment that they were spectacularly good at digging tunnels but I don't know if that was a theoretical prediction or an empirical result.

I meant that the material thrown at the plate were planned to be plasma of more lightweight materials, iirc. (That is different than how to make an atomic bomb directed.)

I haven't seen any real details of directed bomb design discussed. Thanks for the pointer to that book.

If you consider that it was contemporaneous with plowshare, that makes sense - likely a degree of shared research.

Any mechanism that can propel a starship to 0.13c is going to make your day unpleasant if it is pointed at things you have a particular fondness for. Favourite pets, cities, aircraft carriers, that sort of thing.

It is a theoretical "can". :-) It will be a long time before anyone use a fraction of C inside the solar system.

Interstellar flight is a problem after we have gotten to a reasonable cost/kg for both launching from the Earth and for interplanetary travel.

I doubt there will be a pressing need for getting close to even 100 km/s in the inner solar system for decades. (Yes yes, "640K is enough for anyone". :-) )

(Consider: An AU is 150 million km. 100 km/second means 8.64 million km a day; 2 AUs in 35 days! Then you have to lower the speed, even more energy.)

Launching an Orion from the ground is probably not realistic after the 1960s either. It would have the problem with bombs and EMP killing satellites. I've seen suggestions to coast through the sensitive zone without using bombs, but haven't seen any discussion about how many Gs it would mean...

"I doubt there will be a pressing need for getting close to even 100 km/s in the inner solar system for decades"

Quoting https://en.wikipedia.org/wiki/Solar_Probe_Plus :

> Solar Probe Plus or Solar Probe+, previously NASA Solar Probe, is a planned robotic spacecraft to probe the outer corona of the Sun. ... As the probe passes around the Sun, it will achieve a velocity of up to 200 km/s (120 mi/s) at that time making it the fastest manmade object ever, almost three times faster than the current record holder, Juno. ... Launch date: July 31, 2018 (planned).

He he, point. But a probe doing multiple gravity assists to get close to the sun is different from going from point A to point B -- and stopping at B. :-)

> To fly Orion you have to put hundreds of nuclear explosives in orbit and pinky-swear that you're not going to use them to obliterate your enemies.

To reiterate berntb's point: Every nation who has the ability to do this already has ICBMs that can deliver nukes anywhere on the planet in 20 minutes. So what difference does this make?

True, but we still have a treaty that prohibits bombs in orbit, because they'd reduce the warning time enough to threaten deterrence.

Maybe one day we can launch Orion from Mars.

Drive it to Earth, have lots of bombs left.

From a strategic point of view, what's the difference between building on Mars and building it in orbit? Obviously there is a difference because the hostile faction has to seize control of it and move it to Earth, but once they do that, the calculation becomes the same. Do you commit to obliterating them the second they start moving it to Earth?

Best case scenario, Mars is 55 million kilometres from earth. Even using a ridiculous engine going full blast the whole way (in which case your payload will probably disintegrate when it hits the atmosphere) you won't get the transfer time under a month. If you want to actually deliver your payload in a useful manner you'll need twice that at the very least. 2+ months is a long lead time, and the engine you'll need will make the craft impossible to miss.

What's the difference to building it in orbit? Building it in earth orbit you're dropping it from under 500km and your assembly makes 10~20 orbits a day, you can deliver it quickly at pretty much any time.

The former allows countermeasure, the latter not really.

It's not just the war aspect that makes this un-viable. It's the failure modes, several of which include the destruction of life on earth.

I'm talking about just flying the entire Orion ship to Earth, loaded with propellant bombs. Sure, you can try to take it out during the journey, but that may be complicated by it being the highest g ship in existence for the Mars-Earth distance. Once/if it arrives, you have the same strategic problems you have if you build it in orbit.

> I'm talking about just flying the entire Orion ship to Earth, loaded with propellant bombs.

Er… yes?

> Sure, you can try to take it out during the journey, but that may be complicated by it being the highest g ship in existence for the Mars-Earth distance.

That's not really a complication, it's still taking months to make the trip, and while it might be "the highest g ship in existence" that's linear acceleration, it's not a nimble ship performing a tailside in space to avoid projectiles.

> Once/if it arrives, you have the same strategic problems you have if you build it in orbit.

With the pretty damn big difference that you've had several months head start knowing the think was incoming at high speed instead of it just dropping of your head more or less instantly.

Er… yes?

Sorry, I didn't understand why you were talking about bothering to do that and then crashing it into the atmosphere.

If you're crashing it into the atmosphere you're not delivering anything, the ship and its payload will just disintegrate, possibly bouncing back in the process. Even if the payload does detonate on impact, the only purpose an explosion at atmospheric boundaries would have would be EMP.

Galileo safely aerobraked into Jupiter at 47km/s. It takes one hell of a heat shield, but it can be done.

> It takes one hell of a heat shield

That's quite the understatement, the galileo probe was half heatshield by mass. And it took a 230g hit to the face.

Indeed, I didn't understand why you thought I would suggest something pointless. I wasn't suggesting it.

If you want to argue that crashing into the atmosphere would be an inevitable result of the trip, okay, but I don't understand what else it would have to do with my initial comment.

> If you want to argue that crashing into the atmosphere would be an inevitable result of the trip

That's not what I'm arguing FFS, I'm saying that'd be the result of going with the fastest possible trip, which would still leave the target significant time to set up countermeasures.

Here's the rest of the paragraph of my original answer since you apparently stopped at the first period:

> If you want to actually deliver your payload in a useful manner you'll need twice that at the very least. 2+ months is a long lead time, and the engine you'll need will make the craft impossible to miss.

The point is that no matter how you slice it, "the difference between building on Mars and building it in orbit" is the target gets a warning several months in advance in the former case, a few minutes or seconds in the latter. One gives ample opportunity to set up and deploy countermeasures, the other not so much.

So why not build the countermeasure before you build it in Earth orbit? what's the difference then.

[deleted some pointless back and forth]

Because there's no way you'll be able to detect the launch and deploy your countermeasures, even assuming said countermeasures actually work which is unlikely: it's doubtful we can counter ballistic missiles as-is, a payload starting from orbit can launch much closer to the target (rather than half a world away) and is not fighting gravity for half the trip.

Since Orion doesn't need to accelerate constantly, it's in principle quite possible to turn it and fire bombs in the direction of any interceptor, and the thick pusher plate makes it actually pretty durable from some aspects. What happens when two Orion ships duel, I don't know, but shooting it down isn't quite as simple as for most other spacecraft.

> Since Orion doesn't need to accelerate constantly

If it's coasting along it's significantly increasing transit time and thus available window to consider and prepare counter-measures.

> it's in principle quite possible to turn it and fire bombs in the direction of any interceptor

Sure but that means less payload to deliver and even more time for the target to prepare counter-measures.

> and the thick pusher plate makes it actually pretty durable from some aspects.

No matter how thick your pusher plates are they're not going to stand up to the kind of kinetic impact involved.

Empress Sato!!

Even worse: the EMP from an Orion launch would fry a lot of satellites. It's not a practical design for use anywhere near... anything.

You could launch one from the far side of the Moon, but if you have that kind of space presence there are better engine designs. Some have been posted elsewhere in this thread.

Yes, Orion looked a lot more attractive back before civilization became reliant on huge quantities of sensitive electronics in near-Earth space.

Stupid that satellite electronics don't have more radiation shielding. We can refrain from nuclear detonation in orbit all we want, but the sun could still EMP us at any moment. Maybe it's expensive to lift heavy shielding into orbit, but it's more expensive to deal with the consequences of satellite systems outages and repairing/replacing broken equipment.

Anything that can accelerate a starship to a velocity of ~0.1c can have devastating consequences if pointed to earth. Just like planes can be used to destroy skyscrapers. Chicken littles notwithstanding, having a much faster mode of transport is worth the risk.

It may be worth the risk but that doesn't change the fact that what you refer to as "Chicken Littles" represent power players in an intractable political dynamic. The only way this kind of cooperation happens is when humans move past current notions of nation states and begin to think strategically as a species.

Not if an independent organization (think a Corp, not a gov) is the party responsable. They would have no incentive to do such a thing, in fact they would have a negative incentive.

Too bad nuclear tech is basically too regulated for any non governmental organization to experiment inexpensively, not to mention the other nontechnical issues.

On a side note, this is why Independence Day and other interstellar alien invasion movies don't make much sense. The amount of resources necessary to move between solar systems in an even slightly reasonable timeframe is orders of magnitude larger than what you need to obliterate all life on the planet in just a few seconds. The idea that you could meaningfully fight back against an interstellar traveler is silly.

I remember seeing a chart showing that it was possible to reach other stars using a technique like this. Unfortunately the problem is slowing down.

So this is like being concerned how you will spend all that money, once you make it :-)

There's a lot of discussion on interestellar travel, but "we" have almost frozen solar system travel. Yes, there are very nice probes out there doing important work, but manned vehicles are limited to keep ISS's lights on.

The "impossible" tech needed to reach the stars will most probably be discovered or refined while conquering the solar system.

Big sailing ships were developed along four centuries of sailing. The analogy may not be perfect, but I think it's eye opening.

I agree with you in innovation terms, but there's a big economic difference: sailing used free energy and therefore could be developed over the long term by many actors. There are huge energy cost barriers to solar system travel which require bigger bets on economic return to be viable

This was considered by project Orion.

Basically you turn the vehicle around and use the propulsion to slow you down. It would use a heap of fuel to do this but it was possible.

Edit, also, great find of this by the poster.

But since you already used a bunch of your pulse propellant during acceleration, this is partly compensated for by the vehicle weighing less.

In "Death’s End", that’s a science fiction book by Cixin Liu, they use a variant of nuclear propulsion where the nukes are pre-distributed throughout the Solar System instead of being carried by the ship itself. This could free up even more mass and make room for an even bigger magnetic sail to be used for deceleration. Later in the same book, and much later in the fictional timeline, they build a circumsolar particle accelerator, which is easier than you would think [1], and this could be used to produce massive amounts of antimatter in space with a lot more propulsion power than conventional nukes. So I remain optimistic. It’s by the way a remarkable series of books that will freak out a lot of hackers, I recommend sticking with the series to the end.

[1] https://books.google.dk/books?id=A_1oCAAAQBAJ&pg=PA451&lpg=P...

In order to have fuel to do that, you'd need to accelerate it to begin with. It ends up being an exponential increase in the quantity of fuel required, far more than could be carried.

The chart from the paper was pretty convincing. I'll see if I can dig it up. Making the explosions more efficient can only get you so far. I'd prefer to be optimistic about it, but the math was showing that even with an ideal ability to convert mass into energy, it would require too much fuel, to put it mildly.

According to the wikipedia page on Project Orion:

Later studies indicate that the top cruise velocity that can theoretically be achieved are a few percent of the speed of light (0.08-0.1c).[17] An atomic (fission) Orion can achieve perhaps 9%-11% of the speed of light. A nuclear pulse drive starship powered by Fusion-antimatter catalyzed nuclear pulse propulsion units would be similarly in the 10% range and pure Matter-antimatter annihilation rockets would be theoretically capable of obtaining a velocity between 50% to 80% of the speed of light. In each case saving fuel for slowing down halves the max. speed. The concept of using a magnetic sail to decelerate the spacecraft as it approaches its destination has been discussed as an alternative to using propellant, this would allow the ship to travel near the maximum theoretical velocity.[18]

https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...

The referenced paper [18] is informative: http://www.space-nation.org/images/a/a1/Mini-Mag_Orion_and_s...

===

2.3. Deceleration: the canonical issue

While accelerating to a sizable fraction of the speed of light is a daunting task, deceleration is a yet more challenging task. The reason is that no realistic scenario to provide the gravy-train approach to either momentum transfer or pellet energy transfer at distances of 4+ light years seems within known physics. We are therefore challenged with either sending sufficient numbers of pellet-sheath elements so that the spacecraft can decelerate with on-board energy, or to come up with some other innovative concept.

2.3.1. Fueling at departure point

One option is to provide all necessary propellant at the departure location. If the specific power and specific impulse is sufficiently high, this is a potential approach. However, for our MMO concept, it is not particularly feasible. For example, our specific power is certainly high enough, but our specific impulse is dictated by the pellet-sheath combination, which limits the Isp to 128,955 s. While this may seem like a prodigious specific impulse, the ratio of spacecraft speed .1 c to g * Isp is 23. This means that the deceleration propellant must be e^23 times the spacecraft mass, which is an unworkable solution.

2.3.2. Mag Sail deceleration

In 2003 both Andrews and Lenard postulated using a large superconducting ring to intercept charge particles in interstellar space to slow the spacecraft down from high speeds. Additionally, the solar wind emanating from a star system provides an additional source of charged particles that can interact with the magnetic field. Deceleration can actually begin a sizable distance from the target star system. The following two charts indicate deceleration, velocity and time as a function of distance from the target star system. In this case the first phase of the deceleration starts at 21600 AU with a twoturn superconducting carbon nano tube reinforced loop. This loop captures the charged interstellar medium and deflects it to decelerate the spacecraft. This initial hoop size is 500 km in radius and carries 1,000,000 A of current. The spacecraft decelerates from .1 c to 6300 km/s by the time the spacecraft reaches 5000 AU. This will be quite a light show, so if there are any intelligent life forms with an observing system, they should be able to see the arrival.

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